• Author / Uploaded
• Alex Perez

Citation preview

Gas Compressors 14.1 A steady flow compressor handles 113.3 m 3/min of nitrogen measured at intake where p 1 = 97.22 kPaa and t1 = 26.7 OC. Discharge is at 310.27 kPaa. The change if kinetic energy is negligible. For each of the following cases, determine the temperature t2 and the work if the process is: a) Isentropic b) Internally reversible polytropic with n = 1.34 c) An irreversible adiabatic with a compressor efficiency of nc = 80% d) An isothermal. Solve this problem from the view point of energy diagrams. Does – ∫Vdp represents anything asked for? Ans. a) -15,454, b) -15,168, c) -19,318, d) -12,783 kJ/min.

Gas Compressors 14.2

There are required 1902.3 kW of compressor power to handle air adiabatically form 1 atm, 26.7 C to 304.16kPaa. The initial air velocity is 21 m/s; the final is 85 m/s. a) If the process is isentropic, find the volume of air handled, m3/min measured at the inlet conditions b) If the compression is irreversible adiabatic to a temperature of 157.2 OC, with the capacity found in a) find the power input O

Ans. a) 899 m3/min, b) 2377.9 kW

Gas Compressors 14.3 A small blower handles 43.33 m3/min of air whose density is  = 1.169 kg/m3. The static and velocity heads are 16.38 and 1.22 cm of water (at 15.6 OC), respectively. Local gravity acceleration is 9.741 m/s2 a) Find the power input to the air from the blower b) The initial velocity is negligible, find the final velocity Ans. a) 1.24 kW, b) 854 m/min

Gas Compressors 14.4 A large forced draft fan is handling air at 1 atm, 43.3 OC under a total head of 26.6 com of water. The power input to the fan is 224 kW and the fan is 75% efficient. Compute the volume of air handled each minute. Local gravity acceleration is 9.71 m/s2 Ans. 3908 m3/min

Gas Compressors 14.5 Find the cylinder dimension of a single-cylinder, double acting compressor handling 28.32 lts/revolution of air from 99.975 Kpaa to 723.954 kPaa. Compression and reexpansion are in accordance with pV1.35 = C. Use the conventional volumetric efficiency; c = 5% and L/D =1. Ans. 26.77 x 26.77 cm

Gas Compressors 14.6 There are compressed 6.542 m3/min of oxygen from 1 atm, 26.7 OC to 310.29 kPaa by a 35.56 x 35.56 cm single stage, double acting compressor operating at 100 rpm. Compression and reexpansion processes are isentropic and K = 0. Find the volumetric efficiency, the work done on the oxygen, and the heat removed. Solve first by using the conventional indicator card; check by using an energy diagram and considering steady flow Ans. 92.6%, 14.53 kW, 14.57 kW

Gas Compressors 14.7 Find the volumetric efficiency and estimate the approximate clearance of a 45.75 x 45.75 cm, double acting, single cylinder compressor that is turning at 150 rpm and pumping 19.82 m 3 of a gas from 1 atm, 26.7 OC to 675.7 kPaa. Compression and reexpansion are polytropic with pV1.32 = C.

Gas Compressors 14.8 A reciprocating air compressor with a clearance of 6% draws in 4.25 m 3/min of air measured at the suction conditions of 100 kPaa, 57.2 OC. for a discharge pressure of 300 kPaa and an overall adiabatic efficiency of 68%, determine the power of the driving motor Ans. 13.43 kW

Gas Compressors 14.9 For a two stage reciprocating compressor with intercooler, prove that the work will be minimum when the pressure between cylinders (intercooler pressure) is p i = (p1p2)0.5. Here p1 is compressor inlet pressure and p2 is compressor discharge pressure. Further, let the intercooler return the air to the compressor inlet temperature

Gas Compressors 14.10 Air is compressed in a two stage double acting compressor which is electrically driven at 165 rpm. The low pressure cylinder (30.5 x 35.5 cm) receives 6.85 m 3/min of air at 96.53 kPaa, 43.3 OC, and the high pressure cylinder (20.3 x 35.5 cm) discharges the air at 717.16 kPaa. Piston rods ate 5.1 cm in diameter and the isothermal overall efficiency is 74%. Find a) The volumetric efficiency b) The power of the driven motor

Gas Compressors 14.11 There are compressed 11.33 m3/min of air from 103.42 Kpaa, 26.7 OC to 827.36 kPaa. All clearances are 8%. a) Find the isentropic power and piston displacement required for a single stage compression b) Using the same data, find the minimum ideal power for two stage compression when the intercooler cools the air to the initial temperature c) Find the displacement of each cylinder for the conditions of part b) d) How much heat is exchanged in the intercooler e) For a compressor efficiency of 78%, what driving motor output is required Ans. a) 55.4 kW, 15.88 m3/min, b) 47.46 kW, c) 12.43 m3/min, 4.39 m3/min, d) 1423 kJ/min, e) 61.5 kW (2 stage)

Gas Compressors 14.12 Write this computer program. The effect of the polytropic exponent n on the conventional volumetric efficiency of a given compressor is under study. Select specific percentage clearance c and an anticipated pressure ratio p2/p1 and calculate the volumetric efficiencies letting n in the range 1 – 14 (say for air).

Gas Compressors 14.13 The steady flow compressor of a Brayton cycle gas turbine breathes in 45,000 m3/min of air at 15 psia, 60 OF, and compresses it through a pressure ratio of 9.5; the compressor efficiency is 85%. Use item B2 and find: a) Discharge temperature b) Compression power c) Irreversibility and change in availability for po = 15 psia, to = 60 OF.

Gas Compressors 14.14 A low pressure, water jacketed, steady flow compressor compresses 15 lb/ min of air from 14.7 psia and 70 OF to 5 psig and 110 OF. a) Let the process be polytropic, neglect the change in kinetic energy, and find the power. What mass of water is circulated if the temperature rise of the cooling water is 6 OF? How much is ∫Vdp in this system and what does it represent? b) Consider the process as an irreversible adiabatic (no water jacket) with a final temperature of 130 OF (instead of 110 OF) and find the value of m in pV m = C, and the work. What does ∫Vdp represent in this system? Compare works. Ans. a) -129,000 ft-lb/min, 3.64 lb/min of H2O, - ∫Vdp = -165.8 Btu/min. b) 1.577, -216 Btu/min

Gas Compressors 14.15 Air is removed from a large space and given a velocity of 63 ft/s by a fan. The air density is  = 0.075 lb/ft3 and the work done on the air is 0.0155 hp-min/lb air. Find the static head on the fan, in inches of water at 100 OF.

Gas Compressors 14.16 The adiabatic work input required to compress 50 lb/min of air from 14 psia, 80 OF to a higher pressure is 60.4 hp where K = 0. a) If the process is reversible, find the discharge pressure b) If the compression to this pressure is irreversible, where nc = 84%, find the adiabatic work input c) Which of the two these processes will cause the greatest increase in availability?

Gas Compressors 14.17 Water, circulating at the rate of 52 lb/min around the cylinder of an air compressor, enters at 70 O F and leaves at 80 OF, all heat received coming from the air in the cylinder. The compression is internally reversible form 14.7 psia, 80 OF to 330 OF; K = 0. For an air flow of 50 Lb/min, find. a) The power b) S for the air c) The available part of the heat with respect to the air and again as it was received by the water if to = 60 OF Ans. a) 83 hp. b) 0.791 Btu/oR-min. c) 109, 14.3 Btu/min

Gas Compressors 14.18 A 14 x 14 in., horizontal double acting air compressor with 5% clearance operates at 120 rpm, drawing in air at 14.4 psia and 88 OF, and discharging it at 57.6 psia. The compression and reexpansion processes are polytropic with n = 1.33. Sketch the conventional card and determine a) The conventional volumetric efficiency b) The mass of air discharged c) The horsepower input to the air d) How much is - ∫Vdp for the compression process? Ans. (B1) a) 90.8%, b) 19.3 lb/min, c) 29 hp

Gas Compressors 14.19 A 14 x 12 in., single cylinder double acting air compressor with 5.5% clearance operates at 125 rpm. The suction pressure and temperature are at 14 psia and 100 OF, respectively. The discharge pressure is 42 psia. Compression and reexpansion processes are isentropic. Considering the conventional compressor and neglecting the piston rod affect, determine: a) The volumetric efficiency b) The mass and volume at suction conditions handled each minute c) The work in horsepower d) The heat rejected e) The indicated horsepower developed if the compression efficiency is 75%

Gas Compressors 14.20 Carbon dioxide is handled by a rotary type compressor from 15 psia, 90 OF through a compression ratio of 7 with a compressor efficiency of 75%; K = 0. Find the discharge temperature. Solve using item B1 and check the answer using gas tables. Ans. (B1) 1100 oR, (B3) 1009 OR

Gas Compressors 14.21 A compressor handles 3500 ft3/min of carbon dioxide measured at intake where p 1 = 14.2 psia and t1 = 75 OF. At discharge, p2 = 28.4 psia and t2 = 178 OF. The initial velocity is 40 ft/s and the final velocity is 150 ft/s. The process is an irreversible adiabatic. Find: a) H’, U’, S’ b) W’ c) c Ans. a) 8150, 6400 Btu/min, 2.06 Btu/oR-min, b) -8309 Btu/min, c) 82%

Gas Compressors 14.22 A steady flow compressor compresses 2300 ft3/min of methane from 15 psia and 75 OF to 15 psig. The process is an irreversible adiabatic and the change of kinetic energy is negligible. If the compression efficiency is c = 82.7%, find: How much is - ∫Vdp. What is the change of availability per pound of methane flowing if the lowest available temperature is to = 40 OF

Gas Compressors 14.23 A compressor is to be designed with 6% clearance to handle 1500 ft 3/min of air at 14.7 psia and 70 OF, the state at the beginning of the compression stroke. The compression is isentropic to 90.3 psig. a) What displacement in ft3/min is necessary? b) If the compressor is used at an altitude of 6000 ft, and if the initial temperature and the discharge pressure remain in the same as given above, by what percent is the capacity of the compressor reduced? c) What should be the displacement of at compressor at the altitude of 6000 ft to handle the same mass of air as in a)?

Gas Compressors 14.24 A two stage air compressor without a clearance delivers 90 lb /min at 140 psia. At the intake, p 1 = 14.3 psia and t1 = 60 OF. The compression follows pV1.31 = C, and the intercooler cools the air back to 60 OF. Find. a) The optimum intermediate pressure b) The conventional power c) The heat of the various processes (sketch this processes on the TS plane) d) What horsepower would be required for isentropic compression in a single stage machine? e) What is the saving due to the cooling process? Does this appear to be worth while f) If the temperature of the cooling water in the intercooler rises 15 OF, what mass of water is required? Ans. a) 44.75 psia, b) -198 hp, c) -717, -3483, -717 Btu/min, d) -243 hp, e) 45 hp, f) 232 lb/min

Gas Compressors 14.25 A single cylinder, double acting air compressor running at 200 rpm has a piston speed of 600 ft/min. it compresses 60 lb/min from 14 psia and 60 OF to 95 psia. Clearance is 5.5%. for an isentropic compression, find a) v, VD, and W; b) Pm, of the conventional card c) The bore and stroke of the compressor. Solve for W twice using air properties from both Item B1 and Item B2. Compare Ans. (B1), a) VD =983 ft3/min, b)29.9 psia, c)17.35 x 18 in