Job Sheet 4 The Refrigeration Cycle OBJECTIVES In this job sheet, you will plot the refrigeration cycle on a pressure-enthalpy diagram using pressures and temperatures measured on your training system. You will also calculate the coefficient of performance (COP) of your refrigeration system in heating mode. PROCEDURE In the following steps, you will take the different temperature and pressure readings necessary to trace the compression cycle on a pressure-enthalpy diagram. In heating mode, you will measure the temperature at the inlet and outlet of the compressor, the temperature at the outlet of the forced-air evaporator, and the pressure on both sides of the compressor. 1. Perform the following settings on your training system: Main power switch... On Thermostat... Off Valve HV-1... Open Valve HV-2... Open Valve HV-3... Open Valve HV-4... Open Valve HV-5... Open Valve HV-6... Open Valve HV-7... Open Valve HV-8... Closed Valve HV-9... No adjustments required Valve HV-10... Handle in horizontal position Valve HV-11... Open Valve HV-12... Open Valve HV-13... Open Pressure gauge PI-1 selector switch... Right Pressure gauge PI-2 selector switch... Right Desuperheater On/Off switch... Off Priming tank three-way valves... Air distribution register... Open Perform the following settings on the simulated ground heat pump: Main power switch (simulated ground heat pump)... On Temperature controller set point... 3 C (5 F) below ground temperature Festo Didactic 89952-20 101
Wait for the compressor of the simulated ground heat pump to start and then wait about 5 min for the temperature of the water in the tank to get close to its set point. Go to step 2 as you wait. 2. Use the next figure to compare the refrigeration cycle with the heat pump diagram and determine which thermocouples are going to be read during this exercise. Figure 70. Compression cycle. 102 Festo Didactic 89952-20
3. Perform the following settings on the heat pump: Thermostat... Heating mode Temperature set point... 5 C (9 F) above room temperature 4. Measure the temperatures and pressures every four minutes after the compressor indicator light turns on. Complete Table 16. Table 16. Compression cycle. Time (min) 0 4 8 12 16 Compressor inlet ( ) Temperature Compressor outlet ( ) Evaporator outlet ( ) Pressure LP HP 5. Set the thermostat operating mode to Off, then shut down your training system by setting both main power switches to Off. 6. Using Figure 71, plot a graph showing TC-8, TC-9, and TC-11 temperatures as a function of time. Temperature 10 9 8 7 6 5 4 3 2 1 0 0 4 8 12 16 Time (min) Figure 71. Temperature at various locations as a function of time in heating mode. Festo Didactic 89952-20 103
7. Using Figure 72, plot a graph showing LP and HP pressures as a function of time. Pressure 10 9 8 7 6 5 4 3 2 1 0 0 4 8 12 16 Time (min) Figure 72. Pressure as a function of time in heating mode. 8. Using your graphs as a reference, observe a pressure and temperature stabilization after a transitional period of about ten minutes. Using Table 16, copy your results taken after 16 minutes of operation into Table 17. Convert your pressure readings to absolute pressure values. a The standard atmospheric pressure at sea level is 101.3 kpa (14.7 psia). Table 17. Temperature and pressure after 16 minutes of operation. Time (min) Compressor inlet ( ) Temperature Compressor outlet ( ) Evaporator outlet ( ) Pressure Gauge Absolute LP HP LP HP 16 9. Using the values in Table 17, draw the refrigeration cycle on a pressureenthalpy diagram included at the end of this job sheet. Be sure to use the pressure-enthalpy diagram that corresponds to the system of units that you are using. 10. Identify the refrigerant conditions for points A to H shown in Figure 73. Label the points as subcooled, superheated, saturated liquid, saturated mixture, saturated vapor. Fill in the corresponding column in Table 18. Find the enthalpy value corresponding to the different refrigerant conditions identified in Figure 73 and complete Table 18. 104 Festo Didactic 89952-20
Figure 73. Pressure-enthalpy diagram. Table 18. Refrigerant condition and enthalpy. Point Refrigerant condition Enthalpy A B C D E F G H 11. Using your results from Table 18, complete Table 19. Table 19. Refrigeration process. Process name Equation Refrigeration effect Low-pressure superheat Heat of compression High-pressure superheat Heat rejected by condenser Subcooling Expansion 12. Refrigeration devices are rated with a number called coefficient of performance (COP). The COP is the ratio of the refrigeration effect divided by the compression work. (4-2) Festo Didactic 89952-20 105
Using the results from Equation (4-2), calculate the COP for your training system. COP of your training system in heating mode: 13. If you had to select a heat pump based solely on the COP, which one would you select? Would you select the heat pump with the lowest or highest COP? Why? 106 Festo Didactic 89952-20
Job Sheet 4 The Refrigeration Cycle Figure 74. Pressure-enthalpy diagram, SI units. Festo Didactic 89952-20 107
Job Sheet 4 The Refrigeration Cycle Figure 75. Pressure-enthalpy diagram, US customary units. Name: Date: Instructor's approval: 108 Festo Didactic 89952-20