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HALOGENATED HYDROCARBON: A chemical material that has carbon plus one or more of these elements: chlorine, fluorine, bromine, or iodine.

HEAD, STATIC: The static pressure of fluid expressed in terms of the height of a column of the fluid, or of some manometric fluid, which it would support.

HEAD, VELOCITY: In a flowing fluid, the height of the fluid or of some manometric fluid equivalent to its velocity pressure.

HEAT: The form of energy that is transferred by virtue of a temperature difference.

HEAT, LATENT: Change of enthalpy during a change of state, usually expressed in Btu per lb. With pure substances, latent heat is absorbed or rejected at constant pressure.

HEAT, SENSIBLE: Heat which is associated with a change in temperature; specific heat exchange of temperature; in contrast to a heat interchange in which a change of state (latent heat) occurs.

HEAT, SPECIFIC: The ratio of the quantity of heat required to raise the temperature of a given mass of any substance one degree to the quantity required to raise the temperature of an equal mass of a standard substance (usually water to 59oF) one degree.

HEAT, TOTAL (ENTHALPY): The sum of sensible heat and latent heat between an arbitrary datum point and the temperature and state under consideration.

HEAT EXCHANGER: A device specifically designed to transfer heat between two physically separated fluids.

HEAT OF FUSION: Latent heat involved in changing between the solid and the liquid states.

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HEAT OF VAPORIZATION: Latent heat involved in the change between liquid and vapor states.

HEAT PUMP: A refrigerating system employed to transfer heat into a space or substance. The condenser provides the heat while the evaporator is arranged to pick up heat from air, water, etc. By shifting the flow of air or other fluid, a heat pump system may also be used to cool the space.

HEPA FILTER (HIGH EFFICIENCY PARTICULATE AIR FILTER): A disposable, extended medium, dry type filter with a particle removal efficiency of no less than 99.97 percent for 0.3u particles.

HIGH LIMIT: The maximum desirable, or safe, temperature; an alarm given when this temperature is exceeded.

HORSEPOWER: Unit of power in foot-pound-second system; work done at the rate of 550 ft-lb per sec, or 33,000 ft-lb per min.

HOT DECK: The heating section of a multizone system; includes heating coil and duct.

HOT GAS BYPASS: The piping and manual, but more often automatic, valve used to introduce compressor discharge gas directly into the evaporator. This type of arrangement will maintain compressor operation at light loads down to zero by falsely loading the evaporator and compressor.

HUMIDIFIER: A device to add moisture to air.

HUMIDIFIER LUNG: A type of allergic lung condition common among workers involved with refrigeration and air conditioning equipment. The allergy is to two kinds of fungus, Micropolyspora and Thermoactinomyces. Symptoms of the short-term form of the disease include chills, cough, fever, difficult breathing, loss of appetite, nausea, and vomiting. The long-term form of the disease is known by fatigue, cough, weight loss, and difficult breathing during exercise. Also called air conditioner lung.

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HUMIDISTAT: A regulatory device, actuated by changes in humidity, used for the automatic control of relative humidity.

HUMIDITY: Water vapor within a given space.

HUMIDITY, ABSOLUTE: The weight of water vapor per unit volume.

HUMIDITY, PERCENTAGE: The ratio of the specific humidity of humid air to that of saturated air at the same temperature and pressure, usually expressed as a percentage (degree of saturation; saturation ratio).

HUMIDITY RATIO: The ratio of the mass of the water vapor to the mass of dry air contained in the sample.

HUMIDITY, RELATIVE: The ratio of the mol fraction of water vapor present in the air, to the mol fraction of water vapor present in saturated air at the same temperature and barometric pressure; approximately, it equals the ratio of the partial pressure or density of the water vapor in the air, to the saturation pressure or density, respectively, of water vapor at the same temperature.

HUMIDITY, SPECIFIC: Weight of water vapor associated with 1 lb weight of dry air, also called humidity.

HVAC: Heating, ventilation, and air conditioning. A system concerned with the temperature, humidity, cleanliness, and distribution of air.

HYDROCARBONS: the large class of organic compounds that contain both carbon and hydrogen. hydrocarbons include both aromatic and aliphatic compounds, many of which are associated with petroleum products.

HYGROSCOPIC: Absorptive of moisture, readily absorbing and retaining moisture.

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HYPERSENSITIVITY: An altered state or activity in an individual following contact with certain kinds of substances (inanimate or animate).

HYPERSENSITIVITY PNEUMONITIS: See Extrinsic Allergic Alveolitis.

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