Compressed Air Glossary of Terms :: S ::
- 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
- Is one in which the space occupied by the mixture is saturated
with water vapor at the mixture temperature.
- Saturated vapor
- 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
- Devices used between rotating and stationary parts to
separate, and minimized leakage between, areas of unequal
- The stationary seating surface, the inlet of a valve.
- Second law of
- Heat cannot, of itself, pass from a colder to a hotter
- The order in which compressors are brought
- 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
- Shaft input
- The power required at the compressor drive shaft. Losses in
external transmissions such as gears and belt drives are not
- Shaft sleeves
- Devices that may be used to position the impeller or to
protect the shaft.
- Shaft speed
- 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
- Single acting
- The piston only compresses air with its stroke in one
- Single stage
- 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
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
- 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
- Specific Power Consumption.
- Specific energy
- The shaft input per unit of compressor capacity.
- Specific fuel
- 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
- 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
- Screw compressor development center. Licensor to 90% of twin screw
- Stability is 100 minus the surge limit at rated discharge
pressure, where the surge limit is expressed in percentage of
- 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
pressure and temperature (SPT)
- See standard air.
- Start stop control
- 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
- 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
- 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.
- This is a factor expressing deviation of a gas from perfect
- 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.
Thanks to Impact RM for permission to reprint the glossary.
Compressed Air Glossary © Impact RM