Cooling Tower operation

[mcesm2]

Hi
I'm currently looking into optimising the cooling system on one of our process plants and I have a question I hope someone can help me with.

The cooling system is a tempered loop (i.e. warm water from process is recycled into cold water into process to prevent freezing of materials in cool ambient conditions) and has a dedicated mechanical draft cooling tower (3.5MW) to perform the cooling. Due to significant plant uprates in the past the tower is underspecified for the duty it is seeing and there is scope to change the operation of the tower to try to cope with this.

The tower is designed for 500m3/hr with inlet temp to the tower of 25.1'C and return 19.0'C in summer and 16'C in winter, with a wet bulb of 17'C.

What I basically want to know is if I increase the outlet water temp of the tower (and hence the inlet temp to the tower) to say 23'C will I see an increase in the DT across the tower and if so how much?

There is data missing for the tower such as air-flow through the tower so a 'first principals' calc is no use. What I really need is a simple correlation of how changing only tower water outlet temp affects deltaT over tower and hence duty.

Any help on this would be excellent.

Cheers
Shaun.

 

[joerd]

Perry's chemical engineer's handbook has a section on cooling towers. You could probably pro-rate the operation based on that, although it might not be awfully accurate.

Cheers,
Joerd

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

 

[katmar]

Shaun,

This is easy to answer qualitatively, but not no easy quantitatively. If you allow the inlet and outlet water temperatures to rise you increase the driving force and the tower will extract more heat. So for a constant water flow rate the water temperature change will increase.

Predicting how much it will change by is less obvious. In addition to the data you have, you need to know the packing characteristic and the air flow rate. You should be able to get the air flow rate from the tower supplier (or directly from the fan supplier) if you can read the motor current. Once you have the air rate you can estimate the packing characteristic from first principles using your present operating conditions. This is not easy by hand, but there is a very useful (and free) DOS program available on the Chemical Engineering Resources page that does all the calcs for you. BTW - if anyone knows of a Windows equivalent that is available I would love to know about it!

Once you have the packing characteristic you can start varying other parameters to predict the performance. I have done this using winter performance figures to predict summer performance, and it worked well.

Of course, you can always do it the lazy way and just let the tower supplier do the re-rating calcs for you! I did this too, and their numbers compared well with mine.

Hope this helps
Harvey



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