ASHRAE HVAC APPLICATIONS SI CH 29-2015 MINE VENTILATION AND AIR CONDITIONING.pdf

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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 27C Worker efficiency 100%27 twb 29C Economic range for acclimatized workers29 twb 33C Safety factor range; corrective action required33C 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 25.5 and29C wb, depending on governmental regulation, air velocity, andexpected metabolic heat generation rate of workers. Specifying thereject temperature is one of the first step

13、s 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 ventilat

14、ion 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 25

15、00 to 3000 m,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 h

16、eat. 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 inclu

19、de 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 removes

20、 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 betwe

21、enthe 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 becom

22、esimportant when evaluating surface versus underground plants.The preparation of this chapter is assigned to TC 9.2, Industrial AirConditioning.29.2 2015 ASHRAE HandbookHVAC Applications (SI)Coefficient of performance (COP) usually is defined as theevaporator duty divided by the work of compression

23、in similar 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 hois

24、t the 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 drif

25、t does 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 many metres long) or room-and-pillar (multipleproduction faces in a grid of rooms with supporting pillars inbetween) metho

26、ds.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 there i

27、s no heat interchange withthe shaft and no evaporation of moisture.For dry air at standard conditions (15C at 101.325 kPa), the spe-cific heat at constant pressure cpis 1.004 kJ/(kgK). For most work,cpcan be assumed constant, but extreme conditions might warrant amore precise calculation: 1 kJ is ad

28、ded (for descending airflow) orsubtracted (for ascending airflow) to each kilogram of air for every102 m. The dry-bulb temperature change is 1/(1.004 102 1) =0.00977 K per metre or 1 K per 102 m of elevation. The specific heatfor water vapor is 1.884 kJ/(kgK). So, for constant air/vapor mix-tures, t

29、he change in dry-bulb temperature is (1 + W)/(1.004 +1.884W) per 102 m of elevation, where W is the humidity ratio inkilograms of water per kilogram 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

30、 energy equation:q = QEd (1)whereq = theoretical heat of autocompression, WQ = airflow in shaft, m3/s = air density, kg/m3E = energy added per unit distance of elevation change, 1 kJ/(102 mkg)d = elevation change, mExample 1. What is the equivalent heat load from adiabatic compression of140 m3/s at

31、1.12 kg/m3density flowing down a 1500 m shaft?Solution:q = (140)(1.12)(1/102)(1500) = 2306 kWThe adiabatic compression process is seldom truly adiabatic:autocompression is a more appropriate term. Other heating or cool-ing sources, such as shaft wall rock, introduction of groundwater orwater sprayed

32、 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, for the following reasons:The effe

33、ct 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 moisture in the shaft, which suppre

34、sses 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 1.4 K wet bulb per 300 m, and is muchless sensitive to evaporati

35、on 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 themine air. Energy expended in pumps

36、, 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 the nameplate and a utilization fa

37、ctor. For example, a100 kW electric loader operated at 80% of nameplate for 12 h a dayliberates (100 kJ/s)(12 h)(3600 s/h)(0.80) = 3 456 000 kJ/day.Dividing by 24 h/day gives an average heat load over the day of144 000 kJ/h. During the 12 h the loader is operating, the heat loadis doubled to 288 000

38、 kJ/h. The dilemma for the ventilation engi-neer is that, if heat loads are projected at the 144 000 rate, the stopetemperature will exceed the reject temperature for half the day, andthe stope will be overventilated for the other half; if projected at288 000 kJ/h, the stope will be greatly overvent

39、ilated when theloader is not present. Current practice is to accept the additional heatload while the loader is present. Operators get some relief when theyleave the heading to dump rock, at which time the ventilation systemcan partially purge the heading.Diesel equipment dissipates about 90% of the

40、 heat value of thefuel consumed, or 35 000 kJ/L, to the air as heat (Bossard 1982).The heat flow rate is about three times higher for a diesel enginethan for an equivalent electric motor. If the same 100 kW loaderdiscussed previously were diesel powered, the heat would aver-age about 475 000 kJ/h ov

41、er the day, and 950 000 kJ/h duringactual loader operation. Both sensible and latent heat componentsof the air are increased because combustion produces watervapor. If a wet scrubber is used, exhaust gases are cooled by adi-abatic saturation and the latent heat component increases evenfurther.Fans r

42、aise the air temperature about 0.25 K per kPa static pres-sure. Pressures up to 2.5 kPa are common in mine ventilation. Thisis detrimental only when fans are located on the intake side of workareas or circuits.GroundwaterTransport of heat by groundwater has the largest variance in mineheat loads, ra

43、nging 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. Thus, hot drain watershould be stopped at its source or contained in pipelines

44、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 from surrounding rock decreases. In oneMontana mine, water in an open ditch was 2

45、2 K 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 Ventilation and Air Conditioning 29.3of the rock, depresses the dry-bulb temper

46、ature 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 1.26 L/s and 52C. If thewater enters the shaft sump at 29C, what is the rate of heat transfer tothe air?Solution:Heat rate = (1.26 L/s)(1 kg/L)

47、4.1868 kJ/(kgK)(52 29C)= 121.33 kWWall 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 5700C. Heatflows from the core to the surface at an average of 0.07 W/m2. Theimplication for mine engineers is that a geothermal g

48、radient exists:rock gets warmer as the mine deepens. The actual gradient variesfrom approximately 1 to over 7 K per 100 m of depth, depending onthe thermal conductivity of local rock. Table 1 gives depths and max-imum virgin rock temperatures (VRTs) for various mining dis-tricts. Table 2 gives therm

49、al conductivities and diffusivities for rocktypes commonly found in mining. These two variables are requiredfor 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 instantaneous or average heat flux rate.The instantaneous rate is recommended because it is better used forolder tunnels