ACR Journal

August | September 2021 ADVERTORIAL 20 Volume 7 No.5 The same is true for water treatment chemicals and systems: Closed-loop technology can help to reduce or even eliminate the need for chemical treatment of system fluids. While open-loop cooling towers reject heat in a smaller footprint than closed- loop fluid coolers (due to the process fluid being cooled via direct latent heat transfer), closed-loop systems benefit from sustained thermal performance of the entire system. Higher whole-system e ciency over time is achieved because heat transfer surfaces experience less fouling. Closed-loop systems also typically require less pumping horsepower than open-loop systems of similar capacity. With a closed-loop system, installation savings can be achieved due to the reduced pumping horsepower required, the elimination of an intermediate plate-and- frame heat exchanger and the elimination of valves and additional pipework. This is in addition to the lifetime operational savings due to reduced water treatment and chemicals, reduced water consumption and reduced maintenance. Comparing just an open tower to a closed-circuit cooler in terms of price, however, does not tell the whole story. The up-front installation costs and operational costs of an open-loop system should be considered. Because the clean system fluid provided by a closed-loop design reduces maintenance and wear to all connected components, equipment lifecycle is lengthened. Reducing maintenance also results in less downtime, which is particularly important for critical process cooling and data center applications. Compared to open-loop cooling towers, fluid coolers provide more flexibility in terms of where heat-rejection equipment is installed. Closed-loop systems also do not require hydraulic balancing or equalisation. Because of this, fluid coolers can be installed at or below the level of the connected system piping. Conversely, installing a cooling tower below grade or below the pump could result in the tower flooding when the unit shuts down. Closed-loop equipment also provides an advantage for cooling systems operating in subfreezing outdoor temperatures. All open-loop cooling towers must be equipped with basin heaters, a drain-back design or a recirculation system for idle periods in freezing conditions. By contrast, only some types of closed-loop equipment require freeze protection. Closed-circuit coolers can also provide dry sensible heat rejection when outside ambient conditions are favorable. This dry capacity is an added benefit that can reduce the overall water consumption on a project. If the application conditions allow it, fluid coolers can be sized for full design or partial load based on a dry bulb switchover temperature. This means that the recirculating spray pump can be de- energized when the heat load can be fully satisfied by just the fluid cooler fans. While this operational mode greatly reduces water consumption, energy is also saved because the recirculating pump is o . The four primary types of closed-loop heat-rejection equipment are: • Evaporative closed-circuit coolers • Eco/Hybrid closed-circuit coolers • Adiabatic coolers • Dry coolers The cooling load of the system, available equipment space, sensitivity to water consumption, maintenance requirements, and project budget should determine which option is best for the specific application. Evaporative closed-circuit coolers Evaporative closed-circuit coolers eliminate the need for a heat exchanger between the process loop and the heat-rejection equipment. Unlike a cooling tower, where process water is used as the energy transfer medium and is open to the atmosphere, the coil inside a closed-circuit cooler isolates the process fluid. In a closed-circuit cooler, process fluid is circulated through coils within the unit. A water-distribution system cascades water over the tubes of the coil, extracting heat from the process loop via evaporation. Air is drawn or forced across the coils, agitating the falling water and increasing the heat transfer. A small amount of this water evaporates due to latent heat transfer through the tube and fin walls of the coil, removing heat from the system. The cooled process fluid returns to the process or facility via the bottom coil connection. Cascaded water drains to a basin and is recirculated back over the coil. These coolers provide energy-e cient operation in a reduced footprint compared to dry coolers due to evaporation being used as the primary method of cooling. Isolatinga cooling tower fromthe process loop through the use of aheat exchanger allows a cooling tower to be usedwithout requiring themaintenance that an open cooling loop requires. Plate and frame heat exchangers aremost frequently used for this type of design. Evaporative Adiabatic Hybrid Air Cooled The four primary types of closed-loopheat-rejection equipment are evaporative closed-circuit coolers, hybrid closed-circuit coolers, adiabatic coolers and dry coolers. The cooling load of the system, available equipment space, sensitivity towater consumption, maintenance requirements, andproject budget shoulddeterminewhich option is best.