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Appendix D STEADY STATE VELOCITIES During compilation of this manual, it became apparent that there were seme widespread misconceptions concerning maximum permissible air velocities at dust pick-up points. It was believed by many designers that velocities in excess of 200 feet per minute would indeed lift whole grains; thus, systems were limited to that value, usually with extremely disappointing results. To ascertain the minimum vertical air velocities in which various whole grains would remain suspended (steady state velocities), test apparatus were constructed independently by National Agra Underwriters, Inc., and MAC Equipment, Inc. Test methods are described and results are given in the following pages. The MAC tests calculated air velocities from velocity pressures obtained with an inclined manometer, while the National Agra tests employed a "hot-wire" direct reading velocity meter. Variations of results are attributable to differences in moisture contents, kernel sizes and weights, and velocity measurement methods. However, there is sufficient similarity of results to permit designers to use safely inlet velocities up to 800 fpm for the lightest whole grains. A. MAC EQUIPMENT, INC. TESTS* Concept; Air velocities required to maintain various whole grain in suspension. Background; Efforts to develop standards for mechanical dust control systems in grain elevators have uncovered a basic data deficiency in at least one specific area--air velocity at dust pick-up points. Wide disagreement as to the maximum inlet air velocities, which would remove suspended dust and not whole grain, was found. It was discovered that apparently there is no data on steady-state or terminal velocities for whole grains, and that generally accepted "rules of thumb" are currently being employed with often inadequate results. So that inlet velocities at critical pick-up points are not arbitrarily limited below functional levels, pick-up velocities for whole grains must be ascertained. * Courtesy of Russell Brackman, MAC Equipment, Inc., Sabetha, Kansas. 109
110 Procedure; Several methods were used to determine the air velocity effect on whole grain under* various conditions, which to some degree simulate field applications. Method I; To determine air velocity required to hold grain in a suspended state. Method II; To determine air velocities required to (1) separate large, light, foreign material, (2) to unstabilize stationary whole grains, and (3) to lift whole grains. Method III; To determine the inlet velocities, which will (1) lift stationary whole grains, and (2) lift agitated whole grains. Method I A 4-foot clear-plastic tube, open at both ends with an inside diameter of 2.75 inches, is mounted vertically in a wood box as an air plenum. A variable speed fan is arranged to discharge air in the box, which is forced up the tube. Whole grains were dropped in the tube and the fan speed adjusted to a point where the kernels were held in suspension. Average cross-sectional velocity readings were then taken with a hot-wire velocity measuring device at a sampling point 12 inches below the top of the tube. Whole Grain -Tube Variable Speed Fan
Ill Results Grain Steady State Velocity (in fpm) Corn Milo (light) Wheat Oats (light) Oats (heavy) 2,000 1,400 1,700 1,300 1,400 Method II A 4-foot clear-plastic tube, open at both ends with an inside diameter of 2.75 inches, is mounted vertically in a wood box used as an air plenum. A wire mesh cup is placed over the top of the tube. A variable speed fan is arranged to discharge air through the tube. Whole grain and foreign material are placed in the cup and the velocity is increased to a point where the contents become unstable. The cup was removed and readings taken. (See results, column 1.) The cup was replaced and the air velocity increased to a point where the large foreign material departed. The cup was again removed and velocity readings taken. (See results, column 2.) Whole grain was placed in the cup and the fan speed was increased to achieve a velocity adequate to dispel the contents. The wire cup was then- removed and velocity readings were taken. (See results column 3.) Wire Mesh Cup -Tube Variable Speed Fan
112 Results Grain Corn Milo (light) Wheat Oats (light) Oats (heavy) Departure Velocity of Large, Light Foreign Material 700 650 450 450 450 Nonstable Velocity of Whole Grain 1,250 700 650 650 700 Departure Velocity of Whole Grain 1,900 1,600 1,625 1,250 1,300 Method III - Inlet Velocity A 4-foot clear plastic tube, open at both ends with an inside diameter of 2.75 inches, is placed vertically and 3/8 inch from the floor. A variable speed suction fan was connected to the top of the tube. Grain was placed around the inlet of the tube and the fan speed was increased to a point where the grain at the inlet became unstable. Velocity readings were then taken in the 3/8-inch air space and the average was noted. (See results, column 1.) Grain was again placed around the inlet space and agitated with a stirring rod. The fan speed was increased and velocity readings were taken when the grain became unstable. (See results, column 2.) Inlet Velocity Sample Point -Tube Air Flow i 3/81
113 Results Grain Corn Milo (light) Wheat Oats (light) Oats (heavy) Pickup Velocity (Still) 1,700 1,000 1,400 700 1,100 Pickup Velocity (Agitated) 1,250 950 850 800 550 TEST REPORT To determine terminal velocity of grain samples; A) Terminal velocity can be described as that laminar air flow which will suspend the material in a vertical air stream. The material will not rise nor fall at its terminal velocity, which is also referred to as float or suspension velocity. B) Apparatus Orifice Plate Used to Determine Exact Velocity Pressure and Test Velocity 6" Dia. Clear - Plex Tube Inclined Monometer Material Hopper Variable Flow Fan C) Procedure; 1. The air flow is adjusted at the anticipated required velocity. 2. By slightly opening the slide gate on the sample hopper a small amount of material is discharged into the laminar flow air stream.
114 If the material rises the air flow must be decreased. If it falls it must be increased. Air flow adjustments are then made and another sample is tested. By trial and error the exact terminal velocity can be located. D) Terminal velocity of the following grains have been determined to be; 1. corn 1,810 ft/min 2. beans 1,570-1600 ft/min 3. milo 1,610 ft/min 4. wheat 1,525-1,530 ft/min 5. oats 1,200-1,220 ft/min 6. grain dust 170-190 ft/min Other material previously tested for comparison 7. PVC powder 180 ft/ min 8. soda ash 450-500 ft/min 9. foundry dust 250-300 ft/min 10. wheat mids 400 ft/min Note; Because of a variation in grain kernel size, density, and the presence of some cracked kernels, there was a noticeable difference between the terminal velocity of the cracked grain (lower terminal velocities) and heavier kernels (required higher terminal velocities). The velocities listed are an average and are considered to be a representative average terminal velocity. The grain dust sample was taken from MAC dust filter at Sabetha Farmers Cooperative. The suction points on this dust system are from the dump pit and leg boot areas. The dust sample was anticipated to be representative of the typical grain dust as handled in grain dust control systems. B. NATIONAL AGRA UNDERWRITERS, INC. STUDY OF AIR VELOCITIES REQUIRED TO MAINTAIN VARIOUS GRAINS IN SUSPENSION.* ABSTRACT Efforts to develop standards for pneumatic dust control systems in grain elevators have revealed basic data deficiencies. Proper air velocities at dust control pick-up points is an example. There is wide disagreement as to the inlet air velocities, which will prevent the escape of dust without lifting whole grains. Apparently, there is no data on steady-state or terminal velocities for whole grains. Thus "rules of thumb" have been formulated arbitrarily, and performance of many dust control systems has been severely impeded.
115 Methods for Determining Air Velocity Effects on Whole Grains A 4-foot, rigid, clear-plastic tube, 2.75 in. I.D., was mounted vertically from the top of a plenum chamber, 8 in. wide by 10-1/4 in. long by 4-1/4 in. high. A variable speed fan was arranged to discharge air into the chamber. Whole grains were dropped into the top end of the tube and the fan speed adjusted to a point where the grains were held in suspension. Average cross-sectional air velocity readings were taken with a Datametrics Airflow Meter, model 100VT, at a sampling point 12 in. below the top of the tube. Results are as follows; Whole Grain- Drop Point 3/8" Sampling Hole 2.75" I.D. Transparent Tube Plenum Chamber Variable Speed Fan 12" JL 48' * Courtesy of Duane W. Brown, National Agra Underwriters, Inc., Camp Hill, Pennsylvania
116 Results Grain Corn Soybeans Ba r ley Sorghum (milo) Oats Wheat Rice Rice (polished) Test Weight (Ibs/bushel) 56 57 50.5 58.5 37.5 62.3 58.0 Steady-State Velocity (fpm) 2,200 2,200+ 1,500 1,500* 900* 1,500* 1,350* 1,200 *Large trash floats at lower velocities. CONCLUSIONS Even though the results are subject to some variations because of differing test weights, kernel shapes, etc., they provide a range of maximums, which are safely above the velocities needed at dust pick-up points for control of dust. It will be noted that oats have the lowest pick-up velocity (900 fpm) of the common grains tested. Thus, it is not necessary to limit inlet or face velocities at dust pick-up points to the often ineffectual 100 fpm or 200 fpm believed by some designers to be the upper limits.
