heat exchanger 5

[pradeep4u]

Dear All

Increase in fluids flow rate in heat exchangers increase heat transfer rate we all know. Hot fluid flow rate increase will result in increase in heat transfer rate but when cold fluid flow rate increases heat transfer rate decreases why? I am very much puzzled about it? Please guide me

Regards

 

[EdStainless]

No, the heat transfer rate does not decrease, it doesn't change.
If the process is limited by the heat transfer rate on the hot side, and you increase the cold side flow you will have little to no impact on the overall heat transfer. All that will happen is that the cold side fluid will have a lower delta T.
The overall performance will decrease, since you are moving more fluid with no improvement in heat removal.

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P.E. Metallurgy, Plymouth Tube

 

[pradeep4u]

Dear Sir
Thanks for your reply. I want to know that why this phenomenon happens when we increase cold fluid flow rate.
Regards
pradeep

 

[25362]

pradeep4u, it is assumed that the increased cold fluid flow rate doesn't induce some kind of fouling or its redistribution.

 

[pradeep4u]

25362(chemical)
Sorry Sir I did not gt your point. Please explain how fouling increase heat transfer rate.

Regards
Pradeep

 

[EdStainless]

Build a resistance model for the heat transfer.
So you have 7 R's; hot fluid, hot side boundary layer, hot side fouling, tube wall, cold side fouling, cold side boundary layer, cold fluid.
If the first two (or three) are the largest, then changing the last two (or three) will have almost no impact on total heat transfer.
You must understand what is limiting heat transfer before you can change it.

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P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

First, agree with previous deduction that hot side of HX appears to be controlling the overall U here.

The overall U is also affected by the hot side bulk to tube wall viscosity ratio - see the Dittus Boelter or Sieder Tate equation for this effect. In layman's terms, the thicker the film layer at the tube wall, the greater the resistance will be to heat transfer.

As the cold side flow increases, the tube wall gets cooler and the film resistance on the hot side increases also due to the increase in hot side wall viscosity. In this case, it may be that the hot fluid viscosity is significantly affected by temp. So overall U drops, since the increase in cold side "h" is damped by the corresponding decrease in hot side "h".

 

[25362]

You must remember that the overall coefficient "U" will never be higher than the lowest of both heat transfer coefficients by convection, and will in this case be lower than "h_oil".

As georgeverghese explained, if an increase of water flow cools the metal wall and, as a rssult, it increases the viscosity of the oil, reducing its convection heat transfer coefficient "h_oil", the overall U will drop.