As was mentioned in the "Corrosion" section of this manual, the solubility product of calcium and orthophosphate is very low. As a result, if orthophosphate is present in the water phase, it can react with calcium to form monohydrogen, dihydrogen and tricalcium phosphate. These phosphate sludges can bake onto heat exchange surfaces and, like other deposits, can both decrease heat transfer efficiency and yield underdeposit corrosion.

Unfortunately, it is more difficult to predict the formation of tricalcium phosphate than it is to predict the formation of calcium carbonate. However, there are some general guidelines you should follow. First, whenever controlling an inorganic or organic phosphate program, you should always test the orthophosphate concentration in the recirculating water. Second, if the orthophosphate concentration is greater than 2 ppm, you should test for both filtered and unfiltered orthophosphate.

Within reasonable testing error, these two concentrations should be the same. If the filtered orthophosphate is lower than the unfiltered result, it is a good indication that there is a phosphate precipitation problem in the water. You should also note that the makeup water will sometimes contain a detectable concentration of phosphate. Once cycled in the cooling tower, this phosphate, if not properly controlled, can create fouling problems.