Compressor Types .Top of Page
"Compressor bare blocks" or "compressor pumps" that are used commonly through out industry can be divided into four basic groups.
Piston type single stage
The piston or reciprocating compressor is available in either single or multi cylinder and manufactured from cast iron or aluminium.
This compressor pump is used on basically all portable compressor units and on small to medium size workshop compressors. The operating pressure is usually around 100 psi to 120psi (700kPa to 840 kPa) with a maximum pressure of 140psi (980 kPa)
Piston type two stage
This is a similar reciprocating compressor to the single stage type. However it has at least two cylinders. Air is taken into the first cylinder compressed and then discharged through an intercooler into the second cylinder which is of a smaller diameter. This is slightly more efficient than the single stage, particularly where higher pressures are required. The operating and maximum pressures can be higher than the single stage. The two stage compressor pump is used on medium to large workshop compressors and generally coupled to a three phase 415v motor.
This is a rotary compressor in which axial vanes slide radially in a rotor mounted eccentrically within a cylindrical casing. Available in lubricated and non-lubricated construction. The discharge air is normally free from pulsation. The vane compressor can be powered by three phase electric motor or diesel engine. Various operating pressure models are available.
This is also a rotary compressor. Two inter-meshing rotors, each in helical configuration displace and compress the air. Available in lubricated and non- lubricated construction. The discharge air is normally free from pulsation. The screw compressor can be powered by three phase electric motor or diesel engine. Various operating pressure models are available. Screw compressors are now becoming popular as workshop units because of their; efficiency, long life and low noise level.
Compressor Selection .Top of Page
A number of factors should be considered in the selection of a compressor. The primary factors are:
Volume of Free air required-cubic feet per minute(cfm) or liters per second(L/s) - with allowance for future expansion.
The style of compressor unit: Is the compressor to be portable or a fixed installation. If fixed is a three phase 415v supply available.
Estimated operating hours per day / week.
Minimum discharge pressure required to maintain an acceptable working pressure at the point of use.
Any special conditions the compressor must satisfy.
Compressor Air Terminology .Top of Page
The following is a glossary of terms used in association with compressed air.
(cfm) or (L/s). The volumeof air produced by a compressor is expressed in "cubic feet per minute" (cfm) or "liters per second" (L/s).
The conversion factor: cfm x 0.4719= L/s
In determining the (cfm) or (L/s) required refer to "free air delivered" FAD below.
(FAD) The "free air delivered" (FAD) referes to the volume of air actually delivered.
As a general rule smaller compressors, particulary portable 240v power units, express the cfm in piston displacement, which is the swept volume of the machine at a given speed. This is not true FAD. Larger machines either give both figures or disregard the piston displacement. As a guide it is generally assumed that most piston compressors deliver 3.3 cfm per input hp: eg. a compressor powered by a 7.5 hp motor would deliver approximatley 24 to 25 cfm.
(psi) or (kPa) The pressure the air is delivered at is expressed in "poungs per square inch"(psi) or "kilopascals" (kPa).
The conversion factor: psi x 6.9=kPa. If the compressor is to maintain the pressure required it will need to be able to supply the "free air delivered' (FAD) demand required from the installation. If this is not met the pressure will drop to much lower figures.
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