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JOURNAL OF THE MINE VENTILATION SOCIETY OF SOUTH AFRICA Published monthly by the Mine Ventilation Society of South Africa. Secretaries -- Associated Scientific and Technical Societies of South Africa. Kelvin House, 75 Marshall Street, Johannesburg. Telephone 834-1271 P.O. Box 9426 President Hon. Editor Hon. Assistant Editor Hon. Treasurer -- A. W. T. Barenbrug -- J. P. Rees -- R. Hemp -- C. W. Carew Contributions are welcome from members and non-members. The attention of authors is drawn to the Guide to Authors, conventional signs and abbreviations which appear in the Journal from time to time. The opinions expressed by contributors do not necessarily represent the official views of the Society. Volume 19 No. January 1966 Price 50 cents (5/-) DESIGN AND CALCULATION OF EXHAUST SYSTEMS FOR CONVEYOR BELTS, SCREENS AND CRUSHERS By R.D. Wright (Member)* SYNOPSIS In view of the high proportion of capital cost in dust control systems, which may be attributed to the ducting and hooding sec tion of the plant, the principles involved in calculating exhaust volumes must be made on a scientifically established basis. The hitherto acceptable method of applying ruleof-thumb control velocities must be re considered in the light of recognizing the part played by the air induced by the fall of materials and that where so called control velocities have been applied these cannot always be considered completely successful in controlling the dust emission, particularly where large quantities of smaller size ore is handled. Typical examples are given for the calculation of exhaust volumes using the air induction method and instances of exist ing installations are cited. Introduction When granular material falls, (through a chute, for example, on to a transfer con veyor or into a bin), each particle or lump of material imparts some momentum to the air surrounding the falling column of * Sydney, N.S.W., Australia. material in an identical manner to uni directional pulvation. On reaching the ter minal point, i.e. the bottom of the bin, the air streams out of all available openings as the bin is now under positive pressure by the induced air. This air, seeking to escape, carries with it the fine dust suspended in the air caused by the falling material. The dust thus caused may be defined as the finally divided solids which may become air borne from the original state without physi cal or chemical change, other than fracture. It becomes airborne as a result of disinte gration of lumps of the parent material, typically caused by crushing, grinding or from disturbance of already pulverized material, such as at belt transfer points and the like. The effect of material hitting the terminal point, referred to above, is termed pulvation. Hemeon defines pulvation as the pneumatic action that projects fine particles at high velocity from a coagulated state into the air of the immediate vicinity as indivi dual particles and is assisted in forming clouds of dust by the secondary induced air currents which transport the localized dusty air formed in the prior action, away from the sited formation. Journal of the Mine Ventilation Society of South Africa, January, 1966 1 Design and Calculation of Exhaust Systems for Conveyor Belts, Screens and Crushers This phenomenon of air induction may be seen in varying degrees wherever granular material is allowed to fall by gravity. Crushers, belt and bucket operators, vibrat ing screens, chutes, transfer points and bins are typical instances in material handling systems where induction may be observed. It is, moreover, at these points that the mining and quarrying industry must provide adequate means for the controlling of the dust created by the processes, movement, and disintegration of the ore being handled. Of the total costs involved in a typical dust control equipment installation on a crushing and screening house, for example, the collecting units themselves cost in the order of 40 per cent, of the total installed cost of the complete plant. The balance is made up of ducting, fans and hoods. It is therefore important that equal emphasis be placed on the design of the exhaust system as on the choice of the collectors involved. Comparatively little published data is available which gives the background to the reasons for applying various designs formu lae to the problems of materials handling. The original bases for the development of these formulae have been forgotton, and these rule of thumb methods are perpetuated in existing and often outdated codes of practice, publicity material issued by vendors of dust control equipment, and recom mended air volumes appearing in various Handbooks. These were developed many years ago, based on the practical experience, observation and testing of dust control systems, and determining their effectiveness and minimum air volumes required for adequate and effective containment of the dust and the necessary exhaust volumes were specified in terms of " Control Velocity ". Unfortunately, these figures have been applied without discrimination to a very wide variety of applications. No thought was given to the differing dust loadings, different materials, varying volumes of induced air caused by heights of fall, widths of conveyor and quantities of material, and the results have not been as successful in every case as they might have been. As most materials handling dust control systems consist largely of enclosing hoods, the design based on exhaust volumes calcu lated to obtain control velocities would appear to be reasonable. Usually, 150-200 f.p.m. was specified for screen enclosure openings, belt enclosures, crushers, etc. However, it is mandatory that some way of taking into account the air volume put into motion--i.e., by pulvation or secondary air motion caused by the material moving through the process--be found. This has been done in various ways, and reference to established works of reference show that the criteria for conveyor belt enclosures are given as 350-500 c.f.m. per ft of belt width, depending on belt speed. Bins have been specified as 0-5 c.f.m. of cu. volume, and screens 50 c.f.m. per sq. ft. of screen area. However, anomalies have occurred, as it is obvious that if a screen is totally enclosed and has no openings in the enclosures at all, the screen must still be exhausted at 50 c.f.m./sq. ft. of screen area, even though the control velocity criterion predicts a zero required flow. The original control velocity technique, being more often in error than not, proved unacceptable to both dust con trol contractor and client alike, and so the most common way of ensuring a successful installation resorted to was by increasing velocities to 400 and 500 f.p.m. depending on the operations. It must be obvious that the true concept of control velocities as such were no longer used. The new, higher velocities were introduced as design veloci ties required to produce sufficient total exhaust volumes to counteract the volumes and pressures produced within the enclo sures. When these volumes are effective, the true control velocity which exists in the downstream enclosure openings usually is in the order of 100 to 150 f.p.m. despite the design velocity of four to five times this amount. What is actually happening when the control is adequate, is that the air flow induced into the system by falling material is exhausted at the downstream point, and additional volume is pulled into the system at the downstream opening to give a mini mum and actual control velocity of 100-150 f.p.m. It therefore follows that the only proper method of specifying exhaust volumes for these operations hinges on the development of a technique for estimating the volume of air put into motion by materials falling through or into the hand ling system. 2 Journal of the Mine Ventilation Society of South Africa, January, 1966 Design and Calculation of Exhaust Systems for Conveyor Belts, Screens and Crushers Typical belt to belt transfer. Note: no exhaust at head of conveyor No. 1--A or tail of conveyor No. 2--B. This is a completely changed concept, but a logical one, when considered in relation to the induced air path following the material flow. Various theoretical analysis of the prob lem have been made in recent years, and attempts to relate horsepower transferred by falling particles due to drag, to the air resistance produced have been made, notably by Pring, Hemeon & Dennis. Unfortunately, this analysis has proved of little practical value, and empirical formulae developed therefrom have been found to omit some of the important variables. Further investigations have been made, however, and among other things a single important empirical equation has been developed relating to these (previously omitted) variables. Anderson simplified this to read: __ 10Au3RS2 U1 D Where Q1 = induced airflow in standard c.f.m. Au = is the enclosure opening upstream in square feet, (i.e. where the air is induced into the system by the action of the falling material). R = rate of material flow and tons/hr. S = height of fall in feet. D = average particle diameter of material, in feet. This equation has limitations, in that the particle diameter must be greater than , but it is a starting point from which every material transfer exhaust rate may be determined. It raises the design of such systems from the trial and error status too often exper ienced by purchasers of such equipment to a predictable and correct system of engineer ing, producing satisfactory results. The importance of this formula cannot be too strongly emphasized. Journal of the Mine Ventilation Society of South Africa, January, 1966 3 Design and Calculation of Exhaust Systems for Conveyor Belts, Screens and Crushers To illustrate the application of this Anderson formula, it must be advantageous to consider various possible applications and typical situations to facilitate the method used to calculate the exhaust volumes used. Belt-to-Belt and Chute-to-Belt Transfer (See illustration) For these operations, the basic equation may be used directly. There is some justifi cation for reducing the value of S (height of fall in feet) if step-like baffles are placed within the chute to break the fall of the material. Usually, however, such baffles do not stop the fall completely and only serve to produce more dust due to impact. The more important factor in the formula is Au. Induced air flow will be minimized in direct proportion to the degree of tightness of this enclosure opening. It has been found that, with careful design and by extending the enclosure so that it encloses both the feed and return belts at each pulley, it has been possible to reduce this open area to about 0-5 sq. ft. per ft of belt width. It has been suggested that in addition to a volume equal to the induced flow an additional volume equal to 150 c.f.m. per sq. ft. of total hood openings (i.e. upstream plus downstream open area) be exhausted from the enclosure to ensure inward air flow. Often, however, this additional volume is very small in comparison to the induced flow volume predicted by the formula and may be neglected considering the approxi mate nature of the formula, provided, however, that the enclosure downstream (i.e. or around the lower belt) is made as tight as possible. Thus effective control at mini mum exhaust rate depends on the tightness of both top and bottom of the air circuit. The point where the exhaust connection should be attached to the enclosure is at the downstream belt enclosure as far from the actual impact point as possible. No exhaust is necessary at the head pulley of the upper belt unless the material being transferred is hot. In this case thermal head must be taken into account. The technique for chute-to-belt transfer is identical to belt to belt transfer with the only exception being that the chute opening (at the top) be used for Au instead of the open area around the head pulley in a beltto-belt system. At this stage a few words might be said regarding the design of these total enclosures. It should be borne in mind that dust control systems will always be a compromise, to a certain extent, between the dust control engineers and the mainten ance and plant engineer who is concerned with the regular servicing of the machinery components within the dust control system or enclosure. This being so, it would be well for anyone concerned with the design of dust control systems to give careful study to the whereabouts of drive motors pulleys, lubrication points and the like where con stant access must be given without a large amount of labour being expended in the dismantling of heavy, cumbersome or difficult-to-dismantle components of exhaust hoods. Ample access doors must be allowed, or complete sides, or tops must be capable of being disconnected reasonably quickly with a minimum number of fasteners or fastenings to enable the mechanics con cerned with maintenance to have access to the vital parts of the plant. If this is not done it will be found that the sheets, once dismantled, will not be replaced and the efficiency of the complete dust control system will suffer as a result of this laxity. Belt-to-Bin and Chute-to-Bin Transfer The Q1 formula, although not developed directly for this type of application, may be used to approximate the required exhaust volumes for these cases. They differ from the usual transfer in that no downstream open area exists in the air circuit; dust air induced into bins by falling materials must reverse direction and leave by openings at the top, usually the same opening through which it entered, plus cracks, etc. Since part of the kinetic energy of the induced air flow is lost by turbulence, change of direction and some recirculation within the bin, the full Q1 predicted by the formula need not be exhausted. A figure of JQ1 may arbitrarily be used and exhaust may be taken from the top of the bin as far from the bin feed opening as possible. As the bin may be empty, full or anywhere in between during normal operations, a ques tion arises as to the value to use for S. Again as an arbitrary figure S should be used equal to \ the bin height as represent ing the average conditions. These two arbitrary assumptions tend to cancel each 4 Journal of the Mine Ventilation Society of South Africa, January, 1966 Design and Calculation of Exhaust Systems for Conveyor Belts, Screens and Crushers other since when the bin is nearly empty the height of the fall is greater but, at the same time, there is more free volume for kinetic energy dissipation. On the other hand, when nearly full the height of the fall is less but less chance for turbulence losses exists. Screens Screening operations usually require exhaust at three separate points, namely at the screen enclosure itself, at the undersize feed chute to belt transfer point, and at the oversize feed chute to belt transfer. Multiple deck screens may require additional exhaust points. In order to estimate the screen enclosure ventilation rate a Q1 formula has been used with Au equal to the open area around the head pulley of the belt feeding the screen, R the total feed rate to the screen and S the height of fall from the head pulley to the screen centre. With near perfect screen enclosure ventilation of the screen itself at first appears unnecessary; however, because of screen blinding which may be expected, it is best to ventilate at this point rather than at a succeeding downstream location. It is otherwise possible that dusty induced air may force its way out through cracks in the enclosure above the screen. For the undersize feed chute to belt transfer point, the Q1 method may be used with Au the open area around the screen enclosure, that is to say the peripheral crack-like area between the enclosure and the screen, R the rate of flow of undersized material, D the average diameter of undersized conveniently taken at \ the screen cloth opening, and S the height of fall from screen (centre) to the undersized belt. Even with so-called perfect enclosures around screens it has been found that at least one in. clearance openings are usual around the periphery of the screen. For a 6' x 16' screen this gives an open area Au of about 4 sq. ft. which is in the same order as open areas around well designed belt pulley enclosures. In the case of the oversize feed chute to belt transfer point, Q1 has been calculated using the same Au as in the case of the undersize transfer operation. However, R is now the flow rate of oversize only, S the height of fall from screen (centre) to oversize belt and D the average diameter of the oversizes (which is usually approximately the same as that of the material feeding the screen). An addi tional advantage in ventilation at three points of screening operations is that one large volume exhausted at one point is not necessary thus reducing the possibility of excessive dust entrainment at that point. The air volumes calculated by the induction method for screens result in considerably lower quantities being exhausted when compared with conventional, rule-of-thumb volumes calculated on the 50 c.f.m. per sq. ft. of screen area currently in use. Crushers To achieve good dust control at crushers, ventilation must be applied at two points, at the top of the enclosure surrounding the upper portion of the crusher and at the crushed feed chute to belt transfer points. It must be emphasized that dust control at crushers when required will never be ade quate unless the upper portion of the crusher is enclosed and exhausted. This is regard less of the claims which may be made by the operating staff of the installation that constant access must be permitted to the top of the crushers, where blockages may occur. This must be overcome by quickdisconnect access ports or covers which per mit the operators to free the blockages with the minimum of inconvenience. The volume to be exhausted at crushers may be calcula ted by the Q1 method once again; this time using the enclosure open area through which the crusher is fed for Au and obvious values for R, S and D1. The duct take-off should be at the top of the enclosure as remote from the enclosure opening as possible. The crushed feed chute to belt transfer point is ventilated as in any other chute to belt point with one difference. The volume exhausted may be reduced by about -j from that calculated by the Q1 method using Au as the crusher throat opening, S the height of fall from crusher throat to belt, and D the average crushed material size. The reason for this reduction is that the air induced into the system by the fall of the crushed material must enter at the upper enclosure and pass through the crusher throat. Analysis of the airflow through the enclosure using methods developed for estimating compound hood static pressure losses, shows that a portion of the energy developed by the falling crushed material Journal of the Mine Ventilation Society of South Africa, January, 1966 5 Design and Calculation of Exhaust Systems for Conveyor Belts, Screens and Crushers must go toward overcoming a second orifice type loss at the crusher throat (in addition to that which occurs at the initial hood entry and which is inherently taken into account in the Q1 formula). This second static pressure loss reduces the total Q1 induced by an amount directly proportional to the increase in total static loss assuming constant total available horse power developed. This correction for double orifice loss should be made for any transfer operation where there is a natural constriction within the enclosure. For most cases, however, the only important loss which need be considered is at the upstream enclosure opening which loss is already included in the Q1 formula. The methods postulated above have been used with success in the calculation of above ground ore handling plants in a new iron ore mine and for estimating the exhaust require ments for limestone, basalt and dolomite quarries. The secondary crushing plant of the ore operating in question has been com pleted and is operating with success. The limestone plant is now in operation and limited experience of the application of this empirical formula where applied as above performs completely satisfactorily, the one exception appears to be a three rotary impact crusher where the result of dust control using the above formulae has been des cribed as " pretty good ". This means some visible dusting does occur. The research worker concerned with the development of this formula reports that these crushers handle 325 tons per hr of up to 2\" dia stone which is crushed to an average size of -g". The calculated total exhaust rates for the three crushers was 44,800 c.f.m. using the Anderson method. Because of necessary compromises, installed capacity for the three crushers was only 22,800 c.f.m. following the recommendations of the dust collector manufacturer whose equipment was based on the control velocity concept previously described. It may be pointed out here that this suggestion may have been prompted by the dust control manufacturer's desire to sell equipment rather than to be responsible for the results in using too small an air volume. It is reported that the prob lem created was only a minor nuisance since the limestone dust is very low in free silica content, but it does serve to emphasize the limitations of the control velocity con cept when compared with the more accurate " Q1 " or Anderson method. It may be of interest to compare overall exhaust volumes as calculated for systems using the air induc tion technique and the control velocity con cept which was contained in this particular dust collector manufacturer's estimates. The induction technique showed that 4 exhaust rates based on control velocity were inadequate, three were over-de_signed and two were unnecessary, and that there were at least two points where exhaust was required but not provided in the original proposal. For the limestone plant the induc tion technique showed that 254,000 c.f.m. would be required with 48 points of exhaust. The dust collector manufacturer's estimate was for 2000,000 c.f.m. with 59 points of exhaust. It is important to record that the differences in the magnitude of the two total volume estimates for this project was the discovery that based on the induction method several points of exhaust specified in the original dust collector manufacturer's estimates were found to be unnecessary and that the remaining points were under designed. By re-arranging the exhaust points and exhaust volumes it was possible to improve the design considerably with little increase in the total system capacity, with the exception of the three crusher ventilation rates which were found to be insufficient only after installation; the re arrangement of this scheme has proved entirely satisfactory. It might be suggested that more systems be analysed on a similar basis as between the capture velocity and the air induction technique advocated above and that the calculated exhaust volumes be compared with those estimated by the other techniques. Since most if not all of these systems are designed according to damper balanced methods it should be possible to conduct field tests to determine how closely the estimates match the volumes actually required as determined by observation. If the design of material handling ventilation systems is ever to be raised from the trial and error art status which it now has, it is necessary to develop a more basic technology than the ill conceived Control Velocity con cept presently used by the vast majority of dust control engineering companies. The Q1 formula which has been so strenuously 6 Journal of the Mine Ventilation Society of South Africa, January, 1966 Design and Calculation of Exhaust Systems for Conveyor Belts, Screens and Crushers advocated herein, must however, be used with some caution. A thorough knowledge of the process being ventilated is essential since air moving, air constricting or other operations may have side effects on the ultimate volume decided upon for the exhaust rate. Rotary crushers for instance, often act as fans themselves requiring additional exhaust capacity for effective dust control. Fast moving belts induce air by the very fact of their movement in a hori zontal direction into the exhaust hoods. Very large bins which are seldom full may require little or no ventilation at all if well enclosed despite air induction within them, since recirculation can provide all or nearly all of the induction air required. On the other hand, bins which are filled rapidly may require even more exhaust ventilation than the induced air flow to take into account that the volume of air displaced by the material as it fills the bin. It is not within the scope of this paper to deal specifically with actual air volumes of particular applications. What has been presented will enable a new appraisal of such systems to be made, and to assist those concerned with the assessment of designs and proposals to ensure such designs are suitable for the duties for which they are intended. Whilst air volumes to be exhausted can be established from this contribution, it cannot be too strongly emphasized that most designs depend on a careful study and full understanding of the processes at that point. The more complete the enclosure, the more economical and effective the installation will be. Dust particles in the small micron sizes, even if impelled at extremely high original velocities, travel a very short distance in air --a matter of a few inches at the most. Thus the fine dust particles of significance to health follow air currents and are truly " air-borne dusts ". Larger dust particles released at high velocities, (e.g. the larger particles from metal grinding) do have an appreciable trajectory, and cannot be cap tured unless directed into the hood. In all of the foregoing, it has been assumed that no wetting or spray system has been used at the dust source, and of course this is often untrue. Phimister men tioned having installed water atomizers at discharge chutes and transfer belts, in a recent paper delivered to this Society; it is not always possible or desirable, however, to introduce water into the system. When other factors permit, wetting should be carried out as soon after the dust creating source as possible. This is desirable for two reasons; first, the heat created in the production of dust discourages air adsorp tion, and therefore successful wetting is more likely; second, the continuous flood ing of the dust source excludes air, and prevents its absorption by the dust. The provision of sprays operating with air; water mixture results in the super saturating of the atmosphere with very fine water droplets. These have the effect on dust suspensions in that atmosphere, by the particles acting as nucleii upon which the moisture con denses. It follows that steam, for instance, can be used most effectively to limit dusti ness. It is hoped that the wettability of dusts and the application of this principle to the design of dust collectors will form the subject of a later paper. REFERENCES Drinker, P. and Hatch, T.: " Industrial Dust " McGraw-Hill (1963). Wright, R. D.: Relative Size of Particles, Chart Privately Published, 1958. Pring, R. T., Knudsen, J. F. and Dennis, R.: Design and Exhaust Ventilation for Solid Materials Handling. Industrial and Engineering Chamber, Vol. 41, No. 11, November, 1949. Anderson, D. M.: Dust Control Design by the Air Induction Technique. Ind. Med. & Surg., February, 1964. Hemeon, W. C. L.: Plant and Process Ventilation. Industrial Press, New York, 1955. Wright, R. D.: The application of Modern Dust Control Techniques in Reduction Works Dust Control, /. Mine Vent. Soc. of S.A., Vol. 12, No. 9, September, 1959. Phimister, G.: Ventilation and Dust Control in an Underground Crushing-Conveying System. /. Mine Vent. Soc. of S.A., Vol. 16, No. 1, January, 1963. Journal of the Mine Ventilation Society of South Africa, January, 1966 7