Chapter 9

CLIMATIC AND THERMODYNAMIC MODELLING OF RAPID DEVELOPMENT DRIVAGES

A.J. Crossley	I.S. Lowndes
School of Chemical, Environmental and Mining Engineering University of Nottingham UK	School of Chemical, Environmental and Mining Engineering University of Nottingham UK

ABSTRACT

This paper details the construction of a computer based climatic prediction tool currently being developed at the University of Nottingham. The model predicts the psychrometric and thermodynamic conditions within single entry drivages, taking into account the effects of the strata and the machinery on the ventilation air. The interaction between the air travelling through the force ventilation ducting and back down the drivage is considered, through a series of leakage and heat transfer calculations. It is intended that the model may be further developed to include procedures to investigate the effects of applying localized cooling systems. Preliminary results obtained from the model are shown and compared against measurements collected from within a UK rapid development drivage.

KEYWORDS

Mine ventilation, climatic modelling, development drivages

INTRODUCTION

The current trend in the UK towards the adoption of retreat longwall mining methods and the associated rapid development of the required access drivages, has exacerbated the environmental conditions experienced within these workings. There is a recognised need to improve the efficiency in the design and operation of mine ventilation systems in order to maintain an adequate mine environment and climate. Any improvement achieved in the quality, quantity and control of the delivered ventilation will assist in the provision of improved gas and dust dilution and climatic control. Due to the constraints imposed by the mining method, there may be an economic or practical limit to the climatic improvement that may be obtained by the sole use of ventilation air. Where this limit is identified, there may be the need to consider the selective application of mine air cooling systems.

Various methods have been developed to predict the underground climate across a variety of mine workings. Earlier research work considered the heat flow into stopes (Starfield, 1966) and tunnels (Goch and Patterson, 1940; Starfield and Dickson, 1967; Gibson, 1976; McPherson, 1986). More complex situations such as drivages (Voss, 1980; Kertikov, 1997), longwall districts (Voss, 1971; Middleton, 1979) and faces (Longson and Tuck, 1985; Gupta et al., 1993) have also been considered. Advances in available computing power have lead to an increase in the level of detail used to construct the models to represent the many contributing heat sources including the machinery, friction and auto-compression.

This paper details the construction of a computer based climatic prediction tool currently being developed at the University of Nottingham. This work builds upon earlier research conducted at the University of Nottingham (Ross et al., 1997). The model predicts the psychrometric and thermodynamic conditions within single entry drivages, taking into account the heat transfer effects of the strata and the machinery on the ventilation air. The model is currently being correlated against ventilation, climatic and operational data obtained from a number of representative UK deep coalmines. Once fully developed, the model will be used to establish the practical and economic limits of applying single ventilation and integrated cooling systems. The completed model will provide ventilation engineers with a powerful tool for the design of the most efficient and cost effective integrated ventilation and cooling system for rapid development drivages.

DRIVAGE MODEL

The drivage model incorporates a number of modules that represent the effects that the various heat and humidity sinks and sources have on the climate of the ventilating air. The analysis proceeds by representing the volume of the drivage as a number of connected sub-elements of specified length. Two sets of elements are constructed, one set to represent the air traveling along the main (force) ventilation duct, and the other for the air returning back down the main body of the drivage. The resultant flow and climatic conditions are determined within in each element in turn, with the outlet flow conditions from one element forming the inlet conditions of the next. Given a set of initial inlet climatic and flow conditions, the outlet values are determined by considering the effect of each of the heat and humidity sources and/or sinks present within each element. The layout of a typical UK drivage ventilated by a conventional force-exhaust overlap configuration is shown in Figure 1. The schematic illustrates an example overlay of the duct and drivage elements

Figure 1. Typical drivage schematic

For the configuration shown in Figure 1, the calculations follow the direction of the airflow along the force duct and within the drivage. As there may be an exchange of leakage air between the force ventilation duct and the drivage elements, together with heat transfer through the duct walls, an iterative approach is used, whereby the calculations are repeated until a balanced flow and numerically converged solution is obtained. Figure 1 also indicates various climatic zones defined within the drivage. Zone A represents the area in the immediate vicinity of the cutting area. In this region there is significant heat loading due to the strata, broken mineral, water sprays and equipment. This will affect the climatic conditions experienced by the workforce within the zone. Zone B lies beyond the cutting area and incorporates the region where the broken mineral is transferred to the conveyor belt. The machinery and mineral sources within this region will produce a marked effect on the climatic conditions experienced by the ventilation air. The final zone, Zone C, contains the conveyor belt and extends to the entrance of the drivage. In this region, the conveyed broken mineral and the newly exposed drivage walls will produce the greatest impact on the climate.

The underlying principles used in the construction of the drivage model are based upon the climatic and psychrometric calculations used by McPherson (1986, 1993) to represent the climate within through-flow ventilated mine roadways. This theory has been adapted to both model the air travelling through the ventilation ducting and within the drivage. Additional calculations have been incorporated to represent the leakage interaction between these two airflows. Procedures have also been developed to account for the effects of the heat from conveyed coal, the presence of water sprays and the influence of the auxiliary fans.

Heat From Strata

The heat emitted from the surface of the newly exposed drivage walls may have a significant impact on the climatic environment within the drivage. Various factors will affect the rate of heat transfer and these include the cross sectional geometry, the age of exposure, the surface wetness of the walls, the quantity of airflow, the virgin rock temperature and the conductivity and diffusivity of the wall material.

The basic method used to predict the heat pick-up from the strata is the solution of the radial heat conduction equation as presented by McPherson (1993). By applying Fourier's Law and Gibson's algorithm (Gibson, 1976), the dry surface temperature for the rock is calculated from which the heat conducted to the surface may be found. The exposed strata is considered to comprise of dry and wet regions, resulting in both sensible and latent heat transfer to the airstream. An iterative procedure is used to establish a wet surface temperature from which the latent heat transfer to the ventilation air may be evaluated.

Heat From Conveyed Coal

The recently cut mineral will also have an effect the drivage climate. The coal joining the conveyor at the heading is assumed to be at the virgin strata temperature. The use of sprays during the cutting process can result in the cut coal presenting both a significant sensible and latent heat source. As the coal is transported away from the face of the heading, the heat exchange causes the coal surface temperature to drop accompanied by a decline in the rate of heat transfer.

The conveyed mineral is represented as a continuous plane parallel slab of known dimension, which travels at a fixed speed away from the face. This model, was first proposed by Watson (1981) and later developed and applied by Longson and Tuck (1985). It is assumed that the core temperature of the conveyed coal remains at the virgin rock temperature and that there is no heat transfer through the conveyor belt. As with the strata calculations, a surface temperature is predicted, based upon the time since cutting. In applying the model, a wetness factor has been included in the calculations such that latent heat transfer may be calculated by a method analogous to the strata calculations.

Heat From Machinery And Other Sources

The presence of machinery will also affect the climate of the ventilation air. Continuous miners, conveyor drives, crushers, transformers, shuttle cars, coolers etc. are some of the many examples of potential machinery heat sources that may be present in a mechanised drivage. In addition, other factors such as water drainage channels and muck piles may also effect the climate.

The model includes several mechanisms that may be used to simulate the effects of machinery and other heat sources based on the representations developed by McPherson (1986, 1993). The various categories considered are:

• Non-machine spot sources,

• Electric machine spot sources,

• Diesel machine spot sources,

• Linear heat sources.

In each case the sensible and latent (if applicable) heat contributions are found and applied at the appropriate positions. For linear sources, the heat loading is distributed across the corresponding elements.

Sprays

Sprays may be used to suppress dust emission during the cutting process, within dust collectors or coolers installed inline with the exhausting ducts and at mineral transfer points on the conveyor belt. Although the use of these sprays may reduce the amount of dust liberated into the ventilating air, their use may increase the moisture content and thus raise the relative humidity of the airstream. In addition, the use of sprays may result in secondary effects such as an increase in the latent heat emitted from the wetted cut coal.

The spray module is constructed by assuming that a specified proportion of the airstream is influenced by the spray. It is assumed that the air affected by the spray becomes fully saturated and the conditions are found by applying the laws of energy balance, conservation of mass and the second law of thermodynamics (Burrows et al., 1982). The saturated air is re-mixed with the remaining ventilation air, in accordance with conservation of mass and enthalpy.

Fans

Within most long drivages, auxiliary fans are used to draw air into forcing/exhausting ducts by creating a pressure differential between duct and the drivage. The subsequent increase in pressure and compression of the air as it passes through the fan, result in a temperature rise. In addition, frictional losses across the impeller and the fan casing may also result in a heating of the air.

The fan model is based upon the prediction of the resultant temperature rise experienced by the air as it passes through the fan and is based on the analysis presented by McPherson (1993). Given the inlet conditions of the air together with a measure of the fan efficiency and predicted pressure development, the outlet temperatures may be calculated. It is assumed that the inlet air is unsaturated and that the moisture content remains constant across the fan.

Heat transfer through the duct

Sensible heat transfer may occur between the air travelling through the ventilation ducts and that flowing back down the drivage. The direction of the heat flow will depend upon the ventilation configuration, for example, the air returning back down the heading may transfer heat to the air travelling through the forcing duct.

The quantity of heat transferred is evaluated by applying the theory of steady state heat transfer through a cylinder (Burrows et. al., 1982) and combines the effects of convection, conduction and radiation.

Leakage and mixing

The pressure differential that exists between the air flowing through the force duct and that flowing in the drivage may promote a leakage of air from the duct to the drivage. It is assumed that the air leaks uniformly along the length of the duct and as such can be represented by two differential equations (Browning, 1983).

In proceeding from one duct element to another, the climatic calculations are performed to establish the outlet flow conditions for an element, a determined quantity of this air is permitted to leak from this duct element to the adjacent body element of the drivage. The duct calculations proceed to the next element using the reduced airflow quantity. Within the drivage, the leakage air is mixed with the return air flowing in the drivage by applying conservation of mass and performing a balance of the enthalpy terms.

RESULTS

The initial development of the drivage model focussed on the validation of the various individual simulation components. This was achieved by the incorporation of these individual routines within a climatic prediction routine for a single through-flow ventilation model. The results obtained from this model were compared against the predictions produced by a commercially available climatic modelling package. An analysis of the results produced by these studies confirmed that the strata heat, machinery, climatic and psychrometric models all produced a consistent set of data. In addition, ventilation and climatic data collected along the length of a UK colliery conveyor road, were used to validate the conveyed coal module.

A series of studies, employing the ventilation and climatic data collected from a UK colliery drivage were conducted to evaluate the performance of the drivage model. A schematic of the drivage layout is shown in Figure 2.

Figure 2. Schematic of the T16 drivage at Maltby Colliery

The surveyed drivage was at the time of survey approximately 1360m long, with a rectangular cross-sectional profile of 4.3m wide and 3.1m high. Ventilation was provided by means of a conventional force-exhaust overlap system. A series of continuous dry and wet bulb temperature readings were recorded over a 24-hour period at six measurement points along the length of the drivage. To compare the measured data against those predicted by the model, the data was time averaged over two-hour periods. A comparative plot of the measured data and predicted temperature profiles are shown in Figure 3.

Figure 3. Predicted and measured temperatures from the Maltby Colliery T16 drivage

The results produced by the model indicate that there is a significant heating of the air as it passes through the forcing fan and along the duct. This is due to the transfer of heat from the air flowing back from the head-end of the drivage through the forcing duct walls to the ventilating air. As the airflow exits the duct, scours the face and returns back along the length of the drivage, the air is further heated by the various sensible and latent heat sources present within the drivage. The influence of the many concentrated sensible and latent machine heat sources at the head of the drivage may be observed in Figure 4.

Figure 4. Predicted and measured temperatures from the Maltby Colliery T16 drivage - head of the drivage

The ventilation air exiting the forcing duct at the face of the heading is greatly influenced by the dust suppression sprays mounted on the body of the machine and the cutting boom. These sprays produce a cooling effect on the air, which results in a drop in the dry bulb temperature and a slight increase in the wet bulb value (due to an increase in the moisture content). This air is further heated as it passes over and around the continuous miner and through the region containing the bridge belt. A jump in the measured and predicted dry bulb temperature may be observed in the vicinity of the crusher. The heating effect is more pronounced in this region, as the air volume flowing across this region is reduced by the fraction of the total airflow drawn through the exhaust overlap duct. Just beyond this region, the air from the exhaust duct re-mixes with the main airflow. For the auxiliary ventilation configuration considered, the air within the exhaust duct was heated more by the exhaust fan than by the heat transfer through the duct walls. As this air possesses a higher dry bulb temperature (though lower moisture content) at the mixing point, there is a rise in dry bulb temperature and a drop in the wet bulb value. Beyond the location of the main machinery sources, the dry bulb temperature of the air gradually decreases accompanied by a steady rise in the wet bulb temperature. Away from the face of the heading, the contribution of the latent heat, produced from the walls and broken coal, is more significant than the sensible heat emanating principally from the conveyed mineral. Finally, as the ventilation air passes over the conveyor drive located at the entrance to the drivage, there is a slight rise in the dry bulb temperature.

CONCLUSION

A model has been developed to predict the climatic and thermodynamic environment within rapid development drivages. Preliminary correlation studies have been conducted using ventilation and climatic measurements collected from UK collieries. These initial studies have confirmed that the model is able to produce accurate temperature predictions. Further validation studies are being performed to improve the accuracy and applicability of the model

ACKOWLEDGEMENTS

The authors would like to acknowledge the financial support of the European Coal and Steel Community. The authors also wish to record their thanks to the engineers of Protec DMT GmBH and to the environmental engineers of RJB Mining (UK) Ltd for the provision of the underground measurement data. The views expressed within the paper are those of the authors and not necessarily those of the sponsors.

REFERENCES

Browning, E.J., 1983, “An approximate method for auxiliary ventilation calculations,” The Mining Engineer, pp. 129-134

Burrows, J.H.J., Hemp R., Lancaster, F.H., Quilliam, J.H., 1982, Environmental engineering in South African mines. Cape & Transvaal, Cape Town

Gibson, K.L., 1976, “Computer simulation of climate in mine airways,” Ph.D. Thesis, University of Nottingham, UK

Goch, D.S., and Patterson, H.S., 1940, “Heat flow into tunnels,” J. Chem. Metall. Min. Soc. S. Afr., Vol. 41, pp. 117-121

Gupta, M.L., Panigrahi D.C., and Banerjee, S.P., 1993, “Heat flow studies in longwall faces in India,” Proceedings, 6^th U.S. Mine Vent. Symp., R. Bhaskar, ed., SME, Littleton, CO, pp. 421-427

Longston, I., and Tuck, M.A., 1985, “The computer simulation of mine climate on a longwall coal face,” Proceedings, 2^nd U.S. Mine Vent. Symp., P. Mousset-Jones, ed., A.A.Balkema, Rotterdam, pp. 439-448

McPherson, M.J., 1986, “The analysis and simulation of heat flow into underground airways,” Int. J. Min. Geolog. Eng., Vol. 4, pp. 165-196

McPherson, M.J., 1993, Subsurface ventilation and environmental Engineering. Chapman & Hall, London

Ross, A.J., Tuck, M.A., Stokes, M.R., and Lowndes, I.S., 1997, “Computer simulation of climatic conditions in rapid development drivages,” Proceedings, 6^th Int. Mine Vent. Cong., R.V. Ramani, ed., SMME, Littleton, CO, pp. 283-288

Starfield, A.M., 1966, “The computation of air temperature increases in advancing stopes,” J. Mine Vent. Soc. S. Afr., Vol. 19, pp. 189-199

Starfield, A.M., and Dickson, A.J. 1967, “A study of heat transfer and moisture pick-up in mine airways,” J. S. Afr. Inst. Min. and Metall., Vol. 67, pp. 211-234

Voss, J, 1971, ”Prediction of climate in production workings,” Glückauf, Vol. 107, pp. 412-418

Voss, J, 1980, ”Mine climate in mechanised drivages and its determination by advanced calculation,” Methane, climate, ventilation in the coalmines of the European Communities, Vol 1, Colliery Guardian, pp. 285-305

Watson, A.J., 1981, “The contribution of conveyed coal to mine heat problems,” Ph.D. Thesis, University of Nottingham, UK

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