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How Does My Car Air Conditioner Work?

To many car owners, the inner workings of their car's air conditioning systems is somewhat of a mystery. Below are the main parts of your car's air conditioning system and what they do.  

It's interesting to note that air conditioning systems are closed-loop systems, which have no start or end points; in practice, the entire cooling process happens in all parts of the system at once when the compressor clutch is engaged. 

The Compressor

The compressor is a relatively simple engine driven pump, whose function it is to compress the refrigerant gas. In terms of operation, the compressor is equipped with an electronically controlled magnetic clutch; when the clutch is energised by either the driver or the control system, the actual pumping mechanism compresses and circulates the refrigerant against a restriction in the system. When the compressor clutch is deactivated by the driver or the control system, the compression and circulation of the refrigerant stops, which also stops the cooling process of the vehicle interior until the clutch is re-energised.

In a fully functional car air conditioning system, the compressor clutch will automatically cycle on and off (to maintain a preset temperature) in direct response to input data received from one or more thermostats and pressure switches.  

The Condenser

Since the action of compressing the refrigerant gas causes the gas to reach very high temperatures, some of the heat has to be shed to extract the maximum cooling effect of the refrigerant a bit later on in the process. Thus, the hot gas is passed through a heat exchanger where some of refrigerant’s heat is shed to the atmosphere, while more heat is shed when the hot, highly compressed gas changes or condenses into a liquid when it is discharged into the relatively large volume of the condenser.

In almost all cases, the condenser is located in front of the engine-cooling radiator, where it shares the strong positive airflow that is created by either the vehicles motion, and/or the radiator cooling fan.

The Expansion Valve / Orifice Tube

 Although the refrigerant is still in a liquid state when it exits the condenser, it is considerably colder than it was when it entered the condenser. At this point in the cooling process, the liquid refrigerant is transported to the expansion valve or orifice tube (depending on the vehicle) through a high-pressure hose.

 As a practical matter, both expansion valves and orifice tubes are attached to the evaporator, and do the same thing; both allow small, metered amounts of pressurised liquid refrigerant to enter the evaporator, where the small amount of liquid rapidly evaporates. In practice, although the temperature of all liquids decrease sharply when they suddenly change into a gaseous state, the gas used in modern automotive air conditioning systems produce a much larger cooling effect than most other gases per unit of volume, hence their universal use.

The Evaporator 

All automotive air conditioning evaporators are enclosed in a box-like structure that is located under the dashboard. In practice, evaporators are heat exchangers, but unlike radiators that have many small diameter tubes to shed the engine coolants heat more effectively, evaporators use relatively large tubes to extract the maximum benefit from the relationship between the internal volume of the evaporator, and the rate at which the refrigerant cools down when it evaporates into that volume.

As a practical matter, the cold refrigerant now passes through the evaporator, and since it is enclosed in a box, filtered, ambient air can be forced through the fins of the evaporator by a blower motor, which process cools down the ambient air. The now cooled ambient air is then forced through a series of ducts and vents into the passenger compartment, while the refrigerant is returned to the compressor via a low-pressure hose to start the cooling cycle anew.        

The Accumulator / Dryer / Receiver

All automotive air condition systems are fitted with an accumulator, or a receiver /dryer. Which is fitted to any given vehicle depends on the vehicle and the air conditioning system, but regardless of the name, all accumulators / dryers / receivers do the same thing, which is to both filter out impurities from the refrigerant, and to absorb and contain all moisture that may be present in the system.

The only real difference between accumulators / dryers / receivers relates to their positions in the system; accumulators are typically located after evaporators, while receivers / dryers are typically located after condensers.   

NOTE: While the terms filter, accumulator, dryer, and receiver are sometimes used interchangeably, the term “filter” more properly applies to the replaceable filter(s) in the box that encloses the evaporator. This filter is also known as the “cabin filter”, and it should be inspected and or replaced on a regular basis to maintain the efficiency of the air conditioning system.   

What is the importance of accumulators / dryers / receivers?

While most components of well-maintained air conditioning systems, including compressors, will generally provide many years of reliable service, this is not always the case with accumulators / dryers / receivers, because of the very specific functions these components perform.

For instance, all accumulators / dryers / receivers contain bags that contain desiccants that absorb moisture and other impurities. Over time, and particularly when the air conditioning system had been repaired, recharged, or otherwise worked on one or more times the desiccant can become saturated, or the bag can perish.

If the desiccant becomes saturated with moisture and/or other impurities, these substances are no longer removed from the system, which usually causes corrosion to form in the compressor and/or other components. If the desiccant bag ruptures, solid particles can be transported throughout the system, which usually leads to a dramatic decrease in system efficiency as hoses and orifices become clogged, but sometimes also to fatal consequences for the compressor.

Therefore, to prevent major damage to the system, and to maintain the overall efficiency of the air conditioning system, the accumulator / dryer / receiver must be replaced every time the system is recharged, or had been open to the atmosphere, such as when a leak in the system had allowed atmospheric air to enter the system. 

As you can see your cars air conditioning is a complex system so it's important to have it regularly maintained by a qualified professional.