An air compressor is a device that converts power (usually from an electric motor, a diesel engine or a gasoline engine) into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts. There are numerous methods of air compression, divided into either positive-displacement or dynamic-displacement types.
Types of air compressor
Positive displacement compressors draw in and capture a volume of air in a chamber, then reduce the volume of the chamber to compress the air. Reciprocating Piston Compressors, Rotary Screw Compressors, Rotary Vane Compressors, and Scroll Compressors are all positive displacement compressors.
Piston compressors are further divided into two main categories – single acting and double acting.
Single Acting means that air is drawn in and compressed on one side of the piston. The other side is exposed to the crankcase of the compressor. In this case, the downward stroke of the piston draws in the air, and the upward stroke compresses it.
Double Acting reciprocating compressors have compression chambers on both sides of the piston. On the down stroke, air is drawn in on the top of the piston while air is compressed on the bottom side. On the upstroke, air is drawn into the bottom side while air is compressed on the top side. Double acting machines require sealing of the piston rod, so a crosshead is used to eliminate the angular movement of the rod.
Rotary screw compressors are presently the industry standard in plant air compressors from about 25 to 300 horsepower, and are rapidly expanding into both smaller and larger markets, with many manufacturers offering size ranges as low as 3HP on the low end, and over 600HP on the high end.
Rotary screw compressors draw air and lubricant into a void created as two helical rotors mesh together. Once the rotors pass by the inlet port of the pump (called an airend), the cavity decreases in size for the remainder of the rotation, compressing the air-oil mixture.
Rotary sliding vane compressors operate similar to an air motor, with an off-center rotor turning sliding vanes. As the vanes near the area where the distance between the rotor and casing is small, the air is compressed.
Other than the geometry, rotary vane compressors are very similar to rotary screw compressors. They are also oil injected, and require the same separators and oil system components. Separators are generally more marginally sized on vane compressors, leading to increased oil carryover similar to a piston compressor.
Rather than physically reducing the volume of a captured pocket of air, dynamic compressors instead speed up the air to high velocity, and then restrict the air flow so that the reduction in velocity causes pressure to increase. They are oil-free by nature, and some are oil-less. Dynamic compressors include axial and centrifugal types.
Axial compressors use a series of fan blades, similar in appearance to a jet engine, to force air into a smaller and smaller area. Its two primary advantages are straight-through airflow and high volume capability. They are primarily used in medium- to high-volume application applications such as gas turbine engines, natural gas pumping stations, and within chemical plants.
Centrifugal compressors draw in air to the center of an impeller, and then accelerate it outward toward its perimeter. There it impinges upon a diffuser plate and outlet scroll, where velocity decreases and pressure increases. Typical centrifugal compressors used for manufacturing are water-cooled and typically use two or three stages of compression. The impellers are driven at high speeds, up to 75,000 RPM, by lubricated gears. Some are direct-drive and powered by series-wound DC motors or universal motors due to their high-speed capabilities, similar to the suction fans used in vacuum cleaners.
Due to adiabatic heating, air compressors require some method of disposing of waste heat. Generally this is some form of air- or water-cooling, although some (particularly rotary type) compressors may be cooled by oil (that is then air- or water-cooled) and the atmospheric changes also considered during cooling of compressors.
- To supply high-pressure clean air to fill gas cylinders
- To supply moderate-pressure clean air to a submerged surface supplied diver
- To supply moderate-pressure clean air for driving some office and school building pneumatic HVAC control system valves
- To supply a large amount of moderate-pressure air to power pneumatic tools, such as jackhammers
- For filling tires
- 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), and
- To produce a water mists such as in a car wash.
Most air compressors either are reciprocating piston type, rotary vane or rotary screw. Centrifugal compressors are common in very large applications. There are two main types of air compressor's pumps: oil-lubed and oil-less. The oil-less system has more technical development, but is more expensive, louder and lasts for less time than oil-lubed pumps. The oil-less system is most often used in clean manufacturing or testing environments that require an oil-free environment.
- Klenck, Thomas. "How it Works: Air Compressor". Popular Mechanics. Retrieved 30 July 2010.