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Ntu-effectiveness


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#1 rsk

rsk

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Posted 31 July 2008 - 11:42 AM

I want to buy an old used shell & tube heatexchanger, & use it. How shall i evaluate whther this HX will serve my application. I am aware that NTU-effectiveness approach is applied for this analysis please make it clear.
regards

#2 Padmakar Katre

Padmakar Katre

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Posted 01 August 2008 - 10:16 AM

QUOTE (rsk @ Jul 31 2008, 10:12 PM) <{POST_SNAPBACK}>
I want to buy an old used shell & tube heatexchanger, & use it. How shall i evaluate whther this HX will serve my application. I am aware that NTU-effectiveness approach is applied for this analysis please make it clear.
regards


Dear,
What I can suggest you is to first of all collect the Design Data for the Old HE. And perform Rating calculations with HTRI by giving 10% margins on the area and duty for the revised conditions. I hope the service of the exchanger remains the same or the old HE is superior in the design conditions like design temp, design pressure and MOC. The reason have double margin is the age of HE may or may not meet the original design conditions. Waiting for your reply.

#3 siretb

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Posted 04 August 2008 - 03:57 AM

rsk:

I think that you do not need to use the effectiveness, e-NTU or p-NTU approach to assess whether or not an used heatX will do the job.
You must define what your needs are
HEAT DUTY . Temperature levels for the different, likely cases you will have
For this, the regular LMDT/Area (corrected) approach will suffice. Tou will find it described in any textbook.
After you have determined that the heat exchanger type is what you want, that its current condition is good (corrosion, cleanness, ...), that it is good for the pressure service you want, check that you will have enough margin.
I am from the old school. A heat exchanger should be oversized by 40% at least. Not 10%.

The effectiveness approach is, the way I use it, a heat exchanger analysis method, rather than a designing tool. The effectiveness can be seen as the ratio of (actually exchanged load)/(maximum exchangeable load), based of CPHot & CPcold heat capacity ratios for both fluids.
This site has a page on that. Googling "effectiveness, e-NTU p-NTU" will bring you a lot.

If you have doubts for a specific service that a given heatX will work, describe your case in detail and we will help more.




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