Cooling water blowdown how much?

[StoneCold]

I have a cooling water blowdown question. Does anyone have a rule of thumb about the amount of blowdown water from a cooling tower? I know that usually conductivity is the controlling factor but I would like to know if there is any design rule out there that relates the amount of makeup water to the amount of blowdown. I know that the conductivity of the makeup water is about 60 mho's and we are trying to keep our cooling water conductivity below 1500 mhos. So at minimum I need to blow down 1500/60=1/25 . One gallon of blowdown water for every 25 gallons of makeup water. But in reality lots of stuff gets sucked into the tower, etc and the ratio must be lower like one gallon of blowdown for every 10 gallons of makeup water.
In the end I am only looking for a rough number to tell my boss that we made up xxx gallons this week and we need to blow down xxx gallons.
The quantity is an issue because we are having to truck the blowdown water off right now instead of being able to dump it to city sewer.

Thanks
StoneCold

 

[mbeychok]

Stonecold:

First, some comments:

(1) I assume that you are talking about an open, evaporative cooling tower.

(1) Your posted question sounds as if you blow down periodically or batchwise. You should be blowing down continuously by using a level controller to maintain a certain level in your cooling tower basin.

(2) There is no rule of thumb such as you seek. The amount of blowdown depends upon economics, the type of water treatment you use, and the original tower design's evaporative capacity. The amount of blowdown is usually set by the desired cycles of concentration (using chlorides as the indicator of concentration).

(3) As a broad generality, most large, industrial cooling towers operate at about 3 to 5 cycles of concentration. This is not a rule of thumb ... just a broad generality.

The governing equations based on heat and material balances around the tower are:

Cycles = M/(B+W) = M/(M-E) = [E/(B+W)] + 1

where:
Cycles = (chlorides in circulating water) / (chlorides in makeup water)
M = Makeup water, gal/min
B = Blowdown water, gal/min
E = Evaporated water vapor, gal/min = (K)(C)
W = Windage loss water, gal/min
C = Circulating cooling water flow rate, gal/min
K = 0.01 per 10 [°]F [Δ]T
[Δ]T = water temp. entering tower - water temp. leaving tower
W = 0.3 to 1.0% of C (for natural draft towers)
W = 0.1 to 0.3 % of C (for induced draft towers)


Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[cub3bead]

Without a water analysis for a basis you are running the risk of scaling up the system. In fact the drift won't allow you to get to 25 cycles unless you have super efficient mist eliminators.

In general, after about 6 cycles you start getting into an area of dimishing returns. By that I mean that the blowdown volume starts becoming a less percentage of the total makeup. try ploting the makeup curve at a constant evaporation rate and varying the cycles. Plot cycles on the x-axis and flow on the y-axis. the difference between the two curves is the blowdow + drift. Drift is so small to not matter.

Now back to the water analysis. Silica is only soluble to about 150 ppm but can go as low as 100 ppm under some conditions. It will lay down a very nice silica coating on your heat exchange surfaces and is very expensive to remove. Calcium carbonate will also precipitate and cause a lot of grief but it is a little easier to remove by chemical cleaning but not desirable.

All of the equations are in the previous post. Time to do a little algebra.

Otherwise, try calling Betz, Nalco, ChemTreat etc. for recommendations on cycles and treatment chemicals.

 

[quark]

Generally blowdown rate = makeup rate/(COC-1). So for 25gallons of makeup water, you have to blowdown 1.041gallons.

TDS seems to be low in both makeup(36ppm) and circulating water(900 ppm) as well and this may not create much problems. But as suggested in earlier posts, a chemical analysis is much needed. TDS can cause corrosion if it is above 6500ppm and not a likely event in your case. Magnesium silicate is a deadly scale but as a thumbrule, you have to be careful if the product of Mg and SiO2 in circulating water is around 30000ppm. Magnesium silicate scale can form at much lower concentrations if the temperature is high. A pH of 8.2 to 8.5 is good to prevent magnesium silicate formation.

Regards,

 

[StoneCold]

Thanks guys for your comments. I will drill our nalco rep. on what he knows based on our system.

Thanks
StoneCold