HANDHOLE -- An access opening
in a pressure part that permits inspection or tube rolling.
HARD WATER -- Water which contains calcium or magnesium in an
amount which require an excessive amount of soap to form a lather.
HARDNESS -- A measure of the amount of calcium and magnesium
salts in water. Usually expressed as grains per gallon or ppm as CaCO3
or °dH.
HARTFORD LOOP -- A boiler piping configuration which
eliminates unsafe lowering of boiler water level due to back up of boiler water
into the return line.
HEADER -- A small sidewall drum into which tubes are rolled.
Used as distribution or collecting box for the steam generating tubes.
HEAT CAPACITY -- The amount of heat necessary to raise the
temperature of a given mass one degree. Heat capacity may be calculated by
multiplying the mass by the specific heat.
HEATING SURFACE -- Those surfaces which are exposed to
products of combustion on one side and water on the other. This surface is
measured on the side receiving the heat.
HEAT SINK -- Anything that absorbs heat. It is usually part of
the environment, such as the air, a river, or a lake.
HEEL -- The ship tilt to one side.
HFO -- Heavy Fuel
Oil. The heaviest commercial fuel that can be extracted from
crude oil.
HOTWELL -- A tank for the condensate returns and from which
the feed water pump takes its suction. The makeup water often goes into this
tank.
HUNTING -- An undesirable oscillation of appreciable
magnitude, prolonged after external stimuli disappear. Hunting is evidence of
operation at or near the stability limit.
HVAC -- Heating, Ventilation and Air Conditioning
HYDRAZINE -- A colorless fuming liquid, N2H4, derived from diazo acetic acid.
Used in boiler feedwater treatment, to reduce oxygen in the system, the normal
level of hydrazine reserve to be maintained in the water is between 0.1 and 0.3
ppm. The chemical reaction with oxygen can be expressed as: N2H4 + O2 ==> 2H2O + N2
Excess hydrazine, in a boiler, breaks down to produce ammonia that has
beneficial effects in raising the pH of the
steam/condensate system and thus affords some protection to
the system by neutralizing any carbon dioxide.
The reaction can occur in two ways:
3N2H4 ==> 4NH3 + N2
2N2H4 ==> 2NH3 + N2 + H2
Hydrazine should be handled with care since it is
allergenic and carcinogenic to humans.
HYDROKINETER -- A device formerly used in fire tube boilers to
improve the water circulation during firing up. A jet stream from a steam
nozzle near the boiler's bottom forces the colder water at the bottom to move
towards the water surface.
HYDRONIC HEATING -- A system that heats a space using hot
water which may be circulated through a convection or fan coil system or
through a radiant baseboard or floor system.
HYDROSTATIC TEST -- A strength and tightness test of a closed
pressure vessel by water pressure.
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