Cooling towers are used across many types of industries to keep a heat producing process at a controlled temperature. They are common in steel mills, chemical processing plants, power plants, petroleum refineries, food processing plants, and many other industrial facilities. They can also be utilized with water cooled chillers to create a cool and comfortable environment at hotels, airports, college campuses, schools, and hospitals. Cooling towers are becoming a staple HVAC system in many office environments through water source heat pumps.
A cooling tower is an enclosure that removes heat from a process fluid, usually water, by means of evaporation into the ambient air. It does so by bringing a large surface area of water into contact with a rapidly moving stream of air within the enclosure. The air stream is exhausted out of the top of the enclosure and the cooled water is captured in a basin, then returned to the process to gather more heat.
The process of evaporation is driven by the differential of vapor pressure in the water and vapor pressure in the air. Water at a given specific temperature has a unique vapor pressure. Provided that pressure is higher than the vapor pressure of the air next to it, water vapor will move from the water to the air. As the water vapor leaves, the remaining water replaces the vapor almost instantaneously by converting liquid water to vapor. This phase change requires a large amount of heat which lowers the temperature of the water.
To lower the temperature of one pound of water 1° F. requires the removal of 1 BTU of heat. To convert one pound of water to vapor requires over 1000 BTUs of heat, lowering the temperature of over 1000 pounds of water 1° F. The larger the differential vapor pressure between the water and the air, the faster the transfer of vapor. This transfer happens only at the interface (surface area) of the water and air. The larger the surface area, the faster the transfer of vapor. Cooling towers increase the surface area of the water to speed up heat transfer.
The vapor pressure of the ambient air is a critical component in this exchange of vapor. Air at a given temperature and pressure can only hold a unique amount of water vapor. If more vapor is pumped into the air, the vapor will condense forming a cloud or rain (the air temperature will be at its Dew Point). If the air temperature or pressure is raised, so is its capacity to hold water vapor. The condensing vapor gives up its heat to the air, raising the air temperature and increasing its capacity for water vapor. If this air is not moved away from water, the water and air will come to a common vapor pressure and evaporation and heat exchange will cease. If the air is continuously moved away, the evaporation of the water will eventually drop the temperature of the water to a point where its vapor pressure is equal to that of the air and evaporation will cease. The temperature of the water at this point is called the Wet Bulb Temperature.
The term Wet Bulb Temperature comes from the process used to determine it. A small sock wetted with water is placed over the bulb of a thermometer and whirled through the air until the water vapor pressure (unique temperature) reaches equilibrium with the air. The temperature is read and recorded as the Wet Bulb Temperature of the ambient air. This is the coldest that any water sample could come to in this ambient air given enough exposure time.
One can do a simple experiment at home with two samples of water. Measure the temperature of the water samples, then place one in an air tight container and the other in an open container. Make note of the air temperature in the room (we call this the Dry Bulb Temperature). Let the samples stand for several hours (overnight should do) and measure the temperature of the two samples. The sample in the closed container will be near the ambient air temperature (Dry Bulb Temperature). The water temperature has equalized with the air temperature through conductive heat transfer only through the container (no exchange of vapor). The sample in the open container will be lower than the ambient air temperature (usually 10° to 12° F) depending on the Relative Humidity of the ambient air. This is the Wet Bulb Temperature of the ambient air.
As the air in a cooling tower takes on vapor from the water it contacts, it loses its capacity to hold more water vapor. To maximize evaporation across the air/water surface, this air is moved out of the tower and “fresh” ambient air replaces it. Most cooling towers use large mechanically driven fans to move the air across the water surface. Increasing the rate of air exchange will increase the rate of evaporation and therefore cooling capacity. More air, more capacity. Moving more air requires increased fan energy. There is a point at which the increased capacity benefit is overcome by the cost of increased air transfer rate.
Increased capacity can also be achieved through increased air/water surface area. The method of increasing water surface area is accomplished by different methods within the cooling tower. The simplest method is to spray the water into the air stream and let it fall into a basin to be captured and recirculated. The amount of cooling done depends on the droplet size (surface area) and the contact time with the air. To increase the surface area of the droplets, splash type cooling towers use some form of lattice that the air can move through and that the falling droplet will impinge upon and be broken into smaller droplets with an increase in total surface area. The contact time with the air can be increased by increasing the height of the fall path for the water through the air thus increasing the heat rejecting capacity.
A more efficient way of increasing the surface area is to let the water fall onto a vertical plastic sheet mounted perpendicular to the air flow and form a very thin film of water on the sheet. This not only creates a large surface area, but it also slows the fall of water and increases the contact time with the air. The fall time of water over the sheet can be further increased by corrugating the sheet against the water flow which also generates a bit of turbulence to increase heat transfer. This is the method used by today’s “high efficiency” fill packs. They are corrugated sheets assembled together to allow air to flow either horizontally or vertically through the pack while the water falls vertically. Efficiency is a measure of heat rejecting capacity per cubic foot of fill pack. Splash fills require significantly more volume within a cooling tower to achieve the same heat rejecting performance as a corrugated fill pack, but they are far more tolerant of dirty or contaminated water that will foul the fill, choking the air flow and reducing capacity.
A cooling tower is a heat exchanger. There are three basic types of heat exchangers:
Parallel flow – both media streams move in the same direction (least efficient)
Crossflow – media streams move perpendicular to each other (more efficient)
Counterflow – media streams move in opposite directions (most efficient)
There are two basic types of cooling towers in use today – the Crossflow and the Counterflow.
The Crossflow Tower
The crossflow tower delivers its water to the top of the fill pack and the ambient air is moved horizontally across the fill pack along its entire height. The air at the top of the fill pack sees the greatest vapor pressure differential with the hottest water, but the difference is reduced as the water cools falling through the fill and the air at the bottom sees the lowest vapor pressure differential. This produces uneven vapor transfer into the air stream.
The Counterflow tower
The Counterflow tower delivers its water at the top of the fill pack and the air is introduced under the fill pack. As the cold water is leaving the fill, it is contacting all the ambient (lower vapor pressure) air entering the tower. As the ambient air rises through the fill pack its vapor pressure increases, but it is also contacting warmer water with a higher vapor pressure. The vapor pressure differential is consistent for all air streams moving through the fill pack.
When comparing these two types of towers with the same heat load capacity and fan input energy, the Crossflow tower will generally have a smaller footprint, but a taller profile with a higher tower water pump head. The overall energy consumption including fan and pump energy will generally be lower with the Counterflow design.
Induced Draft Cooling describes a cooling tower with the air moving fan placed after the heat exchange has been completed. It pulls the air through the fill pack and is generally located at the top of the cooling tower structure above the water distribution system. The air stream at this point in the heat exchange process is hot and nearly saturated with water vapor making it very corrosive, so care must be taken in the selection of materials placed in contact with the air stream.
Forced Draft Cooling describes a cooling tower with the air moving fan placed at the entrance to the heat exchange process. It pushes the air through the fill pack and out of the tower. It is usually near the bottom of the tower and may be ducted into the fill pack to avoid contact with the falling water. The fan is in the ambient air stream where material selection is not so critical to avoid corrosion.
Field Erected Tower is usually too large to ship fully or partially assembled, so the component parts are sent to the job site to be assembled there on foundations provided by others. The actual assembly may be compromised by workers not fully qualified to do the installation. The design and installation are often unique requiring a field thermal performance test to validate the design capacity of the tower.
Factory Assembled Tower is produced and assembled in a factory and shipped to the job site. It is usually a single piece, but some large designs may require some component assembly at the job site to facilitate shipping size limitations. These “packaged” towers are built to a consistent design by dedicated workers and quality standards to assure consistent performance. Many manufacturers submit their towers to outside performance testing and receive certification for the thermal performance of their designs. Their customers are assured of thermal performance without the need for field testing. They can often be ganged together to provide the same capacity as a large field erected tower.
Major Cooling Tower Components
Drift Eliminators are placed in the heat exchange air stream after the fill pack and water distribution to remove most of the small water droplets carried in the air stream and prevent them from being discharged to the atmosphere. These water droplets contain traces of chemicals used to control bio-growth and corrosion within the wetted areas of the cooling tower and can cause severe damage to equipment, structures, and even vegetation in their fall zone outside of the tower. High quality drift eliminators can reduce this drift to nearly zero.
Fans are usually large propeller type axial fans that efficiently move large volumes of air with low static pressure requirements. Some forced draft designs use centrifugal “squirrel cage” fans to overcome higher static pressure demands such as ductwork or sound attenuation devices.
Fan Shrouds are placed around an axial fan to direct the air efficiently into and out of the fan.
Fill Media is where the heat transfer takes place and comes in many different forms, but basically two types.
The oldest design is a splash fill. It is designed to break up the falling water droplets from the distribution system but minimize the effects of fouling on its surface area which restricts air flow.
Film fills are designed to increase the exposed surface area of the water by causing it to “film” on thin sheets of material that allow air flow between them. They are far more efficient (heat exchange per cubic foot of fill) than splash types, but very vulnerable to fouling which restricts the air path reducing air flow.
Motors are generally industrial grade, totally enclosed, induction type motors suitable for harsh environments.
Since they generally run at a fixed speed not matched to the speed of the air moving fan, they will use some form of speed reduction. This is accomplished through geared speed reducers or belt and pulley systems.
Further speed manipulation to match thermal load demand is often handled with Variable Frequency Drives on the motor, or two speed motors.
Nozzles are the heart of any cooling tower.
They are necessary to evenly distribute the water over the fill pack in such a way as to provide an even liquid-to-gas ratio (water-to-air ratio) throughout the fill pack. This is essential to maximize the heat transfer within the pack. Typical nozzles have fixed orifices (the opening through which the water flows) and throw a round pattern not usually evenly distributed.
A fixed orifice produces a fixed pattern at a specific flow rate. Less flow – smaller pattern. More flow – bigger pattern. They are placed so that their patterns overlap to insure coverage of the entire fill area, but the distribution is not even. A tighter pattern produces more even distribution but requires more nozzles closer together with smaller orifices for lower flow rates per nozzle. The smaller nozzles are more prone to clogging which impairs even distribution. Most towers, big or small, use 1”- 2” fixed orifice nozzles and check them regularly for clogging to insure maximum efficiency.
There are a few nozzles that are designed to be variable flow nozzles that can maintain a nearly constant pattern over a wide flow range. This is useful in systems that have variable process flow rates. Without this feature, a reduction in flow rate would produce areas in the fill that do not receive water and the air is bypassing without doing any cooling. Other areas will receive so little water that it is completely evaporated and leaves a scale behind to choke the air flow and further reduce capacity. Good nozzle design, placement, and maintenance are essential to a cooling tower’s thermal capacity and efficiency.
where the cooled water is collected
This is where the cooled water is collected after it passes through the fill media and is sent to the system circulating pump. It usually takes up the entire footprint of the cooling tower except in some forced draft applications.
Purchase Cost vs. Operational Cost
Evaporation reduces the amount of water in the circulating fluid and requires replacement (makeup water) to keep the system from running dry. Think of a pan of water left over heat until all the water is evaporated. There will remain a deposit of solids coating the pan. These are the dissolved solids that were contained in the water and do not transfer to the air via vapor exchange because they are solids not vapor. In a cooling tower which has constant makeup to replace the water lost to evaporation and maintain the circulating fluid volume, the dissolved solids that stay behind increase the concentration of dissolved solids in the circulating fluid.
If this concentration of dissolved solids is left unchecked, eventually the concentration will exceed the capacity of the water to keep it dissolved (the water’s saturation point) and these solids will begin to precipitate and form solid particles. These particles must be avoided or removed to prevent fouling of the cooling tower fill and scaling in the process heat exchanger walls. Scaling on heat exchanger walls reduces the capacity for heat exchange. Fouling of the fill reduces the air flow through the fill and reduces the heat exchange capacity of the cooling tower. This must be prevented to maintain the efficiency of the heat rejection system.
Cooling tower operators prevent the precipitation of solids by maintaining the concentration of dissolved solids in the circulating water just below its saturation point. This is done by throwing away some of the concentrated water and diluting the remaining volume with “fresh” (makeup) water. This process is called Blowdown.
The concentration of dissolved solids is determined by measuring the electrical conductivity of the circulating water. Pure water has a conductivity near zero. It is the dissolved solids in the water that conduct electricity. An increase in concentration of dissolved solids will produce an increase in conductivity.
Blowdown is initiated when the conductivity reaches a point just short of the saturation point of the circulating water. A drain valve is opened and water is discharged from the system. At the same time, makeup water is injected into the system to maintain the system volume, and the makeup water dilutes the concentration of the circulating water. When the conductivity of the circulating water is reduced to a preset value, the drain valve is closed and blowdown ceases. Further evaporation of water in the cooling tower will raise the conductivity of the circulating water until it again reaches the value to initiate blowdown, and the process is repeated.
The use of some form of biocide is necessary to prevent bio-growth in a cooling tower. Most often, this is some compound of Chlorine or Bromine. It can also be done with Ozone or Ultra Violet light. Anything that will kill bio-growth could be used.
Corrosion occurs in any warm water system where oxygen is present. Both are in abundance in a cooling tower. Chemicals are added to the water to counter act the corrosion but are lost during blowdown and need to be replaced.
To minimize blowdown, chemicals are added to increase the capacity for dissolved solids in the water without scaling. Other chemicals are used to encourage precipitation of the dissolved solids and keep them in suspension instead of plating out as scale.
How much chemical treatment is done is determined by the cost of the chemicals versus the cost of makeup water for blowdown and the cost of disposing of the blowdown. Sometimes, any blowdown is environmentally unacceptable and chemical cost is no longer a factor. It is whatever it takes.
The electrical efficiency of a cooling tower (kW/ton of cooling) is impacted by motor efficiency, pump head, fan efficiency, water distribution, tower design, fill pack design, but most variably by fill pack size. If you want a more efficient tower, buy a bigger tower. Spreading the water flow over a larger fill pack area will increase the water-to-air vapor exchange area and reduce the air flow per square foot of fill thus reducing the pressure required to move the air through the fill pack. A larger tower cooling the same heat load will do it with less fan horsepower.
A cooling tower that can maintain a wetted fill with a reduction in flow rate will essentially give the owner a larger tower for the lower flow rate and reduce the fan horsepower required to cool the load. A constant flow tower will not get a reduction in air pressure drop because the resistance caused by water flow against it has not changed..
All towers have variable fan power requirements based on the load and the ambient wet bulb conditions. To prevent wasting energy by overcooling the process water, fans can be cycled off and/or use a two-speed motor. The most efficient way to match fan power to the current load and wet bulb is to use a Variable Speed Drive and drive the fan at just the right speed to maintain a constant cold water temperature. Most new cooling towers use this VFD technology.
A cooling tower is a great air washer. Any debris, dirt, or bacteria that is in the incoming ambient air stays in the cooling tower. So, its accumulation must be dealt with. The larger and more dense material will settle out in the basin where the water velocity slows. Some of the material will stay in suspension and go out with the blowdown water. Bacteria that finds a sheltered home in the basin sediment or the scale forming on the fill pack will begin a colony to release more bacteria into the system. Legionella in a cooling tower is a major concern. The sediment in the basin must be removed mechanically to eliminate the bio-growth nursery. Frequency depends on dirt loading of the ambient air and the ability of filtration systems to remove it. The material itself is often a bio-hazard and requires special personnel and training to remove it.
Mechanical system failure is the largest threat to the ability of a cooling tower to reject the process heat load. Without an operating fan, a cooling tower is nearly useless and the process will be shut down due to overheating. Motor bearings must be lubricated. Mechanical gear reducers need oil changes. Alignment of motors to gear boxes must be maintained to extend service life of the drive train couplers. Belt driven fans must maintain belt tension and alignment, and bearings must be lubricated. Fan balance must be maintained to prevent vibration causing bearing failure. Electrical infrastructure to the motors must be maintained to prevent loose connections, overload currents, and nuisance trips.
Distribution nozzles must be checked for plugging and wear, and either cleaned or replaced to insure proper water coverage of the fill. Drift eliminators above the distribution should be checked for damage and leakage to maintain proper service.
Chemical treatment should be constantly monitored to insure proper control of scaling and elimination of bio-growth. If either of these start to foul the fill, immediate action should be taken to clean the fill to maintain tower efficiency. Cleaning may be done chemically along with filtration. In some cases the fill will have to be removed and mechanically cleaned or replaced. This will require shutting down the tower for an extended period of time. Multiple cell towers may be able to maintain process cooling with a cell shut down for maintenance. Single cell towers will require a process shutdown.
Most of this maintenance will require shutting down the tower to access the equipment. All the maintenance described here should be done with the tower shutdown for the safety of the operators. All of this service should be done by personnel trained to maintain the specific equipment and accomplish it in a safe manner.
Protecting the environment and its valuable resources are more important now than ever. At the same time most companies are still focused on achieving maximum efficiency. The problem is the two concepts and practices can sometimes be at odds with each other. To achieve the highest level of efficiency you sometimes sacrifice sustainability and being eco-conscience. Those who focus predominantly on sustainability forget the importance of efficiency in achieving savings through energy reduction.
Efficient products can make meaningful contributions to reducing energy consumption and environmental impact without sacrificing the value proposition for the stakeholder. Figuring out how to combine these strategies as sustainable efficiency is much more complicated. The true holistic business strategy is manufacturing a more viable and efficient product that is cost effective while generating a minimal carbon footprint. Moving to sustainability and providing it efficiently makes sense in a resource constrained global economy.
(For useful terms and definitions please visit https://www.cti.org/whatis/glossary.shtml)
(For addition information on cooling towers please visit https://en.wikipedia.org/wiki/Cooling_tower)