What is the difference between closed loop and open loop cooling tower?

The Closed-Loop Advantage

It’s a common question during the early design phase of large mechanical systems: “Is open-loop or closed-loop cooling equipment better suited for this project?”

When it comes to modern heat rejection technology, both open-loop and closed-loop cooling equipment provide a distinct set of advantages for the engineer, installer and building owner. The specific cooling needs of the application, along with the physical parameters of the installation site, budgetary considerations and environmental goals should ultimately determine the type of system that’s best-suited and specified.

Over the past decade, CASEN has pioneered innovation in the closed circuit cooler market, along with advancements and refining of tried-and-true open-loop equipment options. With very real concerns about the higher water consumption of open-loop systems, closed-loop cooling technology is gaining broader appeal every year.

When properly designed for the commercial or industrial process cooling load, both system types can offer unparalleled energy efficiency, reliability and longevity.

Determining which system is best-suited to a certain application is a task left for the specifying engineer and others who are intimately familiar with the needs of the property.

Whether open or closed-loop heat rejection equipment is specified, be certain to select equipment that is CTI (Cooling Tower Institute) certified.

Open-Loop Equipment

The most prevalent type of large-scale heat rejection equipment in use today is the open-loop cooling tower. These systems are known for their expansive range in available capacities and configurations, reasonable first-cost, and energy efficiency. However, there are several trade-offs when compared to closed-circuit alternatives. Among the largest compromises with open-loop technology are water consumption and the level of maintenance and water treatment required.

With open-loop equipment, process fluid enters the top of the cooling tower and flows over the fill (or heat transfer media). At this point, the process water is open to outdoor air and any contaminants present in the atmosphere. Falling from the fill, water collects in a basin before returning to the facility’s cooling loop.

Due to airborne pollutants, incoming contaminants from the makeup water supply, and the presence of absorbed oxygen, proper maintenance of all equipment in the loop is critical. This also heightens the importance of water/fluid filtration and treatment. If the process water in the basin of the open tower is not properly treated, filtrated and maintained, the energy efficiency of the system will be reduced over time due to scaled and/or fouled heat exchangers and chiller tubes.

Open Loop Cooling Tower with Heat Exchanger

There are a variety of applications where an open loop cooling tower is the best option for heat rejection. Facilities that are not facing the need to minimize water consumption may benefit from this system type, as would a property needing to maximize the amount of heat rejection capacity in a limited footprint. However, a closed cooling circuit is still highly recommended for the fluid’s process heating +/or cooling loop(s).

In these situations, isolating the cooling tower from the process loop through the use of a heat exchanger may provide an ideal solution. In this way, the benefits of a cooling tower can be provided without requiring the maintenance that an open cooling loop requires. Isolating the system from the cooling tower through the use of a heat exchanger also eliminates concern for where the heat rejection equipment is installed in respect to the cooling loop.

Plate-and-frame heat exchangers are most frequently used for this type of design. When a heat exchanger is installed, the cooling tower must be sized properly to accommodate the temperature difference between the cooling tower water entering the heat exchanger and the process water that exits within the heat exchanger.

Isolating the cooling tower with a heat exchanger will substantially reduce the need to maintain downstream components. However, this also means that the heat exchanger will require routine maintenance. To ensure that both the thermal performance and the pressure drop across the heat exchanger meet design requirements, heat exchangers should be certified per AHRI Standard 400.

Closed-Loop Technology

CASEN’s wide range of closed circuit coolers, or simply “fluid coolers” provide a heat rejection alternative for engineers or end users who want (or need) to reduce water consumption and equipment maintenance, or a number of other considerations that exist with open-loop cooling applications.

Some cooling applications require a closed-loop system for peak-efficiency long term operation. These types of systems generally include the use of small heat exchangers in terminal units or other connected equipment, making maintenance complicated, if at all possible.

For example, buildings with water-source heat pump loops – widely used for office, hotel and health care facilities – are among one of the largest markets for fluid coolers. Using an open-cooling loop could pose the significant risk of fouling hundreds of heat exchangers in a condominium or similar facility. Closed circuit systems are also prevalent among data centers, battery plants, grow room facilities, high-efficiency chiller applications and multiple different types of industrial process loops.

Water loss through evaporation is either reduced or eliminated, depending on the type of closed-loop cooling equipment selected.

The same is true for water treatment chemicals and/or systems; closed-loop technology can help to dramatically 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 efficiency 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, there are significant installation savings attributed to the reduced pumping HP required, the elimination of an intermediate plate-and-frame heat exchanger, and the elimination of expensive valves and additional pipework. This is coupled with the lifetime operational savings including reduced water treatment/chemicals, reduced water consumption, and reduced maintenance. Comparing just an open tower to a closed circuit cooler in terms of price does not tell the whole story, when considering the up-front additional installation costs and operational costs of an open-loop system

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 data centers and critical process cooling 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 equalization. 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 sub-freezing outdoor temperatures. Some types of closed loop equipment may still require freeze protection of some sort, but 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.

Closed circuit coolers can also provide completely dry sensible heat rejection when outside ambient conditions are favorable. This dry capacity is an added benefit which can greatly reduce the overall water consumption on a project. 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 deenergized 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 since the recirculating pump is off.

The following are four primary types of closed loop heat rejection equipment:
★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 transfer of heat. 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 building via the bottom coil connection. Cascaded water drains to a basin and is recirculated back over the coil.

These coolers provide energy-efficient operation in a reduced footprint compared to dry coolers, due to evaporation being used as the primary method of cooling. Because blowdown of the basin water is reduced on closed-loop systems, water conservation is also improved when compared to open-loop systems. Because evaporative coolers can oftentimes run dry when ambient conditions are favorable during reduced load conditions, water consumption is eliminated entirely during these periods of operation.

CASEN manufactures both induced and forced draft evaporative closed circuit coolers for a wide range of applications. Units are available with cooling capacities from 6 to 1,670 tons.