10-ppt-psychrometry 6n713j

Mine Ventilation Module: X - Psychrometry Dr. Nuhindro Priagung Widodo Teknik Pertambangan Fakultas Teknik Pertambangan dan Perminyakan, Institut Teknologi Bandung 2009 1

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Humidity  The mass of water vapour within a gas

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Amount of Water Vapor In The Air  There is a limit to how much water vapor can enter the

atmosphere. As more water vapor enters the atmosphere, the amount of pressure exerted by that water vapor increases. We call this pressure the vapor pressure.  The higher the temperature of the atmosphere, the more vapor pressure it can withstand.

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 When the vapor pressure maximum is reached, no more

water can enter the atmosphere.  At this point, we say that the atmosphere is completely saturated.  Because the maximum vapor pressure increases with temperature, warmer air can hold more water vapor before becoming saturated.

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 The total pressure of the air parcel is due to the sum of

"partial pressures" of each of the gasses comprising the parcel  The partial pressure due to water vapor is called the "vapor pressure"

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Dew Point  That temperature to which natural air has to be cooled (at

constant pressure) for saturation to occur or for condensation to begin.

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Condensation  Condensation is the opposite of evaporation. It takes place when

water vapor in the air condenses from a gas, back into a liquid form, and leaves the atmosphere, returning to the surface of the Earth.

 Usually in order for condensation to take place, the atmosphere

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must be fully saturated. In other words, the maximum vapor pressure must have been reached. NPW

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Absolute Humidity Vs. Relative Humidity  Absolute humidity : the mass of water vapour per unit mass of

dry air (normally measured in g/kg), on Psychrometric Chart is termed Apparent Specific Humidity.  Relative humidity: mass of water vapour in a volume of air over

total mass capable of being held at that temperature and pressure i.e. the saturated total, Relative Humidity is expressed as %.

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Moisture content (g)

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 Relative humidity does not tell us how

much water vapor is in the air, but rather, what percentage of the maximum vapor pressure has been reached.

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 Most common variable used to describe atmospheric

moisture  RH = water vapor content / water vapor capacity  RH = vapor pressure/saturation vapor pressure = e/es  or RH = mixing ratio/saturation mixing ratio

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 Latent Heat: Heat which is added or removed in order to

change the state of a substance without affecting its temperature (qL)

 Sensible Heat: Heat which changes temperature of

substance - is perceptible to the senses (qs)

 Total Heat: The heat change under any situation is one,

other, or a combination of qL and qs  q = qs + q1 20

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Nature of thermodynamic change on Psychrometric Chart (after Hartman, 1982)

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Percentage saturation (μ)  Percentage saturation is a useful guide to comfort as the human

body tends to respond to relative saturation rather than moisture content,  Is the ratio between the actual moisture content of the sample (gs) and the moisture content of a sample of saturated air (gss), at the same temperature.

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 There are a number of ways one can specify the amount of

moisture (referred to as humidity) in the air  What are they  Absolute humidity  specific humidity  vapor pressure  saturation vapor pressure  relative humidity  mixing ratio  saturation mixing ratio

 wet-bulb temperature  dew-point temperature 25

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 The lowest temperature that can be reached by evaporating water

into the air  Note: the wet bulb temperature will always be less than or equal to the temperature  Example: If you are a runner:  T = 90°F, RH = 90% => high wet-bulb temp.  T = 90°F, RH = 10% => low wet-bulb temp.

 feel more comfortable when wet-bulb temperature is low  Wet-bulb temperature is related to the heat index -->>

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11 potential heat sources underground  Humidity

 Wall-rock heat  Sensible heat flow  Groundwater  Machinery and light  Human metabolism  Oxidation  Blasting

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 Rock movement  Pipelines  Energy losses in airflow

Moisture alone probably will not cause enough problems underground. Things only start to become troublesome when moisture is combined with heat

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 In thermodynamics, an adiabatic process or an

isocaloric process is a thermodynamics process in which no heat is transferred to or from the working fluid.

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(Source: Barenburg, 1974)

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