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Cooling Tower Evaporation Loss

04/29/2018 10:36 AM

Dear Sirs,

In the Evaporation loss formula for Cooling towers below

Evaporation loss (m3/hr) = 0.00085 x Flow (m3/hr) x /T1 -T2) X 1.8

Why the evaporation rate is assumed to be a constant value i.e 0.00085. I have read everywhere but it only give the explanation that " it varies due to season and time, hence a rule of thumb value has been used"

Can anybody suggest the derivation or how did we come to this 0.00085 constant value?

Your help would be highly appreciated.

Regards !

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#1

Re: Cooling tower Evaporation loss

04/29/2018 11:34 AM
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#3
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Re: Cooling tower Evaporation loss

04/30/2018 12:12 AM

I know nothing of this particular topic, but am always trying to learn new stuff. I looked at your link, but saw no clear connection between the (imperial) numbers provided there and the (metric) values in the OP question.

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#2

Re: Cooling Tower Evaporation Loss

04/29/2018 5:03 PM

Like most "Thumb Rules", you have to be careful because you never know where that Thumb has been! In this case it has a decent starting point in "Perry’s Chemical Engineers Handbook", so consult your copy for more details about its derivation.

You can start here.

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#4

Re: Cooling Tower Evaporation Loss

04/30/2018 3:57 AM

The question's answer is in the sentence immediately above it: <...read everywhere...it varies due to season and time...rule of thumb value has been used...>. No further help is needed.

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#5

Re: Cooling Tower Evaporation Loss

04/30/2018 5:44 AM

Firstly, in that formula Evaporation loss (m3/hr) = 0.00085 x Flow (m3/hr) x /(T1 -T2) X 1.8, the 1.8 looks suspiciously like the conversion factor between °F and °C. Why not just say Evaporation loss (m3/hr) = 0.00153 x Flow (m3/hr) x /(T1 -T2)?

It's not too hard to estimate evaporation loss.

For water flow Q, heat lost = Q*ρ*(T1 -T2) kcal/h, where Q = m3/h and ρ = density kg/m3.

At typical cooling tower temperature 20°C, latent heat of evaporation is about 690 kcal/kg, so heat gained by evaporating water = Qevap*ρ*690 kcal/h, where Qevap = evaporation flow m3/h.

Equating the two gives Qevap = Q*(T1 -T2)/690 = Q*0.00145*(T1 -T2) - not bad agreement. By the time you've added a bit for windage loss and safety, even better.

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#6

Re: Cooling Tower Evaporation Loss

04/30/2018 6:09 AM

I've often wondered why the builders of cooling towers don't route the water through to housing estates or factories that can use the hot water to heat buildings ect, A case in point, years ago I worked in a toy manufacturers & all the plastic injection & die cast machines were water cooled, the water being pumped up to a cooling tower to be cooled, Opposite the cooling tower was the boiler house heating water for heat exchangers for washing, heaters for offices & factory assembly shops ect, so they were burning about thousands of gallons of fuel oil to heat water whilst spending huge amounts of money to cool water, a world gone mad.

Bazzer

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#7
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Re: Cooling Tower Evaporation Loss

04/30/2018 6:28 AM

Often, the heating requirements and the cooling requirements don't synchronise.

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#8
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Re: Cooling Tower Evaporation Loss

04/30/2018 6:37 AM

I get that but I think that it could be routed elsewhere when not wanted for heating even to sending it to a water tower as a last resort.

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#9
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Re: Cooling Tower Evaporation Loss

04/30/2018 7:25 AM

The steam leaving a turbine is at quite a low pressure and temperature, in order to get maximum shaft power. The waste heat is at too low temperature for district heating and most other uses.

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#10
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Re: Cooling Tower Evaporation Loss

04/30/2018 8:26 AM

Where I worked, the heat exchangers were interlinked so that hot water (from the moulding tools) was used to pre-heat the hot water systems before it then went to the evaporators.

We also ducted waste heat from the air compressor cooling fans into the factory air system in Winter to improve factory comfort levels.

Both actions were well worth the effort in reduced expenses.

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#11
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Re: Cooling Tower Evaporation Loss

04/30/2018 8:42 AM

In 1986 a factory for a major corporation could have saved £0.5mGBP per annum in fuel bills by using 40degC effluent to preheat boiler feedwater. It didn't happen. The company went bankrupt some time in 2005...

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#12
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Re: Cooling Tower Evaporation Loss

05/10/2018 1:58 PM

Good forward thinking, the Brits take on development is if it aint broke don't fix it!

Bazzer

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