8 replies to this topic

[mrgeido]

Hi,

this may be a simple question but here goes:

with all other factors remaining the same on a tube and shell heat exchanger which would give more heat exchange:

a single tube or twin parallel tubes that are half the length of the single? To put it another way would it be better to be one pass or two passes given the same number of tubes

thanks

There was a post a while ago with six articles, one of which was dedicated to shell and tube heat exchangers. I'm attaching that article. I don't know if your question is answered explicitly, but there's quite a bit of info in there and I think it should give you the resources necessary to answer it.

[Art Montemayor]

mrgeido:

I’ll give this query a try, since I suspect what the real issue(s) are and what concern they could be posing for you. Please let me know if I’m off base.

The main issue is:

Q = (U) (A) (LMTD)

Where,
Q = Amount of heat rate transferred in the heat exchanger;
U = The over-all heat transfer coefficient pertaining to the heat exchanger;
A = The effective heat transfer area within the heat exchanger; and,
LMTD = The Log Mean Temperature difference across the heat exchanger.

Although you don’t state it, you basically are saying that in the case of two similar heat exchangers, Exchanger1 and Exchanger2:

A1 = A2

In other words, the effective heat transfer area within each heat exchanger is equal to the same value.

You are also changing the basic flow configuration between the exchangers. This effectively changes the LMTD in each, such that the LMTD value in each exchanger is different.

Normally what will happen when you change a 1-pass shell/1-pass tube exchanger into a 1-pass shell/2-pass tube unit is that you will increase the Reynolds Number and the fluid’s tube side turbulence. This effect is favorable for an increased tube side film coefficient which, in turn, yields a higher “U”. Unfortunately, you also take a hit on the LMTD – especially if you run into a temperature cross problem. However, more often than not, I’ve found that the heat transfer rate is increased – but at the cost of a pronounced pressure drop increase. (I hope you weren’t thinking of getting something for nothing …)

So to answer your basic query, “would it be better to be one pass or two passes given the same number of tubes”, the answer is: it depends. It depends if you can tolerate the higher pressure drop and if the amount of heat transfer increase is justified considering the lower LMTD, the pressure drop, and other potential effects such as temperature crosses, tube metal erosion caused by higher velocity, excessive tube vibrations, etc., etc.

I hope my response addresses what your query seems to lead to.

PS. I highly recommend the Walter Driedger article on heat exchanger controls. In fact, I highly recommend any of Walter's great control articles which can be found at his website. They should be mandatory reading for any engineer working in the process industry - regardless of discipline.

[Wendy]

Hi Art,

I have sent you a msg asking if I could please have the link to the Walter Dreidger's website, as I failed searching it on Google.

Then I realised I should have asked you by replying this post. so that other people who have the same query can benefit from here ( although i might be the only one....)

Thanks a lot

[JoeWong]

Hi Art,

I have sent you a msg asking if I could please have the link to the Walter Dreidger's website, as I failed searching it on Google.

Then I realised I should have asked you by replying this post. so that other people who have the same query can benefit from here ( although i might be the only one....)

Thanks a lot

Wendy,
http://www.driedger....tx/CE3_STX.html

Hope this help,
JoeWong

[mrgeido]

Hi Art ,

many thanks for the reply, I think I follw your answer...

I believe I can tolerate the pressure drop so will give that option a go.


best regards

Tim

[Art Montemayor]

Wendy:

I sincerely apologize for not specifically giving everyone Walter Driedger's URL. Fortunately, Joe is on top of the situation and gave you the correct URL for Walter's heat exchanger paper. I take this opportunity to thank Joe for his alert action and interest. As you have probably noted, Walter also has published important information about other controls:

0 INTRODUCTION

1 CONTROLLING CENTRIFUGAL PUMPS
First published in Hydrocarbon Processing, July 1995

2 CONTROLLING POSITIVE DISPLACEMENT PUMPS
First published in Hydrocarbon Processing, May 1996

3 CONTROLLING SHELL and TUBE HEAT EXCHANGERS
First published in Hydrocarbon Processing, March 1998

4 CONTROLLING STEAM HEATERS
First published in Hydrocarbon Processing, November 1996

5 CONTROLLING FIRED HEATERS
First published in Hydrocarbon Processing, April 1997

6 CONTROLLING VESSELS and TANKS
First published in Hydrocarbon Processing, March 2000

And all of these can be found by keying in "HOME" on any of his website pages - like the heat exchanger page.

I think you sense the importance of absorbing such knowledge and the potential strength it can lend you in your projects. I would hope that other engineers take that same outlook and profit from soaking up the valuable information that Walter has put out there.

P.S.: Walter has visited our website and even posted a response on one thread that I know of. It is my hope that he continues to visit in the future and perhaps share some of his experienced and valuable knowledge with our young engineers.

[Wendy]

Thank you very much, both Joe and Art
for Walter Driedger's URL and the very good comments and further info.

I am sure it will be great reading for young engineers like myself.

Much appreciated

[Dtan76]

Joe/Ark

Thanks for prompt response. Both of you alway like a good shepherd...