In simple terms, air compressor functions by altering the pressure of a given medium (air) according to an application requirement. Compressors are utilized across numerous industries, including the petrochemical and hydraulic sectors. The standard devices available operate differently, although they are generally used for the same general purpose: pressurizing air. The three general types of compressors are reciprocating air compressors, rotary air compressors and centrifugal air compressors models. Among these types of compressors there several are subcategories to consider, as listed below.
Compressors are available as portable or stationary units.
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Portable units are utilized for smaller processes, and are efficient because they can be plugged in anytime the compressor is needed. They are employed in applications such as framing, roofing and to power pneumatic air tools.
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Stationary units provide compressed air to various points through fixed pipes that can be attached to the machines. These devices, which typically require a well lit, dry and clean area setting, are generally used for heavy duty applications.
Manufacturers design them as lubricated or oil-free machines.
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Lubricated unitsproduce an oil “bath” that lubricates bearings and the cylinder during the crank rotation process.
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Oil-free machinesdo not require external lubrication and theygenerally run hotter than oiled compressors.
Compressors are found as two-stage or single-stage apparatuses.
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Two-stage compressors are usually utilized for heavy duty applications. They typically generate higher levels of compression than single-stage units.
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Less heat is produced from a two-stage compressor than single-stage machines.
Horsepower is an important factor in determining the type of compressor needed for an application.
Types of Compressors
Reciprocating Compressors/Piston Compressors
Reciprocating air compressors function with the aid of pistons and are classified as positive displacement machines. These apparatuses function by storing compressed air inside of tanks. After the air pressure reaches a specified level (which may be preset or adjusted per requirement) the motor shuts off. Either one piston (a single processing unit) or more (double or multi-processing units) are stimulated inside a cylinder to raise gas pressure. A majority of the piston models feature an air tank and manufacturers produce them with an automatic shut-off feature as air is emptied and filled into the tank.
These compressors can be either air or oil cooled as they generate large amounts of heat. Manufacturers supply compressors with horsepower capabilities that generally span from 1-50 for smaller processes, but may exceed 1,000 horsepower for large applications. These devices are efficient for their inexpensive and initial cost and easy installation and do not generate an oil carry-over, which may tend to affect processes.
Rotary Compressors
Rotary air compressors are generally categorized as positive displacement devices. Horsepower ranges from 30 to 200, though some devices exceed that range for larger processes. Rotary compressors depend on two rotors which push gas into a chamber. This process increases the gas pressure as the chamber decreases in size. Rotary air compressors are available as either oil-free or lubricated units, yet some processes may require that the oil be removed from steamed air.
An upside of the rotary compressor device is the low installation and maintenance costs. The drawback of this design is the eventual need for replacement of the rotors. This device also has a short life span, and oil injected models have an oil carryover, which can pollute some processes.
Centrifugal Compressors (Dynamic Air Compressors)
Centrifugal compressors are considered efficient for high duty processes and have a horsepower capability of 100 to 1,000, though some versions may exceed this range, according to application load. They are equipped with high speed impellers that can reach over 50,000 rpms. These devices function differently than the previously mentioned rotary and reciprocating compressors. While the latter models function by compressing air from one volume to another, centrifugal units increase an air’s velocity after raising the pressure. This occurs via a rotor equipped with blades. The process itself involves air or gas being forced into an exterior chamber under a higher pressure.
Unlike the rotary and reciprocating compressor devices, there is a higher initial cost to install these machines, and their higher speed processing may require the purchase of specialized bearings. The upside of this model is that it is designed to produce oil-free air. In addition, because these models do not have pressure pulsations, they are quieter and run smoother than the rotary and reciprocating devices.
Considerations, Cautions and Maintenence
When selecting a compressor, the type of framing equipment to be utilized along with the size of the load of the process may be considered. A continuous run motor or a stop and start model (for processes such as grinders and sprayer pumps) is also an important factor in the compression process.
Some manufacturers advise that compressors that are left running for extended periods of time may overheat, which can cause failure or decrease their lifespan, although some inexpensive models are designed for roughly 100 hours of total operation. While some compressors have the potential to last up to a decade, maintaining the devices properly ensures a functional operation. Large units do not typically require overhauls, but medium-sized compressors require such maintenance every 3-5 years in operation. In the case of maintenance, manufacturers typically supply replacement parts, various types of rings (for piston models) and coolant kits.
