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    BACK TO BASIC: COOLING TOWERS

     

    INTRODUCTION

    The cooling tower is a device in which air is feed through the bottom of the tower. Hot water is falling from the top of the tower in the form of a spray. Air comes directly in contact with hot water and the heat of water is carried away due to evaporation of water. The heat removed from water is called the latent heat of evaporation. Each one pound of water that is evaporated removes approximately 1000 BTU(British Thermal Unit) in the form of latent heat. Cooling towers are used to cool the hot water returning from the comfort cooling system and hot water returning from the process plant, where hot water is generated due to the removal of process heat from the equipment.

    TYPE OF COOLING TOWER:

    (1) Natural Draft Cooling Towers.
    (2) Mechanical Draft Cooling Towers.

    Mechanical draft cooling towers:

    (1) Forced Draft Cooling Towers
    (2) Induced Draft Cooling Towers

    Induced Draft Cooling Towers are of two types:

    (1) Induced Draft Counter Flow Type
    (2) Induced Draft Cross Flow Type

    NATURAL DRAFT COOLING TOWER:

    A natural draft cooling tower is a device known as hyperbolic cooling towers. They are designed to take advantage of the temperature difference between ambient air and hot air inside the cooling tower. These cooling towers are used for large heat duties. They are available in cross flow and counter flow designs.

    Natural Draft Cooling Tower
    Natural Draft Cooling Tower

    MECHANICAL DRAFT COOLING TOWER:

    In these cooling towers, large-diameter fans are used to force or suck the air through circulating hot water. Heat transfer is increased as the hot waterfalls over the large fill surface area that increases the contact time between hot water and air.

    Mechanical Draft Cooling Tower
    Mechanical Draft Cooling Tower

    FORCED DRAFT COOLING TOWER:

    The fan in the forced draft cooling tower is a centrifugal blower type. They are usually located in the plant room and are much quieter than propeller-type fans.

     

    Forced draft tower
    Forced draft tower

    INDUCED DRAFT COOLING TOWER:

    An induced draft cooling tower is a device, in which a propeller-type fan is used. It is located at the exit of the airstream at the fan deck. In this cooling tower, the exit air velocity is three to four times that of the air entering velocity. Fan, motor, and drive mechanisms are located in a humid exit air stream, hence weatherproofing is provided to these components.

    Induced Draft Cross flow cooling Tower
    Induced Draft Crossflow cooling tower

    Induced draft cooling tower
    Induced draft cooling tower

     

     

     

     

     

     

    BASIC COMPONENTS OF COOLING TOWER:

    (1) Fan.
    (2) Drift eliminator.
    (3) Warm water basin.
    (4) Water Distribution Piping.
    (5) Water Sprinkling Nozzle.
    (6) Fill Packing.
    (7) Air Inlet.
    (8) Cold Water Collection Tank.

    MATERIAL OF COOLING TOWER COMPONENTS:

    Various kinds of materials are used for cooling tower components. To achieve (1) Increased corrosion resistance. (2) Reduced maintenance cost. (3) Maximize reliability. (4) Attain a longer life. The material in use are:

    (1) Hot-dip galvanized iron for fan deck and fan shaft.
    (2) FRP Coated basins for hot water.
    (3) Concrete pools for large towers.
    (4) Aluminium for propeller fan.
    (5) PVC for fills and spray nozzles.
    (6) IP55 Motors.

    COOLING TOWER USES:

     Towers are used in many walks of life. Few examples are listed below.

    (1) Power Generation:

     Steam.
    Nuclear.
    Geothermal.

    (2) Hydrocarbon Processing:

    Refineries.
    Petrochemicals.
    Chemicals.
    (3) Food Processing.

    (4) Large building HVAC,( Heating, Ventilation, and Air Conditioning).

    TERMINOLOGY FOR COOLING TOWERS:

    BTU

    BTU is the heat energy required to raise the temperature of one pound of water to one degree Fahrenheit.

    COOLING RANGE

    The temperature difference between hot water entering and cold water leaving the cooling tower.

    APPROACH TEMPERATURE

    This is the temperature difference between the cold water leaving the tower and the wet-bulb temperature. Approach temperature fixes the operating temperature of the tower. This is the most important parameter to determine tower size and cost.

    DRIFT

    The water contained in the airflow and discharged to the atmosphere. Drift loss does not include water loss by evaporation. Proper tower design and operation can reduce drift loss.

    HEAT LOAD

    This is the amount of heat to be removed from the water which is circulated through the tower. Heat load is equal to water circulation in GPM(Gallons per minute), times cooling range, times 500. It is expressed in BTU/hr. Heat load is an important parameter to determine tower size and cost.

    TON

    One cooling ton is 15000 BTUs per hour.

    WET BULB TEMPERATURE

    This is the lowest temperature that water can achieve by evaporation. Wet-bulb temperature is the parameter required in tower selection and design, it should be measured by a psychrometer.

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