A cooling tower is a heat exchanger that is used as a heat rejection device to reduce hot water temperature by transferring waste heat to the atmosphere by evaporation. In this article, the following points will be discussed in brief:
- Need for Cooling Towers
- Commonly used Cooling Water terms
- Basics of Cooling Tower
- Types of Evaporative Cooling Tower
- Factors Affecting Cooling Towers
- Impact of Cooling Water Range
- Natural Draft Cooling Towers
- Advantages and Disadvantages
Need for Cooling Towers
Cooling towers are used for
- Removal of the heat of reaction
- Heat of compression
Commonly used Terms for Cooling Towers
- Dry-Bulb temperature refers basically to the ambient air temperature. It is called “Dry Bulb” because the sensing tips of the thermometer not affected by the moisture of the air.
- Wet-Bulb temperature is the temperature of adiabatic saturation indicated by a moistened thermometer bulb exposed to the airflow.
- Relative Humidity is the amount of moisture in the air, as a percentage of the total moisture the air can contain at the current temperature
- The approach is the difference in temperature between the cooled-water temperature and the entering air wet-bulb temperature.
- The range is the temperature difference between the water inlet and the water exit.
- Cycles of concentration represent the accumulation of dissolved minerals in the recirculating cooling water.
- Blow-down – The portion of the circulating water flow that is removed in order to maintain the number of dissolved solids and other impurities at an acceptable level.
- Fills – Inside the tower, fills are added to increase contact surface as well as contact time between air and water. Thus they provide better heat transfer.
- Drift – Water droplets that are carried out of the cooling tower with the exhaust air.
- Plume – The stream of saturated exhaust air leaving the cooling tower.
- Blow-out – Water droplets are blown out of the cooling tower by wind, generally at the air inlet openings.
Basics of Cooling Tower (Evaporative Type)
- Water-cooled by evaporation
- Airflow required to aid evaporation
- Latent heat of evaporation of water transferred to the air
- Humidity and enthalpy of exit air increases
Factors affecting Cooling Tower Design and Performance (Fig. 1):
- Ambient wet bulb temperature
- Atmospheric conditions (DBT, RH)
- Range & Approach
- L/G Ratio of Tower
- Filling media
From the above curves this is evident:
- Tower size varies directly and linearly with the heat load
- Tower size varies inversely with range
- Tower size varies inversely with the approach
- Tower size varies inversely with wet bulb temperature
Impact of Cooling Water Range
- Reduced LMTD
- Bigger Exchangers
- Lower Velocities on CW side and likely fouling
- Lower pumping cost
- Higher LMTD
- Smaller Exchangers
- Higher velocities on CW side and less fouling
- Higher pumping cost
Types of Cooling Towers (Fig. 2)
Types of Evaporative Cooling Towers
Three types based on airflow generation methods:
- Natural draft, which utilizes buoyancy via a tall chimney
- Mechanical draft, which uses power-driven fan motors to force or draw air through the tower
- Fan assisted natural draft that appears like a natural draft though airflow is assisted by a fan
Natural Draft Cooling Towers (Fig. 3):
- Also called as Hyperbolic Natural draft towers, typically about 400 ft (120 m) high
- Differential pressure between the cold outside air and the hot humid air on the inside of the tower acts as the driving force.
- No fans are used.
- Warm, moist air naturally rises due to the density differential to the dry, cooler outside air.
- The use of natural or mechanical draft towers depends on climatic conditions and operating requirements.
Advantages of Cooling Towers
- No power cost
- No rotating parts
- High construction time
- Used for flows higher than 250,000 GPM
When to choose a Cooling Tower
- Large heat load, e.g. power generation
- In areas of higher relative humidity.
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