Cooling TowerDew point
It is the temperature at which condensation of water vapor from the air begins as the temperature of the air-water vapor mixture falls.
Dew Point Temperature
It is a measure of the latent heat content of air-water vapor mixtures and since latent heat is a function of moisture content, the dew point temperature is determined by the moisture content.
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Latent heat
It is the change in heat content of a substance, when its physical state is changed without a change in temperature.
Sensible heat
It is that heat which when added or subtracted results in a change of temperature.
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Cooling Tower: Basic Concepts
Latent Heat of Vaporization
The latent heat of vaporization of a substance is the quantity of heat required to change 1kg of the substance from liquid to vapor state without change of temperature.
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Dry Bulb Temperature (DBT or DB):
It is an indication of the sensible heat content of air-water vapor mixtures.
Wet Bulb Temperature (WBT or WB):
It is a measure of total heat content or enthalpy. It is the temperature approached by the dry bulb and the dew point as saturation occurs.
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Range:
It is the difference between the cooling tower water inlet and outlet temperature.
Approach:
It is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.
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and ambient wet bulb temperature
or in other words
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Cooling Tower Principle
The evaporation of water by the circulating air results in the cooling of the water.
There is the convective type cooling phenomena occurring in the tower.
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Cooling water is required in the process in order to:
Cool the process fluids which are to be stored in storage tanks.
Cool the process fluids which are used as a cold reflux stream in distillation columns for either maintaining the vacuum conditions in the column or as a reflux stream.
Cool the intermediate streams in a reactor having exothermic reaction and hence maintain uniform temperature conditions in the reactor.
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Cooling Tower
In order to cool the process streams as mentioned above, we need to exchange the heat with the cooling water.
Hence we require a cooling tower to cool the water and send it to the process exchangers for cooling the process streams as per our requirement.
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Cooling Tower
Cooling water is circulated in a closed loop and hence the water consumption is reduced.
The header which supplies the cold water to the process exchangers is called the supply header.
The header from which the hot water outlet of the exchanger returns back to the tower is called is the Return Header.
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Frame and casing
Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components.
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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.
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Cooling tower components
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.
Drift eliminators:
These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.
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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.
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Nozzles:
They 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.
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Fans: