Chapter 2

The Refrigeration Process

2

Learning Objectives

• After completing the material in this chapter, the student should be able to:– Describe the basic vapor-compression

refrigeration cycle.– List the basic components that make up a vapor-

compression refrigeration system.– Describe the function of a compressor.– List various types of compressors.– Describe the function of the metering device.– List three commonly used metering devices.– Describe the function of the evaporator.

3

The Refrigeration Process

• Four major components enable heat transfer:– Evaporator– Compressor– Condenser– Metering device

4

The Refrigeration Process

Components of a refrigeration system.

5

Pressure/Temperature Relationship

• As the pressure of a refrigerant changes, so does the temperature.

• An increase in pressure will cause an increase in temperature.

• An increase in temperature will cause an increase in pressure.

6

Pressure/Temperature Relationship

• This holds true for saturated refrigerants only:– Mixture of liquid and vapor– Refrigerants undergoing a change of state– Superheated and subcooled refrigerant do not

follow the pressure/temperature relationship

7

Pressure/Temperature Relationship

8

Superheat

• It is the amount of sensible heat added to refrigerant after it has boiled into a vapor.

• The vapor is heated above its saturation temperature.

9

Subcooling

• It is the amount of sensible heat removed from refrigerant after it has condensed into a liquid.

• The liquid is cooled below its saturation temperature.

10

Evaporators

• The purpose of an evaporator is to absorb heat from the space that is to be cooled.

• It must be at a lower temperature than the medium being cooled.

• Refrigerant in the evaporator boils from a liquid to a vapor at a low pressure and a low temperature.

11

Evaporators

The evaporator coil is located inside an air handler.

12

Compressors

• Pump the refrigerant through the refrigeration system.

• Create the pressure difference between high and low side of system.

• They are considered to be vapor pumps.• Compressors cannot compress or pump

liquid.

13

Compressors

• Three compressor types:– Reciprocating compressors– Rotary compressors– Scroll compressors

14

Compressors

Principle of a reciprocating compressor.

15

Compressors

Principle of a rotary compressor.

16

Compressors

Principle of a scroll compressor.

17

Condensers

• It is the component responsible for rejecting heat from the system.

• It must be located outside the refrigerated space.

• Refrigerant in the condenser must be warmer than the condensing medium passing over it.

• The refrigerant condenses from a high-temperature, high-pressure vapor to a high-pressure, high-temperature liquid.

18

Condensers

A condenser used in split-type central air conditioning systems.

19

Metering Devices

• Responsible for reducing the pressure and temperature of the refrigerant in a refrigeration system.

• Restricts the flow of refrigerant from the condenser.

• Changes a high-pressure, high-temperature liquid into a low-pressure, low-temperature liquid.

20

Metering Devices• Three types of metering devices:

– Capillary tube• Fixed bore metering device• Does not respond well to load changes

– Automatic expansion valve• Maintains a constant evaporator pressure• Evaporator and spring pressure control valve operation

– Thermostatic expansion valve• Maintains constant superheat in the evaporator• Spring, bulb, and evaporator pressure control valve

operation

21

Capillary Tube

22

Automatic Expansion Valve

23

Thermostatic Expansion Valve

24

Refrigerants in the Residential System

• R-22:– Currently the most popular refrigerant used in

residential air conditioning

• R-410A:– Will soon be commonly used in residential air

conditioning systems

25

Summary

• The vapor-compression refrigeration cycle is a repeating cycle, consisting of a compressor, a condenser, a metering device, and an evaporator.

• The compressor changes the refrigerant from a low-temperature, low-pressure, superheated vapor to a high-temperature, high-pressure, superheated vapor.

• Three common types of compressors are the rotary, the reciprocating, and the scroll.

26

Summary• The condenser rejects heat from the system and

changes the refrigerant from a high-temperature, high-pressure vapor into a high-temperature, high-pressure liquid.

• Condensers used on residential systems are typically air cooled.

• Latent-heat transfers cannot be measured with a thermometer.

• When the temperature of a substance changes, a sensible-heat transfer takes place.

27

Summary

• The metering device controls the flow of refrigerant to the evaporator: – It changes a high-temperature, high-pressure

liquid into a low-temperature, low-pressure liquid

• Three common metering devices are the capillary tube, the automatic expansion valve, and the thermostatic expansion valve.

• The refrigerant in the evaporator absorbs heat into the refrigeration system.