Maintaining efficiency of cooling towers

Cooling towers has a very important role in most of the industries. The primary task of a cooling tower is to reject heat into the atmosphere and suck them out of a system whilst ensuring a smooth inflow of cool air. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water with the help of atmospheric air by evaporative cooling. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling.

Cooling towers fall into two main categories: Natural draft and mechanical draft.

Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally used for water flow rates above 45,000 cu.m/hour. These types of towers are used only by utility power stations.

Mechanical draft towers utilise large fans to force (in forced draft cooling tower) or suck air (in induced draft cooling tower) through circulated water. The water falls downward over fill surfaces, which help increase the contact time between the water and the air – this helps maximise heat transfer between the two. Cooling tower size is affected by the flow, range (hot water temperature minus cold water temperature), approach (cold water temperature minus wet bulb temperature) and wet bulb temperature. When three of these four quantities are held constant, tower size increases in the following manner:

• Directly with the flow
• Directly with the range
• Inversely with the approach
• Inversely with the inlet wet bulb temperature.

Mechanical draft towers: Mechanical draft towers are available in two airflow arrangements: induced draft cooling tower; and forced draft cooling tower. Accordingly to direction of air flow cooling tower are further classified as follows:

• Counter flow induced draft.
• Cross flow induced draft.

In the counter flow induced draft design, hot water enters at the top, while the air is introduced at the bottom and exits at the top. In cross flow induced draft towers, the water enters at the top and passes over the fill. The air, however, is introduced at the side either on one side (single-flow tower) or opposite sides (double-flow tower). An induced draft fan draws the air across the wetted fill and expels it through the top of the structure. Normal capacities range from approximately 10 tons, 2.5cu.m/per hour flow to several thousand tons and cu.m/hour. Towers can be either factory built or field erected.

Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or cells. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells.

According to Paltech Cooling Towers and Equipments Ltd, performance assessment of cooling towers in operational performance assessment, the typical measurements and observations involved are:

• Cooling tower design data and curves to be referred to as the basis.
• Intake air WBT and DBT at each cell at ground level using a whirling pyschrometer.
• Exhaust air WBT and DBT at each cell using a whirling psychrometer.
• CW inlet temperature at risers or top of tower, using accurate mercury in glass or a digital thermometer.
• CW outlet temperature at full bottom, using accurate mercury in glass or a digital thermometer.
• Process data on heat exchangers, loads on line or power plant control room readings, as relevant.
• CW flow measurements either direct or inferred from pump motor kW and pump head and flow characteristics.
• CT fan motor amps, volts, kW and blade angle settings
• TDS of cooling water.
• Rated cycles of concentration at the site conditions.
• Observations on nozzle flows drift eliminators, condition of fills, splash bars, etc.

Components of cooling tower: The basic components of an evaporative tower are: Frame and casing, fill, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans.

Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. For some smaller designs, such as some glass fibre units, the casing may essentially be the frame.

Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximising water and air contact. Fill can either be splash or film type. With splash fill, waterfalls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fill promotes better heat transfer than the wood splash fill. Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.

Cold water basin: The cold water basin located at or near the bottom of the tower, receives the cooled water that flows down through the tower and fills. The basin usually has a sump or low point for the cold water discharge connection.

Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.

Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower–cross flow design– or be located low on the side or the bottom of counter flow designs.

Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalise air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers.

Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in some circular cross-section towers.

Fans: Axial (propeller type) fans are used in towers. Depending upon their size, propeller fans can either be fixed or variable pitch. A fan having non-automatic adjustable pitch blades permits the same fan to be used over a wide range of kW with the fan adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.

Tower Materials: In the early days of cooling tower manufacture, towers were constructed primarily of wood. Wooden components included the frame, casing, louvers, fill, and often the cold water basin. If the basin was not of wood, it likely was of concrete.

Today, tower manufacturers fabricate towers and tower components from a variety of materials. Often several materials are used to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanised steel, various grades of stainless steel, glass fibre, and concrete are widely used in tower construction as well as aluminium and various types of plastics for some components. Wood towers are still available, but they have glass fibre rather than wood panels (casing) over the wood framework. The inlet air louvers may be glass fibre, the fill may be plastic, and the cold water basin may be steel.

Larger towers sometimes are made of concrete. Many towers–casings and basins–are constructed of galvanised steel or, where a corrosive atmosphere is a problem, stainless steel. Sometimes a galvanised tower has a stainless steel basin. Glass fibre is also widely used for cooling tower casings and basins, giving long life and protection from the harmful effects of many chemicals.