Editing Working fluid (section)

{{Short description|Pressurized gas or liquid in a heat engine}}
{{Refimprove|date=May 2010}}

For [[fluid power]], a '''working fluid''' is a [[gas]] or [[liquid]] that primarily transfers [[force]], [[motion]], or [[mechanical energy]]. In [[hydraulics]], [[water]] or [[hydraulic fluid]] transfers force between hydraulic components such as [[hydraulic pump]]s, [[hydraulic cylinder]]s, and [[hydraulic motor]]s that are assembled into [[hydraulic machinery]], [[hydraulic drive system]]s, etc. In [[pneumatics]], the working fluid is [[air]] or another gas which transfers force between pneumatic components such as [[compressor]]s, [[vacuum pump]]s, [[pneumatic cylinder]]s, and [[pneumatic motor]]s. In pneumatic systems, the working gas also [[energy storage|stores energy]] because it is compressible. (Gases also heat up as they are compressed and cool as they expand. Some gases also condense into liquids as they are compressed and boil as pressure is reduced.)

For passive [[heat transfer]], a '''working fluid''' is a gas or liquid, usually called a [[coolant]] or heat transfer fluid, that primarily transfers [[heat]] into or out of a region of interest by [[conduction (heat)|conduction]], [[convection]], and/or [[forced convection]] (pumped [[liquid cooling]], [[air cooling]], etc.).

The '''working fluid''' of a [[heat engine]] or [[heat pump]] is a gas or liquid, usually called a [[refrigerant]], coolant, or working gas, that primarily converts [[thermal energy]] (temperature change) into mechanical energy (or vice versa) by [[phase transition|phase change]] and/or [[heat of compression]] and expansion. Examples using phase change include water↔steam in [[steam engine]]s, and [[refrigerant]]s in [[vapor-compression refrigeration]] and [[air conditioning]] systems. Examples without phase change include air or hydrogen in [[hot air engine]]s such as the [[Stirling engine]], air or gases in [[heat pump and refrigeration cycle#Gas cycle|gas-cycle heat pumps]], etc. (Some heat pumps and heat engines use "working solids", such as rubber bands, for [[refrigeration#Elastocaloric refrigeration|elastocaloric refrigeration]] or thermoelastic cooling and [[nickel titanium]] in a prototype heat engine.)

Working fluids other than air or water are necessarily recirculated in a loop. Some hydraulic and passive heat-transfer systems are open to the water supply and/or atmosphere, sometimes through breather [[air filter|filters]]. Heat engines, heat pumps, and systems using volatile liquids or special gases are usually sealed behind [[relief valve]]s.