The primary difference between pumps and compressors is the fluid they are designed to handle. Looking at the cross sections of pumps and compressors, it is clear they look similar. Pumps and compressors both come in reciprocating, rotary, centrifugal, mixed flow and axial flow types, but pumps are designed to move liquids against frictional or elevation changes and compressors are designed to compress, pressurize and move gases, which results in differing mechanical, hydraulic and pneumatic design considerations.
To further confuse the subject, the term “pump” is sometimes used generically to describe a device (e.g., a bicycle pump, air pump or heat pump) that moves a fluid that is not a liquid. The last example actually has a compressor as its central component but is referred to as a “heat pump” because it moves or pumps thermal energy from one location to another.
The physical properties of liquids and gases have differences in density, viscosity, specific heat and compressibility. It is likely obvious that gases have lower heat transfer capabilities and are lighter and less viscous than liquids, but the compressibility of gases and the incompressibility of liquids may not be commonly understood. A few ways that compressors have design differences than pumps are as follows:
- Compressors handling gas experience significant temperature increase due to the compression of the gas, which is not the case for pumps handling liquid. Therefore, compressors often require cooling systems to remove heat between stages.
- Compressors have a reduction in volume to account for the reduced volume of the gas as it is compressed, which is not the case for pumps.
- Compressors utilize seals that are specifically designed for gases. Specifically, mechanical seals that limit leakage along a rotating shaft are often characterized as noncontacting, which means they are more complex and use a buffer gas for lubrication and cooling.
- Compressors are more typically designed to operate intermittently, pressurizing a storage tank to a set level and then turning off. This results in stored pneumatic power that can be released when called for. Stored energy is not exclusive to compressors, but with pumps, it requires pumping the liquid to and storing it at higher elevation (such as in a water tower) so the energy can be released when called for.
- Designing for safety with proper controls to mitigate the release of energy is always important, but the compressibility of a gas requires greater caution because the potential energy stored in a relatively small volume of compressed gas is immense.
For more, visit the Hydraulic Institute and the Compressed Air & Gas Institute.
