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Compressed Air Glossary of Terms :: S ::

Glossary Index

Society of Automotive Engineers.

Safety valve
A device that limits fluid (liquid and gaseous) pressures by discharging some of the pressurized liquid or gas.

Safety relief valve
An automatic pressure relieving device actuated by the static pressure upstream of the device, which opens in proportion to the increase in pressure over the opening pressure.

Saturated air vapor mixture
Is one in which the space occupied by the mixture is saturated with water vapor at the mixture temperature.

Saturated vapor pressure
Is the pressure existing at a given temperature in a closed vessel containing a liquid and the vapor from that liquid after equilibrium conditions have been reached. It is dependent only on temperature and must be determined experimentally.

Occurs when the vapor is at the dew point or saturation temperature corresponding to its partial pressure. A gas in never saturated with a vapor. However, the space occupied jointly by the gas and vapor may be saturated.

Saturation pressure
Is another term for saturated vapor pressure.

A coating or precipitate deposited on surfaces such as water pipes, steam boilers that are in contact with hard water. Water that contains carbonates or bicarbonates of calcium or magnesium are likely to cause scale when heated.

Standard cubic feet per minute., SCFM or scfm. Flow of free air measured at some reference point and converted to a standard set of reference conditions (e.g., 14.4 psia, 80° F, and 60% relative humidity.) Scfm means cfm at standard conditions. However, standards vary and some care is necessary. In the United states, the usual standard is 14.696 psiA and 60°F. Some chemical engineers will use one ATA and 0°C, but usually will be specific about the reference point. Europeans normally use one ATA and 0°C. It is not the same to all people, therefore it is best that the reference pressure and temperature be definitely specified.

Screw compressor
Is a positive displacement rotary compressor.

Self Contained Underwater Breathing Apparatus

Sea level
This is the average level of the ocean over the entire earth., tidal fluctuation is taken into account when determining sea level.

Devices used between rotating and stationary parts to separate, and minimized leakage between, areas of unequal pressures.

The stationary seating surface, the inlet of a valve.

Second law of thermodynamics
Heat cannot, of itself, pass from a colder to a hotter body.

The order in which compressors are brought online.

Set pressure
The gauge pressure at which a safety valve visibly and audibly opens or at setting which a relief valve discharges an unbroken stream of liquid.

The part of the rotating element on which the rotating parts are mounted and by means of which energy is transmitted from the prime mover.

Shaft input
The power required at the compressor drive shaft. Losses in external transmissions such as gears and belt drives are not included.

Shaft sleeves
Devices that may be used to position the impeller or to protect the shaft.

Shaft speed irregularity
The dimensionless number obtained when the difference between maximum and minimum instantaneous shaft speeds during one period is divided by the arithmetic mean of these two.

Systeme International. The international system of unit measurement.

Single acting
The piston only compresses air with its stroke in one direction.

Single stage compressors
Machines in which air or gas is compressed in each cylinder or casing from initial intake pressure to final discharge pressure.

Silica gel
A desiccant most commonly used in heat regenerative type dryers.

Single stage centrifugal compressors
Machines having only one impeller.

Siphon feed gun
Any paint gun which uses air flowing over an opening to create a vacuum to draw paint up a tube to be atomized.

A type of journal bearing in centrifugal air compressors.

Is the internal leakage within a rotary compressor. It represents gas at least partially compressed but not delivered. It is experimentally determined and expressed in CFM to be deducted from the displacement to obtain capacity.

Slip RPM
Is the speed required of a rotary compressor to maintain a given discharge pressure, supplying leakage only (zero actual output). It is an experience factor.

A soft, black, mayonnaise-like deposit which is typically an emulsion of oil, water, and oil insolubles.

An air operated device for hoisting or pulling. Similar to a winch

Sole plate
A metallic pad, usually embedded in concrete, on which the compressor feet are mounted.

Sonic flow
The point (speed of sound) at which air flow through an orifice can not increase regardless of pressure drop.

Standard operating procedures for the production, purification, storage and distribution of compressed breathing air.

Specific Power Consumption.

Specific energy requirement
The shaft input per unit of compressor capacity.

Specific fuel consumption
The ratio of fuel consumption to compressor capacity.

Specific gravity
This property is the ratio of the specific weight of air or gas to that of dry air at the same pressure and temperature.

Specific heat
The quantity of heat required to raise the temperature of a unit weight of a substance by one degree.

Specific humidity
The weight of water vapor in an air-vapor mixture per pound of dry air.

Specific power
A measure of air compressor efficiency, usually in the form of bhp/100 acfm or acfm/bhp

Specific volume
Is the volume of a given weight of gas, usually expressed as cu ft/lb as SPT conditions.

Specific weight
This property of a gas is the weight of air or gas per unit volume. Unless specified, it refers in compressor practice to the weight per unit volume at conditions of total temperature, total pressure and composition which prevail at the compressor intake.

The number of revolutions per minute of the compressor shaft.

The rod connecting the disc to the lever on a valve.
Screw compressor development center. Licensor to 90% of twin screw compressor market.

Stability is 100 minus the surge limit at rated discharge pressure, where the surge limit is expressed in percentage of rated capacity.

Stack up
The interaction between the stages of a centrifugal compressor.  When a multi-stage compressor is designed, each stage can operate at only one point its characteristic curve. The point is determine by the design conditions of temperature, flow and pressure.  As the design conditions change the point on the stage characteristic curve will shift.  The interaction of each stage curve is referred to as the "stack up" of the compressor.

Steps in the compression of a gas, In reciprocating compressors, each stage usually requires a separate cylinder, in dynamic compressors, each requires a separate rotor disc.

Standard air
Air at a temperature of 68 °F, a pressure of 14.70 psia and a relative humidity of 36 per cent (0.0750 density) (as per A.S.M.E. however in the gas industry the temperature of standard air is usually given as 60 °F. (Unless specifically stated otherwise)

Standard pressure and temperature (SPT)
See standard air.

Start stop control (SPT)
A system in which air supply is matched to demand by the starting and stopping of the unit.

Of a system (or part thereof) is its condition at an instant of time as described or measured by its properties.

The rod connecting the disc to the lever on a valve.

Sterile air
Free from living organisms and especially micro-organisms. Usually used in food and dairy applications.

The unstable operation of a centrifugal compressor below the design point of the compressor.

Standard pressure and temperature. See standard air.

A device used to separate air solids from the stream of air that might become a source of trouble. Adulterants caught in the strainer are blown out through an orifice fitted with a valve or plug. The strainer should be opened periodically for a thorough cleaning.

Stroke total
The difference between the maximum extended height and the compressed height of an air actuator.

Stroke usable
That part of the total stroke which can be utilized repeatedly in actuator applications. It is measured starting at the compressed height and is the difference between the compressed height and the actuator height limit.

Are the connections between the headers and the work station.

Suction pressure
This is the pressure found on the suction side of a refrigeration system.

Supercompressibility factor
This is a factor expressing deviation of a gas from perfect gas laws.

Surface filtration
Filtration that occurs at the surface layer (as opposed to within the body depth) of the filter, and is accomplished by passing the material to be filtered over a grating, screen, sieve or membrane fabric with micro sized holes. The size of the holes in the filter determines what materials will pass through and which materials will be retained by the filter.

Is the reversal of flow within a dynamic compressor that takes place when the capacity being handled is reduced to a point where insufficient pressure is being generated to maintain flow. Also known as pumping.

Surge limit
In a dynamic compressor, surge limit is the capacity below which the compressor operation becomes unstable.

Swept volume
term mainly used by companies selling small compressors because it makes their compressors look bigger than they really are. The swept volume is the actual displacement of the piston, forgetting such losses as bumping clearances, valve clearances, ring losses and the like. It's not unusual to see an advertisement offering a compressor with (say) a delivery of 30 cfm, swept. In reality the compressor will only deliver about 20 scfm..

Synthetic lubricant
A lubricating oil made with synthetic base stocks.

Glossary Index

Thanks to Impact RM for permission to reprint the glossary.

Impact RM

Compressed Air Glossary Impact RM

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