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Breath of fresh airWith no aspect of underground mine safety more fundamental than proper ventilation, various modern systems are being used to ensure airflow is safe for workersBY BREE FREEMANWhen it comes to underground mine workings, few things are more important than adequate ventilation. Every year enormous efforts are made to maintain and improve airflow to working areas. This is to dilute emissions below statutory limits, render the air harmless, carry away hazardous contaminants and provide necessary levels of oxygen for the miners. As a result, ventilation systems form a crucial pan of the design of any mine, the layout of which is determined by the orebody geology, legislation, available manpower, mining methods and the equipment used.Specific environmental issues such as virgin rock temperatures, spontaneous combustion and dust control further complicate theoretical ventilation systems. Moreover, whatever system is eventually planned will inevitably suffer from imperfect implementation and control because of the day-to-day complexities of the extraction operation.“Ventilation in the modem mine is an essential prerequisite, perhaps even more so than in the past.Apart from the obvious biological aspect of preventing depletion of oxygen and the build-up of carbon dioxide in the air, the mine-ventilation system must be able to keep quantities of contaminants below harmful levels,” says Mike Beare, principal mining engineer for consultancy SRK.He explains: “A modern mine has many sources of harmful substances, including fumes from explosives, dust from broken rock and gases present in the rocks themselves. in addition to these, the modem approach to mining (including the use of diesel-engine trucks and loaders) results in large quantities of CO, nitrous fumes and diesel-particulate matter entering the mine airflow. While scrubbers are fitted, they do not remove all the contaminants and the ventilation system has to be designed to be able to dilute these to safe levels.”These issues demonstrate the need for expert design, observes SRK, which also signals the need for ventilation costs to be minimised in the same way as any other cost aspect As a result, various production scenarios need to be addressed at the outset.Central to all of these calculations is the simple fact that airflow is determined by temperature and pressure differences, air flows from high-pressure to low-pressure areas . In a mine, It is caused by pressure differences between the intake and exhaust openings. Airflow follows a square-law relationship between volume and pressure - in order to double the volume of air, four times the pressure must be exerted.CREATING THE PRESSUREThere are two main types of fan:l Axial: these are generally high-volume, low-pressure fans, either directly driven by the motor shaft (with the motor inside the tube body) or remotely driven using belts (with the motor outside the tube body). These are generally adjustable for volume by setting the pitch of the adjustable blades on the rotor and, in some cases, motor speed can be tailored to adjust volume and pressure.l Centrifugal: these are generally high-pressure, low-volume fans that consist of a multi-bladed, squirrel-cage wheel in which the leading edge of the fan blades curves toward the direction of rotation. These fans have low space requirements, low tip speeds and are relatively quiet.AUXILIARY VENTILATIONThe ventilation of dead-end workplaces is the most frequent and Important application of auxiliary ventilation. It is used for both development and exploration work, as well as for production headings with only one entrance. A major inconvenience with any method of auxiliary ventilation during development is the necessity of frequent extension. The auxiliary airstream must be delivered as dose to the face as possible so that it can sweep away any impurities that have been generated. The two main methods of ventilating the faces of dead-end workplaces are erecting line-brattice (air entering on one side of the brattice and returning In the other side) and the installation of a fan, coupled with ventilation tubing.The practice of redirecting the main ventilation system with smaller, local fans is used where a line brattice is not adequate. Tubing, often suspended from timbers or roof bolts (if approved), carries the air to, or away from, the working face (tubing is rigid for exhaust systems and collapsible for forcing systems). This auxiliary ventilation system allows continuous miners to operate without being obstructed by brattice constructions.In addition, booster fans can be located in long airways to boost the airflow volume. These fans can be free-standing and used without using bulkheads.CIRCULATION CONTROLSMine-ventilation systems present a unique challenge in that the workfaces are normally moving away from the source of fresh air.This requires continuous changes to the ventilation system. These controls are needed to distribute the air underground, so that each working section Is ventilated with an adequate supply of fresh air.The various devices work collectively to direct the movement of the air through the main Intakes to the working section and move out through the returns without short-circuiting, which occurs when air from the intake goes directly into the return.ELIMINATING DIESEL EMISSIONSAs Mr Bear commented above, reducing harmful diesel emissions is another vital aspect of improving underground working conditions. But, while diesel-engine manufacturers and machine OEMs should be acknowledged for their combined achievements. In reducing pollutants, there would always be a percentage of emissions that must be strictly monitored, diluted and vented to the surface. What is needed is an alternative to diesel, and some people believe fuel cells could be the key.A 2003 study by the University of Nevada used data obtained from a survey sent to 173 US metal and non-metal underground mines. From a 61% return, the survey included 4,786 diesel units (totalling 478,200kW), collectively consuming about 68MI/y of diesel fuel.As of January 20, the Mine Safety and Health Administration (MSHA) enacted a regulation limiting total DPM (diesel particulate matter) emissions from diesel engines to an Interim 450Ngm of total carbon/cm of ambient air, and suggested it would be reduced further to 160pgm in January 2008.Extensive use of diesel-powered mobile equipment has resulted in the development of mining methods such as drift and fill stoping, which, in most circumstances, require dead-end auxiliary ventilation rather than through-flow ventilation. Since it can be harder and more expensive to ventilate these stopes, and meet the DPM regulations, most mines will have to modify their operating practices.Mines can exploit increased ventilation to help meet the DPM criteria, but this involves considerable costs since fan power Is proportional to cubic-air quantity, and purchasing and installing fans is very expensive.So, perhaps the use of total emission-free machines in our coal mines Is closer than we think.井下通风在矿业安全的方面没有比合适的通风更重要，为了矿工的安全各种各样的先进系统被用于保证风流的通畅。布利弗里曼的观点当谈到煤矿工作时，没有什么事比适当的通风更重要。我们每年都要做很多工作来维护和改进工作空间的风流条件。为了将放射物控制在一定浓度范围里，使空气清洁，使污染物受到控制，为矿工提供必需的新鲜空气。通风系统成为所有矿设计的一个关键因素，与煤炭储量及地质条件、立法、可利用的人力资源、开采的方法和使用的设备等有关。具体的环境问题如地温、自燃和粉尘等使通风系统进一步复杂化。 而且，不管什么系统最终由于渐渐复杂的操作都会不可避免地遭受不完美的实施和控制。“通风在煤矿是一个根本前提，这或许比从前更突出。从生理方面说除了防止空气中氧气量的减少和二氧化碳积累外，矿井通风系统必须能维持污染物的数量在有害的水平之下”， 咨询学校SRK主要采矿工程师麦克Beare说。他解释说： “一个现代矿井有许多危险源，包括由于爆破释放出的有害气体、生产过程中产生的粉尘和岩层本身释放的有毒气体。 除这些，还包括对于采矿过程中产生的大量的CO，亚硝烟和粉尘颗粒。 即使抽放系统合适，它们也不能去除所有污染物，所以通风系统必须被设计成能把这些污染物降低到安全水平”。SRK发现这些问题提出了对专业设计的要求，同时也发出信号需要将通风费用同其他费用一样降低到最小程度。因此，各种生产状况在最初都要考虑到。所有问题的核心是一个很简单的事实：气流取决于气压差，气流从高压地区流向低压地区。在煤矿上，它由井筒之间的压力差造成。 气流的风量和压差之间有一个二次方定律关系为了使风量加倍，必须使压差增加四倍。产生压差风机的二种主要类型： l轴向： 这些通常大风量，低风压的风机要么是被处在风机内部的电动机直接驱动要么是被处于风机外部的电动机通过皮带驱动。风量可以通过调整叶片的角度来调整，并且调整电动机的转速也可以调整风机的风量和风压。l离心： 这些通常高压差，低风量风机是由多片叶轮和螺旋式机壳组成。机壳的作用是吸集从叶轮中甩出的空气。这类风机占用空间小，低转速并且是相对较安静。辅助通风辅助通风系统的最频繁和最重要的作用是保证独头巷的通风，它应用于勘探和掘进，以及盲巷的生产过程中。辅助通风系统的主要不便之处在于生产过程中需要不断的向前延伸。 辅助通风系统必须向尽头输送足够的风量，来保证空气的清洁。 向工作面通风主要有两个方法：一是搭建临时建筑（风流从建筑的一边通过，从另一边返回），二是将局部通风机与风筒相连直接把风送到工作场所。当主通风系统满足不了通风需要时，局部通风机被用来弥补这种不足。风筒，通常布置在巷道的顶端，将新鲜风流送到工作面或将污风从工作面带走。这个辅助通风系统能保证连续的生产，从而避免了因为搭建临时建筑而被迫停止生产的情况。另外，局部通风机允许架设很长的通风路线。而且这些局扇可以独立使用二无需使用隔框。循环控制煤矿通风系统所面临的一个特殊的问题是工作面通常渐渐远离新鲜空气的来源处，这就决定了通风系统也要连续的发生变化。必须得采取措施来控制地下空气的流动从而来保证每个工作面都有充足的新鲜空气来供给。所有设备共同运转来保证新鲜风流从进风井进入，通过工作面最后由回风井排出，而不出现风流从进风井进入后直接从回风井排出的短路现象发生。防止柴油泄露Bear先生前面说过，减少有害的柴油泄露是改进地下工作环境的另一个重要方面。 但是，我们应该承认柴油引擎和机器制造者OEMs在减少污染物方面的成就。为了减少污染物，必须确切地监测到泄露、稀释和释放到地面的百分比。另一关键是选择对柴油，并且人民相信燃料电池可能能解决这个问题。内华达大学的一项2003年的研究使用了173家美国金属和非金属地下矿勘测得到的数据。 61%的数据显示，调查中共同消耗4,786个柴油单位(总计478,200kW)约合柴油68MI/y。在1月20日，矿安全卫生管理(MSHA)在2008年1月制定了限制从柴油引擎泄露到空气中的DPM (柴油粒状物质)的量，建议从450Ngm进一步将减少到160pgm。对柴油发动的流动设备的广泛的应用导致了采矿方法的发展，例如分层进路调顶崩矿采煤法和水平分层充填采矿法，在多数情况下，这都要求盲巷辅助通风而不是贯穿通风。 因为风流通过采空区，所以达到DPM的规定就更加的困难和昂贵，多数矿将必须修改他们的操作规程。煤矿可以通过增加风量来帮助符合DPM标准，但是这需要花费大量的金钱，因为风扇能力与空气体积是成比例的，并且购买和安装风扇是非常昂贵的。因此，我们认为或许在煤矿中使用无遗漏的机器更有可能实现。
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