On the Issue of Combating High Temperatures in Deep Mines
Abstract Generated abstract
This paper examines the problem of controlling elevated air temperatures in deep mines, focusing on heat added by ventilation systems as well as by surrounding rock and other underground processes. It critically evaluates a proposal to reduce heating by sending part of the intake air through a separate high velocity insulated working, using heat balance relations and aerodynamic resistance calculations to estimate fan power, pressure losses, and resulting air temperature increases. The analysis argues that such a scheme would greatly increase energy consumption and add substantial heat to the ventilation air, while offering little reduction in heat inflow from rock. The paper concludes that heating by local fans and by main fans operating in forcing mode should be included in ventilation calculations for deep mines.
Full Text
ON THE PROBLEM OF COMBATING HIGH TEMPERATURES IN DEEP MINES
B. M. TITOV
(Presented by the seminars of the mining and mining-mechanical faculties and the heat-engineering departments of TPI)
The temperature of rocks rises on average by 1°C for every 30–35 m of mine-shaft depth; consequently, if special measures are not taken, in deep mines the mine air may become considerably heated. The technical operating rules in force in the USSR stipulate that the air temperature in stopes and development workings must not exceed 25°C. Thus, in deep mines the problem of combating the high temperature of mine air becomes extremely urgent.
In addition to the heat released from the surrounding rocks, the temperature of mine air is affected by a number of other factors, for example: oxidation processes, the water abundance of workings, the quantity and humidity of the ventilation air supplied, its compression due to increased atmospheric pressure, the method of ventilating mines, the release of heat by electrical motors and its absorption by pneumatic motors, the expenditure of energy in overcoming the aerodynamic resistance of mine workings (pipelines), etc. The influence of each of these factors is studied separately, and then, from the heat balance, the actual temperature of the mine air is found.
Most of the factors indicated above have been sufficiently studied and are correctly taken into account in designing ventilation for deep mines, but nevertheless some of them have still not received due attention. For example, mining engineer P. A. Manukyan, in order to reduce the heating of ventilation air by the surrounding rocks, proposes [1] that part of it be supplied through a special working parallel to the main drift, at a velocity of 18–20 m/sec. He recommends covering the surface of this working with heat-insulating material. In proposing to increase the velocity of air movement in the intake ventilation working, as compared with the haulage working, by approximately three times, P. A. Manukyan is not at all concerned with how this can be done in practice, whether in this case additional heating of the mine air will occur at the expense of the energy expended by the fan motor, and, finally, with the economic aspect of the matter.
The proposal of engineer P. A. Manukyan can be carried out by supplying the ventilation air with two fans of different pressure, or, if one fan is installed, by throttling the haulage working with a ventilation regulator. The variant with two fans is more economical in terms of energy consumption (Fig. 1), but under this ventilation scheme the mine air is additionally heated at the expense of the energy expended by the fan working in the pressure mode. The heat that is transferred by the fan—
... to the ventilation air in a fan operating on suction is dissipated at the surface and does not heat the mine air1. It is probably impossible to implement G. A. Manukyan’s proposal with both fans operating on suction.
Fig. 1. Diagram of the connection of fans and diagram of static pressures.
1—main fan; 2—auxiliary fan.
It is known [2, pp. 78–90] that the increase in the temperature of ventilation air when compressed in a fan is numerically equal to its heating from aerodynamic friction in the pipeline,
\[ \Delta t'=\frac{A}{\gamma c_p}H_{cm}, \tag{1} \]
where
\(A\) — thermal equivalent of work, kcal/kgm;
\(\gamma\) — specific weight of air, kg/m\(^3\);
\(c_p\) — heat capacity of air at constant pressure, kcal/kg °C;
\(H_{cm}\) — pressure required to overcome the aerodynamic resistance of the pipeline, i.e., the static pressure of the fan, kg/m\(^2\).
From the temperature increase \(\Delta t\) and the fan capacity \(Q\), we determine the amount of heat transferred to the ventilation air
\[ Q_m=c_p Q\gamma\Delta t \ \text{kcal/sec}. \tag{2} \]
This amount of heat is equivalent to the power expended in overcoming the aerodynamic resistance of the mine workings,
\[ N'_m=\frac{Q_m}{102A}=\frac{QH_{cm}}{102}. \tag{3} \]
In addition to the heat released during compression, the ventilation air is heated by internal energy losses in the fan; therefore, in formula (3), and consequently also in (1), a correction must be introduced for the fan efficiency:
\[ N_m=\frac{QH_{cm}}{102\eta_{cm}}; \tag{4} \]
\[ \Delta t=\frac{AH_{cm}}{\gamma c_p\eta_{cm}}. \tag{5} \]
Here \(\eta_{cm}\) is the static efficiency, taking into account internal energy losses in the fan.
Let us assume that the air entering the mine is divided equally between the workings AB and DB (Fig. 1), and that its velocity in working DB is three times greater than in AB. We shall take the cross sections of the workings to be circular and the aerodynamic-resistance coefficients to be identical.
For the adopted conditions, the ratio between the diameters of the workings must be
\[ \frac{D_1}{D_2}=\sqrt{3}, \tag{6} \]
where \(D_1\) is the diameter of the haulage working, m;
\(D_2\) is the diameter of the parallel ventilation working, m.
With the same air flow rate, length, and support of the workings, the pressure loss in them is inversely proportional to the ratio of the diameters to the fifth power [4, p. 236].
\[ \frac{H_{st_2}}{H_{st_1}}=\left(\frac{D_1}{D_2}\right)^5. \tag{7} \]
Consequently, after the introduction of P. A. Manukyan’s proposal, the aerodynamic resistance of the intake ventilation working DV will increase by a factor of 15.6,
\[ \frac{H_{st_2}}{H_{st_1}}=\left(\sqrt{3}\right)^5=15.6, \tag{8} \]
and the expenditure of energy required to deliver the ventilation air to point B (Fig. 1) will increase by a factor of 8.3,
\[ \frac{N_2}{N_1}=\frac{0.5N_1+15.6\cdot0.5N_1}{N_1}=8.3. \tag{9} \]
Here \(N_1\) is the power required to deliver the entire volume of ventilation air to point B before reducing the cross section of working DV;
\(N_2\) is the same after reducing the cross section of working DV by a factor of three.
If we assume that initially a power of \(N_1=200\) kW was required to deliver the ventilation air to point B (Fig. 1), then after reducing the cross section of working DV by a factor of three, to perform the same work a power will be required of
\[ N_2=8.3\cdot200=1660\ \text{kW}, \]
and heat will additionally be supplied to the ventilation air in the amount
\[ Q_m=\frac{3600\cdot102\cdot15.6\cdot0.5\cdot200}{427}=1.35\cdot10^6\ \text{kcal/hour}. \]
Such an amount of heat should probably not be neglected in calculations of ventilation for deep mines.
Perhaps the increased expenditure of energy and the additional heating of mine air are compensated by a significant reduction in the inflow of heat from the surrounding rock? It turns out that they are not; as Prof. A. F. Voropaev [4] has shown, with an increase in the velocity of the ventilation jet there is almost no reduction in the inflow of heat from the surrounding rock. Thus, the costs of driving a special ventilation working, the equipment of an auxiliary fan installation, and the additional expenditure of energy are in no way justified.
In addition, it should be noted that P. A. Manukyan gives a fundamentally incorrect explanation of the cause of heating of ventilation air in vertical workings. He believes that a kilogram of air, “falling” down a shaft by 100 m, performs 100 kg/m of work and is thereby heated by 1°C [1].
It seems to us that the motion of air in a shaft cannot be considered analogous to the fall of water, since air does not “fall,” but moves as a result of the pressure difference created by the fan and by natural draft. Heating of the air as it moves downward occurs from its compression, associated with an increase in atmospheric pressure.
The assertion by P. A. Manukyan that, by using pneumatic energy in mines, it is possible to reduce the amount of heat released by mechanisms is valid only if subsequent coolers are installed in the compressor station [5].
Conclusion
The increase in the temperature of the ventilation air is directly proportional to the energy consumed by the fan installation, and, at fan heads of the order of \(200\ \mathrm{kg}/\mathrm{m}^2\), is approximately \(2.5^\circ\mathrm{C}\),
\[ \Delta t=\frac{A H_{st}}{\gamma c_p \eta_{st}} =\frac{200}{427\cdot 1.3\cdot 0.24\cdot 0.6} =2.5^\circ\mathrm{C}. \]
Consequently, the heating of mine air by local-ventilation fans and by main fan installations operating in forcing mode should not be neglected.
Literature
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Manukyan P. A. Combating high temperatures in deep mines. Ugol', No. 5, 1953.
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B. Shtotses and B. Chernyak. Ventilation of deep mines. Translated from the German, ONTI, 1934.
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Skochinskii A. A., Komarov V. B. Mine Ventilation. Ugletekhizdat, 1951.
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Voropaev A. F. On the question of combating high temperatures in deep mines. Ugol', No. 12, 1953.
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Titov B. M. On the question of the influence of the use of pneumatic mechanisms in mines on the temperature of mine air. Proceedings of TPI, vol. 70, Tomsk, 1955.
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Local ventilation fans installed in the mine heat the mine air in all cases. ↩