16+Humidification+Intro

Humidification: Introduction

PDRamoso

ChE 133 WFVW

Outline

• Simultaneous Heat and Mass Transfer

• Humidification Equipment

• Humidity: definitions [notation of Foust]

• Humidity chart [Perry’s 8th ed]

• Processes: graphical representation

Simultaneous Heat and Mass

Transfer

• Change of phase

– Evaporation

– Condensation

– Crystallization

• Transfer of both heat and mass

– Recall: heat of vaporization

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Examples

• Humidification

• Drying (LE2)

• Evaporation

• Crystallization (LE3)

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Direction of Transfer

• In general, the relation between the

humidity and air and liquid

temperatures determine the directions

of heat and mass transfer

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidification/Dehumidification

• Simplest case of SHMT

• Contact of a pure liquid with a gas

– Liquid: water

– Gas: noncondensable (air) and vapor

phases (water vapor)

• Direction of transfer

– Dictated by humidity and temperature

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Common Processes

• Cooling of hot air by contact with cold

water

• Cooling of hot water by contact with

ambient air

– Cooling towers

• Control of humidity in a space

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Spray Chamber

• Liquid is sprayed into the vapor stream

• Gas velocity is low

– High contact time

– Low liquid entrainment

• Usually restricted to small-scale

operations

• Humidity control

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Spray Chamber

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Cooling Tower

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Phase Relations

• Raoult’s Law

– Ideal solution and ideal gas

• With Dalton’s Law,

• Since x=1 (liquid is pure),

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

xPPyP sat

asa

P

Py

sat

as

Absolute Humidity, Y’

• Mass ratio of vapor, dry-basis

[mass vapor]/[mass dry gas]

Note:

a – (water) vapor

b – dry gas (air)

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

b

a

M

MYY

Molal Humidity, Y

• Mole ratio of vapor, dry-basis

[moles vapor]/[moles dry gas]

*rarely used

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Percent saturation, %sat

• Or percent humidity

• Ratio of absolute humidity or molal

humidity

[Y]/[Ys], as percent

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

%100%100%100%

s

b

as

b

a

s Y

Y

M

MY

M

MY

Y

Ysat

Percent saturation, %sat

• In terms of mole fractions,Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

%1001

1%100

1

1%

y

y

y

y

y

y

y

y

sat s

s

s

s

Percent relative humidity, %RH

• Or percent relative saturation

• Ratio of partial pressure of vapor to

partial pressure of vapor at saturation

[Pa]/[Pasat], or [ya]/[ys], as percent

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

%100%100% s

sat

a

a

y

y

P

PRH

%RH vs. %sat

• Which is larger?Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

%100% sy

yRH

%1001

1%

y

y

y

ysat s

s

Humid Volume, Vh

• Volume of 1 lbmol (or 1 mol) of dry gas

plus its associated vapor

• [ft3]/[lbmol dry gas], or [m3]/[mol dry

gas]

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

PaP

KTYV

atmP

RTYV

h

o

h

101325

2730224.01

1

4923591

Molal Humid Heat, ch

• Heat required to raise the temperature

of 1 lbmol (or 1 mol) of dry gas plus its

associated vapor by 1 oF (or 1 oC)

• [Btu]/[lbmol dry gas]-[oF], or [J]/[mol

dry gas]-[oC]

– cb and ca: molal heat capacities of dry

gas, b, and water vapor, a, respectively

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

abh Yccc

Enthalpy of vapor-gas mixture, H

• By analogy to molal humid heat,

• H: enthalpy of 1 lbmol dry gas plus its

associated vapor

• Hb and Ha are molal enthalpies

– Reference conditions:

• Dry air: 32oF, 1atm

• Liquid water: triple point

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

ab YHHH

Enthalpy of vapor-gas mixture, H

• From the reference conditions,

• T0: reference temperature, 32oF for both air and water

• l0: heat of vaporization at the reference temperature, Btu/lbmol

• Molal heat capacities assumed constant, pressure effect on enthalpy is ignored

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

000 TTcYTTcH ab l

Dew Point

• Point at which condensation begins

when the pressure or temperature is

changed over a mixture of fixed

composition

– Usually, T is lowered at constant P

– Dew point temperature, Td

• Temperature at which vapor begins to form

when the gas phase is cooled at constant

pressure

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Wet-bulb Temperature, Tw

• Steady-state temperature attained by

a wet-bulb thermometer under

standardized conditions

– Thermometer is wetted with liquid that

forms the vapor in the vapor-gas mixture

(water)

• Will be related to humidity later

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Adiabatic-Saturation Temperature, Tsa

• Temperature that would be attained if

the gas were saturated in an adiabatic

process

– Recall: adiabatic q=0

– Compare against dew temperature

cooling at constant pressure

• Will be related to humidity later

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

• This equation was developed from overall

material and enthalpy balances between

initial gas condition and adiabatic-

saturation condition

– Thus, it is applicable only for the two points.

It may not describe the path followed by the

gas as it becomes saturated

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity and Tsa

sasasah YYTTc 111 l

• For air-water system only!

• Completely turbulent flow

• For other systems, the behavior in a

saturating contactor would be complex

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity and Tw

11 YYTTc wwwh l

• Thus, Tw = Tsa

• For air-water system only!

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Tsa and Tw

11 YYTTc wwwh l

sasasah YYTTc 111 l

• Only when

• For simple monatomic and diatomic

gases air!

• Implies that mechanisms of heat and

mass transfer are identically

dependent upon flow conditions

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Lewis Relation

h

Y

c ck

h

PrNNSc

Humidity Chart

• Psychrometric Chart

– Graphical representation of different

properties of a gas-vapor system

• Humidity Chart

– Psychrometric Chart for air-water system

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity Chart

• Humidity– dry-bulb temperature

• Wet-bulb temperature

• Enthalpy

• % relative humidity

• Humid volume

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Dry bulb Temp

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Wet bulb Temp

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Enthalpy at Saturation

(+) Deviation (-)

Enthalpy = Enthalpy at saturation +

deviation

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

% Relative Humidity

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humid Volume

Humidity Chart

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Humidity Chart

• Recall from Gibbs’ Phase Rule:

– 2 components, 2 phases

• 2 degrees of freedom

– Need only two parameters to determine

the state of the system

• All other properties could then be read from

the graph or calculated

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Examples

1. Find the properties of moist air when

the dry-bulb temperature is 80oF and

the wet-bulb temperature is 67oF.

ANS:

Humidity: 0.011 lb water/lb d.a.

Enthalpy: 31.6 - 0.1 = 31.5 Btu/lb d.a.

Humid volume: 13.8 ft3/lb d.a.

Relative humidity: 51%

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Examples

2. Find the properties of moist air at

90% RH and dry-bulb temperature of

70oF.

ANS:


Enthalpy: 32.4 Btu/lb d.a.


Wet-bulb temperature: 68oF

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Examples

3. Find the properties of saturated air at

a wet-bulb temperature of 60oF.

ANS:


Dry-bulb temperature: 60oF

Enthalpy: 26.5 Btu/lb d.a.


Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Examples

4. Find the properties of moist air with

humidity 0.020 lb water/ lb d.a. And

60% RH.

ANS:

Dry-bulb temperature: 92.5oF

Wet-bulb temperature: 80.6oF

Enthalpy: 44.5 – 0.14 = 44.36 Btu/lb d.a.


Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

Processes

• Sensible heating/cooling

– Humidity remains constant

– Finding Td

• Adiabatic Humidification

• Heating and Humidifying

– Both T and Y’ increase

• Cooling and Dehumidifying

– Cooling to T below Td

– Removal of water from air

Simultaneous

Heat and Mass

Transfer

Humidification

Equipment

Humidity:

definitions

Humidity chart

Processes:

graphical

representation

16+Humidification+Intro

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