Trends in cooling towers
Cooling towers are one of industry’s primary ways of conserving water and dissipating heat. They are not new technology. On the contrary, they are familiar landmarks at most facilities, enabling process and comfort cooling systems to operate efficiently and effectively.
Perhaps because of their familiarity, however, cooling towers are also often among the most neglected and ignored plant systems. Yet with a little understanding and attention, these effective heat transfer media can provide years of uninterrupted service.
And thanks to improvements in environmental factors such as water treatment, noise control, and plume abatement; in construction and fill materials; and in controls; today’s tower is better equipped than ever to meet both the environmental and cooling water needs of most plants
Reviewing the basics
Cooling towers come in several models and types, each with various characteristics. Selection and size depend on application and heat load. Atmospheric towers use no mechanical device to create airflow. Small, natural induction units are inexpensive and have limited use. Hyperbolic natural draft towers, on the other hand, are very dependable and typically used in large applications such as electric power generation.
More common in industry are mechanical draft towers. In these units, single or multiple fans provide airflow. In forced draft structures (Fig. 1), air is blown through the tower by a fan located in the ambient airstream entering the tower. In induced draft units (Fig. 2), the fan is located in the exiting airstream and draws air through the tower. Hybrid towers combine the features of natural and mechanical units, often giving the appearance of natural draft towers, but being equipped with mechanical draft fans to augment airflow.
Towers are also typed by the flow relationship of air and water within the unit and by construction. In counterflow towers, air moves vertically upward through the fill, counter to the downward fall of water. The enclosed nature of a counterflow tower restricts exposure of the water to direct sunlight, thereby retarding the growth of algae. In crossflow towers, air flows horizontally through the fill configuration across the downward fall of water. Hot water is delivered to inlet basins atop the fill areas and is distributed to the fill by gravity through metering orifices in the floor of the basins.
Field-erected towers are primarily constructed at the site. Prefabricated pieces are shipped to the site for final assembly. Large towers are usually field-erected. Factory-assembled units (Fig. 3), also called packaged or unitary towers, are virtually put together at the point of manufacture, then shipped in as few sections as possible for assembly on site. Small towers are shipped essentially intact. Larger units are shipped as cells or modules, then joined by the user or a contractor on site.
Myriad factors affect cooling tower selection and performance. (See accompanying section, “Tips for selecting a cooling tower.”) Methods for calculating standard design parameters such as wet-bulb temperature, dry-bulb temperature and/or relative humidity, heat load, flow, range, and approach remain unchanged. But although cooling tower technology has stayed relatively stable, certain trends are allowing owners and operators as well as manufacturers to view some aspects of tower construction and operation in a new light. These areas of focus fall into three primary categories: environmental, materials of construction, and controls.
Aesthetics and environmental concerns are among the most important factors affecting cooling tower construction, performance, and operation today. Owners and operators as well as members of communities surrounding towers are demanding construction that blends into the landscape (Fig. 4). More serious environmental issues, however, include noise reduction, plume abatement, and water treatment and chemical use.
Noise control. When a tower has been sized to operate within an enclosure, the enclosure itself has a dampening effect on sound. Sound also declines with distance. However, in some cases, additional noise attenuation measures may be desirable.
Noise is typically of greatest concern at night when members of the surrounding community are sleeping. Such situations can be resolved today with two-speed motors operating the cooling tower fans at reduced speed without cycling during these times. Or, variable speed drives can be used to control the speed of tower fans, preventing the tower from cycling on and off unnecessarily. This relatively inexpensive noise abatement modification can pay for itself quickly through reduced energy costs.
The addition of low noise fans with specially designed airfoils minimizes unwanted noise at all times. In extreme cases, inlet and discharge sound attenuator sections may be required. However, this option is less desirable because it may require an increase in tower size and obstruct normal maintenance procedures.
Plume abatement. Plume abatement is the response by manufacturers to recent demands to reduce the visible fog-like discharge that is a byproduct of evaporation. Although not a health hazard or a pollutant (it is merely water vapor), cooling tower plumes may obscure visibility and be considered unsightly by residents or other companies in the surrounding area.
Plume abatement is accomplished essentially through the addition of a dry heat exchanger placed above the fill and drift eliminators (Fig. 5). Hot water from the process first circulates through the dry air heat exchanger, then through the evaporative fill section. Ambient air is drawn simultaneously through the dry heat exchanger and the warm, wet fill.
Mixing the dry, hot air with the saturated air from the fill in the plenum above the drift eliminators reduces the relative humidity of the air and water vapor mixture leaving the fill. The visible plume is greatly reduced or eliminated, because cooling to near ambient temperatures prevents condensation. In addition, use of a nonmetallic PVC heat exchanger in place of older metal fin-tube units is improving both heat transfer and plume reduction capabilities further.
Water treatment . Increasing and changing restrictions placed by environmental regulations on the quality of water circulated through and/or discharged by cooling towers form another area of concern for owners and operators. Cooling towers are extremely effective air washers. As a result, the quality of water circulated through a tower quickly reflects that of the surrounding air. The added impact of evaporation coupled with increased restrictions on the use of chemicals in blowdown makeup water create continuous water quality challenges.
Governmental curtailment of chlorine levels in blowdown water has forced plants to find alternatives to keep discharges within EPA limits while maintaining adequate control of biological growth. The resulting changes in treatment programs have included the use of such chemical-free processes as ozonation to maintain adequate water quality and still operate within regulatory limits. Because regulations and requirements vary with geographical location, plants considering a review of their cooling tower water treatment programs should consult local experts to determine what restrictions apply to their area and what options are available to them.
Construction materials and fill materials
For field-erected towers, the choice in materials of construction has evolved to requiring less wood units and more fiber glass (FRP) structures. In smaller-sized factory-assembled units, longevity and durability make FRP towers a popular choice. However, a noticeable trend in factory-assembled towers is toward the use of stainless steel instead of galvanized steel.
The more durable stainless steel is better equipped to withstand the attack of water treatment chemicals. As the environmental restrictions have increased — in particular as the use of chromates in water treatment for comfort cooling applications has been eliminated — the option to use basic, simple, inexpensive, and effective corrosion inhibitors is disappearing. Simultaneously, corrosion has become more and more of a problem and the need to protect galvanized steel has grown.
In addition, stainless steel towers are now more economical and thus a more viable product. Large sized units (up to 1100 tons of capacity/cell) are becoming more common and factory-assembled units are being applied in multicell configurations in industrial applications of significant size. In fact, nearly half of all factory-assembled towers today are at least part stainless steel. Fire resistance and a quick delivery time, coupled with added durability, give industrial users a steel tower with the performance they’ve come to expect from wood at less cost.
Fill materials (heat transfer surfaces) are still most commonly composed of PVC, although some use is still made of treated wood lath. Splash-type bars, which break up the water by interrupting its vertical progress, typically are extruded shapes of PVC. Film-type fill causes the tower water to spread into a thin film and flow over large vertical areas to promote maximum exposure to the airflow. It is vacuum-formed or thermoformed from flat sheets of PVC into complicated shapes that are bundled into fill packs. Many types of film fill have integral drift eliminators to reduce the probability of icing during winter.
Today’s sophisticated controls help a cooling tower deliver its full potential. Most recently, systems have seen a dramatic increase in the use of variable frequency drives. Self-contained control products typically include single-speed, two-speed, or variable-speed motor controls plus controls for collection basin heaters and water makeup systems in a single panel. Large towers are frequently monitored from a remote control center.
In some cases, tower controls may be tied to the building automation system (BAS). Although cooling tower monitoring and control are not typically considered sufficiently critical to require precision monitoring, variable speed tower control products are usually designed to interface with any BAS. With today’s emphasis on monitoring and data collection, the most important issue with cooling tower controls remains compatibility. The motor and the controls need to match, especially when two-speed motors are used. Motors are available in several designs and different controls are required for each.
In addition, fuses must be selected to protect the wiring used, and sized to withstand the inrush current required to start the motor. Cooling tower fans are high-inertial loads and motor overloads must be sized to protect the motor under all operating conditions without causing nuisance trips. If not done correctly, temperature control methods can be a source of problems in many systems.
An eye on the future
The continued popularity of cooling towers as an environmentally-friendly means of conserving and creating cold water seems assured. The structure of the industry, however, moves with an eye on today’s trend toward outsourcing plant activities. Although the idea of outsourcing the plant cooling tower may sound foreign now, some experts predict that in the next century plants will buy cold water instead of cooling towers. Tower manufacturers will evolve into service organizations, leasing and operating cooling towers for industry and selling plants cooled water for a specified price. The concept, similar to that of the district heating and cooling plants appearing in the northeast, reflects a tendency by plant owners to outsource activities that are not part of the company’s core competencies and work with vendors who will both provide and maintain the product or system.
Regardless of who ends up operating the equipment, the fact remains that appropriate maintenance and good communication are the critical elements in building and operating an effective, efficient tower system. Establishing upfront what is expected, ensuring the completed system is well cared for, and staying abreast of trends in the industry are ways to ensure finding an appropriate and enduring solution to an age old need.
Plant Engineering magazine acknowledges with appreciation the special input to this article provided by the following companies: Baltimore Aircoil Co., Baltimore, MD; The Marley Cooling Tower Co., Overland Park, KS; and Thermal Care, Niles, IL. A special thank you to The Marley Cooling Tower Co. for providing the cover photo.
Cooling towers effectively conserve water and dissipate heat from industrial operations.
Improvements in controls, noise and plume abatement, and materials have made towers more efficient.
Proper selection and maintenance are critical to smooth performance.
Additional information about cooling towers is available from these sources:
The Cooling Tower Institute encourages water conservation and better cooling towers, among other objectives. Contact the CTI at 530 Wells Fargo Dr., Suite 218, Houston, TX 77090; 281-583-4087; fax: 281-537-1721, www.cti.org .
Educational materials are available from tower authority Bob Burger at The Burger Companies , 1400 Julia Ave., Suite B, McLean, VA 22101-4027; 703-827-5034; fax: 703-893-4935; e-mail: firstname.lastname@example.org; web site: www.coolingtower.com.
Related articles on this subject may be found on the ” Air conditioning, ventilation, and refrigeration” channel at www.plantengineering. com. An expanded version of this article, which includes a special section on tower inspection, also appears on the web site.
Tips for selecting a cooling tower
Cooling tower technology and components have evolved over the last 20 yr to include improvements in reliability, performance, and materials of construction. More compact designs have led to better horsepower/ton cooling characteristics. Here are some tips to consider when selecting a cooling tower.
1. Buy value. A cooling tower must perform day in and day out. Buy a proven cooling tower from a company that can provide service and technical assistance..
2. Consider lifetime costs. A cooling tower’s initial installed cost is only a part of the total lifetime expense of the unit. Be sure to consider operating costs such as electricity, water, filtration, chemical treatments, maintenance, and repair.
3. Consider the 1% design wet-bulb temperature for your tower location. Determine if the 1% design wet-bulb temperature in your location is higher or lower than the 78 F nominal rating. Then, be sure that design temperature is considered in the tower capacity calculation. For example, a cooling tower at nominal conditions that provides 100 tons cooling capacity in Chicago (1% wet bulb of 78 F) provides substantially more cooling tons of capacity in Phoenix (1% wet bulb of 62 F).
4. Use heat exchangers if possible. Keep the tower water circuit isolated from the clean process water circuit to achieve maximum efficiency and reliability. Evaporative cooling towers are very efficient air scrubbers. Airborne contaminants can be drawn into the tower’s cooling water stream. Direct use of tower water can lead to scaling and high maintenance of downstream components. A process system with a heat exchanger keeps the clean water separate from the tower water and avoids contamination problems.
5. Use an indoor pump reservoir in climates where freezing occurs. An indoor pump reservoir is one of the best means for providing freeze protection. An indoor pump reservoir also helps optimize temperature control to the process.
6. Recognize cooling tower maintenance requirements. Just because a cooling tower is built for outdoor use doesn’t mean it can be ignored. A thorough maintenance program helps ensure tower efficiency and reliability so that it will provide years of service.
7. Provide your tower management system with a comprehensive water treatment program. Efficiency and safety of any cooling tower system depend in large part on its water management program. Filtration and water treatment are essential. Consult a local expert for help in establishing a water maintenance and conditioning program that ensures efficiency in your tower. A good water management program reduces the chances for scaling and biological contamination.
8. Enlist the help of experts. The key to using a cooling tower successfully is understanding its place as a critical component in the industrial cooling process. Consult and work with established industrial process cooling equipment suppliers, buy quality products, and maintain them properly.
Information for this section was provided by Thermal Care.
&READERSERVICE> Guide to cooling towers
For literature and product information on cooling towers, circle the appropriate number on the reader service card or visit the company web site. This guide was compiled from information provided by the following cooling tower manufacturers in response to a written request from Plant Engineering magazine.
Circle Company Web site
221 Advantage Engineering ww.advantageengineering.com
222 Arctichill, Inc. www.arctichill.com
223 Baltimore Aircoil www.baltimoreaircoil.com
224 Burger Cooling Tower Co. www.coolingtower.com
225 Ceramic Cooling Tower Co. www.baltimoreaircoil.com
226 Delta Cooling Towers, Inc. www.deltacooling.com
227 Kimre, Inc. www.kimre.com
228 Marley Cooling Tower Co. www.marleyct.com
229 Thermal Care www.thermalcare.com
230 Tower Performance, Inc. www.tpict.com
231 Tower Tech, Inc. www.towertechinc.com&/READERSERVICE>