Heat exchanger manifold design 3

[sfoakes]

My team is designing an unfinned, staggered-tube, cross-flow heat exchanger. With water and air as the two working fluids. It is my job to design the water manifolds in and out of the heat exchanger. All research I have done have shown that manifolds usually consist of a pipe with several outlets along it's length, creating a pressure drop across each. My team leader seems to think that pressure in each tube through the exchanger must be the same so as to create the same flow and heat transfer through each. I am unsure if this is the right approach. He also thinks my idea for the manifold will not work as there will be approx. 50 tubes. What is the best approach I can take to designing this manifold? If anyone can provide any incite on heat exchanger manifold design or any references it would be greatly appreciated. Thanks in advance

 

[rmw]

In a perfect world the pressure drop and velocity through each tube is the same. However in the real world, depending on the size of your Hx, and many fit into this world, you will have a differential pressure from the top of the bundle to the bottom just due to static head (unless, of course your tubes are vertical and tubesheets horizontal).

You can design your manifold so as to minimize or even equalize the pressure losses so that there are no effects on the DP across the tube field.

rmw

 

[davefitz]

a full treatment on this subject can become complex. Some simple rules of thumb can be used:

a) inlet and outlet header config to be a "U" shape, not a "Z" zhape. If inlet fluid originates from the right side , then the outlet fluid exits to the right side. This nuetralizes the change in velocity head along the lenght of the header

b)if water is to be heated, it should flow up. If the water is to be cooled, it should flow down.

c) velocity of fluid in header to be lower than the velocity in the tubes.

d)inlet orifice plates in each tube's inlet generally will help the flow unbalance if the heat exchanger is short and does not have a significant "natural circulation " effect.

 

[sfoakes]

Thankyou. your tips are very helpful. I am thinking of using ANSYS fluent and attempt to model the header. rmv, do you have any tips in designing the header to minimize or equalize the pressure drops.

 

[ione]

“Fundamentals of Heat Exchanger Design” by Ramesh K. Shah and Dušan P. Sekulic. JOHN WILEY & SONS, INC. gives inputs on manifolds/headers design (chapter 12).

If you do not have access to the book, you can try the link below (but some of the most interesting info remain hidden)

http://books.google.it/books?id=beS...EwAg#v=onepage&q=oblique flow headers&f=false