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Air Conditioning 101 - The Basics of AC Systems

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For most automotive do-it-yourselfers, there are three things on their vehicle that they find daunting to deal with: transmissions, electrical, and air conditioning. The AC (or A/C) system is simple, but highly technical. In most areas of the country, those working on these systems require special certification and tools and fines for working on one without that certification are steep.

As a word of caution: be aware of your local and regional laws regarding AC units. You could face serious consequences if you don't follow them.

There is one basic principle that oversees how making cold air out of hot air works: the pressure, temperature relationship. Physics states that when a material changes state (going from a liquid to a vapor, for example), it also changes temperature by either becoming colder or warmer. The idea behind the way an automotive AC system works is that when the refrigerant changes its state, usually due to pressure differences, it sheds heat and becomes colder. No matter the type of refrigerant being used in a car, the idea will be the same.

Currently, HFO-1234yf is the most common refrigerant used in new automobiles. Up until about 2013, CFC R-12 was the most common and up until 1994, HFC R-134a was the most common. Generally, most older systems can use newer refrigerants, but newer systems may not be able to use older refrigerants. The refrigerant type changed to both improve efficiency and for environmental reasons.

Your vehicle's AC system works the same regardless of the refrigerant used or who made the system. The refrigerant is compressed until it becomes a pressurized liquid. It is then suddenly taken out of pressure by suddenly expanding the space it is allowed to occupy, turning the refrigerant back into a gas and shedding a lot of heat in the process. Air is then passed over pipes through which this cold refrigerant is running, cooling the air, which is then sent into the passenger cabin to cool the occupants.

The basic parts of your air conditioning system start with that refrigerant, kept in a closed loop of piping. The AC compressor (an engine accessory) is used to pressurize the refrigerant into a high-pressure liquid. This then passes through some piping and a radiator-style condenser unit, which helps to pull some of the heat away from the liquid. This can create some moisture, which is removed in a dryer. The liquid is then sent to an expansion valve. This side of the AC system is called the "high pressure side."

The expansion valve (also called an "orifice tube") controls how much of the pressurized liquid is decompressed and how quickly that happens. The compressed liquid decompresses into a gas, getting much colder. This then passes through an evaporator, a radiator-like unit that is usually located near the firewall and inside air ducts - most often inside the dashboard. Air is then blown over the evaporator, making the air colder as it is then sent into the passenger cabin. The coolant gas is sent around to the compressor again, finishing the loop on this "low pressure side."

The majority of AC system problems revolve around leaks in the loop, compressor problems, and moisture issues.

In another article, we'll delve into diagnostics on the AC system.

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