Jun 26, 2017 | by Kathy Hudson
An Air Compressor for Every Application: How to Choose
Air compressors are used for everything from food processing to jet engines. While all compressors perform the same basic function (increasing the pressure and reducing the volume of air for large-scale processes), there are many different types. The type you need will depend on the job you need done or the equipment you need powered. Here’s an overview of the various types of compressors and questions to ask yourself to determine which type to choose.
Positive Displacement Compressors
The most common compressors use positive displacement. They work by filling a chamber with air and then reducing the chamber’s volume to increase pressure. Positive displacement compressors are found in everything from bicycle pumps to large industrial equipment.
Reciprocating (piston) compressors are one type of positive displacement compressor. They function via a piston inside of a cylinder. As the piston moves down, air fills the upper portion of the cylinder, and as the piston moves up, the air is compressed. This type of compressor is used for a variety of purposes, including blowing plastic bottles and performing operations in oil refineries, gas pipelines, chemical plants, natural gas processing plants, and refrigeration plants.
There are subcategories of reciprocating/piston compressors: single-stage and two-stage. Single-stage reciprocating compressors contain a single piston and cylinder, while two-stage reciprocating compressors have two pistons, each in their own cylinder. Two-stage compressors function the same way as single-stage, but they add another step for compression to get additional pressure from the second piston.
The rocking piston compressor is similar to the one- and two-stage compressors, but it adjusts the pistons outside standard operation. These units are small, creating lower pressure. They’re very portable. This type of compressor is popularly used for aeration because it doesn’t require oil and operates cleanly.
The rotary scroll compressor is one that works via rotation. It utilizes only a few moving parts, so it doesn’t wear out as quickly as some more-complex designs. The air is compressed by a rotating element inside the housing, and operates at different intensity levels. Two advantages of this type are that these compressors are smooth and extremely reliable. They work well in areas where you don’t have much space or when you’re working with many moving parts. They’re common in vacuum pumps and air conditioners.
Rotary screw compressors are widely used in a variety of applications. They operate by rotating two screws quickly inside the housing, creating pressure. While this type has lower leakage levels than many other types, they do require coolant and lubrication because they generate so much friction. They’re not as efficient as some other types for this reason, but they’re ideal when large amounts of compressed air are needed for an extended period of time. These compressors can produce up to 2000 CFM.
The rotary vane compressor uses a motor that includes a rotor, stator, and a series of blades (vanes) that rotate. The rotary vane design is an older one that works well for medium pressure needs, but many more modern designs perform more efficiently so it’s losing popularity for many applications. This type is still found in automotives and hydraulic pumps, however.
This is an older design as well, created in 1860. A pump contains two rotors with two to three lobes each. One rotor, which is connected to the engine, rotates and pushes on the second rotor to make it spin in the opposite direction. A disadvantage of this type is that operation requires precision and the compressors can have problems if they encounter dirt. This type is fairly low maintenance otherwise, but also isn’t very efficient. They’re commonly used in nuclear power plants.
Negative (Non-Positive) Displacement Compressors
The term non-positive describes negative displacement compressors most accurately, because they use kinetic energy from rotating components to create pressure. The amount of pressure created depends on the speed of the rotating element.
This type of compressor increases pressure by raising the velocity of the air. A central impeller spins up to 60,000 times per minute, transferring energy from the motor to the air by forcing the air outward. These compressors work best when they are operated consistently, but you can scale your use up or down as needed. This type is used in gas turbines and turbochargers as well as in natural gas pipelines and oil refineries. They also frequently supply the compression for air chillers.
This type looks similar to a turbine. Air flows through the series of rotors as the compressor rotates. A central shaft spins, causing half of the rotors to spin, while the other half of the rotors serve as fixed rows of airfoils in between the rotors. Jet engines use this type of compressor.
How to Choose
The type of power you need and where you need it are the two primary factors that will affect the type of air compressor you choose. Ask yourself these questions to make sure you’ve considered all the angles.
How much pressure do you need? If you only require low pressure, you’ll want to look at rotary screw, single-stage reciprocating, and rocking piston.
What’s the maximum CFM usage you’ll need? Keep in mind that you’ll need to calculate numbers from all the tools that are operating at the same time.
Do you need a portable compressor? Rocking piston compressors are a great choice for portable requirements, but other compressors that are low pressure and volume will work well.
Do you need an electric or gas-powered drive system? Electric-powered compressors are generally less expensive both to buy and to run (and maintain), but if you don’t have easy access to electricity, you’ll need to use gas.
Will you use the compressor in concentrated bursts (air nailer, for example) or for longer periods? If you need the compressor to run for longer periods, you’ll need a larger receiver tank.
Will you be operating the compressor in a clean environment? If not, you’ll need a compressor that isn’t finicky when it encounters debris, so stay away from rotary lobes.
These questions should help you narrow down your options and give you insight into the types that would work for your application. From there, you can look into the specifications for the types you’re considering to make a final decision.
Brand new air compressors can be quite costly, so you may want to consider purchasing a refurbished one. If you’d like help deciding what type of compressor is best for your individual situation, we’re happy to talk with you. Give us a call at 864.249.0943!