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Lab #1

PSYCHROMETRIC PROPERTIES OF AIR

January 28, 2002

Objective

Mine air usually contains moisture and this air-moisture mixture varies from place to place and with
time. The pressures and temperatures that exist at the time influence the behavior of this air-
moisture mixture. Increases in pressure or, more effectively, decreases in temperature may result in
condensation of the water vapor to form condensation.

Changes of phase are important within the confines of a closed environment such as in under-

ground airways. Ice to liquid and ice to vapor phase changes occur in mines located in cold climates
and, particularly, if situated in permafrost. However, the vast majority of humidity variations
occurring underground are caused by the evaporation of liquid water or the condensation of water
vapor.

Psychrometry is the study of moisture in air under given conditions and during different

temperature-humidity processes. The purpose of this lab is to familiarize the students with common
psychrometric properties of air and the instruments used to measure these properties.

Pressure, Temperature, and Humidity

When a small amount of water were introduced into a vacuum space, some of it would evaporate
and it would build up a certain pressure in the space, depending purely on the ruling temperature:
the higher the temperature, the more moisture it can hold, just like in a hot summer day, and this
pressure is called vapor pressure. If the space contains as much water vapor as it can hold at the
existing temperature it is said to be saturated with water vapor at that temperature. The behavior of
air-moisture mixture can best be explained by Dalton's Law.

When there is sufficient water present to allow the vapor pressure to build up to the full amount

applicable at that temperature, the space is saturated with water vapor at that temperature. On the
other hand, if there is insufficient water present, the pressure will not build up to this full pressure.
The ratio of the actual vapor pressure to the saturation vapor pressure at the same temperature is
called relative humidity, while the ratio of the actual mass of water vapor that could be contained at
saturation is called percentage humidity – or specific humidity (W), measured in lb/lb or grains/lb
(kg/kg)

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. The density of this air-vapor mixture can be expressed as,

ρ

b

=

ρ

a

+

ρ

v

in lb/ft

3

or kg/m

3

where

ρ

b

is density for moist air,

ρ

a

is density for dry air,

ρ

v

is density for water vapor,

ρ

sat

i s

density of saturated air.

The air and water vapor have weight and each exerts its individual pressure to cause the total

atmospheric pressure. This pressure naturally varies at different elevations above or below a fixed

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One is the ratio of vapor pressure while the other is a ratio of masses; their
numerical values may vary only slightly.

Min-218 Lab
Lab #1: Psychrometric Properties of Air

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datum and is affected by climatic changes of temperature, moisture, etc. So it is necessary to know
three different properties of an air-vapor mixture before its psychrometric state is fully determined –
e.g. its pressure and temperature and one other property which indicates either how much water
vapor or how much heat it contains. For this purpose it is generally most conveniently to measure
the web bulb temperature.

The true wet bulb temperature is the temperature indicated by a thermometer, the bulb of which

is covered with a thin layer of water (usually contained in a muslin wick) and which has an air ve-
locity of at least 590 ft/min (3 m/sec) passing over it. It is therefore the temperature of the wet
thermometer bulb when exposed to air in a certain state. For example, if the air is saturated with
water vapor, the wick on the wet bulb and the water in it will assume the temperature of the air and
the thermometer will indicate exactly the same temperature at the dry bulb thermometer – say 70°F.
The only difference will be that it will take a little longer than the dry bulb thermometer to assume
this temperature.

If the air is not saturated but has a relative humidity of say only 50%, a much more complicated

process takes place. Water from the wet bulb immediately starts evaporating into the air. This
evaporating water requires a considerable amount of heat to turn it from water into water vapor and
the only substances it can draw this heat form are the remaining water, the wick, the thermometer
bulb and the surrounding air. In this process the temperature of the remaining water is thus lowered,
but it is not lowered indefinitely because, as soon as it becomes cooler than the air, heat also starts
flowing from the air to the water. Thus there are two opposing actions taking place at the same
time–heat leaves the water due to evaporation and enters if from the air due to conduction.

After some time two actions must find their point of balance at which as much heat enters the

wet bulb as is leaving it. The temperature of the wet bulb now becomes steady and can be read off.
For example, assuming a barometric pressure of 29.921 in. Hg (absolute), a dry bulb temperature
of 70°F and a relative humidity of 50%, the wet bulb temperature will be 58.5°F. If the relative
humidity had been 80%, the wet bulb temperature would have been 65.7°F while 20% relative
humidity it would have been as low as 50°F.

The Underground Environment

Normally, air has a certain amount of water vapor. This is certainly true in most underground
mines. Usually appreciable changes in the composition of the air due to changes in moisture con-
tent (anywhere from zero to four percent of the mass of the mixture) will occur as the air flows
through the mine airways, resulting in changes in air density.

Moisture alone probably will not cause enough problems underground. Things only start to be-

come troublesome when moisture is combined with heat. Experiences from deep mines in South
Africa show that for given air velocities, work efficiency will drop dramatically (Fig 20-11, p 584).
The text book (pp. 562-572) listed 11 potential heat sources underground, three of which may
cause intolerable environmental conditions: (1) autocompression in shafts, (2) wall-rock heat, (3)
sensible heat flow, (4) groundwater, (5) machinery and light, (6) human metabolism, (7) oxidation,
(8) blasting, and (9) rock movement, (10) pipelines, and (11) energy losses in airflow. (Refer to
Chapter 21 for additional information.)

Moisture alone probably will not cause enough problems underground. Things only start to be-

come troublesome when moisture is combined with heat. Experiences from deep mines in South
Africa show that for given air velocities, work efficiency will drop dramatically (Fig 20-11, p 584).
Text book (pp. 562-572) listed 11 potential heat sources underground, three of which may cause
intolerable environmental conditions: (1) autocompression in shafts, (2) wall-rock heat, (3) sensible
heat flow, (4) groundwater, (5) machinery and light, (6) human metabolism, (7) oxidation, (8)
blasting, and (9) rock movement, (10) pipelines, and (11) energy losses in airflow.

Min-218 Lab
Lab #1: Psychrometric Properties of Air

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Lab Procedure

1)

Obtain barometric pressure reading at the mine portal;

2)

Take velocity measurements at all ventilation stations;

3)

Take wet- and dry-bulb temperature measurements at all stations;

4)

Examine all ventilation control structures and/or obstructions underground.

Instrument Used

Instrumentation necessary for the measurement of the psychrometric properties of air normally
consists of sling psychrometer on which a pair of thermometer are installed in a frame with a handle
by which it can be whirled rapidly. The bulb of one thermometer is covered with a wet cotton wick,
one end of which is immersed in a water reservoir. Air velocity across the wick should be at least
590 ft/min. (or 3 m/sec) and measured by an anemometer.

Lab Report

The report will consist of calculations relating to the measurements made in the lab. These include:

Velocity (V);

Air Quantity (Q);

Relative humidity (

φ

);

Specific Humidity [or Humidity Ratio] (W);

Specific Volume (

ν

);

Air Density (

ρ

);

Mass Flow Rate (MFR); Temperature change/length (T)

The aforementioned values should be calculated in English units and neatly tabulated. Use the

Psychrometric chart to check calculated values for #3, #4 and #5. In addition to the calculations, the
lab report should also contain graphs of:

1)

Velocity vs. Cumulative Distance (CD)

2)

Quantity vs. CD

3)

Relative Humidity vs. CD

4)

Dry bulb temperature vs. CD

5)

Mass Flow Rate vs. CD

The lab should also include a discussion of any irregularities from anticipated results and a dis-

cussion of how these factors affect ventilation. Mark all observations with regard to all ventilation
control structures and/or obstructions.

Further Reading

Psychrometry – The thermodynamics of moist air; Chapter 17 (text, p. 619)
Psychrometric Chart (p. 20)