The chemical feeding rate vary, but are usually within 1-3 ppm for continuous treatment and 5 - 10 ppm for shock treatment. Generally the shock dosing system is designed at 5-8 mg/liter of Cl₂. As soon as a trace of free chlorine HOCl appears at the cooling tower return line, the shock dosing is stopped. The reason is if it is a wooden tower with wooden slats, HOCl above 1 mg/liter will start to attack the cellulose fibers of the wood and can greatly reduce its physical strength, to the point of collapse. So that's why it is a good practice to shut off the chlorinator when cooling tower return shows an HOCl residual of about 0.5 mg/liter.

Now if the tower is plastic or other material, then the 0.5 mg/liter is not so critical, however, it has been shown over the years that the 0.5 mg/liter residual at the tower return is a good indication that you have the biofouling under control.

There are no hard and fast rules for cooling water biofouling control. Each application has its own peculiarities depending on;

• geographical location
• if it is a recirculation system
• if it is a once through system
• if it is fresh water
• if it is seawater

The Cl₂ dosage is based on the max. pumping rate of the cooling water pumps at 5 mg/liter for shock dosing. With once through systems, you also have marine growths like clams, etc. to be concerned about. However, with recirculating systems and functions to consider as follows:

• Other chemicals corrosion inhibitors can cause problems when using an analyzer. Typically the analyzer cell will be plated by the inhibitor which may reduce the cell output, if the level of inhibitor is constant than it can be allowed for which the normal levels of inhibitor found in cooling towers about 10-15 mg/liter.
• Number of times the water recirculates. Shock dosing duration and interval is dependent on the season in relation to the level of algae, etc. in the water. In other words, generally in European climates for example you only shock in the summer - no need in the winter. In tropical climates, you may have to shock dose all year with low level continuous chlorination. It depends on geographical location.
• Generally make-up water does not create a problem as we are only concerned about controlling algae, etc.
• Temperature of water in the basin is not generally a concern as we don't normally chlorinate the basin water. The chlorine solution is normally added at the recirculating pump intakes.
• For the system circulation time of cooling water (duration time), you set up the system to supply a shock dose of 5 mg/liter for as long as it takes the water to reach the cooling tower return, which is generally about 20 - 30 minutes.

So, if recirculation time is 40 minutes say, then you shock dose at 5 mg/liter for 40 minutes and shut-off the system. Now you measure the HOCl residual when it falls off to zero, maybe 6-8 hours, the interval time, you start the shock dosing cycle again. So, you set the duration time to 40 minutes and the interval timer to say 8 hours. Then every 8 hours, the system will shock for 40 minutes and stop..... 8 hours later it will repeat.

There are three variables you can adjust to give the optimum performance for biofouling control:

• Actual chlorine feed rate
• Duration of the shock
• Interval between shocks

Remember that the biofouling control is an art, not a science !!!

1) Type of Chlorination

As previously discussed, there are two methods of chlorination used in the treatment of cooling water. They are as follow:

A choice of continuous or shock treatment must be made with consideration to results desired, chlorine consumed and water to be treated. Shock treatment is usually the choice in the cooling water recirculation system of cooling tower.

1. Continuous Chlorination: This provides a constant feed of chlorine for 24 hours a day and at a low dosage, generally in the range of 1 to 3 parts per million (ppm = mg/l).
2. Shock Chlorination: This provides a periodic chlorination generally at dosage of 5 - 10 ppm. The length of the treatment period and frequency of the treatment is usually adjusted to meet the requirements of each application. A sufficient chlorination should be provided during each treatment cycle. The length of treatment should be no less than the time interval for water to pass through the system. The frequency will vary with ambient and water temperature, location and demand. Sometimes it is necessary to chlorinate continuously at a low level - say 0.5 to 1 ppm dose to maintain the system clean between shock doses but this procedure is generally confined to tropical where there is a continuous presence of biofouling matter. In Korea, a common practice is to shock dose in the summer as biofouling does not become a problem in the winter. Generally three chlorination periods per day are used.
   
   Control of Duration time in The Shock Treatment: The duration is generally set to match the cooling water recirculation time and seasonal conditions. Capital's Shock Chlorination System is using a dual timer.
   
   One timer (t1), 0 - 60 minutes, is used to control the time. The chlorinator is producing chlorine solution by opening and closing the vacuum valve adjustable 0 - 60 minutes. The ejector has water running through it continuously; it is the opening of vacuum valve which allows chlorine gas under vacuum into the ejector and the closing of the vacuum which stops the flow of chlorine to the ejector.
   
   The other timer (t2), 0 - 24 hours, is used to control how often the 0 - 60 minute timer (t1) operates. For instance, t1 could be set at 30 minutes, t2 could be set at six hours then automatically every six hours the chlorinator will feed chlorine gas for 30 minutes, then stop, then six hours later it will repeat.
   
   The system is fully automatic operation, once the optimum duration (t1) time and frequency (t2) time has been decided at site. The duration, frequency, and kg/hr of chlorine used must be set at site to meet the site conditions, this will vary from site to site according to geographical location and type of cooling water application.
   
   The timers we use can have its range changed on site by adjusting electrical jumpers within the timer; t1 and t2 is the same timer with jumpers set to 0 - 60 minutes and 0 - 24 hours. The motorized or solenoid valve located in the gas vacuum line is controlled by a three position selector switch (ON - OFF - AUTO).
   
   When the switch is in the "ON" position, the solenoid will remain open and feed chlorine gas to the ejector. When the switch is in the "OFF" position, the solenoid will remain off (no chlorine feed). When the switch is in the "AUTO" position, the solenoid valve will cycle on and off according to the settings on the "Frequency" and "Duration" timers mounted on the local control panel.

2) Design Factors for Feeding Rate of Chlorine

Chlorination equipment should be sized to provide a chlorine dosage several times that the final TRC [Total Residual Chlorine = Free Residual Chlorine (FRC) + Combined Residual Chlorine (CRC)] level allowed. This provides chlorinator capacity to meet the demand in the system, and losses of chlorine through aeration in the cooling towers. Feed equipment should be sized for the minimum anticipated feed rate, using the below equations:

Pound/day of chlorine = [Recirculation Rate of Cooling Water (GPM)] x [Rate of Dosage (ppm)] x 0.012 or

Kilogram/hr of chlorine = [Recirculation Rate of Cooling Water (CMH)] x [Rate of Dosage (ppm)] x 0.001

The chlorine consumption per day of 10,000 m³/hr cooling water and 6 ppm dosage at three cycles per day and 30 minutes duration per each is:

10,000 m³/hr x 6 ppm x 0.001 = 60 kg/hr of chlorine
60 kg/hr x 30/60 hr x 3 cycles = 90 kg/day of chlorine

3) Design Factors for Chlorine System

Four major factors must be given consideration in the chlorination system. They are:

• Cooling Water Recirculation Flow Rates
• Chlorine Demand
• Nature of the interferences
• System Retention Time

4) Typical Gas Chlorination Dosage Rate

The typical rates in PPM or mg/l are for average Conditions. For special applications, consult with Capital Controls or Chungrok ENC Company.

5) Chlorine Gas Feeder Sizing Parameters

There are several design parameters that will enable a gas feeder to be sized properly. These parameters are:

(1) Flow Rate

The flow of water to be treated is a primary concern for proper system design. Flow rate data based on the recirculating cooling water should be obtained for the following conditions:

- Flow Rate: Constant _______ Variable ________
- Initial Plant Operation ________ GPD ________ (m³/h)
- Average Daily ________ GPD _________ (m³/h)
- Peak Daily ________ GPD _________ (m³/h)
- Future Design Requirement __________ GPD __________ (m³/h)

(2) Chlorine Demand

Accurate demand data is vital to sizing a gas feed system. A chlorine demand profile of the water being treated should always be included in the standard water quality analysis completed prior to plant design.

The impurities responsible for this demand include compounds containing iron, manganese, nitrates and sulfides. When chlorine is added to water, the amount reacting depends on the amount and type of impurities, pH, the amount of contact time, the temperature, and the amount of chlorine applied. The difference between the amount of chlorine applied (chlorine dosage) and the amount of chlorine residual remaining after the reaction is complete is known as the "chlorine demand" of the water.

CHLORINE DEMAND = CHLORINE DOSAGE - CHLORINE RESIDUAL

When the chlorine demand of a water is considered, it is always necessary to know pH, the conditions under which the data was obtained, and the temperature of the water, since the free chlorine (hypochlorous acid, HOCl) depends on pH and temperature of water.

As with flow rate data, chlorine demand data should be obtained for the following conditions:

- Chemical Demand: Constant _______ Variable ______
- Initial Plant Conditions ______ ppm (mg/l)
- Average Conditions _______ ppm (mg/l)
- Peak Conditions _______ ppm (mg/l)
- Future Design Requirements ______ ppm (mg/l)

(3) Feed Rate Required

Proper feeder sizing is a most critical requirement for proper system operation. The gas feed rate should be calculated for each plant condition, i.e. initial, average, peak, etc. The system should be sized for maximum dosage and flow requirement. Since most gas feed equipment is easily down-sized in the field, feed rate capacity conversion parts should be specified to prevent system over sizing during normal operation.

System over-sizing is the most common design error resulting in poor system performance. Because process and treatment facilities are designed for future as well as existing conditions, it is not uncommon to find that a gas feeder sized for maximum design feed rates is grossly over-sized for initial conditions. By using capacity feed rate conversion parts, the proper capacity equipment for initial conditions can be installed, then as flow rates increase, the feed rate capacity can be increased as necessary up to the design maximum.