1、29.1CHAPTER 29MINE VENTILATION AND AIR CONDITIONINGDefinitions 29.1Sources of Heat Entering Mine Air 29.2Heat Exchangers 29.4Mine-Cooling Techniques 29.7Selecting a Mine-Cooling Method . 29.9Mechanical Refrigeration Plants . 29.10Mine Air Heating 29.10Mine Ventilation . 29.11N underground mines, low
2、er worker productivity, illness, andIpotentially death can result from poor working environment con-ditions. It is therefore extremely important to design, install, andmanage underground ventilation systems with the necessary careand attention. Excess humidity, high temperatures, inadequate oxy-gen,
3、 and excessive concentrations of potentially dangerous gases cansignificantly affect the quality of the working environment if notproperly controlled. Ventilation and air cooling are needed in under-ground mines to minimize heat stress and remove contaminants. Asmines become deeper, heat removal and
4、 ventilation problemsbecome more difficult and costly to solve.Caution: This chapter presents only a very brief overview of theprinciples of mine ventilation planning. The person responsible forsuch planning should either be an experienced engineer, or workunder the direct supervision of such an eng
5、ineer. Several English-language texts have been written on mine ventilation since 1980(Bossard 1982; Hall 1981; Hartman et al. 1997; Hemp 1982; Ken-nedy 1996; McPherson 1993; Mine Ventilation Society of SouthAfrica 1982; Tien 1999). The ventilation engineer is stronglyencouraged to study these refer
6、ences.Special Warning: Certain industrial spaces may contain flam-mable, combustible, and/or toxic aerosol concentrations under eithernormal or abnormal conditions. In spaces such as these, there arelife-safety issues that this chapter may not completely address. Spe-cial precautions must be taken i
7、n accordance with requirements ofrecognized authorities such as the National Fire Protection Associ-ation (NFPA), the Occupational Safety and Health Administration(OSHA), and the American National Standards Institute (ANSI). Inall situations, engineers, designers, and installers who encounterconflic
8、ting codes and standards must defer to the code or standardthat best addresses and safeguards life safety.1. DEFINITIONSDefinitions specific to mine ventilation and air conditioning areas follows.Heat stress is a qualitative assessment of the work environmentbased on temperature, humidity, air veloc
9、ity, and radiant energy.Many heat stress indices have been proposed (see Chapter 9 of the2013 ASHRAE HandbookFundamentals for a thorough discus-sion); the most common in the mining industry are effective temper-ature (Hartman et al. 1997), air cooling power (Howes and Nixon1997), and wet-bulb temper
10、ature. The following wet-bulb tempera-ture ranges were derived from experience at several deep westernU.S. metal mines:twb 80F Worker efficiency 100%80 twb 85F Economic range for acclimatized workers85 twb 91F Safety factor range; corrective action required91F twbOnly short-duration work with adequa
11、te breaksHeat strain is the physiological response to heat stress. Effectsinclude sweating, increased heart rate, fatigue, cramps, and progres-sively worsening illness up to heat stroke. Individuals have differenttolerance levels for heat.Reject temperature, based on the heat stress/strain relations
12、hipis the wet-bulb temperature at which air should be rejected toexhaust or recooled. Reject temperature ranges between 78 and 85Fwb, depending on governmental regulation, air velocity, andexpected metabolic heat generation rate of workers. Specifying thereject temperature is one of the first steps
13、in planning air-condition-ing systems. The ventilation engineer must be able to justify thereject temperature to management because of the economicsinvolved. If too high, work productivity, health, safety, and moralesuffer; if too low, capital and operating costs become excessive.Critical ventilatio
14、n depth is the depth at which the air tempera-ture in the intake shaft rises to the reject temperature through auto-compression and shaft heat loads. Work areas below the criticalventilation depth rely totally on air conditioning to remove heat. Thecritical ventilation depth is reached at about 8000
15、 to 10,000 ft,depending on surface climate in the summer, geothermal gradient,and shaft heat loads such as pump systems.Base heat load is calculated at an infinite airflow at the rejecttemperature passing through the work area. The temperature of aninfinite airflow will not increase as air picks up
16、heat. Actual heatload is measured or calculated at the average stope temperature. It isalways greater than the base heat load because the average stope tem-perature is lower than the reject temperature. More heat is drawnfrom the wall rock. Marginal heat load is the difference betweenbase and actual
17、 heat loads. It is the penalty paid for using less than aninfinite airflow (i.e., the lower the airflow, the lower the inlet temper-ature required to maintain the reject temperature and the higher theheat load).Temperature-dependent heat sources (TDHs) depend on thetemperature difference between the
18、 source and air. Examplesinclude wall rock, broken rock, and fissure water (in a ditch or pipe).Temperature-independent heat sources (TIHs) depend only onthe energy input to a machine or device after the energy required toraise the potential energy of a substance, if any, is deducted. Exam-ples incl
19、ude electric motors, lights, substation losses, and the calo-rific value of diesel fuel.Passive thermal environmental control separates heat sourcesfrom ventilating airflows. Examples include insulating pipes andwall rock, and blocking off inactive areas. Active thermal environ-mental control remove
20、s heat via airflow and air conditioningquickly enough so that air temperature does not rise above the reject.Positional efficiency, an important design parameter for minecooling systems, is the cooling effect reaching the work area dividedby the machine evaporator duty. The greater the distance betw
21、eenthe machine and work area, the more heat that the cooling medium(air or water) picks up en route.Percent utilization is the ratio of the evaporator duty of the refrig-eration plant over a year in energy units to the duty if the plant hadworked the entire year at 100% load. This consideration beco
22、mesimportant when evaluating surface versus underground plants.The preparation of this chapter is assigned to TC 9.2, Industrial AirConditioning.29.2 2015 ASHRAE HandbookHVAC ApplicationsCoefficient of performance (COP) usually is defined as theevaporator duty divided by the work of compression in s
23、imilar units.In mines, the overall COP is used: the evaporator duty divided by allpower-consuming devices needed to deliver cooling to the worksites. This includes pumps and fans as well as refrigeration machinecompressors.A shaft is a vertical opening or steep incline equipped with skipsto hoist th
24、e ore, and cages (elevators) to move personnel and sup-plies. Electric cables and pipes for fresh water, compressed air, cool-ing water, pump water, and other utilities are installed in shafts.Drifts and tunnels are both horizontal openings; a tunnel opens todaylight on both ends, whereas a drift do
25、es not. In metal mining, astope is a production site where ore is actually mined. In coal min-ing, coal is usually produced by either longwall (one continuousproduction face hundreds of feet long) or room-and-pillar (multi-ple production faces in a grid of rooms with supporting pillars inbetween) me
26、thods.2. SOURCES OF HEAT ENTERING MINE AIRAdiabatic CompressionAir descending a shaft increases in pressure (because of the massof air above it) and thus also increases in temperature as if com-pressed in a compressor. This is because of conversion of potentialenergy to internal energy, even if ther
27、e is no heat interchange withthe shaft and no evaporation of moisture.For dry air at standard conditions (59F at 14.696 psia), the spe-cific heat at constant pressure cpis 0.24 Btu/lbF. For most work,cpcan be assumed constant, but extreme conditions might warrant amore precise calculation: 1 Btu is
28、added (for descending airflow) orsubtracted (for ascending airflow) to each pound of air for every778 ft. The dry-bulb temperature change is 1/(0.24 778 1) =0.00535F per foot, or 1F per 187 ft of elevation. The specific heatfor water vapor is 0.45 Btu/lbF. So, for constant air/vapor mix-tures, the c
29、hange in dry-bulb temperature is (1 + W )/(0.24 + 0.45W )per 778 ft of elevation, where W is the humidity ratio in pounds ofwater per pound of dry air.The theoretical heat load imposed on intake air by adiabatic com-pression is given in Equation (1), which is a simplified form of thegeneral energy e
30、quation:q = 60QEd (1)whereq = theoretical heat of autocompression, Btu/h60 = 60 min/hQ = airflow in shaft, cfm = air density, lb/ft3E = energy added per unit distance of elevation change, 1 Btu/778 ftlbd = elevation change, ftExample 1. What is the equivalent heat load from adiabatic compression of3
31、00,000 cfm at 0.070 lb/ft3density flowing down a 5000 ft shaft?Solution:q = (60)(300,000)(0.070)(1/778)(5000) = 8,097,686 Btu/hThe adiabatic compression process is seldom truly adiabatic:autocompression is a more appropriate term. Other heating or cool-ing sources, such as shaft wall rock, introduct
32、ion of groundwater orwater sprayed in the shaft to wet the guides, compressed-air andwater pipes, or electrical facilities, often mask the effects of adiabaticcompression. The actual temperature increase for air descending ashaft usually does not match the theoretical adiabatic temperatureincrease,
33、for the following reasons:The effect of seasonal and daily surface temperature fluctuations,such as cool night air on the rock or shaft lining (rock exhibitsthermal inertia, which absorbs and releases heat at different timesof the day)The temperature gradient of rock related to depthEvaporation of m
34、oisture in the shaft, which suppresses the dry-bulb temperature rise while increasing the moisture content of theairThe wet-bulb temperature lapse rate varies, depending on theentering temperature and humidity ratio, and the pressure drop inthe shaft. It averages about 2.5F wet bulb per 1000 ft, and
35、 is muchless sensitive to evaporation or condensation than the dry bulb.Electromechanical EquipmentElectric motors and diesel engines transfer heat to the air. Losscomponents of substations, electric input to devices such as lights,and all energy used on a horizontal plane appear as heat added to th
36、emine air. Energy expended in pumps, conveyors, and hoists toincrease the potential energy of a material does not appear as heat,after losses are deducted.Vehicles with electric drives, such as scoop-trams, trucks, andelectric-hydraulic drill jumbos, release heat into the mine at a rateequivalent to
37、 the nameplate and a utilization factor. For example, a150 hp electric loader operated at 80% of nameplate for 12 h a dayliberates (150 hp)(42.4 Btu/minhp)(0.80)(12 h/day)(60 min/h) =3,663,360 Btu/day. Dividing by 24 h/day gives an average heat loadover the day of 152,640 Btu/h. During the 12 h the
38、loader is operat-ing, the heat load is doubled to 305,280 Btu/h. The dilemma for theventilation engineer is that, if heat loads are projected at the152,640 rate, the stope temperature will exceed the reject tempera-ture for half the day, and the stope will be overventilated for theother half; if pro
39、jected at 305,380 Btu/h, the stope will be greatlyoverventilated when the loader is not present. Current practice is toaccept the additional heat load while the loader is present. Operatorsget some relief when they leave the heading to dump rock, at whichtime the ventilation system can partially pur
40、ge the heading.Diesel equipment dissipates about 90% of the heat value of thefuel consumed, or 125,000 Btu/gal, to the air as heat (Bossard1982). The heat flow rate is about three times higher for a dieselengine than for an equivalent electric motor. If the same 150 hploader discussed previously wer
41、e diesel powered, the heat wouldaverage about 458,000 Btu/h over the day, and 916,000 Btu/hduring actual loader operation. Both sensible and latent heatcomponents of the air are increased because combustion pro-duces water vapor. If a wet scrubber is used, exhaust gases arecooled by adiabatic satura
42、tion and the latent heat componentincreases even further.Fans raise the air temperature about 0.45F per in. of water staticpressure. Pressures up to 10 in. of water are common in mine ven-tilation. This is detrimental only when fans are located on the intakeside of work areas or circuits.Groundwater
43、Transport of heat by groundwater has the largest variance in mineheat loads, ranging from essentially zero to overwhelming values.Groundwater usually has the same temperature as the virgin rock.Ventilating airflows can pick up more heat from hot drain water inan uncovered ditch than from wall rock.
44、Thus, hot drain watershould be stopped at its source or contained in pipelines or in cov-ered ditches. Pipelines can be insulated, but the main goal is isolat-ing the hot water so that evaporation cannot occur. Heat release from open ditches increases in significance as air-ways age and heat flow fr
45、om surrounding rock decreases. In oneMontana mine, water in an open ditch was 40F cooler than whenit flowed out of the wall rock; the heat was transferred to the air.Evaporation of water from wall rock surfaces lowers the surfacetemperature of the rock, which increases the temperature gradientMine V
46、entilation and Air Conditioning 29.3of the rock, depresses the dry-bulb temperature of the air, andallows more heat to flow from the rock. Most of this extra heat isexpended in evaporation.Example 2. Water leaks from a rock fissure at 20 gpm and 125F. If thewater enters the shaft sump at 85F, what i
47、s the rate of heat transfer tothe air?Solution:Heat rate = (20 gpm)(60 min/h)(8.33 lb/gal)(1 Btu/lbF) (125 85F) = 399,840 Btu/hWall Rock Heat FlowWall rock is the main heat source in most deep mines. Tempera-ture at the earths core has been estimated to be about 10,300F. Heatflows from the core to t
48、he surface at an average of 0.022 Btu/hft2.The implication for mine engineers is that a geothermal gradientexists: rock gets warmer as the mine deepens. The actual gradientvaries from approximately 0.5 to over 4F per 100 ft of depth,depending on the thermal conductivity of local rock. Table 1 givesd
49、epths and maximum virgin rock temperatures (VRTs) for variousmining districts. Table 2 gives thermal conductivities and diffusivi-ties for rock types commonly found in mining. These two variablesare required for wall rock heat flow analysis.Wall rock heat flow is unsteady-state: it decays with time becauseof the insulating effect of cooled rock near the rock/air boundary.Equations exist for both cylindrical and planar openings, but this sec-tion discusses cylindrical equations (Goch and Patterson 1940). Themethod can solve for either instantaneo
