A cooling tower is a specialized heat exchanger in which air and water are brought into direct contact to reduce the water’s temperature. As this occurs, a small volume of water is evaporated, reducing the temperature of the water being circulated through the tower.
They represent a relatively inexpensive and dependable means of removing heat from cooling water. 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 other units for further cooling.
Water, which an industrial process or air-conditioning condenser has heated, is pumped to the cooling tower through pipes. The water sprays through nozzles onto banks of material called “fill,” which slows the water flow through the cooling tower and exposes as much water surface area as possible for maximum air-water contact. As the water flows through the cooling tower, it is exposed to air, pulled through the tower by the electric motor-driven fan. When the water and air meet, a small amount of water is evaporated, creating a cooling action. The cooled water is then pumped back to the condenser or process equipment, where it absorbs heat. It will then be pumped back to the cooling tower to be cooled once again.
Cooling Tower Types
Cooling towers fall into two main categories: Natural draft and Mechanical draft.
Natural draft towers use exceptionally 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 m3/hr. Mechanical draft towers utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, which helps increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation.
Mechanical draft cooling towers
Mechanical draft towers are available in the following airflow arrangements:
1.Counterflow Induced draft.
2.Counterflow Forced draft.
3.Crossflow Induced draft.
Counterflow Cooling Towers
Counterflow Cooling Towers are designed to flow vertically upward, counter to the flow of falling water in the fill. Counterflow towers use pressurized, pipe-type spray systems to spray water onto the top of the fill. Induced draft cooling towers have fans typically mounted on top of the unit and pull air through the fill media. Conversely, the air is pushed by blowers located at the base of the air inlet face on forced draft towers.
Crossflow Cooling Towers
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.
Because of this, air does not have to pass through the distribution system, permitting the use of gravity flow hot water distribution basins mounted at the top of the unit above the fill. These basins are universally applied on all crossflow cooling towers.
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. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame.
Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. Fill can either be splash or film type.
With splash fill, water falls 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 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 the same heat transfer in a smaller volume than the splash fill.
Coldwater 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. In many tower designs, the cold-water basin is beneath the entire fill.
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 as in crossflow design or be located low on the side or the bottom as in counterflow designs.
Louvers: Generally, crossflow towers have inlet louvers. The purpose of louvers is to equalize airflow into the fill and retain the water within the tower. Many counterflow 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: Cooling tower fans must move large volumes of air efficiently, and with minimum vibration. The materials of manufacture must not only be compatible with their design but must also be capable of withstanding the corrosive effects of the environment in which the fans are required to operate.
Driveshafts: The driveshaft transmits power from the output shaft of the motor to the input shaft of the gear reducer. Because the driveshaft operates within the tower, it must be highly corrosion resistant. Turning at full motor speed, it must be well balanced and capable of being re-balanced.