Cooling Towers – What Is Piping: All about Piping Engineering

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 TowersCommonly used Cooling Water termsBasics of Cooling TowerTypes of Evaporative Cooling TowerFactors Affecting Cooling TowersImpact of Cooling Water RangeNatural Draft Cooling TowersAdvantages and Disadvantages Need for Cooling Towers Cooling towers are used for Removal of the heat of reactionCondensationHeat 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 temperatureThe 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 evaporationAirflow required to aid evaporationLatent heat of evaporation of water transferred to the airHumidity and enthalpy of exit air increases Factors affecting Cooling Tower Design and Performance (Fig. 1): Ambient wet bulb temperatureAtmospheric conditions (DBT, RH)Range & ApproachCapacityL/G Ratio of TowerFilling media Fig. 1: Curves showing different factors with respect to temperature From the above curves this is evident: Tower size varies directly and linearly with the heat loadTower size varies inversely with rangeTower size varies inversely with the approachTower size varies inversely with wet bulb temperature Impact of Cooling Water Range Higher Range Reduced LMTDBigger ExchangersLower Velocities on CW side and likely foulingLower pumping cost Lower Range Higher LMTDSmaller ExchangersHigher velocities on CW side and less foulingHigher pumping cost Types of Cooling Towers (Fig. 2) Fig. 2: Figure showing the types of cooling towers Types of Evaporative Cooling Towers Three types based on airflow generation methods: Natural draft, which utilizes buoyancy via a tall chimneyMechanical draft, which uses power-driven fan motors to force or draw air through the towerFan 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) highDifferential 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. Fig. 3: Natural draft Cooling Towers Advantages of Cooling Towers No power costNo rotating parts Disadvantages ExpensiveHigh construction timeUsed for flows higher than 250,000 GPM When to choose a Cooling Tower Large heat load, e.g. power generationIn areas of higher relative humidity. Few more resources for you.. Heat Exchangers Pumps Compressors Piping Design and Layout