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Microbial Inhibitor for Cooling Water Treatment

My company is trying to be a vendor of small equipment package, such as chemical injection. I actually have a limited knowledge about this area, but in my understanding, chemical injection package is very simple. It usually consists of chemical storage tank, dosing pump, mixer, and accessories, such as low level switch, controller, and so on. The job became a  bit complicated when we don’t have enough knowledge about that.

However, to improve the ability as a vendor, my company decides to try to enter this area. The good news is I also participate. 

Our first project is to supply chemical injection system for cooling water treatment. 

We already understand that water is coolant. It is used in open re-circulating system, such as cooling towers, spray ponds, fountains; once thru cooling systems, and closed re-circulating system. Because most water sources contain impurities, there are several problems related to cooling water. In this post, I want to share to you my experience of sizing a cooling water treatment, especially microbial inhibitor.

For your information, I am dealing with cooling tower which is open re-circulating cooling system. The system is prone to corrosion, fouling, scale, and microbial contamination.

Why do we need to control microorganisms in cooling tower

The uncontrolled contamination of bacteria, algae, fungi, can lead to deposit formation, which contribute to fouling, corrosion, and scale. So that, the number of bacteria in cooling tower should be controlled to 5000 counts/ml.

How do we control microorganisms in cooling tower?

There are several microbial inhibitor used in cooling tower. Chlorine-based chemical is commonly used. For example, in my system, I use sodium hipochlorite (NaOCl) as microbial agent. On the other day, I considered to used bromine-based microbial inhibitor because it is a stronger agent than chlorine. However, bromine is not commonly used in cooling water treatment. It’s a little bit difficult to find it in the market.

Sizing of cooling water treatment for cooling water treatment?

In the example below I used sodium hypochlorite as microbial inhibitor. I tried to calcuate the dose, dosing pump capacity, chemical storage tank capacity, configuration, and control system. It was a bit challenging. And if I do mistakes in this post, do not hesitate to tell me. 🙂

Step 1. Calculate the condition of cooling tower

I will use the condition in this post as basic to design microbial inhibitor.

Cooling water flow rate        = 1225 m3/h

Evaporation loss                     = 18.74 m3/h (see how I calculate evaporation loss)

Blow down                                  = 3.75 m3/h (I used Cycle of Concentration of 6)

Cooling water make up          = 22.5 m3/h

Step 2. Calculate sodium hypochlorite dosing

Chlorine dosing is calculated as follow.

Chlorine dosing = chlorine demand + chlorine residual.

Make up water for cooling water circulation system requires 1-2 mg/L chlorine injected continuously (source). Residual chlorine is a concentration of chlorine that must be kept in the water distribution system. Residual chlorine is usually 0.2-0.6 mg/L (source) or 0.5 mg/L (source).

In my example, chlorine dosing = 1 mg/L + 0.5 mg/L = 1.5 mg/L

Step 3. Calculate required flow rate of sodium hypochlorite

Required flow rate of sodium hypochlorite (kg/h) =flow of cooling water make up (m3/h) x chlorine dosing (mg/L) x 1000/(concentration of chlorine (%) x 100000)

Then required flow rate of sodium hypochlorite (kg/h) = 22 m3/h x 1.5 mg/L x 1000/(12% x 100000) = 0.28 kg/h

I need to check the concentration of commercially available sodium hypochlorite. In my example, I use 12%.

Step 4. Calculate capacity of pump

I use capacity factor of 3, then capacity of dosing pump is:

capacity factor x required flow rate of sodium hypochlorite

Then the capacity of dosing pump is 0.28 kg/h x 3 = 0.83 kg/h or 0.69 l/h

After finishing calculation, we need to check available dosing pump capacity, such as LMI Milton Roy, Prominent, and so on.

References:

  1. Cooling water treatment Nalco. http://www.nalco.com/documents/Brochures/B-34.pdf

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