WHAT IS COOLING TOWER?
Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade 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 to other units for further cooling.
WORKING PRINCIPLE OF COOLING TOWER.
The function of a cooling tower is to dissipate heat from water-cooled refrigeration, air- conditioning and industrial process systems. Water is typically the heat transfer medium used to dissipate the heat. A cooling tower uses a combination of heat and mass transfer evaporation) to cool the water flowing through the tower. Conductive heat transfer accounts for 20 to 30% of the total heat dissipated. The remaining 70 to 80% of total cooling is the result of evaporative cooling of about 1 to 2% of the recirculating water, depending on the decrease in temperature across the tower. It takes approximately 2,326,000 joules to evaporate 1 kilogram of water (1000 BTU per 1 pound of water).
If this amount of heat is extracted from 454 kilograms (1000 pounds) of water, approximately 0.45 kilogram (1 pound) of water will be evaporated and the temperature will drop 0.55 °C (1 °F). If 4.5 kilograms (10 pounds) of water are evaporated, the water temperature will drop 5.5 °c (10 °F). The water lost by evaporation is replaced with makeup water.
Water is also added to replace water lost through tower drift (loss of water from the tower as a fine mist), leaks in the system (unintentional blow down), and water discharged as intentional blow down. Water that is added to the cooling tower to replace all of these losses is known as cooling tower makeup water.
Cooling Tower Types
Cooling towers fall into two main categories as Natural draft & Mechanical draft.
- Natural draft towers: 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 m3/hr. These types of towers are used only by utility power stations.
- Mechanical draft towers: Mechanical draft towers are available in the following airflow arrangements:
1. Counter flows induced draft.
2. Counter flow forced draft.
3. Cross flow induced draft.
In counter flow induced draft design, hot water enters at the top, while the air is introduced at the bottom and exits at the top. Both forced and induced draft fans are used.
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.
Mechanical draft towers are available in a large range of capacities. Normal capacities range from approximately 10 tons, 2.5 m3/hr flow to several thousand tons and m3/hr. 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.” The number of cells they have, e.g., an eight-cell tower, often refers to
such towers. 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.
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 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 fill. 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
In some forced draft counter flow design, however, the water at the bottom of the fill is
channelled to a perimeter trough that functions as the cold-water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.
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 equalize 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: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller
fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft 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. Galvanized steel, various grades of stainless steel, glass fiber, 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 fiber rather than wood panels (casing)
over the wood framework. The inlet air louvers may be glass fiber, 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 galvanized steel or, where a corrosive atmosphere is a problem, stainless steel.
Sometimes a galvanized tower has a stainless-steel basin. Glass fiber is also widely used for
cooling tower casings and basins, giving long life and protection from the harmful effects of
many chemicals. Plastics are widely used for fill, including PVC, polypropylene, and other polymers. Treated wood splash fill is still specified for wood towers, but plastic splash fill is also widely used when water conditions mandate the use of splash fill. Film fill, because it offers greater heat transfer efficiency, is the fill of choice for applications where the circulating water is generally free of debris that could plug the fill passageways.
Plastics also find wide use as nozzle materials. Many nozzles are being made of PVC, ABS,
polypropylene, and glass-filled nylon. Aluminium, glass fiber, and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are fabricated from galvanized, aluminium, or molded glass fiber reinforced plastic.
Major industries that use Cooling towers
- refineries
- Sugar (sugar industry)
- Distilleries
- Production of energy (gas turbines, reciprocating engines, etc.. Users)
- Chemical Industries
- Pharmaceutical industries
- Food Industries
- Industries canning and juice
- Glass works
- Iron works
- Plastics industry
- rolling mills and wire mills
- iron foundries
- Paper Manufacturing.
- Plastic Manufacturing.
- Chemical and Petrochemical Industry.
- Laminate Sheet Manufacturing.
- Rubber Manufacturing and Processing Industries.
- Steel Plants.
- Metal Casting and Metal Forging Plants.
- Rolling Mills.
- Petrochemical plants
- Power plants
- Chemical plants
- Pharmaceutical industry
- Food processing plants
- Petroleum refineries