Heat_Transfer_04.pdf

7/28/2019 Heat_Transfer_04.pdf

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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.



Figure 10.5-1 Temperature and conversion profiles in vapor

part of cooling tower.

(continue)


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.


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.


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

reversed.

(continue)


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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.


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.


(continue)

This process is carried out adiabatically; the various streams

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



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

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