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
- Condensation
- 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
- Capacity
- 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
Higher Range
- Reduced LMTD
- Bigger Exchangers
- Lower Velocities on CW side and likely fouling
- Lower pumping cost
Lower Range
- 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
Disadvantages
- Expensive
- 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.
Few more resources for you..
It’s cool that natural draft cooling towers are typically 120 meters tall. Honestly, I’d just heard of cooling towers, so it’s really cool to learn about them. Where can I learn more about the other types of cooling towers mentioned?