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DENSITY is the weight of a given volume of gas, usually expressed in lb/cu ft at specific temperature and pressure.

DEW POINT of a gas is the temperature at which the vapor in a space (at a given pressure) will start to condense (form dew). Dew point of a gas mixture is the temperature at which the highest boiling point constituent will start to condense.

DISCHARGE PRESSURE is the total gas pressure (static plus velocity) at the discharge flange of the compressor. Velocity pressure usually is considered only with dynamic compressors.
Note: Pressure may be expressed as gauge or absolute pressures. psiG plus atmospheric pressure equals psiA. Note that psiG does not define a pressure unless the barometric pressure (atmospheric) is also stated.

DISCHARGE TEMPERATURE is the temperature existing at the discharge flange of the compressor.
Note: In a multistage compressor, the various stages will have different discharge pressures and temperatures.

DISPLACEMENT applies only to positive-displacement compressors. It is the net volume swept by the moving parts in a unit of time, usually one minute.

DRY-BULB TEMPERATURE is the ambient gas temperature.

DRY GAS is any gas or gas mixture that contains no water vapor and/or in which all of the constituents are substantially above their respective saturated vapor pressures at the existing temperature. (See Wet Gas).
Note: In commercial compressor work, a gas may be considered dry (even though it contains water vapor) if its dewpoint is low at the inlet condition (say minus 50° to minus 60°F).

DRY UNIT is one in which there is no liquid injection and/or liquid circulation for evaporative cooling or sealing.

ENERGY of a substance is its capacity, either latent or apparent, to exert a force through assistance, that is to do work.

INTERNAL ENERGY is that energy which a substance possesses because of the motion and configuration of its atoms, molecules, and subatomic particles.

KINETIC ENERGY is the energy a substance possesses by virtue of its velocity of motion. It enters into dynamic and ejector compressor calculations, but seldom into positive displacement problems.

POTENTIAL ENERGY is the energy a substance possesses because of its elevation above the earth (or above some other chosen datum plane).

ENTHALPY (Heat Content) is the sum of the internal and external energies.

ENTROPY is a measure of the unavailability of energy in a substance.

FIXED COMPRESSION RATIO is the design (built-in) compression ratio for a rotary unit having this feature.

GAS HORSEPOWER (See Horsepower)

GAUGE PRESSURE is pressure as determined by most instruments and gauges. Barometric pressure must be allowed for to obtain the true or absolute pressure.

GRAVITY (See Specific Gravity).

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