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    Cooling Tower

    Asst.Prof.Dr.Sirirat Wacharawichanant

    Department of Chemical Engineering

    Silpakorn University

    Heat Transfer Operations Introduction

    Cooling towers are heat rejection devices used to transfer

    process waste heat to the atmosphere. Cooling towers may

    either use the evaporation of water to reject process heat and

    cool the working fluid to near the wet-bulb air temperature or

    rely solely on air to cool the working fluid to near the dry-

    bulb air temperature.

    Common applications include cooling the circulating water

    used in oil refineries, chemical plants, power plants and

    building cooling. The towers vary in size from small roof-top

    units to very large hyperboloid structures.

    Introduction

    In a typical water-cooling tower, warm water flows

    countercurrent to an air stream. Typically, the warm water

    enters the top of a packed tower and cascades down through

    the packing, leaving at the bottom.

    Air enters at the bottom of the tower and flows upwardthrough the descending water. The tower packing often

    consists of slats of plastic or a packed bed.

    (continue)

    The water is distributed by troughs and overflows to cascade

    over slat gratings or packing that provides large interfacial

    areas of contact between the water and air in the form of

    droplets and films of water.

    Introduction

    The flow of air upward through the tower can be induced by

    the buoyancy of the warm air in the tower or by the action of

    a fan.

    The water cannot be cooled below the wet bulb temperature.

    The driving force for the evaporation of the water isapproximately the vapor pressure of the water less the vapor

    pressure it would have at the wet bulb temperature .

    (continue)

    The water can be cooled only to the wet bulb temperature,

    and in practice it is cooled to about 3 K or more above this.

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    Introduction

    Only amount of water is lost by evaporation in cooling water.

    Since the latent heat of vaporization of water is about 2300

    kJ/kg, typical change of about 8 K in water temperature

    corresponds to an evaporation loss of about 1.5%.

    Hence, the total flow of water is usually assumed to be

    constant in calculations of tower size.

    (continue) Categorization by air-to-water flow

    Crossflow

    Figure 1 Crossflow.

    Crossflow is a design in

    which the air flow is

    directed perpendicular to

    the water flow (see

    diagram below). Air flow

    enters one or more

    vertical faces of the

    cooling tower to meet

    the fill material.

    Categorization by air-to-water flow

    Water flows (perpendicular to the air) through the fill by

    gravity. The air continues through the fill and thus past the

    water flow into an open plenum area.

    A distribution or hot water basin consisting of a deep pan

    with holes or nozzles in the bottom is utilized in a crossflow

    tower. Gravity distributes the water through the nozzles

    uniformly across the fill material.

    (continue)

    Crossflow

    Categorization by air-to-water flow

    Counterflow

    Figure 2 Counterflow.

    In a counterflow design

    the air flow is directly

    opposite of the water

    flow. Air flow first enters

    an open area beneaththe fill media and is then

    drawn up vertically. The

    water is sprayed through

    pressurized nozzles and

    flows downward through

    the fill, opposite to the

    air flow.

    (continue)

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    Theory and Calculations for Water-Cooling Towers

    In Fig. (10.5-1) the temperature profile and concentration

    profile in terms of humidity are shown at the water-gas

    interface.

    Water vapor diffuses from the interface to the bulk gas phase

    with a driving force in the gas phase of (Hi-H

    G) kg H

    2O/kg

    dry air. There is no driving force for mass transfer in the

    liquid phase, since water is a pure liquid.

    Temperature and concentration profiles at interface

    The temperature driving force is TL-T

    iin the liquid phase and

    Ti-T

    GK or C in the gas phase.

    Theory and Calculations for Water-Cooling Towers

    Temperature and concentration profiles at interface

    Figure 10.5-1 Temperature and conversion profiles in vapor

    part of cooling tower.

    (continue)

    Theory and Calculations for Water-Cooling Towers

    Sensible heat flows from the bulk liquid to the interface in

    the liquid.

    Sensible heat also flows from the interface to the gas phase.

    Latter heat also leaves the interface in the water vapor,diffusing to the gas phase.

    Temperature and concentration profiles at interface

    The sensible heat flow from the liquid to the interface equals

    the sensible heat flow in the gas plus the latent heat flow in

    the gas.

    (continue)Theory and Calculations for Water-Cooling Towers

    The conditions in Fig. 10.5-1 occur at the upper part of the

    cooling tower. In the lower part of the cooling tower, the

    temperature of the bulk water is higher than the wet bulb

    temperature of the air but may be below the dry bulbtemperature.

    Temperature and concentration profiles at interface

    Then the direction of the sensible heat flow in Fig. (10.5-1) is

    reversed.

    (continue)

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    Theory and Calculations for Water-Cooling Towers

    We shall consider a packed water-cooling tower with air

    flowing upward and water countercurrently downward in the

    tower.

    Rate equations for heat and mass transfer

    The total interfacial area between the air and water phases is

    unknown, since the surface area of the packing is not equal

    to the interfacial area between the water droplets and the air.

    (continue)

    Hence, we define a quantity a, defined as m2

    of interfacialarea per m3 volume of packed section or m2/m3.

    Theory and Calculations for Water-Cooling Towers

    This is combined with the gas-phase mass-transfer coefficient

    kG

    in kg mol/sm2Pa or kg mol/sm2atm to give a volumetric

    coefficient kGa in kg mol/sm3 volumePa or

    kg mol/sm3 atm.

    Rate equations for heat and mass transfer

    (continue)

    This process is carried out adiabatically; the various streams

    and conditions are shown in Fig. (10.5-2).

    Theory and Calculations for Water-Cooling Towers

    Rate equations for heat and mass transfer

    Figure 10.5-2 Continuous countercurrent adiabatic water cooling.

    (continue)

    L = water flow, kg water/sm2

    TL= temperature of water, C or K

    G = dry air flow, kg/sm2

    TG

    = temperature of air, C or K

    H = humidity of air, kg water/kg

    dry air

    Hy= enthalpy of air-water vapor

    mixture, J/kg dry air