Air-conditioning. The process by which the
heat and humidity of the air in an enclosed
space are maintained within certain limits, the
air may also be filtered and purified in the
process.
Air, latent heat in. The latent heat of vaporization of the water vapor in air.
Air, saturated. Air that contains all the
water vapor it can hold at a given temperature.
Air, sensible heat of. The heat of the air
only, without regard to the heat of any water
vapor that may be mixed with the air.
Air, total heat of. The sum of both the
latent heat and the sensible heat in any sample
of air.
British thermal unit. A measure of heat
quantity, defined as the quantity of heat required to raise the temperature of one pound
of pure water one degree Fahrenheit at 39.10 degrees
F. the amount of heat required varies slightly
at other temperatures, but for practical engineering purposes, is usually considered as constant. (Abbreviation, Btu.)
Condensation. The change of state from a
vapor to a liquid.
Dewpoint. The temperature at which water
vapor in any sample of air begins to condense.
Ebullition. Vaporization or change of state
from a liquid to a vapor in a rapid, active, and
visible process; also called boiling.
Evaporation. Vaporization or change of
state from a liquid to a vapor in a slow, inactive, and invisible process.
Freezing. The change of state from a liquid
to a solid.
Heat. A vibratory form of energy, perceptible to human beings as the sensations to which
the names cold, cool, warm, hot, and similar
terms are applied.
Heat, intensity of. That characteristic of
heat, the variations in which are indicated by
such terms as cold and hot, or measured by
numbers on the scale of a thermometer.
Heat, latent. The heat that changes the
physical state of a substance; see Latent heat
of vaporization and Latent heat of fusion.
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Heat load. The amount of heat to be removed from a space to be air-conditioned or
refrigerated to satisfy the requirements of the
given installation. The total heat load is the
sum of various separate heat loads coming
from such sources as: sensible heat, latent
heat, crew, electric lights, electrical equipment,
engines and stoves, sunlight, and foodstuffs.
Heat, quantity of. The quantity of heat
possessed by a substance depends on the size,
nature, and temperature of that substance; it
is measured in British thermal units.
Heat, sensible. The heat that raises the
temperature of a substance but does not
change its physical state. It is measured by
the thermometer.
Heat, specific. The heat required to raise
the temperature of a unit mass of a substance
one degree. Vapors and gases have two specific heats, one when the volume is held constant, and one when the pressure is held
constant.
Heat, total. In air-conditioning, the term
means the total heat, including both sensible
and latent heat, to be eliminated from an air
conditioned space. The term heat content is
frequently used as a synonym for it. In the
purely scientific field, the term total heat
means the total heat energy present in a substance on the absolute scale.
Heat transfer. The flow or transmission of
heat from a region of higher temperature to a
region of lower temperatures.
Heat transfer by conduction. The transmission of heat from one part to another of a
single body or substance; or from one body
or substance to another in contact with it.
Heat transfer by convection. The carrying
of heat from one location to another by the
molecules of a substance in motion.
Heat transfer by radiation. The transmission of heat energy through space by the
propagation of wave forms in a medium.
Humidity. Water in the physical state of
vapor mixed in the air. Humidity is invisible;
humidity does not include any water present
in liquid form, however finely divided, such as
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mist or fog. The space that drops of liquid
water occupy does not contain air.
Humidity, absolute. The weight of water
vapor, usually stated in grains per cubic foot
of air.
Humidity, relative. The ratio of the weight
of water vapor in a sample of air to the weight
of water vapor that same sample of air contains when saturated; usually stated as a
percentage.
Humidity, specific. The weight of water
vapor, usually stated in grains per pound of
dry air.
Insulator. Any material that transmits heat
at a very slow rate.
Latent heat of fusion. The heat that
changes the physical state of a substance from
a liquid to a solid, or from a solid to a liquid;
no temperature change is shown by a thermometer during the conversion process.
Latent heat of vaporization. The heat that
changes the physical state of a substance from
a liquid to a vapor, or from a vapor to a liquid;
no temperature change is shown by a thermometer during the conversion process.
Melting. The change of state from a solid
to a liquid.
Pressure. The result of the action of an
external force on an enclosed fluid, which may
be a liquid, vapor, or gas; usually expressed
in pounds per square inch.
Pressure, absolute. The true total pressure
inside an enclosed vapor system, that is, the
gage reading plus standard atmospheric pressure of 14.7 psi.
Pressure, atmospheric. The pressure exerted by the earth's atmosphere. For engineering purposes, usually taken as that which normally exists at sea level, 14.7 pounds per
square inch.
Pressure, back. In refrigeration, back pressure is the difference between the vapor
pressure in the suction line (including the
evaporator) and the head pressure of the
compressor.
Pressure, gage. The pressure indicated by
a pressure gage; the zero mark on a pressure
gage in reality means an interior pressure of
14.7 psi, corresponding to normal atmospheric
pressure.
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Pressure, head. The pressure produced by
the pistons of a compressor.
Pressure, negative. In engineering practice,
a pressure below normal atmospheric or 14.7
psi, is called a negative pressure, or partial
vacuum; usually stated in inches of vacuum.
Psychrometer. An instrument for deter
mining wet-bulb temperature. The form most
generally used, called the sling psychrometer,
has a wet-bulb and a dry-bulb thermometer
attached to a small support. By means of a
handle or chain, the device can be swung
around rapidly. The strong current of air thus
caused evaporates the moisture in the sleeve
of the wet-bulb thermometer, whereby the
wet-bulb temperature is indicated.
Refrigerant. A substance capable of carrying heat, which it picks up at a low temperature level, and is then compressed to a higher
pressure and temperature, where the heat can
be removed by the condensing medium, air
or water.
Refrigeration ton. A unit used in measuring the elimination of heat; one refrigeration
ton is the removal of the heat that would be
required to melt one ton of ice at 32 degrees F in
24 hours.
Saturation temperature. The temperature
at which a liquid substance boils under a
given pressure.
Specific volume. The number of cubic feet
occupied by one pound of a substance at a
given pressure and temperature.
State of a substance. A substance may
exist in three different physical states: solid,
liquid, and gaseous. For example, water (H2O)
can exist in all three states, depending upon
temperature and pressure: solid (ice); liquid;
and gaseous (steam, or water vapor). A substance may change its state on variation of
temperature or pressure, or both.
Superheat. The heat above saturation temperature in a vapor.
Temperature, dewpoint. The temperature
at which the water vapor present in the air
begins to condense, depending upon the
amount of humidity of the air.
Temperature, dry-bulb. The temperature of
the sensible heat of the air, as measured by an
ordinary, or dry-bulb, thermometer.
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Temperature, saturation. The temperature
at which a liquid substance boils under a
given pressure.
Temperature, wet-bulb. The temperature
indicated by a thermometer around the bulb
of which is placed a wet cloth sleeve; a strong
current of air, causing evaporation at the wet
bulb, lowers the thermometer reading by a
definite amount, called the wet-bulb depression, which depends upon the amount of
moisture present in the air.
Thermal capacity. The capacity, or ability,
of a substance to receive and store heat; equals
the specific heat of a substance times its mass.
Thermal conductance. The heat conduction
power of a substance; substances with a very
low thermal conductance are called insulators.
Thermometer, centigrade. An instrument
for measuring temperature in which the scale
is numbered zero at the freezing point of pure
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water and 100 degrees at the boiling point, at standard
sea-level atmospheric pressure,
Thermometer, Fahrenheit. An instrument
for measuring temperature in which the scale
is numbered 32 degrees at the freezing point of pure
water and 212 degrees at the boiling point, at stand
and sea-level atmospheric pressure.
Vapor, dry. A saturated vapor containing
no suspended mist or liquid.
Vapor, saturated. A vapor that is at the
temperature corresponding to the boiling point
of a substance at a given pressure.
Vapor, superheated. A vapor the temperature of which is above the boiling point of
the substance for a given pressure.
Vapor, wet. A saturated vapor that contains some suspended liquid in the form of
mist.
Vaporization. The change of state from a
liquid to a vapor.
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