What is a Cooling Tower?
A cooling tower is heat removal device that uses water to transfer process waste heat into the atmosphere. All cooling towers operate on the principle of removing heat from water be evaporating a small portion of water that is recirculated through the unit. The mixing of warm water and cooler air releases latent heat of vaporization, causing a cooling effect to the water. They are a key component of many refrigeration systems and can be found in industries such as power plants, chemical processing, steel mills, and many manufacturing companies where process cooling is necessary. Also, cooling towers can be used to provide comfort cooling for large commercial buildings like airports, schools, hospitals, or hotels.
Cooling towers are a very important part of many process plants and power plants. The make-up water source is used to refill water lost to evaporation. Hot water from heat exchangers/condenser is sent to the cooling tower. After reducing the temperature of water in cooling tower and is sent back to the condenser /exchangers or to other units for further process
Types of Cooling Towers
Cooling towers might be one of the most vital part of any industrial process. These tall, open-topped, cylindrical structures are responsible for cooling water generated
from industrial or HVAC comfort cooling process. They are classified by the type of draft (natural or mechanical) and by the direction of air flow (counter or cross).
Natural Draft Cooling Towers are usually used for large power plants and industries with infinite cooling water flow. The tower operates by hot air in the tower rising removing waste heat and then releasing it into the atmosphere. These towers are tall and have a hyperbolic shape to induce proper air flow.
Mechanical Draft Cooling Towers have air forced through the structure by a fan that circulates air through the tower. Common fans used in these towers include propeller fans and centrifugal fans. While Mechanical draft towers are more effective than natural draft towers, they consume more power and cost more to operate as a result.
Crossflow Towers has a design that allows the air to flow horizontally through the fill and the tower’s structure into an open plenum area. Hot water flows downward from distribution basins. There is less recirculation because the speed of exit air is higher than entering air. However, fans and motor drive require weather-proofing against moisture which can lead to freezing making it less efficient.
Counterflow Towers has a design where the air is vertically upwards and the counter current with hot water is falling downwards to cool the air. This allows for maximum performance out of each plan area and helps minimize pump head requirements. Also, they are less likely to ice up in cold weather conditions and can save energy in the long run.
Induced Draft towers are typically mounted with a fan at the top of the cooling tower, which allows hot air out and pulls air throughout. The high exiting air velocities reduces the chance of re-circulation. To avoid the entrapment of water droplets in the leaving stream air, drift eliminators are used. Induced draft towers are more efficient as they use 30% to up to 75% less energy compared to forced draft designs.
Forced Draft towers are similar to induced draft but the basic difference is that the exhaust fan is placed at the base of the cooling tower, which allows the air to blow from the bottom. Their use is limited due to water distribution challenges, high horsepower fans and the possibility of re-circulation.
Cooling Tower Terminology
Approach: is the difference between the temperature of the cold water leaving the tower and the wet-bulb temperature of the air is known as the approach. Establishment of the approach fixes the operating temperature of the tower and is a most important parameter in determining both tower size and cost.
Bleed Off: is the circulating water in the tower which is discharged to waste to help keep the dissolved solids concentration of the water below a maximum allowable limit. As a result of evaporation, dissolved solids concentration will continually increase unless reduced by bleed off.
Biocide: a chemical that is designed to control the population of troublesome microbes by killing them.
Blowdown: is the water purposely discharged from the system to control concentrations of salts or other impurities in the circulating water. Units % of circulating water rate or gpm.
British Thermal Unit (BTU): is the heat energy required to raise the temperature of one pound of water one degree Fahrenheit in the range from 32° F to 212° F
Cooling Range: is the difference in temperature between the hot water entering the tower and the cold water leaving the tower is the cooling range.
Cycles of Concentration: compares dissolved solids in makeup water with solids concentrated through evaporation in the circulating water. For example, chlorides are soluble in water so the cycles of concentration are equal to the ratio of chlorides in circulating water to chlorides in makeup water.
Dissolved Solids: total solids that have been dissolved into a liquid. They may be ionic and/or polar in nature.
Drift: is the water entrained in the air flow and discharged to the atmosphere. Drift loss does not include water lost by evaporation. Proper tower design can minimize drift loss.
Heat Exchanger: is a device for transferring heat from one substance to another. Heat transfer can be by direct contact, as in a cooling tower, or indirect, as in a shell and tube condenser. Can also be the tube or fin tubed bundles in a wet/dry tower.
Heat Load: The amount of heat to be removed from the circulating water within the tower. Heat load is equal to water circulation rate (gpm) times the cooling range times 500 and is expressed in BTU/hr. Heat load is also an important parameter in determining tower size and cost.
Makeup: is the amount of water required to replace normal losses caused by bleed off, drift, and evaporation.
Pumping Head: The pressure required to pump the water from the tower basin, through the entire system and return to the top of the tower.
Ton: an evaporative cooling ton is 15,000 BTU’s per hour.
Wet Bulb: is the lowest temperature that water theoretically can reach by evaporation. Wet-Bulb temperature is an extremely important parameter in tower selection and design and should be measured by a psychrometer.