Glossary of Compressed Air Terms :: H :: Glossary Index A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Head adiabatic The energy in foot pounds required to compress adiabatically and to deliver one pound of a given gas from one pressure level to another. Header The main distribution pipe. Head polytropic The energy in foot pounds required to compress polytropically and to transfer one pound of a given gas from one pressure level to another. Head pressure A term used to describe the hot gas pressure on the outlet side of the refrigeration compression. Heat exchanger Is used to cool compressed air or gas. Designed to reduce the temperature and liquefy condensate vapors. Heatless dryer Heatless reactivated dryer. By means of expanding cold dry air to near atmospheric pressure inside the regeneration tower, the dryer air picks up moisture from the saturated desiccant bed and is then purged to atmosphere. Heat reactivated dryers Are categorized as internal or external heat reactivated. Internal type - Process air enters the dryer through the inlet piping, is then dried as it passes through the desiccant bed, and discharges through the outlet piping as dry air. Regeneration of the desiccant is accomplished at atmospheric pressure, using steam or electric heaters, embedded in the desiccant bed. External type - Process air enters the dryer through the inlet piping, is then dried as it passes through the desiccant bed, and discharges through the outlet piping as dry air. Reactivation is accomplished using a reactivation blower or a cooler. Heat recovery Recovering and utilizing the heat content of the compressed air. High efficiency filtration The unofficial filtration industry description for filters designed to trap small size contaminants. High volume low pressure In spray painting applications, Spray equipment which delivers material at a low pressure of no more than 10 PSI (at the air cap), however, with greater volume of air. Horsepower (HP) Is a unit of work equal to 33,000 foot pounds per minute, 550 foot pounds per second, or 746 Watts. Horsepower brake (BHP) The horsepower input to the compressor shaft, or more generally to any driven machine shaft. Horsepower gas 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 ideal The horsepower required to isothermally compress the air or gas delivered by the compressor at specified conditions. Horsepower indicated The horsepower calculated from compressor-indicator diagrams. Applied only to displacement type compressors. Horsepower peak 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. Horsepower theoretical The horsepower required to compress adiabatically the air or gas delivered by the compressor through the specified range of pressures. Hot gas A term applied to the gas found on the high pressure side of the refrigeration system. Hot gas bypass valve A valve which connects the high pressure side of a compressed air system to the suction side and is adjusted so as to maintain a specific pressure on the suction side by controlled bleeding of hot gas to the suction side. Hot start The compressor is started automatically, depending on demand. Control panel is energized with no "pre-start" cycle required, as pre-lubrication pump and buffer (seal) air are always "on". A state of pre-start exists. Steam turbine compressors are "slow-rolling" to maintain "pre-start" turbine temperatures at an adequate, recommended level. "Heavy" instrumentation and monitoring accessories are recommended. Housing A ported enclosure which directs the flow of a gas through the filter element. Humidity The moisture content of air. Humidity specific The weight of water vapor in the air vapor mixture per pound of dry air. Humidity relative The relative humidity of a gas (or air) vapor mixture is the ratio of the partial pressure of the vapor to the vapor saturation pressure at the dry bulb temperature of the mixture. HVLP In spray painting applications, spray equipment which delivers material at a low pressure of no more than 10 PSI (at the air cap), however, with greater volume of air. Hydrocarbons Chemicals containing carbon and hydrogen. Hydrogen chlorofluorocarbons (HCFC) Chemical species slated to replace CFCs in the near future. Hygroscopic cells Are any material with an affinity for moisture. These analyzers use sensing elements that contain moisture adsorbing material. A change in the moisture content of the element is detected by an electric network and is used as a measurement of dew point Glossary Index A B C D E F G H I J K L M N O P Q R S T U V W X Y Z