Expansion valve

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An expansion valve (also called a throttle valve ) is a device that reduces the pressure of the fluid flowing through a local narrowing of the flow cross-section and thus causes an increase in volume or expansion. The pressure reduction takes place without dissipating work and is idealized without dissipating heat , i.e. thermodynamically isenthalpic .


Expansion valves can be regulated or unregulated.

A constriction of the pipeline represents an unregulated expansion valve. Such a constriction, also known as a throttle, is realized in the household refrigerator with a piece of pipe with a very small diameter, also called a capillary .

Regulated expansion valves are used in systems with greater capacity . The cross-section and thus the pressure drop are regulated according to a suitable reference variable . In the case of a refrigeration machine , such a reference variable can be, for example, the pressure in the downstream evaporator , which then results in an approximately constant temperature of the cooled medium.

Use in a compression refrigeration machine

In a cold vapor compression refrigeration machine, such as that used in a refrigerator or in many types of air conditioning , the expansion valve reduces the pressure of the working fluid ( refrigerant ), which usually enters the expansion valve as almost boiling liquid. Here it undergoes a change of state which, as a first approximation , can be assumed to be adiabatic isenthalpic ; this means that neither heat is exchanged with the environment (adiabatic) nor does the enthalpy of the fluid change (isenthalpic). Part of the fluid evaporates as it passes through the expansion valve, while the other part remains in the liquid state. The working fluid then enters the evaporator, where the still liquid portion absorbs heat from the environment and thus realizes the actual benefit of the refrigeration machine.

In large systems, the expansion valve is replaced by a turbine . Since a mechanical useful power is obtained with the turbine , the change of state when passing through the turbine is no longer isenthalpic.


  • Hans-Hermann Franzke: Introduction to machine and system technology. Volume 2 work machines, Springer Verlag Berlin Heidelberg, Berlin Heidelberg 1990, ISBN 978-3-540-50552-5 .
  • Rudolf Plank, Johann Kuprianoff: The small refrigeration machine. Springer Verlag Berlin Heidelberg GmbH, Berlin Heidelberg 1947.

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