Sheet No. Three 1. An ammonia system operating with an evaporating temperature of -30°C and a condensing temperature of 35°C separates flash gas at a temperature of -2°C and delivers it to the condenser through a separate compressor. If the refrigerating capacity is 200 kW (56.9 tons of refrigeration), what are the power requirements if the system operates (a) single stage, and (b) with flash-gas removal? 2. A refrigeration cycle uses Feron-12 as the working fluid. The temperature of the refrigerant in the evaporator is -10˚C and the condensing temperature is 40˚C. The cooling load is 150W. The compressor speed is 720rpm and the volumetric efficiency is 80%. Calculate the mass flow rate of the refrigerant and the displacement volume of the compressor. The following are selected extract of the Feron-12 properties. Temperature, ˚C -10 40
Pressure, MPa 0.22 0.96
Enthalpy, kJ/kg Liquid Vapor 26.8 183.0 74.5 203.1
Specific volume of Sat. vapor, m3/kg 0.08 0.02
3. In a simple vapor compression cycles, the following data were obtained for R-12 refrigerant at various points: Compressor inlet h2 = 183.2kJ/kg v2 = 0.0767m3/kg Compressor discharge h3 = 222.6kJ/kg v3 = 0.00164m3/kg Condenser exit hf4 = 84.9kJ/kg v4 = 0.00083m3/kg The piston displacement volume for the compressor is 1.5liters/stroke and its volumetric efficiency is 80%. The speed of the compressor is 1600rpm. Find: a) Power rating of the compressor (kW), and b) Cooling capacity (kW). 4. A refrigerating plant works between -5˚C and 25˚C. The working fluid is ammonia and has a dryness fraction of 0.62 at entry of compressor. If the machine has a relative efficiency of 55%, calculate the amount of ice formed during a period of 24 hours. The ice is to be formed at 0˚C from water at 15˚C and 6.4kg of ammonia is circulated per minute. Specific heat of water is 4.187kJ/kg and latent heat of ice is 355kJ/kg. Selected properties of ammonia (NH3) is given in the following table; Temperature ˚C 25 -5
Liquid heat, hf kJ/kg 298.9 158.2
Latent heat, hfg kJ/kg 1137.1 1280.8
1
Entropy of liquid kJ/kg.K 1.124 0.630
5. Determine the theoretical COP for CO2 machine working between the temperature range of 25˚C and -5˚C. The dryness fraction of CO2 gas during the suction stroke is 0.6. The following are selected extract of the properties for CO2 Temperature ˚C 25 -5
Heat Liquid 81.17 -7.53
Latent heat, hfg kJ/kg 121.34 245.52
Vapor 202.5 236.88
Entropy kJ/kg.K Liquid Vapor 0.251 .644 -0.042 0.841
How many tons of ice would a machine working between the same limits and having a relative COP (efficiency) of 45% make in 24 hours? The water for the ice is supplied at 15˚C and the compressor takes 8.2kg of CO2 per hour. Specific heat of water may be taken as 4.18kJ/kg.K and latent heat of ice as 335kJ/kg. 6. A F-12 vapor compression refrigeration system has a condensing temperature of 50˚C and evaporating temperature of 0˚C. The refrigeration capacity is 7TR. The liquid leaving the condenser is saturated liquid and compression is isentropic. The enthalpy at the end of isentropic compression is 210kJ/kg. Determine: (a) The refrigerant flow rate, (b) The power required to run the compressor, (c) The heat rejected in the plant, and (d) The COP of the system. You may use the extract of Feron-12 property table given below: hg kJ/kg Fluid entropy Temperature Pressure, hf kJ/kg kJ/kg.K ˚C bar 50 12.199 84.868 206.298 0.3034 0 3.086 36.022 154.364 0.1418
Gas entropy kJ/kg.K 0.6792 0.6960
7. A standard vapor-compression refrigeration cycle operates between an evaporator temperature of -10˚C and a condenser temperature of 40˚C. The flow rate of the refrigerant, Feron-12, is 1.0kg/min and the enthalpy at the end of compression is 220kJ/k. Show the T-S diagram of the cycle, and then calculate: (a) The COP of the cycle, (b) The refrigeration capacity, and (c) The compressor power. You may use the extract of Feron-12 property table given below: Temperature, ˚C Pressure, bar hf kJ/kg hg kJ/kg -10 0.2191 26.85 183.1 40 0.9607 74.53 203.1 8. A food storage locker requires a refrigeration capacity of 50kW. It works between a condenser temperature of 35˚C and an evaporator temperature of -10˚C. The refrigerant is ammonia. It is sub-cooled by 5˚C before entering the expansion valve by the dry saturated vapor leaving the evaporator. Assuming a singlecylinder single-acting compressor operating at 1000rpm with stroke equal to 1.2 times the bore, determine: (a) The power required and (b) The cylinder dimensions. You may use the extract of ammonia property table given below: 2
Tsat, ˚C
Pressure bar
-10 35
2.9157 13.522
Enthalpy, kJ/kg Liquid
Vapor
Entropy, Specific kJ/kg.K volume, m3/kg Liquid Vapor Liquid Vapor
154.056 1450.22 0.82965 366. 1488.57 1.56605
5.7550 5.2086
-1.7023
0.41747 0.09563
Specific heat, kJ/kg.K Liquid Vapor -4.556
2.492 2.903
9. A food storage locker requires a refrigeration system of 2400kJ/min capacity at an evaporator temperature of -10˚C and a condenser temperature of 30˚C. The refrigerant is feron-12 and is sub-cooled by 6˚C before entering the expansion valve and vapor is superheated by 7˚C before leaving the evaporator. The compression is isentropic. The refrigerant compressor is a two-cylinder singleacting operating at 1000rpm with stroke equal to 1.25 times the bore. Take the liquid specific heat as 1.235kJ/kg.K and the vapor specific heat as 0.733kJ/kg.K. Determine (a) Refrigeration effect per kg, (b) Mass of refrigerant to be circulated per minute, (c) Theoretical piston displacement per minute, (d) Theoretical power required to run the compressor in kW, (e) Heat removed through condenser per minute, and (f) Theoretical bore and stroke of compressor. You may use the extract of ammonia properties table given below: Enthalpy, kJ/kg Entropy, kJ/kg.K Specific volume, m3/kg Tsat, ˚C Pressure bar Liquid Vapor Liquid Vapor Vapor -10 30
2.19 7.45
26.9 64.6
183.2 199.6
0.1080 0.2399
0.7020 0.6854
0.0767 0.0235
10. A refrigeration cycle uses Feron-12 as the working fluid. The temperature of the refrigerant in the evaporator is -10˚C and the condensing pressure is 40˚C. The cooling load is 150W. The compressor speed is 720rpm and the volumetric efficiency is 80%. Calculate the mass flow rate of the refrigerant and the displacement volume of the compressor. The following are selected extract of the Feron-12 properties. Temperature, ˚C -10 40
Pressure, MPa 0.22 0.96
Enthalpy, kJ/kg Liquid Vapor 26.8 183.0 74.5 203.1
3
Specific volume of Sat. vapor, m3/kg 0.08 0.02
Pharos University in Alexandria Faculty of Engineering Mechanical Engineering Department
ME‐240 Refrigeration & Air Conditioning (1) Spring 2013 ‐ 2014 8th Semester
Sheet 3.1 Improved & Multi-Stage Vapor Compression System 1) A R‐12 vapor compression system, fitted with a heat exchanger, which subcools the liquid refrigerant coming from the condenser from 32 to 22 . if the vapor leaving the evaporator is dry and saturated at 12 .Draw the flow diagram of the system and represent it on P‐h chart and show the effect of the presence of the heat exchanger on : a) The C.O.P of the system. b) The refrigerating capacity in ton refrigerant if the compressor pumps 25 . 2) For single compressor R‐717 System consisted of flash chamber, heat exchanger. The condenser pressure is 1.6 MPa, subcooler 0.6 MPa and evaporator pressure 0.3 MPa. Vapor temperature increased through the heat exchanger 10 . Make a suitable arrangement for this cycle and determine the following : a) The C.O.P of the system. 3) For a small refrigeration plant of 10 T.R. if inside design air temperature is 2 and the outside design air temperature is 30 .make a suitable selection for a simple ammonia system, including a heat exchanger, single cylinder compressor with stroke to bore ratio 0.9, volumetric efficiency 0.8 and 12 rps. Determine the following: a) The C.O.P of the system. b) Compressor’s dimension. c) The power required for the compressor. 4) The following data are for a compound compression ammonia (R‐717) Machine (Shown in Figure below) with flash inter‐cooler. The Capacity is 100 T.O.R, condenser pressure 13 bar, evaporator pressure 1.0 bar, Inter‐cooler pressure 5 bar. The volumetric efficiency for the low & high‐pressure cylinder = 85%, compression is isentropic. Ammonia enters the low‐pressure cylinder at ‐12 and leaves the condenser at 33 . Determine the following: a) The total power required. b) The C.O.P of the system. c) The total pistons displacement in
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Condenser Ex1 C2 Flash Inter-Cooler Ex2 Evaporator C1
