A refrigeration plant consists of various components that are equally important to maintain high performance levels. In many larger systems, one of those components is the cooling tower, which is used to reject heat from the condensers in a plant into the atmosphere. Cooling towers can be found in various applications, mainly in industrial processes such as power plants, petroleum oil plants, and food processing plants. Cooling towers are also used in HVAC applications, while HVAC systems are used typically in large office buildings, hospitals, and schools. Therefore, many cooling towers are found in these applications.
For industrial processes, cooling towers are utilized to reduce operational costs and ecological factors – because they reduce condensing temperatures. If operated properly, water is only added to compensate for losses that are generated from evaporation, drift, and blow down.
There are three types of cooling tower systems, they are: once-through, open re-circulating, and closed re-circulating cooling water systems. The first type circulates water through the cooling tower once before discharging. This system is usually found in places where water is found in abundance, and it is mostly used by power utility services. On the other hand, open re-circulating systems are open to the atmosphere and continuously cycle water through the cooling towers while mixing with an air stream to reject heat to the atmosphere. The water is cooled down in the tower and is usually supplied to heat exchangers or chillers.
Although there are different types of cooling towers, they all operate under the same principle, which is cooling down water by evaporating part of the water circulating through a process. The water that is entering the cooling tower is cooled by transferring heat and moisture to an air channel supplied from the environment, which passes through a media. This type of cooling tower heat transfer media is called the fill, and it is an essential component of the cooling process.
Cooling towers are good heat rejection devices, but they also consume a very large amount of water. With the increasing costs of water around the globe and the increasing concerns over water scarcity, it is important to well manage the water consumption of a cooling tower. The same cooling principles are applied to evaporative and adiabatic condensers.
Types of cooling towers
There are various types of cooling towers that all fall into two categories, which are: natural draft and mechanical draft cooling towers. Natural draft cooling towers contain large concrete chimneys that introduce air through the media. These types of cooling towers are used generally by utility power stations since they involve water flow rates that are above around 45,000 m3/hr. On the other hand, mechanical draft cooling towers use fans to discharge air from the cooling tower
Mechanical draft towers are considered as ‘conventional’ towers mainly because they are more commonly found. Even though the different types of mechanical draft cooling towers follow similar concepts, they are given different names due to the difference in fan location. For example, the mechanical draft cooling towers are available in three airflow arrangements, which are: counter flows induced draft, counter flow forced draft, and cross flow induced draft. Figure 1 shows the difference between a cross flow and a counter-flow cooling tower. As can be seen in Figure 1, the counter flow towers (b) are designed in a way in which hot water enters at the top, while the air is introduced at the bottom and exits at the top, usually through a fan. In cross flow draft cooling towers (a), hot water also enters from the top of the tower. However, the air enters at the side of the tower, either at one side which in this case is referred to as single-flow tower, or opposite sides (double-flow tower) and passes through the fill while crossing the water flow into open air. Moreover, mechanical draft cooling towers are mostly used because they are available in a wide range of capacities. Depending on the application, capacities range from around 35 kW, with 2.5 m3/hr flow rates to thousands of kilowatts and flow rates.
Unlike the mechanical draft cooling towers, natural draft cooling towers are mainly used for power plants that are located near the load centers. These cooling towers work without any fans, thereby reducing the power costs. In this case, the air flow rate through a natural draft cooling tower depends on the density difference between ambient air and the air inside the tower. However, the best conditions for this type of tower to operate is in humid ambient conditions. Comparing between summer and winter, the effectiveness of the cooling tower was higher in winter when the humidity was high, reaching up to 80%. If operating in a region where the ambient temperature is considerably high and low relative humidity, the density difference would not be adequate for an efficient performance and thus mechanical draft cooling towers would be preferred. The natural draft cooling tower is also available in two types: counterflow and crossflow. Both types have similar functions and rely on the buoyancy effect to flow air through the tower. Moreover, the natural draft cooling tower can either function with a closed circuit or open circuit cooling system. Figure 2 shows the different process, which operate in a similar mechanism, however the heat transfer mechanism functions differently.
Performance indicators
- Cooling Tower Range: Another pointer to determine the efficiency induced draft cooling towers is the cooling tower range, which is determined by deducting the tower’s outlet water temperature from the heated water temperature at the gulf of the cooling tower. Therefore, cooling tower range = Hot Water Temperature – Cold Water Temperature
- Tower Efficiency: The values of the cooling tower approach (cold water temperature – wet bulb temperature) and cooling tower range are can undoubtedly give us the value of cooling tower efficiency.
Thermal Efficiency = Range/(Range + Approach) x 100
As can be seen from the above condition, the effectiveness of the cooling tower is negatively related to the wet-bulb temperature. Since the wet-bulb temperature rises with increasing heat, a warm climate can decrease the efficiency of the tower and vice versa. Cooling tower manufacturers explain the importance of all the above factors during installation. Keep a note of the significant areas to look after.
There are two factors that are strong indicators of the cooling tower efficiency: makeup water quality and the Cycles Of Concentration (COC). Therefore, COC is defined as a dimensionless number that represents the ratio of a parameter corresponding to some minerals in the cooling water to that parameter in the makeup water. These factors will also help identify whether there is room for improving the efficiency of cooling tower. The impurities inside the makeup water lead to some scale formation, which alongside the water composition data could contribute to finding maximum COC (Figure 3). By knowing about the recommended maximum COC, one can simply identify the existence of an opportunity for improving the efficiency of the cooling tower by comparing that optimum COC with the current one. Now, if the current COC is at the proximity of its optimum value, some water treatment technologies such as filtration systems and water softeners might be beneficial to increase the cycles of concentration even higher.
The cycles of concentration can be calculated by any of the following formulas.
COC = Silica in Cooling Water / Silica in Makeup Water
- Evaporation Rate: This is the fraction of the circulating water that is evaporated in the cooling process.
- Drift: This is water that is carried away from the tower in the form of droplets with the air discharged from the tower.
Cooling tower approach
The cooling tower approach is the difference between the cold water temperature at the outlet of the tower and the ambient wet-bulb temperature. This measure is one of the important factors contributing to the cooling tower efficiency.
Approach = Cold Water Temperature – Wet Bulb Temperature
When a cooling tower is designed, some factors are taken into consideration. These factors include the wet-bulb temperature, cooling range, approach to the wet-bulb temperature, water circulation rate, air velocity through the tower’s air passageway(s) and of course tower height. These design parameters play an important role in the efficiency of the cooling tower.
Cooling towers consist of different components with various functions whose proper performance corresponds to the overall performance of the cooling process. Aside from conditions of tower parts, the cooling tower efficiency would essentially depend on the climatic conditions, particularly the relative humidity of the ambient air and its wet-bulb temperature.
Performance enhancers
The benefits of installing a VFD on a cooling tower are listed below.
- Increase system life: VFDs act as soft starters, increasing/decreasing speed at a programmable rate. Usually, fan motors consume a large amount of energy when starting the motor because a high current is delivered suddenly to the motor, VFDs prevent that from happening. This can reduce mechanical wear and prolong system life while saving maintenance costs.
- Automation and motor protection: VFDs have digital and analog inputs that increase the flexibility of motor automation and tracking the performance while measuring the air flow.
- Retrofits possible: Including VFDs in the design of a cooling tower is preferred and acquiring a cooling tower with a built in VFD is the best option. However, VFDs are flexible and can be retrofitted if the cooling tower does not contain a VFD.
- Water and energy savings: Considering the fan law, operating at variable speed would generate significant energy savings since conditions vary and most operating hours are at loads far below design conditions. In addition, appropriate air flow rates are required to avoid consuming more water than supposed to.
- Environmental benefits: VFDs allow the operators to preserve water and reduce carbon footprint.
- Reduced noise: when fans operate at lower speed. Additionally, modern fans tend to often have a lower noise level even when running at full speed due to improved fan design.
Figure 4 shows effect of fan speed on the performance of a cooling tower in case of cold water temperature requirement with various wet bulb temperature of incoming air. By providing the fan speed, cold water requirement can be lowered at minimum wet bulb temperature of incoming air.
Figure 5 shows amelioration in heat transfer by providing fill in the cooling tower. Heat transfer from evaporated water to air can be increased by providing filler of good quality material in cooling tower. The efficiency of cooling tower with fill material is high as compared to the efficiency of cooling tower without fill material. Fill material is used to increase the water and air contact inside the cooling tower. So, the heat loss by water is also high as compared to the cooling tower without fill material. The evaporation loss of cooling tower with fill material is little high, because the water and air contact time is high. Even though losses are generated in the cooling tower, the cooling is achieved due to heat transfer between air and water.
Cooling towers don’t need continuous upkeep. Intermittent attention alone can extend its life. The main part of maintaining cooling tower support is keeping up with water quality. Likewise, keeping mechanical parts greased up and examining the tower to guarantee proper functioning are other areas to look for. Compelling water treatment helps keep the natural draft cooling tower fill clean mosses and bacteria’s to grow on the pipeline. Each cooling tower has an alternate water supply and, consequently, unique water treatment pre-requisites.
Whether or not filtration, UV light, disinfecting, or another water treatment strategy is in force, it is significant that cooling tower users know about their water quality and picks treatment alternatives to continue proper use.
Conclusion
Cooling towers are found in abundancy around the world in many cooling applications since they decrease energy consumption of chillers and refrigeration plants. Cooling towers are also used in HVAC plants and industrial sites unrelated to chillers and refrigeration plants. Nevertheless, the same principles and efficiency parameters would apply to cooling towers in all applications. There are different types of cooling towers, evaporative, and adiabatic condensers, but they all follow the same cooling concept, which is the evaporative cooling process.
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 180 Research Articles in reputed International Conferences and Journals. He has also published 5 reputed books in the area of thermal engineering. 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.
