Change of State: What It Means and Why It’s Vital to A/C Operation

When A/C specialists refer to “change of phase” or “change of state,” they’re talking about the air conditioner’s ability to transform matter from a liquid to a gas or from gas to a liquid.

The air conditioner uses pressure at different locations within the system to create conditions that cause a change of state. Under low pressure, cold refrigerant expands as it soaks heat out of the air. Under high pressure, it can begin the process of rejecting that heat into the outside environment. This process is essential to removing heat from the cab and producing a steady stream of cool air.

Here’s how it works:

There are five components in an air conditioning system: the compressor/clutch assembly, condenser, receiver-dryer, expansion valve, and evaporator, as well as refrigerant. The compressor sucks in cool refrigerant gas and compresses it to a very high pressure (up to 350 psi), raising the temperature of the gas to more than two and a half times the temperature of the outside air.

The hot, pressurized gas flows into the condenser where it cools and changes to a liquid. The refrigerant then passes through the small orifice of the expansion valve and enters the low-pressure environment of the evaporator as a mist.

The evaporator is typically located under the dash. In this low-pressure environment, the liquid refrigerant expands and boils easily (the boiling point of refrigerant is approximately 20 F at 20 psi). Heat from the cab air passes through the metal of the evaporator and is absorbed into the cool refrigerant inside. The vaporized refrigerant leaves the evaporator and makes a second pass through the expansion valve. The cool gas is sucked back into the compressor to begin the circuit again.

When we talk about the “high” side and the “low” side of an A/C system, we’re referring to pressure: the condenser on the low side, the evaporator on the high side, and the compressor in the middle.

Because suction and pressure are so important to the process, any leaks or restrictions in the system can compromise performance and component life.

Checking the air conditioner during an oil change or any scheduled maintenance procedure means the vehicle can receive necessary service while it’s there in the shop, before a costly failure on the road.