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Mining Rule of Thumb

Mining Rules of Thumb have been gathered over 30 years of hard rock mining service provided by Stantec / Mining and predecessor firms.

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About the rules

The primary usage of Rules of Thumb should be in the development of conceptual designs and feasibility studies or, when a quick decision is required in the solution of an operating problem. Although an approximated answer, derived from a Rule of Thumb may solve an immediate problem, Rules of Thumb are not a substitute for the application of sound engineering and design methodologies. Although we firmly believe that the presented Rules of Thumb provide great continuing value to our industry, McIntosh Engineering does not guarantee their validity, nor do we (or the referenced individual sources) accept responsibility for application of the Rules of Thumb by others. Where possible, direct quotes have been provided from individual references; however, it is possible that referenced sources may not have directly stated the Rule of Thumb for which they are assigned credit. Although we have endeavored to accurately quote all individual references contained in the Rules of Thumb compilation, we apologize in advance for any misquotes that may be attributed to individual sources. We will provide updates to the Rules of Thumb compilation, as we become aware of corrections that may be necessary.

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Field: Ventilation and Air Conditioning

Area: Air Conditioning and Refrigeration

  • In the Republic of South Africa, cooling is required when the natural rock temperature reaches the temperature of the human body (98.6 degrees F). A.W.T. Barenbrug
  • A rough approximation of the cooling capacity required for a hot mine in North America is that the TR required per ton mined per day is 0.025 times the difference between the natural rock temperature (VRT) and 95 degrees F. For example, a 2,000 ton per day mine with a VRT of 140 degrees F. at the mean mining depth will require approximately 0.025 x 45 x 2,000 = 2,250 TR. Jack de la Vergne
  • The cold well (surge tank) for chilled surface water should have a capacity equal to the consumption of one shift underground. J. van der Walt
  • At the Homestake mine, the cost of mechanical refrigeration was approximately equal to the cost of ventilation. John Marks

Area: Air Surveys

  • For a barometric survey, the correction factor for altitude may be assumed to be 1.11 kPa/100m (13.6 inches water gage per thousand feet). J.H. Quilliam

Area: Airways

  • The maximum practical velocity for ventilation air in a circular concrete production shaft equipped with fixed (rigid) guides is 2,500 fpm (12.7m/s). Richard Masuda
  • The economic velocity for ventilation air in a circular concrete production shaft equipped with fixed (rigid) guides is 2,400 fpm (12m/s). If the shaft incorporates a man-way compartment (ladder way) the economic velocity is reduced to about 1,400 fpm (7m/s). A.W.T. Barenbrug
  • The maximum velocity that should be contemplated for ventilation air in a circular concrete production shaft equipped with rope guides is 2,000 fpm and the recommended maximum relative velocity between skips and airflow is 6,000 fpm. Malcom McPherson
  • The "not-to-exceed" velocity for ventilation air in a bald circular concrete ventilation shaft is 4,000 fpm (20m/s). Malcom McPherson
  • The typical velocity for ventilation air in a bald circular concrete ventilation shaft or a bored raise is in the order of 3,200 fpm (16m/s) to be economical and the friction factor, k, is normally between 20 and 25. Jack de la Vergne
  • The typical velocity for ventilation air in a large raw (unlined) ventilation raise or shaft is in the order of 2,200 fpm (11m/s) to be economical and the friction factor, k, is typically between 60 and 75. Jack de la Vergne
  • The typical range of ventilation air velocities found in a conveyor decline or drift is between 500 and 1,000 fpm. It is higher if the flow is in the direction of conveyor travel and is lower against it. Floyd Bossard
  • The maximum velocity at draw points and dumps is 1,200 fpm (6m/s) to avoid dust entrainment. John Shilabeer
  • A protuberance into a smooth airway will typically provide four to five times the resistance to airflow as will an indent of the same dimensions. van den Bosch and Drummond
  • The friction factor, k, is theoretically constant for the same roughness of wall in an airway, regardless of its size. In fact, the factor is slightly decreased when the cross-section is large. George Stewart

Area: Clearing Smoke

  • The fumes from blasting operations cannot be removed from a stope or heading at a ventilation velocity less than 25 fpm (0.13m/s). A 30% higher air velocity is normally required to clear a stope. At least a 100% higher velocity is required to efficiently clear a long heading. William Meakin
  • The outlet of a ventilation duct in a development heading should be advanced to within 20 duct diameters of the face to ensure it is properly swept with fresh air. J.P. Vergunst
  • For sinking shallow shafts, the minimum return air velocity to clear smoke in a reasonable period of time is 50 fpm (0.25m/s). Richard Masuda
  • For sinking deep shafts, the minimum return air velocity to clear smoke in a reasonable period of time is 100 fpm (0.50m/s). Jack de la Vergne
  • For sinking very deep shafts, it is usually not practical to wait for smoke to clear. Normally, the first bucket of men returning to the bottom is lowered (rapidly) through the smoke. Morris Medd

Area: Ducts

  • For bag duct, limiting static pressure to approximately 8 inches water gage will restrict leakage to a reasonable level. Bart Gilbert
  • The head loss of ventilation air flowing around a corner in a duct is reduced to 10% of the velocity head with good design. For bends up to 30 degrees, a standard circular arc elbow is satisfactory. For bends over 30 degrees, the radius of curvature of the elbow should be three times the diameter of the duct unless turning vanes inside the duct are employed. H.S. Fowler
  • The flow of ventilation air in a duct that is contracted will remain stable because the air-flow velocity is accelerating. The flow of ventilation air in a duct that is enlarged in size will be unstable unless the expansion is abrupt (high head loss) or it is coned at an angle of not more than 10 degrees (low head loss). H. S. Fowler

Area: Fans

  • Increasing fan speed by 10% may increase the quantity of air by 10%, but the power requirement will increase by 33%. Chris Hall
  • The proper design of an evasée (fan outlet) requires that the angle of divergence not exceed 7 degrees. William Kennedy

Area: General

  • An underground trackless mine may require 10 tons of fresh air to be circulated for each ton of ore extracted. The hottest and deepest mines may use up to 20 tons of air for each ton of ore mined. Northern Miner Press
  • A factor of 100 cfm per ore-ton mined per day can be used to determine preliminary ventilation quantity requirements for most underground mining methods. Hot mines using ventilation air for cooling and mines with heavy diesel equipment usage require more air. Uranium mines require significantly higher ventilation quantities, up to 500 cfm per ton per day. Block cave and large-scale room and pillar mining operations require significantly lower ventilation quantities, in the range of 20 to 40 cfm per ton per day for preliminary calculations. Scott McIntosh
  • Ventilation is typically responsible for 40% of an underground mine's electrical power consumption. CANMET
  • If the exhaust airway is remote from the fresh air entry, approximately 85% of the fresh air will reach the intended destinations. If the exhaust airway is near to the fresh air entry, this can be reduced to 75%, or less. The losses are mainly due to leaks in ducts, bulkheads, and ventilation doors. Jack de la Vergne
  • Mine Resistance - For purposes of preliminary calculations, the resistance across the mine workings between main airway terminals underground (shafts, raises, air drifts, etc.) may be taken equal to one-inch water gauge. Richard Masuda
  • Natural pressure may be estimated at 0.03 inches of water gage per 10 degrees Fahrenheit difference per 100 feet difference in elevation (at standard air density). Robert Peele

Area: Heat Load

  • The lowest accident rates have been related to men working at temperatures below 70 degrees F and the highest to temperatures of 80 degrees and over. MSHA
  • Auto compression raises the dry bulb temperature of air by about 1 degree Celsius for every 100m the air travels down a dry shaft. (Less in a wet shaft.) The wet bulb temperature rises by approximately half this amount. Various
  • At depths greater than 2,000m, the heat load (due to auto compression) in the incoming air presents a severe problem. At these depths, refrigeration is required to remove the heat load in the fresh air as well as to remove the geothermal heat pick-up. Noel Joughin
  • At a rock temperature of 50 degrees Celsius, the heat load into a room and pillar stope is about 2.5 kW per square meter of face. Noel Joughin
  • In a hot mine, the heat generated by the wall rocks of permanent airways decays exponentially with time – after several months it is nearly zero. There remains some heat generated in permanent horizontal airways due to friction between the air and the walls. Jack de la Vergne
  • A diesel engine produces 200 cubic feet of exhaust gases per Lb. of fuel burned and consumption is approximately 0.45 Lb. of fuel per horsepower-hour. Caterpillar and others
  • Normally, the diesel engine on an LHD unit does not run at full load capacity (horsepower rating); it is more in the region of 50%, on average. In practice, all the power produced by the diesel engines of a mobile equipment fleet is converted into heat and each horsepower utilized produces heat equivalent to 42.4 BTU per minute. A.W.T.Barenbrug
  • The heat load from an underground truck or LHD is approximately 2.6 times as much for a diesel engine drive as it is for electric. John Marks
  • The efficiency of a diesel engine can be as high as 40% at rated RPM and full load, while that of an electric motor to replace it is as high as 96% at full load capacity. In both cases, the efficiency is reduced when operating at less than full load. Various
  • Normally, the electric motor on an underground ventilation fan is sized to run at near full load capacity and it is running 100% of the time. In practice, all the power produced by the electric motor of a booster fan or development heading fan is converted into heat and each horsepower (33,000 foot-Lb./minute) produces heat equivalent to 42.4 BTU per minute. (1 BTU = 778 foot-Lbs.) Jack de la Vergne
  • Normally, the electric motor on a surface ventilation fan is sized to run at near full load capacity and it is running 100% of the time. In practice, about 60% of the power produced by the electric motors of all the surface ventilation fans (intake and exhaust) is used to overcome friction in the intake airways and mine workings (final exhaust airways are not considered). Each horsepower lost to friction (i.e. static head) is converted into heat underground. Jack de la Vergne
  • Heat generated by electrically powered machinery underground is equal to the total power minus the motive power absorbed in useful work. The only energy consumed by electric motors that does not result in heat is that expended in work against gravity, such as hoisting, conveying up grade, or pumping to a higher elevation. Laird and Harris

Area: Mine Air Heating

  • To avoid icing during winter months, a downcast hoisting shaft should have the air heated to at least 50C. (410 F.). A fresh air raise needs only 1.50C. (350 F.). Julian Kresowaty
  • When calculating the efficiency of heat transfer in a mine air heater, the following efficiencies may be assumed.
    90% for a direct fired heater using propane, natural gas or electricity
    80% for indirect heat transfer using fuel oil
    Various
  • When the mine air is heated directly, it is important to maintain a minimum air stream velocity of approximately 2,400 fpm across the burners for efficient heat transfer. If the burners are equipped with combustion fans, lower air speeds (1,000 fpm) can be used. Andy Pitz
  • When the mine air is heated electrically, it is important to maintain a minimum air stream velocity of 400 fpm across the heaters. Otherwise, the elements will overheat and can burn out. Ed Summers