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

2. Copyright 2006 Thomson Delmar Learning
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

3. 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

4. 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

5. 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

6. 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

7. 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

8. “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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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
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16. Basic Components and Piping
Discharge Line
Compressor
Condenser
Suction Line
Metering Device
Liquid Line
Evaporator
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17. 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
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18. Copyright 2006 Thomson Delmar Learning
Basic Components
Metering Device
“Baseball Diamond”
Low Side
High Side
Liquid Line
Evaporator
Condenser
Discharge Line
Suction Line
Compressor
Copyright 2006 Thomson Delmar Learning

19. Refrigeration Principles
The End
of Chapter 1
Refrigeration Principles
Copyright 2006 Thomson Delmar Learning