Just how many ways are there to squeeze down a gas? If you look at the entirety of the industrial world, there’s probably a few hundred different devices. If you look at HVAC, luckily the list is a lot shorter. The air conditioning world relies primarily on five compressor designs.
This compressor works pretty much exactly the same as a car engine does, but without the gasoline and combustion. Inside a car you have your cylinders and pistons. When the piston moves up it compresses, when it goes down it sucks. The same principal is used in a reciprocating compressor.
At the base of the compressor there will generally be an electric motor, which turns a shaft. The shaft has a bend in it to allow for offset motion. At the bend there’s a connecting rod, which links the shaft to the piston. When the shaft spins, the piston moves up and down. On the downstroke, fresh refrigerant is pulled in. On the up stroke the refrigerant is compressed and injected into the refrigerant loop.
This particular design is popular in residential scale compressors. There are more parts involved, so there’s a greater chance of hardware failure, but the well understood nature of the piston and cylinder as well as massive manufacturing tolerances make them cheap to manufacture. Consider, steam engines were around in the 1800s, and this is the same principle as their driving pistons. The complexity isn’t very high.
There’s more than one compressor that works like an engine. Rotary compressors work like rotary engines, just no combustion involved either. There are two cylinders involved, an outter and an inner. Between the cylinders, a single vain presses down to create a seal. The inner cylinder rotates off-axis so it’s always touching the outter cylinder and is always in the center of the outter cylinder. As it spins, it divides the space inside the cylinder, one side will have low-pressure, the other will be high pressure. As it spins, gas is sucked in, it passes the divider and compresses the gasses it just sucked in.
This is essentially clever use of shapes in the same way a scroll compressor is clever usage of shapes. The circles moving naturally create larger and smaller spaces as they move, creating natural high and low pressure. There is a trade off with this design. It can only work at low outputs. Where other technologies can work on hundreds of tons, a rotary compressor is best kept below ten tons.
Screw compressors by contrast are capable of absurd levels of compression. If you look at a regular wood screw, you might notice that it tapers. This same taper can be applied to a bigger screw in a compressor or the screw threads can be made narrower. Gasses are sucked into the screw thread in much the same way as they would be pushed or sucked down by a fan blade. As the gasses are forced down the screw, they’re forced into less and less space. This condensed space forces the gasses to be compressed.
This design allows for massive screws to be used, increasing the capacity significantly. The continuous nature of the screws creates a steady output and therefore doesn’t generally need any sort of valve to build up pressure. These types of compressors are generally used at industrial-scales where they operate in the hundreds of tons range.
There is one other solution to high-capacity compressors: centrifugal designs. These compressors are essentially the same as a turbo used on a diesel engine. There is an intake with an impeller that sucks in vast amounts of material, centrifugal forces blast it outwards around the casing, and the immense speed built up is converted to pressure by forcing the refrigerant through a narrow opening. One motor can drive multiple compressors and each compressor can be linked in to another, leading to extreme, staged compressions and high performance.
These compressors are not something to take lightly. Designs of this type tend to arrive on flatbed trucks. Some manufacturers custom design each component for their given installation. They’re typically used in industrial scale chiller systems rather than conventional forced-air configurations.
The last type of compressor common in the HVAC world is the scroll compressor. We’ve covered it before. Essentially two spirals (scrolls) are interlocked. One moves against the other in a circular motion, leading to moving points of contact and forcing a gas into less and less space. In a way it can be compared to a screw compressor, where naturally smaller spaces force compression to occur.
What should you use?
Your particular compressor will vary by installation. Lower capacity applications will largely use scroll and rotary compressors. Reciprocating compressors can be used from light to heavy duty with the right designs. In most cases, you’ll only see screw and centrifugal configurations for enormous, hundred ton and above workloads found in factories, office buildings, and other large environments.