page_title: Air compressor
section_title: Positive displacement
hierarchical_section_title: Air compressor / Displacement type / Positive displacement
caption_attribution_description: English: Technical Illustration of a two-stage air compressor
context_page_description: An air compressor is a device that converts power into potential energy stored in pressurized 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.
context_section_description: Positive-displacement compressors work by forcing air in a chamber whose volume is decreased to compress the air. Once the maximum pressure is reached, a port or valve opens and air is discharged into the outlet system from the compression chamber. Common types of positive displacement compressors are
  Piston-type: air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons. They use one-way valves to guide air into and out of a chamber whose base consists of a moving piston.  When the piston is on its down stroke, it draws air into the chamber.  When it is on its up stroke, the charge of air is forced out and into a storage tank. Piston compressors generally fall into two basic categories, single-stage and two-stage.  Single stage compressors usually fall into the fractional through 5 horsepower range. Two-stage compressors normally fall into the 5 through 30 horsepower range.  Two-stage compressors provide greater efficiency than their single-stage counterparts. For this reason, these compressors are the most common units within the small business community.  The capacities for both single-stage and two-stage compressors is generally provided in horsepower (HP), Standard Cubic feet per Minute (SCFM)* and Pounds per Square Inch (PSI).   *To a lesser extent, some compressors are rated in Actual Cubic Feet per Minute (ACFM).  Still others are rated in Cubic Feet per Minute (CFM). Using CFM to rate a compressor is incorrect because it represents a flow rate that is independent of a pressure reference.  i.e. 20 CFM at 60 PSI.
  Rotary screw compressors: use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, whose volume is decreased as the screws turn.
  Vane compressors: use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume. This type of compressor delivers a fixed volume of air at high pressures.
caption: Technical Illustration of a two-stage air compressor
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