ABSOLUTE HUMIDITY - The weight of water vapor in a given amount of air. Grains per cubic foot
ABSOLUTE PRESSURE - Pressure measured with a base of zero.
ABSOLUTE TEMPERATURE - A temperature scale expressed in degrees F or C using absolute zero as a base.
Referred to as the Rankin or Kelvin scale.
ABSOLUTE ZERO - The temperature at which molecular activity theoretically ceases. -456.69 F or -273.16
AIR CONDITIONING - The process of controlling the temperature, humidity, cleanliness and distribution of the air.
AIR, STANDARD CONDITIONS - Conditions at which BTU ratings for summer air conditioning equipment is rated. 95
F dry bulb, 75 F wet bulb at the condenser inlet and 80 F dry bulb, 67 F wet bulb at the evaporator inlet.
AMBIENT - Refers to the temperature surrounding a body or unit under test.
ATMOSPHERIC PRESSURE - The weight of a 1 square inch column of the earth's atmosphere. At sea level this
pressure is 14.696 pounds per square inch.
BIMETAL - Two metals with different rates of expansion fastened together. When heated or cooled they will warp and
can be made to open or close a switch or valve.
BOILING POINT - The temperature at which the addition of any heat will begin a change of state from a liquid to a
vapor.
BRITISH THERMAL UNIT (BTU) - The amount of heat necessary to change the temperature of 1 pound of pure
water 1 degree Fahrenheit (F).
CAPILLARY TUBE - A refrigerant control consisting of a small diameter tube which controls flow by restriction. They
are carefully sized by inside diameter and length for each particular application.
CENTIGRADE - A temperature scale with the freezing point of water 0 degrees and the boiling point 1000 at sea level.
CHECK VALVE - A valve designed to permit flow in one direction only.
COMPRESSION - The reduction of volume of a vapor or gas by mechanical means.
COMPRESSION RATIO - The ratio determined by dividing the discharge pressure, in PSI, by the suction pressure in
PSI.
COMPRESSOR - A mechanical device used to compress gases. Three main types are: reciprocating, centrifugal and
rotary.
CONDENSATION POINT - The temperature at which the removal of any heat will begin a change of state from a
vapor to a liquid.
CONDENSING MEDIUM - The substance, usually air or water, to which the heat in a condenser is transferred.
CONDENSING UNIT - The portion of a refrigeration system where the compression and condensation of refrigerant
is accomplished. Sometimes referred to as the 'high side'.
CONDUCTION - The transfer of heat from molecule to molecule within a substance.
CONTACTOR - An electromagnetic actuated relay. Usually used to refer to the relay which closes the circuit to a
compressor.
CONVECTION - The transfer of heat by a moving fluid.
COOLING ANTICIPATOR - A resistance heater (usually not adjustable) in parallel with the cooling circuit. It is 'on'
when the current is 'off", adding heat to shorten the off cycle.
COP - Ratio of work performed or accomplished as compared to the energy used.
CRANKCASE HEATER - Electric heating element that is used to heat the compressor crankcase to prevent migrating
refrigerant from condensing and diluting the crankcase oil during the off-cycle.
CUBIC FEET PER MINUTE - A common means of assigning quantitative values to volumes of air in transit, usually
abbreviated CFM.
CYCLE - The complete course of operation of a refrigerant back to a selected starting point in a system. Also used to
describe alternating current through 360 space degrees.
DENSITY - Mass or weight per unit of volume. For example, standard air = .075 pounds per cubic foot.
DISCHARGE LINE - A tube used to convey the compressed refrigerant vapor from the compressor to the condenser
inlet.
DISCHARGE PRESSURE - The pressure read at the compressor outlet. Also called head pressure or high side
pressure.
DRY AIR - Air which contains no moisture vapor.
DRY BULB TEMPERATURE - Temperature read with an ordinary thermometer.
EER [Energy Efficiency Ratio] - The EER rating is a steady state rating, and does not account for the time the unit
operates before reaching peak efficiency. This operating time has an unknown efficiency. It also does not account for
shutting the system down at the end of a cycle ( when the thermostat satisfies) leaving a cold coil in the duct. The
cold coil continues to absorb heat from the surroundings, not the conditioned space. Refrigerant equalizes to the cold
coil that must be pumped out at the beginning of the next cycle. This accounts for some inefficiency at the beginning
of the cycle.
EFFECTIVE TEMPERATURE - An arbitrary concept which combines into a single value the effect of temperature,
humidity, and air movement as sensed by the human body.
ENTHALPY - Total amount of heat in one pound of a substance calculated from accepted temperature base,
expressed in BTU's per pound mass.
EQUIVALENT LENGTH - That length of straight tubing which has the same pressure drop as the fitting, valve or
accessory (of the same nominal size) being considered.
EVAPORATIVE COOLING - The cooling effect of vaporization of a liquid in a moving air stream.
EVAPORATOR - A device in which a liquid refrigerant is vaporized. Some superheating usually takes place.
EVAPORATOR SUPERHEAT - The actual temperature of the refrigerant vapor at the evaporator exit as compared to
the saturated vapor temperature indicated by the suction pressure.
EXTERNAL STATIC PRESSURE - The sum of the static and velocity pressures of a moving air system at the point of
measurement.
FAHRENHEIT - A temperature scale with the freezing point of water 32 F and the boiling point 212 F at sea level.
FEET PER MINUTE - A term assigned to a velocity of a moving air stream, usually express FPM.
FILTER-DRIER - A device that removes moisture, acid and foreign matter from the refrigerant.
FLASH GAS - Instantaneous evaporation of some liquid refrigerant at the metering device due to pressure drop
which cools the remaining liquid refrigerant to desired evaporation temperature.
FREEZING POINT - The temperature at which the removal of any heat will begin a change of state from a liquid to a
solid.
GAUGE PRESSURE - Pressure measured with atmospheric pressure as a base.
HEAT - A form of energy causing the agitation of molecules within a substance.
HEAT EXCHANGER - A device for the transfer of heat energy from the source to the conveying medium.
HEAT FLOW - Heat flows from a warmer to a cooler substance. The rate depends upon the temperature difference,
the area exposed and the type of material.
HEAT OF COMPRESSION - The heat added to a vapor by the work done on it during compression.
HEAT OF THE LIQUID - The increase in total heat (Enthalpy) per pound of a saturated liquid as its temperature is
increased above a chosen base temperature. (Usually - 400F for refrigerants). It is expressed in BTU's.
HEAT TRANSFER - The three methods of heat transfer are conduction, convection and radiation.
HEATING SEASONAL PERFORMANCE FACTOR (HSPF) - The measure of seasonal or annual efficiency of a heat
pump operating in the heating mode. It takes into account the variations in temperature that can occur within a
season and is the average number of BTU of heat delivered for every watt-hour of electricity used by the heat pump
over a heating season.
INCHES OF MERCURY - Atmospheric pressure is equal to 29.92 inches of mercury.
LATENT HEAT - Heat that produces a change of state without a change in temperature; i.e., ice to water at 32 F or
water to steam at 212 F .
LATENT HEAT OF CONDENSATION - The amount of heat energy in BTU's that must be removed to change the state
of one pound of a vapor to one pound of liquid at the same temperature.
LATENT HEAT OF FUSION - The amount of heat energy, in BTU's required to change the state of one pound of a
liquid to one pound of solid at the same temperature.
LATENT HEAT OF MELTING - The amount of heat energy, in BTU'S, that must be removed to change the state of
one pound of solid to one pound of vapor at the same temperature.
LATENT HEAT OF VAPORIZATION - The amount of heat energy in BTU's required to change the state of one pound
of a liquid to one pound of vapor at the same temperature.
LIFT - To elevate a fluid from one level to a higher level.
LIQUID LINE - A tube used to convey the liquid refrigerant from the condenser outlet to the refrigerant control device
of the evaporator.
MANOMETER - A tube filled with a liquid used to measure pressures.
MBH - One MBH is equivalent to 1,000 BTU's per hour.
MEAN TEMPERATURE DIFFERENCES - The mean of difference between the temperature of a fluid receiving and a
fluid yielding heat.
MELTING POINT - The temperature at which the addition of any heat will begin a change of state from a solid to a
liquid.
MERCURY MANOMETER - Used to measure vacuum in inches of mercury.
MICRON - A unit used to measure high vacuums. One micron equals 1/25,400 of one inch mercury.
MUFFLER - Device installed in hot gas line to silence discharge surges.
OIL SEPARATOR - A device for separating out oil entrained in the discharge gas from the compressor and returning
it to the crankcase.
PARTIAL PRESSURE - The pressure exerted by any individual gas in a mixture.
PITCH - The slope of a pipe line for the purpose of improving drainage.
PITOT TUBE - A device comprising a small diameter orifice projecting directly into an air stream measuring total
pressure and surrounded by an annular section with small diameter entrances normal to the flow, measuring static
pressure; both sections are usually connected to a manometer to indicate velocity pressure.
PRESSURE DROP - The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct
or/and lift.
PRESSURE - TEMPERATURE RELATIONSHIP - The change effected in temperature when pressure is changed or
vice versa. Only used at saturated conditions. An increase in pressure results in a temperature increase. A decrease
in temperature results in a pressure decrease.
PUMPDOWN - Process of pumping refrigerant out of the evaporator and suction line at the end of the on- cycle by
closing a solenoid valve in the liquid line and letting the compressor shut-off by the low pressure control.
PSYCHROMETER - A devices having both a dry and wet bulb thermometer. It is used to determine the relative
humidity in a conditioned space. Most have an indexed scale to allow direct conversion from the temperature
readings to the percentage of relative humidity.
PSYCHROMETRIC CHART - A chart on which can be found the properties of air under varying conditions of
temperature, water vapor content, volume, etc.
RADIATION - The transfer of heat without an intervening medium. It is absorbed on contact with a solid surface.
RECEIVER - A vessel for holding refrigerant liquefied by the condenser.
REFRIGERANT - A substance which produces a refrigerating effect while expanding or vaporizing.
REFRIGERANT CONTROL - A device used to meter the amount of refrigerant to an evaporator. It also serves as a
dividing point between the high and low pressure sides of the system.
REFRIGERANT DISTRIBUTOR - A device which meters equal quantities of refrigerant to independent circuits in the
evaporator coil.
REFRIGERANT MIGRATION - The movement of refrigerant through the system to the compressor crankcase during
the off-cycle, caused by its attraction to oil.
REFRIGERANT OPERATING CHARGE - The total amount of refrigerant required by a a system for correct operation.
REFRIGERANT VELOCITY - The rate at which refrigerant is moving at a given point in a system, usually given in feet
per minute (FPM).
REFRIGERATION - The transfer of heat from a place where it is not wanted to a place where its presence is not
desirable.
REFRIGERATION EFFECT - The amount of heat a given quantity of refrigerant will absorb in changing from a liquid
to a vapor at a given evaporating pressure.
RELATIVE HUMIDITY - The percentage of water vapor present in a given quantity air compared to the amount it can
hold at its temperature.
RELAY - A device used to open and close an electrical circuit. The relay may may be actuated by a bimetal
electrically heated strip, a rod wrapped with a fine resistance wire causing expansion when energized, a bellows
actuated by expansion of a fluid or gas or an electromagnetic coil.
REVERSING VALVE - A device in a heat pump that is electrically controlled to reverse the flow of refrigerant as the
system is switched from cooling to heating; also called a four-way valve.
RISER - A vertical tube or pipe which carries refrigerant in any form from a lower to a higher level.
SEER [Seasonal Energy Efficiency Ratio] - The rating is not complete using system of EER so a rating of SEER has
been developed. This rating is tested and verified by a rating agency, and includes the startup and shutdown cycles.
The rating agency is the Air Conditioning Refrigeration Institute ( ARI )
SATURATED VAPOR - Vapor in contact with a liquid.
SATURATION - A condition of stable equilibrium of a vapor and a liquid. SENSIBLE HEAT
Heat that can be measured or felt. Sensible heat always causes a temperature rise.
SIGHT GLASS - A glass installed in the liquid line permitting visual inspection of the liquid refrigerant for the purpose
of detecting vapor in the liquid. They also generally have a device included to monitor moisture content of the
refrigerant.
SLUGGING - A condition in which a quantity of liquid enters the compressor causing hammering and possible
compressor damage.
SPECIFIC HEAT - The amount of heat necessary to change the temperature of one pound of a substance 10 F.
SPECIFIC VOLUME - The volume of a substance per unit of mass; i.e., standard air 13.33 cubic feet per pound. The
reciprocal of density.
STANDARD AIR DENSITY - .075 pounds per cubic foot. Equivalent to dry air at 70 F and at sea level pressure.
STATE CONDITION - Substances can exist in three states - solid, liquid or vapor.
STATIC PRESSURE - The normal force per unit area at a small hole in the wall of a duct.
STATIC TAP - A means by which static pressures of a duct system may be read directly, usually consisting of a small
diameter hole in the side of the duct connected to a manometer.
SUB COOLING - Cooling of a liquid, at a constant pressure, below the point at which it was condensed.
SUBLIMATION - A condition where a substance changes from a solid to a gas without becoming a liquid.
SUCTION LINE - A tube used to convey the refrigerant vapor from the evaporator outlet to the suction inlet of
compressor.
SUCTION LINE ACCUMULATOR - A device located in the suction line that intercepts quantities of a liquid refrigerant
and thereby prevents damage to the compressor.
SUPERHEAT - Heat added to a vapor after all liquid has been vaporized.
TEMPERATURE - A measurement of heat intensity.
THERMISTOR - Basically a semiconductor which has electrical resistance that varies inversely with temperature.
THERMOSTAT - A bimetal actuated switch to close and open a circuit to indicate or terminate operation of a heating
or air conditioning system.
THERMOSTATIC EXPANSION VALVE - Refrigerant control which monitors the flow rate according to the superheat
at the evaporator outlet.
TON OF REFRIGERATION - The amount of heat necessary to completely melt one ton of 32 F ice in 24 hours. 200
BTUs per minute, 12,000 BTUs per hour, 288,000 BTUs in 24 hours. This is based on the latent heat of fusion for ice
which is 144 BTUs per pound.
TOTAL HEAT (Enthalpy) - Total heat energy in a substance. The sum of sensible and latent heat.
TOTAL PRESSURE - The sum of all partial pressures in a mixture of gases.
TRAP - A depression or dip in refrigerant piping in which oil will collect. A trap may be placed at the base of a suction
or hot gas riser to improve oil return up the riser.
VACUUM - Any pressure below atmospheric pressure.
VAPOR BARRIER - The term applied to an impervious layer of material superimposed upon a layer of insulation.
Vapor barriers are always applied on the warm side of the insulation layer.
VAPOR PRESSURE - The pressure exerted by vapor.
VELOCITY PRESSURE - In a moving fluid, the pressure capable of causing an equivalent velocity as applied to move
the same fluid through an orifice such that all pressure energy expanded is converted into kinetic energy.
WATER MANOMETER - Used to measure pressure in inches of water.
WET BULB TEMPERATURE - Temperature read with a thermometer whose bulb is encased in a wetted wick
HVAC terminology made simple
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