6 replies to this topic

[mroy]

I am trying to design a heat exchanger for a client, but am getting stuck.

 

Parameters given by client are :-

 

                                         Shell Side                           Tube Side

 

Fluid                                    Water                                Water

Pass                                    1-pass                               1-pass or 2-pass

Flow                                     225 m3/hr                          300 m3/hr

Inlet Temp                            40                                       28

Outlet Temp                         32                                       32

Pressure                             3 kg/cm2                             Designer to specify.

 

Type of exchanger has to be shell and tube and vertical.

 

I have tried using HTRI but am getting 2-3 shell in series which I don't want. Can anyone help with advice?

 

Thanks

 

[chemproc]

I think you need more degree of freedom. This is what I mean. I would think you should be able to vary the flow on the shell and the tube side. Also your design would be easier if the outlet temperature of one of the stream is not specified.

[srfish]

You do not need more than one shell because there is no temperature cross. With these temperatures you will have a LMTD correction factor of approximately 0.8 for one shell pass and more than one tube pass. You will need more than one tube pass.

[curious_cat]

Does that even make thermodynamic sense? 

 

m Cp deltaT should match; Cp is roughly the same for both fluids. Your heat duties don't seem to match for tube & shell sides. 

[Raj Mehta]

based on the temperatures and the pressure, I am attaching the input & result sheet for your reference. 

 

Hope things are clear now. 

Attached Thumbnails

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[Art Montemayor]

CuriousCat is correct.  Your presumed design inputs are wrong.  There can't be heat exchanged without a heat balance - and your heat balance fails the test.  Refer to the attached spreadsheet.

 

Raj Mehta gives you the HTRI output.  HTRI is programned to do the heat balance automatically.  But Raj Mehta makes a rookie mistake in simply giving you the correct answer to the lack of a heat balance, and failing to tell you what is wrong with your post.   The correct engineering answer to your client is that he/she can't take 225 m3/h of water at 40 oC and heat up 300 m3/h of water at 28 oC with the given outlet temperatures.  either the temperatures change or the mass flow rates change - but the given conditions cannot be logically correct.

 

You don't need HTRI to tell you the input is illogical.  An engineer is expected to ensure that the input given to a computer program that is inherently stupid, is absolutely correct and logical.  Whether you do the basic, necessary logical calculations on a spreadsheet or on the back of an envelope, you should verify the input.  In the meantime, you have wasted HTRI computer time with useless input data.  That, in a nutshell, is why you are stuck.

 

 Heat UN-Exchanger.xlsx   661.72KB   488 downloads

[Raj Mehta]

Mr. Art, I did the basic Heat balance and it was very clear that the either of one of the flow-rate is wrongly specified.  I purposefully did not highlight it, because curious_cat had already highlighted that error in his post. Also I did mention in my post , that the results are developed on the basis of temperature and pressure, so I changed the flowrate of tube side by simple heat balance calculation. 

 

I did HTRI, considering tube side fluid as variable and keeping the same operating conditions of temp & pressure, what flow-rate of tube fluid would he require (which can also be done by mere heat balance) and the results of no of tubes, length, shell dia etc. as reference based on that flow-rate. 

Edited by Raj Mehta, 07 September 2013 - 05:04 AM.