1. Heat Transfer and Refrigeration Cycle
Condenser
Evaporator
Compressor
Fan
Fan
Topics covered in this presentation:
Heat Transfer Principles
Refrigeration Cycle Basics

2. Principles of Heat Transfer
Heat always flows from hot to cold objects.
Rate of heat transfer increases as difference in temperature between two objects increases.
Principle 2
Cold objects have less heat than hot objects of the same mass.
To make an object colder remove heat.
To make an object hotter add heat.
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3. Principles of Heat Transfer
When liquid changes to vapour, a large amount of heat is absorbed.
When vapour changes to a liquid, a large amount of heat is released.
2100 joules is quantity of heat required to raise temperature of 1kg of water by 1°C.
Principle 4
When vapour cooled below dew point it becomes a liquid - condensation.
One kilogram of 100°C vapour has to release 2.26 x 106 joules of heat to change to water at 100°C.
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4. Question 1
How many joules of energy are required to heat 5 kg of water by 10 degrees centigrade?
A) 10500
B) 21000 C)
Correct Answer = C D)

5. Principles of Heat Transfer
Increasing the pressure on a liquid or vapour increases its boiling point.
Decreasing the pressure on a liquid or vapour decreases its boiling point.
Principle 6
When vapour is compressed both its temperature and pressure increase, without additional applied heat.
Example: The radiator cap on a vehicle allows the water in the cooling system to reach a pressure of 1.1 bar. Therefore the boiling point occurs at 125°C.
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6. Question 2
A gas has to lose heat in order to change from a gas to a liquid. Is this true or false?
Answer True or False.
Correct Answer = T

7. Principles Applied to Air Conditioning
A good basic example of how an air conditioning (A/C) system works is
discharging an aerosol can.
Pressurized liquid is held within the can (high pressure side of the system).
The top or nozzle (restriction) is depressed.
The contents are released into the atmosphere (low pressure side).
In the case of a cosmetic (deodorant) aerosol, the contents feel cold when applied to the skin.
If the aerosol was sprayed into the inside of a heat exchanger (evaporator) and air was blown across the heat exchanger, the air would be cooled.
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8. Tube and fin heat exchangers
Basic Air Conditioning System
Compressor
Evaporator
Air movement fans
Condenser
Tube and fin heat exchangers
Fixed Orifice Tube or Thermal Expansion Valve (placed in line near the evaporator to create a restriction causing a pressure drop)
Tubing
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9. Question 3
What material would the evaporator and condenser coils of an air conditioning system be made from?
A) rubber tube
B) metal tube
C) nylon tube
Correct Answer = B
D) glass tube

10. The Compressor The compressor pulls in low pressure vapour.
It pumps out high pressure vapour.
This pressurization raises the temperature of the vapour.
Compressor
Condenser
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11. The Condenser
Compressor
Condenser
The compressor pumps high pressure vapour into condenser.
The high pressure vapour is cooled by the ram air flow.
The vapour condenses into a liquid.
Heat loss changes the vapour into a high pressure liquid.
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12. Expansion Valves Expansion valves restrict refrigerant flow.
Compressor
Evaporator
Condenser
Expansion valves restrict refrigerant flow.
Two main types:
Thermal Expansion Valve (TXV).
Fixed Orifice Tube (FOT).
The compressor pumps against the restriction, so the pressure rises in the condenser.
The compressor sucks against the restriction, so pressure lowers in the evaporator.
Thermal Expansion Valve (TXV)
Fixed Orifice Tube (FOT)
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13. The Evaporator
The evaporator removes heat and humidity from the vehicle’s interior.
Evaporator
Expansion valve
Condenser
Compressor
Low pressure liquid refrigerant flows from the expansion valve into the evaporator.
The liquid absorbs heat, and changes into a low pressure vapour.
The compressor pulls low pressure vapour from the evaporator.
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14. Question 4
Which part, in an air conditioning system, turns the low pressure vapour into high pressure vapour?
A) Compressor
B) Condenser
C) Evaporator
Correct Answer = A
D) Fan

15. Question 5
Where in an air conditioning system does the high pressure vapour turn to high pressure liquid?
A) Compressor
B) Condenser
C) Fixed orifice tube
Correct Answer = B
D) Evaporator

16. Question 7
Where in an air conditioning system does high pressure liquid turn to low pressure liquid?
A) Compressor
B) Condenser
C) Fixed orifice tube
Correct Answer = C
D) Evaporator

17. Question 8
Where in an air conditioning system does the liquid absorb heat?
A) Compressor
B) Condenser
C) Fixed orifice tube
Correct Answer = D
D) Evaporator

18. Question 9
In which part of an air conditioning system does heat leave the system?
A) Compressor
B) Condenser
C) Fan
Correct Answer = B
D) Evaporator

19. Basic Air Conditioning System Operation - 1
LOW 2 BAR
HIGH 12 BAR
VAPOUR 0ºC
VAPOUR 40ºC
LIQUID 40ºC
Condenses vapour to liquid
Evaporates liquid to vapour
Restriction causes pressure drop
LIQUID / VAPOUR Fine mist 0ºC
The compressor creates a high pressure side
and a low pressure side.
Typical high side pressure = 12 bar.
Typical low side pressure = 2 bar.
The pressure drop causes the temperature and the boiling point of the refrigerant to drop.
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20. Question 10 What is the purpose of the fixed orifice tube?
A) To raise the temperature
B) To increase the speed of flow
C) To reduce the pressure
Correct Answer = C
D) To change the vapour into a liquid

21. Basic Air Conditioning System Operation - 2
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LOW 2 BAR
HIGH 12 BAR
VAPOUR 0ºC
VAPOUR 40ºC
LIQUID 40ºC
Condenses vapour to liquid
Evaporates liquid to vapour
LIQUID / VAPOUR Fine mist 0ºC
If 38°C air surrounds evaporator, the refrigerant will absorb heat.
The liquid changes to vapour, and the surrounding air is cooled.
The vapour is pumped into the condenser, so raising both its temperature and its pressure.

22. Basic Air Conditioning System Operation - 3
LOW 30 PSI
HIGH 175 PSI
VAPOR 32ºF
VAPOR 103ºF
LIQUID / VAPOR Fine mist 32ºF
LIQUID 103ºF
Condenses vapor to liquid
Evaporates liquid to vapor
The vapour is now hotter than the surrounding 38°C air.
Heat transfers to the surrounding air.
The heat loss changes the vapour to a liquid.
This is back to the start of the cycle, where it is repeated until the A/C is switched off.
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