COMPUTER SIMULATION. 1. History of Computer Simulation in Ventilation 1

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1 12 COMPUTER SIMULATION The use of computers is having a phenomenal effect on the mining industry. One of the most exciting trends in computer applications is the explosive development of microcomputers beginning around the mid 1970s. Today, more microcomputers and desktop computers are around than terminals. One reason for this increased popularity of microcomputers is their growing computing power and decreasing cost. It is now possible to put a mainframe computer which took up much of an entire floor in the 1960s on every desktop. Today, microcomputers are commonly equipped with 32 to 64 megabytes of RAM (or more), 500 megabytes or even up to several gigabytes of disk storage, and a sophisticated operating system that permits multiple, simultaneous tasks, and multiple users when networked. Using powerful PCs, more and more computer applications in recent years have found their way into the field of environmental simulation (Edwards and Greuer, 1982; Deliac, 1985; Abbas and Scheck, 1991; Greuer and Laage, 1994), interactive network analysis (Agioutantis and Topuz, 1985; Srivastava, et al., 1995; McDaniel and Laage, 1996), monitoring, and centralized control (Firganek, et al., 1975; Tominaga and Umeki, 1991). In ventilation, a small mine with few working faces, problems are simple and easily solved by choosing the appropriate fan. In larger, more complex mines the problem of supplying the working faces with fresh air is beyond the scope of "finger in the wind" analysis. In today's climate of stiffer government regulation and mining company's concern for safety, computer programs which analyze ventilation networks are helping mining companies satisfy their needs. 1. History of Computer Simulation in Ventilation 1 Ventilation network calculations have been performed for several centuries. Due to mathematical difficulties caused by diagonal airways, the preferred method had been a trial and error approach in which junction and mesh equations were made compatible. Then a large number of methods of successive approximation were developed. These methods, while surprisingly efficient in one instance, can become frustratingly inefficient in another. Atkinson's solution in 1854 for a single diagonal airway and Cross's method (1936), with its general applicability and simplicity, became the two most widely known examples. Some of the methods were based on the linearization of the quadratic resistance equation and used in electric analog computers. Practically all of the methods were tested for their utility with digital computers when these became available (Laage, et al., 1995). 1 A portion of this section was written by Dr. Rudolf E. Greuer, Professor of Department of Mining Engineering, Michigan Technological University, Houghton, Michigan, with references cited by the author. 215

2 The use of computers to simulate a mine ventilation system dates to the early days of the electric analogue, which derived from the works using fluid flow models. The electric analogue, based on the similarity between node and mesh equations in ventilation networks with Kirchhoff's laws of electric networks, was a large set-up. This consisted of several resistors, rheostats, voltage sources, and radio valves and was used to simulate the performance of a ventilation network. This analogue method reportedly was used by Pavlovsky in 1918 to study the seepage of water. The first patent for using electric analog computer for water and gas networks was awarded in 1941 in Germany. This computer used filament bulb resistance to model the second power resistance function in the network calculations. Its first use in mining was reported in Germany in 1952, and in 1954 in the U.S. (McElroy, 1954). In , the University of Nottingham (UK) pioneered the idea of combining an electric network simulation of the nodes and mesh equations of ventilation networks with a manual approximation method for the resistance equation. This led to the design of the commercially available "National Coal Board Network Analyzer," which found a wide distribution. Home-built models and modifications, using different approximation methods for the resistance equation, were used in almost all mining countries (Laage, et al., 1995). An electromechanical analog computer, in which the approximation of the resistance equation was automatically performed, was first developed in 1950 at a German coal mine and became commercially available in Thirteen of these computers were installed in German coal mines, and a larger number were installed abroad, for mining as well as gas and water companies. In 1959, electronic function generators for the simulation of the resistance equation were introduced in Japan; in 1960 the German manufacturer adopted this principle. In 1964, a British model became commercially available and may be the only one still on the market. In 1962, the French built an electronic model which was used for several decades in French coal mines (Laage, et al., 1995). Although powerful ventilation planning tools, fully-automatic analog computers are single purpose machines. All-purpose digital computers became commercially available in the late 1950s and, predictably, replaced the majority of the analog computers. The first network calculations with digital computers were performed for waterworks in the U.S. in The first digital ventilation network calculations were reported in Belgium in 1958 and in Germany in Following the lead of gas and water companies, efforts to replace the expensive analog computers with digital computers began in Germany in The same coal company which had pioneered the use of electromechanical analog computers performed almost all of their network calculations on digital computers by the end of By the end of 1969, the majority of analog computer users, representing 80% of the German coal production, had switched to digital computers. From 1961 on, it became customary to include natural ventilation obtained from information on temperatures and elevations in every mesh. Fan characteristics were treated in different ways as storage allowed. A FORTRAN version of a type of standard program became a part of the IBM program library in 1966; in 1967 it was adopted by the British National Coal Board for ventilation planning purposes at its divisional computer centers. It has been used for instructional purposes at Michigan Technological University (MTU) since Although many enhancements and attempts at improvements were made, it basically is still in its original form and is the core of the MFIRE program (Greuer and Laage, 1994). As the availability of digital computers increased, the number of users doing creative work in ventilation planning increased tremendously Unfortunately, much of the work went unreported because of personal, societal, or company constraints. Some of the work that was reported 216

3 included: an advanced program capable of performing high speed calculations for large networks in use in France in 1961; a storage saving program based on the Cross method of flow rate balancing introduced in 1967; network calculations with digital computers started in 1961 in Japan; convergence-improving mesh assemblies reported in 1969; in Russia, attempts with digital computers made in 1963; in 1965 and 1967, reports on different approximation methods; in Great Britain, the first network calculations with the meshes assembled manually reported in 1964; a program with automatic mesh assembly described in 1965; and in the United States, the first program to prove the usefulness of digital computers described in 1963 and an improved version allowing the inclusion of fan characteristics reported in Both these programs still required the manual assembly of meshes. Some of the other earlier papers in this area include those by Hartman and Trafton, 1963; Wang and Hartman 1967; Wang and Saperstein, 1970; Stefanko and Ramani, 1972 and 1973; Barnes, 1975; McPherson 1974 and 1976; Hall, Unsted and Lintott, 1976; and Tien in Many other papers have been written since then and can be referred to at the end of this chapter. As of today, the three most commonly encountered network calculation programs in the U.S. are (1) The Michigan Tech. Program, originally developed in Germany in the 1950s. (After revision and addition by R.E. Greuer and X.T. Chang in the 70s and 80s, this became known as the MFIRE, Version 2.2 program.); (2) The Penn. State Simulator, originally developed by Y.J. Wang in the late 1960s, and widely used in the field; And (3) M.J. McPherson, originally from the United Kingdom, wrote a simulation program in the mid to late 1960s, later turning it into VNETPC. The latest version of this is the VNETPC for Windows, Version 1.0. Other programs also were developed in Japan, the former U.S.S.R., France, and South Africa. By 1970, several of these models were available. Over the past two decades efforts focused on: (1) replacing the Cross method with more efficient approximation methods; (2) combining network calculation for optimization purposes with operations research approaches; (3) making the programs more user friendly, in particular by using interactive graphics; (4) combining network calculations with temperature and concentration calculations; and (5) extending network calculations to transient state conditions. The first objective has been a continual goal since the first days of digital computer use. So far, all results seem to confirm that the Cross method for networks of ordinary size and complexity is as good or better than other methods. The second objective is a very valid one since network calculations are only a means to an end. The third objective is probably the most important one. Efforts to combine ventilation network calculations with the precalculation of temperatures and humidities started in Japan in An early program which included temperatures, humidity, methane and dust concentrations, plus a transient state methane simulator, originated at the University of Pittsburgh in In 1975, at the First International Mine Ventilation Congress, reports were seen on four programs from the U.S. and Great Britain for a combined network, temperature and humidity calculations. At the Third Congress in 1984, a program for temperature, humidity, and radon concentrations was introduced from Australia. Litigation connected with the Sunshine mine fire in the mid-1970s showed that existing programs could only partially simulate the interaction of mine fires and ventilation systems. Although manual non-steady-state temperature precalculations had become a common feature, and steadystate fume concentrations were easy to add as long as no recirculation occurred, manual insertions of thermal draft and throttling effects proved to be cumbersome, and the handling of recirculation to be impossible (Greuer, 1977 & 1979). This led to the development of a new program at MTU in 1975 and The goals of this 217

4 program were to determine the equilibrium between fires and ventilation systems in steady-state conditions at any given time. The crucial heat exchange between rock and air was calculated under non-steady-state conditions. The program was based on mass flow rates and considered natural ventilation in all meshes and throttling effects in all airways. Airflow reversal and fume recirculation also were calculated. This program, sometimes referred to as the MTU/BOM code, was the first building block of MFIRE (Greuer and Laage, 1994). 2. Principles of Network Simulation Currently, there are well established models for ventilation network simulation, as well as for the study of other parameters in mine ventilation. They enable the ventilation engineer to simulate several system alternatives and select the most efficient and cost effective ventilation system. All these programs are based on the Hardy Cross iteration method, which basically is a series of successive approximations based upon two Kirchhoff's laws for electrical circuits and the Atkinson's Equation (Cross, 1936). These laws state that continuity of flow must hold at junctions (conservation of mass), that total change in pressure in a closed circuit must be zero (continuity of potential), and that airflow follows the relationship of head loss equals resistance times that quantity squared, H = RQ 2 (Atkinson's Equation). Recent developments in micro- and minicomputers have further facilitated input/output, real time simulation (Pomroy and Laage, 1988; McDaniel and Wallace, 1997), user interaction, and user-friendliness (Greuer and Laage, 1994; Hartcastle, et al., 1997). They have enabled ventilation engineers to further extend their application into evaluation of underground refrigeration, environmental simulation, and others. Ventilation problems that can be solved by the computer program include: solving complex ventilation systems with many entries, shafts, fans, and working faces; fan selection; determining optimal fan settings for efficient operation; determining the amount of regulation required to control airflow; determining the effect of air leakage on the overall system; selecting optimal fan locations; and, determining possible effects of improvements to airways, such as cleaning rock falls, smoothing airways, and other means of decreasing airway resistance. Progress in the development of hardware and software will continue apace. There will be further improvements in the computing abilities (speed and memory capacity) of personal computers and the power of today's supercomputers will be available in the office machines of tomorrow. One of the most significant recent advances has been the advent of the parallel processor computer. The current generation of machines utilizes a single microchip processor through which passes all of the calculations in sequence; the binary data being transferred from and to memory as directed by the program. Even with the enhanced speed of modern computer chips, the single processor has become a bottleneck. The computing time for one iteration of the complete network is the time taken for every mesh (a closed loop formed by several airways in the network) to pass through the processor, one after the other. The corresponding time on a parallel processor will be reduced to that required for one iteration on the largest mesh only. For example, on current personal computers, a 500-branch (airway) network typically may take several minutes to analyze. On a parallel processor this will be reduced to a few seconds. (McPherson, 1988). The next generation of network programs will provide powerful optimization features. In addition to giving distributions of airflow, pressure drops, airpower losses, etc., such programs will 218

5 give direct advice on the duties and locations of fans and regulators, and the recommended sizes of proposed new arterial airways. Embryotic but practical versions of this type of application for selecting the optimum combination of fans and regulators are already at the testing stage. While ventilation network analysis programs, and variations on them, have dominated software development and application, other programs also continue to be produced. These increasingly will enable ventilation engineers to conduct sophisticated analysis, including shaft design (Rose and Bluhm, 1992; Greyvenstein, et al., 1992), gas drainage system analysis (Zuber, 1997), and air conditioning configurations (Partyka and Koxzkodaj, 1987; Rose and Bluhm, 1992; Marks, 1994). All these will help in working toward the ultimate goal ventilation automation. 3. Post Survey Calculations Computer analysis of an existing ventilation system requires accurate input data which are developed only by detailed ventilation pressure air quantity surveys. For new mine projections, the recommended practice is to use friction values obtained from operating mines in the same area under similar conditions so that pressure loss calculations are representative. 1) Air Quantity Air quantity is obtained by multiplying corrected air velocity (by using calibration chart) and cross-sectional area (after subtracting body area). 2) Air Pressure Air in any section of an airway possesses energy by virtue of the static pressure under which it exists, its elevation above certain datum level of potential energy, and additional velocity energy if it is in motion (Equation 4-1, H T = H s + H v + H z ). By carefully choosing measuring locations where air velocity is less than 400 fpm, the effect of air velocity can be ignored ( 3.6: Air Pressure Measurement). For temperatures of 70 F or less, air density corrections can be neglected without significantly affecting final outcome, since the difference between dry and saturated air density under such temperatures is less than 1%. The remaining pressure would be the combination of static pressure and potential pressure ( 3.6: Air Pressure Measurement). There is another critical element in pressure reading: the effect of surface barometric pressure. The effect of barometric pressure fluctuations on underground pressure systems is instant and significant (McIntosh, 1957; Stevenson and Kingery, 1966; Bruzewski and Aughenbaugh, 1977; Anon., 1988; Neethling, 1989). It has been reported that a powerful thunderstorm has been capable of causing a short term rate of change as high as in. Hg per hour (Francart and Beiter, 1997). To determine true ventilation pressure, this element must be corrected. Figure 12-1 shows a close correlation between the pressure fluctuation underground and barometric change in one year (Kennedy, 1989). 219

6 30 25 Baormeter Underground Time (1,000 hours) Figure Correlation between pressure fluctuation underground and barometric change in a period of one year. Many forms are available for recording pressure survey data. A simplified pressure recording form is shown in Figure Station Surface Time Instrument Reading (ft) Instrument Change (ft) (1) - (1)s Barometric Change Correction (ft) Base Change Base Rdg. (3) - (3)s 1s 3s 5s Elevation Change Correction (ft) Elev. Change (5) - (5)s Ventilating Pressure Air Col, ft In. W.G. (2)-(4)-(6) (7)x(-0.014) Figure A Simplified sample pressure survey form. 220

7 3) Constructing schematics A schematic is a simplified line diagram of the mine. A ventilation network can be viewed as a big plumbing system. The main intakes and exhausts are plotted first, followed by the horizontal connections between the two and, finally, by the raises and ramps between the horizontal levels. Junctions are established wherever two or more branches connect. All branches must have a junction in and out, and each junction must have at least two connecting branches. After the network is plumbed together, the fans and regulators are placed in their respective positions. Constructing the network for an existing mine is fairly easy and straightforward. Any complex circuit can be reduced to one or more branches between the surface and the fan, but avoid over-simplification. Any branch in the mine network can consist of one or more nonisolated entries and still be represented as a single line on the schematic, as shown in a four-unit coal mine in Fig Unit-3 Unit-2 Slope Intake Airshaft Unit-4 Mine Fan Intake Airflow Neutral Airflow Unit-1 Return Airflow Figure Schematic showing a four-unit coal mine. Network construction for a new mine is undertaken after completion of the mining plan and the initial ventilation planning. By this time, the airflow requirements have been specified and layout of the shafts and levels completed. The network is then put together in much the same manner as with an existing mine. Since fans and regulators do not yet exist, these are placed at "best guess" locations initially, and then checked by subsequent simulations. The designer must keep in mind any possible future expansions. The network for a new mine tends to have fewer branches than the network for an existing mine. Selecting the optimum number of branches may be the most delicate part of constructing a network. Too few will not simulate the mine properly and too many will prove cumbersome. Accuracy is not necessarily a function of the number of branches. Some rules-of-thumb for normal branch selection are as follows (Marks and Deen, 1993): The total number of branches must be smaller than the number allowed by the program. Enough capacity should be left for running simulations. If more branches are desired, some of the airways might have to be combined, or the dimension arrays within the program modified. If the designer does not have access to the computer code, the program supplier should be 221

8 approached for a custom version. An individual branch should be input if it carries 1% of the total mine airflow. For example, if a mine is ventilated by 1,000,000 cfm, branches containing more than 10,000 cfm should be input. Branches with less airflow can be grouped into series/parallel sets. It is sometimes best to split single, long, vertical branches of more than 2,000 feet into two or more branches for more accurate thermodynamics and psychrometrics; specifically, to assist in resistance or natural ventilation pressure (NVP) calculations. Density changes are non-linear with depth, and heat transfer and evaporation/condensation are functions of site-specific conditions. Individual branches are often placed where a special process is taking place, such as a significant heat addition, air-conditioning process, or gas inflow. Fans are often located in their own branches. Special branches are used on the tops of exhausts to dissipate heat to atmosphere and to assist in natural ventilation pressure calculations. Special branches also can account for hardto-measure leakage, and can be placed to check the effects of circuit changes on neutral points. Additional branches may be placed in areas of intense activity, or to set up or aid future simulations. Junctions are then systematically numbered by one to several digit numbers, with the surface a common number. In addition, maximum flexibility also should be given for updating the schematic as the ventilation layout or conditions change since each segment of the ventilation system can easily be identified. The final schematic usually is a trade-off between simplified computer input and requirements for a user oriented output. Most importantly, the network is left as closely as possible in the form of the line diagram to maintain a resolution of the various airways. Evaluation of the quantity flow in any branch in the system is then simply a matter of checking the computer output. The air quantities and pressure readings calculated previously now are transferred to the schematic for resistance calculations. Different colors usually are used to differentiate between the two readings. 4) Resistance Calculations Once the air quantity and pressure information are transferred on a schematic map, the resistance for each airway branch in the schematic can be calculated using R = H/Q 2, where H is the pressure in inches of W.G. and Q is the quantity in 10,000 cfm (for instance, Q = 0.65 means Q = 65,000 cfm; see 4.6). Figure 12-4 is a suggested form for such a purpose. Branch Length No. of Starting, Ending, Ave Q, Gain/ R K in ft. Entries Q, cfm Q, cfm in cfm losses factor Figure Table used to calculate mine resistance and K values. 222

9 Keep in mind that altimeters could easily be misread up to 2-ft and errors in elevation can account for an additional 5-ft or more, plus the base altimeter error. Therefore, pressure values could be off as much as 0.1 in. W.G. Pressure readings which are completely out of character for a given location in the mine usually are disregarded entirely, and other readings are averaged to provide a replacement. Also, the quantity of air coming into a mine based on field measurement will rarely equal the quantity exiting the mine, even if allowances are made for methane or compressed air input to the system. The reason for the apparent discrepancies is the accuracy of the indicated quantities, which in turn are based on the accuracy of the individual velocity and area measurements. Even with measurements following the guidelines described in the previous section on quantity measurement, this discrepancy can be minimized, but not eliminated. As a result, the ventilation system must be balanced carefully so that it will satisfy the law of flow continuity: the quantity in equals the quantity out. Realizing also that pressure readings were possibly taken on different days, minor adjustments derived from the common point readings should be applied. 5) Simulation Input Simulation input is prepared using field measurements, complemented by mine records, references, and measured or manufacturer's data on fan curves. Depending on the particular program used, input format could vary, but the basic features usually required in the simulation are branch data, fan data, junction data, and/or special data. Branch Data Depending on the program used and the options desired, branch data can include the type (normal, fixed quantity, fan, airflow inject or reject, dummy, atmospheric), the junction connections in and out (starting and ending), calculated resistance (friction and/or shock losses), or values generated by the program based on the airway's physical characteristics (length, shape, K-factor), and whether or not natural ventilation pressures are present. Of these variables, the resistance is the most critical. The resistance at actual density, R, must be input. For most programs, the actual resistance is taken directly from field measurements or it is derived from knowing the resistance at standard density, R s, and the average branch density. The usual method for obtaining R s is to look up a K- factor at standard density in references such as McElroy. K-factors should, however, be derived from actual measurements from similar openings whenever possible. McElroy's K-factors, measured in the 1920s and 30s, have been found to be slightly high when compared to today's mines, mainly due to the smaller airways used at the time since the K-factor is not merely a function of airway roughness but also of the ratio of airway roughness to the diameter of the opening. The actual resistance can be calculated after obtaining resistance values at standard air density (either calculated using Equation 4-10 or directly from field measurement): R = R s w (12-1) where w = average density, w i +w e 2, lb/ft 3. In VNETPC, using imperial units, the branch pressure drop is reported in milli-inches of water 223

10 and the quantity in kcfm (cfm/1,000). Resistance therefore must follow suit. Resistance is input in "practical units", (milli-inches water per kcfm 2 ). The exponent is avoided. To convert resistance units from regular Atkinson's units, R s, to practical units, R PU : R PU = (0.1)(R s x ) (12-2) As an example, a drift has a resistance value of 2 x in.min 2 /ft 6. The resistance in practical units would be 0.2 milli-inches water per kcfm 2. The Michigan Tech program works on a mass flow basis, and reports the output quantity as cfm at a reference density. Three densities and three resistance therefore are involved: w s = standard density, lb/ft 3 ; w = actual density; w R = reference density; R s = resistance at standard density; R = resistance at actual density; and = resistance at reference density. R R The resistance input to the computer is the reference density, and this is calculated as: R R = R s w R w 2 (12-3) Computer runs also are used for existing mines to derive the equivalent resistance of a complex set of individual branches in series/parallel combination. This must be done before the network is ready to be used for ventilation planning because existing mines may contain thousands of individual airways and it would be far too cumbersome to include all of them in a computer network. When a set of series/parallel branches is to be replaced by a single equivalent branch, the network should be run and the resistance assigned to the equivalent branch modified until the simulated branch airflow matches the actual measured airflow. The network then can be used in normal manner, as long as changes within the set of series/parallel branches are not undertaken. Junction Data This includes elevations and, in some programs, air state point data (temperatures and densities). Junction data are usually required only in programs that calculate NVP internally. Fan Data For each fan, there should be a fan identification number, branch location (where the fan is located), and information on the fan curve (number of points, method of curve-fitting, and the pressure-quantity relationship). The user's manual of each program contains details on how to input fan curves. For most cases, ten points are recommended for each curve since, the more the points, the more accurately the program can re-create the curve. 224

11 All fan curves provided by fan manufacturing companies are plotted based on standard air density (0.075 lb/ft 3 ). If fans are used at locations that do not have standard density, fan curves must be adjusted accordingly. For the pressure-quantity relationship, the abscissa (quantity) is left constant and the pressure is adjusted by: P = P s w (12-4) If special simulations, such as fire modeling, are planned, fan curve input should extend into the second and fourth quadrants. Special Data This category of input is reserved for "real time" analyses. These simulate the spread of gas concentrations from sources such as a methane inflow or from a fire and the heat transfer between air and wallrock. The changing NVP effects caused by a fire on a ventilation system can be simulated. Besides checking system response to an unplanned event, these programs can help in specifying the locations of fire detection sensors, although much of this work is still in the validation stage (Greuer and Laage, 1994). 4. To Start A Simulation Once the network has been constructed and the data input, the network is ready to run. Because of the complexity of the ventilation network (for example, because every single stopping is a potential source for air leakage, resulting in another "airway" in parallel) a considerable number of undefined airways are to be expected. Consequently, there is always the possibility of either overor under-estimating the airway resistance values. Resistance modifications are thus necessary to determine actual representative values before actual planning. Often, the line between this data adjustment or resistance balancing and full use is hazy. A network usually is a continuallyevolving tool and must be updated regularly. Field Data Adjustment The logical way to correct this is to plug in the measured data and adjust the resistance figure by comparing the airflow distribution results with the actual field measurements. The user must acquire confidence in the program before specifying fans or calling for new airways. The procedure is demonstrated as follows: First Computer Printout: Airway R Airflow H, in. W.G , , , , Field Measurements Airway Airflow ,

12 , , ,000 R Values Are Then Changed to: Airway Formula Used Calculations New R 200 H R = Actual Q /(4.83) /(1.95) /(6.13) /(3.91) Second Computer Printouts with New R Values: Airway R Airflow H, in. W.G , , , , R Values Are Then Changed to: Airway Formula Used Calculations New R 200 H R = Actual Q /(4.83) /(1.95) /(6.13) /(3.91) Third Computer Printout with New R. Values Airway R Airflow H, in. W.G , , , , These modifications should be repeated until airflow falls into a reasonably close range to the airflow measured in the field. With an existing mine, the airflow predicted by the computer should come within 10% of the actual flows measured in the mine before the program is used for ventilation planning (Gaines, 1978). For major branches, even better accuracy is desired. A convenient expression for checking the balance of a network program is (Marks and Deen, 1993): 226

13 If S(Q PRED Q ACT ) SQ ACT 0.10, then the network is balanced well enough. where Q PRED = the computer-predicted quantity flowing in a branch. Q ACT = actual measured quantity in a branch. Branch resistance are adjusted to help attain balance. If the branch resistance in question were actually measured with good confidence underground, they should not be changed without a very compelling reason. They should be re-measured. Frequently, a problem with balancing involves the regulators in the circuit. The resistance of regulators should be measured with the gage and tube. If not, a number of computer runs might be necessary to converge on the actual resistance. Even when airflows balance well, the indicated fan pressure may not. The balancing period for a new network often involves trouble at the fan sites. Usually, fixed-quantity branches in the fan locations are used during this time, with the fan curves inputted later. If, for example, the airflows balance well, but the fixed-quantity pressures are low, certain branch resistances might be too low. A more common occurrence is when fixed quantity pressures are too high. This might imply that certain branch resistances are too high. Perhaps the NVPs haven't been calculated properly. Perhaps a couple of parallel branches were forgotten. Unaccounted for leakage can disrupt a network during the airway resistance adjusting period (Marks and Deen, 1993). Typically, a freshly-constructed network contains more branches than necessary. This is a legitimate step in the evolution of a network that should not be bypassed. During or after the break-in period, the number of branches can be reduced through the use of "equivalent" branches. However, too many equivalent branches tend to reduce the resolution of the network. 5. Use of the Program for Planning Computer network simulation provides two things: how the airflows are distributed throughout the network and how much pressure it takes to do the job. Results from a computer run can reveal where resistance bottlenecks are located and which branches might be subject to unacceptably high air velocities. These revelations, in turn, indicate how fans should be sized, and where regulators, parallel airways, or section boosters might be placed (Tien, 1976; Marks and Deen, 1993). Just as the name implies, it is only a simulation. The program simulates, based on the input provided, what the airflow and pressure distribution likely would be in the network. A medium size network containing, say, 800 branches will not be able to simulate with a high degree of confidence what a small rise in one of the mining sections actually will do. This is especially true when a good percentage of the network branches are "equivalent" branches, or single branches composed of a set of individual branches in series/parallel combinations. The larger the proposed change, the more accurate the projection (Marks and Deen, 1993). The potential effects of small changes are often best evaluated by means other than computer simulation. The computer is a wonderful tool for simulating "what if" scenarios based on the same set of data. Interpretation is just as important as the actual running of the simulation. The computer is merely a tool that tests the different hypotheses of the ventilation planner. The human is still responsible for the thinking component of ventilation planning. Assuming that all the bugs have been eradicated, computer-projected airflow balanced well 227

14 with actual measurements, and computer-projected operating points of all circuit fans closely approximating measured operating points, the network is ready for mine ventilation planning. It was stated earlier that the main purpose of network analysis is to test the hypotheses of the designer. A list of possible modeling jobs includes: New mines; New airways in existing mines; Stripping old airways to a larger cross-section; Closing off old airways; Paralleling existing airways; New fans, or new fan positions; Fan blade changes or speed changes; Removing existing fans; Expanding primary circuits; Downsizing primary circuits; Connecting with other mines or primary circuits; Testing the effects of heat loads or air conditioning on NVP; Projecting the spread of contaminants; Conducting fire preplanning exercises; Determining the best locations for fire detection sensors; Playing "what-if" (shafts cave in, fires, etc.); and Conducting sensitivity studies. 228

15 EMPIRICAL RESISTANCE VALUES Appendix ) Resistance Values Developed by MSHA The following table was calculated from field data by Denver Health and Safety Technology Center, Ventilation Division, MSHA. 2 Most of the data were obtained during routine pressure surveys in numerous coal mines over a period of 15 years. Table 12-1 Resistance Values for Underground Coal Mines. Type of Airway or Resistance Values, Ventilation Structure in. W.G. min 2 /ft 6 Intake entry, 100 ft Return entry, 100 ft Belt entry, 100 ft Main fan discharge stack 1/ 0.01 to 0.20 Leakage path-intake side of main fan 6 to Single masonry stopping 2/ 5x10 3 to 7x10 6 Single seal 10,000+ Leakage path-multiple stoppings 8 to 650 Single overcast to Longwall face, 100 ft to / For 72-in and larger diameter fans equipped with stacks specified by the manufacturer. 2/ For 7-ft high by 20-ft wide masonry stopping in good condition with no visible or audible leakage paths. The following summary further described MSHA's findings: a) Resistance for Intakes, Returns, and Belt Entries The friction factors found by experience to be the most useful are from the McElroy table for straight airways excavated in sediments (or coal). A K value of 50 is a good average for intake entries and a value of 75 for return entries, which typically are more obstructed than intake entries. A limited amount of MSHA data available on air-carrying belt entries showed a wide range of variation in K factors, from 75 to 275, with K = 150 being the average. b) Resistance for Mine Fans The discharge duct of a main fan used in the U.S. often is only 90 to 95% efficient in reducing the air velocity (the fans typically will expand 12 in. for every 80-in length, or an 2 Values are extracted from the paper presented by W. Bruce and T. Koenning at the 3rd U.S. Mine Ventilation Symposium, Penn. State University, Oct.,

16 angle of 4.29 ). It constitutes a significant resistance which needs to be accounted for in most ventilation systems. If the fan has no discharge duct, a duct other than specified for the fan, or if the fan diameter is smaller than 72 in., then the duct resistance must be computed from the velocity at the mouth, taking into account the inefficiency of the duct. Example 12-1: Calculate the resistance for a 60-in diameter fan equipped with a 160-in long duct with an 84-in diameter at the mouth of the duct, and having 150,000 cfm of air passing through the duct. Solution: To convert inches to feet, 84-in/12 = 7-ft Duct area at mouth =( 7 2 )2 x π = ( 3.5 ) 2 x = ft 2 Velocity at duct mouth = = 3,898 fpm or = 4,103 fpm at 95% efficiency Pressure loss in duct, H = ( )2 = in. W.G. Resistance, R = H Q 2 = = Which is outside the range shown in the table. c) Resistance to Simulate Mine Fan Leakage For an exhausting main fan, it is quite common to have substantial air leakage from the atmosphere into the return air stream in the vicinity of the intake ductwork and the connection to the return air entry. Such leakage should always be accounted for in computer simulation. The air leakage can be determined from the difference between the air quantity measured (using a pitot tube) within the fan and the air quantity measured just inside the mine. Once the leakage is quantified, resistance values R can be calculated using R = H Q 2 d) Resistance for Masonry Stoppings Values listed in the table are for individual masonry stoppings in good condition. If the stoppings have experienced some deformation, the individual stopping resistance may be as low as 1,000. Single seals will have a resistance value equivalent to the most resistive stoppings. Less substantial stoppings, or stoppings constructed of brattice cloth, may have resistance values in the range of 10 to 1,000. e) Resistance for Multiple Stoppings For simplicity, resistance values for stoppings while simulating are best grouped together and represented by one single resistance path. The values given in the table usually represent 10 to 20 stoppings. f) Resistance for Overcast Resistance values for a single overcast in the table. It is the average of 18 overcast values ranging from to 0.15, or

17 g) Resistance for Longwall Face Figures given in the table represent an average of nine operating longwall faces ranging from 0.19 to 6.62, with the average being The coal seams in the nine mines varied from about 48-in to 8-ft in height. The resistances in the table account for only the face resistance. 3 2) R-values for Illinois and Ohio Resistance values usually are expressed in per 1,000 feet. The following two sets of resistances are for two room-and-pillar operations, one located in Illinois and the other in Ohio. Mine X (Illinois) Intake Airways: 5 entries entries entries entries entry Return Airways: 5 entries entries entries entries entry Neutral Airways: Main entries Submain entries Panel entries Mine Y (Ohio) Mains Intake Airways (2-entries) Return Airways (2-entries) Neutral Airways (2-entries) Submains - Intake Airways (2-entries) Return Airways (2-entries) Panels - Neutral Airways (2 entries) Single Set of Overcast (2 in a row - regular type with 5-ft x 12-ft opening) Resistance Based on theoretical analysis and empirical data, it appears to have a significant amount of resistance present just upstream from the intake end of the longwall face and also downstream from the return end. 231

18 3) R-Values for Longwall Mining in Colorado The following longwall face resistance values and K-factors are based on a study of a Colorado coal mine having a 483-ft longwall face (divided into nine sections): Table 12-2: Resistance on the Longwall Face Sec. no. Length, ft R R/ft Remarks Stageloader & Electricals Maingate end of face Face widens Shearer Tailgate The above values are summarized as follows: Table 12-3: Summary of Longwall Face Resistance Stage Loader/Electricals Face Ends & Tailgate Face Line Calculated friction factors are tabulated as follows: Table 12-4: Friction Factors Sec. no. R K-factors The above values for a mechanized longwall face can be summarized as: Table 12-5: K-factors Condition K-factor Good 200 Normal 275 Rough

19 4) K Factor for Longwall System in Illinois In a study of longwall operations in the Illinois Basin (Jain and Mutmansky, 1987), the following values were suggested K factors: Intake airways: 60 x Return airways: 80 x Concrete-lined airshafts: 25 x Concrete airshafts with buntons: 93,000,000 x Stopping Leakage Factors (typical conditions measured in the field): For less than two-year old stoppings: 250 cfm/100 ft 2 /1 in. W.G. (or 20 x 10-3 m 3 /s per m 2 of stopping area per kpa of pressure difference across the stoppings). For more than two-year old stoppings: 500 cfm/100 ft 2 /1 in. W.G. (or 40 x 10-3 m 3 /s per m 2 of stopping area per kpa of pressure difference across the stoppings). The exponent of the leakage 4 calculation was assumed at a value of 1.43 for the stoppings. Resistance Value for Longwall Gobs: A suggested rule-of-thumb is: 10 x in. W.G./(cfm) 2 /100 ft of gob measured along the diagonal. The leakage exponent is assumed to be Experiments showed that stopping leakage increased with the pressure differential across the stopping according to the following relationship: Q = a P n where Q is air leakage per 100 sq ft stopping, in cfm; P is pressure differential, in inch water gage; a is air leakage at a 1-in. pressure differential, cfm; and n is the exponent which varied from 0.3 to 1.2, depending on the material which the air is leaking. 233

20 CALCULATING RESISTANCE VALUES Appendix ) Estimating Longwall Face Resistance a) Determine the length, L, mean cross-sectional area, A, and perimeter, O, of the face and estimate K-factor from Table Then determine face resistance values, R f using equation: R f = KOL 5.2A 3 b) For each face-end, determine the following shock loss factors where applicable. Sharp right angled bend: X b = 1.4 Sudden contraction at inlet end: X c = A Face-end Obstruction: X ob = [ 0.7(A a ) 1]2 where A is the cross-sectional area of face and a is the cross-section of obstruction facing the airflow. Note: For the majority of longwall faces in the US, the height of the face is the same as that of the airways. No allowance need be made for an expansion loss at the return end unless there is a concentration of equipment at this location. c) Use a shock loss factor of X shear = 4 for each shearer on the face. d) Sum all shock loss factors to give X Total e) Determine the equivalent resistance of the shock losses: R x = (6.211)(X Total ) A 2 f) Determine total resistance values for face: R = R f + R x g) The resistance of the main gate and tail gate should be determined separately, also from: R f = KOL 5.2A 3 using values of friction factor, K, appropriate to the conditions expected, and taking into account the location of equipment close to the face. 234

21 Having determined the full face resistance, including those adjoining lengths of main and tail gates that contain equipment, the resistance may then be used in network simulation for ventilation planning purposes. 2. Stopping Line Leakage For quick estimation purposes, the following figures for stopping leakage can be used: 3,000 cfm per 1,000 feet in main entries having 2 to 4 in. W.G. pressure differential across stopping line; 1,500 cfm per 1,000 feet in submains having 1 in. W.G. pressure differential across the stopping line. 3. Vent Pipe Resistance In metal and non-metal mines, ventilation tubings are usually used at the mining face to assist face ventilation. In the case of coal mining, vent tubing in and around the face area is not very common, probably due to space limitations. But it is quite common to use ventilation tubing during the slope sinking stage before the permanent ventilation circuit is established. Depending on the length of the slope, the tubing used sometimes can be quite long. There is a considerable amount of friction loss in long lines of any type duct, resulting in insufficient ventilation at the bottom of slope. Tubing manufacturers usually supply tubing friction data to help ventilation engineers select the right size tubing to deliver the required amount of air. In order to compensate for laboratory data, a correction factor should be used in calculating pipe resistance: For blowing system: multiply by 3 For exhausting system: multiply by 7, then use Peabody ABC 5 Resistance Chart For corrugated pipe: For a 3-in by 1-in corrugated pipe: reduce effective diameter by 1-in For a 3-in by 2-in corrugated pipe: reduce effective diameter by 2-in When there are obstructions in the airway: Effective Area = OA (A' ) where O = perimeter; A = Original area; and, A' = Actual area. 5 Peabody ABC, Warsaw, Indiana. 235

22 CURRENTLY AVAILABLE SIMULATION PROGRAMS Appendix 12-3 Most of the ventilation programs currently available provide an interactive, understandable interface with the user. They can be categorized into two major groups: mainframe ventilation programs and microcomputer ventilation programs. 1) Michigan Tech. Ventilation Program This program is derived from early versions of programs developed in West Germany in the 1960s. The program, using Hardy Cross iterative technique to balance the network, is very similar to the program of the British National Coal Board issued in Modifications made include identifications of airways, calculations of natural ventilation pressure, efficient fan curve approximations, and streamlined output. 2) MFIRE 2.2 Bureau of Mines This program is derived from early versions of programs developed at Michigan Technological University. It is currently in the microcomputer version and, like its predecessor, is capable of simulating routine mine ventilation network problems and also models the response of ventilation network under the influence of thermal disturbances, such as mine fires (both fuel rich and oxygen rich type) and cooling stations. Fires can be specified as a fixed source of output heat and products of combustion, or allowed to vary as a function of oxygen delivery. In addition, the program calculates the concentration, distribution, and propagation of contaminants in a ventilation system, such as fumes of a fire. Unlike previous network programs, MFIRE accommodates both steady and transient disturbances, including unplanned events, such as fires, and routine events, such as fans starting and stopping or doors opening and closing. The program has extremely flexible input requirements and output options. Natural ventilation, recirculation, and positive and negative thermal and mechanical energy inputs to the ventilation system are accommodated. Mass-based flow rates are utilized with reference to a known temperature and air density at a specified location. Methane evolution also can be specified. 3) Mine Ventilation Services, VNETPC 3.1, CLIMSIM This is an interactive network analysis program with graphic output provided for a pen plotter, but not on the CRT screen. Four methods of entering or calculating resistance are provided. VNETPC, which runs on a mainframe computer, is a similar program for large networks. The CLIMSIM program simulates variations in psychrometric conditions along any mine airway, shaft, or slope that affect climatic conditions in mines. Data requirements include airflow and wet bulb/dry bulb temperatures at inlet, airway geometry and age, rock thermal properties, and power and positions of equipment and cooling plant. Other programs available from Mine Ventilation Services include: NETCOR, correlation between measured airflow and computed airflow; PSYCHRO, psychrometric table or chart replacement; GASSIM, gas emissions and concentrations; BARSUR, thermodynamic relationships for pressure drops, airway resistance, and natural 236