HOW AIRCON WORKS

 To transfer heat from within the car to the outside, airconditioning relies on the principles of the latent heat of evaporation and condensation, where a substance changes state from a liquid to gas (and back again) dependent on the energy absorbed, or shed. In the majority of automotive applications, a toxic refrigerant lies at the heart of a sealed system. In its low-pressure vapour form, it is drawn into the suction side of a compressor. This tends to be belt-driven by the engine crankshaft via the front-end auxiliary drive (FEAD). Through being compressed, both pressure and temperature of the refrigerant rise considerably.

The hot vapour passes into the inlet side of the condenser, which is a basic heat-exchanger, consisting of a continuous loop of thin aluminium tubing that is surrounded by fins, of similar construction to that of a typical automotive radiator. Mounted in front of the radiator for maximum exposure, exterior air passes across the condenser’s wide surface area either using the ram effect of the car moving forwards, or the electric cooling fans. As the incoming air is cooler, heat energy transfers out of the gas via the condenser and into the atmosphere. When a set amount of heat energy is lost, the vapour condenses into a liquid (the latent heat of condensation) as it passes through the condenser.

The liquid refrigerant exits the condenser and enters the receiver-dryer or filter-dryer. This acts as a filter and removes moisture that may have entered via the slightly permeable rubber hoses. Its purpose is to prevent damaging solid particles, including ice, being carried any deeper into the system.

From here, the refrigerant enters an expansion valve just before it passes into the evaporator. You can't access the evaporator easily in most cases, because it's mounted behind the dashboard panel, enclosed typically within the heater box. Its thin aluminium construction is similar to that of the condenser and it also possesses a large surface area.

As the liquid refrigerant leaves the restriction valve and enters the evaporator, it boils at low pressure and absorbs heat from the evaporator body. So, it chills any air blown over it. Air’s ability to carry water is related directly to temperature, so the rapid cooling causes moisture to condense and build on the outside of the evaporator. As a result, this water freezes but, should the evaporator ice-up completely, warm air won't be able to pass through its core. This is one reason why a sensor monitors the evaporator’s core temperature to keep it several degrees Celsius above zero.

When the engine is switched off, the air-conditioning stops working; the surface frost melts and the resultant water exits the car through a special duct. Unfortunately, the evaporator’s large surface area tends to retain moisture, creating a breeding ground for bacteria, the spores of which can be blown into the cabin, accompanied by a bad smell. Back inside the air-conditioning system’s closed environment, the hot vapour exits the evaporator and is sucked back into the compressor and the cycle repeats.