System depressurisation final temperature

[fdomin]

Hello,
I need another point of view to this well discussed subject.
When depressurisation takes place in a vessel, developed temperature inside the vessel can be very low due to a nearly isentropic expansion.
Is this theory also valid for more complex system consisting of piping and several vessels, all being depressurized through a single blowdown valve?
I would tend to consider that as gas flow from the system to the valve, flow regimen is turbulent with pressure drop and heavy mixing, consequently evolution would not be isentropic.
Is this approach more realistic or is something in between these two cases?.

Thanks

 

[sailoday28]

If you assume adiabatic flow and no friction from a finite reservoir(s) and consider everything in this reservoir(s)as homogeneous then the process in the reservoirs is isentropic.
If you consider friction (and/or heat transfer) in the piping downstream of the reservoir, then the downstream conditions are not isentropic.
If you consider friction and no heat transfer, then the process follows the FANNO lines with the piping being fed by isentropic reservoirs.

 

[fdomin]

Thanks sailoday28,
I had forgotten about Fanno lines.
I´m modelling a depressurisation system with Hysys and I find difficult to relate the theory to dynamic depressurisation utility.
This utility uses a "PV Work contribution term" which is more or less the isentropic efficiency of expansion.
So, how much "isentropic efficient" a depressurisation in these kind of systems would be?

 

[sailoday28]

I am not familar with "HYSYS".
IN GENERAL
If the program has the following capabilities then:
Mass out of the adiabatic, const volume source is isentropic. If perfect gas, constant specific heats, then Psource/(M source)^k =constant. Since volume is const.
Pick M(mass) and get new Psource.
M(mass) in source, however varies with time.
Use Fanno line with given mass flow rates w=dmass/dt)for given source stagnation pressures (Psource).
If I can be of further assistance-- Sailoday28@yahoo.com