2 replies to this topic

[RQA21]

Hi all,

I am attempting to quantify the effect of reducing the number of tubes ,in a shell and tube heat exchanger, on the heat duty. The heat exchanger is a single pass. I was thinking about using the relationship Q=UAdeltaTlm. I already know the number of tubes that need to be plugged so I have the area. Can I assume that the overall heat transfer coefficient and the log mean temperature will stay constant for a rough approximation of the reduction in duty? What would be a better way to carry out this calculation? There is also the issue of velocity as it will increase through the tubes due to plugging, and it needs to be kept below a certain value depending on the pipe material and the fluid to avoid erosion.
Any guidance is greatly appreciated.

[Technical Bard]

Yes, you can ratio the areas to get a number of tubes you need, and be roughly correct.  The uncertainty is that the U value will increase as velocity increases.  You can calculate the tubeside coefficient (h) pretty easily for the differing velocities and use that to estimate the change in U value (if the other side is controlling, then it won't matter much). 

[PingPong]

Q = U.A.F.LMTD

 

By plugging part of the tubes U will increase somewhat.

 

If inlet temperatures on both sides stay the same as before then also F and LMTD will increase because outlet temperatures will change. How much depends on the present inlet and outlet temperatures of the exchanger, nr of shellpasses, nr of tubepasses, TEMA type.

 

As a consequence of all the above after plugging part of the tubes Q will decrease less then one would expect when only looking at decrease in A.