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4. A/C COOLING CYCLE
(1) System flow
"Compression -> Condensation -> Expansion -> Evaporation"
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(2) Functions
▶ Compressor
Condition: Gas
Function: Circulates the refrigerant and increases the pressure and temperature for easier
evaporation.
▶ Condenser
Condition: Gas/Liquid
Function: Cools and condenses the refrigerant by using ambient air to liquefy it under high
pressure.
Condition: Gas/Liquid
Function: Keeps the refrigerant free from moisture by separating/collecting the moisture from it. ▶ Receiver drier
▶ Expansion valve
Condition: Liquid/Liquefied gas
Function: Performs adiabatic expansion and flow control for easier evaporation.
1) Basic Operating Principle
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▶ Evaporator
Condition: Liquefied gas/Gas
Function: Cools the air by absorbing the heat from the air around the evaporator.
(3) Description for Each Cycle
▶ Compression
The evaporated refrigerant in the evaporator enters to the compressor. And the refrigerant gas is
compressed until it can be liquefied at ambient temperature.
Thus, the low refrigerant pressure is maintained so that the liquid refrigerant can be evaporated
actively at low temperature (around 0℃). -
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▶ Condensation
The high pressure and high temperature gas (refrigerant) from the compressor is cooled down by
the fresh air entered into the condenser. Then, this gas is converted to liquid and collected in the
receiver drier.
The heat generated from the high pressure refrigerant is dissipated to the ambient air, and it is
called "heat of condensation".
The heat of condensation is the summation of the heat of vaporization (heat that the refrigerant
absorbs from the inside of the vehicle) and the calorific value converted from the amount of work
which is needed to compress. -
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▶ Expansion
The liquid refrigerant lowers the pressure making its evaporation easily accomplished.
This process (lowering the pressure to the level at which evaporation easily takes place before the
liquid refrigerant is sent to the evaporator) is called
"Adiabatic Expansion".
During adiabatic expansion, the expansion valve lowers the pressure of the refrigerant and
determines the correct amount of refrigerant going into the air conditioning evaporator.
That is, the amount of heat, which is needed to stop the evaporation, is determined according to the
cooling load.
The expansion valve detects this and regulates the amount of the refrigerant exactly. -
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▶ Evaporator
The refrigerant is converted from liquid to gas in the evaporator.
(The refrigerant in the form of fog in the evaporator is vaporized actively)
At this time the refrigerant, in the form of liquid, absorbs the heat in the air which is need for
evaporation (latent heat) and is cooled down. Then the blower blows the cooled air inside the
vehicle to lower the temperature.
There are liquid refrigerant from the expansion valve and evaporated refrigerant in the evaporator.
The evaporation temperature can be predicted from the evaporation pressure (i.e. relationship
between saturation pressure and saturation temperature).
It is important to keep the pressure inside the evaporator low, so that the refrigerant is evaporated a
t
low temperature to make sure the completely evaporated refrigerant is entered into the
compressor. -
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