Designing a process cooling system means selecting from a myriad of equipment possibilities. The first decision is whether to choose an open- or closed-loop system for your application. Decisions based on reliability, ease of installation, energy efficiency, water usage and total cost of ownership can come into play when choosing a system.
In the past, given all the options, it may have appeared to be quite confusing. Today, with an optimized selection, it is clear that a closed-loop system can bring benefits not attainable with open-loop systems.
The main focuses of any process cooling application are keeping the equipment up and running and decreasing scheduled and emergency downtime. These efforts can have a large impact on the process and the bottom line. System maintainability and reliability directly impact your ability to maximize uptime.
The fundamental advantage of a closed-loop system is to keep the whole system cleaner, which makes it easier to maintain and improves reliability and performance. An open-loop cooling tower can introduce dirt and other contaminants to the process fluid. Over time, these can accumulate inside of the cooling system loop or directly within the equipment if a heat exchanger is not installed. This requires the arrangement of all the system’s equipment to be selected for operation in a contaminated environment. By contrast, a system running a closed loop prevents the introduction of debris to the process fluid by isolating the fluid inside of a coil in a closed-circuit cooling tower. This allows the fluid to maintain a higher level of quality, which protects the system’s other components and minimizes downtime.
Choosing easy-to-maintain equipment can result in routine maintenance getting done more frequently. This, in turn, helps the performance and reliability of the closed-circuit cooling tower and the entire cooling system.
Increasing the Efficiency of Closed-Loop Circuit Cooling Systems
Once the advantages of a closed-circuit cooling system for reliable performance are understood, it is important to consider how efficiently the system can operate. Water and energy usage are the primary drivers when evaluating a system. By evaluating these factors, you can optimize the daily system performance.
Water is used throughout the cooling system, but the issues for water quality are typically determined by the contents of the process fluid. Water quality and other aspects of the process fluid may require much more attention to detail than water used for makeup or other side-stream applications.
An open cooling tower does not separate the process fluid. This puts a larger burden on water usage and must be monitored carefully. By contrast, when the system is closed, the process fluid is separated from the spray water. This allows for lower quality makeup water at higher cycles of concentration. This reduces water usage while providing conditions conducive to peak performance — all without compromising the quality of the fluid in the closed loop.
In addition to contaminants, an open cooling tower can introduce excess air into the cooling system. Air 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 will result in losses in performance. As the performance decreases, energy usage increases to maintain the specified fluid temperature.
Reducing Operating Costs of Closed-Loop Circuit Cooling Systems
The total cost of ownership for purchasing, operating and maintaining a system is directly related to its efficiency. There is a clear connection between energy efficiency and energy costs. As energy use decreases, the operational costs for purchasing the energy also decrease.
Water costs for cooling processes often have more variables that depend upon many 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 provides two economic benefits: less supplied water must be purchased, and less purchased water must be treated. Because spray-water quality can be lower, it may be possible to reclaim the source of makeup water from another process. This saves on both supply and sewage costs incurred for water in a closed-circuit cooling tower.
Additionally, with a reduction in fouling in the process loop, the lifespan of all the downstream components is maximized. This saves on maintenance and replacement costs for all pieces of equipment throughout the lifecycle of the system.
Beyond operating costs, utilizing a closed-loop cooling system presents opportunities to save on installation costs. Keeping the process fluid in a closed system allows you to prioritize which components require upgraded materials of construction. For instance, suppose that for a given application, the open cooling tower must be made of stainless steel. You may find that when using a closed-loop system, it is possible to use a galvanized closed-circuit cooling tower.
In addition to reducing your material costs, it may be possible to reduce the total number of components needed. For example, because the process side is kept isolated, the heat exchange loop may be eliminated. This saves money on equipment such as the heat exchanger and pump as well as the installation labor for installing and balancing the system.
Aside from the more traditional ways to save, some creative solutions for using a closed-circuit cooling tower can help save on the initial and annual costs of the system. Some examples include:
- Facilities with rigorous maintenance schedules using service options to reduce labor.
- Buildings located in cooler climates taking advantage of dry winter operations.
- Facilities with elevated water costs utilizing a hybrid closed-circuit cooling tower, which offers optional wet, dry or adiabatic operation.
These opportunities tend to occur on a case-by-case basis because everything from the specific process to the local environment may offer the prospect of saving.
Optimizing Process Cooling Equipment Performance
The final consideration is whether there are unique circumstances that could require special equipment or an alternative system configuration. If there are processes for multiple fluids or different temperatures of the process fluid, it is possible to use a single closed-circuit cooling tower to service both processes. Split-coil configurations can meet these needs without requiring a second closed-circuit cooling tower. This eliminates the need for multiple cooling towers while maintaining the advantages of a closed-loop system.
Special coils can be supplied in closed-circuit cooling towers to meet specific codes and standards. Additionally, upgraded coils can be incorporated when serving heavy-duty applications.
In some cases, it makes sense to use an open-loop system for less critical applications and reserve a closed-loop system for the processes with the most critical operating conditions. This is especially relevant if your facility has both process cooling and comfort cooling needs. The process loop utilizes a closed-circuit cooling tower to achieve the advantages where higher quality water is required while the HVAC loop may use an open cooling tower for supplying condenser water to the air handler.
In conclusion, every application has multiple solutions to specific needs. Selecting the right system based on your needs ensures that the solution is right for the application. Closed-loop systems provide increased flexibility for isolating critical components and removing interruptions within the cooling loop. Utilizing these solutions that flow through the entire system can provide greater opportunities to increase the reliability of equipment, simplify system operation, optimize efficiency and lower costs all while reducing the risk of downtime.