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A sidestream filter is a piece of equipment (normally a sand filter) through which a percentage of a cooling system’s total recirculating water is passed for the purpose of removing suspended solids. Normally the filter is sized so that between 3% and 5% of the total recirculating water flow passes through it. It should be sized so that all of the system’s water passes through the filter in 24 hours. Most cooling water sidestream filters are pressure filters. They are placed on line under pressure thus the necessity of pumping the filtered water back to the cooling tower is eliminated.
However, the following benefits are quantifiable: Reduced fouling by suspended matter will improve heat transfer efficiency. Reduced labor costs for physically cleaning the tower because a smaller quantity of suspended solids will settle out in the tower basin. Reduced chemical cost due to less adsorption and loss of treatment chemical on suspended matter. Reduced water cost, if reducing the suspended solids, allows operation of the cooling tower at higher cycles of concentration. In addition to these quantifiable benefits, sidestream filtration can also impact: Corrosion - if fouling by suspended solids settling out in low flow areas or on heat exchangers in low flow areas is prevented (e.g., shell side cooling), there is less potential for corrosion resulting from growth of anaerobic corrosive bacteria, which exist only under deposits (oxygen free environment). Although difficult to quantify until metal needs to be replaced, this often is a major cost. Improved microbiological control - this can be improved as a result of removing organisms and nutrients from the system water filtration.
Silica removal: Silica is removed as it precipitates on magnesium sludge (Mg(OH) 2 ). At the temperatures involved in cold lime softening, silica removal is not highly effective. Typically, only small amounts of silica are removed in this manner.
Suspended Solids Removal: In some instances, suspended solids removal is accomplished as the sludge agglomerates (combines) with incoming suspended solid particles.