Large air-conditioning plant used for production of thermal comfort in commercial building and some other industrial applications produce a large quantity of waste heat in the form of
hot water. In the present scenario, in most of the places, the water supply is limited and thermal pollution is also a serious concern. Considering the recent increase of interest in analysing these problems and solving them for the well being of the environment, the attempt was to deal with the technology, applications of cooling towers. The factors affecting the performance of cooling towers like environmental conditions, cooling water quality needed to be optimised. The performance parameters like range, approach, cooling capacity, evaporation loss liquid to gas ratio (L/G) have been studied when the cooling tower is operated at full load and part load under the same water flow rates.
In vapour compression based large conventional air conditioner, one of the main parts is condenser, which cools the refrigerant. When cooling the refrigerant, the cold water becomes the hot water. The hot water temperature is reduced by cooling towers. When hot water enters into the cooling tower and sprayed by nozzles, hot water is converted into cold water. The effective cooling of water depends upon the dry bulb temperature and wet bulb
temperature, size, height of the cooling tower and velocity of air. A cooling tower is an enclosed device for the evaporative cooling of water by contact with the air. Cooling tower is a heat rejection device. Common application besides to airconditioning includes cooling the circulating water used in oil refineries, petrochemical, and other chemical plants, thermal power stations and HVAC system for cooling buildings. The efficiency and effectiveness of a cooling tower depends on number of parameter like inlet air angle, inlet and outlet temperature of air and water, fill materials, fan speed etc.
Over a last decade, great strides have been made in improving the performance of conventional cooling towers. Heat is dissipated from the surface of a body of water by convection, evaporation and radiation. The driving force is difference in enthalpy rather than in temperature. The water temperature tends to approach the wet bulb temperature rather than the dry bulb temperature of air. This offers an inherent advantage in making it possible to cool the water to a temperature lower than the dry bulb temperature. This cooling is accomplished by a combination of the sensible heat transfer and evaporation of a
small proportion of water. A mechanical draft cooling tower is used to increase the cooling capacity. For the increasing of L/G ratio, the heat transfer rate between liquid and gas also needs to be increased. The ultimate aim is to improve the performance analysis of cooling tower by enhancing efficiency of cooling tower.
Working of Cooling Tower
Cooling towers are a special type of heat exchanger that allows water and air to come in contact with each other to lower the temperature of the hot water. During this process, small volumes of water evaporate, lowering the temperature of the water that is being circulated throughout the cooling tower. In a short summary, a cooling tower cools down water that gets over heated by industrial equipment and processes. The hot water is usually caused by air conditioning condensers or other industrial processes. That water is pumped through pipes directly into the cooling tower. Cooling tower nozzles are used to
spray the water onto to the ‘fill media’, which slows the water flow down and exposes the maximum amount of water surface area possible for the best air-water contact. The water is exposed to air as it flows throughout the cooling tower. The air is being pulled by a motor-driven electric ‘cooling tower fan’. When the air and water come together, a small volume of water evaporates, creating an action of cooling. The colder water gets pumped back to the process or equipment that absorbs heat or the condenser. It repeats the loop over and over again to constantly cool down the heated equipment or condensers.
Types of Cooling Tower
Cooling towers mainly divided into two categories according to need of power requirement for its operation as follows:
Natural draft cooling tower
The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower as hot air moves upwards through the tower (because hot air rises) and fresh cool air is drawn into the tower through an air inlet at the bottom as shown in Fig.1.
Figure 1. Natural draft cooling tower
Mechanical draft cooling tower
Mechanical draft towers have large fans to force or draw air through circulated water as shown in Figure 2. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air. This helps maximise heat transfer between the two. Cooling rates of various parameters such as fan mechanical draft towers depend on
diameter and speed of operation, fills for system resistance etc.
Figure 2: Mechanical draft cooling tower
Cross flow and counter flow design
In cross flow cooling tower systems (Figure 3a) the water vertically flows through the fill media while the air horizontally flows across the falling water. That is why they call it ‘cross flow’, because the air and water cross paths or flows. Because of the crossing of flows, the air doesn not need to pass through the distribution system. This permits the use of hot water flow via gravity and distribution basins on the top of the tower right above the fill media. The basins are a standard of cross flow cooling towers and are applied on all units.
Figure 3: (a) Cross flow of cooling tower
In counter flow cooling tower system processes (Figure 3b), the air vertically flows upwards, counter to the water flow in the fill media. Due to the air flowing vertically, it is not possible to use the basin’s gravity flow like in cross flow towers. As a substitute, these towers use pressurised spray systems, usually pipetype, to spray the water on top of the fill
media. The pipes and cooling tower nozzles are usually spread farther apart so they will not restrict any air flow.
Figure 3: (b) Counter flow cooling tower
A brief comparison between counter flow and cross flow cooling tower is tabulated below as table 1.
Performance Parameters for Cooling Towers
A number of parameters describe the performance of a cooling tower.
• Water/Air Ratio (mw/ma) is the mass ratio of water (liquid) flowing through the tower to the air (Gas) L/G flow.
• Approach is the difference between the temperature of the water leaving the tower and the wet bulb temperature of the entering air.
• Range is the temperature difference between the hot water entering the cooling tower and the cold water leaving. The range is virtually identical with the condenser rise.
• Evaporation Rate is the fraction of the circulating water that is evaporated in the cooling process.
• Drift is water that is carried away from the tower in the form of droplets with the air discharged from the tower.
• Recirculation is warm, moist air discharged from the tower that mixes with the incoming air and re-enters the tower.
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 4: Effect of fan speed on requirement of cooling water temp at various WBT of air
Figure 5: Effect of fill on heat transfer rate in cooling tower.
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.