Water is tested or analyzed to determine the level or concentration of impurities which may contribute to corrosion, deposits, or other undesirable reactions. The analysis is also done to measure the level of corrective treatments being applied to the system. We must, therefore, decide which impurities are significant and in need of control and which tests will be needed for the determination of treatment level. We must also establish a test frequency. Finally, the actual test results must be compared with the prescribed limits and any deviation from these limits must be properly interpreted in order that corrective measures are taken. Although the tests, frequency, and control limits will vary depending on the system to be protected, the principles remain the same. Thus, in the following discussion, we will use a typical open recirculating cooling tower system, but the same observations apply to boilers and other types of systems as well.
Evaporation + Bleed Off = Makeup
Certain constituents in the makeup water are undesirable due to their tendency to cause corrosion or to form deposits. In an open recirculating cooling system, the necessary cooling is accomplished by evaporation at the cooling tower. Evaporation of the water vapor has a concentration effect on all the impurities present in the system water. As the system continues to operate, these impurities will increase in concentration, finally reaching a point where corrective chemical treatment alone cannot adequately protect the system. Depending upon the type and amount of impurities originally present, a concentration limit can be established. The system water is then maintained below this limit by bleeding off a portion of the concentrated system water. Constituents which must be controlled include hardness, silica, and total dissolved solids. Because the evaporating effect of the cooling tower will concentrate all the impurities, it is not necessary to test for all of them. Rather, we determine the most critical impurity and set the concentration limit for this. We could then test for this constituent and maintain its concentration below the prescribed limit. However, because other impurities are concentrated to the same extent, another constituent could be selected as the control and its level monitored. For example, calcium may be the critical constituent and we determine that its concentration is four times the original, or makeup concentration. Often, total dissolved solids are used as the bleed-off control and is determined indirectly by measuring the electrical conductivity of the system water.
To summarize, we can say that the concentration of undesirable constituents is controlled by bleed-off and is adjusted to maintain the concentration of the system water below an established limit. This limit is set for that most critical impurity. We can test for that impurity itself or we can choose another constituent which may be easier to test and control.