Safety of Pressure Systems Regulations:
The Pressure Systems and Transportable Gas Containers Regulations 1989 has now been replaced. See Pressure Systems Safety Regulations 2000.
The term used to describe the filter inside an oil injected screw compressor which removes the hot oil from the hot compressed air. The separator element is a consumable component which traps oil (and dirt) particles down to 1 micron. This means that a separator element may last for years in a clean factory and only months in a dusty factory.
Short Term Exposure Limit (STEL):
These provide guidelines for the short tem exposure to a substance. A worker should not be exposed to a maximum STEL for more than 15 minutes, for 4 times per 8 hour shift. For example, the STEL for mineral oil vapour is 10 mg/cu.m. What concentration are you exposing your Service Engineers to? Also see TWA.
Systeme International. The international system of unit measurement.
This refers to the compressed air lost due to in-efficiency in the design of the compressor. In a reciprocating compressor, some of the air will seep back past the gaps in the piston rings. In a screw compressor, some air will slip back through the small gaps which exist between the screw compressor rotors. This loss is called slippage.
This is a safety relief valve which works backwards and is used in vacuum systems. Instead of venting compressed air out of the system like a normal safety relief valve, it lifts at a pre-set vacuum to allow air into the system thereby protecting the vacuum pump.
Sound Power Level (SPL):
By definition this is the measurement of the power source. The unit is the Watt, to take account of the range we select the picowatt as the reference standard. The power level doesn’t mean that much without knowing the surroundings. A 3 kW fire in a small room produces useful heat. The same 3 kW fire in the middle of a field is pretty useless. Also see SPL.
Sound Pressure Level (SPL):
This is a pressure and by definition must be measured as PSI or something similar. The units chosen are Newtons/sq.m. The exact figure is 2 x 10-5 N/sq.m. This is the unit that is more useful for the ear. It takes account of the surroundings. A gunshot in a cupboard sounds louder than the same gun being fired in a field, even though you may be the same distance away from the gun in each case. Also see SWL.
Specific Power Consumption:
A quick and easy way of comparing compressor efficiencies, as long as you have exactly the right information before you start. The total package power is divided by the actual volume delivered at a specified pressure. This provides a figure of xxx HP per cfm. Some people turn this figure around and quote xxx CFM per HP, or xxx Cu Metres/hour per kW.
A term used on ‘air movers’ such as axial or turbo compressors. Imagine a fan sucking air into a sealed room. Eventually the pressure in the sealed room equals the duty of the fan, although the fan continues to operate it cannot move any more air. The fan has reached its stall pressure.
Standard Temperature and Pressure. This was based upon a standard temperature of 62 deg F and a standard pressure of 14.7 psia, but for compressor testing purposes it has been unified and updated. See Standard Reference Atmosphere.
Standard Reference Atmosphere:
The Standard Reference Atmosphere to all intents and purposes replaces the old STP. Four committees were set up to define the standard, resulting in two different standards. I suppose it could have been worse. ISO8778 is the standard to which test results for compressors and other compressed air equipment is referred. This corresponds to an ambient inlet condition of 1 bara, 20 deg C and a 65% RH. ISO2787 is the standard for air tools and the like, it uses the same reference inlet conditions as ISO8778. The aerospace industry uses ISO2533 which corresponds to ambient inlet conditions of 1013 mbar, 15 deg C and 0% RH. The petroleum industry uses ISO5024 which uses the same reference inlet conditions as ISO2533. Also see CFM – Scfm.
This is indicated by a pressure gauge on a pipe or system. It is not necessarily the total pressure of the system, particularly if the velocities are high. See velocity head.
This is a term frequently used with centrifugal compressors and refers to the separation of air from the surface of the impeller or diffuser. Ideally the design of the compressor should be such that the air flows smoothly through the system, and the air will hug the surface of the impeller. To achieve this the angle of incidence must be zero, or marginally negative. If the angle of incidence becomes positive, then lift-off will occur which creates a wake in the moving air stream. This instability causes pressure fluctuations and hunting.