Alter exisitng heat exchanger conditions
Alter exisitng heat exchanger conditions
(OP)
Hi,
I've been asked to examine the theoretical output conditions of an existing HEX if the cold side input flow and temperature were to change.
I've simply used a temperuate gun on the HEX while it's been running to get the stream temperatures. The flows are recorded by flow meters. It only has one running condition at the moment.
Is it valid to assume constant UA? Altering the level of turbulence will surely change this significantly.
Thanks!
I've been asked to examine the theoretical output conditions of an existing HEX if the cold side input flow and temperature were to change.
I've simply used a temperuate gun on the HEX while it's been running to get the stream temperatures. The flows are recorded by flow meters. It only has one running condition at the moment.
Is it valid to assume constant UA? Altering the level of turbulence will surely change this significantly.
Thanks!





RE: Alter exisitng heat exchanger conditions
RE: Alter exisitng heat exchanger conditions
RE: Alter exisitng heat exchanger conditions
I guess my real question is, if all stream flow rates and temperatures are known, but ALL details of the HEX are not (tube count, heat transfer area etc), then single stream entering has its temperature and flow varied, is it possible to calculate the new output stream temperatures? How is the new UA value calculated?
If you assume temperature of a stream entering changes but flow rate doesn't, the output streams are strait forward to calculate since UA does not change.
I've just assumed constant UA in my calcs. Gives a reasonable result. Just curious how one would perform this task when constant UA isn't assumed.
Cheers!
RE: Alter exisitng heat exchanger conditions
RE: Alter exisitng heat exchanger conditions
RE: Alter exisitng heat exchanger conditions
Makes sense. Let me confirm:
1) 1/U = 1/h1 + 1/h2 + R
2) If stream 1 flow rate changes, then only h1 will change which directly related to velocity change of fluid and remaining values used to describe h1 remain constant (d, Cp, j, etc).
Hence, U = f(velocity)
3) Get HEX temperautres and flows at 2 conditions and find U = f(velocity) relationship
4) Calculate new theoretical output conditions
Yeah this confused me since I know absolutely no details of the HEX other than the input/output flows and temperatures.
Cheers
RE: Alter exisitng heat exchanger conditions
Best wishes,
Sshep
RE: Alter exisitng heat exchanger conditions
The relation is that Nu is directly proportional to h and Nu=a*(RE^b)*Pr(c)
where Re is flow dependant (and large temperature changes may also cause Re and Pr to change since there is a viscosity dependency
I would caution aginst assuming too much here.
RE: Alter exisitng heat exchanger conditions
Good luck
RE: Alter exisitng heat exchanger conditions
RE: Alter exisitng heat exchanger conditions
If you think the correlation obtained above is valid, you can simply extrapolate to get the UA to be used.
RE: Alter exisitng heat exchanger conditions
The general NTU method is as briefly as follow:
you determine the effectiveness of the HE as: e=q/q max
where q max= (C) min *(Thot,in- Tcold,in) where C min = mCp minimimum, it can be the one of the cold fluid or the hot fluid
Besides, NTU = UA / C min
Then once you calculate the e, depending on the type of HE you can calculate the NTU, based on the equations reported on the literature.
However according to your problem I think that is enough for you to determine the e for your HE, which does not change if the only modification to your HE are the T of the fluids, according to: q= e*(Cmin)*(Thot,in-Tcold,in)
Then you can use the regular eq: q= (mCp)(Tout,hot-Tin,hot) , same for the cold fluid.
Hope this helps.
Regards