What Is an HVAC Compressor?

We’ve all appreciated air conditioning on a sweltering summer day, but generally only those who have taken HVAC classes know how the units work. Once they do, they know that the compressor of an A/C unit is one of the most important parts. ¹

What Does a Compressor Do?

By definition, the word “compress” means “to reduce in size, quantity, or volume as if by squeezing.” ² To understand how a compressor works, we first answer the question, “how does refrigeration work?” Refrigeration is the process that allows air conditioners to remove heat from inside a building and carry it outside, thereby cooling the indoor temperature.

Step 1. In an air conditioner, refrigerant absorbs heat from a room in the evaporator coil, causing it to change from a cold liquid to a low-pressure, warm refrigerant gas.

Step 2. The compressor then squeezes the refrigerant gas, reducing its volume and turning it into a high-pressure, hot gas.

Step 3. When the gas enters the condenser, outside air flows over it, removing heat and resulting in a high-pressure, cool liquid.

Step 4. The expansion valve lowers the pressure and temperature of the liquid refrigerant, turning it into a cold gas and sending it back to the evaporator to repeat the heat transfer process.

What are the Types of HVAC Compressors?

HVAC compressors can be categorized by their application. There are five types of compressors used in air conditioning units:

1. Reciprocating: Can be used in appliances, residential A/C, light commercial A/C, commercial A/C, refrigeration, and industrial applications.
2. Rotary: Can be used in appliances and residential A/C units.
3. Scroll: Can be used in residential, light commercial A/C, commercial A/C, and refrigeration applications.
4. Screw: Can be used in commercial A/C, refrigeration, and industrial applications.
5. Centrifugal: Can be used in commercial A/C systems.

The reciprocating compressor has the greatest number of applications. ³

How Do HVAC Compressors Work?

Since the reciprocating compressor has the most applications, we’ll take a look at how it works. First let’s examine its design. Picture an internal combustion engine because that’s what a reciprocating compressor looks like. A central crankshaft drives between two to six pistons located inside of the cylinders. Each cylinder has a suction line for taking in low-pressure refrigerant and a discharge valve for releasing high-pressure refrigerant. The pistons compress the refrigerant. ⁴ Below is a closer look at the process:

Step 1. The crankshaft begins to rotate, pulling the piston downward and reducing the pressure in the cylinder. The difference in pressure inside the cylinder and the suction line causes its valve to open and low-pressure refrigerant to flow into the cylinder. Once the cylinder is full of refrigerant gas, the suction line valve closes.

Step 2. As the crankshaft continues its rotation, it pushes the piston upward, increasing the pressure of the refrigerant gas because it has less space in the cylinder. The difference between the high pressure in the cylinder and the low pressure in the discharge line pushes its valve open. The high-pressure refrigerant escapes.

Step 3. After all of the compressed refrigerant is out of the cylinder, the process repeats. ⁵

HVAC Compressor: An Essential Component

As you can see, the compressor plays an essential role in the refrigeration process air conditioners rely on. As one of an A/C unit’s primary components, you’ll probably come across many compressors once you finish your HVAC training and start working in the field. Understanding how the various types of compressors work will likely prove helpful when that time comes.

Additional Sources
1 – https://www.rsi.edu/blog/hvacr/how-does-an-air-conditioning-unit-work/
2 – https://www.merriam-webster.com/dictionary/compress
3 – Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 163
4 – http://sciencing.com/reciprocating-compressor-work-5002827.html
5 – Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 168-169