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LECTURE – 8

THE CONTENTS OF THIS LECTURE ARE AS FOLLOWS:

1.0 HEAT INDICES IN UNDERGROUND MINES

2.0 DIRECT INDICES

2.1 Wet Bulb Temperature

2.2 Air Velocity

2.3 Dry Bulb Temperature

3.0 EMPERICAL INDICES

3.1 Wet Kata-Thermometer

3.1.1 Kata-thermometer

3.1.2 Construction

3.1.3 Procedure for taking reading

3.1.4 Requirement of wet-kata thermometer reading

3.1.5 Numerical approach

3.1.6 Limitations

3.2 Effective Temperature

3.2.1 Basic effective temperature

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3.2.2 Normal effective temperature

3.2.3 Merits

3.2.4 Limitations

3.3 Wet-Bulb Globe Temperature

4.0 RATIONALLY DERIVED HEAT INDICES

4.1 Specific Cooling Power

REFERENCES

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1.0 HEAT INDICES IN UNDERGROUND MINES

Heat indices give an idea about the degree of comfort in terms of cooling power of

air. Many indices have been proposed from time to time, but most of them are not

capable of giving accurate results and have some limitations associated with them.

The reason being large number of variables involved, complex nature of human

thermoregulation system, whether the person is clothed or not, etc. We can classify

heat indices in three broad categories (Anon, 1993). They are:

o Direct indices

o Empirical indices

o Rationally derived indices

2.0 Direct Indices

These are based on single psychrometric measurement.

2.1 Wet –bulb temperature (aspirated i.e., hygrometer is rotated at

prescribed rpm)

o It is the most important heat index in this category.

o It is used when evaporation is the most dominating way of heat transfer from

human body to the surroundings. In other words, we can say that the results

produced by wet-bulb temperature are reliable to some extent in hot and

humid conditions.

o In mines, where wet-bulb temperature is applied as heat index, the preferred

standard is taken ≤ 27℃ for comfort condition, and limiting value has been

assigned as 32℃.

Limitations

o Readings are obtained by whirling the hygrometer (aspirated), but

human bodies are not subjected to whirling (hence, non-aspirated)

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o Difference in convective, radiative and evaporative properties of

thermometer bulb and those of human body.

2.2 Air velocity

o It is the second most important heat index in this category. Air velocity

alone is not so much significant. But, its combination with wet-bulb

temperature (non-aspirated i.e. hygrometer is kept still in air-current),

produce reliable results in hot and humid mine environment.

o An increase in air velocity is proportionally as effective as decrease in wet

bulb temperature especially at low velocity and dry bulb temperature

under 38℃ (Wyndham, 1961).

o In mines where, air velocity is used as heat index, the preferred standard

is 1.0 - 2.5 m/s for comfortable conditions. The velocity is limited to 2.5

– 3.0 m/s so as to prevent dust getting airborne (Hartman et al, 1982).

2.3 Dry –bulb temperature

o Its use as heat index is rare.

o However, in mines where heat transfer from human body to the

surroundings is mainly through the process of convection and radiation,

its combination with air velocity can be taken as a reliable direct heat

index.

o Dry –bulb temperature above 45℃, starts giving burning sensation to the

skin in air-current (McPherson, 1993).

3.0 Empirical Indices

3.1 Wet – Kata thermometer

3.1.1 Kata Thermometer

It was developed by Dr. L. Hill and his assistants in 1916.

3.1.2 Construction

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It is made up of glass with a large bulb or reservoir at the bottom. To this reservoir,

a long stem is connected having two smaller bulbs at the two ends i.e. one at the

bottom and other at the top. The stem has two mark on the graduations at

35℃ (95℉) and 37.8℃ (100℉). The thermometric fluid used is alcohol. The details of

the dimensions are indicated in Fig.1.

Fig. 1 Kata Thermometer

37.8˚C

35˚C

20 cm

4 cm

2 cm

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3.1.3 Procedure for taking reading

The procedure followed for taking wet-kata reading is shown in Fig.2

Fig. 2 Procedure for taking wet-kata reading

The larger bulb or reservoir is dipped in hot water until the thermometric fluid rises and fills

up the smaller bulb at the top of the stem.

The bulb of the kata thermometer is taken out of the hot water and wiped with cotton to

remove any water drops/ layer on the bulb surface

Using a stopwatch, the time by kata thermometer in cooling from 37.8℃ to 35℃ is recorded

For obtaining rate of heat loss from the surface of kata thermometer, Kata factor (written on

it, provided by the manufacturers) is divided by time taken for cooling from 37.8℃ to 35 ℃.

The larger bulb or the reservoir is covered with wet muslin cloth to record wet-kata thermometer reading and allowed to cool in the mine air

To take dry-kata reading, it is

allowed to cool in the mine air

without application of wet muslin

cloth on the larger bulb/reservoir

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3.1.4 Requirement of wet-kata thermometer reading

Dry-kata reading is different from wet-kata thermometer in terms of mode of heat

transfer taking from kata-thermometer surface. In dry-kata thermometer, heat

transfer from kata-thermometer surface takes place only by convection and

radiation. On the other hand, for wet-kata reading the heat transfer from the kata

thermometer surface takes place by all the three processes i.e. convection,

radiation and evaporation. It is designed in such a way that the wet-bulb kata

reading obtained is a representative of 20 mm wetted bulb (reservoir) at 36.5℃

(much closer to the central core body temperature of humans). Thus, mechanism of

heat transfer from wet-kata thermometer is more close to the mechanism of heat

transfer from human body.

3.1.5 Numerical approach

(A) Relation between wet kata cooling power, velocity of air-current and wet-bulb

temperature (Mishra, 1986)

𝐾 = (14.65 + 35.59𝑣1

3) (309.65 − 𝑇𝑤) 𝑓𝑜𝑟 𝑎𝑖𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑣 ≤ 1 𝑚/𝑠

𝐾 = (4.19 + 46.05𝑣1

3) (309.65 − 𝑇𝑤) 𝑓𝑜𝑟 𝑎𝑖𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑣 ≥ 1 𝑚/𝑠

Where,

𝐾 = kata cooling power (W/m2)

𝑣 = velocity of mine air (m/s)

𝑇𝑤 = wet bulb temperature (K)

(B) Wet kata reading can be approximated from air velocity 𝑣 and wet-bulb

temperature 𝑇𝑤(McPherson, 1993)

𝑤𝑒𝑡 𝑘𝑎𝑡𝑎 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟, 𝐾 = (0.7 + 𝑣0.5)(36.5 − 𝑇𝑤)

Where,

𝐾 = 𝑤𝑒𝑡 𝑘𝑎𝑡𝑎 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑝𝑜𝑤𝑒𝑟(𝑚𝑐𝑎𝑙/(𝑐𝑚2𝑠))

𝑇𝑤 = 𝑤𝑒𝑡 − 𝑏𝑢𝑙𝑏 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (℃)

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𝑣 = 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 (𝑚/𝑠)

(C) Graphical aids to find kata cooling power for a given wet –bulb temperature

Fig.3 Graph to find wet kata cooling power from wet-bulb temperature and

air velocity (after, Hartman, 1982)

In mines, where wet-kata thermometer is used as heat index, the preferred

standard is 4.5 -12.0 mcal/(cm2s). Wet kata cooling power of 4.5, 7 and 12

mcal/(cm2s) may be used for light, moderate and hard works respectively.

3.1.6 Limitations

o Variation of kata factor with temperature.

o Kata thermometer underestimates effect of temperature and humidity of

mine air and overestimates the effect of air velocity.

o The convective and evaporative coefficients of kata thermometer are

twice as that of human body and therefore it cools much faster than the

human body.

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o Kata thermometer has smaller volume to surface area ratio as compared

to human body which further makes it to cool faster than that of human

body.

o Its use as heat index is limited to hot and humid climate.

o A person doing moderate work generally produces 165 W/m2. It means, a

kata reading of 165 W/m2 will provide a comfortable working

environment. However, it is not the case. As discussed earlier, the kata

thermometer cools much faster than that of a human body and therefore

for a comfortable working condition, 165 W/m2 should be multiplied by a

factor. This factor has been arrived at 5 through various experimental

procedures. Therefore to provide a comfortable working environment, a

kata reading of 5 x 165 W/m2 is essential.

o The larger bulb (or reservoir) is fragile in nature.

These limitations have led to its rare use as heat index, now a days.

3.2 Effective Temperature

o It was introduced by American Society of Heating and Ventilating Engineers

(ASHVE). It refers to the combined effect of temperature, humidity and

velocity of air to single empirical temperature scale, reflecting equal

sensation of warmth or cold (Anon, 1993).

o It is defined as the temperature of still saturated air that would give the

same instantaneous thermal sensation as the actual environment under

condition (McPherson, 1993).

o It can be taken approximately equal to 90 % of wet-bulb temperature and

10 % of dry bulb temperature. This means that, the effective temperature,

or the temperature of the saturated air which would have the same cooling

effect on men, of 27/32˚C is 90 % of 27 + 10 % of 32 = 24.3 + 3.2 =

27.5˚C (Le Roux, 1972)

o Two scales are available based on the experiments carried out by ASHVE.

They are-

3.2.1 Basic effective temperature scale

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It is based on the observations made on persons stripped to the waist.

This scale is more applicable to underground mine conditions.

3.2.2 Normal effective temperature scale

It is based on the observations made on persons dressed in normal indoor

clothing. It is more applicable to office work conditions.

Preferably effective temperature should be less than 27℃ and limiting value is 32℃

(Hartman et al., 1982).

3.2.3 Merits

o It can be used as an index of heat in surface industry, mining, military

occupations and even office workers.

o Superior to kata thermometer

3.2.4 Limitations

o It underestimates the effect of humidity, and air velocity in hot and humid

conditions.

o The scale is not applicable to acclimatized persons.

o The scale prepared does not take into account radiant heat.

3.3 Wet-bulb globe temperature

o It has been adopted by ACGIH.

o The mathematical equation for wet-bulb globe temperature is given by

𝑇𝑤𝑔 = 0.7𝑇𝑤 + 0.3𝑇𝑔 𝑤ℎ𝑒𝑛 𝑛𝑜 𝑠𝑜𝑙𝑎𝑟 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑖𝑠 𝑡ℎ𝑒𝑟𝑒

𝑜𝑟

𝑇𝑤𝑔 = 0.7𝑇𝑤 + 0.2𝑇𝑔 + 0.1 𝑇𝑑 𝑤ℎ𝑒𝑛 𝑠𝑜𝑙𝑎𝑟 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑖𝑠 𝑝𝑟𝑒𝑠𝑒𝑛𝑡

Where,

𝑇𝑤𝑔 = Wet-bulb globe temperature (℃)

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𝑇𝑤 = Wet-bulb temperature (℃)

𝑇𝑔 = Black - globe temperature (℃)

𝑇𝑑 =Dry bulb temperature (℃)

The threshold limit values adopted by ACGIH for a moderate work load in hot

environments (Vutukuri and Lama, 1986) is given in Table 1.

Table 1 Threshold limit values adopted by ACGIH

Continuous work 26.7℃

75%work – 25% rest each hour 28.0℃

50%work – 50% rest each hour 29.4℃

25%work – 75% rest each hour 31.1℃

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Fig. 4 Instrument set-up for wet-bulb globe temperature measurement

(after Vutukuri and Lama, 1986)

4.0 Rationally Derived Heat Indices

4.1 Specific cooling power

From heat balance equation for human body we have

𝑀 = 𝑊 + 𝐴𝑐 + (𝑅𝑎𝑑 + 𝐶𝑜𝑛 + 𝐸𝑣𝑝 + 𝐵𝑟)

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For steady state condition, Ac should be equal to zero. Heat loss due to breathing is

almost negligible compared to the other processes. Thus, we can modify the

equation of heat balance for a human body as

𝑀 − 𝑊 = 𝑅𝑎𝑑 + 𝐶𝑜𝑛 + 𝐸𝑣𝑝

The right hand side of the equation is called the cooling power of the environment.

In other words, the rate of cooling experienced by the person is called the cooling

power of the environment.

Cooling power of the environment is dependent on the following factors:

i. Wet bulb temperature, 𝑇𝑤 (℃)

ii. Dry bulb temperature,𝑇𝑑(℃)

iii. Mean radiant temperature,𝑇𝑟(℃)

iv. Velocity of air-current,𝑣(𝑚/𝑠)

v. Pressure due to air or atmospheric pressure 𝑃𝑎 (𝑃𝑎)

vi. Skin temperature,𝑇𝑠𝑘(℃)

vii. Wetted area fraction of the body, 𝑤𝑎

When cooling power is expressed per m2, it is called specific cooling power. The

accuracy in calculating specific cooling power for an environment is largely

dependent on the accuracy with which all the above mentioned parameters (first

five are climatic parameters) are obtained. Mean skin temperature is generally

taken as 35℃. But, it produces better results if mean skin temperature is taken

35.7℃, 35.3℃ and 34.6℃ for light, moderate and hard work respectively.

Fig.5 and Fig.6 shows the variation in cooling power of environment with air

velocity and wet –bulb temperature for acclimatized and un-acclimatized men.

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Fig. 5 Cooling power as a function of wet-bulb temperature and air velocity

for acclimatized men, Dashed lines, wet kata readings (mcal/cm2s), td –

dry bulb temperature, tr – radiant temperature, tw – wet-bulb temperature,

Pa – air pressure (after Vutukuri and Lama, 1986)

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Fig. 6 Cooling power as a function of wet-bulb temperature and air velocity

for un-acclimatized men, Dashed lines, wet kata readings (mcal/cm2s), td –

dry bulb temperature, tr – radiant temperature, tw – wet-bulb temperature,

Pa – air pressure (after Vutukuri and Lama, 1986)

REFERENCES

Anon (1993); “ASHRAE Handbook of Fundamentals”; I-P Edition, American Society

of Heating, Refrigerating and Conditioning Engineers, Atlanta.

Banerjee S.P. (2003); “Mine Ventilation”; Lovely Prakashan, Dhanbad, India.

Hartman, H. L., Mutmansky, J. M. & Wang, Y. J. (1982); “Mine Ventilation and Air

Conditioning”; John Wiley & Sons, New York.

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Le Roux, W. L. (1972); Mine Ventilation Notes for Beginners”; The Mine Ventilation

Society of South Africa.

McPherson, M. J. (1993); Subsurface Ventilation and Environmental Engineering”;

Chapman & Hall, London.

Misra G.B. (1986); “Mine Environment and Ventilation”; Oxford University Press,

Calcutta, India.

Vutukuri, V. S. & Lama, R. D. (1986); “Environmental Engineering in Mines”;

Cambridge University Press, Cambridge.

Wyndhan, C. H. (1961); “Applied Physiological Research in South African Gold

Mining Industry”; Trans. 7th Commonwealth Mng. Met. Congress, Vol. 2; South

African Institute of Mining and Metallurgy, Marshall Town, pp. 775-792.