CHAPTER 19 Humidification Operations - UniMAP Portalportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · • Humidification ... –mas. s of vapor carried ... • Thus,

CHAPTER 19

Humidification Operations

Humidification Operations

• Humidification

and

dehumidification

involve transfer

of

the

material

between

a

pure

liquid

phase and fixed gas that is nearly insoluble

‐

Simpler process than

absorption

and

stripping

as liquid

contain

only

1

component

(thus

no

concentration

gradient

and

resistance

for

mass tranfer)

‐

Both

heat

transfer

and

gas

phase

mass

transfer influences each other

Prepared by, Dr. Nora JULLOK/ UniMAP2

Definitions

1.

Vapor

–

Component

present

as

gaseous

and liquid form; referred as component

A

2.

Gas –

component present only

in

gaseous

form; referred as component

B

• Gas‐vapor mixture follow the Ideal Gas Laws

3.

Humidity

– mass

of vapor carried

by

a

unit

mass of vapor‐free gas





Phase equilibria


Figure 19.1: Equilibria for the system air‐water at 1 atm.


Adiabatic saturator


Humidity Chart

Figure 19.2: Air‐water at 1 atm.Prepared by, Dr. Nora JULLOK/ UniMAP

10

Humidity Chart• Ts

is obtained by trial‐and‐error calculation; for the air water system,

by using humidity chart

• The curved line marked 100% gives humidity of saturated air as a function of air temperature (coordinate of point in this line are found

from Eq. 13)

• Any point above

& to the left of saturation line represent a mixture of saturated air & liquid water.

• Any point below

saturation line represent undersaturated air

• Point on

the temperature axis represent dry air


Humidity Chart (cont..)


• A portion of Humidity Chart.

Figure 19.3: Use of humidity chart.


Use of Humidity Chart1.

Assumption:

A

given

stream

of

undersaturated

air

have

a

temperature T1

and percentage humidity, HA1

2.

Point

a

represent

air

(this

point

is

the

intersection

of

constant temperature line for T1

and constant percentage humidity line for

HA1 )

3.

The

humidity

H1

of

the

air

is

given

by

point

b,

the

humidity coordinate by point a

4.

Dew point – found by following the constant‐humidity line through point

a

to

the

left

to

point

c

on

the

100%

line,

and

then

read

at

point d

on the temperature axis.


Use of Humidity Chart (cont..)


Wet‐bulb Temperature• Evaporation requires energy. The wick and therefore the thermometer bulb

decreases

in

temperature

below

the

dry‐bulb

temperature

(ordinary temperature

measure

with

thermometer)

until

the

rate

of

heat

transfer

from

the

warmer

air

to

the

wick

is

just

equal

to

the

rate

of

heat

transfer needed

to

provide

for

the

evaporation

of

water

from

the

wick

into

the

air

stream.

Figure 19.4: (a) Wet‐bulb thermometer. (b) Gradients in the gas boundary layer.

• The temperature reached is called the wet‐bulb temperaturePrepared by, Dr. Nora JULLOK/ UniMAP

16

Wet‐bulb Temperature (cont..)• Wet‐bulb temperature is a function of:

1. Temperature of air2. Humidity

• Precautions to measure the wet‐bulb temperature:1. The wick must be completely wet, so no dry areas of the wick are

in contact with the gas2.

The

velocity

of

the

gas

should

be

large

enough

(at

least

5m/s)

to

ensure

that

the

rate

of

heat

flow

by

radiation

from

warmer surroundings to the bulb is negligible in comparison with the

rate of

sensible heat flow by conduction and convection from the gas to the bulb.

3.

If

makeup

liquid

is

supplied

to

the

bulb,

it

should

be

at

the

wet‐ bulb temperature.


Wet bulb temperature theory




COOLING TOWERS

Figure 19.5: Natural‐draft cooling tower.


Figure 19.6: Typical cooling towers: (a) crossflow tower; (b) counterflow tower


• A cooling tower is a special type of heat exchanger in which the warm water and the air are brought in direct contact for

‘evaporative cooling’.

• It provides a very good contact of air and water in terms of the contact area and mass transfer coefficient of water vapor while

keeping air pressure drop low.

• Enthalpy of air is lower than enthalpy of water. Sensible heat and latent heat transfer take place from water drop to surrounding air.

Temperature profiles in cooling tower is presented in Figure 19.7


Figure 19.7: Conditions in cooling tower: (a) , (b) at bottom of

tower, (c) at top of tower


Figure 19.8: Flow diagram of countercurrent gas‐liquid contactor


• Thus, cooling is accomplished by sensible heat transfer from water to air and evaporation of a small portion of water.

• The hot water which is coming from heat exchanger is sprayed at the top of the cooling tower.

• Air enters through the louvers at the two opposite walls of the cooling tower.

• During cooling process of water, around 2% water is evaporated.

• Make water is used to compensate the water loss due to evaporation.

• Blowdown is there to drain a part of water containing solid deposit.

• The exit cold water from the cooling tower is used in the heat exchanger

or

other

unit

operation



Figure 19.9: Operating diagram for cooling tower; plot the enthalpy of the air versus water temperature.







Tutorial 4

• Problems:19.1

19.3

19.6

19.10

Prepared by, Dr. Nora JULLOK/ UniMAP 34

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CHAPTER 19 Humidification Operations - UniMAP Portalportal.unimap.edu.my/portal/page/portal30/Lecturer Notes... · • Humidification ... –mas. s of vapor carried ... • Thus,