There are three major categories of compressors:
Centrifugal compressors produce high-pressure discharge by converting angular momentum imparted by the rotating impeller (dynamic displacement), as illustrated in the figure below. In order to do this efficiently, centrifugal compressors rotate at higher speeds than the other types of compressors. These types of compressors are also designed for higher capacity because flow through the compressor is continuous. Adjusting the inlet guide vanes is the most common method to control capacity of a centrifugal compressor. By closing the guide vanes, volumetric flows and capacity are reduced.
Centrifugal Compressor Impeller Diagram
Performance Curve for a Dynamic Displacement Machine
Reciprocating compressors are positive displacement machines, meaning that they increase the pressure of the air by reducing its volume. The relationship between pressure and volume is illustrated in the figure below. This compressor has a crankshaft, connecting rods, and pistons. Single-stage and two-stage reciprocating compressors are commercially available. Single-stage compressors are generally used for pressures in the range of 70 psig to 100 psig. Two-stage compressors are generally used for higher pressures in the range of 100 psig to 250 psig.
Reciprocating Compressor Schematic
A performance curve for a positive displacement machine is provided in the figure below. It can be seen that despite compressor discharge pressure, load remains constant. Load reduction is achieved by unloading individual cylinders. Typically, this is accomplished by throttling the suction pressure to the cylinder or bypassing air either within or outside the compressor. Capacity control is achieved by varying speed in engine-driven units through fuel flow control.
Positive Displacement Performance Curve
Screw compressors are also positive displacement machines that also follow the load profile of the figure directly above. Screw compressors consist of helically-grooved rotors that mesh together as shown in the Reciprocating Compressor Schematic, above. The volume of a trapped mass of air is decreased along the rotor and the pressure is increased. Capacity control for these compressors is accomplished by variable speed and variable compressor displacement. For the latter control technique, a slide valve is positioned in the casing. As the compressor capacity is reduced, the slide valve opens, bypassing a portion of the compressed air back to the suction. Advantages of the rotary screw compressor include smooth, pulse-free air output in a compact size with high output volume over a long life.
Double Helical Rotary Screw Compressor
Compressors may be driven by almost any prime mover a motor, steam turbine, combustion turbine, or internal combustion engine. Historically, electric motors have dominated the market. The use of natural gas engine drives has become a popular alternative. Gas engine-drive compressors can be used as the sole source of compressed air, or they may be used as part of a hybrid system. A hybrid air system is one that employs both electric motor and gas engine-driven air compressors.