Kata cooling
power and effective temperature.
Objective:
Determination
of mine air Kata cooling power and effective temperature and study of dry Kata
cooling power and air velocity relationship.
Introduction:
The cooling power of mine air
determines the capacity of the ambient atmosphere to dissipate the metabolic
heat generated by the humans. The cooling power measured in W/m2
(amount of heat removes from the human body per second per unit surface area)
is dependent mainly on the wet bulb temperature and the air velocity. Certain
empirically constructed nomograms correlate the wet bulb temperature and air
velocity with cooling power. Kata thermometer is an instrument/device to
provide more convenient means of determining the cooling power of underground
mine environment. A cooling power of 800 W/m2 for the environment as
measured with a wet kata thermometer is considered ideal for persons employed
in hard work.
Effective temperature is another popular heat stress
index developed on the basis of subjective thermal sensation. The effect of air
velocity, humidity and temperature is combined into a single empirical
temperature scale reflecting equal sensations of warmth or cold. This
temperature is read from empirically constructed nomograms. For people living
in hot environment an effective temperature of 27 °C correlates to 100 % work efficiency. A dorp
to 65% of work efficiency may be noticed with effective temperatures reaching
32 °C. the dry kata cooling power (heat loss due
to radiation and convection) is related to dry bulb temperature and air
velocity as follows:
k =
(309.65-TD)(8.37+16.74 V1/2
------------------------(for velocity > 1 m/s)
Where,
k –
dry kata cooling power in W/m2
TD
– dry bulb temperature in °K
V –
air velocity in m/s
By knowing the dry kata cooling power one can
compute air velocity, and vice versa using the equation.
Instruments:
Kata thermometer, Assmann Psychrometer, Anemometer,
Stop-watch.
Kata thermometer is devised to
simulate the human heat exchange process with the ambient atmosphere. It
contains a 20-mm diameter and 40 mm long alcohol bulb connected to a glass stem
with a capillary. The alcohol capillary has one small reservoir at each end of
the stem with capillary. By dipping the bulb in warm water alcohol is gradually
made to rise upto the middle of the upper reservoir. Moisture is wiped from the
bulb and the thermometer is suspended under the ambient air under study. The
Kata cooling power, which is a property of the thermometer, is marked on the glass
stem. It gives the heat lost by the air per cm2 of bulb area, as the
alcohol column drops from the 38 °C mark on the stem to the 35
°C mark. This factor in m.cal/cm2
divided the time required in seconds for the alcohol column to drop gives the
cooling power. This is the dry kata cooling power expressed conventionally in
W/m2.
The human body, however, dissipates
heat more significantly through evaporation of sweat. In order to simulate a
sweat-covered body, the kata thermometer bulb is encased in wet muslin. For
this case, once the thermometer is removed from warm water, the excess water
from muslin is gently squeezed before suspending the thermometer. With
evaporation introduced as an additional source of heat loss, the wet kata
cooling power is usually results in a higher value in comparison to the dry
kata observations.
Procedure:
(1)
In
the testing gallery chose the location for cooling power and effective
temperature measurement. Run the ventilation fan until the flow conditions
stabilize.
(2)
Dip
the thermometer gently in the warm water beaker and allow the alcohol to rise
upto the middle of the upper reservoir, make sure that the alcohol column is
continuous.
(3)
Remove
the thermometer, wipe the bulb dry, and suspend the thermometer in the gallery.
Note the time required with a stopwatch, for the alcohol column to drop from
the 38°C mark to the 35 °C mark.
(4)
With
the low remaining unchanged repeat the steps 2 and 3.
(5)
For
the wet kata cooling power encase the thermometer bulb in muslin or cotton.
Follow through steps 2 to 4. Ensure that the muslin is not soggy and dripping.
(6)
Using Assmann Psychrometer obtain two sets of
readings for dry and wet bulb temperatures.Obtain two single point velocity
measurements by placing the anemometer at the same location where the kata
thermometer is suspended. The duct setup in the mine ventilation is utilized to
study the relationship between the dry kata cooling power and air velocity. For
the forcing system in the duct, the inlet to fan can be throttled such that six
different velocity conditions can be obtained.
(7)
For
each velocity condition, obtain two single point velocity observations at the
center of the duct outlet, and also two cooling time observations using dry
kata.
(8)
By changing the velocity conditions with each
set, obtain six sets of readings as in step 7.
Calculations:
(1)
Present
the results of the experiment in appropriate tables.
(2)
From
the first part of the experiment, using the kata factor and mean cooling times
compute the dry and wet kata cooling powers.
(3)
Determine
the effective temperature from the nomogram provided considering the average
dry and wet bulb temperatures and velocity.
(4)
On
a graph paper plot cooling power vs. air velocity (x-axis) using the empirical
relationship 1.
(5)
For
the six sets of observations noted in the second part of the experiment compute
the mean cooling power values (W/m2) and air velocities (m/s).
superpose these experimental results on the graph constructed in step 4.
Remarks:
The
efficacy of the kata thermometer in the simulation of human heat loss process
is a much-argued subject. Ventilation engineer must know difficulties arising
with using kata cooling power values in interpreting ambient air quality.