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Block 13 Condensate Removal

Heat Exchangers and Stall Module 13.1

Module 13.1 Heat Exchangers and Stall

The Steam and Condensate Loop

13.1.1

Heat Exchangers and Stall Module 13.1

Block 13 Condensate Removal

Heat Exchangers and Stall Foreword This Block discusses the removal of condensate from heat exchange equipment supplied by saturated steam and fitted with: o

A temperature control valve on the steam line to the heat exchanger.

o

A steam trapping device on the condensate line from the heat exchanger.

The primary side of the heat exchanger will be referred to as the ‘steam space’, and the steam trapping device will be referred to as the ’trap’. The ‘trap’ can be a ‘steam trap’, a ‘pump trap’, or a ‘steam trap and pump’ fitted in combination. On these installations, a control sensor monitors the temperature of the outgoing heated fluid in the secondary circuit. The control valve endeavours to maintain a temperature determined by the controller, regardless of variations in heat load. The valve achieves this by opening or closing to alter the flowrate of steam, thereby varying the steam space pressure. The discharge from the steam trap may be subject to a lift and /or pressure in the condensate line, or may fall to an open end where it is subjected only to atmospheric pressure. This Block will refer to condensate pressure as ‘backpressure’. The heat exchange equipment can be almost anything that meets the above criteria. Examples include: o

Shell and tube heat exchangers.

o

Plate heat exchangers.

o

Air heating coils or batteries in ductwork.

o

Pipe runs or pipe coils in process equipment, tanks, vats etc.

For brevity, this Block will refer to all such devices as ‘heat exchangers’ or ‘heaters’, and the passage of fluid being heated by the heat exchanger will be referred to as passing through the ‘secondary’ side of the heat exchanger. The performance of steam heat exchangers is often reduced due to condensate flooding the steam space and waterlogging. The two main causes of waterlogging are: o

Fitting the wrong type of trap.

o

Stall.

Important note

Some systems aim to achieve control of temperature by positively encouraging partial flooding of the steam space of the heat exchanger. In these cases, the modulating action of the control valve at the condensate outlet varies the condensate level in the steam space. This changes the area of heating surface exposed to steam, and the effect is to change the heat transfer rate so as to control the secondary outlet temperature. With systems of this type, it is important that the heat exchangers be designed and manufactured specifically to withstand the effects of flooding. Where this is not done, the presence of condensate in the heat exchanger will have an adverse effect on operating performance and will reduce service life. This method of control can have certain benefits if the system is designed correctly. One is that the condensate sub-cools in the heat exchanger before it is discharged. This can considerably reduce the amount of flash steam in the condensate pipework, which may improve the performance of the condensate system and also reduce heat losses. The main operational disadvantage is that systems of this type are slow to respond to variations in heat load. 13.1.2

The Steam and Condensate Loop

Block 13 Condensate Removal

Heat Exchangers and Stall Module 13.1

What is meant by stall? Stall is the reduction or the cessation of condensate flow from the heat exchanger, and occurs when the pressure in the heat exchanger is equal to, or less than, the total backpressure imposed on the steam trap. Lower than expected pressure in a heat exchanger may occur as a result of any of the following circumstances: o

The secondary fluid inlet temperature rising as a result of a falling heat load.

o

The secondary fluid flowrate falling as a result of a falling heat load.

o

The secondary fluid outlet temperature falling due to a lowering of the set point.

As the control valve reduces the steam pressure to meet a falling heat load, the lack of differential pressure across the steam trap causes condensate to waterlog the steam space, as shown in Figure 13.1.1. Condensate return Steam in The control valve is throttling to meet a reduced heat load

Steam in the top of the heater

Hot air coming off the top of the heater Lift and /or back pressure

Fresh air in Air ducting Waterlogged condensate in the bottom of the heater

Cooler air coming off the bottom of the heater The steam trap goes cool or cold

Fig. 13.1.1 An air heater battery suffering the effects of stall

Due to applied safety factors and because heat exchangers are sold in pre-determined sizes, they often have more heating area than required. This has the effect of increasing the heat transfer capability of the exchanger above that required. It also means that the operating steam pressure will be lower than in a comparable heat exchanger perfectly sized for the same duty. The result is that less steam pressure is available to push out the condensate than may be expected. The steam pressure in the heat exchanger is important because it influences the stall condition, which in turn affects trap selection. Before any trap selection and sizing can take place, it is necessary to determine whether or not stall will occur, and if it does, to what degree. If this is not done, it is likely that the heat exchanger will suffer from waterlogging for some or all of its operating life. This, when it occurs, may not be immediately recognised by the observer or operator, as operating performance might not be reduced in an oversized heat exchanger. However, waterlogging can have severe financial consequences, short and long term, unless the heat exchanger is designed to operate this way.

Short-term problems

Consider an oversized heater battery operating as a frost coil and fitted with the wrong type (or size) of trap, as in Figure 13.1.1. In this example, the frost coil is preheating chilled air before it passes on to the main heater battery. Though the frost coil is fulfilling its thermal expectations (because it is oversized for the duty), it will do so with the bottom half of its coils waterlogged. Incoming cold air approaching 0°C (typically flowing at 3 m /s) passing over the coils can easily cause the water in them to freeze. This results in having to repair or replace the heater battery, either causing inconvenience or unexpected outlay. Waterlogging and freezing will not arise if the application is correctly designed. The Steam and Condensate Loop

13.1.3

Block 13 Condensate Removal

Heat Exchangers and Stall Module 13.1

Long-term problems

Traps that are undersized will sometimes show no immediate adverse effects on heater performance if the heater is oversized. Ironically, the wrong type of trap fitted to a heat exchanger can often exaggerate a superficial improvement elsewhere in the condensate system. For instance, a thermostatic or fixed orifice fitted to any heat exchanger will hold back condensate so that it sub-cools below the steam saturation temperature. This will have the effect of reducing flash steam from any natural outlet such as a condensate receiver vent. The casual observer can interpret this as a way to save energy and can easily be tempted to fit these devices. Unfortunately, the situation is not as straightforward as it seems. The reality is that holding back condensate until it sub-cools implies waterlogging to some degree. Condensate that continually floods the steam space will cause corrosion with costly results. The service life of the heat exchanger is reduced, and the overall lifetime costs of the installation will increase. The effects suffered by a waterlogged heat exchanger depend upon the circumstances of the particular installation. The symptoms and effects of stall are itemised later in this Module.

How does stall occur? To understand stall it is necessary to appreciate that saturated steam is a condensing vapour, which gives up its heat as it condenses to water. This condensation always occurs at a constant temperature when the pressure in the steam space remains constant. For example, saturated steam at atmospheric pressure has a temperature of 100°C and will also condense back into water at 100°C, whereas at a gauge pressure of 1 bar, saturated steam has a temperature of 120°C and will condense back into water at 120°C. Steam can also exist inside heat exchangers at below atmospheric pressure i.e. steam at 0.5 bar below atmospheric pressure has a temperature of about 82°C, and will also condense back to water at 82°C. The pressure and temperature relationship of saturated steam is entirely predictable and is documented in steam tables. Basic heat exchanger theory states that the higher the steam temperature above that of the secondary fluid being heated, the greater the potential heat transfer rate. To vary the transfer of heat from condensing steam, the temperature (and thus the pressure) of the steam in the steam space is varied. For example, if a heat exchanger uses steam at 160°C at maximum load, and the load is reduced by 50%, steam at a lower temperature is required. To achieve this, the steam pressure must be reduced, and, in many cases, becomes less than the backpressure. Example: A heat exchanger running at full-load uses saturated steam at 1 bar g (120°C) to heat water from 40°C to 60°C. Full-load therefore occurs when the water temperature rises by 20°C, and the mean water temperature is: 40°C + 60°C Mean water temperature at full-load = = 50°C 2 The difference between the steam temperature and the mean water temperature is termed the Arithmetic Mean Temperature Difference or AMTD, and the heat transfer rate is proportional to this. The full-load AMTD in this example is 120°C - 50°C = 70°C. Consider the situation where the process load falls to 2 /3 load. At full-load, the water temperature rise is 20°C. If the load falls to 2 / 3 full-load, and the outlet water temperature remains constant at 60°C, this means that the temperature rise must be 2 /3 of 20°C Therefore: = 13.3°C At 2 / 3 load, temperature rise = 2 /3 of 20°C and the inlet temperature = 60°C - 13.3°C = 46.7°C

13.1.4

The Steam and Condensate Loop

Block 13 Condensate Removal

Heat Exchangers and Stall Module 13.1

Consequently at 2 /3 load, the return water temperature will have risen to 46.7°C, and so the mean water temperature is now: Mean water temperature at

2 46.7°C + 60° C load = = 53.3°C 3 2

At 2 /3 load, the heat transfer needed will be 2 /3 of that at full-load, and equally the AMTD will be 2 /3 of that at full-load, i.e. 2 2 AMTD at load = x 70°C = 46.7°C 3 3 It follows that the steam temperature at 2 /3 load has to be the mean water temperature at 2 /3 load plus the AMTD at 2 /3 load, i.e. 2 Steam temperature at load = 53.3°C + 46.7°C = 100°C 3 As the temperature of saturated steam at atmospheric pressure is 100°C, this means that the pressure in the steam space is now atmospheric. Consequently, there is no steam pressure available in the steam space to push the condensate through a steam trap. Even if the condensate line fell to an open-ended steam trap, the condensate might not drain out of the exchanger. The condensate will ‘back-up’ the drain line and waterlog the heat exchanger unless proper precautions are taken. If condensate backs up into the exchanger, the surface area available to condense steam is reduced, the heat flow drops and the temperature of the outgoing heated water begins to fall. When the temperature sensor detects this, the controller opens the control valve a little more and the inflow of steam increases. This raises the pressure in the steam space above atmospheric (in this case) and soon becomes high enough to push condensate through the trap. The condensate level falls, but now the steam space pressure is higher than the atmospheric pressure needed to just heat the water to 60°C. The water temperature then climbs. When the sensor detects this, the controller closes down the control valve. The steam space pressure falls to atmospheric - and the flooding begins again. The result is a continual cycling of the water temperature above and below 60°C. If the secondary medium were other than water this could, in many cases, affect its quality.

What are the symptoms and effects of stall? One or more of the following symptoms may be evident:

In summary: 1. Cold or cool steam trap. 2. Hunting control valve. 3. Fluctuating outlet temperature. 4. Stratified heater temperatures. 5. Waterhammer. 6. Reduced heat output. 7. Reduced product quality. 8. Corroding heat exchangers. 9. Leaking heat exchangers. 10. Failing heat exchangers.

In detail: o

o

The steam trap goes cold, or is noticeably cooler than the temperature of the steam pipe inlet to the heat exchanger. The control valve is prone to ‘hunting’, i.e. it cycles regularly somewhere between its open and closed positions.

The Steam and Condensate Loop

13.1.5

Block 13 Condensate Removal

o

o

Heat Exchangers and Stall Module 13.1

The temperature of the secondary fluid flowing from the heat exchanger is less accurate than is expected or required. There is stratification of temperature on the output side of the heat exchanger. This will be more apparent on heater batteries and unit heaters. For example, it is almost certain to be detectable on the air heater battery depicted in Figure 13.1.1. The design is such that the face of the heat exchanger surface is usually accessible, often via an access panel or door in the side of the ducting. If stall is happening, the top of the battery closest to the steam inlet will be very hot, whereas lower down, it will be much cooler or even cold, and the trap will be cool or cold. The temperature of the air flowing through the top of the battery will be noticeably higher than that flowing through the bottom.

o

o

o

o

The heat exchanger makes crackling, banging or thumping noises either continuously or intermittently. Sometimes these noises are associated with severe waterhammer that can cause physical damage to the heat exchanger and any equipment fitted to it. The hot steam condensing into the waterlogged condensate causes the waterhammer and resulting noises, especially when the waterlogging level varies with changes in load. In process applications, the result of one or more of the above symptoms may be poor or unreliable product quality. Increased corrosion. The waterlogged condensate cools to temperatures much lower than the steam temperature at the inlet to the steam space. Carbon dioxide and oxygen dissolve much more readily into cooler water. Carbon dioxide is a common by-product of incorrect boiler water treatment and is carried over into the heat exchanger with the steam. When it dissolves into water it forms carbonic acid, which causes corrosion. Oxygen is present in raw water, and if not completely removed by the water treatment process, it too will get carried over with the steam. Its presence in water, especially cool water in which it will readily dissolve, also aggravates corrosion. Corrosion rates are greatly accelerated when both gases are present. The degree of corrosion will depend upon the heat exchanger material. Copper, carbon steel, and stainless steel will each be affected differently.

o

Mechanical stress. The hot steam in the top of the steam space will cause the heat exchanger to expand there, while the cool water in the bottom of the steam space has the reverse effect. This uneven expansion / contraction can cause mechanical stress to the heat exchanger structure, notably to the soldered, brazed, welded or expanded joints in ‘plate’ and ‘shell and tube’ heat exchangers, and air heater batteries. The most common result is leakage of steam to the surroundings in the former, or into the secondary airflow in the latter. The stress tends to be worse if the waterlogging level continually varies, especially if it varies quickly. The level of waterlogging will vary as the load changes, and as a result; the control valve and steam trap will struggle to achieve stable control. It should be said that a properly engineered plate heat exchanger with gasket joints suitably designed for steam will be very resilient to such stress.

The ultimate effect of stall is increased maintenance and shorter service life of the heat exchanger and associated equipment. This increases overall running costs.

13.1.6

The Steam and Condensate Loop

Block 13 Condensate Removal

Heat Exchangers and Stall Module 13.1

Do all heat exchangers suffer from stall? No. The conditions may be such that there will always be sufficient positive pressure upstream of the steam trap to clear the condensate so stall cannot occur. As a general rule, the higher the secondary temperature above 100°C, and the more stable the running load, (especially if near to the maximum output of the heat exchanger), the less likely for stall to occur. However, each application is unique and will require individual consideration. The only ways to determine the dynamics of the installation are to either plot the application temperatures on a chart or to perform a mathematical calculation. This is explained in Module 13.2, ‘Condensate Removal from Heat Exchangers’. Some applications can appear to operate with partial waterlogging, and show little effect of waterhammer. These tend to be steady load applications, or where the load changes only slightly and very slowly, and /or applications that employ very robust heat exchange equipment. One such example would be large bore corrosion resistant heating coils inside tanks correctly arranged to have a positive fall towards the trapping points. Even in applications of this type, if the installation is designed or corrected to eliminate stall, improved operation, improved reliability, and reduced lifetime costs are virtually guaranteed.

The Steam and Condensate Loop

13.1.7

Heat Exchangers and Stall Module 13.1

Block 13 Condensate Removal

Questions 1. What is the prime cause of stall in heat exchangers? a| Not enough steam pressure upstream of the control valve

¨

b| The heat exchanger pressure is equal to or less than the backpressure

¨

c| The heat exchanger is undersized

¨

d| The condensate discharges to atmosphere

¨

2. What effects will waterlogging cause in some heat exchangers? a| None at all

¨

b| It increases the steam pressure in the heat exchanger

¨

c| It can cause swings in the temperature of the heated fluid and corrode the heat exchanger?

¨

d| It increases the thermal performance

¨

3. In a heat exchanger at full-load, steam temperature is 140°C, inlet temperature is 20°C, and the outlet temperature is 80°C, what is the AMTD? a| 40°C

¨

b| 120°C

¨

c| 60°C

¨

d| 90°C

¨

4. In the same heat exchanger at half load, what is the secondary mean temperature? a| 65°C

¨

b| 50°C

¨

c| 80°C

¨

d| 45°C

¨

5. In the same heat exchanger at half load, what is the steam temperature? a| 70°C

¨

b| 100°C

¨

c| 110°C

¨

d| 95°C

¨

6. If, for the same duty, a larger heat exchanger were used, what would be expected of the steam temperature at full-load? a| It would be higher than 140°C

¨

b| It would be lower than 140°C

¨

c| It would also be at 140°C

¨

d| It would be at 100°C, i.e. atmospheric pressure

¨

Answers

1:b, 2: c, 3:d, 4: a, 5: c, 6: b

13.1.8

The Steam and Condensate Loop

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

Module 13.2 The Heat Load, Heat Exchanger and Steam Load Relationship

The Steam and Condensate Loop

13.2.1

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

The Heat Load, Heat Exchanger and Steam Load Relationship Saturated steam is used to provide primary heat to a process fluid in a heat exchanger. The term heat exchanger is used to describe all types of equipment where heat transfer is promoted from one fluid to another. For convenience, this broad definition will be applied to the term heat exchanger. While shell and tube heat exchangers and plate heat exchangers will be principally referred to, stall may also be relevant to applications including air heater batteries, submerged tank coils, jacketed vessels and storage calorifiers.

Temperature controlled applications In a temperature control application, the inlet temperature of the secondary fluid to the heat exchanger may change with time. This means that in order to maintain a consistent secondary fluid outlet temperature, the heat supplied to the heat exchanger must also vary. This can be achieved by using a control valve on the inlet to the primary side of the heat exchanger, as shown in Figure 13.2.1. Temperature controller set at 60°C

P1

Control valve P2

Temperature sensor Hot water out 60°C

Steam in Steam pressure into heat exchanger

Shell and tube heat exchanger Cold water in 10°C To condensate main

Steam trapping Fig. 13.2.1 Typical temperature control of a steam /water shell and tube heat exchanger

A control valve is used to vary the flowrate and pressure of the steam so that the heat input to the heat exchanger can be controlled. Modulating the position of the control valve then controls the outlet temperature of the secondary fluid. A sensor on the secondary fluid outlet monitors its temperature, and provides a signal for the controller. The controller compares the actual temperature with the set temperature and, as a result, signals the actuator to adjust the position of the control valve. For a constant heating area and heat transfer coefficient, the rate at which heat is transferred from the steam to the secondary fluid for a particular heat exchanger is determined by the mean temperature difference between the two fluids. A larger difference in mean temperatures will create a large heat transfer rate and vice versa. On partially closing the control valve, the steam pressure and the temperature difference fall. Conversely, if the control valve is opened so that the steam mass flow and hence pressure in the heat exchanger rise, the mean temperature difference between the two fluids increases. Altering the steam pressure will also slightly affect the amount of heat energy available in the condensing steam as the enthalpy of evaporation actually falls with increasing pressure. This means that the latent heat available per kg of steam reduces as the steam pressure increases. If steam flow accuracy is required, this must be accounted for. 13.2.2

The Steam and Condensate Loop

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

Example 13.2.1 A manufacturer is to design a heat exchanger in which the specification calls for steam at 4 bar g to heat secondary water from 10°C to 60°C. The water flow is to be constant at all loads at 1.5 L /s. It is assumed that 1 litre of water has a mass of 1 kg, so the mass flowrate = 1.5 L/ s x 1 kg/L = 1.5 kg/ s. The manufacturer uses a heat transfer coefficient ‘U’ for the heat exchanger of 2 500 W /m 2 °C. Take the specific heat of water as 4.19 kJ /kg °C. Determine: (A) The design heat load. (B) The corresponding steam flowrate. (C) The minimum heating area required. Also, if the customer’s minimum heat load occurs when the inlet water temperature rises to 30°C, determine: (D) The minimum heat load. (E) The corresponding steam pressure in the heat exchanger. (F) The corresponding steam flowrate. Calculations: (A) Find the design heat load using the heat transfer flowrate equation (Equation 2.6.5):

Q = m cp ∆T

Equation 2.6.5

Where: Q = Mean heat transfer rate (kW) m = Mean secondary fluid flowrate (kg) cp = Specific heat capacity of the secondary fluid (kJ / kg K) or (kJ / kg °C) DT = Temperature rise of the secondary fluid (K or °C) Q = 1.5 kg/s x 4.19 kJ / kg °C x (60 -10)°C Q = 314.25 kW (B) Find the corresponding steam flowrate at 4 bar g, saturation temperature (T s) is 152°C, and hfg = 2 108.1 kJ / kg (from steam tables). Calculate the required steam flow at the design condition using Equation 2.8.1:

Steam flowrate (kg h) = Steam flowrate (mS ) =

Load in kW x 3 600 hfg at operating pressure

Equation 2.8.1

314.25 x 3 600 kg / h 2 108.1

Steam flowrate (ms) = 536.6 kg /h

The Steam and Condensate Loop

13.2.3

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

(C) Find the minimum heating area to meet the requirement using Equation 2.5.5. Note; the manufacturer uses the Logarithmic Mean Temperature Difference (DTLM) to calculate the minimum amount of heating area to satisfy the design rating: ∆TLM =

Where: DTLM = Ts = T1 = = T2 ln =

T2 - T1 æ Ts - T1 ö In ç ÷ è Ts - T2 ø

Equation 2.5.5

Logarithmic Mean Temperature Difference (LMTD) Steam temperature (°C) Secondary fluid in temperature (°C) Secondary fluid out temperature (°C) The mathematical function known as ‘natural logarithm’ ∆TLM =

∆TLM =

∆TLM =

60 - 10 152 - 10 ö In æç ÷ è 152 - 60 ø

50 142 ö In æç ÷ è 92 ø 50 0.434

DTLM = 115.2°C By re-arranging the general heat transfer equation (Equation 2.5.3: Q = U x A x DT) Equation 13.2.1 can be formulated, where DT can be represented by the mean value DTM. A= Where: A = Q = U = DT M =

Q U ∆TM

Equation 13.2.1

Heating area (m²) Mean heat transfer rate (W) Heat transfer coefficient (W / m² °C) Mean Temperature Difference. Note: DTM may be either DTLM (LMTD) or DTAM (AMTD). A=

314 250 W 2 500 W/m2 °C x 115.2°C

A = 1.09 m² For the purpose of this example it will be assumed that the heat exchanger is designed to have exactly this area of 1.09 m². (D) Find the minimum heat load, when the inlet water temperature is 30°C, using the heat transfer flowrate equation (Equation 2.6.5) as used in Part ‘A’ of these calculations: Q = m cp ∆T

Equation 2.6.5

Qmin = 1.5 kg / s x 4.19 kJ/ kg °C x (60 - 30)° C Q min = 188.5 kW

13.2.4

The Steam and Condensate Loop

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

To calculate the corresponding steam flowrate, it is first necessary to determine the steam temperature at the minimum load condition. It is possible to use the DTLM design figures to accurately predict the steam temperature for any load condition, but this requires the use of logarithmic calculations. However, once the exchanger size is fixed and the design temperatures are known, it is much easier to predict operating temperatures using what could be termed a heat exchanger Temperature Design Constant (TDC). The TDC method does not require logarithmic calculations. Please note: TDC cannot be used on those applications where the secondary flowrate varies or where control is achieved by varying the condensate level in the steam space. Note: When sizing a heat exchanger it is normal for heat exchanger manufacturers to use the DTLM method. Once sized, by knowing the heating area and the full-load operating temperatures, TDC can be used to accurately predict all operating temperatures resulting from changes in load, as can be seen in the following text. Operating temperatures can also be predicted graphically by using what is termed a ‘Stall Chart’. This method is discussed in Modules 13.5, 13.6, and 13.7.

Temperature Design Constant (TDC)

For any type of steam-heated exchanger with the secondary liquid flowing at a constant rate, TDC can be calculated from the test figures quoted by the manufacturer for full-load. If these data sets are not available and the heat exchanger is already installed in service, TDC can be calculated by observing the steam pressure (and finding the steam temperature from steam tables) and the corresponding secondary inlet and outlet temperatures at any load. TDC is the ratio of the steam to water temperatures at the inlet and outlet; and is shown in Equation 13.2.2. TDC =

Ts - T1 Ts - T2

Equation 13.2.2

Where: TDC = Temperature Design Constant Ts

= Steam temperature

T1

= Secondary fluid inlet temperature

T2

= Secondary fluid outlet temperature

In Example 13.2.1 at full-load conditions: The steam pressure = 4 bar g The inlet water temperature (T1) = 10°C The outlet water temperature (T2) = 60°C Steam temperature at 4 bar g (Ts) = 152°C Ts - T1 TDC = Ts - T2 152 - 10 152 - 60 142 TDC = 92 TDC = 1.543 5 for this particular heat exchanger TDC =

The Steam and Condensate Loop

13.2.5

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

The TDC equation can be transposed to find any one variable as long as the other three variables are known. The following equations are derived from the TDC equation (Equation 13.2.2). To find the steam temperature at any load use Equation 13.2.3: Ts =

(T2 x TDC) - T1 TDC - 1

Equation 13.2.3

To find the secondary fluid inlet temperature at any load use Equation 13.2.4: Equation 13.2.4

T1 = Ts - [ TDC (Ts - T2 ) ]

To find the secondary fluid outlet temperature at any load use Equation 13.2.5: TS - T1 ù T2 = TS - éê ë TDC úû

Equation 13.2.5

For any heat exchanger with a constant secondary flowrate, the operating steam temperature can be calculated for any combination of inlet temperature and outlet temperature. In Example 13.2.1 the secondary outlet temperature remains at 60°C, and minimum load occurs when the inlet temperature is 30°C. What is the steam temperature at minimum load? Inlet temperature

= 30°C

Outlet temperature = 60°C Using Equation 13.2.3:

Ts =

(60 x 1.543 5) - 30 0.543 5

Ts =

62.61 0.543 5

Steam temperature (T s) = 115.2°C (E) Find the corresponding heat exchanger steam pressure and enthalpy at minimum load From steam tables: A steam temperature of 115.2°C corresponds with a steam pressure of 0.7 bar g. The specific enthalpy of evaporation at 0.7 bar g (hfg) = 2 215 kJ / kg (F) Find the steam flowrate at minimum load: From (D) the minimum heat load is 188.5 kW. From (E) the hfg is 2 215 kJ /kg. Using Equation 2.8.1:

Steam flowrate (kg /h) =

Steam flowrate (mS ) =

kW rating x 3 600 hfg at operating pressure

Equation 2.8.1

188.5 x 3 600 kg / h 2 215

Steam flowrate (m s) = 306.4 kg / h at minimum load 13.2.6

The Steam and Condensate Loop

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

Block 13 Condensate Removal

Questions 1.

What determines the rate of heat transfer for any given heat exchanger?

a| The heat transfer coefficient

¨

b| The mean temperature difference between the two fluids

¨

c| The heating surface area

¨

d| All of the above

¨

2.

How is the temperature of steam controlled in a heat exchanger?

a| By a steam trap

¨

b| By changing the steam pressure upstream of the control valve

¨

c| By controlling the steam flow and pressure in the steam space

¨

d| By an adjustable safety valve

¨

3.

What is LMTD?

a| Logarithmic Maximum Temperature Difference

¨

b| Latent Mean Temperature Difference

¨

c| Logarithmic Mean Temperature Difference

¨

d| Lowest Minimum Temperature Difference

¨

4.

What is the basic difference between LMTD and TDC?

a| None

¨

b| LMTD is used to accurately calculate a required heating surface area while TDC can be used to easily predict operating temperatures

¨

c| LMTD is easier to use

¨

d| Using TDC is more accurate than using LMTD

¨

5.

What effect does lowering the steam pressure have?

a| The steam temperature rises

¨

b| It has no effect on the temperature but increases the latent heat

¨

c| The steam temperature falls

¨

d| The total heat in steam increases

¨

6.

Knowing the heat load, what other factor has to be known to accurately determine the steam mass flowrate to any piece of equipment?

a| The temperature of steam

¨

b| The total heat in the steam

¨

c| The enthalpy of evaporation of the steam at the evaporating pressure

¨

d| The specific volume of the steam

¨

Answers

1: d 2: c, 3: c, 4: b, 5: c, 6: c The Steam and Condensate Loop

13.2.7

Block 13 Condensate Removal

13.2.8

The Heat Load, Heat Exchanger and Steam Load Relationship Module 13.2

The Steam and Condensate Loop

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

Module 13.3 Oversized Heat Exchangers

The Steam and Condensate Loop

13.3.1

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

Oversized Heat Exchangers The effect of oversizing a heat exchanger The previous calculations (Module13.2) assumed that the heat exchanger had been sized on the perfect heating area to meet the specification. This would mean that the heat exchanger was exactly sized for the duty. This is highly unlikely in practice as the designer or specifier will usually add other factors, including those for fouling and uncertainty of maximum operating loads. It is also unlikely that manufacturers can supply heat exchangers to match a specification exactly. As undersized heat exchangers are impractical they are usually bought oversized. The operating conditions laid down in Example 13.2.1, Part ‘C’, have been reconsidered in Example 13.3.1 by adding 15% to the required heating area to account for contingencies. Required heating area is calculated to be 1.09 m² (Example 13.2.1, Part ‘C’) therefore the specified heating area for Example 13.3.1 is to be 1.09 + 15% = 1.254 m². The minimum size that the manufacturer can supply has a heating area of 1.31 m², representing an actual heating area of some 20% above that required. A larger heating area requires less steam pressure for the same heat transfer rate, and because of this the steam pressure in an oversized heat exchanger will be lower for the same heat load. As the steam pressure is less, the steam temperature is less, and the heat exchanger LMTD (Logarithmic Mean Temperature Difference) will also be less. To determine the steam temperature for the design condition, it is first necessary to find the new LMTD (DTLM) for the larger heating area (see Example 13.3.1).

Example 13.3.1

The DTLM can be found by re-arranging Equation 13.2.1 to give Equation 13.3.1

$ 

∆70  

 8∆70

Equation 13.2.1

 8$

Equation 13.3.1

Where: DTM = Mean temperature difference. Note: DTM may be either DTLM (LMTD) or DTAM (AMTD) Q

= Mean heat transfer rate (W)

U

= Heat transfer coefficient (W / m² °C)

A

= Heating area (m²)

∆70  

13.3.2

N:  ƒ& :P ƒ&[P

The Steam and Condensate Loop

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

From Example 13.2.2, at full-load: The secondary inlet temperature (T1) = 10°C The secondary outlet temperature (T2) = 60°C The new steam design temperature can now be determined using Equation 2.5.5: 7 7

∆7/0

 7V 7  ,Q    7V 7 

Equation 2.5.5

Where: DTLM = 95.95°C T1

= 10°C

T2

= 60°C

TS

= Steam temperature °C   7V   /Q    7V  



   76    /Q    =   76       76    /Q    =    76   

By taking antilogs of both sides of the equation . . . 76  76  76  76 

= H

= 

76   76   76 76    [  76 6WHDPWHPSHUDWXUH76

   

 

Steam temperature (TS) = 133.1°C This temperature corresponds to a steam pressure of 1.95 bar g. When the heat exchanger was perfectly sized in Module 13.2, the steam pressure was 4 bar g. In this example, with a heat exchanger 20% oversized, the steam pressure is 51% less. Now that the steam pressure has been predicted at the full-load condition, it is possible to calculate the steam flow at full-load. The Steam and Condensate Loop

13.3.3

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

By using Equation 2.8.1 find the steam flowrate at the full heat load of 314.25 kW. At 1.95 bar g, steam tables state that the enthalpy of evaporation is 2 164.6 kJ / kg.

6WHDPIORZUDWHNJ  K  = 6WHDPIORZUDWHV  

/RDGLQN:[   KIJ DWRSHUDWLQJSUHVVXUH

Equation 2.8.1

[ NJ  K 

6WHDPIORZUDWH V   NJ  KDWIXOO  ORDG The steam flow was 536.6 kg / h in the perfectly sized heat exchanger (Example 13.2.1), so it can be seen that there is a slight drop (2.5%) in mass flowrate. This is due to the steam having a slightly larger enthalpy of evaporation in the larger heat exchanger due to its lower pressure.

Determine the TDC for the larger heat exchanger

Now that the steam temperature has been determined for the oversized heat exchanger (using the LMTD equation [Equation 2.5.5]), it is now possible to find its TDC, using Equation 13.2.2. 7'& 

76 7 76 7

7'& 

 

7'& 

 

Equation 13.2.2

Where: TDC = Temperature Design Constant TS

= 131.1°C

T1

= 10°C

T2

= 60°C

TDC = 1.684 At the minimum heat load: When the heat exchanger was perfectly sized in Example 13.2.1 the steam temperature was 115.2°C at the minimum heat load of 188.5 kW. Because the oversized heat exchanger in this example is about 20% larger, the steam temperature will also be less at the minimum heat load. The minimum heat load remains the same as in Example 13.2.1 and occurs when the secondary inlet temperature rises to 30°C. From Equation 13.2.3: 76  

7 [7'& 7 7'&

Equation 13.2.3

Where: TS

= Steam temperature °C

= 133.1°C

T1

= Secondary fluid inlet temperature °C

= 30°C

T2

= Secondary fluid outlet temperature °C = 60°C

TDC = Temperature Design Constant

13.3.4

= 1.684

The Steam and Condensate Loop

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

76  

[  

76  

 

TS = 103.8°C Comparing the two heat exchangers at minimum load, the steam temperature has dropped from 115.2°C in the perfectly sized heat exchanger to 103.8°C in the oversized heat exchanger. From steam tables, this steam temperature corresponds with a steam pressure of about 0.15 bar g, and hfg = 2 247 kJ / kg. The steam pressure in the perfectly sized exchanger (at 115.2°C) was 0.7 bar g. By using Equation 2.8.1, it is possible to find the steam flow at the minimum heat load of 188.5 kW.

6WHDPIORZUDWHNJ  K  =

6WHDPIORZUDWH6  

/RDGLQN:[   KIJ DWRSHUDWLQJSUHVVXUH

Equation 2.8.1

[ NJ  K 

Steam flowrate (mS) = 302 kg / h at full-load The minimum steam flow was 306 kg / h in the perfectly sized heat exchanger (Example 3.2.1), so it can be seen that there is a marginal drop in mass flow in the oversized heat exchanger at the minimum heat load. This is due to the steam having a slightly larger enthalpy of evaporation in the larger heat exchanger due to its lower pressure.

The steam pressure, the steam trap, and effective condensate removal

As the steam gives up its heat across the heat transfer surface to the secondary fluid, it condenses in the steam space. Condensate passes out through the outlet of the heat exchanger, and through a steam trap, which traps the steam in the steam space whilst allowing the condensate to be freely discharged. If the heat exchanger has not been specifically designed to operate with condensate flooding the steam space, the steam pressure needs careful consideration to ensure the heat exchanger is properly drained of condensate. Any waterlogging of the steam space will reduce the effective heating surface area, and the heat transfer requirement may be satisfied only if the exchanger is sufficiently (perhaps accidentally) oversized. The capacity of the steam trap will depend upon its type, its orifice size and the differential pressure across it. Differential pressure provides the energy to push the condensate through the trap, and is the difference between the steam pressure in the heat exchanger, and the backpressure exerted on the outlet of the trap by the condensate system. If the steam trap drains by gravity via a properly sized pipe to a vented condensate receiver or an open end, the backpressure should be very near atmospheric. Under these conditions, the differential pressure on a sizing chart can simply be read as the gauge pressure in the heat exchanger. If, however, there is a lift after the trap (a rise in the trap discharge line), or the trap discharge line is undersized, or this line is pressurised for any other reason, the backpressure may, at times, be greater than the pressure in the steam space. When this is so, the differential pressure across the trap is reversed and is deemed to be a ‘negative differential pressure’. The trap capacity is now zero.

The Steam and Condensate Loop

13.3.5

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

As can be seen in the above calculations, the steam pressure in any heat exchanger is governed by its size and the secondary conditions. As the capacity of the steam trap depends on the differential pressure, it follows that changes in the steam pressure and backpressure effect the capacity of the steam trap at all times. As the differential pressure reduces, the capacity of the steam trap will fall. Provided the differential pressure is positive and the steam trap is selected and sized with this in mind, waterlogging and its associated problems will not occur. Sizing the steam trap for the oversized heat exchanger The conditions that need consideration are: o

Full-load :

523 kg / h at 1.95 bar g in the steam space

o

Minimum load: 302 kg / h at 0.15 bar g in the steam space

o

Backpressure:

Atmospheric pressure (0 bar g) Controller

Control valve Vacuum breaker

Secondary out

Steam in Heat exchanger Secondary in

Static head above trap usually 0.5 to 1 m

Float type steam trap

Must drain by gravity to atmosphere Fig. 13.3.1 Static head and vacuum breaker method of dealing with stall

Consider, on the float trap capacity chart Figure 13.3.2, a DN25 (1") FT14-4.5 ball float steam trap. It can be seen that it will pass 850 kg / h at a differential pressure of 1.95 bar. It may also be seen that at a differential pressure of 0.15 bar it will pass about 370 kg / h. In this example, consider the trap fitted to the oversized heat exchanger and draining by gravity to a vented condensate receiver, as depicted in Figure 13.3.1. To ensure proper drainage, the steam trap has to be able to cope with all loads between the full-load and minimum load conditions.

13.3.6

The Steam and Condensate Loop

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

As the condensate backpressure is atmospheric in this example, the minimum steam space pressure of 0.15 bar g is always higher than the backpressure. It can be seen from the capacity chart (Figure 13.3.2) that the trap has enough capacity at the minimum and maximum loads, so the DN25 (1") FT14-4.5 ball float steam trap is big enough. If, however, in this example, the backpressure were higher than the minimum steam pressure of 0.15 bar g, the system would stall somewhere within the normal operating range. (This would only require a lift of just more than 1.5 metres after the trap to cause this). Accordingly, the trap would have to be selected and sized depending upon the amount of backpressure. With larger amounts of backpressure it may be necessary to fit a pump - trap. Advice on how to select the correct trap for a heat exchanger is given in Module 13.4. 1500

1000

Trap capacity at 1.95 bar Dp

Maximum flow Trap capacity at 0.15 bar Dp

Condensate kg/h

Minimum flow

2 DN

5(

1"

)

1 FT

4-

4 .5

500 400

300

D

200

5 N2

(½

",

(1

¾

")

")

1 FT

1 FT

4-

4-

10

4.

5

4 -1 14 T F 1 ") (1 DN 0 -1 25 14 DN T F ") ," ¾ (½ 4 20 -1 N D 14 , T F 15 ") DN ," ¾ (½ 20 N ,D 15 DN

5,

100

2 DN

0

50 40 30

20 0.1

0.2

0.3

0.5

DP at minimum load (0.15 bar)

1

2

3

4

5

10

14

DP at maximum load (1.95 bar)

DP - Differential pressure (bar) Fig. 13.3.2 FT14 ball float steam trap capacity chart showing data for Example 13.3.1 The Steam and Condensate Loop

13.3.7

Oversized Heat Exchangers Module 13.3

Block 13 Condensate Removal

Questions 1. Why is it usual to fit an oversized heat exchanger? a| It can cope with contingency loads

¨

b| It can cope with future fouling effects

¨

c| Because this is what the supplier will offer

¨

d| All of the above

¨

2. The oversized exchanger had about 20% more heating area than the one perfectly sized in Example 13.2.1. What was the percentage drop in steam pressure at the same minimum heat load? a| About 78% drop in pressure

¨

b| About 24% drop in pressure

¨

c| About 10% drop in pressure

¨

d| The steam pressure remained the same in each heat exchanger

¨

3. For the same heat load on two heat exchangers, why is the mass flowrate of steam always less in the exchanger with greater heating surface? a| Because the control valve is smaller

¨

b| The steam pressure is less and so the enthalpy of evaporation is more

¨

c| Because there is more heating area in the oversized exchanger

¨

d| The steam pressure is less and so the steam will be drier

¨

4. What is the effect of higher backpressure on a steam trap? a| None whatsoever

¨

b| It reduces the steam pressure in the heat exchanger

¨

c| It reduces the capacity of a steam trap

¨

d| It increases the differential pressure across the steam trap

¨

5. What effect does a lowering steam pressure have? a| The steam temperature rises

¨

b| It has no effect on the temperature but increases the latent heat

¨

c| The steam temperature falls

¨

d| The total heat in steam increases

¨

6. In Example 13.3.1, if the backpressure were 2 bar g, what size should the trap be? a| Larger than 1”

¨

b| 1”

¨

c| A pump-trap should be used

¨

d| 2”

¨

Answers

1: d, 2: c, 3: b, 4: c, 5: c, 6:c

13.3.8

The Steam and Condensate Loop

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

Module 13.4 Example: Selecting the Trap

The Steam and Condensate Loop

13.4.1

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

Example: Selecting the Trap Example 13.4.1 Selecting the trap

A factory requires a steam / water heat exchanger operating at a nominal 4 bar g to heat process water circulating at 1 L / s (1 kg / s) from 10°C to 80°C, giving a design load of 293 kW. The process is such that a minimum heat load occurs at 60% of the full heat load. This is a permanently running process line with no future load increase. Two suppliers are asked to provide a heat exchanger. The following information is important to selection: o

o

o

Supplier ‘X’ can provide a heat exchanger with a heating area of 2 m2, a ‘U’ value of 2 500 W/ m2 °C and duty of 350 kW when operating with steam at 4 bar g and with a water flow of 1 L / s. Supplier ‘Y’ is able to provide a heat exchanger with a smaller heating area more suitable for the design heat load of 293 kW, when operating with steam at 4 bar g and with a water flow of 1 L / s. The ‘U’ value is 2 500 W / m² °C. The heat exchanger condensate line will lift 5 metres to a condensate return pipe that falls en route to a vented receiver, and having a total backpressure of 0.5 bar g. Note: A one metre column of water under atmospheric pressure will exert a pressure at the bottom of the column of approximately 10 kPa or 0.1 bar g. Any lift in the condensate discharge line will thus exert a static lift due to the column of condensate held in the line, in addition to any pressure in the condensate system.

It is necessary to determine the system operating conditions to select and size the trap for proper condensate removal from both heat exchangers under any operating load condition. The following questions need to be answered for proper condensate removal: (A) Will stall occur during normal operation? (B) At what load will stall occur? Check the application heat load at the design condition. From the heat transfer flowrate equation (Equation 2.6.5):  &S ∆7

Equation 2.6.5

Q = 1.5 kg / s x 4.19 kJ / kg °C x (80 -10)°C Heat transfer rate (Q) = 293 kW (293 000 W) Consider supplier ‘X’ A 350 kW heat exchanger with a 2 m2 heating area.

What will be the steam space pressure in this heater at this design heat load? It is first necessary to determine the LMTD (DTLM) for a 2 m2 heating area. From Equation 13.2.1:

$ 

 8∆70

P  

 :  :P ƒ&[ D 70

D 70  

 :  :P ƒ&[P

Equation 13.2.1

DTM = 58.6°C 13.4.2

The Steam and Condensate Loop

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

The steam design temperature can now be calculated, by use of Equation 2.5.5: ∆7/0

Where: DTLM = T1 = T2 = = TS

7 7  7V 7  ,Q    7V 7 

Equation 2.5.5

58.6°C 10°C 80°C Steam temperature °C  

   76    ,Q    76  

  76    ,Q       76    76    ,Q       76  

By taking antilogs of both sides of the equation: 76   76  76   76 

 H  

76      76  76



Steam temperature (TS) = 110°C This saturation temperature is equivalent to a steam pressure of 0.45 bar g. This pressure is less than the 0.5 bar g backpressure, and the system will permanently stall. In this case, if a ball float steam trap were fitted, condensate would permanently flood the heat exchanger, its level modulating relative to changes in load. Operating performance might be unsatisfactory as the secondary outlet temperature will tend to fluctuate, and the heat exchanger might fail prematurely due to corrosion. If the system is permanently running under stall conditions, a ball float steam trap is the wrong choice for this application, and a pump-trap should be fitted instead.

The Steam and Condensate Loop

13.4.3

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

Consider supplier ‘Y’ For the manufacturer to size the heating area that best matches the design condition, it is necessary to find the minimum heating area that will satisfy the operating full-load. It is first necessary to determine the rated LMTD for the heat exchanger with a steam space pressure is 4 bar g (TS = 152°C). From Equation 2.5.5: ∆7/0

Where: DTLM = T1 = = T2 TS =

7 7  7V 7  ,Q    7V 7 

Equation 2.5.5

LMTD 10°C 80°C 152°C

∆7/0

∆7/0

   ,Q         ,Q     

∆7/0

 ,Q 

∆7/0

  ƒ& 

By using Equation 13.2.1 the minimum heating area can now be determined for the rating of 293 kW.

$  Where: A = Q = = T2 DTM =

 8∆70

Equation 13.2.1

Heating area (m²) Heat transfer rate (kW) Heat transfer coefficient (W / m² °C) DTLM

$ 

   [

+HDWLQJDUHD$  P

From his standard range, supplier ‘Y’ can provide a plate heat exchanger that meets the specification with a heating area of 1.198 m2. This is oversized (by about 5%) and steam pressure will therefore be less than 4 bar g at the full-load operating condition. In practice, heat exchangers are likely to be specified at least 10% over capacity. It is for this reason that the operating steam pressure (not the quoted normal working pressure) should always be established before selecting and sizing the steam trapping device. The reputable manufacturer should be willing to supply this information, or, at least, the heating area, the ‘U’ value, and the heat output. From this data, the rated LMTD can be calculated, from which the operating pressure can be found. 13.4.4

The Steam and Condensate Loop

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

Find the LMTD for the heat exchanger with a heating area of 1.198 m²:

$ 

∆70 ∆70

D 70

 8∆70

Equation 13.2.1

 8$  :  :Pò ƒ &[Pò

ƒ&

The operating steam temperature at full-load can now be found by use of Equation 2.5.5: 7 7

∆7/0

Where: DTLM = T1 = T2 = TS =

 76 7  ,Q    76 7 

Equation 2.5.5

97.8°C 10°C 80°C Steam temperature°C  

   76    ,Q    76  

  76    ,Q       76    76    ,Q       76  

By taking antilogs of both sides of the equation: 76   76  76   76 

 H

  

76     76  6WHDPWHPSHUDWXUH76  ƒ& This saturation temperature is equivalent to a steam pressure of 3.4 bar g at the design condition. As this pressure is more than the constant 0.5 bar g backpressure, the system will not stall at full-load.

The Steam and Condensate Loop

13.4.5

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

What is the steam flowrate (ms) at full-load? The steam mass flowrate will depend upon the steam space pressure, which is 3.4 bar g at full-load, with an enthalpy of evaporation of 2 122 kJ / kg. From Equation. 2.8.1:

6WHDPIORZUDWHNJ K =

/RDGLQN:[  KIJ DWRSHUDWLQJSUHVVXUH

6  

Equation 2.8.1

[  NJ  K  

6WHDPIORZUDWH 6  NJ  KDWIXOOORDG What is the TDC? It is now necessary to find the heat load at which the system will stall. In order to do so, it is necessary to calculate the TDC for this heat exchanger from the design conditions. From Equation 13.2.2: 7'& 

Where: TDC = T1 = T2 = TS =

76 7 76 7

Equation 13.2.2

Temperature Design Constant 10°C 80°C 147°C   7'&       7'& 

   

The stall condition

At stall, the pressure in the steam space will equal the 0.5 bar g backpressure. The saturation temperature of steam at 0.5 bar g is 111.6°C. From Equation 13.2.4 the inlet temperature can be found: 7  7V  [ 7'&7V 7  ]

Where: = T1 T2 = TS = TDC =

Equation 13.2.4

Inlet temperature °C 80°C 111.6°C 2.045 7   [ [  ]

7   7 ƒ&DWVWDOO

13.4.6

The Steam and Condensate Loop

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

What is the heat load at stall? From the heat transfer flowrate equation (Equation 2.6.5): 

&S ∆7

Equation 2.6.5

 NJ  V[N- NJ ƒ&[ ƒ& +HDWWUDQVIHUUDWH  N: : As full-load is 293 kW, the percentage stall load    [ RIIXOOORDG 

The selection of the trapping device will depend on whether the minimum heat load is higher or lower than the stall load. The minimum load is quoted as being 60% of the full-load of 293 kW, therefore: Minimum load = 0.6 x 293 kW = 176 kW Stall load

= 138 kW

As the minimum load is greater than the stall load, the system will never stall. It is therefore practical to fit a ball float steam trap, as there will always be a positive differential pressure across it. However, the ball float steam trap has to be sized to carry both the full-load and the minimum load, and it is therefore necessary to calculate the steam flows and the corresponding steam space pressures at both conditions. It is first necessary to calculate the secondary inlet temperature at the minimum load. This can be predicted by use of Equation 13.4.1: 7;   [ 7 7 [ ] 7

Equation 13.4.1

Where: Tx = The secondary inlet temperature at any load factor ‘x’ T1 = The secondary inlet temperature at full-load T2 = The secondary outlet temperature at full-load x = The load factor. For example; the minimum heat load of 60% is equivalent to a load factor of 0.6 7   [ 7 7 [ ] 7 7   [   ]  7   [[ ]  7  ƒ&

The Steam and Condensate Loop

13.4.7

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

The minimum load condition From Equation 13.2.3:

76  

7 [7'& 7 7'&

76  

[  

76  

 

Equation 13.2.3

Where: TDC = 2.045 T2

= 80°C

T1 TS

= 38°C = Steam temperature °C

6WHDPWHPSHUDWXUH76  ƒ&DWPLQLPXPORDG

This is the steam temperature at the minimum load of 176 kW, and is equivalent to a steam pressure of 1.0 bar g. The condensate pressure is 0.5 bar g. The differential pressure across the ball float steam trap at minimum load therefore equals 1.0 bar g - 0.5 bar g = 0.5 bar. What is the steam flowrate (mS(min)) at the minimum heat load of 176 kW? The minimum steam flowrate will depend upon the steam space pressure, which is 1.0 bar g with an enthalpy of evaporation of 2 201.1 kJ / kg. From Equation 2.8.1:

6WHDPIORZUDWHNJ K =

/RDGLQN:[  KIJ DWRSHUDWLQJSUHVVXUH

6  

Equation 2.8.1

[  NJ  K  

6WHDPIORZUDWH 6  NJ  KDWWKHPLQLPXPKHDWORDGRIN: As it has been established that this system will not stall, a ball float steam trap is suitable. It is now necessary to size a ball float steam trap for operation up to the maximum system differential pressure of 3.5 bar and pass . . . a) the full-load of 498 kg / h with a differential pressure of 3.4 bar g - 0.5 bar g = 2.9 bar g. b) the minimum load of 288 kg / h with a differential pressure of 1.0 bar g - 0.5 bar g = 0.5 bar g. It can be seen from the ball float steam trap sizing chart (Figure 13.4.1) that a DN25 (1") FT14-4.5 will satisfy both of these conditions, and could be selected. However, if the minimum heat load were less than the stall load, then a pump-trap would have to be selected. The methods of selecting trapping devices are further discussed in Module 13.8, ‘Practical methods of preventing stall’.

13.4.8

The Steam and Condensate Loop

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal 1500

1000

Trap capacity at 1.95 bar Dp

Maximum flow Trap capacity at 0.15 bar Dp

Condnesate kg/h

Minimum flow

DN

( 25

) 1"

F

4 T1

-4

.5

500 400

300

D

200

5 N2

(½

",

(1

¾

")

")

1 FT

1 FT

4-

4-

10

4.

5

4 -1 14 T F 1 ") (1 DN 0 5 -1 2 14 DN T F ") ¾ ", (½ 4 20 -1 N 14 ,D T 5 F 1 ") ¾ DN ", (½ 20 N ,D 15 N D

D 5,

100

N2

0

50 40 30

20 0.1

0.2

0.3

0.5

DP at minimum load (0.15 bar)

1

2

3

4

5

10

14

DP at maximum load (1.95 bar)

DP - Differential pressure (bar) Fig. 13.4.1 FT14 ball float sizing chart showing data for Example 13.4.1

The Steam and Condensate Loop

13.4.9

Example: Selecting the Trap Module 13.4

Block 13 Condensate Removal

Questions 1. If the heat exchanger from supplier ‘X’ had a heating area of 1 m² instead of 2 m², what would have been the LMTD for the same secondary full-load conditions? a| 33.6°C

¨

b| 29.3°C

¨

c| 117.2°C

¨

d| 107°C

¨

2. If the heat exchanger from supplier ‘X’ had a heating area of 1 m² instead of 2 m², what would have been the steam temperature at the full heat load of 293 kW? a| 55°C

¨

b| 100°C

¨

c| 117.2°C

¨

d| 165.7°C

¨

3. If the heat exchanger from supplier ‘X’ had a heating area of 1 m² instead of 2 m², would the system still stall at the full heat load of 293 kW? a| Yes, because the backpressure is higher than the steam pressure

¨

b| No, because the full-load pressure is 6.1 bar g

¨

c| No, because there is more heating area in the oversized heat exchanger

¨

d| The steam pressure is less, consequently the steam will be drier

¨

4. What is the effect of higher backpressure on a ball float steam trap? a| None whatever

¨

b| It reduces the steam pressure in the heat exchanger

¨

c| It reduces the ball float steam trap capacity

¨

d| It increases the differential pressure across it

¨

5. What can be done to stop any heat exchanger waterlogging? a| Increase the pressure upstream of the steam control valve

¨

b| Ensure the condensate discharges to atmospheric pressure

¨

c| Calculate the stall point and fit the correct trapping device

¨

d| Increase the size of the steam trap pipework

¨

6. Steam pressure in a heat exchanger at minimum load is 0.8 bar a. What is the differential pressure across the ball float steam trap if the backpressure is 0.1 bar g? a| +0.7 bar

¨

b| -0.7 bar

¨

c| +0.9 bar

¨

d| -0.3 bar

¨

Answers

1: d, 2: a, 3: b, 4: c, 5: c, 6: d

13.4.10

The Steam and Condensate Loop

The Stall Chart - Constant Flow / Varying Inlet Temperature Module 13.5

Block 13 Condensate Removal

Module 13.5 The Stall Chart - Constant Flow Secondary - Varying Inlet Temperature - Constant Outlet Temperature

The Steam and Condensate Loop

13.5.1

The Stall Chart - Constant Flow / Varying Inlet Temperature Module 13.5

Block 13 Condensate Removal

The Stall Chart By definition, stall will occur when the steam pressure in the heat exchanger is less than or equal to the condensate backpressure. Good results are obtained from heat transfer calculations as shown in Module 13.4. Those not wishing to use a mathematical approach can use a simpler method to arrive at a practical result. This method is graphical and involves the use of a ‘stall chart’. It gives slightly less accurate results, but is perfectly adequate for most practical purposes. A reduction in heat load is usually due to an increasing inlet temperature or a reducing secondary fluid flowrate, and requires a fall in steam pressure for control to be maintained. Sometimes stall may be caused by a combination of these, or perhaps a fall in outlet temperature due to a change in the set point.

Constant secondary flowrate with varying inlet temperature In this type of heat exchanger, the secondary flowrate and outlet temperature remain constant while the inlet temperature varies with changes in heat load. Steam flow controlled by control valve Steam

Flow temperature sensor placed in secondary outlet Hot water out Constant secondary flow through heat exchanger Cold water in

Steam trapping device

Condensate

Fig. 13.5.1 Shell and tube heat exchanger with primary control valve

At full-load the inlet temperature will be at its lowest. With a constant secondary flow through the heat exchanger, any reduction in the heat load will cause the inlet temperature to rise. The stall chart can show how the steam temperature and the inlet temperature change as the heat load changes, and predict the inlet temperature at stall and the minimum load condition. Under full-load conditions, the temperature difference between the steam and secondary fluid will be large. Conversely, under no-load conditions there is no heat exchange so the steam and secondary fluid must be the same temperature, and the temperature difference between them is zero. By proportionality, it follows that at 50% load this temperature difference is 50% of its maximum value. From this basic principle of proportionality, two straight lines can be drawn onto a chart to represent all these conditions. At full-load the lines are furthest apart, showing that the temperature difference is at a maximum. At no-load the lines converge to a single point, showing that the temperature difference is zero. A typical stall chart is shown in Figure 13.5.2. It considers a steam temperature of 120°C heating a constant flow of secondary water from 20°C to 80°C. Note that the steam temperature of 120°C is arrived at by one of two means: o

o

It has been calculated from LMTD design figures, as per the calculations in Module 13.4, which take into consideration the heat exchange surface area. The steam space pressure has been observed during operation and the temperature calculated.

Firstly, the steam temperature in the heat exchanger under full-load conditions (Point A) is marked upon the left vertical axis in the stall chart in Figure 13.5.2. Secondly, the desired secondary fluid outlet temperature is marked on the right vertical axis (Point B). The secondary fluid inlet temperature (Point C) at full-load is then marked on the left vertical axis. 13.5.2

The Steam and Condensate Loop

The Stall Chart - Constant Flow / Varying Inlet Temperature Module 13.5

Block 13 Condensate Removal

If a straight line then joins the points A and B, the line AB will represent how the steam temperature alters relative to changes in heat load. Similarly, if a straight line joins the points B and C, the line BC will represent the changing inlet temperature of the secondary fluid as the heat load varies. 200 180 160

Temperature °C

140 120

A

100 80

B

60 40 20

C 0 100

80

60 40 Percentage heat load

0

20

Fig. 13.5.2 Constant flowrate / Varying inlet temperature - Stage 1

It is then necessary to add a horizontal line to represent the equivalent steam saturation temperature of the condensate backpressure. This temperature should be marked on the right vertical axis, as shown in the Figure 13.5.3 (Point D). A straight line should then be drawn in to connect this point with the same temperature on the left vertical axis at point E. 200 180 160

Temperature °C

140 120 100

A D E

80

B

60 40 20

G C

F 60 80 0 40 20 Percentage heat load Fig. 13.5.3 Constant flowrate / Varying inlet temperature - Stage 2

0 100

The condensate backpressure takes into account the pressure in the condensate system plus any static pressure that may be due to a lift in the condensate discharge line from the bottom of the heat exchanger. A column of liquid will exert a pressure at its base due to its own mass. This is often referred to as ‘static lift’ when it is exerted on the outlet of the trap. The Steam and Condensate Loop

13.5.3

The Stall Chart - Constant Flow / Varying Inlet Temperature Module 13.5

Block 13 Condensate Removal

A 1 metre column of water under atmospheric pressure will exert a pressure at the bottom of the column of approximately 10 kPa or 0.1 bar g (actually 9.806 65 kPa or 0.098 066 5 bar). Any lift in the condensate discharge line will thus exert a static lift due, to the column of condensate held in the line, in addition to any pressure in the condensate system. The horizontal line DE will either intersect the line AB, or will be above point A on the chart. The point of intersection between the lines AB and DE represents the ‘stall point’, where the steam pressure and the backpressure are the same. If the line DE is on or above point A, the system permanently operates under stall conditions. (In vacuum condensate systems, or when B is greater than 100°C, point D may also be below point B, if this is the case, the system will not stall at any heat load). A vertical line should then be dropped down from the stall point. The point at which this vertical line crosses the bottom horizontal axis (Point F) marks the percentage stall load relative to the full heat load. The percentage stall load can also be quickly calculated using Equation 13.5.1. 6WDOOORDG 

'% [ $%

Equation 13.5.1

Where: A = The steam temperature in the steam space at full-load B = The secondary fluid outlet temperature D = The backpressure equivalent saturated steam temperature The vertical line connecting the stall point with point F will also intersect the line BC. If a horizontal line is drawn from this intersection point to the left vertical axis, this will mark the secondary inlet temperature at which stall occurs (Point G). Example 13.5.1 The steam pressure in a heat exchanger at full-load is observed to be 7 bar g. Condensate pressure is 1 bar g, and there is a lift after the trap of 10 m. At full-load, the secondary fluid enters the heat exchanger at 25°C and leaves the heat exchanger at 80°C. 1. What is the percentage heat load at stall? 2. What is the secondary inlet temperature at stall? The saturation temperature of saturated steam at 7 bar g is 170°C. Therefore the steam temperature in the heat exchanger at full-load is 170°C. This can then be plotted as point A in Figure 13.5.4: 200 180 A 160

Temperature °C

140 120

D E

100 80 60

B G

40 20 C 0 100

13.5.4

F 60 80 40 20 Percentage heat load Fig. 13.5.4 Stall chart for Example 13.5.1

0

The Steam and Condensate Loop

The Stall Chart - Constant Flow / Varying Inlet Temperature Module 13.5

Block 13 Condensate Removal

1. What is the percentage heat load at stall? The secondary fluid outlet temperature of 80°C should be plotted as point B in Figure 13.5.4, while the secondary fluid inlet temperature at full-load of 25°C should be plotted as point C. The lift in the condensate line of 10 m creates a backpressure of 1 bar, in addition to the 1 bar g pressure in the condensate system. Therefore, the total system backpressure is 2 bar g. As the saturation temperature of steam at 2 bar g is 135°C, the horizontal line DE representing the backpressure is added at this temperature. The stall chart in Figure 13.5.4 shows that the percentage heat load at stall (Point F) is approximately 61%. The mathematical calculation can be validated by use of Equation 13.5.1: 6WDOOORDG 

'% [ $%

Equation 13.5.1

Where: A = The steam temperature in the steam space at full-load

= 170°C

B = The secondary fluid outlet temperature

= 80°C

D = The backpressure equivalent saturated steam temperature = 135°C 6WDOOORDG 

'% [ $%

6WDOOORDG 

 [ 

6WDOOORDG 

 [ 

6WDOOORDG  2. What is the secondary inlet temperature at stall? The stall chart in Figure 13.5.4 also indicates that the inlet temperature at stall (Point G) is about 46°C or 47°C. The mathematical calculation can be validated by use of Equation 13.4.1: 7;   [ 7 7 [ ] 7

Equation 13.4.1

Where: Tx = The secondary inlet temperature at any load factor ‘x’ T1 = The secondary inlet temperature at full-load T2 = The secondary outlet temperature at full-load x = The load factor. For example; the minimum heat load of 61% load is equivalent to a load factor of 0.61 7   [ 7 7 [[ ] 7 7   [  [ ]  7   [  [ ] 

7  ƒ&

The Steam and Condensate Loop

13.5.5

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Inlet Temperature Module 13.5

Questions 1. What causes stall? a| The steam pressure is more than the condensate pressure

¨

b| The condensate pressure less than the steam pressure

¨

c| The condensate pressure is more than or equal to the steam pressure

¨

d| The steam trap is too small

¨

2. What is the relationship between the steam and secondary fluid inlet temperature at full-load? a| The difference is large

¨

b| The difference is small

¨

c| The difference is zero

¨

d| The secondary inlet temperature is higher than the steam temperature

¨

3. What is the temperature difference between the steam and secondary fluid inlet temperature at 75% load? a| It is 75% of the temperature difference at full-load

¨

b| It is 25% of the temperature difference at full-load

¨

c| It is at a minimum

¨

d| It is exactly the same

¨

4. Figure 13.5.4 shows the backpressure at 2 bar g. What would the stall load be if the condensate pressure was atmospheric? a| 10%

¨

b| 22%

¨

c| 30%

¨

d| 80%

¨

5. Also, what would be the secondary inlet temperature? a| 25°C

¨

b| 45°C

¨

c| 55°C

¨

d| 68°C

¨

6. If, at full-load, the steam pressure were 1 bar g (120°C) instead of 7 bar g, what would be the approximate stall load for an atmospheric backpressure? a| 20%

¨

b| 30%

¨

c| 40%

¨

d| 50%

¨

Answers

1: c, 2: a, 3: a, 4: b, 5: d, 6: d

13.5.6

The Steam and Condensate Loop

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

Module 13.6 The Stall Chart - Varying Flow Secondary - Constant Inlet Temperature - Constant Outlet Temperature

The Steam and Condensate Loop

13.6.1

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

The Stall Chart Varying flowrate with constant inlet / outlet temperature Not all heat exchangers are required to operate with a constant secondary flow. Sometimes, due to the configuration of the secondary pipework, as the heat load changes, the liquid flowrate through the heat exchanger will vary while the inlet and outlet temperatures remain constant. At full-load, the flowrate through the heat exchanger will be at its maximum. Any reduction in the heat load must lead to a reduction in the flowrate through the heat exchanger. In practice, this could mean either a 3-port diverting valve fitted in the secondary return line, bypassing the heat exchanger, or a 3-port mixing valve fitted in the flow line, (see Figure 13.6.1). Flow temperature controlled by 3-port mixing valve Outlet temperature controlled by a steam control valve

Hot water out Bypass balancing valve

Steam

Cold water in Condensate Fig. 13.6.1

The stall chart can also be used in these types of installations, but the construction method is slightly different to that used for constant secondary flow. This method is described below. The first part of this method is very similar to that shown in Example 13.5.1. With reference to Figure 13.6.2, the steam temperature in the heat exchanger under full-load conditions (Point A) should be marked on the left vertical axis. The desired secondary fluid outlet temperature should then be marked on the right vertical axis (Point B). The secondary fluid inlet temperature (Point C) should also be marked on the left vertical axis. The horizontal line representing the system backpressure must also be marked on this chart. This temperature should be marked on the right vertical axis at point D, with a straight line connecting it to the same temperature on the left vertical axis at point E. 200 180 160

Temperature °C

140 120 100

A E

D

80

B

60 40 20

C

0 100

60 0 40 20 Percentage heat load Fig. 13.6.2 Varying flowrate / Constant inlet temperature - Stage 1

13.6.2

80

The Steam and Condensate Loop

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

With reference to Figure 13.6.3, the secondary load line BC should be drawn connecting points B and C. A horizontal line should then be drawn from where BC crosses the 50% load ordinate, to the right axis. This represents the mean secondary fluid temperature, and is shown as point F. The mean secondary fluid temperature point F should then be connected by a diagonal straight line to the steam temperature point A in the heat exchanger under full-load, creating the line AF. 200 180 160

Temperature °C

140 120 100

A D E

80

B

60 40 20

F C

G 80

0 100

60 0 40 20 Percentage heat load Fig. 13.6.3 Varying flowrate / constant inlet temperature - stage II

The backpressure line DE will either intersect the steam line AF, or be above point A on the chart. The point of intersection between the lines AF and DE marks the stall point, where the steam pressure and the backpressure are the same. A vertical line may be dropped down from the stall point, to indicate when the stall condition occurs. The point at which this vertical line crosses the bottom horizontal axis (Point G) should mark the percentage load. As in the previous example, if the line DE is above the point A, stall occurs under all load conditions. The percentage stall load can also be calculated using Equation 13.6.1: % + &  '        6WDOOORDG  [ % + &   $       

Equation 13.6.1

Where: A = Steam temperature at full-load B = Secondary fluid outlet temperature at full-load C = Secondary fluid inlet temperature at full-load D = Equivalent backpressure steam temperature

The Steam and Condensate Loop

13.6.3

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

The minimum steam temperature It should be noted that the lowest operating steam temperature equals the set point temperature at point B. This occurs at 70°C in the stall chart, Figure 13.6.4, and is represented by point H on the steam line AF. 200 180 160

Temperature °C

140 120 100

A D E

80

H

B

60 40

F

20

C 0 100

G 80

60 0 40 20 Percentage heat load Fig. 13.6.4 Minimum steam temperature equals the set point

In practice, as the heat load decreases, and the steam temperature approaches the secondary control temperature at point H, changes in steam temperature occur slowly rather than the rapid step change suggested at point H in Figure 13.6.4. The steam temperature will tend to fall in a similar way to that shown in Figure 13.6.5. It is difficult and unnecessary to draw this line on a stall chart, whereas Figure 13.6.4 is practical and easy to use. Referring to Figure 13.6.4, it can be seen in this example that the steam temperature at any load less than 37% is 70°C. In truth, the gradual fall in steam temperature is more like that depicted in Figure 13.6.5, but the difference is so small as to be insignificant with regard to selecting and sizing the trapping device. 200 180 160

Temperature °C

140 120

A

100 80

B

60 40 20

C

0 100

13.6.4

60 40 20 Percentage heat load Fig. 13.6.5 The decay of steam temperature at low loads 80

0

The Steam and Condensate Loop

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

Example 13.6.1 The steam pressure inside a heat exchanger with a varying secondary flowrate at full-load is 8 bar g, the pressure in the condensate line is 0.5 bar g, and there is a lift of 7 metres after the trap. At full-load, the secondary fluid enters the heat exchanger at 30°C and leaves the heat exchanger at 90°C with a flowrate of 3.64 L / s. What is the percentage load at stall, and what is the secondary flowrate through the heat exchanger at stall? The saturation temperature of the steam at 8 bar g is 175°C. Therefore the steam temperature in the heat exchanger at full-load is 175°C. This should then be plotted as point A in Figure 13.6.6. The secondary fluid outlet temperature of 90°C should be plotted as point B, while the secondary fluid inlet temperature of 30°C should be plotted as point C. 200 180 A 160

Temperature °C

140

D

120 E 100

B

80 60

F

40 20 C 0 100

G 80

60 40 Percentage heat load

20

0

Fig. 13.6.6 Stall chart for varying flow / constant temperature

The lift in the condensate line of 7 m creates a differential pressure of 0.7 bar, in addition to the 0.5 bar g pressure in the condensate line. Therefore, the total system backpressure is 1.2 bar g. As the saturation temperature of steam at 1.2 bar g is 123°C, the horizontal line DE representing the backpressure is drawn at this temperature in Figure 13.6.6. In this example the percentage load (Point G) is approximately 55%. This means that the secondary liquid flowrate must reduce to 55% of the maximum flowrate for stall to occur, that is, 55% of 3.64 L / s = 2 L / s. This can be verified mathmatically by using Equation 13.6.1.

The Steam and Condensate Loop

13.6.5

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

6WDOOORDG 

'  

%&    

%&  $     

Equation 13.6.1

[

Where: A = Steam temperature at full-load

= 175°C

B = Secondary fluid outlet temperature at full-load = 90°C C = Secondary fluid inlet temperature at full-load

= 30°C

D = Equivalent backpressure steam temperature

= 123°C

6WDOOORDG 

  

    

       

6WDOOORDG 

 [ 

6WDOOORDG 

 [ 

[

6WDOOORDG  Most heat exchanger applications will either be varying flowrate or varying temperature as described above and in the previous Modules in Block 13. There may, however, also be instances where both the flowrate and the inlet temperature of the secondary fluid vary. In these examples it becomes more difficult to determine their combined effect by interpretation of the stall chart. Systems such as these can be analysed by comparing the results from both methods shown above and using the worst case.

13.6.6

The Steam and Condensate Loop

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

Block 13 Condensate Removal

Questions 1. What is the difference between the constant and variable stall charts? a| Nothing

¨

b| The steam line is constructed differently

¨

c| The backpressure line is at different pressures

¨

d| The secondary line is constructed differently

¨

2. If the backpressure line is higher than Point A on the steam line what does this mean? a| The system will never stall

¨

b| The system will constantly stall

¨

c| The system is constantly in vacuum

¨

d| The heat exchanger is too big

¨

3. If the backpressure line is lower than Point B the steam line what does this mean? a| The system will never stall

¨

b| The heat exchanger is too small

¨

c| The system is constantly in vacuum

¨

d| The system will constantly stall

¨

4. If, in Example 13.6.1, the condensate backpressure were atmospheric, at what percentage load would stall have occurred? a| 18%

¨

b| 28%

¨

c| 35%

¨

d| 55%

¨

Answers

1: b, 2: b, 3: a, 4: c The Steam and Condensate Loop

13.6.7

Block 13 Condensate Removal

13.6.8

The Stall Chart - Varying Flow / Constant Inlet Temperature Module 13.6

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Module 13.7 The Stall Chart - Constant Flow Secondary - Varying Inlet Temperature - Varying Outlet Temperature

The Steam and Condensate Loop

13.7.1

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Block 13 Condensate Removal

The Stall Chart Constant Flowrate / Varying Outlet Temperature All systems discussed up to this point assume that the secondary fluid outlet temperature remains constant. In some applications, the outlet temperature may change with time. This will also change the heat load and affect the stall point. Such changes often occur in process applications, and also heating calorifiers that change their outlet temperature to compensate for changes in ambient conditions. If the highest heat requirement occurs when the control temperature (the set point) is at a maximum, any reduction in the set point will cause a reduction in the heat load. A reducing set point will tend to increase the stall load, as shown in the following calculations. Once the design conditions are known, the effect of reducing the set point can either be calculated mathematically as shown below or illustrated on a stall chart by means of proportionality.

Example 13.7.1

Initially, secondary water at a rate of 1.5 L / s enters a heat exchanger at 20°C and leaves at 70°C. It is observed via a pressure gauge on the steam inlet that the pressure in the steam space under these conditions is 5.2 bar g (TS = 160°C). The condensate drains down to a vented receiver in a plant room below the installation. (T(back) = 100°C). If the set point is reduced to 60°C, what is the effect on the stall point and the steam load at stall?

Calculating the effect of reducing the set point arithmetically

It is first necessary to establish the heat exchanger TDC from the full-load operating conditions and by use of Equation 13.2.2: 7V 7 7V 7

7'& 

The T1 T2 TS

Equation 13.2.2

full-load conditions are: = 20°C = 70°C = 160 °C (steam temperature at 5.2 bar g)

Therefore:

7'& 

 

7'& 

 

7'& 

 

7'&  How does the stall load change with a lowered set point? Firstly, consider the stall load with the higher set point of 70°C The design conditions are: = 20°C T1 T2 = 70°C TS = 160°C T(back) = 100°C

13.7.2

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

For a constant secondary flowrate, the stall factor can be calculated from Equation 13.5.1: 6WDOOORDG 

'% [ $%

Equation 13.5.1

Where: A = The steam temperature at full-load with a 70°C set point (TS) B = The secondary fluid outlet temperature (T2) D = The backpressure equivalent saturated steam temperature (T(back)) 6WDOOORDG 

'% [ $%

6WDOOORDG 

 [ 

6WDOOORDG 

 [ 

6WDOOORDG    6WDOOIDFWRURI  With the set point at 70°C Full heat load (Q) = m cp DT (kW) Full heat load (Q) = 1.5 kg / s x 4.19 kJ / kg°C x (70 - 20) °C Full heat load (Q) = 314 kW Heat load at stall = 0.333 3 x 314 kW Heat load at stall = 105 kW The condensate discharges to atmosphere, and the hfg at atmospheric pressure is 2 257 kJ / kg. Steam load at stall =

N:[ V  K  N- NJ

Steam load at stall = 168 kg / h with the set point at 70°C Secondly, consider the stall load with the lower set point of 60°C The steam temperature can be predicted for any load by use of Equation 13.2.3: 7V  

Where: TS = T2 = TDC = T1 =

7 [7'& 7 7'&

Equation 13.2.3

Steam temperature (°C) Secondary outlet temperature = 60°C Temperature Design Constant = 1.555 Secondary inlet temperature = 20°C 7V  

[  

7V  

  

7V  ƒ&ZLWKWKHVHWSRLQWDWƒ&

The Steam and Condensate Loop

13.7.3

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Block 13 Condensate Removal

At the reduced set point of 60°C, the stall factor can again be calculated by use of Equation 13.5.1: 6WDOOORDG 

'% [ $%

Equation 13.5.1

Where: A = The steam temperature at full-load with a 60°C set point (TS) B = The secondary fluid outlet temperature (T2) D = The backpressure equivalent saturated steam temperature (T(back)) 6WDOOORDG 

'% [ $%

6WDOOORDG 

 [ 

6WDOOORDG 

 [ 

6WDOOORDG  6WDOOIDFWRURI With the set point at 60°C Full heat load Q = m cp DT (kW) Full heat load Q = 1.5 kg / s x 4.19 kJ / kg°C x (60 - 20) °C Full heat load Q = 251 kW Heat load at stall = 0.555 5 x 251 kW Heat load at stall = 140 kW The condensate discharges to atmosphere, and the hfg at atmospheric pressure is 2 257 kJ / kg. Steam load at stall =

N:[ V  K  N- NJ

Steam load at stall = 223 kg / h at the reduced set point of 60°C It can be seen from the above calculations that when the set point is reduced from 70°C to 60°C, the stall load increases from 168 kg / h to 223 kg / h. It is therefore important that if the application is such that the set point will be reduced, the trapping device, i.e. ball float steam trap or pump-trap, is selected on the stall condition at the lower set point.

13.7.4

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Illustrating the effect of reducing the set point by the stall chart method

The stall chart in Figure 13.7.1 shows the secondary temperature line CB and the corresponding steam line AB for this application (Example 13.7.1) with the higher set point of 70°C. 200 180 160

A

Temperature °C

140 120 100 80 B

60 40 20

C

0 100

60 40 20 Percentage heat load Fig. 13.7.1 The full-load condition with 70°C set point 80

0

As mentioned at the beginning of this module, once the operating conditions are known, the effect of reducing the set point can be illustrated on the stall chart by means of proportionality. This is shown in Figure 13.7.2 by marking the reduced secondary outlet temperature of 60°C (Point D) on the secondary load line CB and drawing a line ED parallel to, and below, the dotted steam line AB. 200 180 160

A

Temperature °C

140 120 E 100 80 60

D

40 20

B = 70°C D = 60°C

C

0 100

60 0 40 20 Percentage heat load Fig. 13.7.2 Defining the steam temperature for 60°C set point 80

It is observed that the new steam line DE cuts the left hand side of the stall chart at 132°C (Point E), and this is the steam temperature when the set point is reduced to 60°C, for a constant secondary flowrate. The steam line DE represents steam temperature for reducing heat loads when the set point is 60°C. The Steam and Condensate Loop

13.7.5

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Block 13 Condensate Removal

Once the new steam temperature of 132°C has been established, it is possible to draw the new steam line DE from 132°C to 60°C and the secondary temperature line CD from 20°C to 60°C. This stall chart, Figure 13.7.3, represents the steam and secondary inlet temperatures when the set point is at 60°C, consequently zero load now occurs on this stall chart when the secondary temperature is 60°C. 200 180 160 140

E

Temperature °C

120 100 80 D

60 40 20

C

0 100

80

60 40 Percentage heat load

20

0

Fig. 13.7.3 The steam line and secondary line for a 60°C set point

By superimposing the backpressure line of 100°C (line HJ) onto Figure 13.7.4 it is now possible to depict the new stall load and the corresponding inlet temperature, with a set point of 60°C. The stall load is approximately 55% (Point F) and the inlet temperature at which stall occurs is approximately 38°C (Point G). 200 180 160

Temperature °C

140 E 120 100

J

H

80 D

60 40 20

G C

0 100

F 80

60 40 Percentage heat load

20

0

Fig. 13.7.4 The backpressure line is added

13.7.6

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

By combining Figure 13.7.1 and Figure 13.7.3, it is now possible to observe how the reduction in the outlet temperature from 70°C to 60°C has affected the stall load. In the stall chart below (Figure 13.7.5), it is possible to draw both steam lines AB (160°C to 70°C) and ED (132°C to 60°C). It can be seen that the backpressure line (JH) cuts the two steam lines in different places. The steam line for the higher heat load (with the 70°C set point) is cut at approximately 33% (Point F1), whilst the part load line (with the 60°C set point) is cut at approximately 55% (Point F2). 200 180 160

A

Temperature °C

140 120 E 100

H

J

80

B D

60 40 20 0 100

F2

F1

60 40 Percentage heat load

80

20

0

Fig. 13.7.5 The change in stall load

It is important to remember that the above percentages refer to different heat loads. At full-load, the outlet temperature is 70°C and the heat load was calculated in the first part of Example 13.7.1 to be 314 kW, and at the reduced load, when the set point is reduced to 60°C, the heat load was calculated to be 251 kW. For example When the set point is 70°C: The heat load is 314 kW, and stall occurs at 33.33% of this load. Steam load at stall is  [

N:[ V  K  NJ  KNJ  KE\FDOFXODWLRQ  N- NJ

When the set point is 60°C: The heat load is 251 kW, and stall occurs at 55.55% of this load.

N:[ V  K  NJ  KNJ  KE\FDOFXODWLRQ  N- NJ It is observed that the steam load at stall increases as the set point is reduced. Steam load at stall is  [

The Steam and Condensate Loop

13.7.7

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Block 13 Condensate Removal

The stall chart can also illustrate the inlet temperatures for both stall conditions. This can be useful when carrying out diagnostics on heat exchangers with stall problems. In the stall chart below (Figure 13.7.6), it can be seen how the inlet temperatures can be observed for each of the stall conditions. 200 180 160

A

140 E Temperature °C

120 100

J

H

80

B D

60 G1 40 20

G2 C

0 100

60 40 Percentage heat load

80

20

0

Fig. 13.7.6 The difference in inlet temperatures at the two stall points

With the 70°C set point, an inlet temperature above 53°C (Point G1) will produce a stall. With the 60°C set point, an inlet temperature above 38°C (Point G2) will produce a stall. The validity of these figures can be confirmed by use of the calculation method - Equation 13.2.4: 7  7V  [ 7'&7V 7  ]

Equation 13.2.4

At the higher set point, T2 = 70°C T1 = 100 - [1.555 (100 - 70)] T1 = 100 - [1.555 (30)] T1 = 100 - 46.7 T1 = 53.3°C At the lower set point, T2 = 60°C T1 = 100 - [1.555 (100 - 60)] T1 = 100 - [1.555 (40)] T1 = 100 - 62.2 T1 = 37.8°C

13.7.8

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Summary

It can be seen from the above information that the stall load will increase as a result of a reducing set point. In fact, stall load will continue to increase to a maximum until the steam pressure falls to equal the condensate backpressure. It is also possible to predict the set point at, and below which permanent stall occurs. The effect can be predicted in the stall chart below (Figure 13.7.7). Stall occurs when the steam temperature is the same as the condensate backpressure, which, in this example, is 100°C, (Point K). In Figure 13.7.7 it is possible to predict the outlet temperature at 100% stall, by projecting the steam temperature line from 100°C (Point K) parallel to the full-load steam line AB, creating line (KL). Where the new steam line KL cuts the secondary load line BC at point M, the outlet temperature can be observed, to be approximately 49°C. If the set point is reduced to (or below) 49°C, stall would be permanent for this example. 200 180 160

A

Temperature °C

140 120 100 K 80

B

60 M

40 20

C

0 100

L

60 0 40 20 Percentage heat load Fig. 13.7.7 The outlet temperature at 100% stall load (for Example 13.7.1) is approximately 49°C 80

Selecting the correct trapping device

The object of predicting steam pressures and their corresponding steam loads is to enable the selection of the correct trapping device for any application. In this instance, the trapping device would be selected on the following information. Maximum steam load

= 543 kg / h with the set point at 70°C

Steam pressure at this load = 5.2 bar g Condensate backpressure = 0 bar g (atmospheric pressure) \Trap differential pressure = 5.2 bar at maximum steam load Stall steam load

= 168 kg / h when the set point is 70°C

Stall steam load

= 224 kg / h when the set point is 60°C

Differential pressure at stall = 0 bar A ball float steam trap can be specified if it meets the following two criteria satisfying the initial brief in Example 13.7.1:1. It can pass the full-load condition, i.e. 543 kg / h at 5.2 bar differential pressure 2. It can pass the maximum stall load, i.e. 224 kg / h at the 60°C set point

The Steam and Condensate Loop

13.7.9

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Block 13 Condensate Removal

Creating a static head above the ball float steam trap

At the stall condition, with the steam pressure inside the heat exchanger equalling the backpressure, a differential pressure would not exist to push the condensate through a ball float steam trap. Because of this, pressure has to be manufactured on the upstream side of the trap by means of a static head. Static head must be available between the heat exchanger condensate outlet and the trap inlet to generate enough differential pressure to enable the trap to pass the stall load of 224 kg / h. In order to allow condensate to drain easily from the exchanger, a vacuum breaker is fitted to its steam inlet downstream of the control valve (Figure 13.7.8). It can be seen in Figure 13.7.9 that a DN25 (1”) FT10-10 ball float steam trap will accommodate these criteria. However, the trap requires a minimum of 4 metres head above the trap inlet to pass the stall load. A 4 metre head might not be available in practice, and, if so, a larger trap would need to be specified. Refer to Figure 13.7.8. For the purposes of Example 13.7.1, if the available head were only 200 mm then it can be seen from Figure 13.7.10 that a DN40 (1½”) FT10-10 ball float steam trap would be required.

Control valve P1

P2

Temperature sensor Vacuum breaker

Flow

Heat exchanger

DN25 (1”) FT needs 4 m head DN40 (1½”) FT needs 200 mm head

Return

Steam trap Fig. 13.7.8 The trap size depends on the static head

Footnote:

Should the backpressure have been greater than atmospheric pressure, due perhaps to a lift after the trap and /or a pressurised condensate line, then the same sizing routine could be carried out. Depending upon the amount of backpressure, it may be that even the largest sized steam trap cannot pass the required amount of condensate at stall. Under these circumstances, a ball float steam trap cannot be specified, as the heat exchanger will flood at part loads. Instead, a pump-trap must be used, which is able to clear the condensate from the heat exchanger into the condensate system at any heat load.

13.7.10

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

1500

1000

DN25 (1”) FT14-10 trap capacity at 5.2 bar Dp Maximum load 543 kg / h

D

5 N2

300

DN

200

Condensate kg/h

4-

4. 5

500 400

Stall load 224 kg / h

" (1

T1 )F

25

DN

1

(

) 1"

D 5,

1 FT

N2

0

4-

(½

10

",

¾

")

1 FT

4-

4.

5

4 0 -1 -1 14 14 T T F F ") ") (1 ¾ 5 ", 2 (½ DN 4 20 -1 N D 14 , T F 15 ") DN ¾ ", (½ 20 N ,D 15 N D

100

50 40 30

20 0.1

0.2

0.3

0.5

1

2

3

DP at stall load (0.4 bar) (4 metres head)

4

5

10

14

DP at maximum load (5.2 bar)

DP - Differential pressure (bar) Fig. 13.7.9

The Steam and Condensate Loop

13.7.11

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Block 13 Condensate Removal 5 000 4 000 3 000

2 000

DN

Condensate kg / h

1 000

DN40 (1½”) FT14-10 capacity Stall load 224 kg / h

DN

50

50

DN

500

40

DN

400

DN

5

DN DN

200

100

DN

200

(1

25

25

T 14

FT1

") (1 ½

(1

4 -4

FT ½ ")

") 5 (1

25

DN

50 100

2

F (2 ")

") 0 (2

40

300

(2

T1 ") F

T1 ") F

(1 "

(1

-1 0

14-

4 -1

FT1

FT1

4 -4

T1 ") F

4 .5

4

4 -1

4 -1

14 ) FT

300

.5

0

4

.5 H

-1 0

4 -1

C

HC

4H C

400

500

600

800

1 000

Available head 200 mm above the trap Differential pressure (mm w.g.) Fig. 13.7.10

13.7.12

The Steam and Condensate Loop

Block 13 Condensate Removal

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

Questions 1. How does a reduction in set point affect stall? a| It does not affect it at all

¨

b| It reduces the percentage stall

¨

c| The system operates under permanent stall conditions

¨

d| It increases the percentage stall

¨

2. How does a reduction in the set point affect the steam pressure? a| It reduces the steam pressure

¨

b| It does not affect it at all

¨

c| It increases the steam pressure

¨

d| The steam pressure equals the backpressure

¨

3. In Example 13.7.1, with the set point at 60°C what would the steam temperature be if the system were operating at 30% heat load? a| 100°C

¨

b| 110°C

¨

c| 120°C

¨

d| 80°C

¨

4. In Example 13.7.1, with the set point at 60°C if the minimum possible heat load were 70%, would the system stall? a| Yes, at all loads

¨

b| Only if the set point were increased

¨

c| No

¨

d| Yes, but only on loads higher than 70%

¨

5. If, in Example 13.7.1, the set point were reduced to 40°C, what would be the approximate steam temperature if the inlet temperature remained at 20°C? a| 66°C

¨

b| 45°C

¨

c| 55°C

¨

d| 76°C

¨

6. If, in Example 13.7.1, the set point were reduced to 40°C, what would be the approximate steam temperature if the inlet temperature rose to 30°C? a| 28°C

¨

b| 38°C

¨

c| 48°C

¨

d| 58°C

¨

Answers

1: d, 2: a, 3: d, 4: c, 5: d, 6: d The Steam and Condensate Loop

13.7.13

Block 13 Condensate Removal

13.7.14

The Stall Chart - Constant Flow / Varying Outlet Temperature Module 13.7

The Steam and Condensate Loop

Block 13 Condensate Removal

Practical Methods of Preventing Stall Module 13.8

Module 13.8 Practical Methods of Preventing Stall

The Steam and Condensate Loop

13.8.1

Practical Methods of Preventing Stall Module 13.8

Block 13 Condensate Removal

Practical Methods of Preventing Stall If stall conditions are inevitable, potential problems can be overcome by designing the installation around one of three basic solutions: 1. Ensure the steam pressure in the steam space can never drop below atmospheric pressure, and that the condensate can drain by gravity to and from a ball float steam trap. 2. Accept that the pressure in the steam space may be less than the backpressure, and provide an alternative means of removing condensate, by installing a pump-trap. 3. Ensure the pressure in the steam space is stable and higher than the backpressure. This will entail having the temperature control system on the secondary side of the system. Taking these three options in turn:

1. Installations that ensure the conditions in the steam space can never drop below atmospheric pressure, and that the condensate can drain by gravity to and from a steam trap: 1a) Condensate removal by vacuum breaker method (see Figure 13.8.1) The steam trap cannot be subject to any backpressure higher than atmospheric, and must drain condensate either to an open end (which may be wasteful), or to a nearby vented receiver and pump, enabling the energy contained in the condensate to be reclaimed. There are two criteria that must be satisfied: o

o

A vacuum breaker must be fitted to the steam inlet to the heat exchanger after the control valve. The trap must be installed at a discreet distance below the heat exchanger outlet such that sufficient static head is created to pass the requisite amount of condensate when stall occurs. A distance of between 0.5 to 1 m is usually sufficient; however, smaller distances can be accommodated with larger traps, if less head is available. Controller Control valve Vacuum breaker

Sensor Secondary flow out

Steam in Heat exchanger Secondary flow in Static head above trap usually 0.5 to 1 m

Float type steam trap

Must drain by gravity to atmosphere Fig. 13.8.1 Static head and vacuum breaker method of dealing with stall

13.8.2

The Steam and Condensate Loop

Block 13 Condensate Removal

Practical Methods of Preventing Stall Module 13.8

1b) Auxiliary drain trap method (see Figure 13.8.2) A standard float trap set is installed with condensate returning to a condensate system, which is either pressurised and / or elevated above the trap. An auxiliary float trap may be fitted, discharging condensate via an open end to drain. When there is sufficient steam pressure to overcome the backpressure, the main float trap will function, but when stall occurs, condensate will back-up and drain through the auxiliary float trap thus preventing condensate flooding back into the heat exchanger. As this condensate will drain to waste, this method should only be used if stall occurs infrequently. The auxiliary trap should be sized on static head to pass the stall load as in method 1a, and the ‘main’ trap should be the same size, but fitted at least 150 mm below the auxiliary take-off tee-piece. Apart from the obvious disadvantage of energy loss, this method also requires available head between the trap inlets and the heat exchanger outlet.

Controller

Control valve Vacuum breaker

Steam in

Secondary flow out Condensate discharge against a lift or backpressure

Heat exchanger Secondary flow in

Static head above auxiliary trap usually 0.5 to 1 m

Minimum 150 mm

Auxiliary float type steam trap

Main float type steam trap about 150 mm below auxiliary trap Must drain by gravity to atmosphere Fig. 13.8.2 Auxiliary drain method of dealing with stall

The Steam and Condensate Loop

13.8.3

Practical Methods of Preventing Stall Module 13.8

Block 13 Condensate Removal

2. Installations which allow the steam pressure in the steam space to drop below the backpressure, but where the condensate can drain by gravity to a pump-trap arrangement: 2a) A pump and float trap installed in combination (see Figure 13.8.3) This method uses a pump and float trap installed in combination. It is better suited to heat exchangers with nominal heating capacities in excess of 1.5 MW (nominally 2 500 kg / h of steam). The steam pressure changes relative to changes in heat load. At high loads the steam pressure will be higher than the backpressure, but at low loads it will be lower. The pump is a mechanical pressure-powered type, in which an auxiliary steam supply automatically takes over to provide the motive power to discharge the condensate when stall occurs. If the steam space pressure is higher than the backpressure, condensate passes through the pump body to the float trap, which allows the condensate to discharge. This method is more practical and economical on larger installations; for example, those using condensate drain lines of 40 mm or more. Controller

Sensor

Control valve

Secondary flow out Steam in Heat exchanger Secondary flow in

Condensate discharging against a backpressure

Exhaust Reservoir pipe

SPIRAX SARCO

Pressure powered pump

Float-thermostatic trap Fig. 13.8.3 Combination pump and steam trap method of dealing with stall

13.8.4

The Steam and Condensate Loop

Block 13 Condensate Removal

Practical Methods of Preventing Stall Module 13.8

2b) A pump-trap with constant flow heat exchanger (see Figure 13.8.4) The secondary flowrate does not change as it passes through the heat exchanger, consequently the steam pressure changes relative to changes in the secondary inlet temperature. At high loads the steam pressure will be higher than the backpressure, but at low loads it will be lower. This method uses a pump-trap device, which offers the functions of a pump, steam trap and check valves in one body. The Spirax Sarco APT14 automatic pump trap is designed to occupy a minimum amount of space, and can be fitted to heat exchangers with nominal heating capacity of up to 1.5 MW. It is most suited to installations with condensate drain lines up to 25 mm, but can be used on drain lines up to 40 mm in some circumstances. A typical installation is shown on Figure 13.8.4.

Pressure reducing valve

Separator

Control valve

Safety valve

Secondary flow out

DP17

Steam in High limit cut-out Steam to pump

Condensate

Steam plate heat exchanger

Condensate

Secondary flow in

Automatic pump trap Fig. 13.8.4 Pump-trap method of dealing with stall

2c) A pump-trap device with varying flow heat exchanger (see Figure 13.8.5) This method is similar to 2b), but the secondary flow through the heat exchanger varies with the heat load, due to the action of the secondary mixing valve. The heat exchanger delivers a constant temperature water flow which is blended by the secondary mixing valve according to load. As the secondary flow varies, the steam pressure changes to maintain a constant outlet temperature, such that, at high loads, it is above the backpressure, and at low loads it is below.

Pressure reducing valve

Separator

Control valve

Safety valve

Flow out

Steam in High limit cut-out Steam to pump Condensate

Condensate

Automatic pump trap

Secondary flow in Steam plate heat exchanger

Fig. 13.8.5 Pump-trap method of dealing with stall

The Steam and Condensate Loop

13.8.5

Practical Methods of Preventing Stall Module 13.8

Block 13 Condensate Removal

3. Installations which ensure the steam pressure is kept constant and can never drop below the backpressure, and that the condensate can drain to and from a steam trap: 3a) Steam trap with temperature control valve in secondary circuit (see Figure 13.8.6) This method requires temperature control to be carried out with a 3-port mixing or diverting valve in the secondary circuit. The steam supply to the heat exchanger is held at a constant pressure (usually less than 1 bar g) with a pressure control valve, and as such, condensate can always be cleared from the heat exchanger against a lower backpressure. This method is not always practical or possible. It is unsuitable on steam / air heater batteries or liquid systems where the secondary system is at such a low pressure that it is unable to prevent the liquid from boiling. Like all methods, it has both advantages and disadvantages, which must be assessed before an option can be chosen.

Pressure reducing valve

Separator

Safety valve

Control valve

Secondary flow out

DP17

Steam in Steam plate heat exchanger

Secondary flow in

High limit cut-out

Condensate

Condensate Fig. 13.8.6 Constant steam pressure - Secondary temperature control

3b) Steam trap and modulating valve in condensate drain line (see Figure 13.8.7) Condensate drainage is achieved with a modulating valve in the condensate drain line. This method also maintains the desired steam pressure in the steam space regardless of load conditions. However, it encourages (instead of eliminates) waterlogging in the heat exchanger, as control is achieved by deliberately flooding the steam space with condensate as the load reduces. Usually this method is only considered if: o

The heat load is steady or changes very slowly.

o

The heat exchanger is designed to withstand the effects of waterlogging.

o

The likely stratification of temperatures of the secondary fluid is acceptable.

Separator

Pressure reducing valve

Safety valve Air vent

DP17

Secondary flow out

Steam in High limit cut-out Condensate

Secondary flow in

Control valve Steam plate heat exchanger

Condensate Fig. 13.8.7 Constant steam pressure - Condensate level control

13.8.6

The Steam and Condensate Loop

Block 13 Condensate Removal

Practical Methods of Preventing Stall Module 13.8

On / off control should not be used with heat exchangers

An on / off temperature control valve does not modulate depending on heat load, but is either fully open or fully closed. An example would be a solenoid valve. When open, full steam pressure will be maintained in the heat exchanger to clear the condensate against the backpressure. At first glance, this method of control would seem to overcome any backpressure problems, but is not recommended on processes such as heat exchangers, where the secondary fluid has to be heated to its required temperature as it passes through. There are three main reasons for this: o

o

o

An ‘on / off’ control system is activated by a thermostat which relies upon a product overtemperature to achieve control. As steam has high heat content, a significant amount of heat can be held in the steam space after the solenoid valve has shut. The overall effect is a higher product temperature than required. Should the thermostat setting be lowered to counteract this effect, the ‘on’ temperature may be lower than the system parameters may require. It can result in poor control of the system temperature and the potential for product spoilage. The continual and rapid changes in pressure and temperature will impose thermal and mechanical stresses upon the heat exchanger which will probably reduce its service life. It is never a good idea to subject steam systems to an instantaneous increase in pressure. Any condensate present in the steam space and condensate pipe is instantly pushed, by the sudden inrush of steam, through the system towards the steam trap. This can cause waterhammer, and damage the heat exchanger and steam trap.

On / off control is normally only suitable for ‘non-flow’ or ‘batch’ type heat exchange processes, notably tanks with robust heating coils, or jacketed pans, where the desired steam pressure is applied over a long heating up period (usually over many minutes or even hours). The rise in product temperature is much slower than that experienced with flow-type systems that are expected to heat the product in the short time it takes to pass through a heat exchanger.

The Steam and Condensate Loop

13.8.7

Practical Methods of Preventing Stall Module 13.8

Block 13 Condensate Removal

Conclusion

The most suitable type of steam trap for heat exchange equipment in general, and especially if stall is likely, is a ball float steam trap with integral balanced pressure air vent. If there is any likelihood of stall, a pump-trap is generally the most effective way of dealing with it, as it benefits from being: o

Simple.

o

Cost effective.

o

Compact.

Please note: The diagrams in this Module are schematic only, and for simplicity do not contain all the ancillary equipment that would be necessary or advisable for a specific installation. The exception is Figure 13.8.8, which shows a detailed, actual, installation of an APT14 automatic pump-trap.

Note: Motive steam supply must be trapped and free of condensate

Soft sealing check valve

100 mesh strainer

Condensate outlet

Motive IN

Exhaust OUT

Condensate inlet APT14 Automatic pump trap

Secondary liquid outlet

Spirax Sarco sized length of pipe to act as a reservoir Secondary liquid inlet

Minimum installation head 0.2 m from base of pump Recommended the reservoir is installed at least 1 pipe diameter below the process outlet, but as high as possible above the APT inlet.

Fig. 13.8.8 Detailed installation of a pump-trap with plate heat exchanger

Footnote

The subject of stall can become somewhat complex, especially when selecting and sizing the most appropriate equipment and designing its installation such that it can be guaranteed to work when commissioned to do so. This Module is not so much intended to make the reader an expert in the subject of stall, but rather:

13.8.8

o

To allow him or her to understand what it is.

o

To understand why it exists.

o

To know what can be done to prevent it.

o

To know who to contact for proper advice.

The Steam and Condensate Loop

Block 13 Condensate Removal

Practical Methods of Preventing Stall Module 13.8

Questions 1. Which of the following methods can be employed to prevent the effects of stall? a| Prevent vacuum formation in the steam space and drain to atmosphere

¨

b| Maintain the condensate backpressure below the steam space pressure

¨

c| Ensure condensate removal by installing a pump-trap

¨

d| All of the above

¨

2. When using the vacuum breaker method of preventing stall, which of the following methods is ideal to ensure the backpressure is maintained below atmospheric pressure? a| Install the auxiliary steam trap

¨

b| Drain the condensate to the atmosphere

¨

c| Drain the condensate to a vented receiver to allow energy recovery

¨

d| Replace the float trap with a balanced pressure thermostatic device

¨

3. Which of the following functions is provided by an automatic pump-trap? a| Check valve

¨

b| Float type steam trap

¨

c| Pressure powered pump

¨

d| All of the above

¨

4. When should a separate pump, steam trap and reservoir combination be used to prevent stall? a| In small heat exchangers where backpressure may be greater than steam pressure

¨

b| In large heat exchangers where backpressure may be greater than steam pressure

¨

c| In vacuum breaker systems where the condensate is drained under gravity

¨

d| All of the above

¨

5. What is the advantage of placing a modulating valve in the condensate drain line? a| The desired steam pressure is maintained regardless of the load conditions

¨

b| It can be used to effectively deal with rapid heat loss changes

¨

c| It is not necessary to control the steam flowrate

¨

d| Stall is prevented regardless of the backpressure

¨

6. Why should on / off control not be used on heat exchangers where the secondary fluid is heated whilst it flows through the heat exchanger? a| Product damage may result from continued heating after the steam valve is closed

¨

b| Rapid opening of the steam valve may cause waterhammer

¨

c| On / off control cannot accurately maintain product temperature

¨

d| All of the above

¨

Answers

1: d, 2: c, 3: d, 4: b, 5: a, 6: c The Steam and Condensate Loop

13.8.9

Block 13 Condensate Removal

13.8.10

Practical Methods of Preventing Stall Module 13.8

The Steam and Condensate Loop