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Hi all,
I have a problem with calculating the unmeasured steam flow entering a falling film reboiler (forced) of a distillation column. The only corresponding measurements available are sump temperature, pressures of the steam supply side and after the control valve. My first guess was Bernoulli. However, I believe in this case, with the given steam supply and after valve pressure, Bernoulli equation would not give realistic solution for the condensing steam. I think, the system should be formulated with energy balance and heat transfer mechanism instead. But in this case there is an unknown heat transfer coefficient (U).
Energy balance: Q=m.deltaH=U.A.deltaT
deltaT=Tsump-Tsteam
Tsteam=f(psteam) from Antoine equation lnpsteam=A+B/C+Tsteam
U=unknown and should be estimated!
A= Area of the heat exchanger=known
deltaH=Enthalphy of vaporization=from the steam tables
I look forward to hearing your opinions/suggestions about the problem.
Erman
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Steam Flow Calculation With The Measured Steam Pressure
Started by Guest_Erman Sevinc_*, Feb 21 2006 03:57 PM
4 replies to this topic
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#2
Guest_Arvind Prasad_*
Guest_Arvind Prasad_*
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Posted 22 February 2006 - 09:16 AM
You are right that this can be solved only by heat balance. Bernoullis equation is used for isothermal conditions. To findout the U for your equipment you can read about unsteady state heat transfer in "Process heat transfer" by DQ Kern. You are welcome for further clarifications
Prof Arvind Prasad(India)
Prof Arvind Prasad(India)
#3
Guest_Kiran G Parihar_*
Guest_Kiran G Parihar_*
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Posted 23 February 2006 - 05:33 AM
If you have DP across the control valve and the details of control valve, you can calculate the flowrate using ISA 75.01 Control valve equations !!!. Why do you need to calculate via energy balance (maybe I did not got the question well)???
Kiran G Parihar
Kiran G Parihar
#4
djack77494
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Posted 23 February 2006 - 01:40 PM
Erman,
I cannot understand why a flow measuring instrument would not have been placed in your steam line. Lacking that, however, I think your choices are by heat balance or (possibly) by hydraulics. Whatever pressure measurements you have (sounds like just two) can determine pressure differentials. Then, if you know the resistance, you can calculate the flowrate. That may not be possible in your case because you must know the resistance. If your resistance includes a modulating valve that would be expected to be opening and closing, then the resistance is not fixed and this method will not work. Use the heat balance method until you get a chance to put a flow meter in the line.
BTW, Q = U.A.deltaT is not a energy balance equation; it is a heat transfer equation. Approaching the problem by heat transfer is difficult and error prone. I would recommend against it. I would think it easier to estimate the boil-up you are achieving in the reboiler, calculate the heat gain needed, and then calculate the steam needed to provide this heat. That's a true energy balance, and the heat transfer equations are totally avoided.
Doug
I cannot understand why a flow measuring instrument would not have been placed in your steam line. Lacking that, however, I think your choices are by heat balance or (possibly) by hydraulics. Whatever pressure measurements you have (sounds like just two) can determine pressure differentials. Then, if you know the resistance, you can calculate the flowrate. That may not be possible in your case because you must know the resistance. If your resistance includes a modulating valve that would be expected to be opening and closing, then the resistance is not fixed and this method will not work. Use the heat balance method until you get a chance to put a flow meter in the line.
BTW, Q = U.A.deltaT is not a energy balance equation; it is a heat transfer equation. Approaching the problem by heat transfer is difficult and error prone. I would recommend against it. I would think it easier to estimate the boil-up you are achieving in the reboiler, calculate the heat gain needed, and then calculate the steam needed to provide this heat. That's a true energy balance, and the heat transfer equations are totally avoided.
Doug
#5
proinwv
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Posted 23 February 2006 - 08:19 PM
Hi,
I agree with Kiran. If you can measure the inlet and outlet conditions to a control valve you can calc the flow from that. Your manufacturer should have a program to do so, with their valve constants included. The caveat is if the valve is not at 100% stroke it might be difficult to determine the Cv. I would consult them on that.
These calcs should be within 10%, if that is good enough.
Also, why not an orifice meter in the lnlet line do do the same, but with more accuracy?
I agree with Kiran. If you can measure the inlet and outlet conditions to a control valve you can calc the flow from that. Your manufacturer should have a program to do so, with their valve constants included. The caveat is if the valve is not at 100% stroke it might be difficult to determine the Cv. I would consult them on that.
These calcs should be within 10%, if that is good enough.
Also, why not an orifice meter in the lnlet line do do the same, but with more accuracy?
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