MadSci Network: Physics

Re: Joule-Thomson effect on water

Date: Thu Jun 1 14:09:13 2000
Posted By: Allan Harvey, Staff,National Institute of Standards and Technology
Area of science: Physics
ID: 957558161.Ph

The Joule-Thomson effect is a bit misunderstood.  People associate it with 
cooling, but lowering pressure at constant enthalpy (the definition of the 
effect) can produce either cooling or warming, depending on the fluid and 
its state.  For most vapors (including steam), Joule-Thomson expansion 
does lower the temperature.  But for liquids at temperatures well below 
their critical temperature, the Joule-Thomson coefficient is typically 
negative, meaning that a decrease in pressure actually raises the 
temperature.  So, if you are talking about taking pressurized cool water 
and lowering the pressure (say from 100 bar to 1 bar), you would actually 
raise the temperature by about two degrees.

Of course you can get cooling due to the latent heat if you lower the pressure enough so that the liquid evaporates. This evaporative cooling is the basis of the vapor-compression refrigeration cycle in common use. It is also the basis of industrial water cooling towers. But at the temperatures you talk about, getting water to evaporate requires either a substantial vacuum or else evaporating it into air of low humidity.

As for nozzles, you get the same effect as long as the pressure is dropped with negligible heat transfer to and from the fluid, so there is not really an "ideal" design from that standpoint. While I'm not familiar in detail with refrigeration equipment, I'm sure that those people do worry about nozzle design, but it would mainly be for other factors such as minimizing pumping power requirements and preventing clogs and leaks.

Allan Harvey, Physical & Chemical Properties Division, NIST "Don't blame the government for what I say, or vice-versa."


Admin note:
Oswald Campbell adds the following:

Joule Thompson effect When a perfect gas flows through a nozzle with constant inlet and outlet pressures, the temperature of the gas is the same before and after it flows through the nozzle. There is a temperature drop, however, during the passage of gas through the nozzle itself. At this point internal energy is transformed into kinetic energy with an accompanying temperature drop. However, for real gases there is a sustained change in temperature, even though the energy content of the gas remains constant. Water I assume would behave in a similar manner. I am told that there are heat pump systems (a method by which the heat in the water of a lake is used to heat a home even in the dead of winter) which function in this manner.

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