Heat transfer coefficient V flow rate in heat exchangers

[NeilSF]

I am trying to determine why the cooling circuit for our air compressors is not sufficient, and if there is a way of optimising it.

We have a fin fan cooler, cooling the cooling water circuit.

I wish to model the heat transfer in the system at different cooling water flow rates. I suspect the cooling water flow is higher than design rates (and therefore there's higher heat transfer than is designed for). I wish to determine how the system will behave if I vary the low rates through the coolers.

Is there a way of obtaining a reasonable correlation between heat transfer coefficient and flow rate?

Thanks,

NF.

 

[vpl]

You mean besides Q=m_dotc_p?T?

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[25362]

NeilForster: since this is apparently a heat exchanger effectiveness issue, I suggest to read J.P. Holman's Heat Transfer, McGraw-Hill. The IV Ed. has example 10-7 similar to the subject you are bringing up.

 

[EdStainless]

You can treat each part of the system as a lumped factor. You don't need to seperate individual factors.
That said, you will need an accurate flow measurement and inlet and outlet temps at each location.
If you are not getting enough cooling it raises some questions.
Did you ever get enough cooling?
Has the compressor duty changed?
How clean is the outside of you finfan? Ever pressure wash the fins?
My guess, you fins are fouled and your flow rate is low from pump wear.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[MortenA]

Calculating the heat transfer coefficient U*A from HX properties (type, layout, etc) and know fluid properties is standard text book stuff that any heat transfer text book (incl. Perry) will tell you about.

Rule of thumb is a little hard since a HX is like selling rubber bands by the meter:

Vary any parameter and you get a new situation - that can be countered etc.

Usually the "heat resistance" will be much large on one side than the other - thus changing the "other" flow rate will not change UA much - but it may change LMTD and thus change the duty this way.

Best regards

Morten

 

[AndreChE]

Try Aspen to simulate the cooler and make a sensitivity analysis for everything you want

AndreChE

 

[TD2K]

The GPSA data book, page 9-10, has a section showing the effect changes in various properties has on that side's heat transfer coefficient where heat transfer is due to sensible heat.

For example, if you increase the mass flow rate, the tube side coefficient increases by (G1/G2)^0.6 for turbulent flow. On the shell side, the correlation is (G1/G2)^0.8