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How it works: automotive air conditioning

Few vehicle systems are as unfairly vilified for their effect on the environment as air-conditioning

Published: February 29, 2012, 5:00 PM

Spring is just a few weeks away (by the calendar at least) and not far behind it will come air-conditioning season. Few vehicle systems are as misunderstood, or unfairly vilified by myths and misconceptions about their effect on the environment as is air-conditioning. So let's shed some light on the subject.

The air-conditioning (AC) system in your car, truck or SUV works the same way as the refrigeration system in your refrigerator, with your vehicle's interior taking the place of the inside of your fridge.

A typical AC system comprises just four primary components: a compressor; a condenser; an expansion valve; and an evaporator.

They are connected in a closed system that circulates a refrigerant with very specific properties.

The compressor, which is similar in design to an air compressor, has typically been belt-driven by the engine and that remains the predominant arrangement.

Increasingly in today’s vehicles, however, AC compressors are electrically driven, particularly in hybrids or those with automatic stop-start systems.

The compressor’s role, as its name implies, is to compress the gaseous refrigerant and that compression process causes the fluid to heat up. (Remember the Ideal Gas Laws from high school Physics?)

From the compressor, the hot gas passes through the condenser — in effect, a separate radiator that usually sits in front of the coolant radiator in your vehicle. As air flows over the condenser, it cools the refrigerant, in relatively terms – it’s still very hot – causing it to change state from a gas to a liquid.

It then passes through an expansion valve, which controls the flow of the cooler liquid entering the evaporator.

The evaporator is another radiator, mounted in the heater/air-conditioning housing in your vehicle, usually under and ahead of the instrument panel on the passenger's side.

As the refrigerant passes through the evaporator, it is heated by warm air – from either outside or within the passenger compartment – passing over the radiator's coils.

The more important effect, from our perspective as occupants of the vehicle, is that in the process of warming the gas in the evaporator, the air passing through it is itself cooled. That's the air that's blown into the passenger compartment to cool us.

There is also a dehumidifying effect on the air, which helps further in making it fell cool.

The gaseous refrigerant from the evaporator is then returned to the compressor to start the cycle all over again. En route, it passes through an accumulator/drier, which is a sort of safety valve to prevent any liquid refrigerant from reaching the compressor and also removing any water from the gas.

Throughout that process, just as in your refrigerator, the refrigerant remains contained and under high pressure. And as long as it is contained, it cannot directly impact the environment, either in terms of ozone depletion or climate change. It can only do so if it is released into the atmosphere.

Just like your refrigerator, if your air-conditioning system is working well, that's proof that its refrigerant is contained. If you do have to have it serviced, however, be sure to have it done by a certified technician with all the proper tools to keep the refrigerant contained.