Chapter 4

THE INFLUENCE OF HEATING AIRFLOW THROUGH DOWNCAST SHAFTS

ON THE STABILITY OF AIR STREAMS IN A MINE

L. Kloc Ruda Sl. Coal Company, ul. E.Kokota 168, 41-711 Ruda Sl. 11 Poland	N. Szlązak University of Mining & Metallurgy in Cracow, Department of Underground Mining, Cracow, Poland
	A. Tor Jastrzebie Coal Company, Jastrzebie, ul. Armii Krajowej 56, Poland

ABSTRACT

In winter, air flowing down downcast shafts must be heated in order not to allow for the icing of shaft guides. Heating air results in natural ventilation pressure, NVP, which can affect the change in quantity of airflow and even the change in the direction of airflow. On the basis of numerical calculations, the influence of NVP on the stability of air streams is characterised as well. The results of theoretical solutions are compared to the measurements taken in mining excavations

KEYWORDS

Stability, Natural Ventilation Pressure, Heating of Air, Safety of Work

INTRODUCTION

The phenomenon of airflow in excavations without any mechanical sources of pressure differential has been well known since the earliest times of underground mining. Airflow in such cases is caused mainly by the process of heat exchange between the airstream and the rocks surrounding the excavation, and also by changes in the contents and humidity of the air. The influence of other purely mechanical factors can be another reason. The combined interaction of all these factors is called natural ventilation pressure (NVP) or natural draught (Budryk, 1961).

The direction of NVP in excavations, where air flows from a higher to a deeper lower point, first of all in downcast shafts, is opposite to the direction of the flow the pressure of main fans are attempting to create. The negative value of pressure in a downcast shaft can be of a different value and, depending on the structure of a ventilation system, may influence the quantity of air flowing. A similar phenomenon may also take place during underground fires. In addition, according to mining regulations, air temperature in downcast shafts must be maintained above +2^oC to prevent ropes and guides from icing (Regulation, 1995). Disturbances in air distribution are dangerous in methane-rich mines, where air distribution into ventilation areas must be remain constant. In order to avoid those phenomena it is necessary to determine the value of NVP in inclined excavations (mainly in shafts) in winter and summer.

At present different sets of ventilation systems of great complexity can be considered, through the use of computer programmes designed to analyse the air distribution in a ventilation system. On the basis of practical observations it can be seen that due to heating of air in shafts disturbances in ventilation systems occur. It was observed that during the heating of air in downcast shafts in some conditions its quantity becomes less. There are also some fluctuations of airflow, which results in the direction of airflow not remaining constant. Such factors as: an increase in temperature of air and the depth of a shaft may influence the overall process.

In winter, in order to avoid the icing of machinery in a downcast shaft and to heat air flowing into a mine heating equipment is used. When the quantity of air is small, air heaters are installed just below surface level throughout the aeration system in the whole perimeter at support. As there are some disadvantages to this solution (difficulties with installing pipelines in a shaft), another solution is used. This consists of some part of the fresh air (25-30%) being blown into a shaft through a special heating drift, where heaters increasing the temperature of the air to approximately 70^oC are installed. A heating drift is sited tangentially to a shaft at a depth of several meters below surface level and this is where the mixing of hot and cold air takes place. As the measurements show a distance of 80-120 m is necessary for the hot and cold air to mix; therefore we may suppose that there are great differences in temperature in the upper part of a shaft, which worsens, among others, the conditions in which support operates.

In order to have air mixed quickly, it is indispensable to deliver heated air into a shaft uniformly.

On the basis of two examples the further part of this paper presents the necessity to regulate heating of airflow through downcast shafts in order to preserve the stability of the main streams of air. One example presents the disturbances in airflow in winter in one of the shafts while air in the remaining shafts is heated non-uniformly. The other refers to the disturbances in the direction of airflow in an excavation connecting downcast shafts while air is heated in one shaft.

DETERMINING THE VALUE OF VENTILATION PRESSURE

J. Pawinski and J. Roszkowski in their work (1973), took into consideration the problem of determining the value of natural ventilation pressure with regard to the influence of heating. NVP can be obtained from the following dependence:

(1)

where: T₂ - temperature of heated air (after getting mixed with cold air), K, T₁ - temperature of air flowing into a mine during the coldest period, K, γ₁ - specific gravity of air, N/m³, z - height co-ordinate, m.

Drawing explicit conclusions concerning the disturbances in the directions of airflow becomes difficult when air is delivered through two or three downcast shafts with a few levels and is also exhausted through two or more ventilation shafts. Computer record of the ventilation system is based on the results of measurements of thermodynamic parameters of air in a mine. Using a computer simulation of air distribution in a ventilation system can solve this problem. Therefore calculating NVP in particular branches is possible by means of the method by H. Bystron (1981). The pressure related to particular branches is expressed by the following dependence:

(2)

where: p_sw, p_sd - pressure of dry air creating in a ventilation system atmosphere isentropically bedded if there are no sources of pressure in the system, respectively for cross-section of outflow (p_sw) and inflow (p_sd) of a branch, Pa, z_w, z_d - levelling heights of the centre of the area of cross-section of a branch, respectively for outflow (z_w) and inflow (z_d) of a branch, m, g - acceleration of gravity, g=9.81, m/s², ρ_m - air density in a branch presented as average air density in cross-section of outflow (ρ_w) and inflow (ρ_d) of a branch, kg/m³.

Both the record of the structure of a ventilation system and the computer programme for stimulating air distribution allow for forecasting the stability of air streams depending on the value of the assumed ventilation pressure.

The value of temperature of heated air can be determined when we know the boundary value of ventilation pressure in downcast shafts, during which the straight direction of airflow in branches of the system is preserved. Simultaneously the maximum parameters of heating equipment installed in downcast shafts can also be obtained.

DETERMINING THE TEMPERATURE
OF HEATED AIR

The value of ventilation pressure or its increase, which is the result of local heating, is a function of the quantity of airflow Q. If volumetric airflow rate were equal to zero, ventilation pressure should not occur. However as the quantity of air increases, NVP approaches a certain value, which while Q→∞, is equal to:

(3)

where: T_pp- temperature of heated air (after getting mixed with cold air), K, T_pa- temperature of air flowing into a mine in the coldest period, K, γ - specific gravity of air, N/m³, z - height co-ordinate, m.

On the basis of equation (3)temperature of air after getting mixed T_pp can be determined, depending on the value of NVP required. When temperature of air after getting mixed is known, temperature of heated air can be obtained fromthe equation of energy balance:

(4)

where: c - specific heat of air, J/kgK, T - temperature of air, K, m - mass airflow rate, kg/s.

Parameters of heated, atmospheric and mixed air were marked by the following indexes:

_po - heated air,

_pa - atmospheric air,

_pp - air in a downcast shaft (mixed).

If c_ps= c_po= c_pp, the following dependence is obtained:

(5)

Adequate temperature of heated air should be ensured in order to preserve an adequate value of NVP in downcast shafts. It can be achieved by means of regulating the parameters of heaters or regulating the quantity of air flowing through these heaters. On the basis of computer records of ventilation systems of two mines the results of simulating the influence of ventilation pressure on the stability of air streams in those systems are presented in the further part of this work.

EXAMPLE I

In mine Z there is a set of two downcast shafts I and II located centrally and connected with each other on two levels. The air used from the levels mentioned is exhausted into two ventilation shafts IV and IX. Figure 1 presents a simplified ventilation model of this mine.

In winter, air flowing into a mine is heated in such a way that the temperature of airflow is not lower than
+2 ⁰C. At that time some disturbances related mainly to

the change in the quantity and direction of airflow in shaft II occur. It is related to a more intensive heating of air in shaft II than in shaft I. The reduction in the quantity of air flowing into a mine is observed in shaft II to level I. However, in the segment of shaft II between levels I and II there is not only the reduction in the quantity of air, but also the change in its direction.

Measurements of thermodynamic parameters were taken in shafts I and II in order to determine NVP in downcast shafts. The measurements were taken in spring. The table 1 presents the results of measurements of air temperature in downcast shafts.

During this period much more air flowed into the mine through shaft I than through shaft II. About 14,300 m³/min flowed through shaft I and about 5,000 m³/min through shaft II. On the basis of the measurements taken ventilation pressures in a shaft were calculated according to dependence (2). The values of NVP in particular segments of shafts are presented in the Table 2.

The pressures determined are of opposite direction to the total pressures of the main fans and for further calculations they are assumed as having a negative value. The measurements taken allowed for the determination of the boundary value of NVP in shaft II, in case of which the disturbances in the direction of airflow between level I and II occur.

In computer records of a ventilation system the values of NVP in particular segments of shafts were taken into consideration.

Figure 1. Simplified scheme of mine Z

Table 1. Results of measurements of air temperature in downcast shafts in mine Z

Level	Shaft	Dry temperature [^oC]	Wet temperature [^oC]
Surface level	I	3.2	0.6
		II	3.2	0.6
	Belowa heater	I	Not heated	Not heated
		II	26.2	16.2
	Level I	I	9.8	6.8
		II	14.8	10.0
	Level II	I	11.2	8.4
		II	18.2	14.0

Table 2. Values of NVP in particular segments of shafts in mine Z

Shaft	NVP, [Pa]
		till level I	between level I & II
I	-180	-10
II	-360	-90

The simulation of air distribution was conducted assuming in shaft II between levels I and II in turn one after another ventilation pressures of opposite direction of airflow. The calculations were done for the values ranging from -50 Pa to -250 Pa. Moreover, the calculations of uniform heating of air in both shafts assuming the value of NVP at the entry to shafts at -360 Pa were done.

As there is a great number of results of calculations, only the results of the change in the quantity of air in shaft II between level I and II are presented in Figure 2.

Figure 2. Dependence of the quantity of air on NVP between levels I & II in mine Z

Solid line represents the changes in the quantity of air as the intensity of heating (NVP) in shaft II increases and broken line during simultaneous heating in shaft I and II.

The change in the direction of airflow in shaft II between level I and II is more intensive during a smaller NVP in shaft II (heating) than during a simultaneous increase in pressure in shafts I and II.

EXAMPLE II

The ventilation system in mine Y is presented in a simplified ventilation scheme in Figure 3. In mine Y fresh air enters levels I and II through downcast shafts I and V and level I through shaft II. Air is distributed into ventilation areas and functional chambers. After ventilating used air is taken out into ventilation shaft IX. In a ventilation system some airflow disturbances in drift C connecting shaft bottoms I and V at level II occur.

In order to determine the influence of pressure on those disturbances, the measurements in the ventilation system of the mine were taken. The values of the measured temperature of air are presented in Table 3.

Table 3. Results of measurements of air temperature in downcast shafts in mine Y.

Level	Shaft	Dry temperature [^oC]	Wet temperature [^oC]
Surface level	I, II, V	-7.0	-9.2
Level I	I	12.2	10.0
		II	11.4	9.0
		V	11.4	9.4
	Level II	I	16.2	11.2
		V	11.6	9.4

In total about 6,400 m³/min of air enters the mine through shafts I and II, out of which nearly 3,000 m³/min enters level II through shaft I. This air is used to ventilate mining district and functional chambers in bottom shaft IX. Shaft V is located at a certain distance from the shafts mentioned above and about 2,300 m³/min of air enters the mine through it, out of which about 1,700 m³/min of air enters level II.

On the basis of the measurements taken ventilation pressures in downcast shafts were calculated. The values obtained are presented in the Table 4.

In order to determine the influence of NVP on airflow direction in drift C connecting downcast shafts, the calculations of airflow distribution were done.

Figure 3. Simplified scheme of mine Y

Table 4. Values of NVP in particular segments of shafts in mine Y

Shaft	NVP, [Pa]
		till level I	between level I & II
I	-434	-10
II	-412	0
V	-410	-60

Natural ventilation pressures obtained were inserted into branches of a ventilation system and airflow distribution was calculated. Calculated volumetric airflow rate in drift C was equal to 193 m³/min. The air was flowing towards shaft I into ventilation shaft IX. The influence of ventilation pressures on the direction and quantity of air in drift C connecting bottom shafts I and IV was taken into consideration.

The calculations proved that if air in shaft V is heated more intensively than in shaft I, there are changes in the quantity and direction of airflow in drift C. The calculations were conducted on the basis of the assumption that in shaft I NVP is reduced by 100 Pa, beginning from value in the table 4. The results of calculations are presented in figure 4 in the form of curves of changes in the quantity of air in drift C (solid line). Broken curve shows the changes in the quantity of air during the uniform change in NVP both in shaft I and V.

On the basis of the graph it can be concluded that for the value of NVP in shaft I equal to -310 Pa airflow in drift C is stopped. Such a situation occurs for original pressures in shafts II and V.

Fig. 4. Change in the quantity of air in drift C depending on NVP in shafts I & V

DISCUSSION ON THE RESULTS
OF CALCULATIONS

In the case of mine Z the increase in NVP in the segment of shaft II between levels I and II results in the reduction of the quantity of air entering the mine through shaft II while the quantity of air in shaft I increases simultaneously. For NVP between level I and II in shaft II equal to -180 Pa airflow is stopped and consequently airflow in this branch takes the opposite direction for bigger values. During uniform heating airflow is stopped for NVP equal to -230 Pa, which is equivalent to heating of air in a heater to approximately 40°C.

Therefore it can be concluded that if air entering shafts is uniformly heated in both shafts, the stability of airflow in shaft II between level I and II increases.

In the case of mine Y the change in the direction of airflow in drift C occurs for smaller values of NVP below the surface level of shaft I. Smaller values of NVP entail smaller and smaller intensity of heating of air in shaft I. On the basis of the calculations it can be concluded that the change in the intensity of heating of air, simultaneously in the value of NVP in shaft II does not influence significantly the direction or quantity of airflow in drift C in mine Y mentioned above.

The greater the values of ventilation pressures in downcast shafts, that is the more intensively air is heated, the greater negative ventilation pressures; therefore the quantity of intake air entering through downcast shafts is smaller. Non-uniform heating of air in downcast shafts results consequently in disturbances in air distribution in excavations connecting bottoms of those shafts. The reduction in the quantity of air in ventilation areas affects unfavourably methane or temperature hazards. Such a phenomenon is unfavourable and all possible actions should be taken to limit it.

Therefore it is necessary to record the temperature of heated air, which allows for monitoring the value of NVP.

CONCLUSIONS

Ventilation pressure in downcast shafts affects unfavourably the stability of airflow directions in shafts and also reduces the intensity of ventilating mining districts. It is of great importance in the case of methane or temperature hazard. Methane emission in mining districts, where ventilation intensity changes, can be hazardous to mining works.

Winter is the most unfavourable period when air must be heated in order to prevent shaft guides from icing. In summer, ventilation pressures in downcast shafts are similar and therefore their mutual interaction becomes level.

Different parameters of heaters in winter can affect unfavourably the stability of the main airflow directions.

On the basis of all the considerations above, there should be an attempt at fulfilling the following conditions:

• Heating of air in all downcast shafts should be uniform, that is air temperature below the outlet from heaters should be similar,

• Regulation of heating will contribute to preserving an adequate stability of airflow directions. In order to monitor the uniformity of heating of air, temperature sensors should be installed where atmospheric air gets mixed with heated air.

• Heating can be regulated by reducing the quantity of airflow in heaters by means of changing the parameters of fans of heaters.

• There should be an attempt at separating streams of air so that they do not get mixed in the area close to downcast shafts.

REFERENCES

Budryk W., 1929, “Depresja cieplna,” Przeglad Gorniczo-Hutniczy nr 11 & 12. (Polish text)

Budryk W., 1961, Wentylacja kopaln. Czesc I. Przewietrzanie kopaln, Wydawnictwo Gorniczo - Hutnicze, Katowice. (Polish text)

Bystron H., 1981, “Metoda regulacji systemu przewietrzania generujacego depresje cieplna,” Archiwum Gornictwa. T 26, 1981 z 4. (Polish text)

Pawiński J., Roszkowski J., 1973, “Kryteria stabilnosci pradow w szybach wdechowych podczas ogrzewania powietrza,” Prace Komisji Gorniczo - Geodezyjnej PAN, Gorn. Z. 12, Krakow. (Polish text)

Regulation of Polish Mining Laws, 1995, "Rozporządzenie Ministra Przemyslu i Handlu z dnia 14 kwietnia 1995 r. w sprawie bezpieczenstwa i higieny pracy, prowadzenia ruchu oraz specjalistycznego zabezpieczenia przeciwpozarowego w podziemnych zakladach gorniczych (§ 538)"

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I SZKOŁA AEROLOGII GÓRNICZEJ 1999

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PROCEEDINGS OF THE 7^TH INTERNATIONAL MINE VENTILATION CONGRESS

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THE INFLUENCE OF HEATING AIRFLOW THROUGH DOWNCAST SHAFTS

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