Cooling-Tower Design for Critical Applications

Editor's note: The following is an adaptation of "Design Considerations for Cooling Tower Systems with Critical Demands," presented at the 2010 Cooling Technology Institute Annual Conference, held Feb. 7-11 in Houston. To purchase the full paper, go to http://bit.ly/clPzdL.

Several factors need to be addressed prior to determining the appropriate cooling-tower system for a facility with critical demands. Locations, such as data centers, medical centers, airports, and electronic-chip manufacturers, require special considerations in addition to the requirements of an average cooling-tower system.

This article reviews the normal design considerations encountered in designing custom-built cooling-tower systems with critical demands and includes special design considerations necessary for mission-critical cooling facilities. Such considerations include building codes, customer requirements, environmental constraints, operational components, fire resistance, safety compliance, minimum maintenance, and a long service life that are mandatory for these critical-cooling applications.

Design Considerations
There are numerous factors to contemplate when designing any custom-built cooling-tower system. Designing a cooling-tower system with critical demands just intensifies the design process. Not only do the normal considerations for all cooling-tower systems apply, but the special demands that make the cooling system "critical" must be addressed as well.

Building Codes
Building codes are the foundation on which all cooling-tower systems are designed. Examples of building codes include:

• The International Building Code, which is developed and maintained by the International Code Council independent of the jurisdiction responsible for enacting the building code.
• State and local building codes.
• Hospital building codes.

Building codes dictate the basic design of the cooling-tower system. Once basic requirements are established, a cooling-tower-system design must be evaluated to determine if it meets/exceeds the critical demands of the system. For example, let's say a building code requires a cooling-tower system to meet 90-mph wind loads. Because this is a cooling-tower system that has critical demands, the wind load might need to be increased to 120 mph to ensure a stronger cooling-tower system.

Customer Requirements
A cooling-tower system's location influences several of the issues that must be addressed during design. Therefore, much thought should be given to a system’s placement. The requirements for a "grass-root tower" built in a clear field are different than those for a tower constructed to fit in or around existing buildings and equipment. In the current market, real-estate limitations result in cooling-tower systems being constructed not only at grade, but on rooftops. Basins may need to be elevated to meet location requirements.

Often, a customer will request a cooling tower be built to replace an existing tower. A new tower with increased thermal capacity is designed to fit within the old cooling tower's water basin. Additionally, the new tower design may be restricted by the old cooling tower's horsepower and pump head. Because the new cooling tower is replacing an existing cooling tower, the demolition of the old cooling tower and the installation of the new cooling tower usually are scheduled with limited downtime for the entire project.

Some of the more common customer requirements include:

• An access platform around the basin or piping/valves.
• A parapet wall to hide mechanical equipment.
• An inlet screen.
• A ladder and cage.
• A stairway.
• Conduit support.
• Colored walls to improve aesthetics.
• Internal walkway inspection.

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Environmental Constraints
There are three main environmental issues—water, acoustics/sound, and power consumption (horsepower) to be considered.

Water. Water usage can be a costly commodity for a cooling-tower system. In some cases, the amount of water usage or water loss can influence the taxation of a facility.

One area to review is the type of drift eliminator (DE) to be used in the cooling tower. A DE limits the amount of water droplets that can pass into the atmosphere.

Figure 1 shows how water droplets impact the side wall of a pack as air and water pass through a DE. As water droplets collect on the side wall, a water film is formed, causing droplets to drain back into the cooling tower.

Drift rate can range from 0.02 to 0.0005 percent. Just specifying drift rate is not sufficient for achieving the desired result from a DE. The amount of drift loss can vary depending on the quality of a DE installation. A DE must be installed with no openings between packs or between the DE packs and the structure around which they are installed.

Photo A shows a tight fit, with very few penetrations passing through a DE.

Acoustics/sound. Several sources, such as a fan, gearbox, motor, or water spray/splash impact, may contribute to a sound issue.

There are several methods of sound reduction, each impacting cost. Usually, the greater the desired sound reduction, the greater the cost impact. Also, the cost impact normally is greater than the noise reduction.

Some sound-reduction methods include:

• Reducing fan speed. As fan speed is reduced, the number of fan blades can be increased.
• Utilizing special low-noise fan blades/wider blades.
• Increasing fan stack height.
• Increasing tower size, which reduces the fan horsepower.
• Designing air inlets. (They must be horizontal with the tower.)
• Installing parapet walls.
• Installing walls in front of an air inlet. A wall should be installed at a distance from a tower equal to the height of the air inlet for proper airflow into the tower.
• Installing sound attenuators on exit-air inlets (above the fans) and/or air inlets.

Power consumption (horsepower). Methods of reducing power consumption include:

• Utilizing a two-speed motor, which allows motor speed to be decreased when demand on a cooling tower is down.
• Utilizing a variable-speed drive, which provides a much larger range of control and has the ability to match motor speed with the demand required.
• Reducing the cooling tower’s footprint. The amount of air required to cool a certain amount of water remains constant regardless of the footprint, assuming the conditions and fill media remain the same. The work required to move air through a tower increases as the size of the tower decreases, and vice versa. The more efficiently water is cooled, the lower the power demand. However, there are limitations to how the size of a cooling tower’s footprint can reduce power consumption.
• Selecting a high-efficiency fan allows more motor power to be applied to the work of pulling or moving air through the fill in a tower.

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Operational Components
A major design consideration must be the operational components of a cooling tower. These are issues that affect a tower’s day-to-day operations, as well as service and maintenance. Major operational components for most cooling towers include:

• A vibration switch.
• An oil-level switch.
• A motor heater.
• A viewing port in the fan stack.
• An internal mechanical-service walkway.
• A gearbox, fan, and drive shaft.
• Mechanical-equipment removing devices, such as a davit or trolley and beam.
• An extended fan deck.

Fire Resistance
Tower location can affect fire concerns. Concern increases if a cooling tower has restricted access that could prevent the maneuverability of fire-protection services. Also, the greater the demand placed on a tower by the owner, the greater the concern for the tower's fire resistance. Another consideration when determining the need for a fire-resistant cooling tower is the designated firefighting team's response time.

Once the need for a fire-resistant cooling tower has been identified, the degree of protection must be determined. The following areas provide fire resistance. Listed from the least to the greatest fire protection, the following areas provide fire resistance:

• Materials with a flame-spread rating of 25 or less.
• Materials that are self-extinguishing.
• Sprinkler systems.
• A 20-min fire-resistant wall system.
• A 30-min fire-resistant wall system.
• A Factory Mutual-approved cooling-tower system.

Safety Compliance
Safety is one of the most over-looked elements in cooling-tower design and construction. A critical cooling-tower system certainly needs to address safety concerns. By paying attention to safety requirements and issues during the cooling-tower-design process, safety compliance often can be achieved more easily and inexpensively. Safety always should be incorporated in tower design.

Safety compliance should include:

• Self-closing gates at handrail access points.
• An anti-skid fan deck.
• A grating around the tower.
• Barriers (guardrails) for openings.
• Guards around moving equipment minimize trip hazards when installing equipment.

Planning during the design process allows conduits to be placed outside of the handrail system, which eliminates the hazard.

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Minimum Maintenance and Service Life
The conditions within a cooling tower are complex. There are several factors that play a role in determining the frequency and type of maintenance a cooling tower requires to operate efficiently. The selection of material that will be used to construct a cooling tower, the water and chemicals used in a cooling tower, and the surrounding environment will affect the maintenance and service life of a cooling tower.

Cooling-tower-frame-structure recommendations for minimum maintenance and extended service life include:

• Concrete.
• Concrete and fiberglass.
• Fiberglass.

Metal recommendations to eliminate or reduce corrosion include:

• Structure hardware and anchor plates—304- or 316-grade stainless steel, silicon bronze, or Monel.
• Drive frame (torque tube)—hot-dip galvanized (HDG), HDG and epoxy, or 304-grade stainless steel.
• Fan hub—HDG or HDG and epoxy.
• Fan-blade hardware—304- or 316-grade stainless steel or Monel.
• Fan-stack hardware—304- or 316-grade stainless steel or silicon bronze.
• Motor and gearbox hardware—304- or 316-grade stainless steel or Monel.

Conclusion
Taking the time to identify the demands required for a cooling-tower system to operate efficiently is a vital part of the cooling-tower-design process. There are certain requirements with which all cooling towers must comply, but critical-demand cooling towers have special requisites that must be addressed. The design process is the logical time to identify and incorporate the necessary elements for the construction of a cooling-tower system that provides comfort and longevity.

Bibliography
ASTM. Standard test methods for fire tests of building construction and materials. ASTM E119. West Conshohocken, PA: ASTM International.
International Code Council. (2006). International building code 2006. Washington, DC: International Code Council. NFPA. (2005). Standard water-cooling towers. NFPA 214. Quincy, MA: National Fire Protection Agency.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan Spencer at megan.spencer@penton.com.

President of Composite Cooling Solutions LP (CCS), David M. (Mike) Bickerstaff has three decades of cooling-tower-industry experience, including the erection of cooling towers and air-cooled condensers for HVAC, process, and power applications. A construction-management strategist, he has held senior construction-management roles with Marley Cooling Technologies and Ceramic Cooling Tower Co. The vice president of marketing, building trades, and light industrial for CCS and the president of Bowman Engineering & Equipment Co., Frank J. Bowman Jr. has more than four decades of sales- and marketing-management experience in the cooling-tower industry. He has served as vice president of international marketing for Baltimore Aircoil and director of marketing for Ceramic Cooling Tower Co.