Assignment 12

ASSIGNMENT NO.1

Final paper will be set from these assignments (only values changed), so it is important for you to complete these assignments

Date of submission: September 18

The Reversed Carnot Cycle 1. Why is the reversed Carnot cycle executed within the saturation dome not a realistic model for refrigeration cycles? 2. A steady-flow Carnot refrigeration cycle uses refrigerant-134a as the working fluid. The refrigerant changes from saturated vapor to saturated liquid at 30°C in the condenser as it rejects heat. The evaporator pressure is 160 kPa. determine a. the coefficient of performance b. the amount of heat absorbed from the refrigerated space c. the network input

Ideal and Actual Vapor-Compression Refrigeration Cycles 1. Why is the throttling valve not replaced by an isentropic turbine in the ideal vaporcompression refrigeration cycle? 2. It is proposed to use water instead of refrigerant- 134a as the working fluid in airconditioning applications where the minimum temperature never falls below the freezing point. Would you support this proposal? Explain 3. A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.14 and 0.8 MPa. If the mass flow rate of the refrigerant is 0.05 kg/s, determine a. rate of heat removal from the refrigerated space and the power input to the compressor b. rate of heat rejection to the environment c. COP of the refrigerator. 4. Refrigerant-134a enters the compressor of a refrigerator as superheated vapor at 0.14 MPa and -10°C at a rate of 0.05 kg/s and leaves at 0.8 MPa and 50°C. The refrigerant is cooled in the condenser to 26°C and 0.72 MPa and is throttled to 0.15 MPa. Disregarding any heat transfer and pressure drops in the connecting lines between the components, determine a. rate of heat removal from the refrigerated space and the power input to the compressor b. isentropic efficiency of the compressor c. coefficient of performance of the refrigerator. 5. A commercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at -30°C by rejecting its waste heat to cooling water that enters the condenser at 18°C at a rate of 0.25 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 65°C and leaves at 42°C. The inlet state of the compressor is 60 kPa and -34°C and the compressor is estimated to gain a net heat of 450 W from the surroundings. Determine a. quality of the refrigerant at the evaporator inlet b. refrigeration load c. COP of the refrigerator d. theoretical maximum refrigeration load for the same power input to the compressor 6. Refrigerant 134a is the working fluid in an ideal vapor-compression refrigeration cycle that communicates thermally with a cold region at 0oC and a warm region at 26oC. Saturated vapor enters the compressor at 0oC and saturated liquid leaves the condenser at 26oC. The mass flow rate of the refrigerant is 0.08 kg/s. Determine (a) the compressor power, in kW, (b) the refrigeration capacity, in tons, (c) the coefficient of performance, and (d) the coefficient of performance of a Carnot refrigeration cycle operating between warm and cold regions at 26 and 0?C, respectively. 7. If the minimum and maximum allowed refrigerant pressures are 1 and 10 bar, respectively, which of the following can be used as the working fluid in a vaporcompression refrigeration system that maintains a cold region at 0oC, while discharging energy by heat transfer to the surrounding air at 30oC: Refrigerant 22, Refrigerant 134a, ammonia, propane?

Cascade Refrigeration 1. What is cascade refrigeration? What are the advantages and disadvantages of cascade refrigeration? 2. How does the COP of a cascade refrigeration system compare to the COP of a simple vapor-compression cycle operating between the same pressure limits? 3. Consider a two-stage cascade refrigeration system operating between the pressure limits of 0.8 and 0.14 MPa. Each stage operates on the ideal vapor-compression refrigeration cycle with refrigerant-134a as the working fluid. Heat rejection from the lower cycle to the upper cycle takes place in an adiabatic counter-flow heat exchanger where both streams enter at about 0.4 MPa. If the mass flow rate of the refrigerant through the upper cycle is 0.24 kg/s, determine a. mass flow rate of the refrigerant through the lower cycle b. rate of heat removal from the refrigerated space and the power input to the compressor c. coefficient of performance of this cascade refrigerator 4. A vapor-compression refrigeration system uses the arrangement shown in Fig. for two-stage compression with intercooling between the stages. Refrigerant 134a is the working fluid. Saturated vapor at -30?C enters the first compressor stage. The flash chamber and direct contact heat exchanger operate at 4 bar, and the condenser pressure is 12 bar. Saturated liquid streams at 12 and 4 bar enter the high- and lowpressure expansion valves, respectively. If each compressor operates isentropically and the refrigerating capacity of the system is 10 tons, determine a. power input to each compressor, in kW. b. coefficient of performance.

5. Figure shows the schematic diagram of a vapor-compression refrigeration system with two evaporators using Refrigerant 134a as the working fluid. This arrangement is used to achieve refrigeration at two different temperatures with a single compressor and a single condenser. The low-temperature evaporator operates at -18oC with saturated vapor at its exit and has a refrigerating capacity of 3 tons. The higher temperature evaporator produces saturated vapor at 3.2 bar at its exit and has a refrigerating capacity of 2 tons. Compression is isentropic to the condenser pressure of 10 bar. There are no significant pressure drops in the flows through the condenser and the two evaporators, and the refrigerant leaves the condenser as saturated liquid at 10 bar. Calculate a. mass flow rate of refrigerant through each evaporator, in kg/min. b. compressor power input, in kW. c. rate of heat transfer from the refrigerant passing through the condenser, in kW.

6. An ideal vapor-compression refrigeration cycle is modified to include a counterflow heat exchanger, as shown in Fig. Ammonia leaves the evaporator as saturated vapor at 1.0 bar and is heated at constant pressure to 5oC before entering the compressor. Following isentropic compression to 18 bar, the refrigerant passes through the condenser, exiting at 40oC, 18 bar. The liquid then passes through the heat exchanger, entering the expansion valve at 18 bar. If the mass flow rate of refrigerant is 12 kg/min, determine a. refrigeration capacity, in tons of refrigeration. b. compressor power input, in kW. c. coefficient of performance. Discuss possible advantages and disadvantages of this arrangement.

Vapor-Compression Heat Pump Systems 1. On a particular day when the outside temperature is 5oC, a house requires a heat transfer rate of 12 kW to maintain the inside temperature at 20oC. A vaporcompression heat pump with Refrigerant 22 as the working fluid is to be used to provide the necessary heating. Specify appropriate evaporator and condenser pressures of a cycle for this purpose. Let the refrigerant be saturated vapor at the evaporator exit and saturated liquid at the condenser exit. Calculate a) mass flow rate of refrigerant, in kg/min. b) compressor power, in kW. c) coefficient of performance. 2. A vapor-compression heat pump with a heating capacity of 500 kJ/min is driven by a power cycle with a thermal efficiency of 25%. For the heat pump, Refrigerant 134a is compressed from saturated vapor at -10oC to the condenser pressure of 10 bar. The isentropic compressor efficiency is 80%. Liquid enters the expansion valve at 9.6 bar, 34oC. For the power cycle, 80% of the heat rejected is transferred to the heated space. a) Determine the power input to the heat pump compressor, in kW b) Evaluate the ratio of the total rate that heat is delivered to the heated space to the rate of heat input to the power cycle. Discuss.

Gas Refrigeration Cycle 1. Explain the working and applications of Gas Refrigeration Cycle in detail. 2. An ideal gas refrigeration cycle using air as the working fluid is to maintain a refrigerated space at -23°C while rejecting heat to the surrounding medium at 27°C. If the pressure ratio of the compressor is 3, determine a. the maximum and minimum temperatures in the cycle b. coefficient of performance c. rate of refrigeration for a mass flow rate of 0.08 kg/s. 3. Air enters the compressor of an ideal Brayton refrigeration cycle at 100 kPa, 270 K. The compressor pressure ratio is 3, and the temperature at the turbine inlet is 310 K. Determine a) net work input, per unit mass of air flow, in kJ/kg. b) refrigeration capacity, per unit mass of air flow, in kJ/kg. c) coefficient of performance.