Fouling factor in seawater/water plate heat exchanger 1

[juanchispa]

I´m develping a seawater/water plate heat exchanger.
My problem is that as I´m not a process or mechanical engineer I don´t know some things.
I have to cool water with unespicified flow rate yet, it comes from 70 m high to the plate heat exchanger so I don´t know if it is an impossible question. And seawater comes from 20 meters under this HE. So there is a quite big pressure difference.

In this moment my main problem is that I don´t know how calculate the U. I´m using CuNi10 as my plate HE material and the flow rate of the seawater should be around 2 m/s in order to prevent biofouling. And my maintenance period is once per year. The fouling factor per seawater I found: 8,8 m2K/W and how should I reach to the required U?

Thanks guys. Cheers

 

[Montemayor]

Chispa:

Any fouling factor allowance depends on the TYPE of fouling you are getting (or expect to get). Fouling cannot be treated generically. It makes a huge difference if you are confronting dramatic fouling of a heating surface such as the formation of sea barnacles or crustaceans. This type of growth and accumulation is totally unacceptable for a Plate Heat Exchanger (PHE) and this is the type of "fouling" you can expect if you are dealing with sea water under certain conditions. PHEs are great devices to employ - but I think you are applying this one in the wrong application. The very close spacings between surfaces in a PHE make it a device that can't tolerate the introduction of solids and much less the incremental growth of solids within it.

A shell and tube (a much more rugged unit) type of exchanger is hard enough to manage in such an application and I believe the PHE is out of its league here.

 

[panduru]

Where can I find very reliable figures for starting assumptions of fouling factors and HT co-effs. for ST exchangers?

The sources I have come across so far have only a few sample figures, that too in the form of a very broad range.

And, of course, I understand they come from experience.

Any pointers most welcome and will be appreciated.

 

[Montemayor]

Panduru:

Your post is another subject and should be in another, independent thread. Mixing threads with different subject matter results in chaos and total confusion. Nobody gains. Everybody loses.

Fouling is not the same in all applications. I don't know why it is that engineers try to generalize it when they know that it can be slime, solids, polymers, rust, crustaceans, crud, etc., etc.. It is vitally important that you identify the type of fouling you expect. The fouling factor you employ should correspond to the type of fouling. I can't make it more clearer than that.

Please start a new thread with your specific problem and needs so that it can be addressed specifically there.

 

[panduru]

Thank you, Montemayor.

 

[juanchispa]

What I don´t know is how calculate the "U" because I need some factors which are involved there. In the fouling factor I found 8,8 m2K/W for sea water itself. Should have another for specific biofouling? Another for conduction throw metal.
So problem are numbers, I found this number for seawater fouling but I don´t know more. That´s the last thing for finish my design.

Why PHE? Because of space and weight as well as easier to clean. This kind of HE is required as well as this low maintenance period. If not of course tube HE would be better.

Thanks

 

[Montemayor]

Chispa:

A Fouling Factor is simply a contingency value introduced into the expected heat transfer resistance(s) in order to buy time as the unit begins its cycle of getting "fouled" (dirty to the point where heat transfer is diminished).

The Fouling Factor's effect is to inflate the size of the required heat transfer area (it yields a bigger than required exchanger - based on clean flow). The net effect of any fouling is to incrementally increase the resistance to heat transfer as time goes on. Therefore, you are faced with trying to quantitfy a factor without specifying the time involved in order to cease the operation. You are the sole judge of determining just how conservative you want to make your Fouling Factor. The higher the Fouling Factor (or more properly: the more Fouling you design for), the bigger the exchanger and - sometimes - the more time you can expect to have the exchanger in service.

However, I should warn you: The use of fouling factors is often the cause of fouling itself. Many past experts in the field have shown that excessive fouling factors just aggravate the effect of fouling and, in many cases, stimulate it.

Depending on the type of fouling you experience, you will certainly need the PHE's characteristic of "easy cleaning". You will be doing a lot of that, if I don't miss my guess. Seawater cooling is a mess and a nemesis to any heat exchanger - particularly to a PHE. I hope you are not using Stainless Steel plates with seawater. Stainless will ultimately fail under the chloride service.