chapter 2 the refrigeration process. 2 learning objectives after completing the material in this...
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Chapter 2
The Refrigeration Process
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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.
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The Refrigeration Process
• Four major components enable heat transfer:– Evaporator– Compressor– Condenser– Metering device
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The Refrigeration Process
Components of a refrigeration system.
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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.
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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
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Pressure/Temperature Relationship
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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.
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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.
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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.
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Evaporators
The evaporator coil is located inside an air handler.
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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.
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Compressors
• Three compressor types:– Reciprocating compressors– Rotary compressors– Scroll compressors
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Compressors
Principle of a reciprocating compressor.
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Compressors
Principle of a rotary compressor.
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Compressors
Principle of a scroll compressor.
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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.
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Condensers
A condenser used in split-type central air conditioning systems.
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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.
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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
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Capillary Tube
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Automatic Expansion Valve
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Thermostatic Expansion Valve
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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
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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.
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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.
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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.