Shell & Tube Heat Exchanger Design 3

[VARUN555]

Hello,

I am working on checking the design for a shell and tube heat Exchanger. We have 4 shell and tube heat exchangers to cool the quench tank water from 160 deg f to 120 deg f using two water chiller connected to 1st & 2nd as well as 3rd & 4th heat exchanger. The temperature of cold water is 80 deg f. The pump flow rate for hot water is 375GPM & that for the cold water is 40GPM & 75 GPM respectively. The problem is that the water temp of the quench tank after coming out of heat exchanger is 150 deg f. The area of the heat exchanger is correct as per the calculations. What measures I need to take to decrease the temperature of the quench tank. Do I need to increase the flow rate of the cold water and decrease the flow rate of the hot water or do I need to increase the quench tank area. Thank you.

 

[Dhurjati Sen]

Can't you increase the number of passes? Or you can't replace the exchangers as such and has been asked to carry out minor process changes?

DHURJATI SEN
Kolkata, India

 

[MJCronin]

VARUUN ....

IMHO, You are in a position where you need a "Heat Exchanger Network" software modelling tool.

There are several programs available, You may be able to get a STHX fabricator to model your particular system and evaluate practical alternatives

https://www.chemstations.com/Contac...kOckBXicCh4-ZHWnuKg_U0UL8CFrEbBoCCHcQAvD_BwE.

https://www.youtube.com/watch?reload=9&v=kuY2-dtXizU

Please also review the concept of practical limitations on heat exchanger "approach". You may be at the economic limits of a very small approach temperature ..... Being trapped by this thermal limitation is a commonly assigned problem for HX engineering newbies by many uncaring, arrogant MBA managers

https://www.enggcyclopedia.com/2019/05/heat-exchanger-approach-temperature/

Keep us posted about your final design solution and the reasons for selecting it..

Best regards

MJC

MJCronin
Sr. Process Engineer

 

[EdStainless]

Has this system ever worked?
You need to start with a basic heat balance. If you flowing 115gpm of water at 80F and trying to cool 375gpm of 160F water. So for every degree that the quench water cools your cooling water will heat 3 degrees. At first look I don't see how you can get the quench water below 140F under the best conditions?
It may be fouling on the tubes reducing heat transfer.
You do need to model this.
We need a layout of the system (flow paths, rates, temps).
What are the flow velocities in the tubes?
Lots of questions.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Latexman]

What tipped you off, a thermodynamically impossible scenario?

Heating - 375 gpm x 500 pph/gpm x 1 Btu/lb/F x (160-120 F) = 7,500,000 Btu/hr

Max. Cooling - (40+75 gpm) x 500 pph/gpm x 1 Btu/lb/F x (160-80 F) = 4,600,000 Btu/hr

Houston, we have a problem!

Good Luck,
Latexman

 

[TiCl4]

I was going to post the numbers, but Latexman beat me to it. You don't have enough chiller water flow to handle the heat load; you are running an ~61% deficit.

Oh, and that 61% deficit includes an infinitely large heat exchanger (approach temperature = 0 degrees). I have no idea what current square footage or Hx design you have, but with a more reasonable approach temperature you are likely going to need to double (or more!) your chiller water flow to handle the heat load and achieve a sufficient heat transfer coefficient with your given Hx area.

Also, what exactly does the quote below mean? If you had done any basic calculations you would have realized you are trying to do the impossible.

The area of the heat exchanger is correct as per the calculations...

What calculations???

 

[georgeverghese]

If your heat exchanger thermal design is correct, then it is likely you have fouling on the quench water side of the HXs'(salts dropping out with lower temp). Talk to to a water treatment specialist who may recommend some antiscale chemical for batch or continuous injection in to the quench water feed to the HX.

Can you show us the summary thermal design datasheet for the HXs'? From your post, it sounds like you have 2 sets of 2 HX's. i.e 2 in series, 2 in parallel on quench water side, total 4 shells?? What about arrangement on cold water side? A simple setup arrangement sketch of these units with flow paths may help.

 

[Latexman]

It could be a simple misunderstanding. A PFD with stream information might clear the matter up. A heat balance would be most excellent.

Good Luck,
Latexman

 

[VARUN555]

Hello,



Thank You for your response. Please find the attached Heat-Exchanger layout.

https://postimg.cc/V5DVtxWt

 

[Latexman]

Please indicate the stream flow rates and temperatures.

Good Luck,
Latexman

 

[georgeverghese]

So you've got:

On the quench water side: 2 shells in series on cold water side, followed by 2 shells in series on chilled water side
On the coolant side: 2 shells in parallel on the cold water side: 2 shells in parallel on chilled water side.

" The problem is that the water temp of the quench tank after coming out of heat exchanger is 150 deg f."

Coming out of which HX - the last one? And how did you calculate this?
What simulation program did you use, and did you correctly configure the mechanical setup in each HX in the simulator?