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Hi everyone!
Do you know whether it is possible to estimate the rate of evaporation in a wet cooling tower? A rule-of-thumb indicates that the evaporation rate is approximately 1% of the water flow-rate per each 10°F of cooling range. Otherwise, this rule-of-thumb doesn't take into account the meteorological conditions as the dry-bulb temperature and the relative humidity of the inlet air.
If you have heard about some studies about this, I would be pleased to know more about them.
Thank you.
To find out the evaporation loss in percentage, the formula is
Evaporation loss rate I =Absolute humidity at cooling tower exit-absolute humidity at tower inlet*1/ (L/G)*100.
L/G= LIQUID GAS RATIO.
OR
Evaporation loss (m3/hr)=0.00085*1.88*circulation rate (m3/hr)*(T1-T2)
Thank you for your answers Brian_UK and moideen.
The formula from: www.eng-tips.com, www.retscreen.net and the second one of moideen are almost all the same, only the significant digits differ. I know this formula and I think it gives a good idea of the evaporation loss rate, but it's not really accurate because the meteorological data isn't used.
The formula from www.jutzi.com may be more accurate: ER (gpm) = F * DT (°F) * RR (gpm) / 1000, where ER=evaporation rate and RR=recirculation rate. F depends on the meteorological data. But do you how the function to determine F? Because I looked for it a couple of weeks ago and I've never found it. I only found a graph with no accuracy, so I can't used it.
I know a formula similar to your first one moideen. First you determine the flow rate of air:
ma = ( mw(h3-h4) ) / ( (h2-h1) - (w2-w1)h4)
where:
ma: air mass flow rate
mw: water mass flow rate
h: enthalpy
w: specific humidity
1: inlet air
2: outlet air
3: warm water
4: cool water
Then, the evaporation loss can be determined by using this equation: EL=ma(w2-w1).
But there are two many unknowns, we need to know the air outlet temperature to determine the air mass flow rate. But this temperature depends on the air mass flow rate. I'm lost.
I agree, 'F' does seem to be a factor that is varied according to who is using it and for what purpose. ;)
Hi, Kakolio :)
I do not know why do you need precise values of evaporation loss .... because I think that is not possible to calculate neither with simple nor with complex equation .... we can only estimate the rate of evaporation ....Quote:
Originally Posted by Kakolio
why!?!
.... because cooling system is alive system and input values change every second ... we can only know how much water we utilize for i.e. one month period ... it is possible to install metering devices on fresh water inlet pipe and another one on blow-down pipe (still we have possible leaks and droplets removed by draft - unknown values) to calculate approximately how much water evaporate and that is that ... we need those data to determine equipment for chemical treatment of cooling water ....
... in another words rule-of-thumb or newest available formulas for new systems are quite applicable (old systems are another story, but still OK) and my warm suggestion to you is ... don't feel lost ;)
Best regards, Josip :)