Intorduction Humidification Lecture Slides PDF

8/10/2019 Intorduction Humidification Lecture Slides PDF

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Transfer

PMMT

Dr. Muhammad Tayyeb Javed

DCE-PIEAS

d o c s i t y . c o m


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Humidification

Increasing the amount of vapours present in the

as stream is calledHumidification

Reducing the amount of vapours present in thegas stream is calledDehumidification

e.g. drying of solids, air conditioning, gas

drying

Humidity, H (mass of vapour associated with a unit mass of dry gas)

Humidity of Saturated Gas,H0(when it is saturated with vapour at a

g ven empera ure

Percent Age Humidity, (H/ H0*100)

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HumidificationHumid Heat, s (heat required to raise unit mass of dry gas and its

associated vapour through unit temperature difference at constant

pressure)

Humid Volumeis the volume occupied by unit mass of dry gas and its

associated vapour

Dew Pointis the temperature at which the gas is saturated with vapour.

As a gas is cooled, the dew point is the temperature at which

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Percentage Relative Humidity

, w

When gas is almost saturated or when

partial pressure is a small portion of

total pressure, then % age humidity is

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Wet Bulb Temperature At equilibrium the rate of heat transfer from the gas just balances that

required to vaporize the liquid and the liquid is said to be at the wet-

bulb temperature

temperature and the rate of flow of gas past the liquid surface

Rate of heat transfer from gas to liquid

The liquid evaporating into the gas is transferred by diffusion from the

interface to the gas stream as a result of a concentration difference (c0-

c), c0is conc. of the vapour at the surface and c is conc. in the gas stream

Rate of evaporation is

Pwand Powmay be expressed in terms of respective

humidit if these both ressures are smaller as

compared to P. Then (P-Pw) and (P-Pwo) may bereplaced by a mean partial pressure of gas PA

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Wet Bulb TemperatureEquating

At normal temperatures, (h/hD) is virtually independent of the

gas velocity provided this is greater than about 5 m/sec.

e we - u empera ure w epen s on y on e empera ure

and the humidity of the gas and values normally quoted are

determined for comparatively high gas velocities

For Air-Water system, the ratio (h/hDA) is about 1.0 kJ/kg K andvaries from 1.5 to 2.0 for organic liquids

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Adiabatic Saturation TemperatureIf the gas is passed over the liquid at such a rate

that the time of contact is sufficient for

equilibrium to be established, the gas will

become saturated and both phases will be

brought to the same temperature

In a thermally insulated system, sensible heat

equals latent heat

As a result of continued passage of the gas, the

temperature of the liquid gradually approaches

an equ r um va ue w c s nown as e

adiabatic saturation temperature

if the liquid is continuously circulated, the

empera ure o e qu a e op an o om,and of the gas at top, approaches the AST

Temp and humidity differences are maximum at

the bottom and zero at the to

The rate of transfer of heat and

mass decreases from bottom to top

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Adiabatic Saturation Temperature

Approximately linear relationship between humidity and temperature for allmixtures of gas and vapour having the same adiabatic saturation temperature

A curve of humidit versus tem erature for ases with a iven AST is known as

adiabatic cool ing line

Straight lines of slope (s/) is obtained

These lines are not exactly straight and parallel because of variations inand s ompar ng o equa ons, s equa o w en s= DA This is the case for most water vapour systems and accurately so whenH =

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h/h s = b is sometimes known as s chrometric ratio, & 1 for air water

system

For other systems it is greater than 1 (air-organic liquid 1.3-2.5)

WBT is always higher than AST

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Adiabatic Saturation TemperatureWhen Sc and Pr for a mixture of gas-vapour are

approx ma e y equa o , ew s re a on:

If unsaturated gas is brought contact,

simultaneous transfer of heat and mass

takes place

The temp of gas falls and its H

increasesThe temp of the liquid approaches the

WBT corresponding to the particular

con on o e gas a a momen

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Adiabatic Saturation Temperature For a liquid other than water, the adiabatic saturation temperature is

less than the WBT and therefore in the initial sta es the tem erature

of the liquid rises

As the gas becomes humidified, however, its WBT falls and

consequently the temperature to which the liquid is tending decreases

as evaporation takes place

In due course, therefore, a point is reached where the liquid actually

reaches the WBT of the gas in contact with it oes no rema n a s empera ure, owever, ecause e gas s

not then completely saturated, and further humidification is

accompanied by a continued lowering of the WBT

fall until the gas is completely saturated

The liquid and gas are then both at the AST

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Adiabatic Saturation Temperature The air-water system is unique, however, in

that the Lewis relation holds quite

accuratel so that the AST is the same as

the WBT

If unsaturated gas is brought into contact

with water at the AST of the gas, there is no

change, and it remains in a condition of

dynamic equilibrium through the whole of

the humidification process,

represents the conditions of gases of

constant WBT as well as constant AST

The change in the condition of a gas as it is This is particularly useful because

um e w wa er vapour s ere orerepresented by the adiabatic cooling line and

the intermediate conditions of the gas during

the process are readily obtained

normally obtained in practice

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Humidity DATA for Air-Water System The following quantities are plotted against temperature

Humidity

Specific volume of dry gas(linear fn of temperature)

The saturated volume (increases more rapidly with temperaturethan the specific volume of dry gas because both the quantity and the

specific volume of vapour increase with temperature)

Latent heat of vaporization

Humid heat

slightly curved since s is a function of humidity)

Each adiabatic cooling line represents the composition of all gases

whose adiabatic saturation temperature is given by its point of

intersection with the 100 % humidity curveFor the air-water system, adiabatic cooling lines represent conditions of

constant WBT as well and, enable the change in composition of a gas to

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Psychrometric or humidity chart, refers to a particular total pressure of system

Based on either the temperature or the enthalpy of the gas

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PMMTcopyrights@ Muhammad Zaman, DCME, PIEAS 13



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Determination of Humidity

Chemical Methods

Determination of Wet Bulb Tem erature

Determination of Dew Point Measurement of the Change in Length of a Hair or

Fibre

Measurement of Conductivity of a Fibre easuremen o ea o sorp on on o a ur ace

Electrolytic Hygrometry

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Capacitance Meters

Observation of Color Chan es

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AST Vs WBT

The adiabatic saturation tem erature is defined as

the temperature obtained by an air-water vapormixture if it becomes saturated with water vapor in

an a a a c process . ,

The true wet-bulb temperature is determined by a,

described in Nellis and Klein (2009).

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Intorduction Humidification Lecture Slides PDF