A water chemistry model for open recirculating cooling water systems

A steady state computer model is proposed to provide a visual approach to modeling the cooling water chemistry of open recirculating cooling water systems. Using only the chemical composition of a cooling tower's makeup water, the model calculates and plots chemical characteristics of the cooling water system as a function of pH and cycles of concentration, the two control parameters of a cooling tower. The model also plots an "Operations Window," a graph of pH versus cycles of concentration that shows important operating limitations for cooling towers. With additional cooling tower operating and cost data, the model may also be used to calculate and plot process flow rates and operating costs.; The computer model calculates cooling water chemistry assuming chemical and atmospheric equilibrium in an "open" cooling tower, but only chemical equilibrium in a "closed" cooling tower basin. Chemical charge balances are performed on the makeup water with the removal or addition of sodium and on the cycled makeup water with the removal or addition of carbon. The pH and total alkalinity of the cooling water after acid addition are calculated assuming either a constant total alkalinity versus pH slope, or fractional retention of the produced carbon dioxide.; A cost analysis performed using the model shows that the cost effectiveness of using sidestream softening in open recirculating cooling water systems increases dramatically as the cost for the treatment of cooling tower blowdown increases, especially for cooling water systems using hard and highly alkaline waters as makeup water. However, the cost of silica removal in the sidestream softener prohibits the use of sidestream softening of cooling water whenever silica removal is also required. Also, the cost for replacing cooling water treatment chemicals removed in the sidestream softener may prohibit the use of sidestream softening of cooling water unless the use of these treatment chemicals is no longer required in the softened cooling water or the use of other treatment chemicals that cannot be removed in the sidestream softener are adopted.