Properly maintained evaporative condensers and closed-circuit cooling towers are imperative to ensuring energy-efficient, reliable process cooling. However, these cooling products often are located in remote areas where they can be easily neglected. As a result, they can become susceptible to corrosion from scale, algae and other solids, all of which can have a negative impact on the equipment performance over time.
Many facility operators have found that induced-draft evaporative condensers and closed-circuit cooling towers based on an advanced coil technology can provide a solution. Designed to reduce scale formation and fouling for a cleaner coil and sustained peak thermal performance, these advanced systems are helping to ensure reliable, efficient process cooling in a range of applications.
Fouling occurs when solids -- often calcium and magnesium compounds -- build up on the coil's exterior surface as water evaporates within the unit. By reducing this fouling tendency, the advanced coil technology is able to sustain its peak heat transfer capability over the life of the equipment and maximize its longevity (figure 1).
Four facets of the technology contribute to its reduced scale formation and fouling tendency:
The Air and Water Flow in a Parallel Path. The coil system maintains better water coverage over the tubes because the air and spray water flow in a smooth, parallel, downward path over the coil. With this parallel flow, the spray water is not stripped from the underside of the tubes by the upward airflow, as it is on many conventional designs. The improved water coverage eliminates scale-producing dry spots on the coil (figure 2).
Water Flow Over the Coil Is Increased. The coil system has a spray water coverage of at least 10 gal/min/ft2 of coil plan area, compared to the 4 to 5 gal/min/ft2 of spray coverage on many conventional systems. This level of coverage provides continuous flooding of the primary heat transfer surface for decreased fouling potential. The chief engineer for a refrigerated warehouse noted a significant difference in coil wetting between an evaporative condenser with the improved coil technology and the other condensers in his facility.
Evaporative Cooling Occurs Primarily in the Fill. The design incorporates a combined-flow technology that uses both primary and secondary heat transfer surfaces. The primary heat transfer surface is a serpentine coil, which relies largely on sensible conduction heat transfer. As a result, it is less susceptible to scale formation than other designs that are more reliant on latent (evaporative) heat transfer. More than 60 percent of the latent heat transfer in the advanced systems occurs in the secondary surface, which effectively moves the evaporation process away from the primary heat transfer surface.
Spray Water Is Colder. Spray water at a colder temperature has a lower propensity to form scale because scale-forming compounds remain in solution rather than being deposited as solids on the exterior coil surface. In the advanced equipment, the use of a secondary heat transfer surface keeps the spray water flowing over the coil six to eight degrees colder than on other closed-circuit cooling tower and evaporative condenser designs. This colder spray water alone typically reduces the scaling potential by 25 percent compared to other systems.
Maintenance ConsiderationsNo matter how advanced the equipment, regular preventive maintenance is still key to preventing downtime. Equipment that is easy to maintain is more likely to be maintained.
The water level control, pump suction strainer and spray nozzles in the advanced evaporative condenser and cooling tower are accessible for inspection and adjustment from outside the unit while it is operating. Inspection and cleaning of the water distribution system and coil can be accomplished quickly and easily without removing the eliminator sections.
The effect of scale buildup or fouling on the performance of a conventional closed-circuit cooling tower or evaporative condenser can be significant. With only 1/32" of scale on the coil, a unit can be robbed of 27 percent of its heat transfer capability. As the scale thickness increases, capacity losses rise substantially.
Induced-draft crossflow designs that incorporate an advanced coil technology and a maintenance-friendly configuration can minimize scale formation and fouling to ensure sustained thermal performance and optimized coil life. PCE