Apart from regular Centrifugal and Reciprocating Compressors, there are other types of compressors which are frequently used in process industry. Here is a brief introduction to various types of these compressors.
Rotary Lobe Compressor
Rotary lobe compressor consists of two synchronized lobed rotors turning within a casing. These rotors do not touch each other or the casing. The outflow is not contaminated. Flow from these compressors is uniform.
Rotary Screw Compressor
Air or other gas enters pockets formed between mating rotors and a casing wall. The pockets rotate away from the inlet, taking the gas towards the discharge end.
The rotors do not touch each other or the casing wall.
Outflow is uncontaminated in the ‘dry type’ of machine, in which power is applied to both rotors thru external timing gears.
In the ‘wet type’, power is applied to one rotor, and both rotors are separated by an oil film, which contaminates the discharge.
Flow is uniform.
Liquid Ring Compressor
This type of compressor consists of a single multi-bladed rotor which turns within a casing of approximately elliptic cross section.
- A controlled volume of liquid in the casing is thrown to the casing wall with rotation of the vanes.
- This liquid serves both to compress and to seal.
- Inlet and outlet ports located in the hub communicate with the pockets formed between the vanes and the liquid ring.
- Special Advantage : Wet gases and liquid carryover including hydrocarbons which are troublesome with other compressors are easily handled.
- Additional cooling is seldom required.
- Condensible vapor can be recovered by using liquid similar to that in the ring.
- Flow is uniform.
- Mainly manufactured by Nash Engineering Company.
Rotary Scroll Compressor
A scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases.
The vane geometry may be involute, archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range.
Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls.
This type of compressor was used as the supercharger on Volkswagen G60 and G40 engines in the early 1990s.
Rotary Vane Compressor
Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape.
As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades.
- Rotary Vane compressors are, with piston compressors one of the oldest of compressor technologies.
- With suitable port connections, the devices may be either a compressor or a vacuum pump.
- They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines.
- Dry vane machines are used at relatively low pressures (e.g., 2 bar or 200 kPa; 29 psi) for bulk material movement while oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar (1,300 kPa; 190 psi) in a single stage.
- A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.
- Rotary vane compressors can have mechanical efficiencies of about 90%.
- Flow is uniform.
Reciprocating Diaphragm Compressor
A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane,
instead of an intake element.
The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in contact with the gas being compressed.
The degree of flexing and the material constituting the diaphragm affects the maintenance life of the equipment. Generally stiff metal diaphragms may only displace a few cubic centimeters of volume because the metal can not endure large degrees of flexing without
cracking, but the stiffness of a metal diaphragm allows it to pump at high pressures.
Rubber or silicone diaphragms are capable of enduring deep pumping strokes of very high flexion, but their low strength limits their use to low-pressure applications, and they need to be replaced as plastic embrittlement occurs.
Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications.
Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like airfoils to progressively compress the working fluid. They are used where there is a requirement for a high flow rate or a compact design.
- The arrays of airfoils are set in rows, usually as pairs: one rotating and one stationary.
- The rotating airfoils, also known as blades or rotors, accelerate the fluid.
- The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage.
- Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number.
- Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation.
- Axial compressors can have high efficiencies; around 90% polytropic at their design conditions.
- However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials.
- Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants.