Plate Exchanger's Heat Transfer

[114425]

Hi all,

I am currently encountering a problem on the calculation of overall heat transfer coefficient on plate heat exchanger. I am using condensing steam to heat up water-like liquid. When I checked with Perry's, the typicall overall heat transfer coefficient should be around 3000-5000; however, when I do my calculation, my overall heat transfer coefficient is only around 300ish. The hot steam side is in turbulant flow conidition while the cold side is in laminer flow. Typically fouling factors and plate resistance are used. Does perry's value usually refer both phase in turbulant condition, and that's why my calculation is way off?

Thank you very much,

 

[speco]

114425,

Are you sure you are using the same units? A high overall transfer coefficient for a plate and frame would normally be in the 1000 to 1500 BTU/(hr-sq.ft.-Deg F) range. It would be substantially less if one side were in laminar flow.

Regards,

Speco

 

[114425]

yeah i am pretty sure with units; but yeah, it is a very laminer flow with only about 400 reynolds number while other side being 5g/s of steam flowing in; that's why my U end up very small

 

[ione]

Not that surprisingly dealing with water-like fluid (for water Pr < 10 for temperatures below 100 °C) and such a low Re = 400. The cold side heat transfer coefficient dominates over the hot side.

 

[114425]

hmmm so i guess it is normal with such low overall heat tranfer coefficient? In this case, should i choose another type of heat exchanging system with higher U?

 

[speco]

114425,

It really depends on your overall objective. You are not likeley to find another type of heat exchanger with a higher heat transfer coefficient. However, within the realm of plate exchangers, there is a lot of variability. Some plates have a more horizontal chevron pattern, resulting in higher coefficients. Some plates are taller and narrower, and therefore have higher velocities, thus more turbulence. There's also a difference in the distance between plates between styles and manufacturers.

Regards,

Speco

 

[rmw]

You are never going to get on a square foot for square foot (or unit of area) basis a Hx plate or not that will match a liquid heat exchange coefficient equal to what a phase change with its inherent heat transfer coefficient on the other side will give up.

As Speco has said, through a variety of selections regarding plate features, you can maximize what the liquid side can do and hence get all you can get out of the Hx - any Hx.

Remember too that it is not just a matter of coefficient vs. coefficient. You have to be able to get the steam into the plate area and normally PHE plate passages and nozzle entry passages are quite narrow. This can be mitigated by selecting wide spaced plates on the vapor side and narrow spaced plates on the liquid side. This is commonly done with applications like sugar where not only is the steam vapor, but low pressure vapor at that with huge specific volumes.

Remember too that steam most normally always has some non-condensibles in it and over time these will build up and blanket heat transfer surface if you don't put specific means in to take the non-condensibles off.

I have known PHE manufacturers who refused to sell their units for condensers (but they would sell them to heat liquids with steam so we never told them that it was for condensing). PHE manufacurers who have good experience with condensing vapors have adequate designs for air (non-condensibles) offtake.

rmw

 

[IRstuff]

5 GRAMS/s ? against a liquid flowing at what rate? and what temperature rise?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[114425]

To rmw,

Thanks for the suggestion. I will continue to check on those information.

To IRstuff,

5g/s of steam and about 10g/s of water at aim to rise to 80C from room temperature.

 

[114425]

I been searching in the forums about the plate heat exchanger system. Some of the posts suggesting an article called "Consider the Plate Heat Exchanger" by Chand and Raju, but I am having touble finding it from some online library. I been searching on Science Direct and Wiley but can't find this journal at all. Anyone mind giving me a hand on this problem as well? Thankz in advance!

 

[IRstuff]

Ostensibly, there seems to be enough heat content to do the job, so how much time do the streams spend in the exchanger?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[vpl]

114425

You might have to pay for the issue, but here's the web site for Chemical Engineering Magazine:

http://www.che.com/themagazine/.

If you contacted them, they probably have that article.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[114425]

To vpl,

I been searching this website as well, but no search results was displayed. I think it is probably this article was published on 1980 and the website is limited from 1985-present's papers.
but still thank you very much for your help.

 

[vpl]

114425

What I meant was you might need to contact the magazine folks and ask them if they have a back copy. This might not be electronic. It will probably not be free. But if you really want it, that would be the way to go.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.