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[dylant]

I was told by my professor that 10-20 m/s is a typical velocity of fluid flowing inside a shell and tube heat exchanger.
Can someone please explain to me?

Thanks,
Dylan

[Art Montemayor]

Dylan:

The faster you push a fluid, the higher the expected Reynolds Number; the higher the Reynolds Number, the higher resulting the turbulence, the expected convective film coefficient, and the pressure drop.

Everyone of the above listed values is OK or good - with the exception of the last one (the pressure drop). The pressure drop suffered is usually the prime trade off or the "price of the lunch" when you are designing heat transfer equipment. I've never considered fluid velocity as an item of importance when specifying a heat exchanger. In fact, I've never seen the fluid velocity expressed or calculated in a heat exchanger specification sheet. And I've seen hundreds of them. What does always appear in a heat exchanger spec sheet is the allowable pressure drop. And this, as I've said, is the criterion that engineers employ to design a heat exchanger application. You essentially want as high a pressure drop as you can reasonably obtain. This is, of course, for improving the convective heat transfer coefficient - and the resultant "U".

If your prof really said "10-20 m/s is a typical velocity of fluid flowing inside a shell and tube heat exchanger", then he/she is being vague and generalizing. Note that the fluid has not been identified as liquid or vapor/gas. This is an important ommission. 65 ft/sec is a hellova fast velocity for a liquid inside a baffled shell, whereas it might be OK for a gas. I frankly don't see liquid going that fast in a heat exchanger as necessary (or allowable) in the real world.

The specific comment (or advice) I would offer you is: Always design on the basis of the allowable tube side and shell side pressure drops. What ever velocity results from that basis is what you get, and that's the end of the story. In other words, fluid velocity in a heat exchanger is normally of secondary importance - except where it can cause serious tube vibration or metal erosion.

For further basic and useful information on heat exchangers (including a mention of velocity values), go to the following page on this Website, which, I believe was written by Chris Haslego - a heat exchanger design veteren in his own right:

http://www.cheresour...esignexzz.shtml