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HEAT is energy transferred because of a temperature difference. There is no transfer of mass.

HORSEPOWER is a unit of work equal to 33,000 foot-pounds per minute.

HORSEPOWER, THEORETICAL is the work theoretically required to compress and deliver a given gas quantity in accordance with a specified process.

HORSEPOWER, INDICATED is that obtained by indicator card analysis of compression or expansion of a cylinder of a reciprocating compressor. It is the same as gas horsepower.

HORSEPOWER, GAS is the actual work required to compress and deliver a given gas quantity, including all thermodynamic, leakage and fluid friction losses. It does not include mechanical losses.

HORSEPOWER, PEAK is the maximum power required by a given compressor when operating at a (1) constant discharge pressure with variable intake pressure, or (2) constant intake pressure with variable discharge pressure.

HUMIDITY, in normal usage, has to do with moisture (water vapor) in the atmosphere. There are two engineering terms involved:

HUMIDITY, RELATIVE is the ratio of the actual partial vapor pressure in an air-vapor mixture to the saturated vapor pressure at the existing dry-bulb temperature mixture.

HUMIDITY, SPECIFIC is the ratio of the weight of water vapor in an air-vapor mixture to the weight of dry air. It is usually expressed as pounds of vapor per pound of dry air.

IDEAL GAS follows the perfect gas laws without deviation. Practically, there are no ideal gases, but it is the basis from which calculations are made and corrections applied.


INLET PRESSURE is the total pressure (static plus velocity) at the inlet flange of the compressor. Velocity pressure is usually considered only with dynamic compressors. (See note under Discharge Pressure).

INLET TEMPERATURE is the temperature at the inlet flange of the compressor.
Note: In a multistage compressor, the various stages may have different inlet temperatures.

INTERCOOLING involves the cooling of gas between stages of compression (1) to reduce the temperature, (2) to reduce the volume to be compressed in the succeeding stage, (3) to liquefy condensable vapors, and (4) to save power.

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