LECTURE 2 THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS: 1.0 HUMIDITY 2.0 DIFFERENT WAYS OF EXPRESSING HUMIDITY. 2.1 Relative Humidity

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1 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 Page 1 of 9

2 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 to 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 lesser 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: Relative humidity = e e sd 100%.(1) Using equation e = e sw P b (t d t w ) kpa..(2) Page 2 of 9

3 and e s = exp ( t t )Pa.(3) described in Lecture 1, equation (1) can be written as: Relative humidity = e sw P b (t d t w ) exp ( t d ) t d kpa kpa 100 % 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. Specific humidity = e kg P b e kg dry air..(4) Where, e = Vapour pressure or Partial pressure due to water vapour (kpa) P b = 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 m = 103 e 461.9T.(5) Page 3 of 9

4 Where, m = absolute humidity (kg/m 3 ) T= temperature in Kelvin (K) e = Vapour pressure or Partial pressure due to water vapour (kpa) This is rarely used in analysis. Volume of air passing through ventilation system keeps changing because of variation in temperature 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 Lecture 2]. What can be inferred from the above sentences are as follow: - At saturation point e and e sw are same. - Relative humidity of the air is 100%. - Specific humidity = 0.622e sw/(p b e sw) 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 system. 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 Page 4 of 9

5 also called saturation ratio or percentage humidity. Mathematically, it can be expressed as Degree of saturation= e P b e e sd P b e sd 100% = e P b e e sd P b e sd 100%..(6) Where, e = Vapour pressure or Partial pressure due to water vapour (kpa) P b = Barometric pressure (kpa ) e sd = Saturation vapour pressure at wet bulb temperature (kpa) We can see that e and e sd are very small compared to P b, 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 not 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 that 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. Page 5 of 9

6 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 details. Table 1 Methods of measuring water vapour content in air S. No. Method Apparatus /Instruments used Details /Principle used 1. Thermodynamic method Psychrometers Measuring wet bulb and dry bulb temperatures, and applying various equations 2. Using hygroscopic substances 3. Condensation method 4. Absorption method a. Chemical method a. Electrical methods Hair hygrometers Dew-point hygrometers. Electronic psychrometers or humidity meters Substances generally used are organic like bones, hair etc. They are prone to volumetric and elasticity change when exposed to moisture. Cooling of air lowers the temperature but not the actual vapour pressure (AVP) of water content in it. We know that saturation vapour pressure(svp) is proportional to temperature. Lowering of temperature causes lowering of SVP, and a time comes when at a particular temperature SVP equals AVP. 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. Some of the compounds or semiconductors show change in their resistivity and other electromagnetic properties in moist Page 6 of 9

7 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 instruments 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 It 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 It is recorded by thermometer having 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. Wetbulb 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). Page 7 of 9

8 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 energy. This causes depression in the temperature near the thermometer 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 slower 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. Page 8 of 9

9 McPherson, M. J. (1993); Subsurface Ventilation and Environmental Engineering ; Chapman & Hall, London. Misra G.B. Calcutta, India. (1986); Mine Environment and Ventilation ; Oxford University Press, Vutukuri, V. S. & Lama, R. D. (1986); Environmental Engineering in Mines ; Cambridge University Press, Cambridge. Page 9 of 9