Evaporative coolers can use low-quality water to provide high-efficiency cooling.

Closed-loop, evaporative cooling systems vary in size from small packaged, skidded units to large, field-erected systems.

For industrial facilities around the world, water quality and availability have become important considerations. Many engineers assume that if their facility cannot obtain additional fresh water, the only available process cooling technology option is a dry cooler. However, closed-loop, evaporative cooling and condensing systems can optimize the use of scarce or poor-quality water resources. Because low-quality water  - for example, cooling tower and boiler blowdown, reverse osmosis discharge, demineralized water, and treated wastewater as well as reclaimed and produced water - can be used as spray water makeup, even sites with limited water availability have a choice. A plant can improve or increase capacity by using this cooling technology without having to purchase additional water; in fact, closed-loop, evaporative coolers often allow plants to reduce existing fresh water usage.

The drenching water spray of a closed-loop, evaporative cooler allows low-quality water to be used as makeup water.

System Design Benefits

Closed-loop, evaporative coolers use evaporative or latent (phase change) heat transfer to remove the process heat. As a result, the process outlet temperatures approach the designed wet bulb temperature. Conversely, dry coolers use sensible (no phase change) heat transfer, so the process outlet temperatures of these systems approach the designed dry bulb temperature. There usually is a large difference between wet and dry bulb temperatures. For example, a typical 85°F (29°C) dry bulb day at 60 percent relative humidity would have a corresponding wet bulb temperature of 65°F (18°C), which means that a closed-loop system would allow for a much lower process outlet temperature.

Because the closed-loop system is an evaporative cooling device, it will use as little as one-fifth of the plant space required for a dry cooler and will require much less horsepower to operate. Reduced horsepower requirements lead to lower operating costs, less noise contribution and a smaller carbon footprint. The increased heat transfer efficiency of such systems as compared to dry coolers allows for smaller physical equipment and therefore lower acquisition costs.

Closed-loop cooler heat transfer surfaces also require little maintenance. The tubes are all prime surface and do not use fins. Also, the systems use widely spaced tubular surface coils, which are less prone to fouling and plugging than the closely spaced fins of a typical dry cooler.

Closed-loop, evaporative coolers use evaporative or latent heat transfer to remove the process heat. As a result, the process outlet temperatures approach the designed wet bulb temperature for efficient cooling.

Water Efficiency

The closed-loop system rejects heat by evaporation (latent cooling). The process fluid or vapor to be cooled or condensed flows through the closed-loop tube bundles. Water from the unit basin is sprayed downward over the tube surfaces while fans induce airflow over the bundles in a co-current direction. On the tube surface exterior, evaporative cooling occurs at the outside water film boundary. The saturated airstream leaving the tube bundle then makes two 90-degree turns into the fan plenum. The reduction in velocity returns most of the large water droplets to the basin. The saturated air is then discharged through the fan stacks at a high velocity (1,500 ft/min).

Due to the closed-loop design, wide tube spacing and high drenching water spray rate, low-quality water (even containing suspended solids) can be used as makeup water. Because the open-loop spray water passes over the tube exterior only, it does not contaminate the process stream.

As part of a wet/dry system, using a closed-loop cooler is another way to minimize water use.

Minimizing Overall Water Use

Several design options can be considered to reduce the overall water use in a process cooling system while still maintaining cooling efficiency.

Combining a closed-loop system with a dry cooler is one option. In this configuration, the dry cooler can accomplish the first part of the cooling or the highest-temperature portion, and the closed-loop cooler system can be used as an efficient trim cooler to finish the remainder of the cooling and achieve the desired process outlet temperature even in the hottest ambient conditions. This cooling combination can attain the low process outlet temperatures that a dry cooler alone would have difficulty accomplishing. The spray water can be turned off during colder ambient periods to allow the dry cooler to do all the cooling without requiring any water.

Using a closed-loop cooler as part of a wet/dry system is another way to minimize water use. With this design, widely spaced fins (four to five per inch) are used on the tube bundle strictly for dry cooling or condensing during colder ambient periods. An optimal ambient temperature is selected, below which the unit can be run completely dry. For instance, a plant might specify that the system operate dry at 80°F (27°C) dry bulb or lower and operate in the wet mode with the spray water turned on when the ambient dry bulb exceeds 80°F either on a seasonal or day/night basis. This type of system has the advantage of achieving low process outlet temperatures while realizing some of the space savings and operating cost reductions of a wet system. Wet/dry systems allow significant water savings while offering operator flexibility in choosing whether to use water depending on ambient conditions and the plant’s cooling needs.

In conclusion, water conservation will continue to have an increased impact on plant design and operation. Due to the innovative design and custom manufacturing capabilities provided by closed-loop evaporative coolers and condensers, these systems can help maintain plant performance while using water streams that might be unusable with other cooling technologies, thereby providing the opportunity to conserve water.