What is an Air Compressor?
An air compressor is a pneumatic device that converts power (using an electric motor, diesel, or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces more and more air into a storage tank, increasing the pressure. When the tank’s pressure reaches its engineered upper limit, the air compressor shuts off.
The compressed air, then, is held in the tank until called into use. The energy contained in the compressed air can be used for a variety of applications, utilizing the kinetic energy of the air as it is released and the tank depressurizes.
When tank pressure reaches its lower limit, the air compressor turns on again and re-pressurizes the tank. An air compressor must be differentiated from a pump because it works for any gas/air, while pumps work on a liquid.
How Air Compressors Function
Air compressors can power a variety of work, from inflating a tire to operating a nail gun. Learn how to find a compressor that handles your jobs.
Single-stage, piston-type air compressors are the most common models for home use and work well for many applications around the home or workshop. An electric motor or gasoline engine drives a piston, which compresses air and forces it into a storage tank.
As the piston forces more air in, the air pressure rises. Once the pressure reaches a specified level, the compressor stops running. As you use the stored air to power a tool, the compressor restarts to build the air pressure back up.
Two-stage compressors have two pistons. The first compresses the air and pushes it through a check valve to the second piston, which compresses it further and delivers it to the tank. These compressors are usually heavy-duty, commercial models that can deliver a greater volume of air at higher pounds per square inch (PSI) levels. They’re good choices for continuous use in shops or to power multiple tools at once.
How do Air Compressors Work?
Air compressors work by forcing air into a container and pressurizing it. Then, the air is forced through an opening in the tank, where pressure builds up. Think of it as an open balloon: the compressed air can be used as energy as it’s released.
They’re powered by an engine that turns electrical energy into kinetic energy. It’s similar to how a combustion engine works, using a crankshaft, piston, valve, head, and a connecting rod.
There are different types of air compressors and each one has a different specialty. Generally, the differences aren’t too severe: it all boils down to the way a compressor handles air displacement.
Types of Air Compressors.
Air compressors are categorized as either positive displacement or dynamic displacement, based on their internal mechanisms. The four most common types of air compressors you will see are:
- Rotary Screw Compressor
- Reciprocating Air Compressor
- Axial Compressor
- Centrifugal Compressor
We’ll cover what each is best used for below, so you can make an informed decision for your project.
Positive Displacement Compressors
Positive displacement compressors encompass a variety of different air compressors that generate power via air displacement. Air compressors in this category work with different internal mechanisms, but the principle for each is the same.
A cavity inside the machine stores the air brought from outside, and then slowly compresses the cavity to increase the air pressure and potential energy.
1. Rotary Screw Compressors
A common type of displacement compressor, rotary screw compressors are some of the easiest types of air compressors to take care of, as they are equipped with an internal cooling system and don’t require much maintenance. They are typically large, industrial-sized machines that can be either lubricated with oil or run oil-free.
Rotary screw air compressors generate energy via two internal rotors that turn in opposite directions. The air gets trapped between the two opposing rotors, and builds up pressure within the housing. Because of the internal cooling system, these air compressors are designed for continuous use, and range in power from 5 horsepower up to 350 horsepower.
2. Reciprocating Compressors
Another popular type of displacement compressor is the reciprocating compressor. These are typically found at smaller work sites such as garages and home construction projects. Unlike the rotary screw compressor, the reciprocating compressor is not designed for continuous use.
A reciprocating air compressor has more moving parts than a rotary screw compressor, and these parts are lubricated with oil for smoother movement.
These types of air compressors work via a piston inside a cylinder, which compresses and displaces the air to build pressure. Reciprocating compressors can come in single or multi-stage variations, which affects the pressure ranges they can achieve.
When you need more power, the multi-stage compressor is the way to go. While single-stage compressors will get the job done for smaller projects such as woodworking and metalworking, multi-stage compressors provide the power needed for intense construction, such as auto assembly and maintenance. Multi-stage reciprocating compressors can reach up to 30 horsepower.
Dynamic air compressors generate horsepower by bringing in the air with rapidly rotating blades and then restricting the air to create pressure. The kinetic energy is then stored as static within the compressor.
3. Axial Compressors
Axial air compressors are not typically used in construction projects but are instead found in high-speed engines on ships or planes. They have a high-efficiency rate but are much more expensive than other types of air compressors, and can get up to many thousands of horsepower, which is why they are mainly reserved for aerospace research.
4. Centrifugal Compressors
Centrifugal air compressors slow and cool the incoming air through a diffuser in order to build up potential energy. Because of the multi-phase compression process, centrifugal compressors are able to produce a high amount of energy in a relatively small machine.
They require less maintenance than the rotary screw or reciprocating compressors and some types can produce oil-free air. They are typically used for more demanding construction sites such as chemical plants or steel manufacturing centers, as they can reach around 1,000 horsepower.
Air Compressor Power Supply
Electric compressors are the most common models. They require less maintenance than gasoline-powered models, are quieter, and work in any dry area with a ready electrical supply. Electric compressors are suitable for working indoors.
Many home-use compressors function on a 120-volt household current, but larger models may have different requirements. A portable electric compressor requires a suitable extension cord, which limits mobility.
Depending on the model, inflators plug into a 120-volt household outlet or a 12-volt vehicle accessory outlet. Gasoline-powered air compressors are a good choice for outdoor work areas where electricity is limited or unavailable. They typically have more horsepower than electric models, so they can generate greater PSI.
Air Compressors and Air Tools
Powering air tools is the key function of an air compressor. Consider the tools you want to use now and those you might need in the future. Examples include:
- Nail and staple guns
- Impact wrenches
- Air hammers/chisels
- Paint sprayers
- Rotary tools/grinders
Most air tools have specific requirements for air volume and pressure. A compressor must meet these requirements for the tool to function properly. When choosing an air compressor, consider the tools you want it to power.
Determine which one requires the highest CFM at the highest PSI. Add 50% to the required CFM for a margin of safety, and look for a compressor that meets the requirement. For example, if a tool requires 3 CFM at 90 PSI, select a compressor that delivers at least 4.5 CFM at 90 PSI.
Air Compressor Features
In order to choose the right model, you should understand the air compressor features that can handle your jobs and simplify your work:
- An oil-free pump reduces maintenance and doesn’t mix oil into the compressed air.
- A belt-drive system provides quieter operation than a direct-drive system.
- Thermal protection stops the motor to prevent damage from overloads.
- Adjustable exhaust allows you to direct exhaust away from your work area.
- Multiple couplers allow you to handle different tasks, without connecting and disconnecting tools.
- Included accessories and tools such as hoses, nailers and blow guns add value to your investment. Not all compressors come with air hoses.
- You can purchase auxiliary air tanks to increase air storage capacity.
Applications of Air compressors
Air compressors have many uses, including supplying high-pressure clean air to fill gas cylinders, supplying moderate-pressure clean air to a submerged surface supplied diver, supplying moderate-pressure clean air for driving some office and school building pneumatic HVAC control system valves.
Supplying a large amount of moderate-pressure air to power pneumatic tools, such as jackhammers, filling high-pressure air tanks (HPA), for filling tires, and to produce large volumes of moderate-pressure air for large-scale industrial processes (such as oxidation for petroleum coking or cement plant bag house purge systems).
Air compressors are also widely used in oil and gas, mining, and drilling applications as the flushing medium, aerating muds in underbalanced drilling, and in air pigging of pipelines.
Most air compressors either are reciprocating piston type, rotary vane, or rotary screw. Centrifugal compressors are common in very large applications, while rotary screw, scroll, and reciprocating air compressors are favored for small and medium-sized applications.