Air compressors take direct air and compress it to a desired pressure for an industrial or commercial application. There are three methods used for compressing air:
1. The piston compressor. This is the traditional method of having a piston press down on a given volume of air which is then fed into a container for storage. Piston compressors often have 2 or 3 pistons to reduce the pulsing effect as each piston releases its compressed air. The pulsing effect of the piston is a disadvantage and it is almost imperative to store the compressed air and remove it from a
container where the pulsing effect is minimized. The advantage of piston compressors is the high pressure possible (10 to 30+ atm, 147 to 450 + psi).
2. Rotary or screw compressors. Here two screws rotate within each other and the air trapped between the threads is compressed and discharged at one end.
The mechanical motion is smooth and in one direction. It is the preferred method for high volume, continuous operation. For most designs the maximum pressure created is around 14 atm (200 psi).
3. For very high volume of compressed air, centrifugal compressors should be considered. Here turbine blades rotate at high speed to compress the air. The amount of compression in a single stage is not high, so commercial centrifugal compressors come in 2, 3, and 4 stage designs. Air pressure available ranges from 2 to 10.5 atm (30 to 150 psi). The capacity of a piston or rotary compressor ranges from less than 5 hp to over 500 hp. They can be specified and installed with little assistance. Centrifugal compressors range from 200 hp to over 5000 hp and require assistance in the specification and installation. Centrifugal compressors are mentioned here but are not listed.
The piston and rotary (screw) compressors come in “oil free” and “oil inject” designs. The oil inject models use the oil to extend the product life due to less friction. The oil provides a better seal and gives higher pressure capability. But with improvements in material technology and precision machining, oil free compressors have become more reliable. Oil free is the preferred design in many industries, such as pharmaceutical and PET blow molding.