Cooling towers is a heat rejection device that cools and process water using the evaporative cooling principal in whom it extracts waste heat to the atmosphere through the cooling of a water stream to a lower temperature. This process allows a small portion of the water being cooled to evaporate in to 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 air temperature and its relative humidity to 100%, and this air is discharged to the atmosphere.

They act as giant air washers. Finely misted water is sprayed in to a column of moving air. In the cooling tower, this mist is intended to increase the surface area of the water so that heat can be transferred from the water to the air more effectively. These giant towers are intrinsic to almost every industry, especially the process plants, as they represent a relatively inexpensive and dependable means of removing low-grade heat from cooling tower. In a process plant cooling towers are used to cool the process water coming from heat exchanger. The make up water source is used to replenish the water lost to the evaporation. Hot water from heat exchanger is sent to the cooling towers. The water exits the cooling tower and sent back to the heat exchangers or to the other units for further cooling as this process cools the water to the ambient temperature only.

Cooling towers can be categorized in to two main types:

Natural draught cooling towers: Natural draught designs use very large cooling area to introduce air through the media. In this design water is sprayed in a certain area from a suitable height to transfer heat through interaction of atmospheric air. The size of the cooling towers depends upon the differential pressure between the cold outside air & the hot humid air inside of the tower as the driving force. No fans or fill media are used. In a big power plant they use very large concrete chimneys for interaction of air through the water. Due to the tremendous size of the cooling towers (500 ft height and 400 ft. diameter at the base) they are generally used for water flow rates above 2,00,000 gal/min.

Mechanical draught cooling towers: Cooling towers with fans are referred to as mechanical draught- induced or forced draught depending on fan location. Mechanical draught cooling towers are most commonly used. These towers have long piping runs that spray the water downwards. Large fans pull air across the dropping water to remove the heat. As the water drops downward on to the fill or slats in the cooling tower, the drops break up in to a finer spray. On the colder days, tall plumes of condensation can be seen. On warmer days only small condensation plumes will be seen.

Cross flow & counter flow cooling towers: Most towers employ counter flow design but cross flow designs are also available. The differences between these two designs are governed by the relationship of air & water flow inside the cooling tower. In counter flow cooling towers, water falls by gravity over the fill media and air passes vertically upwards. By contrast, in cross flow cooling towers waterfalls by gravity over the fill media while air flows horizontally. Most counter flow cooling towers utilize a plastic film fill media that reduces both pump head and horsepower cost; cross flow towers typically utilize a splash bar fill media. However, it is possible to find either type of fill media in both types of towers.

Open circuit cooling towers: A direct or open circuit cooling tower is an enclosed structure with internal means to distribute the warm water fed to it over a packing or ‘fill area’ The fill area provides a vastly expanded air water interface for heating of the air and evaporation to take place. The water-cooled as it descends through the fill by gravity while in direct contact with air that passes over it. The cooled water is than collected in a cold-water basin below the fill from which it is pumped back to the process to absorb more heat. The heated and moisture laden air leaving the fill area is discharged to the atmosphere at a point remote enough from the air inlets to prevent its being drawn back in to the cooling tower.

Close-circuit cooling towers: An indirect or closed circuit cooling tower involves no direct contact of the air and the fluid, usually water or a glycol mixture, being cooled. Unlike the open cooling tower, the closed circuit cooling tower has two separate fluid circuits. One is an external circuit in which water is re-circulated on the outside of the second circuit, which is tube bundles (closed coils) which are connected to the process for the hot fluid being cooled and returned in a closed circuit. Air is drawn through the re-circulating water cascading over the outside of the hot tubes, providing evaporative cooling similar to an open cooling tower. In operation heat flows from the internal fluid circuit, through the tube walls of the coil, to the external circuit and then by the heating of the air and evaporation of some of the water, to the atmosphere. Operation of the closed circuit cooling towers is therefore very similar to the open cooling tower with one exception. The process fluid being cooled is contained in a “closed” circuit and is not directly exposed to the atmosphere or the re-circulated external water.

Package & field erected cooling towers: Most heating and cooling application require cooling towers below 10,000 gal/min. Towers of this type, called package cooling towers, usually are mass produced in factories with FRP structure & casing. This type of cooling tower is manufactured so it can be transported easily to the job site.

The towers requiring a thermal duty, beyond the capabilities of a package cooling towers are larger, requiring them to be manufactured, shipped & assembled at site. These are field-erected towers and generally used in most industrial and utility applications. Field erected mechanical draught towers can handle flow rates from 10,000 to 3,50,000 gal/min.

Common applications for cooling towers are providing cooled water for air conditioning, manufacturing and electric power generation. The smallest cooling towers are designed to handle water streams of only a few galloons of water per minute supplied in small pipe, while the largest cool hundreds of thousands of galloon per minute supplied in pipes much as 15 feet in diameter on a large power plant. Some of the important applications are: 1. Central Air Conditioning 2. Diesel Generating Sets 3. Industrial Furnaces 4. Injection molding M/c 5. Any process which require water cooling in industries

The performance of a cooling tower is directly proportional to the efficiency of the heat transfer. There are some of the common causes that reduces the heat transfer co-efficient as:

Scale menace: When water evaporates from the cooling tower, it leaves scale deposits on the surface of the fill from the minerals that were dissolved in the water. Scale build up acts as a barrier to heat transfer from the water to the air. Excessive scale build up is telltale sign of water treatment problems.

Clogged spray nozzles: Algae and sediments that collect in the water basin as well as excessive solids get in to the cooling water and can clog the spray nozzles. This cause uneven water distribution over the fill area and reduced heat transfer surface area. This problem is a sign of water treatment problem again and clogged strainers.

Poor airflow: This reduces the amount of heat transfer from the water to the air. Poor airflow can be caused by debris at the inlets or outlets of the tower or in the fill. Other causes of the poor in flow are loose fan and motor mountings, poor motor and fan alignment, poor gearbox maintenance, improper fan pitch, damage to fan blades or excessive vibrations. Reduced airflow due to poor fan performance can ultimately lead to motor or fan failure.

Poor pump performance: An indirect or closed circuit cooling tower uses a cooling tower pump. Proper water flow is important to achieve optimum heat transfer. Loose connections, failing bearings, cavitations, clogged strainers; excessive vibrations and non-design operating conditions result in reduced water flow, reduced efficiency and premature equipment failure

Re-circulation of exhaust air: Re-circulation is an adulteration of the atmospheric air entering the tower by a portion of the atmospheric air leaving the tower. This adulteration by the exhaust air raises the wet bulb temperature of the entering air above that of the ambient air, reducing the tower’s overall performance.

Re-circulation is always more in forced draught arrangement than in the induced draught. Further in the induced draught design it is more in the cross flow design than in a counter flow one. In a cross flow cooling tower fan draws air through the entire louver face and a pressure difference is created along the vertical plane of the structure. Air entering the upper section is closer to the fan and passes through the cooling tower at a higher velocity than air at lower levels. This effect produces negative pressure at upper levels of the tower and under typical atmospheric wind condition, sets up re-circulation of exhaust air. Hot moist air with a high wet bulb temp. is pulled back in to the tower, which greatly reduces its performance.

In a counter flow cooling tower air intake louvers are near the base, a considerable distance from the fan discharge plum, so it is difficult for exhaust air to circulate in normal condition. It can only re-circulate if the discharge plum hits a barrier adjacent to the fan outlet

One of the advancement in cooling towers is construction with fiber-reinforced plastic (FRP). Fiberglass has been used in the cooling tower piping, fan stacks & siding for many years with great success due to its low maintenance requirements, resistance to moisture and material properties that allow a range of water temperature and pH. Currently the fastest growing segment of the cooling tower market is structure built with pultruded FRP sections. This inert inorganic material is strong lightweight, chemically resistant, and able to handle a range of pH values. FRP is stronger than Douglas fir and redwood, and because it is available in long lengths, it allows a cooling tower to be designed and built with a minimum number of airflow obstructions.

This enhances performance and reduces the number of connections and field labor erection costs; Fire retardant FRP can eliminate the cost of a fire protection system, which can equal 5 to 12 percent of the cost of a cooling tower. One of the most important advancement is use of FRP fans in cooling towers. These fans are providing higher aerodynamic efficiency and significant saving in operating cost up to 20%. The special characteristics of reinforced fiberglass provide excellent resistance to corrosion & erosion against humid air and water droplets.

Fan blades are made up of fiberglass reinforced epoxy resin & are hollow in construction. These have been designed with most efficient aerofoil sections thickness to achieve maximum lift & minimum drag co-efficient. Light and strong blades also increases the life of mechanical drive arrangement.

TR : TR means tones of refrigeration when it comes to the cooling tower terminology. We only ask about TR of the cooling tower if it is applicable for refrigeration or air conditioning application. One TR of cooling tower could reject 3780 Kcal/Hr heat.

Range: Range of a cooling tower means the difference between Inlet (hot water ) temp. & Outlet (cold water) temp. of the cooling tower. It is also called delta T of the cooling tower sometimes.

Approach: The difference between cold water temp. & Wet bulb temp. of the atmosphere is called approach of the cooling tower. This is normally 4 C to 5 C according to application requirement. The smaller the approach the size will be larger of the cooling tower.

Wet-bulb temp. : The temp. of the atmosphere with a wet cotton around the bulb of thermometer means the wet bulb temp. It is the most important factor considered while designing a cooling tower. For better performance of the cooling tower it should be designed with maximum wet bulb temp. of the area.

L/G Ratio : The ratio of heat rejected by liquid i.e. water to the heat gained by gas i.e. air is called L/G ratio of the cooling tower.