Open cooling towers expose process cooling water to the atmosphere, typically as part of a chiller system loop (see Figure 1). These open towers use an efficient, simple, and economical design. All components in an open system must be compatible with the oxygen introduced via the cooling tower.

Closed circuit cooling towers completely isolate process cooling fluid from the atmosphere. This is accomplished by combining heat rejection equipment with a heat exchanger in a closed circuit tower (see Figure 2). A closed loop system protects the quality of the process fluid, reduces system maintenance, and provides operational flexibility at a slightly higher initial cost.

When deciding which system is best for an application, several factors should be considered.

PERFORMANCE

If an application must produce full capacity throughout the year, maintaining a clean, reliable system loop is critical. Isolating the process fluid in a closed loop system prevents airborne contaminants from entering and fouling the system. Sustaining optimum performance in an open loop system will require regular maintenance to assure similar efficiency. High-efficiency chillers and heat exchangers rely on clean process water to function properly and are significantly impacted by even small amounts of fouling.

EXPENSE

The initial equipment cost of an open loop system will be less than a comparably sized closed loop system, since the open system does not include the intermediate heat exchanger component. However, the higher first cost of a closed loop system will be paid back during years of operation through the following savings:

Cleaner process fluid results in a cleaner internal surface area, and higher efficiency components in the system (e.g., chiller).

Reduced system maintenance costs.

Reduced water treatment costs for evaporative equipment.

Operating in "free cooling" mode during the winter to save energy consumption.

MAINTENANCE

Since the process fluid of a closed loop system is completely isolated from the environment, routine maintenance is only required on the heat rejection equipment itself. The need to shut down the system periodically to clean the heat exchanger is dramatically reduced, if not entirely eliminated. Providing clean process fluid to the system will extend the life of other components in the system (condenser bundles, compressors, etc.).

WATER TREATMENT

Maintaining proper process fluid quality in a system may involve several steps, such as chemical treatment, filtration equipment, and the addition of clean make-up water. A closed circuit cooling tower can provide the following advantages over an open cooling tower:

Lower volume of recirculating water to treat.

Process loop requires minimal treatment.

During periods of dry operation, the need for make-up water is eliminated.

OPERATIONAL FLEXIBILITY

Closed circuit towers allow for the following modes of operation not possible with open towers:

Free cooling operation without the need for an intermediate heat exchanger: Chiller turned off.

Dry operation: Conserve water and treatment chemicals, prevent icing and plume.

Variable pumping: Closed condenser water loop allows for variable speed pumping to conserve energy.

Figure 3. Chiller loop with open tower/heat exchanger combination.

CLOSED CIRCUIT TOWER vs.
OPEN TOWER/HEAT EXCHANGER

Sometimes, an open cooling tower is paired with a heat exchanger (see Figure 3) to capture some of the benefits of closed loop cooling. Choosing closed circuit cooling towers over this open tower/heat exchanger combination may still be a better choice for the following reasons:

Total cost: Addition of a heat exchanger (pump, piping, etc.) to the open tower loop brings the initial cost much closer to that of the closed circuit tower system.

Single piece of equipment: Compact design of the closed circuit tower conserves space in a self-contained package, compared to multiple locations for the tower/heat exchanger arrangement.

Maintenance: Narrow spacing in heat exchanger (e.g., plate and frame) may trap solids introduced by the open tower, requiring frequent, time-consuming cleaning to assure optimum performance

Dry operation: Open tower/heat exchanger system cannot be run dry in the winter.

These guidelines provide some general information to help decide whether a closed circuit cooling tower is better suited for a particular application than an open tower, with or without a heat exchanger. For additional assistance with a project, contact your local cooling tower representative.

Excerpted and reprinted with permission from the Baltimore Aircoil Company (BAC) Product & Application Handbook. For more information, visit www.baltimoreaircoil.com.

Publication date: 05/29/2006