Cooling Towers in HVAC

An industrial cooling tower is a critical component of many refrigeration systems and can be found in industries such as power plants, chemical processing, steel mills, and many manufacturing companies where process cooling is necessary. Also, a commercial cooling tower can be used to provide comfort-cooling for large commercial buildings like airports, schools, hospitals, or hotels.

An industrial cooling tower can be larger than an HVAC system and is used to remove the heat absorbed in the circulating cooling water systems used in power plants, petroleum refineries, petrochemical plants, natural gas processing plants, food processing plants, and other industrial facilities.

With the increased rate of the population all over the world, there has been a huge rise in the rate of needs and requirements by the world for manufactured products. This has forced the industrial sector to manufacture more and more products every day, which generates more manufacturing process heat. The machines and processes of industries that generate tremendous amounts of heat must be continuously cooled so that those machines can continue to operate efficiently. The most efficient, effective, and least expensive solution to removing this heat is the installation of an industrial cooling tower.

Large areas require more cooling than small ones do. Traditional HVAC systems operating by themselves will not be able to handle large areas easily. An important point to consider with cooling towers is that their size should be directly proportional to the area they have to cool. A size that’s just right will be large enough to cool your building or equipment without being too large and having excessive overhead or installation costs.

Cooling towers are an investment that will have a high upfront cost but will have energy cost savings in the long run. HVAC systems can only handle so much and a properly designed, comprehensive HVAC and cooling tower system will pay for itself. An additional factor to consider is maintenance for your cooling tower. If you perform regular preventative maintenance your system will be operating as efficiently as possible. The cost of maintenance is considerably less than any problems leading to an inefficient or broken tower.

Types of Cooling Tower Systems

Cooling tower systems are often vital to industrial processes. These tall, open-topped, cylindrical structures are responsible for cooling water generated from industrial or HVAC comfort cooling airflow. The different types of cooling towers are identified by the classification of the draft (natural or mechanical) and by the direction of airflow (counter or cross).

Natural Draft Cooling Tower Systems: These are usually used for large power plants and industries with infinite cooling water flow. The tower operates by removing waste heat by way of rising hot air that is then released into the atmosphere. These towers are tall and have a hyperbolic shape to induce proper airflow.

Mechanical Draft Cooling Tower Systems:  These towers have air forced through the structure by a fan that circulates air through the tower. Common fans used in these towers include propeller fans and centrifugal fans.

While mechanical draft towers are more effective than natural draft towers, they consume more power and cost more to operate as a result.

Cross flow Cooling Tower Systems:  These have a design that allows the air to flow horizontally through the fill and the tower’s structure into an open plenum area. Hot water flows downward from distribution basins. However, fans and motor drive require weather-proofing against moisture that can lead to freezing making it less.

Counter flow Cooling Tower Systems: These have a design where the air moves upwards and the counter-current, with hot water, falls downward to cool the air. This allows for maximum performance out of each plan area and helps minimize pump head requirements. Also, a counter flow cooling tower system is less likely to ice up in cold weather conditions and can save energy in the long run. All Delta Cooling Towers are counter flow and all include these advantages.

Induced Draft Cooling Tower Systems: These are typically mounted with a fan at the top of the cooling tower, which allows hot air out and pulls air throughout. The high exiting air velocities reduce the chance of re-circulation. To avoid the entrapment of water droplets in the leaving stream air, drift eliminators are used. Induced draft towers are more efficient as they use 30% to up to 75% less energy compared to forced draft designs.

Forced Draft Cooling Tower Systems: These cooling tower systems are similar to induced draft, but the basic difference is that the air-moving fan is placed at the base of the cooling tower, which allows the air to blow through from the bottom. Their use is limited due to water distribution challenges, high horsepower fans, and the possibility of re-circulation.

Materials for Cooling Towers

Water-cooled systems are primarily made from three materials: Metal, fiberglass, and plastic. As you know, metal can rust and corrode, and whatever’s inside of it can begin to leak over time. To no surprise, a metal cooling tower only has an average shelf life of up to only 15 years and requires maintenance with epoxy paint, sealants, and more. That maintenance can lead to downtime for your business. This is why metal is now being replaced with better technology. Fiberglass cooling tower manufacturers, although providing a better alternative to metal, are still prone to cracks and wear which can lead to long-term higher maintenance costs.

Key Components of Cooling Towers

This section explains how the components of a cooling tower work together.

Water Distribution: Hot water from the chilled-water system is delivered to the top of the cooling tower by the condenser pump through distribution piping. In an open tower, the hot water is sprayed through nozzles onto the heat transfer medium (fill) inside the cooling tower. Some towers feed the nozzles through pressurized piping; others use a water-distribution basin and feed the nozzles by gravity. In a closed-loop tower, the water from the condenser loop runs through tubes in the tower and is not exposed to the outside air. Water for cooling the tubes circulates only in the tower. In the open tower, a cold-water collection basin at the base of the tower gathers cool water after it has passed through the heat transfer medium. The cool water is pumped back to the condenser to complete the cooling-water loop. In the closed tower, the condenser water cools as it moves through the piping in the tower and returns to the chiller plant.

Heat Transfer Medium (Fill): Cooling towers use evaporation to release waste heat from an HVAC system. In an open tower, hot water from the condenser is slowed down and spread out over the fill. Some of the hot water is evaporated in the fill area, or over the closed-circuit tubes, which cools the water. Cooling tower fill is typically arranged in packs of thin corrugated plastic sheets or as splash bars supported in a grid pattern.

Air Flow: Large volumes of air flowing through the heat-transfer medium help increase the rate of evaporation and the cooling capacity of the tower. The cooling-tower fans generate this airflow. The size of the cooling-tower fan and airflow rate are selected to achieve the desired cooling at design conditions of condenser-water temperatures, water flow rate, and wet-bulb temperature. Cooling towers may have propeller fans or squirrel-cage blowers. Small fans may be connected directly to the driving motor, but most designs require an intermediate speed reduction provided by a power belt or reduction gears. The fan and drive system operate in conjunction with the control system to control start/stop and speed. Variable-Speed Drives (VSDs), when added to the fan motors, control fan speed and more precisely regulate the temperature of the water as it leaves the tower.

Drift Eliminator: As air moves through the fill, small droplets of cooling water become entrained and can exit the cooling tower as carry-over or drift. Devices called drift eliminators remove carry-over water droplets. Cooling-tower drift becomes annoying when the droplets fall on people and surfaces downwind from the cooling tower. Efficient drift eliminators virtually eliminate drift from the air stream.

Safety Issues of Cooling Towers

Water Treatment: Cooling-tower water must be regularly treated, generally with chemicals, to prevent the growth of harmful bacteria, minimize corrosion, and inhibit the buildup of scale (mineral deposits) on the fill.

Maintenance Personnel: Cooling towers are often placed in precarious locations, and inspection ports can be located in awkward or exposed locations. This can create a hazardous working environment. Be sure to implement adequate fall-prevention measures and procedures. In addition, always follow lock-out and tag-out safety procedures.

Best Practices for Efficient Operation of Cooling Towers

Always consult the manufacturer’s manual for the cooling-tower. One of the excellent sources of information and standards for cooling towers is the Cooling Technology Institute. Here are some recommendations for operating any cooling tower more efficiently:

• Implement a preventive-maintenance program: This includes regular water treatment and maintenance of the mechanical and electrical systems. See the Maintenance Schedule for Cooling Towers, below for more information.
• Reduce the temperature of water leaving the tower: The temperature of water leaving the cooling tower should be as cold as the chiller manufacturer will allow for entering condenser water. Newer chillers usually tolerate colder temperatures for water returning from the cooling tower. Check with your chiller manufacturer’s representative or manual and set the entering condenser-water temperature (same as the leaving cooling tower temperature) as low as possible.
• Operate cooling towers simultaneously: Direct water through all towers regardless of the number of chillers operating. Tower fans should be staged on as required. Operating the towers simultaneously will use less energy in most situations than staging towers individually. This strategy is particularly effective with VSDs on the fans. When a fan VSD reaches 40% speed (adjustable), the next fan stages on and operates in parallel, both now running at a minimum speed of 20%. Balance water distribution between multiple towers (or cells within a single tower enclosure) and within each tower or cell. Water often flows down only one side of the tower, or one tower may have more flow than an adjacent tower. This increases the temperature of the water returning to the chiller and reduces the efficiency of the tower.
• Consider a condenser water reset strategy: The temperature set point of the water leaving the cooling tower should be at least 5°F (adjustable according to the design) higher than the ambient wet-bulb temperature. If the Direct Digital Control (DDC) system has a wet-bulb temperature sensor, this can be done automatically. Otherwise, the operator should consider manually adjusting the set point seasonally.
• Close the bypass valve before starting the cooling-tower fans: Make sure the DDC control sequence prevents the tower fans from starting before the cooling-tower bypass valve is fully closed. If the bypass valve isn’t fully closed, hot water leaving the chiller short circuits into the water returning to the chiller, adding unnecessary load to the compressor.
• Trend log the temperature of the water leaving the tower: Use the trend logging capability of the DDC to track the temperature of the water leaving the tower. Higher than normal temperatures may indicate that the tower in not operating properly.
• Effective water treatment: Effective water treatment eliminates harmful bacteria and bio-film and controls scale, solids, and corrosion. Bleed or blow down-the continuous flow of a small portion of the re-circulating water to a drain to eliminate dissolved solids-is insufficient by itself to control scale and corrosion and is always ineffective in controlling biological contamination. A regular chemical-treatment program is always recommended for controlling biological organisms, scale, and corrosion.
• Prevent scale deposits: When water evaporates from the cooling tower, the minerals that were dissolved in it are left behind as scale deposits on the surface of the fill. Scale build-up inhibits heat transfer from the water to the air, which reduces the fill’s effectiveness. Excessive scale build-up is a sign of inadequate water treatment.
• Prevent or clean clogged spray nozzles: Algae and sediment that collect in the water basin as well as excessive solids that get into the cooling water can clog the spray nozzles. This causes uneven water distribution over the fill and uneven airflow through the fill, which reduces evaporation. These problems indicate improper water treatment and clogged strainers. Kits are available to replace older, smaller distribution nozzles or troughs with large-orifice, clog-free designs.
• Ensure Adequate Airflow: Poor airflow through the tower reduces the transfer of heat from the water to the air. Poor airflow can be caused by debris at the inlets or outlets of the tower or in the fill, loose fan and motor mountings, poor motor and fan alignment, poor gearbox maintenance, improper fan pitch, damage to fan blades, or excessive vibration. Reduced airflow due to poor fan performance can ultimately lead to motor or fan failure.
• Ensure Adequate Pump Performance: A closed-loop cooling tower uses a pump to transport water over the tubes for evaporative cooling. Proper water flow is important to achieve optimum heat transfer. Loose connections, failing bearings, cavitation, clogged strainers, excessive vibration, and operating outside of design conditions result in reduced water flow, reduced efficiency, and premature equipment failure.

Conclusions

• Cooling towers are specialized heat exchangers, but instead of the usual conduction – convection heat transfer of shell and tube heat exchangers, it generates cooling by bringing water and air into contact. This cooling is achieved through evaporative cooling and sensible heat transfer.
• There are different types of cooling towers depending on their air flow generation, air-to-water flow, heat transfer method, and construction. Each of these types has its own set of advantages and disadvantages.
• There are three important factors that determine cooling tower efficiency: relationship between range and approach, wet-bulb temperature, and cooling load.
• Because of the continuous evaporation of water, several problems arise, such as scaling and biological fouling. Common water treatment methods are water blow down, filtration, water softening, and chemical addition.

Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is a recognized Ph.D. Supervisor at Gujarat Technological University (GTU). He has published more than 245 Research Articles in reputed International Conferences and Journals. He has also published 10 reputed books and book chapters in the area of thermal engineering. Working as Academic Editor for the Journal of Materials Science Research and Reviews. Presently, he is an Associate Professor at GEC, Dahod, Gujarat Technological University, GTU, Ahmedabad (Education Department, State of Gujarat, India). His area of research is Desiccant Cooling, ANN, TRNSYS, and Exergy.