The purpose of a cooling water treatment programme is to control the problems that I outlined in part 2. Sourcing a a water treatment service company that offers a complete range of scale and corrosion inhibitor formulations, biocides and ancillary equipment is imperative to control these problems.
Water Softeners
If the available make-up water is very hard then removal of most of the calcium, which is the principal action associated with scale formation, will reduce the tendency for scale to form. For large systems, treating all the make-up water can be too expensive and may not be required. The softened water can be blended with raw water to provide a suitable make-up water, with a reduced tendency to form scale. Removal of calcium ions is usually carried out using a base exchange water softener.
Base exchange water softeners contain ion exchange resins which, when in contact with water, replace calcium ions with sodium ions. The system is fully automated using very reliable control valves that automatically regenerate the softener, based either on the time in operation or volume of water treated. Time control is usually only appropriate for intermittently operated systems and volume control is most cost effective as the resin is only regenerated when most of its exchange capacity is exhausted. Where soft water is required continuously or there is a large demand, the softener should be a "duplex" design, having two exchange columns such that one is in operation while the second is being regenerated
Chemical Dosing
Control of scale and corrosion is most commonly achieved by using chemical inhibitors. To function correctly, these chemicals need to be maintained at the correct concentration in the system. However, the concentration at which they are dosed to the system has to take account of the concentration factor. Thus in a system with a concentration factor of 4, an inhibitor requiring 100ppm would be dosed at 25ppm in the make-up water. However, even if a system maintains a constant concentration, the volume of make-up water added will vary depending on ambient weather conditions and the load on the system.
Accurate dosing of inhibitors is best achieved using a system that monitors make-up water quantity (usually from a water meter signal) and signals to a pump, the volume of chemical to add.
System Concentration Factor
Maintaining a constant system concentration factor is important for good system control; chemical treatments will have been recommended for the particular water chemistry at that concentration. At the least, variations in concentration will result in under or over concentration of treatment, which could make it ineffective or give an undesirable interaction with other
treatment chemicals.
Control of concentration is usually achieved by monitoring system conductivity (which is proportional to dissolved solids) and when a pre-fixed value is reached, opening an electronically controlled valve to remove water from system to drain,until a second (lower) conductivity value is reached when the valve will shut. This will result in the concentration varying between two limits depending on the
conductivity water meters points. The major requirement to ensure the correct operation is to clean and calibrate to conductivity probe regularly.
Biocide dosing
Control of microbial fouling is important for two reasons:
a) to prevent biofilm formation that reduces system efficiency and increases scale and corrosion rates; b) to ensure that microbial growth is controlled and proliferation of legionella is prevented.
The dosage of oxidising biocides such as Chlorine or Bromine, is often controlled by measuring the redox value of the system and dosing product to maintain a redox value that equates to a free chlorine or bromine concentration, in the recommended range of 0.5 - 1.0 mg/l chlorine or 1.0 - 2.0 mg/l bromine. The redox value of the system does not directly measure chlorine or bromine, but is the sum of all the reducing and oxidising species in the system. So that in a well controlled system, increase in redox value equates to an increase in chlorine or bromine concentration.
The fluctuations in concentration will reflect the rate of biocide addition in relation to the system capacity and recirculation. Fast dose rates will lead to greater variation around the target value as shown.
Cooling System Controllers
The ability to monitor and control the various elements of a programme incorporating cooling water treatment chemicals are key to the successful
control of the system itself. A single controller that is capable of monitoring redox, conductivity, water meter, and time makes it simpler and easier to ensure a system is operating correctly.
