A working fluid is a fluid inside a closed system that facilitates its function, such as heating, cooling, or electricity generation. A simple example is the coolant in a refrigerator. The coolant passes through a series of tubes and transitions between different states in response to temperature changes. This allows for the transfer of heat away from the refrigerator to keep the contents cool and at a stable temperature.
Working fluids can take a number of forms. When designers develop a system, they consider the most appropriate working fluid for their needs. The fluid needs to have certain characteristics such as reliable performance within a given temperature range, minimal corrosive properties, and so forth. Data tables on materials can provide information about different available fluids and their properties to help designers decide on the best choice for their needs. It may be possible to run a system with different kinds of fluids, or it might have a design that only allows for a very specific fluid formulation.
The steam engine is another example of a system that relies on a working fluid. Heating water turns it to steam, generating pressure and creating energy to make electricity or drive an engine. The engine in this case needs a constant supply of new water, as the steam evaporates as it moves through the engine. Other working fluids work in a closed state and should not need refilling unless the system has a leak.
Hydraulic systems also rely on a working fluid. Brakes in vehicles use hydraulics to convert the pressure of the driver's foot into enough energy to stop the car. The pressure of the working fluid determines how effectively the brakes work. A hole in the system can create a leak, lower pressure, and make the car harder to stop, as the system does not have enough pressure to actuate the brakes. Similar hydraulic assist systems can be seen on heavy doors in some settings to make them easier to operate.
Some working fluids are potentially hazardous. It may be necessary to drain a system in an enclosed environment to capture the fluid and prevent contamination. Operators must also exercise care when working around the fluid and need to wash their skin carefully after interacting with it so they do not develop skin irritation. Older systems may use outdated working fluids that have been banned by the authorities in response to health and safety concerns, like freon in old refrigeration systems.