Free Cooling
Free Cooling
(OP)
Hi there,
hope all is fine.
Please I would have to calculate an heat exchanger in free cooling.
This means that in this case there is no forced convection but only natural convection with one of the two fluids.
The heat exchange is driven by the temperature difference.
I suppose that epsilon-NTU method can't be used, since Cmin is 0, so NTU is infinite and efficiency is 1.
Please, can anybody of you suggest me how to calculate this type of heat exchanger ?
A practical example would be the simplest solution.
Many thanks
hope all is fine.
Please I would have to calculate an heat exchanger in free cooling.
This means that in this case there is no forced convection but only natural convection with one of the two fluids.
The heat exchange is driven by the temperature difference.
I suppose that epsilon-NTU method can't be used, since Cmin is 0, so NTU is infinite and efficiency is 1.
Please, can anybody of you suggest me how to calculate this type of heat exchanger ?
A practical example would be the simplest solution.
Many thanks





RE: Free Cooling
RE: Free Cooling
Why can't you calculate the natural convection heat transfer coefficient for the fluid in natural convection? For example, I'm pretty sure I remember seeing how to calculate the natural convective heat transfer coefficient from a horizontal tube bundle in my university heat transfer textbooks. I'll have to check tomorrow as that particular book and my Perry's is at the office.
RE: Free Cooling
Since there is no air flow in front of the coil, I consider Cmin = 0
Please, is it wrong ?
To Mr.TD2K
I would be grateful if you could give me your feedback.
Many thanks
RE: Free Cooling
You could approach the problem using Churchill and Chu correlation that is reported at the link below
ht
RE: Free Cooling
many thanks
As a matter of fact I am afraid of not understanding.
If I well understand this correlation provides a Nusselt number, that is an heat exchange coefficient.
Now: the problem is that I can even have the internal and external coefficient, the two heat exchange areas - internal and external - the two temperatures, but I am missing the final temperature for the two fluids in order to apply LMTD.
Please, may I ask you an example of how applying this correlation ?
Many thanks
RE: Free Cooling
TTFN

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RE: Free Cooling
I think that still one equation is missing since there are three unknowns
Unknown:
1 - t cold outlet
2 - t hot outlet
3 - capacity
available equations
1 - Q = m*c*(tin-tout) (for water)
2 - Q = UA*LMTD
It seems one equation is missing since it can't be utilized the one similar to 1 for air since air mass flow is not known.
What do you think about ?
RE: Free Cooling
please, could you post an example of calculation ?
It would be extremely useful for me.
All in a all I am stuck to the post of Mr. TeeAr
Thanks in advance