Kata cooling power and effective temperature.
Determination of mine air Kata cooling power and effective temperature and study of dry Kata cooling power and air velocity relationship.
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)
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.
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.
(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.
(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.
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.