Vehicle air conditioning made easy

Want to know how your vehicle air conditioning systems work? For a clear overview, you need look no further.

It is the intention of this explanation of vehicle air conditioning to give the basic principles of how it works and not go into too much detail about enthalpy diagrams etc which can be easily looked up if so desired via Google.

First it must be understood that heat cannot be destroyed nor created and can only be moved from one place to another. That is heat has to be taken from within the cabin of the vehicle to the outside where it can be transferred to the ambient air. Also vehicle air conditioning is the same principle with the same components no matter which vehicle it is fitted to.

Latent and Sensible heat

Every substance has 2 basic forms of heat, Latent and Sensible. Sensible heat  can be measured by a thermometer i.e. as a substance cools the temperature will fall and visa versa. However Latent heat is the heat given off or absorbed when a substance changes state (solid to liquid and liquid to vapour and visa versa) but the actual temperature does not change. When the refrigeration gas changes state from a liquid to a vapour (-26.6oc for R134a at atmospheric  pressure) it requires 215.9 kj/kg of energy but the temperature will remain at -26.6oc until all the liquid has turned into a vapour when the temperature will start to rise (sensible heat). It also follows that 215.9 kj/kg of energy will be given off when the gas goes from vapour to liquid again at -26.6oc
Now that’s all the technical stuff over and done with so let’s move onto how the system works.

Compressor

We will start at the compressor. This is driven by the engine usually via a belt, the purpose of which is to compress the gas up to 6- 10 Bar (100-150 psi) may be even higher if the ambient temperature is high. There are different styles of compressor but all achieve the same result. Piston, radial piston, vane and scroll. Most vehicles have the radial piston type. Then there are magnetic  clutch type with fixed piston displacement, magnetic  clutch type with variable piston displacement via the pressure of gas returning to the compressor and the new type of constant drive with variable piston displacement via a pulsed signal from the A/C  computer. Generally they are reliable pieces of kit if maintained  but as all things they can go wrong sometimes with spectacular results when they break up and seize.

Condenser

This high pressure gas then flows into the condenser which is positioned in front of the engine radiator so it can be cooled by air passing through it. The air flow can be assisted by the engine fan or electric fan which is more often required when the vehicle is stationary or moving slowly. As the gas flows through the condenser it cools giving of heat until it condenses to a liquid giving of latent heat as mentioned before.

Receiver Dryer

The liquid then flows into the receiver dryer. This is a small canister containing a desiccant that is capable of absorbing any water from the liquid refrigerant. It will also act as a simple filter and a receiver for the liquid to help allow a constant flow to the expansion valve.

Expansion Valve
This then moves us onto the expansion valve. The purpose of which is to restrict the flow and allow a pressure drop and allowing the liquid boil and vaporize. The 3 main types are TXV, no longer used in new vehicles, fixed orifice tube, which is a narrow tube mounted in the pipe work and the block valve which can vary the amount of liquid depending on the pressure of the gas (and hence the temperature)leaving the evaporator

Evaporator

Now that takes us nicely to the evaporator. This compact radiator is mounted in the heater box in the air flow before the heater matrix. Warm air flows around the evaporator where the liquid is turning into a gas, and as mentioned before for the liquid to vaporize a lot of energy is required which is obtained from absorbing heat energy from the air flowing through the evaporator. The air is thus cooled and directed via the vents into the cabin.
The refrigerant in the form of a gas then flows back into the compressor and the whole process starts again. It’s that simple and is how all air conditioning systems work in vehicles. I know nothing is just ‘that simple’ well the basics are straight forward but the systems of controlling the temperature, vent flaps, compressor etc.  are mind blowing.