Free Cooling 1

[AO1958]

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

 

[ione]

How could Cmin be equal to zero?

 

[TD2K]

What's the configuration of the heat exchanger?

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.

 

[AO1958]

to mr Ione.
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

 

[ione]

Cmin is not equal to zero. The problem in applying the e-NTU method in your case is that you haven't any flow on the outside(or better you have just a flow due to buoyancy).

You could approach the problem using Churchill and Chu correlation that is reported at the link below

http://www.thermalfluidscentral.org...p/Natural_convection_on_cylinders_and_spheres

 

[AO1958]

To Mr. Ione.
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

 

[IRstuff]

It's an iterative process, unless you have a solver. You assume values for the temperatures and iterate until the energy transferred is the same at all interfaces.

TTFN
faq731-376

 

[TeeAr]

Hi

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 ?

 

[AO1958]

To Mr IRStuff

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