AC Compressors: How they Work, Types, Failures


AC compressors play a crucial role in air conditioning systems, as they are responsible for compressing and circulating the refrigerant, which facilitates the cooling process. Here’s a summary of how AC compressors work, the different types, and common failures:

How AC Compressors Work

  • Compression: The compressor draws in low-pressure, gaseous refrigerant from the evaporator coil. It compresses the refrigerant, increasing both its pressure and temperature.
  • Condensation: The hot, high-pressure refrigerant then flows to the condenser coil located in the outdoor unit.
  • Heat Release: In the condenser coil, the refrigerant releases heat to the surrounding outdoor air, causing it to condense into a high-pressure liquid.
  • Expansion: The high-pressure liquid refrigerant passes through an expansion valve or metering device, where it undergoes a rapid drop in pressure and temperature.
  • Evaporation: As it enters the evaporator coil in the indoor unit, the refrigerant absorbs heat from the warm indoor air, turning it back into a low-pressure gas.
  • Air Circulation: The blower fan in the indoor unit then circulates the cooled air throughout the living space.

Types of AC Compressors

  • Reciprocating Compressors: These compressors use a piston and cylinder mechanism to compress the refrigerant. They are common in residential air conditioning systems.
  • Scroll Compressors: Scroll compressors use two spiral-shaped scrolls to compress the refrigerant. They are known for their efficiency and are often found in both residential and commercial systems.
  • Rotary Compressors: Rotary compressors use a rotating blade to compress the refrigerant. They are compact and often used in smaller air conditioners and window units.
  • Variable-Speed Compressors: Also known as inverter compressors, these can vary their speed and output to match the cooling needs, offering enhanced energy efficiency and precise temperature control.

Common AC Compressor Failures

  • Compressor Overheating: Overheating can occur due to a lack of refrigerant, poor airflow, or electrical issues, leading to damage or failure.
  • Electrical Problems: Faulty wiring, capacitors, or relays can cause compressor electrical failures, preventing it from starting or running efficiently.
  • Refrigerant Leaks: Refrigerant leaks can lead to low refrigerant levels, causing the compressor to overwork and potentially fail.
  • Compressor Short Cycling: Rapid on-and-off cycling can wear out the compressor quickly, often due to issues with the thermostat or a blocked filter.
  • Compressor Lockup: Mechanical failure or a seized compressor can result in a complete breakdown of the unit.
  • Contaminants: Contaminants in the refrigerant can damage the compressor and other components over time.

The average cost of replacing an AC compressor is $1200 including labor. The cost can however be as low as $700 or as high as $3400 depending on size, brand, location, type among other factors.

A properly-sized and well-maintained air conditioner compressor will last for up to 15 years or even longer if it is well-suited for your area’s climate. Wrongly sized compressors and those that are not regularly maintained will of course not last that long.

How Does and Air Conditioning Compressor Work?

You could say that the compressor is the heart of the air conditioner. Its functions just like the heart of a human body.

For an air conditioner to cool the air inside your house, a refrigerant has to be circulated between the inside and outside of the house. Copper tubes are used to make the system a closed loop.

A refrigerant is a fluid ideally with a low boiling point. That means that the fluid is able to turn to its gas state easily and also turn back to the liquid state just as easily.

Central and ductless (mini-split) air conditioners have an indoor and outdoor unit. The indoor unit is where the evaporator coil is located while the outdoor unit comprises of the condenser coil and the compressor.

Packaged air conditioners like window and portable air conditioners have all the components in one unit.

When the refrigerant enters the evaporator coil inside the house, it is usually in it is liquid state, meaning it is quite cold. The refrigerant cools the hot indoor air by absorbing its heat and in the process evaporates to its gas/vapor state.

A low-pressure high-temperature refrigerant gas then enters the compressor unit. But why do we need the compressor in the first place?

The compressor increases the pressure of the refrigerant, allowing it to move through the condenser coils, where it will discharge the heat it absorbed from the house.

In thermodynamics (heat transfer) heat can only move from a point of high concentration to a point of low concentration (like from the fireplace to your skin and not vice versa).

Although the refrigerant coming from the house is hot, the outside air is equally hot. That is where the compressor comes in.

When you compress a gas, its pressure will definitely increase but that is not the only thing that happens. Its temperature increases as well.

An air conditioner compressor therefore compresses the refrigerant and in the process raises its temperature. That creates a temperature differential between the refrigerant and the outside air.

The refrigerant leaves the compressor and enters the condenser coil as a high-pressure superheated gas.

As the refrigerant moves through the condenser coil, a fan blows air over the coils which aids in the transfer of heat from the refrigerant to the surrounding. The shell of the condenser unit has metal fins similar to those of a car radiator to help in the dissipation of heat to the surrounding.

That is the main reason the air around the outside AC unit is always hot.

By the time the refrigerant is exiting the condenser coil, it will have turned back to its liquid state and ready to go back to the evaporator coil and remove more heat from indoor air.

The cycle goes on and on until the air in your house is sufficiently cooled. And basically that is how an air conditioner compressor works.

Types of Air Conditioner Compressors

The following are the 5 types of air conditioner compressors and how they work:

1. Reciprocating Compressors

These compressors work just like a combustion engine. They use a piston and a cylinder and are powered by a motor connected to a crankshaft.

When the piston moved down, the refrigerant is sucked inside the cylinder and as the piston is moving up, the refrigerant is compressed and pushed into the condenser coil.

Depending on your needs, you can get a compressor that has 2, 4 or even 8 cylinders. Reciprocating cylinders have been used for a long time in residential AC units but modern ACs are favoring scroll compressors.

2. Scroll Compressors

As I mentioned above, scroll compressor are being preferred now more than reciprocating compressors. They are reliable and precise, and therefore more efficient in their working.

A scroll compressor is made up of 2 scrolls where one is stationary while the other one is free to rotate.

The refrigerant enters channels founds between the two scrolls but as it is swirled by the rotating scroll, the channels become smaller in size resulting in compression of the gas.

The refrigerant leaves through the center of the 2 scrolls as a high-pressure superheated gas before entering the condenser unit.

3. Screw Compressors

As a homeowner, you will not have this type of compressor in your residential AC system. It is made for use in buildings where large amount of air needs to be cooled like factories or office blocks.

Screw compressors comprise 2 helical rotors which pull the refrigerant from the compressor intake to the outtake. As the refrigerant moves inside the 2 rotors, the spaces become increasingly smaller, which is what results in the compression of the gas.

4. Rotary Compressors

Rotary compressors are made up of one or more blades which are mounted on a shaft inside a cylinder. The shaft is then positioned off-center inside the cylinder.

The design of the shaft and blades inside the cylinder ensures that the refrigerant intake has a big space between the shaft and the cylinder while the outtake has a small space.

As the shaft rotates, the blades move the refrigerant from the intake to the outtake where it is compressed and sent out to the condenser coil.

5. Centrifugal Compressors

Centrifugal compressors increase the velocity/kinetic energy (pressure) of the refrigerant as it flows through an impeller. They have few moving parts and are made to cool large amounts of air.

Basically, as the impeller discs rotate rapidly, the refrigerant is sucked to the center of the impeller where its velocity increases and is compressed inside the housing before being discharged to the condenser coil.

Main Reasons for AC Compressor Failure

Unfortunately, AC compressors are prone to premature failures. Failures in AC compressors are caused by several reasons which if avoided can make the compressor last for a long time.

The following are the main causes of failure in air conditioner compressors:

1. Too Much or Little Refrigerant

If your refrigerant is leaking or if the HVAC technician did not charge the system with enough refrigerant in their last service, you will have insufficient refrigerant in your AC system.

Insufficient refrigerant means that your AC compressor will work harder than it needs to do, to keep cooling your home. At the end of it all, the compressor will overheat and fail prematurely.

Too much refrigerant will also damage the air compressor. As I mentioned, the refrigerant should only enter the compressor in its gas state. Too much refrigerant in the system means that not all of the refrigerant will vaporize and some of it in liquid form will enter the compressor.

2. Frozen or Dirty Coils

The refrigerant needs to move freely inside both the condenser and evaporator coils. If one or both of the coils are blocked, the refrigerant will flow back to the compressor (in its liquid state) and damage it.

The condenser coil is especially susceptible to blockage by dust (since it is located outside) meaning heat from the refrigerant will not be properly dissipated. Ultimately this will result in overheating.

The evaporator coil can also freeze up, meaning that the refrigerant will be unable to absorb heat from the indoor air. Turning off the unit to let the ice thaw might help in this case.

3. Oil Leaks/Contamination

To run smoothly, an AC compressor uses lubricating oils to avoid friction between moving parts. Pump failure or oil leaks can cause oil penetrate areas where only the refrigerant is allowed.

As I mentioned, the compressor is only designed to compress gases and never liquids. Presence of oil inside the compressor will without a doubt result in failure.

Wrap Up

And basically that is how an air conditioner compressor works. If you want yours to last long, make sure that it is periodically inspected (the entire AC system) by a licensed technician.

I hope you enjoyed reading this post.