Upgraded Cooling System Halves Energy Costs

By Peter Montagna, King Industries, Inc.

ChemicalProcessing.com

New layout and speed control also provide other benefits

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Cooling water plays a crucial and varied role at many plants — from temperature control of exothermic reactors to removal of pollutants via vent condensers. Sites require a reliable supply to maintain production and often to avoid safety and environmental incidents. Batch operations, with their many startups and shutdowns, pose an additional challenge in ensuring that cooling water is available when and where it’s needed. Such units, especially in pilot plants, may stay idle for an extended time. When they run, cooling demand can change from nonexistent to peak capacity within just a few minutes.

So, most companies will opt for a cooling system that has excess capacity. But with today’s high energy costs, too much excess capacity can lead to a loss of competitive advantage. It’s therefore critical to find a reliable, capable and economical source of cooling water.

Recently King Industries faced this challenge. Because our plant has variable production demands, it was important to install a system that was responsive to process demand yet energy efficient. Our upgrade gave us more cooling capacity while cutting energy costs by more than half.

Design basis

Figure 1. Cooling system had two sections served by separate pumps.

The plant’s existing cooling system included two towers, each with a centrifugal circulation pump operating at constant 3,500 rpm. The first tower fed three continuously operated units and a fourth batch unit that was infrequently used (Figure 1a). It provided up to 330 gal/min of cooling water. The second tower fed four batch units in a pilot plant area, each with highly variable demand (Figure 1b). This second tower provided only 120-gal/min peak capacity, which severely limited operations. Each tower had a fan equipped with a 5-hp motor spinning at 1,750 rpm.

Demand for pilot plant operations increased, necessitating a boost in cooling water flow to the area. So, we decided to replace the two towers with a single new one capable of providing cooling water to all eight users (Figure 2). Given current labor and business limitations, however, the cooling system would rarely run at maximum capacity. So, it was essential that it operate as economically as possible at varying capacities.

Our engineers evaluated each process unit, targeting a maximum cooling water temperature increase of 10°F. For continuous units, they used the normal energy removal rate to determine required cooling water flow (given in Figure 1a). For batch units, they calculated maximum cooling water flows (Figure 1b). This totaled 860 gal/min for the eight units, or 72% more flow than the original two tower system. We then applied a modest 20% factor to allow for any future expansion or contingency. This brought the targeted maximum flow rate for the new system up to 1,030 gal/min or 106% more than the original two tower system.

Figure 2. Single pump with VFD provides greater flow at lower cost.

We then sized the cooling tower itself. Based on the proposed circulation rate of 1,030 gal/min and a 10°F temperature rise, we determined the maximum heat removal rate for the tower to be about 5.15 MM BTU/hr, or 430 tons. With the help of the manufacturer we chose a tower based on this maximum heat removal rate and the maximum expected wet bulb temperature for the geographic area. The new tower included two air circulation fans equipped with 5-hp inverter duty motors capable of spinning at up to 1,750 rpm.

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.

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