© Festo Didactic 89952-20 101

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.

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

The Refrigeration Cycle

Job Sheet 4

OBJECTIVES

PROCEDURE

Job Sheet 4 – The Refrigeration Cycle

102 © Festo Didactic 89952-20

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.

Job Sheet 4 – The Refrigeration Cycle

© Festo Didactic 89952-20 103

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)

Temperature Pressure

Compressor inlet

( )

Compressor outlet

( )

Evaporator outlet

( ) LP HP

0

4

8

12

16

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.

Figure 71. Temperature at various locations as a function of time in heating mode.

0

1

2

3

4

5

6

7

8

9

10

0 4 8 12 16Time (min)

Tem

pera

ture

Job Sheet 4 – The Refrigeration Cycle

104 © Festo Didactic 89952-20

7. Using Figure 72, plot a graph showing LP and HP pressures as a function of time.

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)

Temperature Pressure

Gauge Absolute

Compressorinlet

( )

Compressoroutlet ( )

Evaporatoroutlet

( ) LP HP LP HP

16

9. Using the values in Table 17, draw the refrigeration cycle on a pressure-enthalpy 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.

0

1

2

3

4

5

6

7

8

9

10

0 4 8 12 16

Pre

ssu

re

Time (min)

Job Sheet 4 – The Refrigeration Cycle

© Festo Didactic 89952-20 105

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)

Job Sheet 4 – The Refrigeration Cycle

106 © Festo Didactic 89952-20

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?

Job Sheet 4 – The Refrigeration Cycle

© Festo Didactic 89952-20 107

Figure 74. Pressure-enthalpy diagram, SI units.

Job Sheet 4 – The Refrigeration Cycle

108 © Festo Didactic 89952-20

Figure 75. Pressure-enthalpy diagram, US customary units.

Name: _______________________________ Date: ____________________

Instructor's approval: ______________________________________________