Water CT mainly consists of a casing, heat and mass transfer fill matrix and water and air circulation system. Hot water flows in counter flow with ambient air, transferring heat and mass to the ambient air through a fill matrix and the resultant cold water is collected at the bottom. Small CTs are generally single integral and skid mounted. The cooling towers are specified by the TR or MWt of heat handled, approach to the wet bulb temperature, cooling range and quantity of water handled (m3/h). Water CTs are generally built from 5 TR to any capacity (say 220 kilo TR).
The type of heat rejection in a cooling tower is termed evaporative in that it allows a small portion of the water being cooled to evaporate into a moving air stream to provide significant cooling to the rest of that water stream. The heat from the water stream transferred to the air stream raises the temperature of air and its relative humidity and this air is discharged to the atmosphere. Evaporative heat rejection devices such as cooling towers are commonly used to provide significantly lower water temperatures than achievable with air cooled or dry heat rejection devices, like the radiator in a car, thereby achieving more cost-effective and energy efficient operation of systems in need of cooling.
If cooled water is returned from the cooling tower to be reused, some water must be added to replace or make-up the portion of the water that evaporates. Because evaporation consists of pure water, the concentration of dissolved minerals and other solids in circulating water will tend to increase unless some means of dissolved-solids control, such as blow-down, is provided. Some water is also lost by droplets being carried out with the exhaust air (drift), but this is typically reduced to a very small amount by installing baffle-like devices, called drift eliminators, to collect the droplets. The make-up amount must equal the total of the evaporation, blow-down, drift, and other water losses such as wind blowout & leakage, to maintain a steady water level.
Historical Development of Cooling towers
With the invention of steam engine, heat energy was used to generate power and cooling tower became a necessity. James Watt’s patent for condenser, discovery of combustion, followed by mechanical refrigeration, internal combustion and fi nally electric power generation resulted in rapid development of industries. All of these generate waste heat that must be removed and dissipated to ambient, which necessitated the development of cooling towers.
The technique of evaporative cooling can be traced back to ancient times when rivers, seas, lakes, ponds, etc. were utilized as a medium of supply of cooling water. With the limited industrial activity of the past ages and plentiful resources, cold water was used in once through mode, discharged back and forgotten. Later, considering the thermal pollution, directly discharging the hot water back to its source was environmentally unacceptable. Hence hot process water was either to be cooled before the discharge or cooled and recycled. When topographical considerations were taken into account in power plant site selection, large ponds or canals were employed to hold, cool, recirculate or discharge process water, which required large area. To reduce the area required, spray systems were introduced to aerate the water in holding plants and to promote faster cooling by generating more water surface.
The next logical development came when it was discovered that spraying downwards in a box, instead of upwards, lower temperature could be achieved. Shortly after this, instead of relying on prevailing winds for air movement, aerodynamically designed fans were incorporated. As mechanics and hydrodynamics of water cooling became better understood, fill or packing material was included in designs to slow the vertical fall of water and provided greater air / water interfacial contact for more cooling. Through the development of the mechanical draft cooling tower, the land area required were brought down up to 1000 times compared to a cooling pond or lake.
Energy Performance Analysis
The Energy performance parameters of few industrial cooling towers, which were used for air conditioning applications, were measured and presented in Table 1. Digital relative humidity meter, temperature indicator and Ultrasonic water flow meters were used for measurement. The approach, range and eff ectiveness of cooling towers were computed. The observations and suggestions are given below.
- The CT fills are fully chocked with algae & scales and need to be replaced. The nozzles need to be cleaned to remove the scales. The water distribution across the cooling tower should be uniform. It is suggested to ensure that the approach of CT is 4-5OC. Presently, it is 7.4-14.9OC.
- Presently, three CTs are run for one chiller. It is suggested to run only 2 CTs for one chiller, as per design. The water flow to idle CT needs to be closed.
- Raw water is added as make up in Cooling tower (CT) and no water treatment is in place. Scaling is observed on condenser, CT fi lls and piping. Use of herbal liquid is recommended to avoid the deposit of scales and to dissolve the existing scales in cooling water (CW) line and condenser. Alternatively chemical de-scaling can be carried out once in an year.
- The operating principle & historical development of cooling towers are discussed in this article.
- Timely maintenance of cooling towers and ensuring an approach of 4-5 OC will lead to substantial energy saving in cooling applications.
- Treatment of cooling water is essential in order to ensure continued performance of cooling towers.