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microHydro
06-11-2012, 10:03 PM
I've sized a water/refrigerant shell and tube heat exchanger. I need some practical advice how to configure the tube and baffle pattern. Here is a 3d model of my design:

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The box is 14' x 3' x 7". The baffles for the water side are 16" apart, and 7 inches high. I choose this spacing based on the Hazen Williams Head Loss, but the biggest guess in the equation was the length of the water path. Specifically the extra equivalent length for the u turns. In this case the path length was 14.5 feet and the u-turn equivalent length was 58 feet, for 72.5 feet total.
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There are 3 layers of tubes.
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The PID section below shows more detail. Note the low delta T available. The water flow actually ranges from 200 to 350 GPM. There is very little head available on the water side, about 18".
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Please advise how you would improve this design.

Brian_UK
06-11-2012, 11:28 PM
Sorry, slightly confused over your project.

Why are you trying to cool down a river?

microHydro
07-11-2012, 02:27 AM
It's a water source heat pump. What can I clarify for you?

Having never built a heat exchanger like this, I need the practical experience of someone who has. Applying design equations is not the same as having practical experience.

hookster
07-11-2012, 07:15 AM
I will look at this from a maintenance point of view.
I just wondered how you were going to approach silting and cleaning on the water side or is it disposable?
The tube return ends will be a nightmare to achieve stability against vibration and chafing. The large surface area in design footprint leads itself to a plate heat exchanger.
Have you considered air release mechanism and cavitation erosion on the baffle plates as fluid stream is diverted.
what is the fluid pressure drop?

microHydro
07-11-2012, 07:51 AM
Hi hookster!

Thanks for your insight. Silting is a problem in the spring. I expect several inches of sand to deposit in the bottom. I plan for the top cover to be removable, and to wash it out with a fire hose.

I rejected a plate heat exchanger because of the strong probability that it will freeze.

On the water side, I calculated the pressure drop at 1/2 a psi using the Manning open channel formula, and about .7 psi with Hazen Williams. Of course, the coefficients are pure guesses, and I'm unsure of the equivalent length of the u bends. The inlet pressure is a bit less than 1 psi.

Could you please elaborate on the problems of air release and cavitation? Also vibration and chafing of the tube return ends. The water velocity is 1.2 ft/sec through these passage dimensions.