Wetted Surface Area
Wetted Surface Area
(OP)
All:
I am in the process of calculating surface area for 3 stacked shell & tube heat exchangers and hence calculate the relief rate. Shell length is 24ft and ID is 37".
Should I take my total length to be 72 ft(24*3) and ID -37" and determine the surface area for horizontal cylinder.
Also I am trying to determine the liquid flow rate for my tube rupture case, can anyone provide a method to determine this.........
Help would be appreciated.
Regards,
sp77
I am in the process of calculating surface area for 3 stacked shell & tube heat exchangers and hence calculate the relief rate. Shell length is 24ft and ID is 37".
Should I take my total length to be 72 ft(24*3) and ID -37" and determine the surface area for horizontal cylinder.
Also I am trying to determine the liquid flow rate for my tube rupture case, can anyone provide a method to determine this.........
Help would be appreciated.
Regards,
sp77





RE: Wetted Surface Area
Yes assuming tha the HX is liquid on the shell side then the area=area of all HX's.
Is it also liquid in the tubes?
Anyway: Decide how large your rupture is (a hole in the pipe or total sewerance óf the pipe and then do an "orifice calculation" to determine the flow rate with the downstream pressure equal to the PSV set point plus margin (10%) and the upstream equal the max presuure on the tube side and the tube ID (or hole size) as the orifice size. I would disregard any other dP since they most likely will be quite small compared to the loss in the rupture.
Best regards
Morten
RE: Wetted Surface Area
Using the orifice formula W = 1891*Y*d1^2*C*(dP*rho)^0.5, I calculated the flow rate to be 133,700 lbs/hr.
Also I read that as aconservative approach, the flow should be based thru 2 orifices. Could you please explain that.
If so then it would be aound 267,450 lbs/hr.
Again thanks for the help
Regards,
sp77
RE: Wetted Surface Area
In the first set of bullitpoints the "c" bullit reads:
"The high pressure fluid is assumed to flow both through the tube stub reamining in the tube sheet and through the other longer section of tube"
Disregarding the fact the flow from the opposite side oif the rupture will be less due to pressuredrop in the long pipesection is a conservative measure. If you are cutting it close then maybe you can get a somewhat ower flow from the "long" side - but also a more complicated calculation.
Read the section before (3.18.2) carefully with regards to set pressures etc.
Best regards
Morten
Best regards
RE: Wetted Surface Area
RE: Wetted Surface Area
Best regards
Morten
RE: Wetted Surface Area
Regarding your question about whether you should use 3*24=72 ft, the heat absorption equations were developed recognizing the fact that larger vessels were less likely to be completely engulfed by flames compared to a smaller vessel. Hence, the wetted area is raised to a fraction of a power, for example A^0.82 is used to determine total heat input, btu/hr.
What this means is you will get a lower heat input rate by considering your 3 exchangers as being one long exchanger because (3*24*37/12)^0.82 < 3*(24*37/12)^0.82. I believe the proper way to determine your heat input would be to consider the heat input to each exchanger separately using the latter approach.
RE: Wetted Surface Area
EGT01 -you are right I need to evaluate both the scenarios. Also I have one question , how do we determine whether the fluid is supercritical? And if it is how do I determine the vapor properties for my fire case scenaroio?
Regards,
sp77.
RE: Wetted Surface Area
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For information about how to handle relief of supercritical fluids, see the discussion in this thread...
thread1203-137692
Unfortunately, I see that the links are no longer working. The details of the article from Chemical Engineering Progress are...
"Rigorously Size Relief Valves for Supercritical Fluids", by Ryan Ouderkirk, Chemical Engineering Progress, August 2002
Keep in mind, that you may have a supercritical fluid upstream of your relief valve but you need to consider what happens to the fluid as it "depressures" through the relief valve into the relief valve discharge system.
RE: Wetted Surface Area