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Chapter 2. The Refrigeration Process. 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.
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Chapter 2 The Refrigeration Process
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.
The Refrigeration Process • Four major components enable heat transfer: • Evaporator • Compressor • Condenser • Metering device
The Refrigeration Process Components of a refrigeration system.
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.
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
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.
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.
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.
Evaporators The evaporator coil is located inside an air handler.
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.
Compressors • Three compressor types: • Reciprocating compressors • Rotary compressors • Scroll compressors
Compressors Principle of a reciprocating compressor.
Compressors Principle of a rotary compressor.
Compressors Principle of a scroll compressor.
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.
Condensers A condenser used in split-type central air conditioning systems.
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.
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
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
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.
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.
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.
