1. Heat Transfer and Refrigeration Cycle
Compressor
Evaporator
Condenser
Fan
Fan
Topics covered in this presentation:
Heat Transfer Principles
Refrigeration Cycle Basics
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2. Principles of Heat Transfer 1
Heat always flows from hot to cold objects, whether they are in direct contact, or through a heat conductor.
21°C
21°C
The rate of heat transfer increases as the difference in temperature between two objects increases.
65°C
An example is heat flow from a pan of hot water to the surrounding cooler air.
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3. Principles of Heat Transfer 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.
The mass of an object does not change with heat content change.
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4. Principles of Heat Transfer 3
The universe is composed of matter, which exists in 3 states: vapour, liquid and solid.
When liquid changes to vapour (or vice versa), a large amount of heat is absorbed or released, without a change in temperature.
At 100°C water can be either liquid or vapour, the amount of contained heat determines which state.
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5. Principles of Heat Transfer 3
Heat energy/quantity is measured in joules.
2100 joules of heat energy is required to raise the temperature of 1kg of water by 1°C.
A kilogram of 100°C water has to absorb 2.26 x 106 joules of heat to change to 100°C vapour.
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6. Principles of Heat Transfer 4
When vapour is cooled below its dew point it becomes a liquid. This is known as condensation.
A kilogram of 100°C vapour has to release 2.26 x 106 of heat to change to 100°C water.
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7. Principles of Heat Transfer 5
Increasing the pressure on liquid or vapour increases the boiling point.
Decreasing the pressure on liquid or vapour decreases the boiling point.
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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8. Principles of Heat Transfer 6
When a vapour is compressed, its temperature and pressure increases, without additional heat being applied.
Example: If refrigerant vapour is compressed from 2 BAR to 12 BAR, its temperature increases from 0°C to 55°C (the boiling point also increases).
If heat is removed from the 55°C vapour it will condense into a liquid at that temperature.
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9. Principles Applied to Air Conditioning
All A/C systems use a restriction in the high pressure line (liquid) to cool the system, the restriction causes the liquid to become a mist (vapour), as the liquid changes to vapour it becomes cold due to the drop in pressure. An everyday example of this principle is when an aerosol can is discharged.
Pressurized liquid is held within the can (high pressure side of the system).
As the top is depressed (restriction) the contents are released into the atmosphere (low pressure side). In the case of a cosmetic (deodorant) aerosol the contents feel cold 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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10. Tube and fin heat exchangers
Basic Air Conditioning System
Compressor
Air Movement fans
Evaporator
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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11. The Compressor Pulls in low pressure refrigerant vapour.
Pumps out high pressure refrigerant vapour.
Compressor
Pressurization raises the temperature of refrigerant vapour going into the condenser.
Condenser
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12. The Condenser
Sits at the front of the vehicle. The compressor pumps high pressure refrigerant vapour into the condenser.
High pressure refrigerant vapour is cooled by ram air flow.
Compressor
Condenser
As condenser is cooled, vapour turns to liquid.
Heat loss changes refrigerant vapour into high pressure refrigerant liquid.
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13. Fixed Orifice Tube (FOT)
Expansion Valves
Compressor
Provides a restriction in the refrigerant flow.
Evaporator
Condenser
Two main types:-
Thermal Expansion Valve (TXV).
Fixed Orifice Tube (FOT).
Compressor pumps against restriction, pressure rises in condenser.
Compressor sucks against restriction, pressure lowers in evaporator.
Fixed Orifice Tube (FOT)
Expansion valve restriction acts as a regulator between low and high pressure sides of system, partially vapourising the refrigerant liquid.
Thermal Expansion Valve (TXV)
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14. The Evaporator
The evaporator absorbs heat and humidity from the vehicle’s interior.
Low pressure liquid refrigerant flows from the expansion valve into the evaporator.
Evaporator
Condenser
The liquid heats up, and changes into low pressure refrigerant vapour.
Compressor
Compressor pulls low pressure refrigerant vapour from evaporator.
Moisture in the air forms droplets on evaporator fins. These droplets collect at the lowest point and are drained away under the vehicle.
Expansion valve
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15. Basic Air Conditioning System Operation - 1
Compressor pumps more refrigerant into high side than can flow through restriction.
LOW 2 BAR
HIGH 12 BAR
VAPOUR 0ºC
VAPOUR 40ºC
This creates high and low pressure areas.
Typical refrigerant pressure drop through restriction is 12 BAR down to 2 BAR.
Evaporates liquid to vapour
Condenses vapour to liquid
Pressure drop causes a temperature drop from 40°C to 0°C.
LIQUID / VAPOUR Fine mist 0ºC
LIQUID 40ºC
Restriction causes pressure drop
The boiling point also drops.
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16. Basic Air Conditioning System Operation - 2
If 38°C air surrounds the evaporator, heat will be transferred to the 0°C refrigerant.
LOW 2 BAR
HIGH 12 BAR
VAPOUR 0ºC
VAPOUR 40ºC
This gain in heat changes refrigerant liquid to vapour.
Surrounding air is cooled as a consequence.
Evaporates liquid to vapour
Condenses vapour to liquid
Compressor pumps vapour into condenser, raising temperature and pressure.
LIQUID / VAPOUR Fine mist 0ºC
LIQUID 40ºC
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17. Basic Air Conditioning System Operation - 3
Refrigerant vapour is now hotter than the 38°C surrounding air.
LOW 2 BAR
HIGH 12 BAR
VAPOUR 0ºC
VAPOUR 40ºC
Heat transfers from the condenser to the surrounding air.
Heat loss changes refrigerant vapour to liquid, the cycle repeats.
Evaporates liquid to vapour
Condenses vapour to liquid
Oil is circulated through the system to lubricate compressor parts.
LIQUID / VAPOUR Fine mist 0ºC
LIQUID 40ºC
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