Traditionally, chemical, fertilizer and gas manufacturers, power plants and other facilities with heat-generating industrial operations have used metal-mesh screens to filter debris and prevent pumps from clogging during industrial processes critical to reducing heat buildup. The most common options include static-mesh and traveling screens. Static-mesh screens typically are lower in cost but require manual removal and cleaning. By contrast, rotating traveling screens are operated by motors with timing mechanisms for periodic cleaning. Because they are automated, traveling screens require less attention from plant personnel.
While more efficient from a labor standpoint, the up-front cost of traveling systems can be significant, especially for facilities managers facing budget constraints.
Such was the case at El Dorado Chemical’s facility in El Dorado, Ark. A recent plant renovation and overhaul included the construction of two new cooling towers to circulate water and remove heat during the chemical production of ammonium nitrate and sulfuric acid.
Each cooling tower sits above a giant concrete basin that is 8’ deep and roughly the size of a football field. Water is pumped from the basin and water-retention pond to the top of the cooling tower, where it cascades over heat exchangers. The heat exchangers are used to draw heat from the coolant, which is pumped to the cooling tower from the chemical plant.
El Dorado’s original plant specifications called for the installation of a traveling water-screen system with 24 individual screens (12 per cooling tower). The system would be used to filter leaves, grass, branches, trash and other debris that gathered in the retention basin. After receiving initial bids in excess of $500,000 for the desired specifications, El Dorado and Leidos, its Oklahoma City-based engineering consultant, challenged bidders to reduce costs and design an alternative. Following the second bidding process, Cambridge EnTech, Cambridge, Md., proposed a custom-manufactured system that cut up-front costs and fell in line with El Dorado’s budget.
Analyzing the flow rate of the pumps and using hydrodynamic calculations based on in-housing testing, Cambridge worked with Leidos and El Dorado for several months to design a solution that fit both cooling and financial needs. The company drew from experience in producing screens for other cooling tower projects, including those at a solar thermal power facility in the Mohave Desert and a methane gas plant in Louisiana.
Design Modifications Reduce Equipment Cost and Improve Operation
Among the changes were a custom frame, chain-drive belt, hollow shafts, additional support member and revamped blow-off pumps.
Custom Structural Frame. Instead of using standard-sized structural members such as rolled I-beams, Cambridge laser-cut steel sheets and formed its own structure. After welding, the support frames were epoxy coated for corrosion resistance. The smaller members also allowed for more open area on the screen for water to flow. The increased flow meant the pumps did not need to work as hard to filter debris.
Chain-Drive Conveyor Belt. The chain-drive option provides strength along the edges to pull the belt out of the water. It also takes the load and supports the fine weave of the mesh, which filters debris down to 0.25”. Without the chain drive, the mesh would tear.
Hollow Shafts. The mesh was supported using hollow stainless steel shafts. The design, which used fewer raw materials, reduced the weight and torque on the motor. This produced energy savings and reduced maintenance costs.
Support Member. The company fabricated a special support member to go in the middle of the concrete opening for the sump. This resulted in a 13’ wide opening rather than an opening 6 to 7’ wide.
Blow-Off Pipes. In the traveling water screen design, water used for the blow-off pipes was directed back to the sump so that it could be recirculated with the cooling water.
Cambridge’s proposal reduced the number of 6’ screens from 24 to 16. The end design included eight per cooling tower, or one pair for each pump. Three pumps would run at a time, and a fourth was kept as a safety spare. Each 13’ wide pair then was coupled so that only one motor and gearbox was required for operation on each tower to reduce the equipment cost.
The screens rotate and a compressed-water spray-bar blows the filtered debris into a catch basin, which is emptied periodically by plant personnel. With this design, each screen pair is capable of filtering more than 20,000 gal/min of water.
Cambridge’s proposal reduced equipment and manufacturing costs while producing energy, water and electrical installation savings. The company reviewed the manufacturing process for its screens and modified it in ways that lowered up-front and operating costs (see sidebar). The company reflected these savings in the final winning bid to El Dorado.
Cambridge worked with on-site contractor Global Industrial to install the water screens in June 2015. Engineers from Cambridge were on-site more than two weeks during the process. Follow-up visits are planned in 2016 to make adjustments and ensure the operation is running correctly.
In addition, Cambridge offered a customized solution to correct a related problem at the El Dorado plant. During concrete pouring for the retention pond, four of the openings for the sump were made 1’ too wide. In less than two weeks, Cambridge fabricated custom stainless supports to close each gap and solve a critical issue for the construction and installation teams. PC
To learn more about traveling water screens from Cambridge EnTech, Cambridge, Md., call 866-863-9562 or visit www.cambridgeentech.com.