Passive Cooling - A process whereby a secondary refrigerant  is pumped directly to a evaporator and cooling is accomplished without a the operation of a mechanically driven refrigeration system.

Package Heat Pump - A refrigeration system that  has all required components  in one cabinet.

Packless Valve - A valve that does not use packing to prevent leaks.

Power - The rate of doing work measured in watts , horse power ...

Pour Point - The temperature at witch oil will not change is the container is tilted.

Performance Factor - The ratio of heat removed compared to the energy used to do it.

Photocell - A device that changes resistance with light.

Pressure - The force exerted per unit area.

Primary - The input side of a transformer.

Psychrometer - a device used to measure web bulb temperature.

Pyrometer - A device used to  measure high temperatures.

Psychometric Chart - A chart used to determine the specific volume , heat content. dew point, relative humidity , absolute humidity  and dry and wet bulb temperatures.

PSIG- Pounds per square inch gauge pressure.

PSIA- pounds per square inch absolute pressure .  PSIG + Atmospheric pressure (14.7)

Purge - A momentary opening to allow a fluid to escape

Pre Purge - The flushing of air through the combustion and heat exchanger prior to the burner firing.

Post Purge - The flushing of air through the combustion and heat exchanger after the burner has completed firing.

Primary Air - air that mixes with a fuel prior to the fuel being burnt.