Basic Theory and Principles of Operation

To be effective, the automotive air conditioner must control four conditions within the vehicle interior:

	It must cool the air	It must circulate the air
	It must purify the air	It must dehumidify the air

These functions are essential if passenger comfort is to be maintained when the ambient temperature and humity is high. By performing these functions, the air conditioner maintains the body comfort of the passengers.

Understanding Heat

What is Heat
To understand just how an air conditioning system works, we must first undertand the nature of heat.
For a simple definition we may say that heat is energy. The meshing of gears, the turning of wheels cause friction which results in heat. Combustion (fire) gives off heat. The burning of the sun radiates heat to the earth's surface.
Heat in the correct amount will provide life and comfort. Heat in the extremes - either too much or too little - will be uncomfortable.
The control of temperature means control of comfort.
Air conditioning is a method of controlling heat.

When is Heat HOT?
When is Heat COLD?
All substances contain heat!
Something "feels" hot when it is warmer than our own body temperature.
When something contains less heat than our bodies, we say it feels cold!
Cold is merely removal of some heat.
Science tells us that a measurement called "Absolute Zero" is a point at which all heat is removed from an object (approximately -273°C). Any substance above this absolute zero retains some heat.

The average person requires a comfort zone of approximately 21°C to 26°C, with a relative humidity of 45% to 50%. In this temperature and humidity range, we feel most comfortable. All objects within this range are comfortable to touch.

Heat Measuremant
A temperature reading gives us the heat intensity of a substance and not the actual quantity of heat.
Heat quantity is measured in "KILOCALORIES" (KCAL). One KCAL is the most amount of heat required to raise the temperature of one kilogram of water by one degree Celsius (at sea level). This quantity measurement is used in air conditioning to describe heat transfer during changes of sate.

What Causes Heat To Move?
Heat always moves from the hotter object to the colder one. Whenever there is a transfer difference between two objects, the heat energy will be transferred from the warmer object to the cooler one until both objects stabilise at the same temperature.
This is known as the law of heat transfer, and is the basis of air conditioning operation.
When a hot cup of coffee is set aside for some time, it becomes cold. Heat moves out of the hot (90°C) coffee and into the cooler (25°C) surrounding air. In time the coffee will reach the temperature of the surrounding air.

How does heat get inside a vehicle?
When a car is driven or parked in the sun, heat enters the vehicle from many sources. These sources include:

 

• Ambient air


• Sunlight


• Engine heat


• Road heat


• Transmission


• Exhaust system

This condition occurs within the A/C evaporator.

Condensation
Is the term used to describe the opposite of the evaporation process. If you take a vapour and remove enough heat from it, a change of state occurs. The vapour becomes liquid.
The change of vapour to a liquid is called condensation

This condition occurs within the A/C condenser.

Freezing
Is another change of state. Freezing results when heat is removed from a liquid substance until it becomes solid. Remember that anything above -273°C still contains some heat.

In an air conditioning system freezing must be avoided. Otherwise component damage will occur.

Pressure & Temperature Relationship
To increase or decrease the boiling point of a substance, we must alter the pressure on the substance. Increasing the pressure, increases the boiling point.
To decrease the boiling point, decrease the pressure.

A good example is the automotive cooling system.
The pressure cap keeps the radiator from boiling over by increasing the pressure on the coolant.
Example: 110kPa radiator cap allows the coolant temperature to reach 126°C before boiling.
The substance used in the air conditioning system, called refrigerant, also boils at different temperatures depending on the pressure that it is under.

Principles of Air Conditioning

High Pressure Side
Low pressure refrigerant vapour entering the compressor is compressed to become high pressure/temperature refrigerant vapour. This is then circulated along with lubricationg oil to the condenser. As the high pressure/temperature vapour travels through the condenser, heat is released to the cooler ambient air passing over the condenser tubes condensing the vapour into a liquid. This high pressure/temperature liquid then travels through the filter drier onto the thermal expansion valve where a small variable orifice provides a restriction against which the compressor pushes.

Low Pressure Side
Suction from the compressor pulls the high pressure/temperature liquid refrigerant through the small variable orifice of the thermal expansion valve and into the low pressure side of the A/C system. The refrigerant is now under low pressure and becomes a low pressure/temperature vapor where heat from the cabin being blown over the evaporator coil surface is absorbed into the cooler low pressure refrigerant. The refrigerant is then pulled through the evaporator and into the compressor. The A/C cycle begins again as the refrigerant vapour is compressed and discharged under pressure.

Heat Transfer
Refrigerant in the LOW PRESSURE side is COLD and can absorb large quantities of heat from the air moving over the evaporator.
Refrigerant in the HIGH PRESSURE side is HOT and the cooler ambient air moving over the condenser can absorb heat from it.