How do air conditioners work?

Air conditioners and refrigerators work the same way. Instead of cooling just the small, insulated space inside of a refrigerator, an air conditioner cools a whole car or house.

Air conditioners use chemicals that easily convert from a gas to a liquid and back again. This chemical is used to transfer heat from the air inside of a home to the outside air.
The machine has three main parts. They are a compressor, a condenser and an evaporator. The compressor is located on the engine, because it is typically belt driven.  The condenser is located at the front of the car to take advantage of air coming in from the moving vehicle. The evaporator is located closer to the cabin of the car, usually under the dash or near the firewall. 

The working fluid arrives at the compressor as a cool, low-pressure gas. The compressor squeezes the fluid. This packs the molecule of the fluid closer together. The closer the molecules are together, the high its energy and its temperature.

The working fluid leaves the compressor as a hot, high pressure gas and flows into the condenser. If you looked at the condenser, look for the part that has metal fins all around. The fins act just like the radiator in the car and help the heat go away, or dissipate, more quickly.

When the working fluid leaves the condenser, its temperature is much cooler and it has changed from a gas to a liquid under high pressure. The liquid goes into the evaporator through a very tiny, narrow hole. On the other side, the liquid's pressure drops. When it does it begins to evaporate into a gas.

As the liquid changes to gas and evaporates, it extracts heat from the air around it. The heat in the air is needed to separate the molecules of the fluid from a liquid to a gas.

The evaporator also has metal fins to help in exchange the thermal energy with the surrounding air.
By the time the working fluid leaves the evaporator, it is a cool, low pressure gas. It then returns to the compressor to begin its trip all over again.

Connected to the evaporator is a fan that circulates the air inside the car to blow across the evaporator fins. Hot air is lighter than cold air, so the hot air in the car rises to the top of a room.

There is a vent there where air is sucked into the air conditioner and goes down ducts. The hot air is used to cool the gas in the evaporator. As the heat is removed from the air, the air is cooled. It is then blown into the car through other ducts usually at the floor level.

This continues over and over and over until the room reaches the temperature you want the car cooled to. The thermostat senses that the temperature has reached the right setting and turns off the air conditioner. As the room warms up, the thermostat turns the air conditioner back on until the car reaches the temperature

A few causes of low cooling efficiency or no cooling at all at idle are:

Lack of air flow across the condenser: Make sure the electric cooling fan motor near the condenser is coming on, or in models that are equipped with a fan blade make sure this fan is turning and is turning very fast.

Low Freon levels: Freon level and pressure should be checked by your certified air conditioning mechanic.

Overheating: If the engine is running hot or overheating, it can have a noticeable negative affect on the air conditioning system. Some cars have two electric cooling fans, one for the air conditioning condenser and the other for the radiator. Make sure they are both working properly. Usually at idle on a hot day with the AC on both fans will be on.

When the vehicle is traveling at freeway speeds, the compressor is pumping the Freon throughout the system much faster and harder than at idle. There is a dramatic increase in air flow across the condenser due to 55 mph winds, and the engine is usually operating at a cooler, more efficient temperature as well, thus allowing the air conditioning system to operate efficiently. Note: An air conditioning system that is somewhat low on Freon can still feel comfortable at freeway speeds due to the added air flow across the condenser which can overcome the ill effects of slightly low Freon. Periodic air conditioning performance checks by your mechanic are the best way to keep the system in great shape.