Heat Emissions from Mining Machinery: Implications for Microclimatic Conditions in Underground Workings

Abstract

The thermal regime of underground mines, shaped by air temperature, velocity, and relative humidity, is a crucial factor for production and the health and safety of miners. While many aspects of this thermal regime have been thoroughly studied in the literature, local heat sources from mechanized equipment, such as load–haul–dump machines, conveyors, and auxiliary fans, have received comparatively little attention despite their significant impact on the thermal environment in mining development areas and stopes. This paper presents findings from a comprehensive study of the microclimatic air parameters in several nickel–copper and potash mines. We focus specifically on variations in air temperature in areas where mining equipment is operational. The heat output from different types of equipment, including load–haul–dump units, cutter–loaders, drilling rigs, conveyors, and auxiliary fans, has been quantified. We established empirical relationships for heat emissions from these machines and conducted a comparative analysis of their heat outputs. The main advantage of these relationships is their simplicity and the minimal number of input parameters required, making them practical for use in the field.

1. Introduction

The thermal regime of mine workings (or mines in general) is a quantitative characteristic that describes the values and patterns of change of all microclimatic parameters in the mine atmosphere and the temperature of the surrounding rocks [1,2]. The thermal regime of the mine should ensure workplace safety and preserve the health of all miners [3,4,5].

An analysis of the literature [2,6,7,8] indicates that the following factors are principally involved in the formation of the thermal regime of mines:

• Rock mass temperature, as determined by the geothermal characteristics of the area and the history of heat transfer between the mine air and rock mass.
• Temperature and humidity of the atmospheric air at the surface.
• Hydrostatic compression or expansion of air when moving along vertical or inclined mine workings.
• Moisture condensation and absorption in the air.
• Latent heat of ore oxidation.
• Heat exchange with compressed air pipelines.
• Metabolic heat produced by miners.
• Local sources of heat: mechanized equipment (machines, fans), blasting operations, and heat from the hydration of backfill materials.

Factors 1 to 7 have been comprehensively studied and described in many previous publications [9,10,11,12,13]. Not all factors are equally relevant to all mining environments; instead, the contribution of certain factors differs depending on the geological and mining technical features, as well as the climate of the region where the mining enterprise is located. The severity of any given factor also strongly depends on the type of mine workings. In vertical mine shafts, the main factors are 2–4, whereas in remote mine workings, factors 2–4 play a lesser role, while factors 1 and 5–8 can be very significant.

Local heat sources (i.e., factor 8) have been studied comparatively less in the literature. Existing studies are most often of a qualitative nature. The heat from the hydration of backfill materials is a significant source of heat, and its simulation is usually carried out by specifying empirical dependencies for the heat source [14], or using models like DeScutter and Taerwe [15,16], which take into account the influence of temperature and the degree of hydration. The release of heat during blasting operations and explosions is usually not studied separately. In these problems, more attention is paid to the calculation of the time of harmful gases removal or the calculation of the shock waves’ propagation [17,18].

In previous studies [19,20], it has been noted that diesel engines emit a significant amount of heat into their environment through the exhaust system, the engine cooling system, and the body of the engine. Based on the description presented in these studies, the total power of an engine (i.e., the total amount of energy received by the engine) is the sum of three equal energy components:

• The energy used to perform useful work by the engine.
• Energy consumed in the form of heat loss through the exhaust system.
• Energy released in the form of heat loss through the cooling system and the motor housing.

In general, previous authors have assumed that all three energy components are approximately equal [20]. Thus, the heat release from vehicles with internal combustion engines (ICEs) corresponds to two-thirds of the energy input supplied to the engine (in the form of fuel), which is, in turn, proportional to the rated power of an ICE. Therefore, if the rated power is known, the heat release from the engine into the environment can be easily determined.

Ne et al. [21] showed that heat release from electric equipment during operations depends on the heat dissipation coefficient. It is proposed that this coefficient depends on the technical characteristics of the equipment and should be determined in situ within a concrete mine environment for a separate heat source. McPherson [22] identified local (spot) and distributed (linear) sources of heat; the local heat sources in the mine are sensible heat sources (electric equipment) and a combination of sensible and latent heat sources (diesel engines). Latent heat is released into the air by means of water vapor generated by the engine. The value of latent heat is calculated as the product of the mass of water vapor multiplied by the latent heat of water evaporation. The mass of water vapor is calculated taking into account the rated power of the vehicle and the amount of fuel consumed. In addition, fuel consumption is an empirical parameter—it is equal to the product of the rated engine power (kW/h) multiplied by an empirical constant (0.3 L of fuel per kW engine rating per hour). The real power of the vehicle is then calculated taking into account the water/fuel ratio, which depends on the type of engine, the exhaust gas cooling system, and the maintenance of the vehicle. The value of this factor varies in a wide range from 3 to 10. As a result, the total heat release from an ICE is taken to be 2.83 kW per 1 kW of rated power. The sensible heat release component is defined as the difference between the total and latent heat release [23].

It should be noted that, at present, there are extensive published results of both theoretical and experimental studies of the influence of mining vehicles with both ICEs and electric engines (EEs) on the ventilation of underground working areas of mines, in addition to the removal of toxic and explosive gases and diesel particulate matter [19,24,25,26,27,28]. An analysis of methods for calculating the heat flow released during the mining vehicle fires in underground workings is provided in [29]. A theoretical study of the efficiency of dust suppression in a stope was carried out in [30] taking into account the operation of load–haul–dump (LHD) machinery. However, an analysis of the existing literature showed that the influence of mechanized equipment on the thermal regime of underground mine workings remains poorly understood. In the literature to date, there are no representative experimental studies of the thermodynamic parameters of air used to ventilate underground workings where diesel and electric equipment are operating. However, this issue is extremely important for mine ventilation, in particular, due to the current trend in the global mining industry for a constant increase in the intensity of mining operations, achieved primarily through the introduction of numerous pieces of high-performance equipment in mines. Practical experience shows that the operation of mechanized equipment can lead to local air temperature increases in working areas, where the miners are located, by more than 9 °C [2,31]. At that, it becomes difficult to quantify a temperature rise for a specific equipment using existing calculation approaches. At present, accurate estimation of the effect of heat release from mechanized equipment is a tangible difficulty in parametrizing mathematical models of heat and mass transfer in mine ventilation networks and in further predicting the microclimatic parameters of air in mine workings. For this reason, machine learning methods are now also beginning to be applied to assess adverse microclimatic parameters in mine workings and the thermal stress they cause for miners [32,33].

This work aims to provide a detailed study of the influence of local heat sources on the formation of the thermal regime of underground working areas in a number of potash and nickel–copper mines. The concept of this study is to use the results of experimental measurements of the microclimatic parameters in the vicinity (upstream and downstream) of operating mechanized equipment. Using these data, the heat release from mechanized equipment of different types was determined and analyzed.

2. Experimental Study of the Thermal Regime of Mines

2.1. Objects of Study

An experimental study of the formation of the thermal regime was carried out in the following mines:

• Copper–nickel mines of the Talnakh and Oktyabrsky deposits in the Norilsk industrial region, which use room-and-pillar methods with full backfilling of the worked-out area and the drilling-and-blasting technique.
• Potash mines of the Starobinsky deposit in the Republic of Belarus, which use the board-and-pillar method with long working faces and complete collapse of the worked-out area—mechanized mining.
• A potash mine of the Gremyachinskoye deposit in the Volgograd region, which uses the room-and-pillar method with full backfilling of the worked-out area—combination of mechanized mining and drilling-and-blasting operations.

These mines differ in terms of the mining technology used, the equipment (type, power), thermal conditions, and ventilation arrangements. For this reason, the experimental data collected from this set of mines will enable us to examine and analyze a wide range of factors and patterns in the formation of microclimatic conditions. This will enhance the broader applicability of this study’s results.

2.2. Experimental Techniques and Tools

The purpose of field studies was to determine the distribution of microclimatic air parameters in the mine workings. The following instruments were used to carry out experimental measurements.

• Temperature Humidity Meter Fluke 971. The absolute error in measuring the temperature in the interval from −20 °C to +60 °C is 0.1 °C and the absolute error in measuring the relative humidity in the interval from 5 to 95% is 2.5%. Purpose: measurement of air temperature and relative humidity in mine workings.
• Infrared and Contact Thermometer Fluke 568. The absolute measurement error of the infrared channel is ±0.1 °C for negative temperatures (t < 0 °C) and ±1.0 °C for positive temperatures (t > 0 °C). Purpose: determination of the surface temperature of the local sources of heat release.
• Anemometer APR-2, manufactured by the “NPF Ecotechinvest” company. The absolute error of the APR-2 anemometer is ∆ = 0.2 + 0.05v, where v is the air speed, m/s. Purpose: determination of the mean air velocity in the cross section of mine workings.
• Laser distance meter Leica DISTO D2. The absolute error of the distance measurements is 1.5 mm under normal conditions; under unfavorable conditions, such as bright sunlight or measuring uneven surfaces, ∆ = 0.15L mm (L is the measured length, m). Purpose: determination of the cross-sectional area of mine workings.
• Handheld weather station Kestrel 5000AG. Absolute error of temperature measurements: ±0.5 °C; absolute error of humidity measurements: ±2%. Purpose: measurement of the temporal dynamics of air temperature and humidity in the working areas of mines.
• FLIR 660 thermal imager. This portable device allows for measuring the temperature field with a spatial resolution of 640 by 480 points and has an acceptable accuracy (less than 2%) within the temperature range we consider in this study (from +2 to +50 °C). Purpose: determination of temperature distribution on the surfaces of heat sources.

Measurement points were located in uncluttered straight sections of mine workings. At least three measurements were made at each measurement point in order to determine the average aerodynamic and thermophysical parameters of the air flow. At an air velocity of more than 0.3 m/s, the velocity measurement was carried out by bypassing the entire cross section with a measuring device; otherwise, the measurement was carried out at separate points in the cross section (9 or 16 points).

The processing and graphical presentation of the experimental data collected at the mines, as well as the construction of the consolidated thermograms, were carried out using Microsoft rates of various types of equipment at the studied mines.

2.3. Consolidated Results of Experimental Studies

Experimental studies included measuring the dynamics of changes in air temperature and relative humidity along the airway paths from the mouth of the intake shaft to the ventilation shafts through the underground working areas (see Figure 1, Figure 2, Figure 3, Figure 4 and Figure 5). Measurements were made during both warm and cold periods of the year. The measurement results are presented in the form of thermograms, where the main areas and factors influencing the formation of the thermal regime are highlighted. These thermograms were then used to assess the impact of different heat release factors on the microclimatic parameters of the air in the mine as a whole.

The experimental studies undertaken allow the general patterns of the formation of microclimatic conditions in various mine types to be identified. In general, these patterns are as follows. The initial parameters of the air at the entry of the intake shaft are determined by the climatic conditions of the region, depending on its geographical location, and the time of year and day. In the presence of an air heating system at the intake shaft mouth, the inlet air parameters change; this is especially noticeable in the winter season, when the air must be heated up to +2 °C.

Subsequently, when the air descends along the shaft, it is noticeably heated due to the effects of hydrostatic (or auto-) compression, with a gradient of 0.00667–0.00769 °C/m. This gradient varies depending on the intensity of moisture phase transitions, in addition to heat and mass exchange of air with groundwater on the shaft walls and supports. In the case of ventilation shafts, the temperature of the rising air drops due to the almost adiabatic expansion of the air. The main factors that determine the microclimatic parameters of air in the shafts are the initial parameters of the air (for intake shafts), hydrostatic compression or expansion of the air, and the phase transitions of moisture.

With further movement of air along the main intake airways, the air exchanges heat with the surrounding rocks and takes on their temperature, which depends on the geothermal gradient of the site and the depth of the mine workings. The distance that the air flows to take on the temperature of the rocks depends on the initial temperature difference between the air and the surrounding rock mass, the air velocity, and the area of the heat exchange surface. In practice, this distance does not tend to exceed 2 km [31].

The amount of air supplied directly to the working areas decreases due to its distribution over a large number of mine workings. As a consequence, the intensity of heat release from different heat sources increases—the majority of the mining equipment is concentrated in working areas that are remote from the main air intake and return airways; therefore, a large volume of released heat is diluted by only a small volume of air. This leads to a sharp increase in air temperature directly in the development working areas and stopes.

Furthermore, contaminated and heated air flows from separate working areas down to the main return airways, where the temperature of the air flows drops and approaches the temperature of the rock mass. When contaminated air enters the ventilation shaft and rises, its temperature continues to drop, as noted above.

Based on the experimental measurements, the most unfavorable microclimatic conditions are observed in the development workings and stopes. These are the main working areas where miners are constantly present. This result is logical; however, our studies allowed us to quantitatively estimate how much worse the microclimatic conditions are in the more remote working areas and stopes. The air temperature increase in the stopes after the main intake airways ranges from 5 to 8 degrees Celsius. Additionally, the air humidity in the stopes can either increase significantly (up to 67%) or drop to 29%.

These conditions are primarily due to the influence of local heat sources, which include mechanized equipment with ICEs and EEs and auxiliary fans.

Thus, in order to calculate forecasted scenarios and develop effective measures to minimize air heating in working areas, it is essential to clearly understand the contribution of specific equipment to the resulting high temperatures. It is important to know the maximum heat emissions from specific types of equipment. A detailed experimental study of these local heat sources is given below.

Note that the heat from blasting operations and the heat of hydration of backfill materials are not considered in this study. This is because these heat sources have a very localized effect, both in time and space. Heat from blasting operations has a direct impact during the specific shift when blasting is carried out, which is not considered in this paper. The heat of hydration from backfill materials primarily affects the surrounding rock mass, while its direct effect on the ventilation system is assumed to be minimal, since the backfilled workings are usually isolated and not ventilated.

3. Technogenic Sources of Heat Release in Mines

The following intense sources of heat release are used during mining operations in the development workings and stopes of the mines considered in this study:

• Vehicles with ICEs;
• Vehicles with EEs;
• Auxiliary fans.

The thermal influence of these sources is most pronounced precisely in the place of their operation in the working areas of mines. As they move away from the working areas along the path of the air stream, their thermal influence weakens due to heat exchange with the rock mass and mixing with air leakages from other mine workings. These processes occur mainly in main return airways of ventilation levels, where polluted air flows to the ventilation shafts. The course of heat exchange processes here is still determined by how much the air is heated up after interacting with mining equipment in the working area. The increase in air temperature during the passage of the working area is usually an unknown parameter determined by a specific type of mining equipment. The study of the range of values of this parameter for various types of equipment is of great interest to us.

3.1. Vehicles with Internal Combustion Engines

In this study, self-propelled mining vehicles are considered. These are most often equipped with an ICE and serve to transport people, loads, and other technological equipment, e.g., drilling machines, machines for barring, etc.

Figure 6 shows infrared thermal images of various underground mining vehicles during their operation in the workings of different mines. The thermal images were obtained from experimental studies of heat sources using a FLIR 660 thermal imager. During the survey, the emissivity was 0.95. This coefficient was determined from the reference table in the documentation for the thermal imager. As shown, the vehicles are intense sources of heat, with individual working parts of some vehicles reaching temperatures in excess of +50 °C.

During the operation of ICEs, part of the heat released in the process of fuel combustion is converted to useful work, while the rest of the heat is emitted into the mine air. When the vehicle is moving, the useful work performed by the engine is used to overcome the frictional force and, as a result, it is also converted to heat and emitted in the mine air. The part of the work associated with overcoming the potential force of gravity when lifting the vehicle up, for example, is extremely small due to the gentle angles of inclination of the transport workings and small height difference compared to the total length of the transport route. Thus, it can be assumed that all the energy released during the combustion of fuel in this scenario is ultimately converted into heat.

Another question is how much of this heat is transferred directly to the air. When a mining vehicle interacts with its environment, a part of the heat is removed into the mine atmosphere, a part is absorbed by the surrounding rock mass, and a part (latent heat) is used in phase transitions of moisture in the mine atmosphere, taking into account the release of fuel combustion products. The exact determination of these parts depends on external conditions and machine design, as well as uncertainty in the distribution of dissipative energy losses in the “engine-transmission-wheels-body-rocks” system, and it is an extremely difficult problem. The solution to this problem is most often not needed in practice.

Experimental measurements were carried out to determine the power of heat release from the operation of LHD and drilling machines with ICEs for various types of underground mines. The relative humidity of the air, its flow rate, and the increase in the air temperature after passing through the heat source were measured in the mine workings. Data regarding the specific fuel consumption of the machines and their passport characteristics were also collected.

The measurements of air microclimatic parameters were made along the air-flow path both before the studied unit of mining equipment and after it at some distance (at a distance of at least 30 m). The distance was chosen in order to measure the microclimatic parameters in such a cross-section where the air flow had time to level out sufficiently along the cross-section of the working.

The power of the local heat source at a known air density, specific heat capacity, and air-flow rate, as well as the air temperature increment, is determined from heat balance equation: all heat released from mining equipment into the atmosphere of a mine working is spent on increasing the internal energy of the air (and, consequently, its temperature).

Table 1. Results of measurements of actual heat release from vehicles with ICEs.

Vehicle Type and Name	Air Flow Rate, m3/s	Temperature Increase, °C	Power of Heat Emission, kW	Rated Power, kW	Maximum Heating Power from Fuel Combustion, kW	${\mathit{K}}_{\mathit{i}\mathit{c}\mathit{e}}$ Coefficient
MoA3-7405-9586 dump truck	17	5.6	123	140	336	0.31
MT 433R dump truck	21	8.9	242	260	723	0.28
TORO 400D LHD	19	6.7	165	170	473	0.30
TORO 151D LHD	18	3.9	91	110	244	0.32
PAUS PFL30S LHD	28	2.8	101	109	303	0.28
ML-110 LHD	23	6,4	191	235	540	0.30
Sandvik LH-410 LHD	30	5.0	194	235	540	0.31
ST-14 LHD	25	7.5	243	275	638	0.32
ST-1030 LHD	21	6.5	177	220	488	0.31
Boomer H282 drilling rig	18	3.0	70	80	177	0.34
Sandvik DD311d drilling rig	10	4.5	58	85	189	0.26
Mean value:						0.3

shows the calculated values of heat release from vehicles with ICEs for the measured air-flow rate and temperature increase values. The rated engine power is also given. All measurements were carried out when the vehicle was operating at the maximum engine speed (unloading rock mass from the face, transporting loads, moving up an inclined mine), which corresponds to the rated power of the vehicle. $K_{ice}$ is the dimensionless coefficient of heat release, equal to the ratio of heat released into the mine atmosphere to the total amount of heat released during fuel combustion.

Based on the experimental results, a significant increase in air temperature by more than 9 °C occurs locally in mine workings due to the operation of vehicles with ICEs, which leads to significant effects on the microclimatic conditions in underground working areas. The actual heat release values from the operation of the vehicles compared well with their rated power; this is likely because the experiment was carried out at maximum engine speed. As shown in Table 1, the amount of heat removed by the air blowing over the vehicle is significantly less than the maximum amount of heat released during the combustion of diesel fuel. The ratio of heat emission to the air to the maximum heat from fuel combustion was found to be approximately constant and varies in a narrow range from 0.26 to 0.34, with a mean value of 0.3.

When a mining vehicle operates in the underground environment, heat is dissipated in three main ways: into the mine atmosphere, into the surrounding rock mass, and through phase transitions of moisture in the air, along with the release of fuel combustion products. Accurately determining the proportions of heat dispersed through these pathways depends on external conditions and vehicle design, as well as the complex distribution of dissipative energy losses across the ‘engine-transmission-wheels-soil’ system—a task that poses significant challenges [34].

Therefore, this study aims to develop a phenomenological model to calculate heat release from internal combustion engines and then calibrate it using experimental data. Considering the justification for the complete release of fuel combustion heat into the environment, the following expression is derived:

$$W_{ice}=P_a·C_f·K_{ice},$$

(1)

where $P_a$ is the average fuel consumption, kg/s; $C_f$ is the calorific value of the fuel, MJ/kg.

The introduced dimensionless coefficient of heat release $K_{ice}$ is an empirical parameter determined in the course of experimental observations. Table 1 shows the values of the coefficient $K_{ice}$, calculated based on the experimental measurements of the air microclimatic parameters both downstream and upstream of the vehicle.

The average fuel consumption is calculated based on the technical characteristics of mining machines as follows [35]:

$$P_a=N_{ice}·p_s·f_p,$$

(2)

where $N_{ice}$ is the rated power of the ICE, kW; $p_s$ is the specific fuel consumption, kg/(kW∙h); and $f_p\left(t\right)$ is a dimensionless coefficient equal to the portion of power generated by the engine from its rated power, which takes a minimum value at idle speed and a maximum at maximum engine speed. The coefficient $f_p\left(t\right)$ is determined based on the types of work performed by the vehicle; in the most pessimistic case, it should be equal to 1.

Substitution of expression (2) into (1) gives the final expression in the following form:

$$W_{ice}=N_{ice}·p_s·f_p·C_f·K_{ice}$$

(3)

If we express the thermal power in (3) in terms of the temperature increase $\mathsf{\Delta }T$, then from the obtained equation we can determine the expression for the coefficient $K_{ice}$:

$$K_{ice}={\displaystyle \frac{c·\rho ·Q·∆T}{N_{ice}·p_s·f_p·C_f}}$$

(4)

where $c$ is the specific heat capacity of the air blowing over the mining vehicle, kJ/(kg∙°C); $\rho$ is the air density, kg/m³; Q is the air-flow rate in the mine working with the mining vehicle, m³/s; and $\mathsf{\Delta }T$ is the increase in air temperature after it passes an operating vehicle, °C.

Formula (3) can be used in the heat balance equation when simulating heat and mass transfer processes in mine workings in the presence of vehicles that emit heat. Formula (4) can correspondingly be used to parameterize Formula (3) (i.e., determine the coefficient $K_{ice}$) from experimental data.

It was noted above that the coefficient $K_{ice}$ varies within narrow limits and has an average value of 0.30. In our opinion, such a small value of the coefficient (less than 1) is due to:

• The difference between the actual power and the rated power of the vehicle;
• Variable load on the engine over time;
• The influence of moisture exchange processes (i.e., increasing the moisture content of the air);
• Partial absorption of released heat by the surrounding rocks at the section of the mine working between the measuring points.

Overall, such an effective coefficient makes it easy to estimate the possible range of air heating from the operation of vehicles with ICEs, taking into account the understandable technical parameters of the equipment, which are usually known.

3.2. Vehicles with Electric Engines

Similar to the case of ICEs, a part of the energy consumed by vehicles with EEs is used in useful work (i.e., angular momentum transfer to the moving and rotating parts of the vehicles) and a part of the energy is released in the form of heat on the windings in EEs, as well as in cable lines. Similarly, the part of the work that acts to overcome gravity when the vehicle moves up is also extremely small due to the gentle angles of inclination of the mine workings. For this reason, the majority of the work is spent on disintegrating the rock mass (ore breaking with combined complexes) and overcoming the frictional forces (friction in mechanisms during ore breaking and transportation).

Given the complexity of the mechanisms of interaction of the individual structural parts of vehicles and the non-stationarity of the physical and technological processes, in addition to the lack of universal models that adequately describe the physical processes occurring during the destruction of rock mass and friction, it is not possible to construct a workable theoretical model for determining the proportion of heat transferred to the mine atmosphere during the operation of vehicles with EEs. Therefore, by analogy with the previously considered case of ICEs, we instead construct an empirical model for calculating heat release from machines with EEs, based on this study’s experimental observations.

Figure 7 shows the obtained thermal images of a cutter–loader (executive body), a conveyor line, an EE of a conveyor drive drum, an electrically driven drilling machine, and a mine transformer substation. Based on the thermal images, an unambiguous conclusion can be made regarding the intense local heat release from the EE of vehicles and other mechanized equipment, in addition to the distributed dissipative heat release along the entirety of the conveyor line.

The calculated values of heating power due to heat emissions from operating EEs for the considered mechanized equipment are shown in

Table 2. Results of measurements of actual heat release from equipment with EEs.

Vehicle Type and Name	Rated Power, kW	Power of Heat Emission, kW	${\mathit{K}}_{\mathit{e}}$ Coefficient
SL-300/400 cutter–loader	1901 *	72	0.038
SL-500C cutter–loader	1749 *	84	0.048
Ural-20R cutter–loader	710	28	0.039
Ural-20R cutter–loader	710	24	0.034
Sandvik DD311 drilling rig	70	5	0.071
DHMS BTP1-P drilling rig	110	13	0.118
Mean value:			0.058

* The calculated value of the rated power of the SL cutter–loaders machines also includes the rated power of the cooling system and the roof support for a longwall (1009 kW).

. Table 2 also shows the rated power of the equipment and the calculated dimensionless coefficients of heat release from the EE operation. $K_e$ is the dimensionless coefficient of heat release, equal to the ratio of heat release into the mine atmosphere to the total rated power of the EE.

As previously, it is assumed that the heat release from the operating equipment with EEs is proportional to their rated power:

$$W_e=K_e·N_e$$

(5)

where $N_e$ is the rated power of the electric motor, kW.

Based on empirical data, the mean value of the heat release coefficient for equipment with EEs (${\mathrm{K}}_{\mathrm{e}}$) is 0.058. The value range for this coefficient was found to be quite broad, i.e., from 0.034 to 0.118. For the Ural-20R cutter–loader, the value of the coefficient $K_e$ was calculated to be the smallest. In general, ${\mathrm{K}}_{\mathrm{e}}$ for cutter–loaders is lower than for drilling rigs. At the same time, the absolute value of heat release for cutter–loaders is higher. This difference in ${\mathrm{K}}_{\mathrm{e}}$ values is due to the different severity of heat exchange with the rock mass and the evaporation of moisture for cutter–loaders and drilling rigs. The latter are more compact and more intensively ventilated by the air flow than cutter–loaders, which occupy a larger section of the mine working. During the operation of drilling rigs, the holes can also be washed with water. The presence of water can also affect the temperature distribution in the mine working, leading to an increase in the ${\mathrm{K}}_{\mathrm{e}}$ coefficient.

3.3. Conveyors with Electric Engines

As a result of studying heat release from the drives of conveyor lines, the following features were noted. The installed conveyors were operating completely horizontally in all the considered mines; therefore, all the power consumed by the EE during operation of the conveyors is converted into heat due to dissipative processes. In this case, the nature of heat release is uneven. Part of the heat is released in the EE (due to the efficiency factor of the EE), while the rest is converted into heat due to friction forces in the rollers and along the entire length of the conveyor belt.

Table 3. Results of experimental measurements of the actual heat release from EEs of conveyor lines.

Conveyor Drive	Air Flow Rate, m3/s	Temperature Increase, °C	Power of Heat Emission, kW	Rated Power, kW	${\mathit{K}}_{\mathit{e}}$ Coefficient
KLSh-1000	5.1	1.8	12	150	0.080
KLSh-1000	3.9	1.9	10	150	0.067
KLSh-1000	2.8	3.0	11	150	0.073
KL-600	4.1	1.1	6	150	0.040
KL-600	2.0	3.0	8	150	0.053
Mean value:					0.063

presents the results of experimental measurements of the air-flow rate and the increase in temperature during the operation of conveyor line drives, in addition to the calculated power of the heat release and coefficient of heat release. The coefficient of heat release is the ratio of heat emission power and rated power.

Overall, the mean value of the coefficient of heat release for the EE of conveyor lines was 0.063. The total heat release is divided into two parts—the heat release near the conveyor drive and heat release distributed along the length of the conveyor. Taking into account the obtained experimental data, the expression describing the heat release power (kW) at the conveyor drive takes the following form:

$$W_{dr}=K_e·{\displaystyle \frac{\left(100-\eta \right)}{\eta }}·N_e$$

(6)

where $N_e$ is the total electric power consumed by the conveyor drive, kW; and $\eta$ is the efficiency factor of EEs, %.

The rest of the heat released uniformly along the length of the conveyor (kW/m) is described by the following expression:

$$W_{line}=K_e·{\displaystyle \frac{\eta }{100}}·{\displaystyle \frac{N_e}{L}}$$

(7)

where L is the length of the conveyor line, m.

The results of experimental studies of heat emission from the EEs of cutter–loaders, drilling rigs, and conveyor lines also show that the amount of heat emitted in the mine atmosphere is much lower than the rated power of electrical equipment (by an order of magnitude), consistent with our findings. The significant discrepancy between the rated power and the observed power of heat emission can be explained by the following reasons:

• The difference between the actual power consumption and the rated one;
• Variable loads on the EEs;
• Partial absorption of heat release by the surrounding rock mass.

In this instance, there is one factor fewer than in the case of transport with ICEs. In the case of EEs, it is assumed that heat cannot be removed from the engine in the form of moisture evaporation into the air. Despite this, the heat release coefficient for EEs was found to be much lower in all cases than the equivalent value for ICEs. The lower value of the heat release coefficient of equipment with EEs in comparison with vehicles with ICEs is largely due to the different efficiency factors of ICE and EE mining equipment (note that for EEs, the efficiency factor is higher, as we do not consider power loss in the electrical substation and cables).

3.4. Auxiliary Fans

In addition, experimental studies of heat release from auxiliary fans were carried out for the considered mines. The auxiliary fans were installed in the development workings near the junctions with the stopes. A ventilation pipeline was connected to the auxiliary fans and stretched from the fan to the face of the stope. The purpose of the auxiliary fans is to ventilate the face of the stope after drilling and blasting operations or when the mechanized equipment is operating in it.

Figure 8 shows thermal images of the auxiliary fans operating in underground conditions.

Table 4. Results of experimental measurements of the heat release from the EEs of auxiliary fans.

Fan	Air Flow Rate, m3/s	Temperature Increase, °C	Power of Heat Emission, kW	Rated Power, kW	${\mathit{K}}_{\mathit{f}\mathit{a}\mathit{n}}$ Coefficient
VME-6	7	2.6	23	25	0.92
VME-8	10	3.7	47	50	0.94
VME-10	15	5.30	101	110	0.91
VME-12	21	4.1	109	110	0.99
KorfmannGAL 7-300/300	8	5.2	54	60	0.90
FBD No. 8.0/2*55	13	3.0	49	55	0.89
Mean value:					0.93

shows the results of the experimental measurements of heat release from the EEs of auxiliary fans. In this instance, the temperature increase was calculated using two measurement points: (1) before the electric drive of the fan and the fan itself; and (2) after the air flow exits the ventilation pipeline. The coefficient of heat release $K_{fan}$ is the ratio of heat emission power and rated power of the fan.

On the basis of the above observations, unlike the previously considered equipment with ICEs and EEs, the power of heat release from auxiliary fans is closely comparable to the rated power of the fan drive. Therefore, almost all of the rated power is transferred to the air flow in the form of heat; thus, when predicting the thermal regime of the mine, it is recommended to take the coefficient of heat release (i.e., ${\mathrm{K}}_{\mathrm{f}\mathrm{a}\mathrm{n}}$) for auxiliary fans equal to 1.

The difference between the heat release coefficients obtained empirically for the EEs of fans and other equipment (cutter–loaders, drilling rigs, conveyors) can be explained as follows. The energy consumed by the fan motor from the electrical network is almost entirely used to perform work—it is spent by rotating the impeller and overcoming the frictional resistance of the pipeline. The work of frictional resistance forces is also converted into heat. A small fraction of the energy is released as heat on the fan motor windings. Thus, the electric energy of the fan motor is completely converted into the heat.

The experimental values of ${\mathrm{K}}_{\mathrm{e}}$ are still below unity. We attribute this to the fact that part of the heat of the air flow has time to be absorbed by the rock mass.

4. Discussion

The experimental data described in Section 3 are necessary, first, for the parametrization of mathematical models of heat and mass transfer in mine ventilation networks. The empirical coefficient $K_e$ makes it possible to calculate the integral thermal effect from the operation of mining equipment with a given nameplate power. Such an estimate will be useful when 1D mathematical models of heat and mass transfer in mine ventilation networks are used. It is important to note that, nowadays, 3D models are often applied for analyzing individual sections of the mine ventilation networks (blind headings, distribution of air flows near fans, etc.) [36,37,38]. However, a 3D approach is almost never used in the analysis of extended ventilation networks (1000 branches and more). Such networks are usually studied using 1D models of heat and mass transfer [39,40]. There are a number of reasons for this, mentioned in [41].

One-dimensional models are very important in a certain class of mine ventilation problems. So, when analyzing the temperature field in the large network of mine workings, we need to solve the following equation for each mine working [10]:

$$\rho c\left(S{\displaystyle \frac{\partial T}{\partial t}}+Q{\displaystyle \frac{\partial T}{\partial x}}\right)=\lambda S{\displaystyle \frac{{\partial }^{2}T}{\partial x^2}}+\alpha P\left(T_m-T\right)+W$$

(8)

where $\rho$ is the air density, kg/m³; $c$ is the specific heat capacity of the air, J/(kg·°C); $S$ is the cross-sectional area of the mine working, m²; $T$ is the air temperature, °C; $\lambda$ is the effective thermal conductivity, W/(m·°C); $\alpha$ is the heat transfer coefficient, W/(m²·°C); $T_m$ is the temperature of the surrounding rock mass, °C; $W$ is the heat source, W/m; $t$ is time, s; $x$ is the coordinate along the mine working under consideration.

Equation (8) is solved for each mine working No. $\mathrm{i}$, and the index $\mathrm{i}$ is omitted in (9) for simplicity. Airflows ${\mathrm{Q}}_{\mathrm{i}}$ are determined from the solution of the system of equations representing the first and second laws of Kirchhoff [10]. Equation (8) is applicable to describe heat transfer processes only if there are no intensive sources of mass transfer (evaporation or condensation of moisture). If there are such sources, Equation (8) should be written in terms of specific enthalpy and supplemented with the moisture content balance equation [42,43].

The rock mass wall temperature, as shown in Equation (8), is generally a function of time and can be determined by solving the equation for unsteady-state heat diffusion in the rock mass. Similar models are described in several studies [13,15], including previous work by the authors of this paper [10].

The number of mining equipment units, their type and position in the ventilation network of the mine are constantly changing as mining progresses. This is reflected in the design documents for the development of a particular mineral deposit. To quickly determine how the temperature field changes in the system of mine workings, it is more convenient to select the required position on the ventilation model and set the typical value of heat release (or heat release coefficient). Using the data described in Section 3, it becomes possible to set the value of $\mathrm{W}$ in (8) if information is known about a particular mining equipment located in the mine. The operation mode of the equipment (no-load running, full power, etc.) can also be taken into account by introducing Formula (3). In our opinion, this approach is very convenient for developing models of mine ventilation networks.

Taking into account specific operating modes of mining equipment in some cases will require additional experimental studies, since the heat emission coefficients determined in this study were obtained for full load conditions. We assume that in most cases, taking into account the fact that mining equipment does not operate at full capacity can be carried out using the formulas proposed in our work by appropriately selecting the values of $N_{ice}$, $N_e$, etc.

Another important aspect is the parametrization of models of ventilation networks for the designed mines. In this case, it is not possible to conduct experimental measurements and determine the parameters of the model. The tabular values of the $K_e$ coefficients will partially solve this problem regarding technogenic heat sources. Here, when parametrizing the model, it will also be necessary to know many other parameters (air resistance of mine workings, heat transfer coefficients, etc.), but these issues are not the subject of this study. For designed mines, these parameters are also determined based on tabular data [44,45].

5. Conclusions

This study presents the results of experimental measurements of the temperature and relative humidity of air used to ventilate the workings of various types of mines. We demonstrate that the formation of microclimatic parameters of the air in underground working areas is significantly influenced by local sources of heat generation by mechanized equipment with ICEs or EEs (e.g., LHD, cutter–loaders, drilling rigs, conveyors, and auxiliary fans).

A coefficient of heat release is introduced, which is equal to the ratio of heat released into the mine atmosphere to the total amount of heat released during fuel combustion (for ICEs) or to the rated power of the mechanized equipment (for EEs). The values of heat power and the heat release coefficient were calculated using the experimental data for various mining equipment units (28 units in total).

We identify that there are significant differences between the values of the coefficients of heat release for equipment with EEs and ICEs. If for equipment with an internal combustion engine, the heat release coefficient is on average 0.3, then for equipment with EEs, it is 0.058. This is interpreted to be due to the fact that ICEs have lower efficiency factors than EEs.

A low heat release coefficient is also noted for conveyor drives operating from the electrical network (on average, 0.063). In addition to this point source of heat, it is important to consider that the conveyor belt itself is a distributed source of heat.

The results of this study have important implications for determining the power of heat release from local sources, i.e., mechanized equipment used for underground mining. The study outcomes can be used to provide parametric support for a mathematical model that allows for adequate simulation of the effects of operating mechanized equipment on the thermal regime in the working areas of the mines.