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.
PRECHARGED LINES - Refrigerant line's which are filled with refrigerant and are sealed at both ends. The seals are broken when the lines are installed and the line charge becomes part of the total system charge.
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.
QUICK CONNECT - Name given to the end connections on precharged lines which screw on to mated fittings of the outdoor and indoor sections. Tightening the quick connections ruptures the seals on the fittings and the line charge becomes part of the total system charge.
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 undesirable.
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.
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 - 0.075 pounds per cubic foot. Equivalent to dry air at 70 oF 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 of fusion absorbed by melting 1 short ton (0.893 long ton or 0.907 t) of pure ice at 0 °C (32 °F) in 24 hours. Equivalent to the consumption of one ton of ice per day during the transition from stored natural ice to mechanical refrigeration.
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.