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Section 3 HOODS, TRANSITIONS, AND DUCTWORK A pneumatic dust-collection system has four main components; the hoods and other enclosures; the ductwork; the filter or other dust collector; and the exhaust fan. The system also includes transitions, the fittings that link hoods and other enclosures to the ductwork. Each component must be properly designed, fabricated, installed, and operated. If any one of them is defective in some way, the system will not work well. This section covers important aspects of dust-collection hoods, transitions, and ductwork. Capture Velocity Air must flow into hoods or other suction inlets fast enough to carry dust with it. This minimum capture, or pickup velocity is 200 fptn. It has been thought that 200 fpm is the maximum permissible inlet velocity. Air at higher velocities, it was believed, would capture whole grain as well as dust. However, recent experiments have shown this belief to be incorrect (Appendix D). Minimum pickup velocities for eight grains were found to range from 900 fpm (for oats) to more than 2,200 fpm (for soybeans). There is thus no need to limit air inlet velocities to 100 or 200 fpm, which has been fairly common practice. In fact, air at velocities below 200 fpm often will not capture airborne grain dust. A velocity of at least 200 fpm is not the only requirement at suction inlets in a pneumatic dust-collection system. Some minimum volume flowâin cubic feet per minuteâis also required. For a given hood or other enclosure, the required minimum flow of aspiration air is the sum of three quantities; 1. The volume of air displaced by entering grain (1.25 cfm times the number of bushels of grain entering per minute) . 2. The volume of air entrained by the entering grain. 3. The volume of air required to provide the minimum capture velocity (200 fpm times the area of the opening into the enclosure in square feet) . The required volume of aspiration air will differ at different hoods or other pickup points. Volume flows and velocities also must differ elsewhere in a pneumatic system if it is to operate properly. For example, the volume flow in a main duct is the sum of the volume flows in the branch ducts leading into it. 11
12 The total suction on a system is determined by the fan. At a given total suction, however, volume flow and velocity at various points in a system depend on its design. As noted in Section 2, decreasing the diameter of a duct increases air velocity at constant volume flow. To double velocity at constant volume flow, the cross-sectional area of the duct must be cut in half. Similarly, decreasing the inlet area of a hood increases the velocity of the entering air at constant volume flow. The movement of air can also be varied by inserting a restriction, or blast gate, in a duct. Hood and Transition Design Several basic points must be considered in the design and installation of dust-collection hoods. First, air moves from all directions toward hoods or other openings under suction. The pattern of movement toward a plain, circular opening is shown in cross section in Figures 3-1 and 3-2. In these figures, the direction of air movement from points near the hood is indicated by the lines (streamlines) leading into the opening. The curved lines marked with percentages (velocity contour lines) indicate velocity relative to the velocity at the opening, or face velocity. The contour line marked 100 percent represents the face velocity. Let us say, for example, that the face velocity is 300 fpm. In that case, the velocity at any point on the contour line marked 60 percent is 180 fpm, and so on. In other words, velocity toward the hood from any direction declines sharply with increasing distance from the opening. A hood's performance depends on the associated air movement (volume flow and velocity) and the size and shape of the opening. These characteristics are interrelated. Figures 3-1 and 3-2, as specified above, depict air movement near plain, circular openings. However, the same principles apply to other types of openings. A method of calculating airflow or velocity near hoods of various types is given in "Industrial Ventilation"1. We have already seen that the minimum capture velocity for grain dust is 200 fpm. On this basis, air entering a hood should have a velocity of at least 200 fpm around the perimeter of the opening. Velocity should increase to not more than 800 fpm at the entrance to the transition linking the hood to its branch duct. At higher velocities, energy losses from air friction and turbulence become excessive. For the same reason, transitions should be tapered so that velocity increases gradually to that required in the branch duct. Industrial Ventilation," A Manual of Recommended Practice, 16th Edition, American Conference of Governmental Industrial Hygienists, Committee on Industrial Ventilation, Lansing, Michigan, 1980.
13 flange o so too % of dtomttor FIGURE 3-1 FIGURE 3-2 Velocity contours (expressed in percentage of opening velocity) and streamlines for circular openings. (from "Industrial Ventilation," 16th Edition, 1980)
14 Hood Placement Hoods should be designed and placed so that larger pieces of airborne grain avoid capture and settle back into the grain stream while the smaller airborne dust particles are captured by the hood (Figures 3-3 and 3-4). It is extremely important, in order to maintain the recommended minimum inlet velocity of 200 fpm, that hoods have flexible side skirting in contact with the conveyor belt. The designer, when calculating air volumes, should remember that he must design for grain displacement and entrained air as well as the open area of a hood to maintain a minimum inlet velocity of 200 fpm. (See Table 3-1 and Figures 3-5, 3-6, and 3-7.) It is also recommended that the upstream and downstream ends be skirted so as not to interfere with the maximum grain stream. As a rule of thumb, the hood transition should be at least 12 inches from the grain stream. However, the proper distance and hood design will depend on a number of factors including belt speed, belt width, method of feeding the belt, idler spacing, and the physical room available. In many cases where there are a number of belt loaders loading onto a single belt in series, the installation of a continuous belt cover with suction being applied at several points along the belt is the best method of controlling the dust. (With this method or in any hood design, extreme caution should be used by the designer to avoid any internal horizontal surfaces which could collect dust.) Hoods should be installed vertically, where possible. A vertical hood is entirely self-cleaningâthat is, airborne particles not captured will fall from the hood by gravity. If practical, the transition should taper evenly on all four sides, with a maximum taper of 30 degrees. In some instances, hoods must be installed horizontally because of physical constraints. The bottom of a horizontal suction hood should be sloped at least 55 degrees from the horizontal. Examples of improperly designed hoods are shown in Figure 3-8. Ductwork Design The ductwork in a pneumatic system connects all dust-collection points to the exhaust fan. Each combination of hood, transition, and branch duct is a simple dust-collection system in itself. Thus a complex system is really an arrangement of simple systems connected to a common main duct. After the first step in designing a system, which is to select the proper hood or other enclosure for each dust pickup point, the volume flow and velocity at each hood determine the size of the branch duct from that hood. The main duct is then sized to handle the combined airflows from the branch ducts at a velocity high enough to transport dust.
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18 I if 6 "0 9"0 13"0 f h " d T t T I /*" Â«. I 1 Filter ' - Lf jl ! M ^ i IH| f V Fan â¢-I j Irn } V ^ To Dust Bin FIGURE 3-5 Typical dust-collection system.
19 34 6 8 I0 2000 .02 .03 04 .06 .08 .I .2 .3 .4 .6 .8 I 23468 I0 FRICTION LOSS IN INCHES OF WATER PER I00 FT on Standard Air of 0.075 Ib per cu ft dentitv flowing through overage, clean, round, galvanized metal ducts having approximately 40 iwnli pi' IOO ft.) Caution: Do not extrapolate below chart. FIGURE 3-6 Tube sizing chart. (Friction of air in straight ducts for volumes of 10 to 2000 cfm.)
20 34 6 8 I0 Â°Â°OI .02 03 .04 .06 .08 .I .2 .3 .4 6 8 I '' 2 34 68 I0 FRICTION LOSS IN INCHES OF WATER PER I00 FT (Based on Standard Air of 0.075 fb per cu ft density flowing through average, clean, round, galvanized metal ducts having approximately 40 joints per 100 ft.) FIGURE 3-7 Tube sizing chart. (Friction of air in straight ducts for volumes of 1000 to 100,000 cfm.)
21 a) Emission areas not covered by suction hood. b) Hood situated too far from belt and is not skirted. FIGURE 3-8 Improperly designed suction hoods.
22 Each component of a system causes a decrease in static pressure, or pressure loss, relative to atmospheric pressure due to friction of air flowing in the duct. It is this pressure decrease, as pointed out in Section 2, that permits atmospheric pressure to force air through the system. The sum of the pressure losses in the components of a system is the static pressure of the system. This static pressure, or suction, must be provided by the exhaust fan selected for the system. It is important to recognize that ductwork is more than a conduit for airâa good deal of solid particulate material can be transported along with the air. Proper design of hoods and other enclosures will insure that only light, flotation dust is captured during normal operation. However, during abnormal conditions, larger particles of grain can also enter the system. Therefore, ductwork should be designed to transport abnormal amounts of solids without clogging. In addition, clean-out doors should be provided in all ducts that could be carrying grain. Duct Velocity The single most important design specification for ductwork is the velocity. It was noted above that air must flow through a duct at least fast enough to transport the captured dust. This minimum velocity is called the transport velocity. Normally, a velocity of 4,000 fpm all the way to the fan will assure good operation without clogging problems. However, there is room for judgment through a range of 3,500 to 4,500 fpm. At some pickup points, where abnormal conditions are unlikely, transport velocities at the lower end of the range may be employed. At other pickup points, such as elevator boots, occasional surges are more likely. Ductwork from these points should be designed for velocities at the higher end of the range. Minimizing Horsepower In vertical runs that are self-cleaning, velocities considerably lower than 3,500 fpm may be used. Lower velocities require less pressure drop and so reduce horsepower demand. Other ways to minimize horsepower demand may be detected by carefully examining the entire system. For example, a long run of small branch duct with low airflow will require much more pressure drop than the average branch. If the entire system is designed to provide the high suction needed by this branch, the required horsepower will be higher (often by 20 to 30 percent) than would otherwise be needed. In such cases, it might be better to install a small, separate collector close to the distant source of dust. In this way the main system would be freed from operating at unduly high suction. Bends and Branch Entries To maintain a uniform velocity in ductwork, generous radii should be provided at all bends and branch entries. Also, abrupt enlargement or contraction of duct diameters should be avoided. Sharp changes in the direction and diameter of ducts increase friction and turbulence, which upset flow. Turbulence and abrupt changes in direction also may cause larger particles to settle and plug the duct, regardless of the nominal air velocity. Furthermore, increases in friction and turbulence increase the horsepower demand on the system. Guidelines for design of elbows and other duct fittings appear in Figures 3-9, 3-10 and 3-11.
23 Balancing the System A pneumatic system must be designed so that airflow is properly distributed among the branches. Otherwise, the system will not produce the airflow required at each hood or other pickup point. Designing for proper distribution of airflow is called balancing the system. To achieve proper airflow distribution, therefore, the static pressures in all ducts entering a junction must be the same. A system can be balanced in three basic ways; the balance method; the blast-gate method; and the plenum method. In the balance method, each branch is designed to have the correct pressure loss up to the main duct. The designer simply calculates the duct diameter that will produce the desired pressure loss. The balance method requires more design time than the other methods. However, it avoids the need for adjustable restrictions in ducts, which can lead to improper operation. A system designed in this way has limited flexibility because changes or additions require recalculation of the entire system. Also, extra high velocity is often needed in short branches near the filter or other collector to achieve enough pressure loss to maintain balance. Unusually high velocities can lead to excessive wear by abrasion. In some cases, however, effective use of high-loss flexible hose can result in proper pressure loss without extra high velocity. The blast-gate method achieves balanced airflow by means of an adjustable restriction, or blast gate, in each branch. This approach simplifies design calculations. However, it complicates start-up because each blast gate must be adjusted for proper flow. When each blast gate has been adjusted, it should be locked in position with a pin to prevent tampering. When blast gates are placed in horizontal runs they should be placed at the top of the duct so that they will not act as dams and knock occasional large particles from the flowing air. The plenum method employs a low-velocity plenum, or duct, equipped with a dust-removal system. Air need not flow at transport velocity because any airborne material that settles is conveyed away. This approach is especially useful where many branches are required in a relatively compact space. Which method should be used to balance a pneumatic system? There is no universal answer. The blast-gate method is the most popular in the grain industry, primarily because of its operational flexibility. However, each installation should be evaluated on its own merits by a competent engineer. In a few cases, the correct duct system is none at all. That is, the best choice is to use bin-vent collectors at individual locations. Materials of Construction Low-carbon steel is the construction material used most commonly in pneumatic dust-collection systems. The metal must withstand normal corrosion and also erosionâa high-velocity stream of grain dust can be extremely abrasive. Also, ductwork and other enclosures under suction must withstand pressure equivalent to the difference between the internal static pressure and atmospheric pressure. The metal in each duct should be of a gauge that will withstand the maximum negative pressure that can be imposed on the duct.
24 Good 21/2 dia center lint radius Idio CLR ELBOW RADIUS Elbows should be 2 or 21/2 diameters centerline radius except where space does not permit. -*-w Good Bad ASPECT RATIO Keep AR(*L)high in using rectangular duct Vmâv Tâ 3- Vm=Minimum transport velocity PROPER DUCT SIZE A = Cross sect ion ana Sin the duct to hold the selectet transport velocity or higher. FIGURE 3-9 Principles of duct design. (from "Industrial Ventilation", 16th Edition, 1980)
25 K'max L I ~T~ I Poor Bad BRANCH ENTRY Branches should enter at gradual expansions andatananaje of XT or less (preferred) to 45* if necessary. Good I Â£ Fair BRANCH ENTRY Bad Branches should not enter directly opposite each other. FIGURE 3-10 Principles of duct design. (from "Industrial Ventilation", 16th Edition, 1980)
26 Taptnd Mtt FAIR FAIR â<^ 1, â 1 _k V C VÂ» '-' _ -X 1 / / ' / ' \ ^iZi / 'âDuet turn FAIR FAN INLET A straight inlet is best: if an elbow inlet is necessary, provide on inlet box and duct turn vanes to eliminate air spin or uneven loading of the fan wheel Inlet boxes should not be used for dust-laden air. I I > BAD FIGURE 3-11 Principles of duct design. (from "Industrial Ventilation", 16th Edition, 1980)
27 Suggested gauges for straight ducts are given in Table 3-2. It may be wise to use heavier gauges in areas where ductwork is exposed to bumping by forklifts or other materials-handling equipment. The metal in elbows and angles should be at least two gauges heavier than in straight ducts of equal diameter. The metal in hoods should be at least two gauges heavier than in straight connecting branches. TABLE 3-2 Suggested Gauges for Ductwork Under 15"'Static Pressure3 Duct Diameter Gauge Under 8 inches 20 8-18 inches 18 18-30 inches 16 More than 30 inches 14-12 a See "Round, Industrial Dust Construction Standards," Sheet Metal and Air Conditioning Contractors, National Association, Inc., Vienna, VA., 1981, particularly where static pressure exceed 15".