Packaged terminal air conditioning
A combination heating and cooling unit designed for a single room or zone.
Package units
Complete refrigerating system including compressor, condenser, and evaporator located in refrigerated space.
Packing
Sealing device consisting of soft material or one or more mating soft elements. Reshaped by manually adjustable compression to obtain or maintain a leak-proof seal.
Partial pressures
Condition where two or more gases occupy a space and each one creates part of the total pressure.
Parts per million (ppm)
Unit of concentration of one element in another.
Pascal (Pa)
Unit of pressure in the metric system.
Pascal's Law
Pressure imposed upon a fluid is transmitted equally in all directions.
Passive solar heating system
A solar energy system that is dependent upon the radiation striking directly on the surface to be heated.
PCB
See Polychlorinated Biphenyl.
Peltier Effect
When direct current is passed through two adjacent metals, one junction will become cooler and the other will become warmer. This principle is the basis of thermoelectric refrigeration.
Perimeter drier
An electrical resistance heat wire located in a freezer door to prevent condensation on the exterior of the cabinet and around the freezer door.
Perimeter hot gas tube system
System that has a tube located on the surface of the outer portion of the cabinet to prevent condensation from forming.
Permanent magnet
Material that has its molecules aligned and has its own magnetic field; bar of metal that has been permanently magnetized.
Permanent split capacitor motor
A motor with no relay, in which current flows through both the starting and running winding, making the motor sensitive to line voltage and resulting in low starting torque.
Permeable
Having openings that allow the passage of liquid or gas.
PH
Measurement of the free hydrogen ion concentration in an aqueous solution. A pH of 7 is neutral.
Phase
Distinct functional operation during a cycle.
Phase loss monitor
Motor protection device for polyphase motors that measures current flow to detect phase loss.
Phial
Term sometimes used to denote the sensing element on a thermostatic expansion valve.
Photoelectricity
Physical action wherein an electrical flow is generated by light waves.
Photon
Particle of electromagnetic energy found in solar radiation.
Photostatically
Method by which the molecular formation of an element changes due to light.
Photovoltaic cell
See Solar cell.
Piercing valve
A type of service valve used on hermetic units.
Piezoelectric
Property of quartz crystal that causes it to vibrate when a high frequency (500 kHz or higher) voltage is applied. Concept is used to atomize water in a humidifier.
Piston
Close-fitting part or plug that moves up and down in a cylinder.
Pitot tube
Tube used to measure air velocities.
Planck's constant
Constant value (6.626 x 10-34 J s) which, when multiplied by the frequency of radiation, determines the amount of energy in a photon.
Plenum chamber
Chamber or container for moving air or other gas under a slight positive pressure.
Pneumatic system
An air conditioning system in which pneumatic motors are operated by pressurized air lines.
Pollen count
A measure of the amount of pollen in the air.
Polychlorinated Biphenyl (PCB)
Dielectric fluid used in capacitors and transformers that is very toxic. Use of PCB in transformers and capacitors is strictly regulated by the Environmental Protection Agency.
Polyphase motor
Electrical motor designed to be used with a three- or four-phase electrical circuit.
Polystyrene
Plastic used as an insulation in some refrigerated structures.
Polyurethane
Any synthetic rubber polymers produced from the polymerization of an HO and NCO group from two different compounds. Often used in insulation and molded products.
Ponded roof
Flat roof designed to hold a quantity of water, which acts as a cooling device.
Porcelain
Ceramic coating applied to steel surfaces.
Portable service cylinder
Container used to store refrigerant. Two most common types are disposable and refillable.
Positive pressure
A pressure greater than atmospheric.
Positive temperature coefficient thermistor (PTC)
Electronic thermistor that increases in resistance as temperature increases.
Potassium permanganate
Chemical used in carbon filters to help reduce odors.
Potential, electrical
Electrical force that moves, or attempts to move, electrons along a conductor or resistance.
Potential energy
Energy related to an object's position.
Potential relay
Electrical switch that opens on high voltage and closes on low voltage.
Potentiometer
Instrument for measuring or controlling by sensing small changes in electrical resistance.
Pound-force
Force applied to a 1-lb. Mass to give it an acceleration of 32.173 ft./82 (gravitational acceleration).
Pour point
Lowest temperature at which a liquid will pour or flow.
Power
1-Time rate at which work is done or energy emitted. 2- Source or means of supplying energy.
Power burner
A burner that has air blown into it by a blower.
Power element
Sensitive element of a temperature-operated control.
Power factor
Correction coefficient for the changing current and voltage values of ac power.
Power saver switch
A switch that disconnects heaters in a refrigerant cabinet.
Ppm
See Parts per million.
Precooler condenser
Used to cool the refrigerant prior to entering the main condenser.
Pressure
Energy impact on a unit area; force or thrust on a surface.
Pressure, absolute
See Absolute pressure.
Pressure, atmospheric
See Atmospheric pressure.
Pressure, back
See Back pressure.
Pressure cycling switch
Pressure-controlled switch located on the inlet line of the evaporator to prevent rapid cycling of the compressor.
Pressure drop
Pressure difference at two ends of a circuit, or part of a circuit.
Pressure gauge
Instrument for measuring the pressure exerted by the contents on its container.
Pressure, gauge
Pressure above atmospheric pressure.
Pressure, head
Force caused by the weight of a column or body of fluids.
Pressure-heat diagram
Graph of refrigerant pressure, heat, and temperature properties. (Mollier's diagram).
Pressure limiter
Device that remains closed until a certain pressure is reached, then opens and releases fluid to another part of system or breaks an electric circuit.
Pressure motor control
Device that opens and closes an electrical circuit as pressures change.
Pressure-Operated Altitude (POA) valve
Device that maintains a constant low-side pressure, independent of altitude of operation.
Pressure, operating
Pressure at which a system is operating.
Pressure regulator, evaporator
Automatic pressure regulating valve mounted in the suction line between the evaporator outlet and the compressor inlet. Its purpose is to maintain a predetermined pressure and temperature in the evaporator.
Pressure, suction
Pressure in low-pressure side of a refrigerating system.
Pressure switch
Switch operated by a change in pressure.
Pressure water valve
Device used to control water flow. It is responsive to head pressure of refrigerating system.
Primary air
In a combustion system, the air mixed with fuel prior to ignition.
Primary coil
A tube-and-fin circular coil that contains a water-glycol solution, which surrounds the ignitor and burner. This coil is used in a water-gylcol gas forced-air furnace.
Primary control
Device that directly controls operation of heating system.
Process tube
Length of tubing fastened to hermetic unit dome, used for servicing unit.
Product heat load
Sum of specific, latent, and respiration heat loads.
Products of combustion
The material produced when a substance is burned.
Propane
Volatile hydrocarbon used as a fuel or as a refrigerant.
Proportional
Being in the proper relative quantity or balance.
Protector, circuit
Electrical device that will open an electrical circuit if excessive electrical conditions occur.
Proton
Particle of an atom with a positive charge.
Psi
Pounds per square inch.
Psia
Pounds per square inch absolute. Absolute pressure equals gauge pressure plus atmospheric pressure.
Psig
Pounds per square inch gauge.
Psychrometer
Instrument for measuring the relative humidity of atmospheric air. Also called Wet Bulb Hygrometer.
Psychometric chart
Chart that shows relationship between the temperature, pressure, and moisture content of the air.
PTC
See Positive temperature coefficient thermistor.
Puffback
The ignition of vaporized oil in the firepot.
Pulley
Flat wheel with a “V” groove. When attached to a drive and drive members, the pulley provides a means for driving the compressor.
Pulse
Term referring to one cycle of ignition and combustion of a gas-air mixture in a pulse combustion furnace.
Pulse combustion process
Repeated ignition of a gas and air mixture in a high efficiency gas furnace.
Pulse furnace
Furnace that has a “tuned” (resonant) combustion chamber. Part of the energy normally lost through the flue is returned to start next “pulse” of combustion.
Pump
Any one of various machines that force gas or liquid into-or draw it out of-something as by suction or pressure.
Pump, centrifugal
Pump that produces fluid velocity and converts it to pressure head.
Pump, fixed displacement
A pump in which the displacement per cycle cannot be varied.
Pump, reciprocating single-piston
A pump having a single reciprocating (moving up and down or back and forth) piston.
Pump, screw
Pump having two interlocking screws rotating in a housing.
Pump down
The act of using a compressor or a pump to reduce the pressure in a container or a system.
Purging
Releasing compressed gas to the atmosphere for the purpose of removing contaminants.
Pyrometer
Instrument for measuring high temperatures.