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Cooling Tower Efficiency Calculation


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#1 Charan_123

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Posted 08 February 2017 - 11:03 AM

Hello everybody,

 

Is there any practical way to calculate the evaporation loss and drift loss of water from a cooling tower ?

 

and 

 

How do we calculate the efficiency of a cooling tower?

 

Thank you

 

Sincerely hoping for some insight. 

 



#2 sgkim

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Posted 10 February 2017 - 06:31 AM

There would be no way to calculate the drift loss from the cooling tower, but the evaporation loss can be estimated from the material and energy balance in the cooling tower.

 

Material Balance:  m1 = m2 + E ................(1) 1= in-flow,  2=out-flow, E=evaporation loss

Energy Balnce:     h1*m1 = m2*h2 + E*λ....(2)  h1, h2 enthalpies of water of stream 1 and 2,  λ.=heat of vaporization of water at cooling water return temperature,  sensible heat of air is neglected.

 

then,   E = m1*(h1-h2)/(λ-h2) = m1*(Cp)*(t1-t2)/(λ-h2) ≒ m1* (Δt) / λ ....(3)   t1-t2 = Δt = temperature difference of cooling water, λ-h2≒ λ

 

You yourself can define the efficiency of cooling tower.

 

Stefano



#3 SPC

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Posted 10 February 2017 - 10:22 AM

You can also you Perry's Chemical Engineer' Handbook.



#4 Saml

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Posted 10 February 2017 - 11:06 AM

The problem with "efficiency" is that the cooling tower will always remove the heat that the process sends into the cooling tower.

 

A more "efficient" tower will work closer to the calculated equilibrium with air, while the less efficient will be hotter.

 

So the issue is how cold the cooling water is, not how much heat is removed, that is essentially unchanged (the change is a second order effect)

 

So you can work with either the "approach" to the wet bulb.

 

Others use the term "capability" that is calculated as follow:

- Take the actual inlet temperature, outlet temperature, Wet and Dry Bulb Air Temperature and  L/G ratio. With this information, get the flow that can be cooled from the design curve.

- Measure the actual flow and divide it by the previously obtained one.

 

The nice point is that it corrects for several factors, mainly water flow whereas a simple "approach temperature" do not.

The drawback is that since it is the relationship of 2 flows, you will have to give endless explanations of why your tower cannot process more flow. That is it difficult to understand people not involved with calculations, that 80% capability does not mean "the tower should remove 20% more heat" or "the tower should circulate 20% more water" but simply that "the tower should operate 1,5°C cooler"



#5 breizh

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Posted 10 February 2017 - 10:19 PM

Hi ,

to support your work

Good luck

Breizh



#6 Charan_123

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Posted 02 March 2017 - 09:18 AM

Thank you very much breizh, Saml, sgkim, SPC for your valuable input.

.






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