Reducing the flow rate of spray water by throttling the flow with a regulating valve or reducing the speed of the pump motor will lower the heat-transfer capacity of the condenser. Tests suggest that the condenser capacity near its normal operating point varies as the flow rate to the 0.22 power:
thus if the flow rate is reduced by 20% the capacity of the condenser would drop to 95% of its original value. At lower flow rates the drop in capacity is more precipitous until a complete interruption of the spray-water flow to dry operation drops the condenser capacity significantly.
Usually reducing the spray water flow rate is not recommended. If the rate is dropped much below the design value, areas of the tubes may become alternately dry and wet. The result is excessive scaling on that tube surface. Avoidance of scale is also one of the reasons for opposing cycling of the pump for capacity control. The second reason is that the frequent stopping and starting of the motor accelerates its wear.