Corporate Buyers Guide

Reciprocating piston air compressors are positive displacement machines, which means that they increase the pressure by reducing the volume of a gas. Reciprocating compressors have a piston driven by a crankshaft which is driven by an external motor. The piston moves up and down in a cylinder to compress the gas. As it moves down, the inlet valve opens to allow gas be drawn into the cylinder. When the downward movement stops, this valve closes and the upward movement of the piston begins. This is where the gas is compressed. When the piston nears the top of its movement, the outlet valve opens allowing the compressed gas to leave the cylinder.

Most piston compressors on the market today have two pistons with each one feeding into the compressed gas line or tank. The compressor feeds the compressed gas in short intervals causing pressure pulsation of the compressed gas. With two or more pistons feeding the compressed gas line, the pulsation becomes more of a ripple. It is always recommended that single or double pistons feed the compressed gas into a tank. Depending on the size of the tank, the pressure pulsation will be dampened or eliminated.

The storage tank allows the gas to cool down. The compression creates heat in the gas which should be dissipated. Also, the moisture in the gas drawn into the compressor leaves a high moisture level in the compressed gas. This, plus oil vapors from the compressor lubrication all can be turned to liquid and removed when the air cools down.   

Reciprocating compressors can be single stage, two stage, or multi-stage. Single stage compressors compress the gas using only one cylinder. Two stage compressors have two cylinders and take the compressed gas from the first cylinder and feed it into the second to compress it further. Multi-stage compressors have more than two cylinders in series, with each one taking the compressed gas from the cylinder before it and compressing it further

Like most complex products, rotary screw compressors have a number of options. First, there are the oil free versus regular lubricated models. Next, depending on capacity, they may be direct or belt driven. Then there is the constant vs variable speed option. The oil free compressors are pretty well standard in the food and pharmaceutical industries and to some extend with the companies that provide products to those industries. The other two options depend on the specific need of the customer.

When specifying a compressor, the output pressure and air capacity are the two main parameters. The pressure rating is in either psi or psig for US built rotary screw compressors. European built models would be rated in atm. For the US, the psig (psi gage) is the pressure above sea level and more accurate reading. Many catalogs say psi but actually mean psig. The atmosphere (atm) reading is the pressure above sea level. The capacity rating is stated as cfm, acfm or FAD. Acfm (actual cfm) and FAD (free air delivery) state the volume of air drawn into the compressor and not the volume of compressed air. Compressed air at 73 psig has about 1/5 of the volume of the air that entered the compressor. If the manufacturer rates their compressors in cfm, this point needs clarification.

Much of what has been said above, applies to any compressor. The characteristics of rotary screw compressors are first of all their high air capacity. The two screws meshing and continuously moving in one direction draw a lot of air into the screw cavities. As the air moves along, the cavities get smaller compressing the air.  Second, the screws have a long edge in contact with the outside walls which permits air leakage. Therefore the pressure rating of rotary screw compressors does not go much above 250 psig.