Cooling Tower Blowdown and Make Up Water Calculation - Confirmation

[Pavan Kumar]

Hi All,

I want to confirm my Blowdown and And Make up water calculations that I did for the Cooling Tower in our plant. Attached please find the sketch for the cooling tower with the IN and OUT streams ( please excuse me if it is shabby.) The cooling tower is supplied with Make Up water, M gpm. The evaporation and drift losses are E and D gpm respectively. The circulation water rate is C gpm. The Cooling tower supplies F gpm of cooling water to another tower which does not have its own Make Up water source ( This may sound strange but that's how it is and I am going to correct this situation. First I got to do the mass balance and hence this post.)

Also due to excess Make up water being supplied there is an over flow to the sewer at B gpm.

As per the guidance by Mr. Milton Beychok in the link below, I did the mass balance calculations and need confirmation.

https://www.eng-tips.com/faqs.cfm?fid=1152

Mass Balance:

Make Up water = M gpm
Evaporation Losses = E gpm
Drift Losses = D gpm
Overflow (Blowdown) = B gpm
Water to second tower = F gpm
Circulation water = C gpm
Temperature of water return to Cooling Tower = T1 Deg F
Temperature of water Out of Cooling Tower = T2 Deg F

Salt Concentration in Circulation Water = Xc
Salt Concentration in Make Up Water = Xm

M = E + D + B + F

Evaporation Rate, E = 0.00085 * C * ( T1-T2) gpm

Drift Loss , D = 0.2% of Circulation Rate = 0.002 *C gpm

Dissolved Salt Balance:

Xm * M = ( Xc * D) + ( Xc * B ) + ( Xc * F ) = Xc * ( D + B + F)

Cycles = Xc/ Xm = M / (D + B + F) = ( E + D + B + F ) / ( D + B + F) = [ 1 + E / (D=B+F) ]

From this B = [ E / ( Cycles -1 ) ] - ( D + B)

Now with my project Numbers:

C = 4025.55 gpm
T1 = 25.36 Deg C = 77.648 Deg F
T2 = 22.72 Deg C = 72.896 Deg F

E = 0.00085*C*(T1-T2) = 0.00085 * 4025.55 * (77.648-72.896) = 16.26 gpm

D = 0.002 * 4025.55 = 8.0511 gpm

F = 18.45 gpm ( as measured by our flow meter )

Xc = 428 Micro siemens / cm ( Concentration directly proportional to conductivity)
Xm = 319 Micro siemens / cm

Cycles = Xc / Xm = 428 / 319 = 1.34

B = [ E / ( Cycles - 1 ) ] - ( D + F )

= [ 16.26 / ( 1.34 -1) ] - ( 8.0511 + 18.45 )
= 47.8 - 26.5011 = 21.29 gpm


Make Up = E + D + B + F

M = 16.26 + 8.0511 + 21.29 + 18.45
= 64.05 gpm

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
It sounds good ! Just mass balance and Math to apply .
Why did you use conductivity measurement instead of Chloride to calculate the concentration ratio RC ? at least you should confirm that those ratios are equivalent (should be) .
Note : your RC is very low , refer to the standard given to you by the company in charge of the cooling water treatment .
a good habit is to check Calcium and compare R chloride to R calcium to ensure your system is in equilibrium with Calcium precipitation .
R conductivity is not going to tell you much about the chemistry of water within your system .

Hope this is helping you
Pierre

 

[Pavan Kumar]

Hi pierreick,

Electrical conductance is the only measurement of dissolved solids we do at our plant and I feel it should be equivalent to the concentration of solids. I will check with my manager if I can get the measurement of chlorides and calcium separately. Does RC means concentration ratio?. The conductivity ratio and hence the concentration ratio of circulating water to make up water is low because we is a continuous overflow from the cooling tower ( undesirable but has been operated this way for some reason, please see the sketch attached in my previous post) and there is continuous stream drawn as make up water for second cooling tower ( again undesirable but has been operated this way).

Effectively the cycles of concentration or concentration ratio is low at 1.34.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi Pavan ,
You are correct RC = concentration ratio.
About Blow down , you may be able to reduce the flow based on analysis performed by your chemicals supplier .
my view
Pierre