looking for some information on shell and tube heat exchangers

[bntii]

I have a basic design question about heat exchangers.
In the shell and tube type, which is extracts more heat:

a) 12 tubes of 12" length

or

b) 6 tubes of 24" length

I am having trouble getting my head around this design issue which involves as I see it questions of flow volume and time for the heat transfer.

Thanks all

[CDK]

Heat transfer is a function of surface area, conductance, delta-t and flow volume. In the example the surface area is approximately the same, not considering the area lost by seals.
Since you do not specify the other factors I assume they are also the same, so the efficiency of the two designs will be equal.
The long tubes may perform marginally better because with less tubes it is easier to obtain equal speed distribution.
And of course less tubes makes the construction less expensive.

[bntii]

Thanks CDX,

I was looking into this question as I have a engine which is not getting enough cooling. The exchanger is a 2 pass type with straight 1/4" tubes.
After talking to a few folks it seems that I may need to get a different core bundle with a more tubes of a smaller diameter.
Does anyone have a source for replacement bundles for exchangers?

Thanks all

[CDK]

Bowman marine is the source for heat exchangers, core bundles and connecting parts. Their US agent is mesamarine.com.

[TedZ]

Heat transfer is very complicated. Another factor is the distance between the tubes. Flow outside the tubes is less restricted with greater distance.

The implication is that smaller tubes will have a higher specific surface and they will if there is enough of them. Flow around them, as mentioned, might be restricted and the gain negated.

We generally overdesign heat exchangers a lot assuming they still have to work when scaled. We use a three way valve to eliminate over cooling and by watching the position of that valve you get invaluable info on when descaling is necessary.

My $.02

Ted

[FAST FRED]

Make sure the H-E is hooked up right. You want the coldest sea water to greet the hottest eng output , in an attempt to keep the sea water below 140F at all times.

Otherwise salt will come out of solution and slow the water flow.

FF

[SeaJay]

I don't know if you guys have had a chance to look at Dave Gerr's new book, Boat Mechanical Systems Handbook, but it is excellent. He has a chapter on Heat exchangers and like everywhere else in the book, he gives you the distilled engineering data to do basic calculations (and then some). The tables and calculations are too lengthy to post here, but the book is worth every bit of its price.

Gerr doesn't need me to be a shill for his work, but I think he is really to be commended for this effort. I've got a pretty extensive library on boat related subjects but outside of mechanical engineering text books, I've seen nothing as detailed. But more important, it is focused on boats and systems that we actually use. Get you hands on a copy if you can.