We’ve all enjoyed the comfort of air conditioning on a sweltering summer day, but generally, only those who have undertaken HVAC training truly understand how these complex units operate. Once familiar with the mechanics, it becomes clear that the compressor of an A/C unit is undeniably one of its most vital components.
What Does an AC Compressor Do?
By definition, the word “compress” means “to reduce in size, quantity, or volume as if by squeezing.”1 To grasp how an AC compressor works, we first need to understand the fundamental principle of refrigeration. Refrigeration is the process by which air conditioners remove heat from inside a building and transfer it outside, thereby cooling the indoor temperature.2
Here’s a step-by-step breakdown of the compressor’s role in this process:
- Step 1: Heat Absorption (Evaporator Coil)Inside your home’s air conditioner, the refrigerant absorbs heat from the indoor air as it passes through the evaporator coil.3 This heat absorption causes the cold liquid refrigerant to change into a low-pressure, warm refrigerant gas.
- Step 2: Compression (The Compressor’s Job)The compressor then squeezes this low-pressure, warm refrigerant gas.4 This action significantly reduces its volume, consequently transforming it into a high-pressure, hot gas.5 This increase in pressure and temperature is crucial for the next stage of the cooling cycle.
- Step 3: Heat Rejection (Condenser Coil)When this high-pressure, hot gas enters the condenser coil (located in your outdoor unit), outside air flows over it.6 This cooler outside air absorbs the heat from the refrigerant gas, causing it to cool down and condense back into a high-pressure, cool liquid.7
- Step 4: Pressure Drop (Expansion Valve)Finally, the liquid refrigerant passes through an expansion valve. This valve drastically lowers the pressure and temperature of the liquid refrigerant, causing it to revert to a cold gas or a mix of liquid and gas. This now-cold refrigerant is sent back to the evaporator coil indoors to repeat the entire heat transfer process, continuously drawing heat out of your home.8
What are the Types of HVAC Compressors?
HVAC compressors can be broadly categorised by their application across various cooling systems. There are five primary types of compressors commonly used in air conditioning units:
- Reciprocating: Highly versatile, these compressors are found in a wide range of applications, including household appliances, residential A/C, light commercial A/C, commercial A/C, general refrigeration, and industrial systems.9
- Rotary: Primarily used in smaller applications such as household appliances and residential A/C units.
- Scroll: Common in residential, light commercial A/C, commercial A/C, and refrigeration applications.10
- Screw: Typically found in larger commercial A/C, refrigeration, and industrial applications.
- Centrifugal: Exclusively used in large-scale commercial A/C systems, particularly for chillers.
Among these, the reciprocating compressor boasts the greatest number of applications.
How Do HVAC Compressors Work? (Detailed Look at Reciprocating Compressors)
Given that the reciprocating compressor has the most widespread applications, let’s take a closer look at how it functions. Imagine the design of an internal combustion engine, as a reciprocating compressor shares a similar appearance. A central crankshaft drives anywhere from two to six pistons, each housed within its own cylinder. Each cylinder is equipped with a suction line (intake valve) for drawing in low-pressure refrigerant and a discharge valve for releasing high-pressure refrigerant.11 The pistons are the components responsible for compressing the refrigerant.12
Below is a closer look at the process:
- Step 1: Suction StrokeThe crankshaft begins to rotate, pulling the piston downwards within the cylinder. This downward motion creates a reduction in pressure inside the cylinder. The resulting pressure difference between the cylinder and the suction line causes the intake valve to open, allowing low-pressure refrigerant gas to flow into the cylinder. Once the cylinder is full of refrigerant gas, the suction line valve closes.
- Step 2: Compression and Discharge StrokeAs the crankshaft continues its rotation, it pushes the piston upwards. This upward movement compresses the refrigerant gas within the cylinder because it now has less space. The pressure inside the cylinder dramatically increases. The difference between this high pressure in the cylinder and the lower pressure in the discharge line pushes the discharge valve open, allowing the high-pressure, hot refrigerant gas to escape the cylinder.
- Step 3: Cycle RepetitionOnce all of the compressed refrigerant has been expelled from the cylinder, the discharge valve closes, and the piston begins its downward stroke again, initiating the next cycle. This continuous, repetitive process is what allows the compressor to efficiently move and pressurise the refrigerant throughout the air conditioning system.
HVAC Compressor: An Essential Component
As you can see, the compressor plays an absolutely essential role in the refrigeration process that air conditioners rely on to cool our spaces.13 As one of an A/C unit’s primary components, you’ll undoubtedly encounter many different types of compressors once you complete your HVAC training and begin working in the field. A solid understanding of how the various types of compressors work will prove incredibly helpful as you diagnose and service these vital systems.