Thermodynamic of Depressurising Water
Thermodynamic of Depressurising Water
(OP)
we're looking at temperature build-up in a pump minimum flow recycle loop and I've been reliably informed that the temperature of water will increase when water is depressurised i.e. when letting water pressure down from 100 bar to 30 bar the temperature could increase by 1 degC (I've used random figures here but you get the idea).
I've never come across this before, so I have the following questions for the forum:
- Has anybody come across this before?
- What're the thermodynamics behind it?
- And, most importantly, where can I do some background reading on it to fully understand it?
Thanks for your help.
I've never come across this before, so I have the following questions for the forum:
- Has anybody come across this before?
- What're the thermodynamics behind it?
- And, most importantly, where can I do some background reading on it to fully understand it?
Thanks for your help.





RE: Thermodynamic of Depressurising Water
Best regards
Morten
RE: Thermodynamic of Depressurising Water
I don't have a complete table of J-T coef for water, however, in general, it is possible for the J-T coef can be +,0, or negative.
Regards
RE: Thermodynamic of Depressurising Water
Brian Bobyk - Hoerbiger Canada
RE: Thermodynamic of Depressurising Water
The wikipedia article is very informative: http://
J-T coefficients of water and many other substances can be obtained from the NIST webbook at: http://webbook.nist.gov/chemistry/fluid/
RE: Thermodynamic of Depressurising Water
Best regards
Morten
RE: Thermodynamic of Depressurising Water
The process is not JT, as you are not expanding the liquid much, as does occur with a gas.
RE: Thermodynamic of Depressurising Water
remember our laws of thermo....
RE: Thermodynamic of Depressurising Water
As I am given to understand, practically all fluids (liquids included) undergoing a J-T throttling process, will heat up when their reduced temperatures are below 0.5, regardless of pressure.
RE: Thermodynamic of Depressurising Water
Where is the energy to shear coming from? From within the fluid itself. Your shearing explanation leads precisely to Joule Thomson effect.
It all depends on your initial state. Whether, J-T effect leads to cooling or heating simply depends on the sign of the J-T coefficient.
RE: Thermodynamic of Depressurising Water
And JT coefficient is just the thermo dyn. reduced to a constant. The cooling comes from expansion (PV=nRTZ) and the offset is due to the process as describe by FredRosse.
You e.g. get isentropic expasion (partially) in an oil/gas well - since some of the pressure difference from bottom to top comes from the staic collumn in the well. E.g. a 5000 metres deep well with an average density of 300 kg/m³ woudl experience 150 bar dP just because of the static column (grossly simplified since the density changes from bottom to top but still). Another (partly) isenthalpic process is expansion across an expander (as in a turbo-expander). Here the gas get colder because some of the work is "taken from the system" instead of heating the gas as it will in an expansion across a valve (as several others has explained)
Best regards
Morten
RE: Thermodynamic of Depressurising Water
RE: Thermodynamic of Depressurising Water
The valve is a resistance - and work is required to let down the pressure. The work dosnt "leave the system (fluid)" so the enthalphy is constant. The opposite is an ideal expander where all the work is "taken out of the system" (via the shaft) - in this system the enthropy is constant.
The two roads for expansion has a significant temperature difference for a gas - but not for a liquid (som but not a lot).
Best regards
Morten