• Author / Uploaded
• Kristiani Dewi Pasaribu

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Problem / Discussion Evaporation - Refrigeration Robi Andoyo, STP., M.Sc., Ph.D

Tomato juice at 5.5 kg/s feed flow rate and 60 0C inlet temperature is concentrated in a double-effect forward feed evaporator using steam in the first effect at a pressure of 198.5 kPa (absolut). The heat transfer area, the overall heat transfer coefficient, and the pressure in each effect are shown in the following table. The solids content and the heat capacity of the feed are 11% and 3900 J/kg 0C respectively. Calculate the steam flow rate, the solids content at the exit of each effect, the steam economy, and the flow rate of cooling water in the condenser.

Assume : The condensate in each effect exits at the condensation temperature. The operation is steady state.

Step 1 Draw the process diagram

Step 2 Combine the heat transfer rate equation with an enthalpy balance in the first effect to calculate the steam consumption

= 198.5 kPa

• The saturation temperature in the 1st effect, Tsat1, is found from steam tables for Psat = 90 kPa to be equal to 97 0C = Tb1 • Substitute values, and calculate ms

Step 3 Calculate the solids content

Hv1 equal to the enthalpy of saturated steam, Hv1 saturated. Thus: Hv1 at 97 0C = 2671 kJ/kg (from Steam tables)

• Find mv1: A mass balance on the product gives mf = mo1 + mv1 or mv1 = mf - mo1 • Find the enthalpy of liquid streams Using 0 0C as a reference temperature, the enthalpy of the liquid streams is: H = c pT Thus :

Solve mo1, substitute values, and calculate the mass flow rate at the exit of the first effect

mo1 = 3,6 kg/s

Calculate the solids content at the exit of the first effect from a solids mass balance

0,16 3,6

At the exit of the second effect • Write an enthalpy balance around the second effect

3,6 3,6 1,9

970

1,9

• The saturation temperature in the second effect, Tsat2, is found from steam tables for Psat = 17.9 kPa to be equal to 57.6 0C.

3.6

1.9

3.6

1,8

• Substitute values and calculate the evaporation rate in the second effect mv2 = mi2 - mp = 3,6 – 1,8 = 1,8 kg/s

• Calculate the solids content at the exit of the second effect from a solids mass balance

0,16

3,6 1,8

Step 4 Calculate the steam economy 1,9

1,77

It wished to freeze 15 ton of fish /day from an initial temp. of 10 0 C to a final temp. of -8 0C using a stream of cold air. Estimate the max capacity of ref. Plant required. If the heat transfer-coefficient from the air to evap. Coil (over all) is 22 J.m-2.s-1.0C-1. Calculate the surface area of evaporator coil required if the logarithmic mean temp drop across the coil is 12 0C. Spesific heat of fish is 3.18 kJ/kg.0C above freezing and 1.67 kJ/kg.0C bellow freezing & latent heat is 276 kJ/kg

Enthalpy change in fish: (10 x 3.2) + (8 x 1.67) + 276 = 31.8 + 13.4 + 276 CpL.(TL-Tref) + Cpf.(Tf-Tref) + λ = 321.2 kJ kg-1

Heat removed in freezing: 15 x 1000 x 321.2 = 4.82 x 106 kJ day-1

Average rate of heat removal: (4.82 x 106)/(24 x 60 x 60) = 55.8 kJ s-1

Step 6 Calculate the cooling water flow rate in the condenser • Write an enthalpy balance in the condenser

Rate of heat transfer

q = U A ∆TLM A = q / U ∆TLM = (55.8) / (22 x 12) = 0.21 x 103 m2 = 210 m2.