Expansion factor
Expansion factor
(OP)
Can anyone comment on this;
I have a typical surge tank, out in the open, half filled with water and half with air.
The air volume is required to expand / contract in-line with the surge pressures that arise in the connected pipeline.
Does the expansion / contraction of the air follow an isothermal process or an adiabatic process or is it, in reality, somewhere in between ?
Would insulating the tank make any difference to this ?
Comments appreciated.
I have a typical surge tank, out in the open, half filled with water and half with air.
The air volume is required to expand / contract in-line with the surge pressures that arise in the connected pipeline.
Does the expansion / contraction of the air follow an isothermal process or an adiabatic process or is it, in reality, somewhere in between ?
Would insulating the tank make any difference to this ?
Comments appreciated.





RE: Expansion factor
Heat exchange between the lowest air level (where it touches the cooler water) would cool down the trapped air in any event over a period of an hour.
(If the expansion tank were vented, then there is no change in pressure of the air volume above the water.)
RE: Expansion factor
In any event, the only significant pressure build up will be from fluid entering the tank from the pipeline.
In rare ocassions, and when the tank is very large, the ambient temperature and sun radiation, might expand the gas and cause an overpressure. But this is very rarely an issue for the vessel itself. However, depending on the process line sensibility to pressures, it might be an issue.
Answering your question, it would be an adiabatic expansion or compression
<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying " Damn that was fun!" - Unknown>>
RE: Expansion factor
Best regards
Morten
RE: Expansion factor
rmw
RE: Expansion factor
PS: It is adiabatic compression
RE: Expansion factor
It's not a code issue that I have, it is one of modelling the behaviour of the air charge correctly. Within the surge analysis package I have there is an input box for the expansion factor. The software provider offers no real advice on what input to use other than to say to use a factor between 1.2 to 1.3 on the basis that in their view the process falls between a true isothermal one (factor=1) and a fully adiabatic one (factor=1.4).
I guess it is a safe option if you are not sure either way as the results obviously fall in the middle.
I was just trying to get a better handle on things and be more correct.
RE: Expansion factor
To answer your question:
Does the expansion / contraction of the air follow an isothermal process or an adiabatic process or is it, in reality, somewhere in between ?
ANSWER: It is virtually always somewhere in between.
Closer to adiabatic if the compression/expansion is fast, closer to isothermal if the process is slow. Of course, the influence of exterior heating or cooling of the expansion tank (as mentioned previously) could change the results. The mass (and specific heat) of the tank walls, and the heat transfer coefficient between the interior gas and the tank walls also play here.
Would insulating the tank make any difference to this ?
Insulating the tank would make a difference. However the mass of the tank itself (and the liquid inside the tank) may be more relevant for many configurations.
In theory, if you could have an expansion tank with zero mass, or zero specific heat, perfectly insulated, and perfect insulation between the expansion tank gas and the liquid inside the tank, then you would have a purely adiabatic expansion/compression.