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Automotive Air Conditioning

How does it work?

The first automotive manufacturer to offer an air-conditioning system for their vehicles was the Packard Motor Car Company from the USA in 1939. Due to multiple design problems, the system was not very successful. It had no thermostat to regulate the temperature and was very bulky and expensive. The idea, however, caught on and was further developed during the 1950’s. By the end of the 60’s more than half of all new cars sold in the USA came standard with air conditioning. Aftermarket AC installations also became quite common.

During the 70’s and 80’s it was discovered that the coolant Freon used in air-condition systems of that era was damaging to the Ozone layer. Therefore, by 1996, legislation was passed that required all new automotive AC units to run on an alternative coolant (R-134a or HFC-134a) in order to protect the environment.
Air conditioning has worked in much the same way since its inception, namely to cool the air and to remove humidity. Hence, an air conditioning system normally consists of three main components: a compressor, a condenser and an evaporator.

High-pressure side

Automotive air condition systems are a nearly closed loop with a high-pressure and low-pressure side.

The first component of the AC loop is the compressor which is a belt-driven pump attached to the crankshaft. The coolant enters the compressor as a low pressure gas. As the belt drives the pump, the coolant gas is compressed and forced into the second component in the loop: the condenser.

The purpose of the condenser is to act similarly to the radiator in your car, namely to radiate heat away from the system. The coolant enters the condenser as a compressed gas. Compressing the coolant gas and moving it to the condenser creates heat. By running the coolant through the condenser, this heat is radiated away from the system, cooling the compressed gas until it becomes a liquid again, similar to how steam cools down to form water. The condensed coolant is now a high pressure liquid which is almost ready to do its job of cooling the air in the car.

Next in line is the Receiver-Dryer. This is a little reservoir containing desiccants – small granules that attract water. The liquid coolant is passed through this reservoir to remove any water that may have made it into the AC loop. Water has to be removed as it could form ice crystals in the coolant. These crystals could potentially damage the AC system and its components.

This concludes the high-pressure side of the loop. Now let’s move on to the low-pressure side.

Low-pressure side

The first component on the low-pressure side is the Thermal Expansion Valve (TXV). The high-pressure coolant liquid flows from the Receiver-Dryer through the expansion valve, where it expands, thereby reducing the pressure of the coolant. The valve regulates the flow of the coolant to allow the system to operate steadily. This puts strain on the moving parts of the valve which can cause the valve to wear-out over time until it needs to be replaced.

Some vehicles have an Orifice Tube instead of a Thermal Expansion Valve. The tube serves the same purpose as the valve by lowering the pressure of the coolant. Orifice Tubes have no moving parts and therefore last longer, but can become clogged with debris. Systems with Orifice Tubes turn the AC system on and off automatically to regulate the coolant flow.

The next component in the AC loop is the Evaporator. While all the other parts of the AC system are housed in the engine compartment, the Evaporator is normally located above the passenger-side foot-well of the cabin. It looks similar to a radiator and its job is to absorb the heat from the air in the cabin.

Coolant enters the evaporator as a cold, low-pressure liquid (ideally at 0° Celsius). The coolant does not freeze at this temperature, but it does have a very low boiling point. The absorbed cabin heat causes the coolant to boil and revert back into a gas. In this gaseous form, the coolant can absorb a large amount of heat. A fan blows on the outside of the evaporator coil. This blows cool air back into the cabin, while the expanded, heated coolant gas moves out of the cabin, thereby removing the heat from the passenger compartment. The coolant gas then returns to the compressor and the cycle begins again.

Systems with an Orifice Tube have an Accumulator between the Evaporator and the Compressor. Orifice Tubes sometimes let too much coolant into the evaporator. This can cause some of the coolant not to boil and therefore to exit the Evaporator as a liquid instead of a gas. The Accumulator traps this excess coolant liquid to prevent it from entering the Compressor. The Compressor can only compress gases, not liquids.

The Evaporator also removes humidity from the air in the cabin. The water vapour condenses on the outside of the evaporator coil together with dust, pollen and other small particles in the air. This “dirty” water is then disposed of by expelling it from the vehicle and is the liquid sometimes seen dripping from the under-body of a car after a long trip with the AC on.

Recharging

Even though the AC works in a closed loop, it can happen that coolant gas is lost over time. This can occur if any of the components, hoses or seals are damaged or worn. In this case you should have your AC system “recharged” and the damaged/worn parts replaced by a qualified workshop.

Recharging means that new coolant gas is added to the AC system. This normally entails removing all the remaining coolant from the AC system. A vacuum is then created in the system to check for leaks. Finally, the correct amount of coolant gas is refilled into the system. This completes the “recharge” and the AC system is once again ready to cool your vehicle during those hot summer days.

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