Re: Joule-Thomson effect on water

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."

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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.