There are two parallel centrifugal pumps which pump about 25000 ton/h water toward a dry cooling tower. The system is using a sigma-pressure indicator system (to be sure that the coolers are completely filled up and prevent any extra pressure in flexible junctions). The outlet butterfly valves are designed for three positions, include 1. closed, 2. completely open and 3. half open (usually when one of them is out of service)
The cooling tower is designed for the ambient temperature of about 16 C. In the winter, due to the extra decrease in ambient temperature (usually between -5 C to 10 C), we have to close the air dampers before air coolers. However, the electrical consumption of the pumps is constants, because there is not a mass flow control system or an electric speed control driver.One good suggest is decreasing in mass flow rate of the pumps. but there is a problem with controlling the outlet valves position.
One good suggest is decreasing in mass flow rate of the pumps, while can open the dampers more than usual. in this case, the electrical consumption of the pumps decrease. But there is a problem with controlling the outlet valves position and how to define a new sigma-pressure index for best operation conditions.
I will appreciate if anyone can help me.
Is it feasible to vary one of the pump's throughput by switching it on and off? Or would this likely damage the pump or the controller? What sort of motors do they use?
Dear Mr. Maine
It is not possible to switch the pumps on and off, the controller system can not accept this action. Moreover, when there are all air coolers in service, it is not possible to turn one of the pumps off, because the water level dropped. The pumps are equipped with a constant speed 6 kV electromotor driver.
I am looking for a way to decrease the mass flow rate, using outlet butterfly valves and adapt the system with this changes.
You can try to partially recirculate the flow at the outlet of the pump. It's possible to manage the amount of the flow recirculated by means of a simple gate valve. However, my feeling is that electrical consumption isn't reduced. I'm afraid that the only way is to use an electric inverter driver, if possible, to control the pump speed and consequently reduce the energy consumption. I'm interested in the solution you will adopt.
The purpose of the cooling tower is to cool water, isn't it? So I don't understand why you want to reduce the water flow at lower ambient temperatures. When you reduce the water flow, the delta T will increase. Is this what you want?
Dear Salvatore
Thank you for your answer, but it is not a good way to recirculate a part of the flow at the outlet of the pump because the electrical consumption will not change.
Dear Ron
We are looking for a way to decrease the electrical consumption during cold days. In fact, the amount of released heat by the cooling tower is constant. It happens by a special delta T (now about 10 oC) and a special flow rate (about 7000 lit/s). We can increase the delta T and decrease flow rate, while the amount of released heat keep constant. It leads a decrease in electrical consumption of the pumps.
In that case the only sure way to save electricity is to decrease the rpm of the pumps. Reducing the mass flow by throttling the valves will only save a small amount of energy, if any, depending on the pump curve, and the working point the pump is on. If you are lucky the pump will now operate in a point at the right side of the maximum efficiency curve. If that is the case throttling will bring the operating point to the left in the diagram, towards the optimum efficiency.
As said reducing the speed will surely bring you savings, proportional to the third power of the speed ratio. But modifying the pump driving system is costly, So the pay back time will depend heavily on the number of hours the speed can be reduced.
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