The main difference between pumps and compressors is that the fluid delivered by compressors -- air -- is compressed and under pressure at the time it is delivered, even if there is no load on the system. Most devices used to compress air are very similar in concept and -- perhaps even in hardware -- to hydraulic pumps, and selection considerations are similar.

The only other substantive difference is that most hydraulic systems are powered by a single pump that is actually a part of the system, whereas a host of pneumatic systems are often powered by a single compressor, which is almost a "utility" in the plant like water or electric service. Nevertheless, many small compressors are available for specific, discrete jobs; typically they are positive-displacement compressors. Dynamic, or nonpositive-displacement compressors are typically larger, facility-type units.

Compressors are fairly simple devices, capable of long periods of maintenance-free operation if properly integrated into pneumatic systems. Yet time and again they suffer from early failures because obvious precautions were ignored during system design. Four basic rules can provide substantial improvement in compressor life with only moderate design effort:

  • Pumps and compressors should be sized to provide at least the required pressure and flow, and preferably 10 to 25% more.
  • Filters should be selected to protect the pumping unit, and sometimes to protect downstream components or products as well.
  • Relief valves should be selected to keep pressure or vacuum at appropriate levels.
  • Pumping units should be placed in a clean, cool, dry environment.

Bellows compressors consist of a welded metal bellows connected to inlet and outlet ports with check valves. These compressors typically cover the pressure range up to 10 psig, and are used in pollution detecting and measuring devices, gas-sampling instruments, and medical applications. Lubrication is not needed, allowing high purities to be maintained.

Vane compressors are simple machines with few moving parts. Like their hydraulic counterparts, vane pumps, the compressors are inexpensive, with low operating cost, and low starting-torque requirement. They are compact and relatively vibration free, with little pulsation in the compressor output. The sliding vanes are closely fitted in the rotor slots and wear very little during operation. These compressors are available in power ranges from 10 to 500 hp, at pressures to 150 psi.

Reciprocating compressors consist of a piston moving within the cylinder to trap and compress the gas. In principle, such a unit is like an automobile engine, with the pistons compressing the gas and valves controlling its inlet and outflows. Sizes range from less than 1 to over 5,000 hp. Reciprocating compressors have good part load efficiencies and are useful for wide variations in operating conditions.

Diaphragm compressors are a modification of the reciprocating compressor. Compression is performed by the flexing of a metal or fabricated diaphragm which is caused by the motion of a reciprocating piston in a cylinder under the diaphragm. The space between the diaphragm and the piston is usually filled with liquid.

Lobed-rotor compressors have two rotating elements that revolve in opposite directions in a chamber. In most compressors, the rotors do not actually touch and do not drive each other, being driven instead by timing gears. Because the rotors do not actually touch, air leaks between them at a small but constant rate. This leakage, called "slip," is constant for a given compressor at a given pressure. For highest efficiency, these compressors should be operated at maximum speed. They are available in power ranges from 7 to 3,000 hp, delivering pressures to 250 psi. Because the internal lobes do not contact, they need no lubrication.

Liquid piston compressors have no moving parts in wearing contact. A rotor with multiple forward-curved blades rotates in an elliptical casing. Fluid, trapped within the casing, is carried around the inner periphery by the blades. Space between the blades changes volume due to the elliptical fluid path, and the inner surface of the liquid ring trapped between the blades serves as the face of a liquid piston. These compressors accept liquid slugs and fine particles without serious damage. Lubrication is required only in bearings located outside the pump housing. These compressors deliver up to 150 psi throughout the range of 10 to 500 hp.

Centrifugal compressors are best suited to moving large volumes of air at relatively low pressures. Basically, they consist of a high-speed rotating impeller, a diffuser section where velocity is reduced and pressure increased, and a collector section that further reduces velocity and increases pressure. Centrifugal compressors can handle high flow demands well, but when demand decreases much below rated flow and output pressure rises, the compressors can surge. In surge, the pressure field at the compressor outlet varies randomly. If allowed to continue, this condition can damage bearings, blades, and even the housing itself. Centrifugal compressors typically use from two to six stages, supplying from 400 to 3,000 cfm at speeds to 20,000 rpm.

Regenerative blowers (also known as peripheral blowers) use a disclike impeller with blades mounted around its outside edge. As the impeller revolves, air is drawn into the space between the blades. Centrifugal force moves the air in a spiral path outward to the housing, where it slips by the initial blade and returns to the base of the succeeding blade, where the process is repeated. In some models, a flow splitter creates two flow paths, so that the air must make two circuits around the impeller. In other models, the splitter is omitted, and the air makes only one circuit before exiting. Regenerative blowers provide air flows up to 1,000 cfm and pressures to 8 psi.

Helical compressors look like two giant screws meshing together; they work much like hydraulic screw pumps. Maximum pressure from these machines is approximately 125 psi in single-stage configurations. Helical compressors may be either oil flooded or dry.

Dry helical compressors, like lobed units, require timing gears to maintain proper clearance between the rotating elements. These units are most efficiently operated at high continuous speeds.

Flooded compressors do not require any timing gears, because the oil-laden screw surfaces can drive each other. However, oil separators are needed to remove the oil from the air as it leaves the compressor. They are available over a power range of about 7 to 300 hp.

Single-screw compressors are based on the same principle as helical compressors. As the central screw rotates, air trapped between the screw teeth is compressed against the star-shaped rotors. These compressors tend to have low vibration and noise levels, and low discharge pressures. Lubrication is required.