Closing the Cooling Tower Loop
Closed-loop cooling systems may provide greater efficiency benefits than open-loop systems.
When it comes to process cooling, many choices are available for designing and customizing a system that best fits the needs of a given application. Perhaps
the most basic choice is whether an open- or closed-loop system should be used for the cooling application. When making the decision, many criteria come into play, including reliability, ease of operation, energy efficiency, water usage and total cost of ownership. Traditionally, it may appear to be a balancing act to pick the best combination of benefits, but for applications with flexibility and a focus on the bigger picture, a closed-loop system may offer across-the-board benefits that should not be ignored.
The primary focus of any process cooling application is keeping the manufacturing process itself up and running. Cooling system maintainability and reliability — thereby decreasing scheduled and emergency downtime — directly impact the ability to maximize process uptime. The cleaner the whole system stays, the easier it is to maintain and the more reliably it will perform.
A fundamental advantage of a closed-circuit cooling tower is that it protects downstream components against fouling. An open-loop cooling tower can introduce dirt, pollen and other contaminants to the process fluid. Over time, these can build up inside the cooling loop — or worse, directly within the process equipment if a heat exchanger is not employed. This requires that the arrangement of all connected equipment, including pumps and filters, be selected for operation in a contaminated environment.
A system running a closed loop prevents the introduction of foreign particles into the process fluid by isolating the fluid inside the coil of the closed-circuit cooling tower. This allows the fluid to maintain a higher quality that protects the downstream equipment and minimizes fouling throughout the system.
The easier a system is to maintain, the more likely it will receive the service it needs to stay operational. A closed-loop system offers an avenue to simplified maintenance and reliable cooling.
After evaluating the ability of a cooling system to perform reliably, it is important to consider how efficiently the system can operate. Water consumption and energy usage are two primary indicators of efficiency. By addressing these areas, it is possible to optimize the everyday system performance.
Water is used throughout the cooling system, but the limiting factor for water quality typically is determined by the composition of the process fluid. The total suspended solids, total dissolved solids, pH and other aspects of the process fluid can require more attention than water used for makeup or other side-stream applications.
Because a traditional open cooling tower does not separate the process fluid, it puts a larger burden on water usage. The cycles of concentration must be monitored carefully, and the quality control of the water becomes a critical aspect of system performance.
In a closed-loop system, the process fluid is separated from the spray water, which allows for lower quality makeup water at higher cycles of concentration. This reduces the overall water usage of the system while simultaneously providing suitable conditions to achieve peak performance and higher quality fluid in the process loop.
In addition to contaminants, an open cooling tower can introduce excess air into the system. Air that is trapped in the process fluid negatively affects its ability to transfer heat. If a heat exchanger is used to protect the process loop from air or contaminants, this also will degrade thermal efficiency. As the thermal efficiency decreases, energy usage must increase to maintain the appropriate fluid temperature.
Another process cooling option — air-cooled systems — rely solely on sensible heat transfer. Because of this, they operate at considerably lower thermal efficiencies than evaporatively based systems. Air-cooled systems supply higher temperature fluid than a cooling tower because they are dependent upon the ambient dry-bulb temperature. If the temperature of process fluid supplied to a chiller increases by 1°F, this can result in a 3 percent loss in efficiency for the chiller.
Where possible, using an evaporatively cooled system with the fewest thermal losses provides the best opportunity to minimize energy consumption and control the system’s performance.
Directly related to the efficiency of the cooling process is the total cost of ownership, which includes purchasing, operating and maintaining the system.
There is a clear correlation for translating energy efficiency to energy costs. As the usage of energy decreases, the operational cost for purchasing the energy also decreases.
The water-related costs of a cooling process have more intricacies that depend upon many different aspects of the system. In a closed-loop system, the ability to use lower quality spray water means that less makeup water can be used. This saves not only on the supply cost of water, but it also saves on the chemical treatment associated with that water. Furthermore, because spray water quality can be lower, it may be possible to source the makeup water from a separate process. This can save on both supply and sewage costs incurred for water in the closed-circuit cooling tower.
Additionally, because the fouling potential is minimized in the process loop, the lifespan of all the downstream components are maximized. This saves on maintenance and replacement costs for individual pieces of equipment throughout the lifecycle of the system.
In conclusion, for every application, there are multiple ways to meet the associated critical needs. By evaluating the combined benefits of a system, you can better align which options are right for you. A closed-loop system provides increased flexibility for isolating critical components and removing bottlenecks in the cooling loop. By employing solutions that flow through the entire system, you provide greater opportunities to increase the reliability of your equipment, simplify system operation, optimize efficiency and lower costs.