AC compressorAn air conditioner compressor is the component in the system that raises the temperature and pressure of the vapor refrigerant that leaves the evaporator coil. Its important that the compressor raises the pressure of the vapor refrigerant so that it creates a pressure difference, the pressure difference is needed in order for the refrigerant to flow. High pressure fluids flow towards lower pressure fluid. Basically whats happening is the compressor is raising the pressure of the refrigerant so that the refrigerant will flow to the lower pressure refrigerant in the evaporator coil.

The compressor raising the pressure will also increase the temperature. The direction of heat transfer is from a higher temperature substance to a lower temperature substance, the lower temperature being in the evaporator coil and the hotter being in the compressor and condenser.

The temperature of the freon being increased is very important, because the refrigerant will get so hot that the hot air outside will be cooler even in the summer when it can be up to 120 degrees! The air being cooler outside allows the heat in the refrigerant to transfer to it when it goes through the condenser coils. The heat in the refrigerant was transferred originally from the heat inside the home at the evaporator coil. Its important that the compressor increase the temperature so that it can finish removing the heat from inside the home to the condenser.

Air conditioner and heat pump compressors are known as vapor compressors because they are not meant to have any liquid, and liquids are not a compressible fluid. Any liquid that may enter the compressor will result in reduced efficiency and capacity and will typically cause mechanical damage to the compressors internal components.

A reciprocating compressor use pistons, cylinders, and valves to compress the refrigerant. The piston moves back and forth in the cylinder. Reciprocating just means back and forth. Reciprocating compressors vary in size and capacity depending on the requirements of the system.

The compressor is a dividing point between the high and low-pressure sides of the system and houses components such as the suction and discharge valves. The evaporator is on the low pressure side and the compressor and condenser are on the high pressure side. The suction valve connects the compressor to the low pressure side of the system via the suction line that carries refrigerant into the compressor. The discharge valve connects the compressor to the high pressure side of the system via the discharge line which carries refrigerant after it has been compressed. The suction and discharge valves open and close depending on the pressure difference of them and allow the vapor refrigerant to enter and leave the compression chamber at the proper moment.

reciprocating compressor

The Reciprocating Compressor Process

Well start describing the process of the compressor when the piston is at its highest possible position within the cylinder. The position is know as top dead center. At top dead center both the suction and the discharge valves are in the closed position and the refrigerant in the compression chamber is equal to the discharge pressure.

Re-expansion: The compressor motor will start to turn and the piston begins to move down in the cylinder. The piston going down, increases the amount of space or volume, the refrigerant is in. Also the pressure begins to decreases because the amount of refrigerant that was already in there is now in a larger space. This causes the refrigerant to expand. Refrigerant expanding is why this part of the process is called re-expansion.

Suction: The pressure of the refrigerant continues to drop until it reaches a point just below the suction pressure of the system. Suction pressure is the low-pressure side of the system. At that pressure the suction pressure will now be greater than the compression chamber and the suction valve will open. As the piston continues to move downward suction gas is drawn into the compression chamber. Suction will continue until the the piston stops moving in the downward direction. When the piston reaches its lowest point in the cylinder, bottom dead center, the suction portion of the cycle ends.

Compression: As the compressor continues to operate, the piston starts to move upward in the cylinder. This upward motion of the piston pushes the suction valve closed, trapping refrigerant in the cylinder. The piston continues to move upward, reducing the volume of the cylinder and increasing the pressure of the refrigerant. Compression will continue until the pressure of the cylinder is slightly greater than the pressure of the refrigerant in the discharge line.

Discharge: When the cylinder pressure is greater than the discharge pressure, the discharge valve will be pushed open, allowing the high pressure refrigerant to be pushed out of the cylinder into the discharge line as the piston continues to move upward. Discharge will continue until the piston reaches top dead center, where the discharge refrigerant will push the discharge valve closed as the piston again starts to move downward.

The cycle repeats it self as long as the system is energized.

Rotary compessors are cylindrical and have the discharge line at the top of the compressor. This type of compressor operates with a roller connected directly to the shaft of the motor. This roller rotates at the same speed as as the motor within the shell of the compressor, and rubs against the inside of the compression chamber and forms an area in which refrigerant vapor is trapped during compression. There is a movable vane that creates a seal with the roller due to a spring pressure that constantly pushes the vane up against the roller as it rotates. The vane separates the discharge and suction sides of the cylinder. Like the reciprocating compressor, refrigerant is introduced into the compressor via the suction line and discharged from the compressor through the discharge line.

rotary compressor

Rotary Compressor Process

To help visualize the compression process, we start where the the roller is blocking off the discharge port and the refrigerant from the suction port is able to enter the cylinder. Refrigerant enters. As the roller rotates the the suction or intake port is sealed off from the refrigerant that was just let in the cylinder. No more refrigerant is coming in at that moment. As the roller turns there is less space for the refrigerant, the pressure and the temperature will increase because its being compressed. As the roller turns more, the refrigerants pressure will increase above the discharge lines pressure, opening the discharge valve. The high pressure high temperature refrigerant vapor will leave the compressor through the discharge line. While the refrigerant is being compressed the suction valve opens when the roller is not blocking the suction valve, letting more refrigerant in to the cylinder. This creates a continuous flow of compression.

scroll compressor spirals

Scroll compressors work efficiently with two perfectly manufactured spirals. One of the scrolls is stationary, while the other scroll vibrates or wobbles. As the scroll that vibrates moves, refrigerant vapor is pushed and compressed toward the center of the compressor where it is discharged from the compressor.

The configuration of the scrolls forms multiple chambers, each of which is at a different stage of compression, allowing the compressor to operate smoothly and continuously. Scroll compressors are becoming a popular choice for replacement, because they are better able to handle any liquid that may enter the device.

One of the scrolls are stationary and the other floats giving the scroll some play. This play will allow the floating scroll to move if liquid, which is not compressible, should it enter the compressor. Damage to the compressor is avoided since the scroll’s movement and play increases the volume of the chamber, thereby accommodating the volume of the liquid. Basically since there’s a little extra room, in case liquid enters, there will be enough room for it. The pistons on a reciprocating compressor move in a well-defined path and the introduction of liquid could result in a major component failure. Liquid entering the scroll compressor will, however, reduce the capacity of the system.

Another benefit of scroll compressors is that they do not use suction and discharge valves. The scrolls, as they age, tend to “wear in”, making them more efficient, as opposed to valves, which wear out with time. Since there are no suction or discharge valves, scroll compressors are equipped with a a low-mass, disc-type check valve at the discharge port tht prevents the high-pressure refrigerant from traveling back through the compressor during the off cycle.


Silberstein, Eugene. “Compressors.” Residential Construction Academy: HVAC, Second Edition. Clifton Park, NY: Delmar, 2012. 29-33. Print.

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