Lecture 2

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LECTURE 2

THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS:

1.0 HUMIDITY

2.0 DIFFERENT WAYS OF EXPRESSING HUMIDITY

2.1 Relative Humidity

2.2 Specific Humidity

2.3 Absolute Humidity

3.0 DEW POINT

4.0 DEGREE OF SATURATION

5.0 MEASUREMENT OF WATER VAPOUR IN AIR

6.0 THERMODYNAMIC METHOD OF MEASURING HUMIDITY

6.1 Dry -Bulb Temperature

6.2 Wet -Bulb Temperature

7.0 CONCEPT OF WET BULB TEMPERATURE AND

BAROMETRIC PRESSURE

7.1 Concept of Wet -Bulb Temperature

7.2 Concept of Barometric Pressure

REFERENCES

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In this lecture let us discuss some more terminology related to psychrometry.

1.0 HUMIDITY

It is defined as the water -vapour content of the air. It is expressed in various ways.

They are explained later in this lecture . Normal atmospheric air in most of the cases

is humid. Humid air is also called moist air. The capacity of air to hold moisture

increases with temperature. But, one should not be in confusion that in summer , air

should be more humid compared to other seasons . This is because air may have

more capacity t o hold water, but we require source of water as well. Also, even if

air has high water content at higher temperature, but it may not be saturated or

have higher relative humid compared to air having low water vapor content but

more relative humidity at les ser temperature. This is because relative humidity is

not only governed by temperature of air/atmosphere (dry bulb temperature) but

also wet bulb temperature. We will learn about dry bulb temperature and wet bulb

temperature later on in this lecture.

Now, let us discuss the different ways of expressing humidity.

2.0 DIFFERENT WAYS OF EXPRESSING HUMIDITY

2.1 Relative Humidity

It is defined as the ratio of vapour pressure at a temperature to the saturation

vapour pressure at that dry bulb temperature. It should be kept in mind that for

calculating relative humidity, saturation vapour pressure is taken at dry bulb

temperature and not at wet bulb temperature. Numerically it can be expressed as:

% .(1)

Using equation

..(2)

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and

Pa .(3)

described in Lecture 1, equation ( 1) can be written as:

We can see that, relative humidity is unit less. It is represented in percentage.

2.2 Specific Humidity

It is defined as the mass of water vapour present in kg per kg of dry air.

Mathematically it is expressed as follow.

..(4)

Where ,

e = Vapour pressure or Partial pressure due to water vapour (kPa)

Pb = barometric pressure (kPa)

2.3 Absolute Humidity

It is defined as the amount of water vapor present in a unit volume of air, usually

expressed in kilograms per cubic meter. It is mathematically expressed as

.(5)

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Whe re,

m = absolute humidity (kg/m3)

T= temperature in Kelvin (K)


This is rarely used in analysis. Volume of air passing through ventilation system

keeps changing because of variation in tem perature and pressure. Thus, use of

absolute humidity is discouraged.

3.0 DEW POINT

It is defined as the temperature at which air attains saturation and a further

addition of water vapour leads to dew formation because of condensation of water

vapour. It is rarely used to indicate the moisture content of the air/ atmosphere.

The temperature recorded in this case is dry bulb temperature . But let me tell that

at dew point , dry bulb and wet bulb temperature are same [ as per equation (2) in

Lecture 1 and Le cture 2 ] . What can be inferred from the above sentences are as

follow:

- At saturation point e and esw are same.

- Relative humidity of the air is 100%.

- Specific humidity = 0.622e sw/ (Pb esw) kg/kg dry air

- Dry bulb temperature and wet bulb temperature are same.

- Dew point suggests that if further moisture is added to the atmosphere, it

will condense into mist/dew. This indicates that both evaporation and

condensation process are taking place at equal rate. Thus, there cannot be

any net evaporation in the sys tem.

4.0 DEGREE OF SATURATION

It is stated as the ratio of weight of water vapour in air at given conditions to the

weight of the water vapour in air at saturation, keeping temperature constant. It is

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also called saturation ratio or percentage humidity . Mathematically , it can be

expressed as

Degree of saturation= ..( 6)

Where,


Pb = Barometric pressure (kPa )

esd = Saturation vapour pressure at wet bulb temperature (kPa)

We can see that e and esd are very small compared to Pb , so we can neglect them

and equation ( 6) becomes equal to equation ( 3). From this , we can take degree of

saturation approximately equal to relative humidity, but numerically they are no t

similar.

Of all the humidity terminology discussed , specific humidity is most widely used.

Now, let us discuss some of the very conceptual points.

- Water vapour is not a chemical constituent of air. It is like an impurity to air

like dust, smog, etc.

- Instead of saying air is saturated, it is better to say that space is saturated.

Actually it is the space which becomes saturated , and not air. It means th at

even if we evacuate a system and fill it with water vapour, the system can

hold the same amount of water vapour that air of the same volume can hold

at that temperature. But, conventionally we call it as air is saturated.

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5.0 MEASUREMENT OF WATER VAPOUR IN AIR

There are various methods of measuring water vapour in air. Table 1 lists the

various methods used with fewer detail s.

Table 1 Methods of measuring water vapour content in air

S. No. Method Apparatus / I nstruments used

Details / Principle use d

1. Thermodynamic

method

Psychrometers Measuring wet bulb and dry bulb

temperatures , and applying various equations

2. Using hygroscopic

substances

Hair

hygrometers

Substances generally used are

organic like bones, hair etc. They are prone to volumetric and elasticity change when exposed to

moisture.

3. Condensation method

Dew -point hygrometers

Cooling of air lowers the temperature but not the actual

vapour pressure (AVP) of water content in it. We know that saturation vapour pressure(S VP) is

proportional to temperature . Lowering of temperature causes

lowering of SVP, and a time comes when at a particular temperature SVP equals AVP.

4. Absorption method

a. Chemical method

. Some of the chemical compounds

(like calcium chloride, silica gel, etc.) are hygroscopic in nature.

Through them a measured amount of air is passed. These compounds absorb water from the air. The

increase in their weight gives a direct measure of the water vapour

content of the air.

a. Electrical methods

Electronic psychrometers or humidity

meters

Some of the compounds or semi -conductors show change in their resistivity and other

electromagnetic properties in moist

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air. Based on this principle , these

instruments work.

Besides these methods, water vapour content in air can be measured using

principle of diffusion and optical properties of some substances. But these methods

are rarely used. The most commonly used is thermodynamic method.

6.0 THERMODYNAMIC METHOD OF MEASURING HUMIDITY

It is the only method used in mines. The instrument s used are called hygrometers

or psychrometers. These instruments have a pair of thermometers, one of them

having its bulb covered with wet muslin cloth. The thermometer with wet bulb

muslin cloth on its bulb records wet -bulb temperature and the other one records

dry -bulb temperature. These two temperatures along with barometric pressure is

used in calculating humidity. Let us understand the three terms separately.

6.1 Dry-Bulb Temperature

I t is the temperature recorded by using a conventional thermometer. The

thermometer without muslin cloth in the psychrometer records dry -bulb

temperature. It just reads the ordinary temperature of the air and is a measure of

sensible heat content of the air . Its unit is F or C or kelvin (K).

6.2 Wet-Bulb Temperature

I t is recorded by thermometer havin g wet muslin cloth on its bulb. The temperature

recorded is in general lower than dry -bulb temperature because of cooling effect of

the evaporating water of wet muslin cloth . They are equal only when air is in

saturation and no net evaporation of water from wet muslin cloth takes place . Wet -

bulb temperature can never be higher than dry -bulb temperature. From the

definition point of view, it is defined as the temperature at which water vapour

evaporating into the air can bring down the air in saturation adiabatically at that

temperature . It is a measure of the evaporating capacity of the air. Its unit is F or

C or kelvin (K).

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7.0 CONCEPT OF WET-BULB TEMPERATURE AND BAROMETRIC PRESSURE

7.1 Concept of Wet-Bulb Temperature

The water molecules in the wet muslin cloth take up energy from the neighboring

molecules and evaporate into the air. The evaporating molecules leave the

thermometer surface with reduced en ergy. This causes depression in the

temperature near the thermomete r bulb. Thus, lower temperature is recorded.

Thus, a difference between the temperature at the bulb and the atmosphere exists.

This causes flow of heat from the air through convection. Initially this flow of heat

from air to bulb with wet muslin cloth is s lower than the rate of heat loss from the

bulb with wet muslin due to evaporation. But, a stage comes when the rate of heat

loss and rate of heat gained in the two opposite processes equal. At this point, no

further depression in temperature of wet -bulb is observed. At equilibrium, the

temperature of thermometer with wet muslin cloth on its bulb is taken as wet -bulb

temperature.

7.2 Concept of Barometric Pressure

It is simply the pressure recorded by a barometer at a particular place. It is usually

expressed in kPa.

REFERENCES

Banerjee S.P. (2003) ; Mine Ventilation; Lovely Prakashan, Dhanbad, India.

Deshmukh, D. J. (2008); Elements of Mining Technology, Vol. II; Denett & Co.,

Nagpur, India.

Hartman, H. L., Mutmansky , J. M. & Wang, Y. J. (1982); Mine Ventilation and Air

Conditioning; John Wiley & Sons, New York.

Le Roux, W. L. (1972); Mine Ventilation Notes for Beginners; The Mine Ventilation

Society of South Africa.
http://www.google.co.in/search?tbo=p&tbm=bks&q=inauthor:%22Jan+M.+Mutmansky%22

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McPherson, M. J. (1993); Subsurface Ventilation and Environmental Engineering;

Chapman & Hall, London.

Misra G.B. (1986) ; Mine Environment and Ventilation; Oxford University Press,

Calcutta, I ndia.

Vutukuri, V. S. & Lama, R. D. (1986); Environmental Engineering in Mines;

Cambridge University Press, Cambridge.

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Lecture 2