A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at -7°C. The high-pressure air is cooled to 27°C by rejecting heat to the surroundings. It is further cooled to -15°C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, determine (a) the lowest temperature that can be obtained by this cycle, (b) the coefficient of performance of the cycle, and (c) the mass flow rate of air for a refrigeration rate of 12 kW.

A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at -7°C. The high-pressure air is cooled to 27°C by rejecting heat to the surroundings. It is further cooled to -15°C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, determine (a) the lowest temperature that can be obtained by this cycle, (b) the coefficient of performance of the cycle, and (c) the mass flow rate of air for a refrigeration rate of 12 kW.

Refrigeration and Air Conditioning Technology (MindTap Course List)

8th Edition

ISBN:9781305578296

Author:John Tomczyk, Eugene Silberstein, Bill Whitman, Bill Johnson

Publisher:John Tomczyk, Eugene Silberstein, Bill Whitman, Bill Johnson

Chapter28: Special Refrigeration Applications

Section: Chapter Questions

Problem 15RQ: Why is two-stage compression popular for extra-low-temperature refrigeration systems?

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A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at -7°C. The high-pressure air is cooled to 27°C by rejecting heat to the surroundings. It is further cooled to -15°C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, determine (a) the lowest temperature that can be obtained by this cycle, (b) the coefficient of performance of the cycle, and (c) the mass flow rate of air for a refrigeration rate of 12 kW.
A gas refrigeration system using air as the working fluid has a pressure ratio of 4. Air enters the compressor at -7°C. The high pressure air is cooled to at 27°C by rejecting heat to the surroundings. It is further cooled to at -15°C by regenerative cooling before it enters the turbine. Assuming both the turbine and the compressor to be isentropic and using constant specific heats at room temperature, Assume isentropic efficiencies of 0.75 for the compressor and 0.80 for the turbine. Cp= 1.005 kJ/kg·K and k = 1.4 determine: (a) the lowest temperature that can be obtained in the cycle, (b) the coefficient of performance of the cycle, and (c) the mass flow rate of air for a refrigeration rate of 12 kW
2. Refrigerant-134a enters the compressor of a refrigerator at 100 kPa and -20°C at a rate of 0.5 m³/min and leaves at 0.8 MPa. The isentropic efficiency of the compressor is 78 percent. The refrigerant enters the throttling valve at 0.75 MPa and 26°C and leaves the evaporator as saturated vapor at 26°C. Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the power input to the compressor, (b) the rate of heat removal from the refrigerated space, and (c) the pressure drop and rate of heat gain in the line between the evaporator and the compressor. Answers: (a) 2.40 kW, (b) 6.17 kW, (c) 1.73 kPa, 0.203 kW
R-134a enters the compressor of a refrigerator at 140 kPa and -10 ˚C at a rate of 0.3m3/min and leaves at 1 MPa. The isentropic efficiency of the compressor is 78%. The refrigerant enters the throttling valve at 0.95 MPa and 30 ˚C, and leaves the evaporator as saturated vapor at -18.5 ˚C. Show the cycle on a T-s diagram with respect to saturation lines, and determine (a) the power input to the compressor, (b) the rate of heat removal from the refrigerated space, and (c) the pressure drop and (d) rate of heat gain in the line between the evaporator and the compressor.
Consider a gas refrigeration system with air as the working fluid. The pressure ratio is 5.5. Air enters the compressor at 0°C. The high-pressure air is cooled to 35 °C by rejecting heat to the surroundings. The refrigerant leaves the turbine at -95°C and then it absorbs heat from the refrigerated space before entering the regenerator. The mass flow rate of air is 0.55 kg/s. Assuming isentropic efficiencies of 90% for both the compressor and the turbine, determine (a) the effectiveness of the regenerator, (b) the rate of heat removal from the refrigerated space, and (c) the COP of the cycle. Also, determine (d) the refrigeration load and the COP if this system operated on the simple gas refrigeration cycle. In this cycle, take the compressor and turbine inlet temperatures to be 0 and 35 °C, respectively, and use the same compressor and turbine efficiencies. Use constant specific heat for air at room temperature with Cp = 1.005 kJ/kg K and k = 1.4.
A gas refrigeration cycle with a pressure ratio of 3 uses helium as the working fluid. The temperature of the helium is -10°C at the compressor inlet and 50°C at the turbine inlet. Assuming adiabatic efficiencies of 80 percent for both the turbine and the compressor, determine (a) the minimum temperature in the cycle, (b) the coefficient of performance, and (c) the mass flow rate of the helium for a refrigeration rate of 18 kW.
Refrigerant-134a enters the compressor of a cooling system as superheated vapor at 0.18 MPa and 0°C with a flow rate of 0.15 kg/s. It exits the compressor at 0.8 MPa and 60°C. Post compression, the refrigerant is cooled in the condenser to 28°C and 0.7 MPa. Subsequently, it's throttled to 0.16 MPa. Neglecting any heat transfer and pressure drops in the pipelines between the components, represent the cycle on a T-s diagram concerning saturation lines. Calculate: (a) The rate of heat extraction from the cooling area and the energy input to the compressor. (b) The isentropic efficiency of the compressor. (c) The Coefficient of Performance (COP) of the cooling system.
. Refrigerant-134a at a rate of 0.08 kg/s enters the compressor of a refrigerator as superheated vapor at 0.18 MPa and 0 ℃ and leaves at 0.9 MPa and 80 ℃. The refrigerant is cooled in the condenser to 31.3 ℃ and 0.8 MPa and it is throttled to 0.18 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, a) Show the cycle on a T-S diagram b) Determine the rate of heat removal from the refrigerated space and the power input to the compressor c) Determine the adiabatic efficiency of the compressor d) Determine the coefficient of performance of the refrigerator.
R134a is used as a working fluid in a cooling system with a cooling capacity of 50 kJ / s operating according to the ideal vapor compression refrigeration cycle. The refrigerant enters the compressor as saturated steam at 120 kPa pressure and is compressed to 650 kPa pressure. Mass flow of R-134a is 0.10 kg / s. Calculate the efficiency coefficient of the cycle.
A vapor-compression cycle is used as a refrigerator to extract 8 kW of heat from a cold space at 0°C and rejects heat into a space at 25°C. R-134a enters the compressor at 140 kPa superheated by 4°C above the saturation temperature and leaves at 900 kPa. It exits the condensor as a saturated liquid. Assume that pressure losses are negligible and that the compressor is isentropic. Report your answers to four significant digits using rounding. QUESTION 1 Report the maximum possible COP. QUESTION 3 Report the power required by the compressor. QUESTION 2 Report the mass flow rate in kg/s. QUESTION 4 Report the COP for the cycle.
A gas turbine unit consists of a compressor, heat exchanger, combustion chamber and turbine. The compressor and turbine are connected with a shaft. The unit operates between pressure limits of 100kPa and 625kPa. The cycle temperatures are: 28°C at compressor inlet, 625°C at the turbine inlet and 375°C at the turbine exhaust. The isentropic efficiency of the compressor is 85% and the effectiveness of the heat exchanger is 85%. If the net power is 1.5MW, calculate: (a). the temperature at compressor delivery (b). the isentropic efficiency of the turbine (c). the thermal efficiency of the cycle (d). the air-fuel ratio (e). the specific fuel consumption (f). the daily fuel consumption at the given power.
A vapor-compression cycle is used as a refrigerator to extract 8 kW of heat from a cold space at 0°C and rejects heat into a space at 25°C. R-134a enters the compressor at 140 kPa superheated by 4°C above the saturation temperature and leaves at 900 kPa. It exits the condensor as a saturated liquid. Assume that pressure losses are negligible and that the compressor is isentropic. Report your answers to four significant digits using rounding. 1. Report the maximum possible COP. 2. Report the mass flow rate in kg/s. 3. Report the power required by the compressor. 4. Report the COP for the cycle.