Deposit Control

Scale Inhibitions

Five general methods are available for controlling mineral scale formation in open recirculating cooling systems. These are:

• §         External pretreatment of the make-up water

• §         External side stream or full flow treatment of the circulating water

• §         Blowdown control

• §         Chemical treatment of the circulating water to reduce the reactivity of one of more reacting species

• §         Chemical treatment of the circulating water to "stabilize" reactive species so that they will not precipitate from solution.

The optimum scale control program developed for any specific system must depend on the make-up water composition and its availability, operating parameters in the cooling systems, the number of concentration cycles to be carried in the circulating water, and sometimes on effluent considerations. Some systems, for which plenty of very soft make-up water is available, do not require any scale control program.

For other systems, simple stabilization chemical treatment is sufficient. On the other hand, in many parts of the country the make-up water is both hard and in short supply, so that it must be conserved.

The most cost-effective scale control program for recirculating cooling systems in such cases may include, for example, partial softening of the make-up water, strict control of concentration cycles, side stream filtration or softening and stabilization chemical treatment of the circulating water. For further chemical treatment, contact the makers of chemicals. The blowdown control shall be only discussed here.

Increasing the blowdown rate from a recirculating cooling system is a simple way to reduce the levels of calcium and alkalinity in the water, thus reducing the calcium carbonates scaling potential. However, this is frequently not a cost-effective option. Increased blowdown, which means operating the cooling system at lower cycles of concentration, requires increased make-up water and products more wastewater for disposal. Increased make-up leads to increased corrosion inhibitor usage and may require more frequent biocide applications.

Blowdown control is, however, a critical part of any good scale control program in open recirculating cooling systems. It is important to strike a technically practical and cost-effective balance between the hardness that can be removed from the make-up water by pretreatment, the cycles of concentration that can normally be achieved, the amount and quality of blowdown water that can be tolerated and the costs of acid and stabilizing treatment chemicals.

The ability of the plant to control the system is also an important factor. Widely varying blowdown rates can make any scale control program costly and/or ineffective. Also, lapses in feed of stabilizing chemicals can lead to serious scaling problems if the system is operating under supersaturated conditions.

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