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