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Glossary of Technical Terms - P

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Packaged Boiler: A boiler having all components assembled as a unit

Packaged Unit: A complete refrigeration or air conditioning system in which all the components are factory assembled into a single unit

Parallel: Circuit connected so current has two or more paths to follow

Parallel Circuit: An electrical circuit in which there is more than one different path across the power supply

Partial Pressures: Condition where two or more gases occupy a space and each one creates part of the total pressure

Pascal’s Law: A pressure imposed upon a fluid is transmitted equally in all directions

Perm: The unit of permeance

Permeance: The ration of water vapor flow to the vapor pressure difference

PH: A term based on the hydrogen ion concentration in water, which denotes whether the water is acid, alkaline, or neutral

Photo electricity: A physical action wherein an electrical flow is generated by light waves

Pilot: A small flame which is used to ignite the gas at the main burner

Pilot Generator: A device used to generate voltage in a millivolt heating system

Pilot Safety Valve: A valve that will shut off main gas if the pilot flame is not proved

Pilot Switch: A control used conjunction with gas burners. Its function is to prevent operation of the burner in the event of a pilot failure

Pinch-Off Tool: Device used to press walls of tubing together until fluid flow ceases

Piston: Close fitting part which moves up and down in a cylinder

Piston Displacement: Volume obtained by multiplying area of cylinder bore by length of piston stroke.

Pitch: Pipe slope

Pilot tube: A device that senses static and total pressure

Polarity: The condition denoting direction of current flow.

Polystyrene: Plastic used as insulation in some refrigerator cabinet structures

Pounded Roof: Flat roof designed to hold quantity of water

Potential: The amount of voltage between points of a circuit.

Potentiometer: A variable resistor

Pour Point (oil): Lowest temperature at which oil will pour

Pour Point (Oil): Lowest temperature at which oil will pour.

Power: Time rate at which work is done or energy emitted

Power Element: Sensitive element of a temperature operated control

Power Factor: The rate of actual power as measured by a wattmeter in an alternating circuit to the apparent power determined by multiplying amperes by volts.

Pressure: Force per unit area

Pressure Limiter: Device which remains closed until a certain pressure is reached and then opens and releases fluid to another part of system

Pressure Motor Control: A device which opens and closes on electrical circuit as pressures change to desired pressures

Pressure- Operated Altitude (POA) Valve: Device which maintains a constant low side pressure independent of altitude of operation

Pressure-Reducing Device: The device used to produce a reduction in pressure and corresponding boiling point before the refrigerant is introduced into the evaporator.

Pressure Reducing Valve: A diaphragm operated valve installed in the makeup water line of a hot water heating system

Pressure regulator: A device for controlling a uniform outlet gas pressure

Pressure Relief Valve: A device for protecting a tank from excessive pressure by opening at a predetermined pressure

Primary Air: The combustion air introduced into a burner which mixes with the gas before it reaches the port

Primary Control: One type of operating controller for an oil burner

Primary Voltage: The voltage of the circuit supplying power to a transformer

Process Tube: Length of tubing fastened to hermetic unit dome, used for servicing unit

Propane: A hydrocarbon fuel

Psi: Pressure measured in pounds per square inch

Psia: Pressure measured in pounds per square inch absolute

Psig: A symbol or initials used to indicate pressure in pounds per square inch gauge

Psychrometer or Wet Bulb Hygrometer: An instrument for measuring dry-bulb and wet-bulb temperature.

Pshychrometeric Chart: A chart that shows relationship between the temperature and moisture content of the air.

PTC: Positive temperature coefficient; a thermostat used as a start relay

Pump: A motor driven device used to mechanically circulate water in the system

Pump Down: A service procedure where the refrigerant is pumped into the receiver

Pump Head: The difference in pressure on the supply and intake sides of the pump created by the operation of the pump

Purging: Releasing compressed gas to atmosphere through some part or parts for the purpose of removing contaminants from that part or parts

Pyrometer: Instrument for measuring temperatures

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