Introduction:
Psychrometry
is the science concerning the thermodynamic properties of air- water mixture.
Most of the air conditioning problems require the determination of the
psychrometric properties of the air. For a given atmospheric condition known as
“state point”, at the known barometric pressure, any two-psychrometric
properties of air fix the state point. The easiest to measure & employ are
the dry & wet bulb temperatures. Following this. Other properties such as
vapour pressure, mixing ratio, relative humidity, air density, & enthalpy
etc. are computed.
Instruments:
Aneroid (no liquid) barometer is commonly
used for the atmosphere pressure measurement in the mines owing to its portable
& rugged nature. It consists of an airtight chamber with flexible diaphragm
retained under partial vacuum conditions. The collapse and motion of the
diaphragm is restricted by a spring element. Any increase or decrease in the
atmospheric pressure results in the diaphragm moving inward or outward
respectively from its normal position. The movement is magnified to 200 times through
a system of levers connected to a pointer moving over dial over which
graduations are in pressure units.
Assmann psychrometer, or
a similar instrument, whirling hygrometer is designed to obtain dry & wet
bulb temperatures of the ambient air. The device has two thermometers mounted
side by side in a rigid frame. One thermometer whose bulb is exposed reads the
“dry” bulb temperature. The bulb of the other thermometer is encased in muslin
or a cotton sleeve. It reads the “wet” bulb temperature. As the sleeve is
dampened & the instrument is exposed to a current of air. Evaporation takes
place from the damp sleeve thereby cooling the thermometer bulb. One the
evaporation rate reaches a steady state value the temperature read by the
thermometer becomes constant which is known as the wet bulb temperature. In
order to facilitate the dispersion of the vapour formed around the bulb, air
with certain velocity is to be passed around the bulb. This is achieved by
whirling the thermometer as in the case of whirling hygrometer, or by forcing
air over the bulbs by using a small spring operated fan as in the case of
Assmann psychrometer.
Procedure:
(1)
Dampen
the sleeve on the wet bulb thermometer with distilled water.
(2)
Tap
gently on the outer casing of the aneroid barometer to cause a slight
deflection of the needle to indicate that the barometer is in the working
condition.
(3)
Position
the Barometer properly at the observation station & note the atmospheric
value, after waiting for about half an hour in which time creep related error
in the needle deflection may be minimized.
(4)
Whirl
the hygrometer (in a shade if the observation is outdoors) at about 3
revolutions per minute for at least a minute. Read the temperatures rapidly,
immediately after whirling is stopped starting with the wet bulb temperature
first because it has a tendency to climb up quickly. Several readings may be
taken until consistency in the observations is attained.
(5)
When
using the Assmann psychrometer, wind the clockwork with the key provided until
the fan operates at full speed. Hold the instrument away from the body, &
read the thermometer after a minute. Repeat the readings at short time
intervals till they are consistent.
Calculations:
Let B, TD, and TW denote the barometric pressure in kPa, Dry bulb temperature in°C, & wet bulb temperature in °C respectively. Based on those three known psychrometric properties certain other important properties may be computed as follows:
·
Vapour pressure : The saturation vapour pressure (PS) at TW may be obtained from psychrometric
tables, or from the relationship
PS = 0.6105 exp{(17.27 * TW)/(TW+ 237.3)} kPa.
The
actual (partial) vapour pressure (PV) for the atmospheric conditions is given
by:
PV
= PS – 0.000644* B* (TD – TW) kPa.
W
= 0.622 * PV / (B – PV) kg/kg ----------(Mixing Ratio)
·
Air density: Using the PV value, the air density (r = mass of the moist air per
unit volume) is computed as :
RH = (PV/PS) * 100 %
·
Enthalpy :The heat content of air(h) is
given by
H = 1.005 * TD
+ W( 2501.6 + 1.884 * m) kg/kg.
Also read the values of vapour
pressure, mixing ratio, relative humidity, specific volume (reciprocal of
apparent density) & enthalpy from the psychrometric chart provided &
compare with calculated values.
Remarks:
(1) A rapid assessment of the psychrometric properties can be facilitated by reading the values from a psychrometric chart. The procedure may, however, provide somewhat erroneous results, partly because the charts are designed for certain standard barometric conditions.
(2)
The
bulbs of the hygrometer shall be protected from direct radiation falling on
them from intense light sources, human body, & from surrounding objects, in
order to obtain correct temperature estimation.
(3)
Students
must, on their own, learn to compute the other psychrometric properties such as
specific humidity, dew point temperature, and saturation ratio.