# Student CD for Commercial Refrigeration for A/C Technicians

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Student CD for Commercial Refrigeration for A/C Technicians
Chapter 1 Refrigeration Principles

Chapter 1 Overview Temperature ranges of refrigeration Refrigeration cycle Relate refrigeration to air conditioning Relationship between a refrigerant’s pressure and its temperature Newer refrigerants used in commercial refrigeration systems Relationship between the 4 basic components of a refrigeration system Copyright 2006 Thomson Delmar Learning

Common Space & Product Temperatures
Air Conditioning = 75° High temperature refrigeration = 55° Medium temperature refrigeration = 35° Low temperature refrigeration = -10° Extra low temperature refrigeration = -25° Copyright 2006 Thomson Delmar Learning

Simple A/C System A compressor + two tanks + metering device is used to illustrate a simple A/C system: High pressure stays the same, but . . . Refrigerant: Temperatures drop as discharge gas cools Condenses as heat is rejected Sub-cools before entering the TEV Low pressure stays the same, but . . . Temperatures rise as liquid vaporizes and absorbs heat in the evaporator Then the vapor superheats after all refrigerant has evaporated Copyright 2006 Thomson Delmar Learning

Simple A/C System AMBIENT AIR 95o 278 psig 69 psig RETURN AIR 75o 175o 60o 165o Compressor 50o SuperHeat 125o 40o 278# 69# 125o This is a simplified Air Conditioning system. Suppose we have a tank of R22 on the right and release it into a room. The tank drops in pressure and temperature, absorbing heat from the air surrounding the tank. The loss of refrigerant can be prevented by using a compressor to suck the heated refrigerant into itself. Because we need to have refrigerant return to a liquid state before it can be used for cooling we must compress it first. This raises the temperature of the vapor well above the room temperature. The room air passing over the left tank will condense the hot vapor into a liquid. The liquid then goes through a metering device dropping the pressure and temperature below that of the room. The right tank can absorb the heat of the room, starting the cycle all over again. NOTE: The compressor and the metering devices are the dividing lines between the high pressure and low pressure sides of the system. SubCool 115o 40o 105o 40o Copyright 2006 Thomson Delmar Learning

Simple Medium Temperature Refrigeration System
Compare the previous A/C system illustration to the following refrigeration system: Are the high side pressures and temperatures of both systems different? Why or why not? Are the low side pressures and temperatures of the two systems different? Why or why not? Copyright 2006 Thomson Delmar Learning

Simple Medium Temperature Refrigeration System
AMBIENT AIR 95o 278 psig 49 psig RETURN AIR 35o 175o 45o 165o Compressor 35o SuperHeat 125o 25o 278# 49# 125o This is a simplified Air Conditioning system. Suppose we have a tank of R22 on the right and release it into a room. The tank drops in pressure and temperature, absorbing heat from the air surrounding the tank. The loss of refrigerant can be prevented by using a compressor to suck the heated refrigerant into itself. Because we need to have refrigerant return to a liquid state before it can be used for cooling we must compress it first. This raises the temperature of the vapor well above the room temperature. The room air passing over the left tank will condense the hot vapor into a liquid. The liquid then goes through a metering device dropping the pressure and temperature below that of the room. The right tank can absorb the heat of the room, starting the cycle all over again. NOTE: The compressor and the metering devices are the dividing lines between the high pressure and low pressure sides of the system. SubCool 115o 25o 105o 25o Copyright 2006 Thomson Delmar Learning

“Standard” A/C System with fixed metering device
The following slide is an example of what happens to refrigerant in an A/C system: Compressor discharges hot gas Gas condenses to liquid, releasing heat Metering device lowers pressure Refrigerant vaporizes, absorbing heat Returns to the compressor Copyright 2006 Thomson Delmar Learning

Standard A/C System R-22 69 psig 278 psig 175º 40º 60º 125º CONDENSER 125º EVAPORATOR 115º 40º Refrigeration Cycle for a Typical R22 A/C System High Side: High pressure vapor leaves the compressor at a high temperature. When it enters the condenser it first gets rid of the sensible heat of compression and motor heat it picked up in the compressor. This is known as de-superheating. Once it gets rid of that sensible heat it reaches its condensing temperature. In our example, when the vapor has reached 125º it starts condensing into a liquid (changes state) as it rejects the latent heat it picked up in the evaporator. Any additional cooling of the liquid before it leaves the condenser without a change of state is called subcooling. It often picks up additional subcooling in the liquid line before the metering device. Low Side: As it passes through the metering device the high temperature liquid drops in pressure and temperature as it changes to a dense vapor containing tiny droplets of liquid. About 25% of the liquid flashes off dropping the temperature of the remaining liquid from 115º to 40º. That temperature (evaporating temperature) remains the same through the evaporator absorbing latent heat from the refrigerated space as it boils off. When all the refrigerant is boiled off, or vaporized, it is now a “saturated vapor”. Any additional heat it picks up after that is called superheat because only sensible heat can be absorbed when there is no change of state. The vapor continues through the suction line picking up superheat until it gets to the compressor where the cycle starts all over. 50º AMBIENT AIR 95o RETURN AIR 75o Copyright 2006 Thomson Delmar Learning

Fully Condensed Liquid
Standard A/C System R-22 Total Superheat 20° 69 psig 278 psig Super Heated Vapor 175º 40º 60º 125º Evaporation Starts CONDENSER Condensing Starts 125º EVAPORATOR Fully Evaporated 115º 40º Fully Condensed Liquid Coil Superheat 10° 50º Sub-Cooled Liquid AMBIENT AIR 95o RETURN AIR 75o Copyright 2006 Thomson Delmar Learning

Commercial Refrigeration System with a TEV metering device
Comparing A/C to Refrigeration systems: Same ambient, same high side pressures and temperatures Lower space temperature, low side pressures and temperatures are lower than those on the A/C system Copyright 2006 Thomson Delmar Learning

Typical Walk-In Refrigerator (R22)
50 psig 280 psig 175º 25º 45º TEV 125º CONDENSER 125º EVAPORATOR 115º 25º Refrigeration Cycle for a walk-in refrigerator using R22 Note: the main difference between A/C and refrigeration is the evaporator temperature. The condensing temperatures are the same, except freezers and high efficiency A/C units. High Side: High pressure vapor leaves the compressor at a high temperature. When it enters the condenser it first gets rid of the sensible heat of compression and motor heat it picked up in the compressor. This is known as de-superheating. Once it gets rid of that sensible heat it reaches its condensing temperature. In our example, when the vapor has reached 125º it starts condensing into a liquid (changes state) as it rejects the latent heat it picked up in the evaporator. Any additional cooling of the liquid before it leaves the condenser without a change of state is called subcooling. It often picks up additional subcooling in the liquid line before the metering device. Low Side: As it passes through the metering device the high temperature liquid drops in pressure and temperature as it changes to a dense vapor containing tiny droplets of liquid. About 25% of the liquid flashes off dropping the temperature of the remaining liquid from 115º to 25º. That temperature (evaporating temperature) remains the same through the evaporator absorbing latent heat from the refrigerated space as it boils off. When all the refrigerant is boiled off, or vaporized, it is now a “saturated vapor”. Any additional heat it picks up after that is called superheat because only sensible heat can be absorbed when there is no change of state. The vapor continues through the suction line picking up superheat until it gets to the compressor where the cycle starts all over. 35º AMBIENT AIR 95o BOX TEMPERATURE 35 Copyright 2006 Thomson Delmar Learning

Typical Walk-In Refrigerator (R22)
Total Superheat 20° 50 psig 280 psig Super Heated Vapor 175º 25º 45º TEV 125º Evaporation Starts CONDENSER Condensing Starts 125º EVAPORATOR Fully Evaporated 115º 25º Fully Condensed Liquid Coil Superheat 10° 35º Sub-Cooled Liquid AMBIENT AIR 95o BOX TEMPERATURE 35 Copyright 2006 Thomson Delmar Learning

Current refrigerants used in most new refrigeration systems
Walk-in refrigerators – R22 and R404A Walk- in freezers – R404A Reach-in refrigerators – R22, R404A, 134a Reach-in freezers – R404A Note: after 2010, manufacturers will not be allowed to produce equipment with R22 Note: R404A must be charged in a liquid state Copyright 2006 Thomson Delmar Learning

The Four Basic Components of a Refrigeration System
Metering device Lowers the refrigerant temperature below the space temperature Evaporator Absorbs heat as refrigerant evaporates Compressor Increases the refrigerant temperature above the ambient temperature Condenser Rejects heat from the refrigerant as vapor condenses Copyright 2006 Thomson Delmar Learning

Basic Components and Piping
Discharge Line Compressor Condenser Suction Line Metering Device Liquid Line Evaporator Copyright 2006 Thomson Delmar Learning

Refrigeration System “Baseball Diamond”
The 4 components can be illustrated in the shape of a baseball diamond Half the system is high pressure Half the system is low pressure Copyright 2006 Thomson Delmar Learning