2009 Tutorial Sheet 1

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Refrigeration Systems (Tutorial 1): Autumn 2009 1. A single stage vapour compression refrigeration system working with ammonia operates at an evaporator temperature of –17.8oC and condensing temperature of 32.2oC. Assume a saturated cycle with the compression process being polytropic with an index of 1.24. Assume no pressure drops in piping and compressor valves and no temperature changes from the exit of the evaporator to the inlet of the compressor. Calculate: a) Coefficient of Performance (CO
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  1. A single stage vapour compression refrigeration system working with ammoniaoperates at an evaporator temperature of –17.8 o C and condensing temperature of32.2 o C. Assume a saturated cycle with the compression process being polytropic withan index of 1.24. Assume no pressure drops in piping and compressor valves and notemperature changes from the exit of the evaporator to the inlet of the compressor.Calculate: a) Coefficient of Performance (COP) and compare with the COP forisentropic compression (index of compression = 1.286) Refrigeration Systems (Tutorial 1): Autumn 2009 The following data are given for ammonia:At –17.8 o C: saturation pressure = 209.4 kPa, specific volume of saturated vapour =0.5694 m 3  /kg and enthalpy of saturated vapour = 1420.9 kJ/kgAt 32.2 o C 2. Compare the COP of an ammonia based vapour compression refrigeration systemwhich uses wet compression with that of the one which uses dry compression. In bothcases the evaporator temperature is –20 o C and the condenser temperature is 30 o C.Assume no subcooling and the compression process to be isentropic. In wetcompression the refrigerant vapour enters the compressor in such a condition that it issaturated vapour upon leaving the compressor. Show both the cycles on T-s and P-hdiagrams. Use the following property data:: saturation pressure = 1245.0 kPa, enthalpy of saturated liquid = 333.1 kJ/kgat –20 o C:saturated liquid: enthalpy = 108.594 kJ/kgsaturation pressure = 190.74 kPa,saturated vapour: enthalpy = 1437.23 kJ/kg, entropy = 5.9025 kJ/kg.Kat 30 o Csaturated liquid: enthalpy = 341.769 kJ/kg: saturation pressure = 1168.6 kPasaturated vapour: enthalpy = 1486.14 kJ/kg, entropy = 5.2624 kJ/kg.Kat 1168.6 kPa and entropy of 5.9025 kJ/kg.K   : enthalpy = 1709.73 kJ/kg 3. In the vapor compression cycle a throttling device is used to reduce the pressure ofthe liquid refrigerant (from condenser pressure to evaporator pressure). Determine thepercent improvement in the COP of the cycle if, in place of a throttling device, anexpansion engine could be used to expand saturated liquid refrigerant (HCFC-22)isentropically from a condensing temperature of 35 o C to an evaporator temperature of0 o C. Assume that the compression is isentropic from the saturated vapor state at 0 o C tocondenser pressure corresponding to 35 o C. Show both the cycles on P-h diagram. Thefollowing are the refrigerant property values:at 0 o C (Saturation pressure = 0.498 MPa):  Saturated liquid and vapor enthalpies = 200 kJ/kg & 404.87 kJ/kg respectivelySaturated liquid and vapor entropies = 1.00 kJ/kg.K & 1.751 kJ/kg.K respectivelySaturated liquid enthalpy = 243.14 kJ/kg, Saturated liquid entropy = 1.146 kJ/kg.K, andat 35 o C (Saturation pressure = 1.356 MPa):Enthalpy at compressor exit = 429.9 kJ/kg4. The compressor of a 25 TR (refrigeration capacity) vapour compression refrigerationsystem is cooled by circulating cooling water through the water jacket surrounding thecompressor. The refrigerant (ammonia) at the exit of condenser is saturated liquid ( P =12.3 bar, h f = 350.4 kJ/kg ) and it is saturated vapour at the exit of the evaporator ( T = -12.2 o C, h g = 1448 kJ/kg ). The pressure and temperature of the superheated refrigerantat the exit of compressor are 12.3 bar and 77 o C , respectively ( h v = 1617 kJ/kg ). Thepower input to the compressor is 17.3 kW . Find the required mass flow rate of coolingwater if its temperature rises by 5.6 o C as it flows through the water jacket surroundingthe compressor. Take the specific heat of water as 4.18 kJ/kg .5. A R 22 based refrigeration system operates between an evaporator temperature of – 20 o C and a condensing temperature of 32 o . The system consists of a Liquid-to-SuctionHeat Exchanger (LSHX), which has a heat exchanger effectiveness of 0.77. Assumethe compression process to be reversible, adiabatic and the refrigerant condition at theexit of evaporator and condenser to be saturated. Draw the system schematic and showthe cycle on T-s and p-h diagrams. Using the data given below find: (a) Specificrefrigeration effect, (b) Volumic refrigeration effect, and (c) COP. At –20 o C Enthalpy of saturated vapour = 397.467 kJ/kg : Saturation pressure = 244.83 kPa At 32 o C Enthalpy of saturated liquid = 239.23 kJ/kg : Saturation pressure = 1255.2 kPaA constant specific heat (c p ) value of 0.654 kJ/kg.K may be assumed for the refrigerantvapour at 244.83 kPa. The specific volume of refrigerant vapour at the entry and exit ofthe compressor are 0.1107 m 3  /kg and 0.027 m 3  /kg, respectively.6. The compressor of a 25 TR (refrigeration capacity) vapour compression refrigerationsystem is cooled by circulating cooling water through the water jacket surrounding thecompressor. The refrigerant (ammonia) at the exit of condenser is saturated liquid( P=12.3 bar, h f = 350.4 kJ/kg ) and it is saturated vapour at the exit of the evaporator  ( T= − 12.2 o C, h g = 1448 kJ/kg ). The pressure and temperature of the superheatedrefrigerant at the exit of compressor are 12.3 bar and 77 o C , respectively and thecorresponding enthalpy is 1617 kJ/kg . The power input to the compressor is 17.3 kW .Find the required mass flow rate of cooling water if its temperature rises by 5.6 o C as itflows through the water jacket surrounding the compressor. Take the specific heat ofwater as 4.18 kJ/kg .7. Saturated liquid ammonia is expanded in a throttle valve from 36 o C to a temperatureof -30 o C . Find a) the dryness fraction of ammonia at the exit of the throttle valve b) Voidfraction (ratio of volume of vapour to total volume) at the exit of throttle valve, c)Throttling loss and d) Entropy change in the process.8. Refrigerant R134a has a normal boiling point of 26.07 o C . Find the saturation pressureof R134a at -30 o C assuming an average latent heat of vaporization of 117 kJ/kg.Molecular weight of R134a is 102.03 kg/kmol.9. Saturated R134a at 40 o C is expanded in a throttle valve to a final temperature of -30 o C. Assuming that the energy required for generation of flash gas is supplied byR134a itself, find average Cp value of R134a liquid. Check the value obtained with theactual Cp value from property data.
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