The next step was to determine how much pressure rise, or total dynamic head (TDH), the circulation pump needed to generate to provide this cooling water circulation rate. Total dynamic head (TDH) is the sum of overall elevation change (i.e., static head) and friction loss (i.e., dynamic head). The system returned cooling water to an open air-cooled tower. Thus, the difference in height between the pump suction and the tower return line gave the static head, in this case, 15 ft. Dynamic head loss in piping systems is a function of flow rate, pipe size, pipe length, piping configuration and components — we found the overall flow resistance at the normal system flow rate of 570 gal/min to be about 100 ft.
So, the centrifugal pump needed to generate about 115 ft of head at this flow rate. The pump would need to produce significantly more head if every user were demanding cooling water at its maximum flow; however, we deemed this scenario unlikely. Instead, we chose a pump based on a reasonable compromise, generation of about 120 ft of head at 1,000 gal/min. It’s equipped with a 50-hp inverter duty motor capable of spinning up to 1,750 rpm. Installing a larger impeller would enable the pump to generate additional TDH, if needed at a later date.
Many companies have realized significant energy savings by adjusting speed in response to varying process demand. So, we opted to use variable frequency drives (VFD) on both the circulation pump and the cooling tower fans to help maximize efficiency.
We adopted a simple strategy to adjust cooling water flow in the various branches. We use cooling water supply and return temperatures as measures of the energy removal rate within a process, and aim to control cooling water flow within a branch to target a rise of 10°F.
To accomplish this, a temperature element and control valve were installed on each of the eight branches on the return line to the main header (Figure 2). A temperature element also was put on the supply line near the circulation pump. This allowed for the continual measurement of temperature rise for each of the eight units being fed.
Each control valve is forced to stay open at least 20%, to provide a minimum amount of flow through each branch. As cooling water within a particular branch rises more than 10°F above the supply temperature, its flow control valve opens further. This allows for greater flow through the branch needing water, slightly reducing flow through the other seven branches as resistances to flow change.
As demand within a branch continues to rise, the valve continues to open. The control system continually monitors the positions of all eight control valves. If any valve opens more than 80%, pump speed increases. As pump speed rises, so does flow and total dynamic head generated. This increase in head results in greater water flow through all branches. If a branch doesn’t require greater flow, its control valve closes (to its minimum position of 20%, if necessary). This directs more flow to the branch in need. The pump speed is set to run at a minimum of 975 rpm, to ensure some flow through the system.
The VFDs on the air circulation fans in the cooling tower allow us to compensate for changes in cooling water supply temperature, which can vary seasonally from near freezing to 80°F or higher, by altering speed. The old setup also used two 5-hp fans but both operated continuously at 1,750 rpm.
The new fans are programmed to operate from 25% to 100% of maximum speed over a 25°F temperature range, which differs for each: 45°F to 70°F for one, and 50°F to 75°F for the other. A two degree dead band prevents each of the fans from cycling on and off. Each of the fans shuts off at two degrees below its low point temperature (43°F and 48°F, respectively) and restarts once the temperature rises to the minimum value. When the supply temperature exceeds the maximum value in the range, the fan remains operating at full speed.