working fluid
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For
fluid power Fluid power is the use of fluids under pressure to generate, control, and transmit power. Fluid power is subdivided into hydraulics using a liquid such as mineral oil or water Water is an Inorganic compound, inorganic, Transparency an ...
, a working fluid is a
gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The molecules in a solid are closely packed together and c ...

gas
or
liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material density is constant within a fluid par ...

liquid
that primarily transfers
force In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force ...

force
,
motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon in which an object changes its position (mathematics), position over time. Motion is mathematically described in terms of Displacem ...

motion
, or
mechanical energy In physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "p ...
. In
hydraulics Hydraulics (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
,
water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , even though it provide ...

water
or
hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery Hydraulic machines use liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually De ...
transfers force between hydraulic components such as
hydraulic pump Hydraulics (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10. ...
s,
hydraulic cylinder A hydraulic cylinder (also called a linear ) is a mechanical that is used to give a unidirectional through a unidirectional stroke. It has many applications, notably in construction equipment (), , and civil engineering. Operation cylinder ...
s, and
hydraulic motor A hydraulic motor is a mechanical actuatorAn actuator is a component of a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular mach ...
s that are assembled into
hydraulic machinery Hydraulic machines use liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are a Pha ...
,
hydraulic drive system Hydraulic machines use liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers to a fluid flow, flow in which the material de ...
s, etc. In
pneumatics Pneumatics (from Greek ‘wind, breath’) is a branch of engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehic ...
, the working fluid is
air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...

air
or another gas which transfers force between pneumatic components such as
compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

compressor
s,
vacuum pump A vacuum pump is a device that draws gas Gas is one of the four fundamental states of matter (the others being solid, liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation ...

vacuum pump
s,
pneumatic cylinder animated pneumatic cylinder (Computer Aided Design, CAD) Image:Kolv (pneumatisk).PNG, Schematic symbol for pneumatic cylinder with spring return Pneumatic cylinder(s) (sometimes known as air cylinders) are mechanical devices which use the power o ...
s, and
pneumatic motor A pneumatic motor (air motor), or compressed air engine, is a type of which does by expanding . Pneumatic motors generally convert the compressed air energy to mechanical work through either linear or rotary motion. Linear motion can come from ...
s. In pneumatic systems, the working gas also
stores energy
stores energy
because it is compressible. (Gases also heat up as they are compressed and cool as they expand; this incidental heat pump is rarely exploited.) (Some gases also condense into liquids as they are compressed and boil as pressure is reduced.) For passive
heat transfer Heat transfer is a discipline of thermal engineering Thermal engineering is a specialized sub-discipline of mechanical engineering Mechanical engineering is an engineering Engineering is the use of scientific method, scientific pri ...

heat transfer
, a working fluid is a gas or liquid, usually called a
coolant A coolant is a substance, typically liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. Fluids are ...

coolant
or heat transfer fluid, that primarily transfers
heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these ...

heat
into or out of a region of interest by conduction,
convection Convection is single or multiphase fluid flow In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including ''aerodynami ...

convection
, and/or
forced convection Image:Schneekanone.jpg, Forced convection by a fan in a snow machine. Forced convection is a mechanism (engineering), mechanism, or type of transport, in which fluid motion is generated by an external source (like a pump, fan, suction device, etc.) ...
(pumped
liquid cooling Liquid cooling refers to cooling by means of the convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid In physic ...
,
air cooling Air cooling is a method of dissipating heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than Work (thermodynamics), thermodynamic work or Mass transfer, transfer of matter. The various ...
, etc.). The working fluid of a
heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these qu ...

heat engine
or
heat pump A heat pump is a system used to heat or cool an enclosed space or domestic water by transferring thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, ...

heat pump
is a gas or liquid, usually called a
refrigerant A refrigerant is a working fluid For fluid power, a working fluid is a gas or liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an appl ...
, coolant, or working gas, that primarily converts
thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concepts, such as the internal energy of a system; heat or sensible heat, which are defined as types of energy transfer (as is ...
(temperature change) into mechanical energy (or vice versa) by
phase changePhase change may refer to: * Phase transition, the transformation from one thermodynamic state to another. * Phase-change memory, a type of random-access memory. * Phase change (waves), concerning the physics of waves. {{disambiguation ...
and/or
heat of compression A compressor is a mechanical device that increases the pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, fr ...
and expansion. Examples using phase change include water↔steam in
steam engine from Stott Park Bobbin Mill, Cumbria, England A steam engine is a heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energ ...

steam engine
s, and
chlorofluorocarbon Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated Alkane, paraffin hydrocarbons that contain only carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatility (chemistry), volati ...
s in most
vapor-compression refrigeration Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant A refrigerant is a working fluid used in the refrigeration cycle of air conditioning Air conditioning (also A/C, air conditioner) ...
and
air conditioning Ceiling mounted cassette AC Air conditioning, often abbreviated as A/C or AC, is the process of removing heat In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work ...
systems. Examples without phase change include air or hydrogen in
hot air engine A hot air engine (historically called an air engine or caloric engine) is any heat engine In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to e ...
s such as the
Stirling engine File:Stirling Animation.gif, 136px, upBeta-type Stirling engine, with only one cylinder, hot at one end and cold at the other. A loose-fitting displacer shunts the air between the hot and cold ends of the cylinder. A power piston at the open end ...

Stirling engine
, air or gases in 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 valves.


Properties and states

The working fluid selection, working fluid's properties are essential for the full description of thermodynamic systems. Although working fluids have many physical properties which can be defined, the thermodynamic properties which are often required in engineering design and analysis are few. Pressure, temperature, enthalpy, entropy, specific volume, and internal energy are the most common. If at least two thermodynamic properties are known, the state of the working fluid can be defined. This is usually done on a property diagram which is simply a plot of one property versus another. When the working fluid passes through engineering components such as turbines and
compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

compressor
s, the point on a property diagram moves due to the possible changes of certain properties. In theory therefore it is possible to draw a line/curve which fully describes the thermodynamic properties of the fluid. In reality however this can only be done if the process is reversible process (thermodynamics), reversible. If not, the changes in property are represented as a dotted line on a property diagram. This issue does not really affect thermodynamic analysis since in most cases it is the end states of a process which are sought after.


Work

The working fluid can be used to output useful work (physics), work if used in a turbine. Also, in thermodynamic cycles energy may be input to the working fluid by means of a compression (physical), compressor. The mathematical formulation for this may be quite simple if we consider a cylinder in which a working fluid resides. A piston is used to input useful work to the fluid. From mechanics, the work done from state 1 to state 2 of the process is given by: : W = -\int_^ \mathbf \cdot \mathrm\mathbf where ''ds'' is the incremental distance from one state to the next and ''F'' is the force applied. The negative sign is introduced since in this case a decrease in volume is being considered. The situation is shown in the following figure: The force is given by the product of the pressure in the cylinder and its cross sectional area such that :\begin W &= -\int_^ PA \cdot \mathrm\mathbf \\ &= -\int_^ P \cdot \mathrmV \end Where ''A⋅ds = dV'' is the elemental change of cylinder volume. If from state 1 to 2 the volume increases then the working fluid actually does work on its surroundings and this is commonly denoted by a negative work. If the volume decreases the work is positive. By the definition given with the above integral the work done is represented by the area under a pressure–volume diagram. If we consider the case where we have a constant pressure process then the work is simply given by :\begin W &= -P \int_^ \mathrmV \\ &= -P \cdot \left(V_2 - V_1\right) \end


Selection

Depending on the application, various types of working fluids are used. In a thermodynamic cycle it may be the case that the working fluid changes state from gas to liquid or vice versa. Certain gases such as helium can be treated as ideal gases. This is not generally the case for superheated steam and the ideal gas equation does not really hold. At much higher temperatures however it still yields relatively accurate results. The physical and chemical properties of the working fluid are extremely important when designing thermodynamic systems. For instance, in a refrigeration unit, the working fluid is called the refrigerant. Ammonia is a typical refrigerant and may be used as the primary working fluid. Compared with water (which can also be used as a refrigerant), ammonia makes use of relatively high pressures requiring more robust and expensive equipment. In air standard cycles as in gas turbine cycles, the working fluid is air. In the open cycle gas turbine, air enters a compressor where its pressure is increased. The compressor therefore inputs work to the working fluid (positive work). The fluid is then transferred to a combustion chamber where this time heat energy is input by means of the burning of a fuel. The air then expands in a turbine thus doing work against the surroundings (negative work). Different working fluids have different properties and in choosing one in particular the designer must identify the major requirements. In refrigeration units, high latent heats are required to provide large refrigeration capacities.


Applications and examples

The following table gives typical applications of working fluids and examples for each:


See also

*List of gases *Water power engine


References

* {{Steam engine configurations, state=collapsed Engines Fluid mechanics Thermodynamics