![]() The approach controller TDIC-1 in Figure 1 (where TDIC stands for temperature difference indicating controller) serves to maintain its setpoint (SP-1) by throttling the air fan speed to keep the approach at the optimum point.įigure 1 also shows the optimum range, which corresponds to the optimum approach if all other conditions remain unaltered. ![]() Therefore, the system is optimized when it is operating at that point. At the same time, the fan costs tend to drop as the approach increases.įigure 1: In cooling towers with optimization controls, if the speeds of the pumps and fans are reduced from 100% to 80%, their operating cost will be cut in half, and if their speeds are reduced to 50%, their operating cost drops to 15%.Īfter adding up the two curves and obtaining the total cost curve, the optimum approach is at the minimum point on that curve. Therefore, as the approach rises, more and more water must be pumped and consequently the pumping costs will rise. This will cause the temperature of the cooled process fluid (Tp) to rise, which in turn will cause its controller (TIC-4) to further open the coolant valve, CV-4. Optimized controlsĪs shown in Figure 1, as the approach, and therefore cooling water temperature (Tctws) rises, the temperature difference across the process cooler (Tp – Tctws) is reduced. Other costs of operation include water losses: blowdown of 0.5-3.0% of circulated flow rate, evaporation of 1% for each 12.5 ☏ (7 ☌) of cooling range, and drift of 0.02% to 0.1% of circulated flow rate. The initial investment cost of cooling towers is about $40 per GPM of capacity and the energy cost of operation is about 0.01 BHP/GPM, or about $6 per year per GPM if optimized, and about $12 per year per GPM if not. If the speeds of the pumps and fans are reduced from 100% to 80%, their operating cost is cut in half, and if their speeds are cut in half, the operating cost drops to 15%. Therefore, it is desirable to use variable-speed pumps and fans. Because both the pump and the fan are sized for the maximum process load and worst weather conditions, operating them at full capacity when the load drops is wasteful. ![]() Optimization minimizes the sum of these costs. The operating cost of cooling tower operation is the sum of the energy costs of operating the cooling water pumps and the air fans. The controlled variable is the temperature of the cooling water that is sent back to the process and the manipulated variable is the air flow through the tower, which can be changed either by adjusting the speed of variable-speed fans or by starting and stopping a number of constant-speed fans. ![]() The load on a cooling tower depends on the flow and temperature of the water returning from the process. Here, I focus on the control and optimization methods, and on the operating cost reductions that can be obtained by applying these methods. They include range, which is the ∆T between return and supply water temperatures, and the approach, which is the ∆T between supply water and air wet bulb temperatures. Part 1 described the operation of the different cooling tower designs and the nature of the cooling process itself, plus the terms used in connection with it. Read Part 1: Cooling tower safety vs optimization: The operational and safety aspects of cooling towers. ![]()
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