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This is the most comprehensive guide to cooling towers on the internet.
Here‘s what you‘ll learn:
Cooling towers are specialized heat exchangers that remove heat from water mainly by means of latent heat loss from evaporation while coming into contact with an airstream. Aside from evaporative cooling, water is also cooled by sensible heat transfer due to the temperature difference between air and water. Cooling towers are basically heat exchangers, but instead of the usual conduction-convection of shell and tube or plate heat exchangers, it generates cooling by bringing water and air into contact.
In order to better understand the further topics, here are some basic terminologies when dealing with cooling towers.
This is the difference between the water temperature at the entry and exit from the cooling tower.
The rate at which heat is removed from the water.
The quantity of fresh water which must be supplied to the water system to replace the same amount of water that evaporated.
The air droplets that become entrained or carried by the air stream coming into contact with water.
The effluent mixture of heated air and water vapour discharged from a cooling tower.
The difference between the temperature of water leaving the tower and the wet bulb temperature of the air entering.
The ambient air temperature at 100% relative humidity. This is measured by a psychrometer where a thin film of moisture covers the bulb of the thermometer which is then exposed to flowing air. The resulting measurement is usually cooler as compared to the measurement from an ordinary thermometer depending on the relative humidity.
Water that is deliberately removed from the system in order to remove parts of the accumulated solids due to the continuous water evaporation, and sludge due to impurities and bacterial growth.
Occurs when the discharge air re-enters the system by mixing with fresh air. This lowers the efficiency of the cooling tower.
Cooling tower parts may be further broken down into structural, mechanical, and electrical. Structural components are static equipment such as the basin, tower framework, fan deck, casing, and louvers. Mechanical parts, on the other hand, are the rotating equipment: fans, driveshafts, and speed reducers. Electrical parts consist of motors and their controls.
Sometimes referred to as wet deck or surface, this increases the surface area of the cooling tower to allow optimum contact surface and contact time between the air and the water, while providing minimum restriction to airflow.
The distribution system depends upon the type of cooling tower according to air-to-water flow. Cross-flow cooling towers use a gravity-flow distribution system where the water is distributed at an elevation above the fill. For counter-flow cooling towers, pressurized water spray systems are utilized.
The drift eliminators prevent the escape of water droplets by creating sudden changes in the path of the air stream. As the air stream passes, large water droplets impact the walls of the drift eliminator, causing the water to fall back into the cooling tower.
Air intake louvers are a part of the cooling tower that serves as a block for water splash-out, noise, and debris. They are a common part of cross-flow cooling towers, and are located above the cold water basin, at the bottom of the panel where the fill is packed, and are placed completely around the tower.
The function of the casing is to provide housing and to transmit loads to the tower frame. It also acts to contain water within the cooling tower.
The fans force air either in or out of the cooling tower, depending on the type of draft required for the application. The main driver is the motor. Torque from the motor is transferred by the driveshaft to the fan or to a gearbox. Large cooling towers with big and heavy fans require gearboxes to increase the torque, which eliminates the need for heavy motors, and in turn eliminates the need for a stronger structure. In some cases, belt and pulley assemblies are used instead of gear boxes.
The fan stack, also referred to as the fan cylinder or fan cover, enhances the efficiency of the fan. This is done by a well-shaped air inlet to create smooth air flow into the fan. It also helps the discharge of air at higher elevations minimizing recirculation and interference.
This acts as a support to the fan cylinders and as a structural member to transmit loads throughout the tower frame.
Valves are needed to control the water level inside the cooling tower. Valves used in cooling towers are isolation valves, flow-control valves, and make-up regulator valves.
The collection basin is located at the bottom of the cooling tower. The primary purpose of the basin is to collect falling water that has not evaporated or lost due to drift. Usually, the basin also serves as the foundation of the cooling tower. This is where chemical treatments for the circulating water are added.
This is a deep pan or small basin with holes or nozzles located above the fill. This is a part of the distribution system of a cross-flow cooling tower where gravity acts to distribute hot water evenly across the fill material.
The tower frame supports the whole cooling tower and transmits all loads to the foundation. Common materials used for the frame are concrete and wood. Fiberglass and stainless steel are often used.
Cooling towers are generally classified as follows:
Air Flow Generation. Cooling towers vary on how air flow is introduced into the system. This may be by natural, mechanical, or hybrid draft. Mechanical draft cooling towers are further divided into forced draft and induced draft.
Air-to-Water Flow. This classification divides cooling towers into cross-flow and counter-flow cooling towers. Their difference is the method of how water comes into contact with the air stream.
The table below summarizes the characteristics of each type.
Characteristics of Cross-flow and Counter-flow Areas of Consideration Cross-flow Counter-flow 1. Size • Takes more space but can be constructed lower than counter-flow • Takes a smaller area than cross flow 2.Maintenance and Operation • Access to nozzles is available anytimeHeat Transfer. In chapter 1, it was mentioned that there are two methods of how heat is removed from water: loss of latent heat through evaporative cooling, and cooling through sensible heat transfer. Sensible heat transfer in a cooling tower is either by conduction, convection, or a combination of both. Classification by heat transfer methods differentiates cooling towers on how these principles are applied.
Build or Construction. This classification divides cooling towers according to how the structure is erected or manufactured.
In designing a cooling tower, there is a myriad of properties to consider, from psychrometry to heat and mass balance. Aside from studying the properties of air and water coming in and out of the system, one must also consider the resulting physical characteristics of the tower can the tower create a natural draft, or will the tower be structurally sound, and are parts susceptible for fouling and corrosion? This chapter focuses on the common parameters that affect the performance of cooling towers.
Cooling tower efficiency = (CW return temperature - CW supply temperature) / (CW return temperature - Air wet bulb temperature) × 100%
Consequently,
Cooling tower efficiency = Range / (Range + Approach) × 100%
From these, it can be seen that a cooling tower with a smaller approach is more efficient. Cooling towers usually have a 5 to 10⁰F approach. While a small approach is desired, investment cost may be impractical since the size of the cooling tower increases exponentially as the approach is being lowered.
Usually, the range and cooling water flow rate are the parameters being balanced. This is because the heat load is already given from consumer demand, and ambient air wet-bulb temperature may not be manipulated. Increasing the range will make the cooling tower efficient. This can be done by increasing the cooling water return temperature, or by lowering the cooling water supply temperature. In either of the cases, usually, one temperature is constant due to the requirement of end users. Of these two options, increasing the cooling water return temperature is more practical since the temperature difference between air and water in contact will be much larger. The larger the temperature difference, the more heat can be dissipated.
If the only option is to lower the cooling water supply temperature, the result will also lower the approach. In turn, the design will require a much larger tower.
From the previous point, it is seen that a high ambient wet-bulb temperature will decrease the approach. Thus, at locations where there are high wet-bulb temperature conditions present, larger cooling towers are required for a given cooling load.
In order to save energy, one method is to use fan speed control. Heat transfer rate is increased by higher air velocity. If not much heat transfer or evaporative cooling is needed, fan speeds can be reduced. This can be done by using variable speed drive motors, two-three speed fan motors, and adjustable pitch fan blades. Another option is to design a multi-cell cooling tower. In this case, one cell may be on standby during off-hours or at times of low demand.
Cooling towers, like any other equipment, must be routinely inspected and maintained to ensure adequate and efficient delivery of required cooling while maintaining (or even extending) their expected service life. The following are some maintenance inspection practices for cooling towers.
Any cracks or damage on the basin, casing, fan deck, and tower frame. Address all inspection findings accordingly.
CTI Standard 201 "sets forth a program whereby the Cooling Technology Institute will certify that all models of a line of Evaporative Heat Rejection Equipment offered for sale by a specific Manufacturer will perform thermally in accordance with the Manufacturer's published ratings" (CTI.org, 2018). A CTI certification guarantees that a cooling tower has been inspected by a CTI licensed testing agent and found to perform up to CTI standards and manufacturer specifications.
As the water evaporates, impurities dissolve in the water and become more concentrated. Make-up water is added, which will later evaporate and leave behind more impurities. These are dissolved minerals that will later build scale to the parts of the cooling tower in-contact with water. Not only is the cooling tower affected, but also the consumers, such as heat exchangers and condensers.
Aside from scale build-up, biological fouling tends to form on surfaces. Evaporative cooling towers are particularly prone to biological fouling where an air stream carrying microbes is scrubbed by the water. Concentrated minerals present in the water due to evaporation provide a favorable environment for these microbes to grow.
Depending on the application and water parameters, methods of water treatment largely vary. There are proprietary water treatment chemicals and filtrations systems being sold on the market targeting a range of problems. The following are some common cooling water treatment methods.
In any industrial plant, heat is generated from equipment used in different processes. The need to remove undesirable heat is a common occurrence in the industrial manufacturing processes. The same is true for commercial and residential buildings, where comfort cooling, refrigerated storage, and equipment preservation are required. If the undesirable generated heat is not removed or rejected, the machinery, equipment, and air conditioning systems will not work properly.
Because of their high efficiency, cooling towers are a widely used heat rejection system. Cooling towers vary in size depending on the application; from small chiller units for residential cooling, to 200-meter tall structures serving power generating plants. Below are some cooling tower applications.
HVAC is used for comfort cooling of residential and commercial areas. Heat generated from people, equipment (computers, servers, etc.), lighting, solar radiation, and outdoor ambient air is absorbed by the cooling system and rejected to the cooling tower.
This is for cold storage in the food and beverage industry, pharmaceuticals, air and gas generation, etc. This is similar to an HVAC system where a refrigerating unit absorbs heat from a closed space and rejects that heat to the cooling tower.
Power generation plants use steam as the working fluid. In order to generate power, water is heated to steam by coal, natural gas, or even nuclear radiation. This heat is then converted to mechanical energy. However, some of this heat cannot be converted to energy and must be removed to complete the steam cycle. This is where cooling towers come into play.
This is similar to a power plant. Condensers, heat exchangers, and cooling jackets all absorb heat from processes. This heat is then carried by water to be rejected through the cooling tower.
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