Thermodynamic problem, with time factor
Thermodynamic problem, with time factor
(OP)
Hello All,
I have a thermodynamic question. If you have steam at 365 F, 150 psi gage, total volume 2.3434 ft3. There are two vales on the tank 2" diameter pipe. I want to know how long it will take for the pressure to drop to 10 psi gage if the vales are opened. I really need a procedure to calculate this. I have looked at USUF processes etc in thermo books, but there are no equations trhat have time in it.
Please help
Thanks
...SPK
I have a thermodynamic question. If you have steam at 365 F, 150 psi gage, total volume 2.3434 ft3. There are two vales on the tank 2" diameter pipe. I want to know how long it will take for the pressure to drop to 10 psi gage if the vales are opened. I really need a procedure to calculate this. I have looked at USUF processes etc in thermo books, but there are no equations trhat have time in it.
Please help
Thanks
...SPK





RE: Thermodynamic problem, with time factor
now, just what is the downstream pressure - atmospheric or 10 psig? also, what is the final temperature? those two factors need to be considered!
you calc is straightforward, u just need to clarify the rest of the problem.
as far as a formula, any thermodynamics textbook has the energy formula.
if this is your homework, suggest you learn the definitions and learn how to apply them. perhaps i already provided too much info.
-pmover
RE: Thermodynamic problem, with time factor
RE: Thermodynamic problem, with time factor
It's less of a thermodynamic question, and more of the classical tank "blowdown" problem. Close the thermo book, and open the fluid flow book, and find one on chemical processes (I use Felder and Rousseau) or dynamic modelling. You need to write an unsteady-state mass balance around the tank. Characterize the mass in the tank and the flow out of the tank as a function of the tank pressure and the outlet valve and piping system. Combine these, solve the differential equation, and bada bing bada boom, you got the answer. If the 2" valves go straight to atmosphere, I'd guess the blowdown takes less than 10 seconds on that small tank.
Good luck,
Latexman
RE: Thermodynamic problem, with time factor
Use a spreadsheet and cacl. stepvise. Decrease timesteps until you have a reliable result.
Best regards
Morten
RE: Thermodynamic problem, with time factor
RE: Thermodynamic problem, with time factor
I am not a process engineer. You mention about spread sheet calculations. Are there any available on the web that would do something like this.
Thanks
RE: Thermodynamic problem, with time factor
Sounds like you are in over your head on this one. I recommend you farm it out.
Good luck,
Latexman
RE: Thermodynamic problem, with time factor
such as isentropic condition to estimate steam release within one control volume(ie) the vessel; the second control volume (ie) two gate valves -half opened- and 2" piping, 10'long discharging to atmosphere was considered isenthalpic in nature to apply the first law of thermo. and in which I used initial and final states of the vapor to determine the time element.
RE: Thermodynamic problem, with time factor
Could this be solved with the application of Bernoulli equation for solving unsteady state problems? If so go to http://www.svce.ac.in/~msubbu/FM-WebBook/Unit-II/Efflux...
RE: Thermodynamic problem, with time factor
0=d(mu)/dt + Wo*(u+Pv), where Wo= flow out = -dm/dt
for a perfect gas, Pv= ZRT, and u=Cp(T-T')
0= [2Cp(T-T')+ZRT]*dm/dt + mdT/dt
dT/dt / [2Cp(T-T')+ZRT] = -(1/m) dm/dt
the flow rate out can be calcualted via choked flow if the vale is the primary restriction, Wo=63.4*0.67*Cv*SQRT(Xt*Pi/sv,i), or if the flow is frictionally choked thru a long pipe then use the Fanno relationships.
RE: Thermodynamic problem, with time factor
RE: Thermodynamic problem, with time factor
The problem statement is a tank volume of 2.343 ft3; this is a constant volume problem.
If the steam is saturated, then the perfect gas equations are not used and the calculation procedure would need to use the steam tables to determine the amount of steam condensed during the depressurization.
RE: Thermodynamic problem, with time factor
RE: Thermodynamic problem, with time factor
RE: Thermodynamic problem, with time factor
Transient problems have been discussed on this site before. Search on keyword "Fliegner"; choose "Search Posts (keyword)" from the pull-down. You should get at least four other threads.