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By Andrew Mackenzie, Paul Rantala

Data is now appearing in literature that indicates many current paste fill systems in Canada suffer from plugged lines and severe wear rates. Both of these conditions suggest: design problems with the pipe distribution system, backfill composed of poorly graded material and/or possibly poor operator practices. Problems that can be prevented in the design stage, but still may be readily solvable with current technology. However, if left untreated will lead to an increasing frequency of down time. The compatibility of the distribution system with the paste fill rheology is fundamental and essential for the reliable delivery of fill. Either many mine operators are not familiar enough with the technology to provide a critical review that could foresee problems down the road, prior to construction and or North American designers are not clarifying their designs to their customers. The objective of this paper is to highlight key concepts to mine operators with the purpose of better enabling a critical review of contractor's/consultant's designs. There is a great deal of understanding about the rheology of backfill over a range of pulp densities from hydraulic slurry to dense to paste. However, outside of deep mines (especially in South Africa) neither hydraulic or dense fill distribution systems have suffered the problems of paste fill. Paste fill is fundamentally different from hydraulic fill principally in its transportation characteristics. These differences have a direct impact on the design of the distribution system. By understanding the practical implications of these differences the operator can play a more active role in their design while better appreciating how to best operate their own system.

Jan 1, 2001

By E. R. Reeves

The mining industry has recognized the importance of engineering controls as a primary means of preventing hearing loss. The noise control treatments most commonly observed on drills and bolters during this study were windshields, sound absorbing material, and hydraulic motor covers. These controls were evaluated on machines at underground metal mines to determine the amount of noise reduction achieved by each control. The results indicate that absorbing material has very little effect on the noise level. The noise reduction attributable to the motor covers was dependent on the material used to create the cover. Properly installed windshields were the most consistent control.

Jan 1, 2004

By Birger Schmidt

INTRODUCTION Urban tunneling in soil poses a unique set of problems to planners, designers and contractors. One of the important concerns is the impact of tunneling on the environment: not only the effects of noise, vibrations, construction traffic and surface traffic diversions, but also the effects of ground movements and dewatering on existing structures and facilities. Underpinning or other protection of structures often consume more than 10% of the total construction cost of an urban rapid transit tunnel, and another unknown percentage is expended on construction procedures designed to minimize these environmental effects. Ideally, many basic planning and design decisions should be based on reliable predictions of ground movements due to tunneling. Some of these decisions are: the choice of tunnel alignment and depth to minimize risks and effects of ground movements; selection of schemes to protect existing structures through soil improvement or underpinning, or outright acquisition and possible' demolition; relocation of utilities; selection of procedures for construction monitoring and project control; and the choice of construction and lining methods. To make rational decisions of this nature, the owner and his designers must perform analyses that will balance risks and costs of a variety of alternatives; but to make meaningful risk analyses, they must employ rational methods of predicting ground movements and their effects and must know the reliability of these predictions. Of course, the risk analyses also must consider politics, public relations, insurance and liabilities.

Jan 1, 1974

By S. A. Kupferman

Cement raw materials used in southern California and Arizona cement plants occur in a wide variety of geologic environments. Limestone, the primary material making up 75-80% of the raw materials mix used in cement manufacture, is mined solely from Paleozoic Era deposits. During this time, 570-245 million years ago, vastly more limestone was formed in California and Arizona than at any other time (continent was in southern latitudes). The general geology of each limestone deposit associated with the eight southern California and two Arizona cement plants will be reviewed. Many of these deposits are geologically complex, having been subjected to Mesozoic deformation, metamorphism and instrusive events, along with Cenozoic uplift and deformation. All of the cement plants in this region require the use of secondary additives in order to meet the chemical requirements for cement. The kinds of additives vary greatly, depending upon the chemistry of the limestone deposit, cement specifications, and the cement manufacturing process. In some cases, clay deposits with suitable chemistry on or near the limestone sources are utilized. Where alkali limitations or need for alumina-rich materials exist, then specialty clay deposits are used. Iron-rich sources in the region include iron ore deposits and slag or mill scale from local steel mills. The geologic setting and location of these additives, along with gypsum and silica-rich deposits presently in use, will be mentioned.

Jan 1, 1987

By H. Colijn

Introduction Bulk commodity prices are ever increasing. This forces buyers and sellers in world markets to look more carefully at methods for obtaining an accurate accounting of commodity transactions in commerce and materials utilization in processes. Weighing and sampling inaccuracies can result in loss of income, product quality, and cost controls. So weighing and sampling accuracies are extremely important. They are the key components of managing the quantity and quality control aspects of bulk materials handling. This article will confine itself to the weighing systems and will deal with various aspects of system design, calibration, and operations. Emphasis will be placed on new developments and requirements in this field. Weighing methods To measure the gravimetric pull on a body being weighed (the load), the gravimetric force must be countered by an opposing force. Measuring this counter-balancing force thus becomes the measurement of weight. The conventional even-arm balance shows the concept of balancing an unknown mass with an equal known counterbalancing mass. But the loads encountered in industry generally range up to many thousands of pounds. So it is obvious that the even-arm approach is not practical. Thus, with the exception of direct load-bearing load cells, most of the weighing equipment uses some form of lever system. This is usually a combination of first- and second-class levers, to gain a mechanical advantage. These levers permit the load force to be counter-balanced and hence measured within a practical range. Lever systems not only perform this force-reducing action, they also serve to focus the load force at a single point, for convenient measurement. They align the force vertically in one direction so that measurement accuracy will not be affected by unrelated side forces. There are five basic methods of gravimetric weight measurements. That is, there are five basic types of opposing forces. • The first is the standard weight counterbalance. It uses a known weight to counterbalance the load directly or through a lever system.

Jan 9, 1988

Unless otherwise indicated, pounds are US avoirdupois, feet are US, and gallons are US.

Jan 1, 1985

By S. Harteis, D. Dolinar

Many mining operations rely on bulkheads to provide a barrier between impounded water or slurry and active mine workings. However, bulkhead failures could cause and have caused catastrophic flooding that puts the underground workforce at risk. The National Institute for Occupational Safety and Health1 (NIOSH) in collaboration with the Mine Safety and Health Administration (MSHA) is conducting research to evaluate the adequacy of existing design practices for water and slurry bulkheads. A key component of this research effort is documentation of the performance history and design parameters for bulkheads installed in underground coal mines during the last 20+ years. This research is part of a larger effort to develop general design guidelines and procedures for constructing and maintaining bulkheads that will help to ensure their long-term structural integrity, while significantly reducing the inundation risk for miners. This paper presents the results of NIOSH’s research to identify state-of-the-art bulkhead designs, including design criteria, leakage monitoring systems, and emergency warning systems. Underground observations and evaluations of existing bulkheads suggest that the most important design factors influencing their performance are the interface of the bulkhead with the surrounding strata and the potential magnitude of the hydraulic pressure to which they may be subjected. When a bulkhead has failed, leakage has generally been through the surrounding strata or along the bulkhead/strata interface, with the failure potential along the interface increasing with hydraulic head. It was also determined that development of a monitoring program to ensure the long term structural integrity of bulkheads is an important safety consideration. Current monitoring procedures range from weekly visual inspections to constant monitoring via pressure transducers and fluid level indicators. Where possible, these devices are used in conjunction with a computer based mine monitoring system to alert mine personnel when an emergency condition exists at the bulkhead installation.

Jan 1, 2006

By J. Wes Blakely

In the mid-1950s the importation of foreign oil began to encroach on traditional coal markets such as railroad steam engine fuel, home heating, electrical generating plants, and others. Many of the smaller coal companies with substantial coal reserves subsequently found that they could no longer continue to mine economically and merged in various ways with larger established coal mining concerns. As the seats of top management and authority were removed from the mines to large metro centers such as New York, Pittsburgh, and Chicago, the more personal relationship between mine workers and management that had existed prior to these mergers began to be eroded and replaced by " remote control " mine management. The disappearance of this face-to-face contact of upper management with mine employees tended to create barriers blocking simple effective communications between coal mine management and labor. In examining the human relations aspects of the coal mining industry, many elements, some concerned with work and others not, have depicted the industry as almost devoid of good applied human relations and communications practices, and, in truth, little has been done to alleviate this situation. In past years many coal producing companies, with the exception of one of the largest, failed to adequately define the term "communications" as it applied to their companies, whether internally or externally. Internally many still place heavy reliance on written communications such as company-wide newspapers to do the job of communicating with their employees which they feel ought to be done. One company puts an inordinate belief in its company "newspaper" as the primary instrument to stimulate its work force into increasing productivity. Primarily aimed at the workers, who greatly outnumber the salaried administrative people, this publication mainly documents the fortunes of individual mine workers who are felt to have achieved something with their lives, e.g., a mine worker who had managed to put four children through universities, the philosophy behind this being to inspire other workers to achieve worth- while goals.

Jan 1, 1981

By Miroslav Cervenka

The Coal, characterized by coal quality parameters as for example, ash content (Ad), moisture (Wrt), heat content (Qr1), sulphur content (Sd) and other parameters is still in many countries the basic source of energy. Energy is life, especially in modern society, where the demand for ecologically clean energy is very high; and therefore, there is great importance in effective coal utilization Direct utilization - burning of coal for energy generation, is still one of the most effective methods. In most cases, it is one of the cheapest energy transformation methods in spite of the fact that this energy transformation is connected with the deterioration of our surroundings. Deterioration can be substantially reduced if the burning of coal at power stations, heat power stations and other coal consumers is optimized. That means the burning is done in steam boilers, the technical state of which is very good; the coal has minimimum .variations with respect to the projected values; and the burning process is controlled and checked not only from a regulation point-of-view but also from an ecological point-of-view. The instrumentation capable of measuring the characteristic coal quality parameters in real time exists and it forms the basic assumption for the work our research team has made. The proper application of this instrumentation is able to provide information for burning process optimalization. It is the main topic of this paper, which is the compilation of the information on our work -made during the last five years. The whole information is based on practical experience and applied systems in Czechoslovakia in the mines and power engineering industry. See [Cervenka, 1989]

Jan 1, 1992

By Howard M. Cotton

Early in 1977, Amoco Production Company and Houston Chemicals, a subsidiary of Pittsburg Plate Glass (PPG) Industries, started producing iodine commercially in northwestern Oklahoma. This is significant because it is the first plant in over 40 years to be built in the United States with iodine as its sole commercial product. With the exception of a limited amount of iodine produced by Dow Chemical in Michigan, the United States is dependent upon imports from Chile and Japan for its iodine needs. The two million pounds of iodine produced annually from the Woodward plant will supply over one-third of the United States' requirements. Routine analysis of subsurface brines by the Formation Water Lab at the Amoco Production Company Research Center in Tulsa, Oklahoma, revealed unusually high concentrations of certain potentially economic minerals. Investigations determined that, of the minerals in question, iodine appeared to offer the best potential. The brines in question are located in the Morrow sands (basal Pennsylvanian) at a depth of 7,175 to 7.400 feet, in an area where Amoco has been active in a gas play for the past 20 years. The regional iodine concentration ranges from 10 to 700 ppm and averages around 300 ppm with the project area. The project is located 8 miles north and slightly east of the town of Woodward, where the well-developed, porous, and permeable sands of the Woodward trench contain large volumes of brine.

Jan 1, 1978

By David K. Parrish, Duane A. Labreche

Analyses of jointed, cracked, and fractured rock masses are needed in a variety of applications, including estimates of reduced elastic modulus for damaged rock, estimates of permeability changes as a function of stress changes, and estimates of relaxed regions due to excavation. A numerical model of a jointed rock mass, the compliant joint model, was used to study the response of a jointed rock mass near a circular excavation. The joint constitutive model was linear elastic in shear and nonlinear elastic in the relationship between normal stress and joint closure. Initial stresses were geostatic and representative of stresses in a granitic jointed rock mass approximately 300 m deep. The joint properties were based on values that are representative of joints in granite. The results from numerical simulations were compared with analytical results for stress near a circular opening in an elastic medium having reduced Young's Moduli to simulate jointing effects on the rock mass. The differences in response between the two models resulted from the differences in the symmetry of elastic properties, the shear modulus of the continua, and the constitutive relations in the two models. For example, the non- linear anisotropic elastic continuum described by the compliant joint model resulted in vertical stress concentrations near the side of the circular opening that were 25 percent higher than the vertical stress concentrations in an isotropic linear elastic medium. The equivalent Young's Moduli in the vertical and horizontal directions of the compliant joint continuum deviated by as much as 50 percent from the values assigned to the modeled region prior to excavations. The perturbations of the elastic Young's Moduli extended to distances of two to three radii from the opening.

Jan 1, 1986

By Andrew S. Lain

Before you begin mining aggregates in Oklahoma, you must first look at the market and ask these questions: (1) Is there a need for additional aggregate production? (2) Can we produce the aggregate efficiently enough to compete? (3) Is the deposit close enough to the market? If you can answer "yes" to each of these three questions, then you are ready to start the process for mining aggregates in Oklahoma. The first consideration to be made is whether the aggregates produced will need to meet state specifications. If you do not want to meet state durability standards, there are numerous formations you can mine and produce aggregates. We will deal only with the formations that will meet state durability standards. These formations are located in the southern half of Oklahoma and in the Tulsa area. Some of the formations that contain limestone, dolomite, and sandstone that will meet state durability tests are West Springs Creek, Kindblade, McKenzie Hill, Butterly-Royer, and Rush Springs Formations. Granite and rhyolite also are found in Oklahoma and can be used as construction aggregates. Once an area has been identified as having durable material, and drilling has verified its properties, a permit must be secured from the Oklahoma Department of Mines, Oklahoma Department of Environmental Quality, and any other applicable agency before construction of a plant occurs. There are many different brands of crushers available, but all work on one of two crushing principles-either compression breakage or impact breakage. Each type has its application in different industrial-mineral types and the desired end product. A typical small crushing plant would start with a jaw crusher for primary reduction, and then employ one standard cone and one shorthead cone for secondary reduction. One grizzly would be in the circuit with one finished-product screen and three to four finished-product stackers. Natural sands and gravels are found in rivers and old river channels. These too need to be tested to see if spec product can be produced. All of our sand operations have very little gravel; thus, we do not process the gravel. The sand is processed through classifying tanks for gradation.

Jan 1, 1999

By J. D. Miller, C. L. Lin

As the techniques and resolution for three-dimensional spatial analysis have advanced in the last decade, it is now possible to map in detail the microstructure of multiphase materials in three-dimensional digital space. In this regard, three-dimensional X-ray microtomography offers a unique imaging capability. Spatial resolution on the order of 10 µm can be achieved with the use of microfocus X-ray generators. Recently, a state-of-the-art cone beam X-ray microtomography system was installed at the University of Utah for the quantitative analysis of multiphase materials in three dimensions. This facility was designed to obtain 2,048 by 2,048-pixel reconstruction over a 10-mm diameter, while also allowing for the imaging of somewhat larger (40-mm) objects. The system is capable of handling high-density materials, even materials having a density as high as 8.0 g/cm3. This unique, one-of-a-kind instrument can be used to obtain three-dimensional spatial reconstruction for many applications, such as three-dimensional liberation analysis, pore structure analysis of particle beds during filtration and the air-void system of concrete structures. The utilization of X-ray microtomography not only allows for the quantitative analysis of multiphase systems but also allows, for the textural characterization and determination of phase continuity. This paper presents information regarding the current use of this new facility and a review of potential applications for the advanced analytical system. Key words: Cone beam X-ray microtomography, Three-dimensional analysis, Material characterization

Jan 1, 2003

By Anthony D. Hammond

INTRODUCTION A two-dimensional approach for modeling narrow-vein deposits using the TECHBASE® software was implemented at Sunshine Mining for its use in the evaluation of mineral properties. This 2-D approach prevents the overestimation of high-grade tons typically associated with the use of fixed 3-D blocks and reduces the size of the model in computer memory. The method consists of constructing a two-dimensional model oriented along the average plane of the mineralized structure. This approach works well for single, multiple and intersecting veins with the condition that each vein is roughly planar with variation in dip or strike less than 30" in order to maintain a reasonable resolution in the process of projecting data onto a plane (Figure 1). Also, a precise identification of the hanging-wall and foot-wall location is critical in ensuring an accurate representation of the vein. By using various modeling techniques, the two-dimensional model estimates metal content, thickness, tonnage, economic and dilution parameters which will be ultimately used in the identification of minable reserves. The following is a generic description of the steps involved in modeling a steeply dipping vein. DATA ENTRY A typical project database consists of drillhole coordinates, survey information, assay data and any other quantitative or qualitative data. This database can be created inside the software or imported from any spreadsheet. TECHBASE® utilizes the concept of "fields" and "records" (columns and rows) for storing data. The use of "calculated fields" enables the user to expand on the manipulation of the database with minimum memory requirements. Another important feature of TECHBASE® is the ability to replicate the structure and format of the "tables" for its use in another project and the capability of running a series of repetitive tasks by means of batch and "runlog" files.

Jan 1, 1998

By B. M. Sadashiva Murthy

Increasing quantities of nutrients and pesticides in agricultural runoff in recent years has caused the more developed western nations to become concerned about diffuse agricultural pollution. Thus, the problem of diffuse pollution is of greater importance and concern for any integrated water quality management programme. The above mentioned sources and activities causing pollution to groundwater cannot be completely avoided. The best solution is to entail a firm understanding of the principles of solute transport through porous media. The complex factors that control the movement of contaminants in porous media and the resulting behavior of contaminant plume are usually intricate to assess because of interaction of many factors such as dispersion, advection, sorption, degradation, volatilization, etc. As accurate quantification of solute under field conditions is inherently difficult, the fate of nitrate and phosphate have been analyzed in laboratory column and channel studies. From the analysis of the breakthrough curves, the dispersion co-efficient for nitrates and phosphates through sandy soil was found to be 0.262 m2/d and 0.114 m2/d for column studies, 0.321 m2/d and 0.231 m2/d for channel studies respectively. The column studies revealed that the concentration of nitrates and phosphates increased steadily in the first few days (around 5 days) and then it increased rapidly after 8 to 10 days. However, it attained equilibrium after 20 days. Similarly, the channel studies revealed a steady increase in concentration of nitrates and phosphates in the first few days (1 to 2 days) and rapid increase up to 15 days and there after it attained equilibrium. In the study, one-dimensional analytical model has been developed which include the advection, dispersion, and a characteristic term called ?total elimination rate?. The comparative study of column and channel studies, and model output (in which the term total elimination rate ?K? is considered as zero) exhibited a variation of 30 to 50 % in leaching characteristics of pollutants (nitrates and phosphates).

Jan 1, 2008

Compared to alternative ways of dealing with coal fines, oil agglomeration has always been a commercial bridesmaid and never yet a bride. Yet interest is high in the process, as shown by a number of pilot plants in operation that are funded by industry as well as government. In the past in the US, alternative ways to handle coal finer than 100 to 200 mesh included flotation, or no treatment except dewatering, or discarding the fines as waste. There are now a number of factors that tilt the economics more toward fine coal recovery. Today, the cost of mining a ton of underground coal runs from $22-30, and discarding even 10% as unrecovered fine coal is a real economic loss. A number of attempts (notably by EPRI) have been made to quantify the effect of coal cleaning on utility boiler operations. Transportation and ash handling costs, frequency of unscheduled outages, fouling and slagging problems, and boiler tube erosion are all lowered by cleaning coal. The advantage of coal cleaning on these parameters has been shown, but the dollar amount varies with many assumptions and is difficult to pin down. The message to a utility, however, has not been lost and the trend toward cleaner steam coal is accelerating. This may lead to fewer installations that only wash the coarse coal, and add back the unwashed fine coal until a maximum contractual ash limit is reached. And, as the proportion of coal fines continues to increase by continuous and longwall mining, it may become impossible to clean the coal sufficiently without cleaning the fines. Flotation and oil agglomeration are the only processes that presently deal directly with cleaning the very finest coal fractions. Flotation has proven its effectiveness in cleaning blackwater and in separating fine coal from mineral matter. It has the achievable potential for far greater pyrite rejection than is now obtained in commercial preparation plants. While flotation will continue to be the predominant method for fine coal cleaning, there are conditions and opportunities where oil agglomeration can be a

Jan 8, 1980

By Vernon R. Degner

The past two decades has seen the application of the mechanical flotation machine, originally developed primarily for metal1iferous ore beneficiation, increasingly applied to cleaning coal in the 28x0 T. Mesh size range. Internal cell hydrodynamic studies of mechanical flotation machines of all sizes, as well as computer aided modeling studies of flotation kinetics has, within the last five years, established a basis for optimizing coal flotation circuit design and machine operation, and has a1 so pointed a way toward improvements in mechanical flotation machine design for the future. Consideration of the trend toward increased fines produced by current mining techniques, as well as the environmental need for a progressively cleaner product, lends support to basing the design of the mechanical coal flotation machine of the future on finer feed; below 100 microns in size. Future types of mechanical flotation machines, which may satisfy the fine coal cleaning requirements for a significant improvement in product grade while maintaining the combustible recovery levels of current technology, are discussed.

Jan 1, 1988

By J. W. Stevenson

The need for a separate ventilation department within the management team was recognized by the management of U. S. Pipe and Foundry before the company was purchased by Jim Walter Corporation in January, 1970. Because the coal beds of the deep underground coal properties owned by Jim Walter Resources lay 1300 to 2200 feet below the surface, efficient mine ventilation practices are essential through shaft openings varying from 16 to 22 feet in diameter. The extremely high methane gas quantity indicated to be in the coal bed because of the depth of cover and documented by various U. S. Bureau of Mines tests and demonstrated by early mining proved the need to implement this early management decision. At U. S. Pipe and Foundry mine ventilation was a function of the Mining Engineering Department with ventilation personnel assigned to the mine receiving their day to day work assignments from the mine manager or mine foreman. At Jim Walter Resources, the General Manager - Ventilation reports to the Senior Vice President, Mining Engineering, who reports directly to the President, Mining Division, Figure 1 (Company Organization Chart). Position descriptions are static; however positions are dynamic and consequently the degree of emphasis of specific duties and responsibilities change depending not only upon the aptitude of the individual performing the job but also upon his ability to meet the needs of his supervisors; and ultimately upon his ability to react to the economic forces directing our society. The following describes the duties and responsibilities of the General Manager - Ventilation, Figure 2.

Jan 1, 1980

By V. D. Ayurov, N. O. Kaledina

The coal mines monitoring should be based on the usage of simple and evident models, letting in the total unity to reflect a structure of organization of mine, system ability of its construction and specific character of its functioning. Such models are offered to create on the basis of extracted hierarchies of objects of mine. All objects of mine are broken on levels of technological hierarchy. For objects of each level of hierarchy there are made balanced models of entering and leaving material flows. Then these balanced models are coordinated among themselves by the help of the inter hierarchical relations. So the structured-qualitative characteristic of the object of monitoring, reflecting in a kind of the table structure of formation of material flows on mine, their intensity and measure of transformation is formed under transition from one level of hierarchy of mine to another. An example of real us- age of hierarchical balanced models on mine is given. The mathematical models of coal mine monitoring should reflect objective laws of their structural organization and functioning. Therefore under the constructing of mathematical models it is important such a way to reflect a structure of the mine organization, that in totality the models has been reproduction of system ability of its construction. The knowledge of the separate parties of functioning of mine, reflected in particular mathematical models, should "to be packed" in combined unity. Or else, they should be connected in system, convenient for recognition by the person. Considering the mine, the expert allocates multi-dimensionality of the objects of making its elements. He always has the knowledge of laws of development of mining work that is connected with pithy stratification of researched object. In dependence of sparial-temporary level of abstracting there are allocated the horizon, wing of a mine field, layer, panel, extracting face, longwall in coal mine. The human thinking on each of these levels operates with a set of concepts peculiar to him, characteristics, definitions.

Jan 1, 1996

By George Aul, Richard Ray

Short-hole horizontal degasification systems have been utilized in Island Creek's Virginia Division mines since 1986. These systems remove methane from the coal seam in advance of longwall mining. Without an in-seam methane drainage program, methane liberation rates from the longwall faces may exceed 0.47 cu m/s (1,000 cfrn), requiring 47 cu m/s (100,000 cfm) of air for adequate dilution capability. Optimization of the degas systems has enabled methane liberation levels and the associated ventilation requirements on the longwall faces to be reduced by 50 to 80 percent. Since the inception of the first degas system at the Virginia Pocahontas No. 6 Mine, Island Creek has developed an overall methane drainage program designed to maximize gas production and minimize ventilation requirements for methane dilution. Gas production from the degas systems is affected by four main design criteria: (1) hole location, (2) hole length and spacing, (3) collection system capacity, and (4) time available for drainage. Drilling equipment and materials for the gas collection system are selected to meet the necessary criteria while minimizing manpower requirements. Benefits gained from reducing methane liberation rates from the longwall faces are fourfold: (1) improved safety due to lower methane concentrations in the face area, (2) fewer production delays caused by excessive methane concentrations, (3) lower fan operating costs resulting from reduced air requirements, and (4) enhanced worker comfort resulting from lower air velocities and less dust generation.

Jan 1, 1991