MH INDUSTRIAL EXHAUST FANS - Aerovent

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Dust Collecting & Pneumatic Conveying Pneumatic conveying embraces both dust collecting and conveying. In a typical system the amount of material transported is low compared to the amount of air used. Usually, a large quantity of fast-flowing air is needed to assure entrainment of material. Conse-quently, the materials moved have little effect on the performance of the fan. Where the sole purpose of the pneumatic system is to convey as much material as possible, different factors are involved. Dust Collecting & Fume Removal All airborne material, except fumes and the finest of dusts, will settle in ducts and fans if a minimum velocity is not maintained. Since in dust collecting the air quantity is determined by the hood size and velocity, it is usually most economical to maintain the lowest duct velocity that will keep the material in suspension. Raising the velocity raises the horsepower. For example, assume 1,000 CFM through a 12" pipe at 1,280 FPM with 0.2" resistance per 100 feet. Doubling the velocity while maintaining the same CFM would require an 8 1 ∕2" pipe at 1.2" resistance per 100 feet. Some state codes give minimum velocities. Where no code applies, the figures shown in Table 9 may be used. When choosing the fan size for a dust collecting system, be certain that the velocity at the fan inlet and outlet is not lower than the minimum velocity required to keep the dust suspended. Material Conveying The problem of inducing the material into a conveying system is often a difficult one. The best overall method is one that feeds the material into the airstream evenly by either mechanical or gravity means. It is often required that the fan pick the material up as well as convey it. One misbelief frequently encountered is that the ability of a system to pick up material is due to the fan’s suction pressure. Suction in itself is useless. It is the velocity moving past the material that induces it to flow. For this reason, it is important not to plug up the entrance of the duct with material to be conveyed. It should be remembered, when figuring entrance loss to a conveying system, that where an appreciable amount of bulky material is to be moved, it may reduce the effective area of the inlet and thus increase the entrance velocity and loss. Since the purpose of a conveying system is to move a lot of material (as contrasted to dust collecting), the ratio of material to air volume is quite important. Experience has established good minimum velocities for common materials and these are given in Table 10. For those applications not listed, Chart B gives CFM/lb. of material and minimum velocities based on the weight of the material. Table 9. Dust Collecting & Fume Removal Duct Velocities VELOCITY MATERIAL (FPM) 1. Grinding Dust ........................................5000 2. Foundry ..............................................4500 3. Sandblast Dust .......................................4000 4. Wood Flour ..........................................2000 5. Sander Dust ..........................................2000 6. Shavings, Dry .........................................3000 7. SHAVINGS, WET ........................................4000 8. SAWDUST, DRY .........................................3000 9. SAWDUST, WET .........................................4000 10. WOOD BLOCKS .........................................4500 11. HOG WASTE ............................................4500 12. BUFFING LINT, DRY ......................................3000 13. BUFFING LINT, WET ......................................4000 14. METAL TURNINGS .......................................5000 15. LEAD DUST ............................................5000 16. COTTON ...............................................3000 17. COTTON LINT ...........................................2000 18. WOOL .................................................4000 19. JUTE LINT .............................................3000 20. JUTE DUST .............................................3500 21. GRAIN DUST ...........................................3000 22. SHOE DUST ............................................4000 23. RUBBER DUST ..........................................3500 24. RUBBER BUFFINGS ......................................4500 25. BAKELITE MOULDING POWDER ............................3500 26. BAKELITE MOULDING DUST ..............................2500 27. OVEN HOOD ............................................2000 28. TAIL PIPE EXHAUST .....................................3000 29. MELTING POT & FURNACE ................................2000 30. METALLIZING BOOTH ....................................3500 31. SOLDERING FUMES ......................................2000 32. PAINT SPRAY ...........................................2000 33. CARBON BLACK .........................................3500 34. PAPER ................................................3500 Table 10. Material Conveying Duct Velocities VELOCITY MATERIAL (FPM) 1. WOOD CHIPS ...........................................4500 2. RAGS .................................................4500 3. GROUND FEED ..........................................5000 4. POWDERED COAL .......................................4000 5. SAND .................................................7500 6. WOOD FLOUR ..........................................4000 7. SAWDUST .............................................4000 8. HOG WASTE ............................................4500 9. PULP CHIPS ............................................4500 10. WOOD BLOCKS .........................................5000 11. CEMENT ...............................................6000 12. COTTON ...............................................4000 13. WOOL .................................................4500 14. JUTE ..................................................4500 15. HEMP .................................................4500 16. VEGETABLE PULP, DRY ...................................4500 17. PAPER ................................................5000 18. FLOUR ................................................3500 19. SALT ..................................................6000 20. GRAIN ................................................5000 21. COFFEE BEANS ..........................................3500 22. SUGAR ................................................6000 10 Aerovent Bulletin 705
Dust Collecting & Pneumatic Conveying Wherever material is airborne, the fan must provide the energy to move the material. In small concentrations this is negligible, but in most conveying systems it is important. It is reflected as an increased resistance. Chart A gives the ratio of friction loss of the conveying system to the same system handling clean air The material conveying capacity of a system varies as the square of the velocity since it is a function of the velocity pressure. The economics of a given installation determine whether the minimum or a higher velocity shall be used. It is sometimes thought that a larger fan is naturally better than a smaller one. This is far from correct, since material is just as liable to settle in a fan as in a duct. If the inlet and outlet velocities of a fan are at least as high as the minimum conveying velocity, no settling should occur in the fan. Abrasion & Corrosion Certain materials can be highly abrasive by nature and, depending on the concentration and properties of the abrasive, can greatly affect the selection of the type of fan and wheel design. Proper fan and wheel selection alone may not satisfy the abrasive conditions. Special abrasive, impact, corrosion, and heat resistant materials or coatings may be required to further increase the life of the fan. Although Aerovent has the experience to recommend various materials or coatings to increase the life of the product, it must be recognized that to date no materials have been discovered that completely resist these conditions. Therefore, resistance to them is one of degree only. Chart A. Friction For Conveying FRICTION MULTIPLIER 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 MULTIPLY FRICTION FOR CLEAN AIR BY MULTIPLIER 1.0 0 10 20 30 40 50 60 70 80 90 100 CUBIC FEET OF AIR PER MINUTE PER POUND OF MATERIAL Chart B. Average Velocities & Air Volumes For Conveying Materials CONVEYING VELOCITY IN FT. PER MIN. 6500 6000 5500 5000 4500 4000 3500 3000 CU. FT. OF AIR PER LB. OF MATERIAL 100 90 80 70 60 50 40 VALUES ARE FOR BULK MATERIALS OF AVERAGE SURFACE CONDITION. WEIGHTS ARE NOT DENSITIES. CONVEYING VELOCITY FT. PERMIN. CU. FT. OF AIR PER LB. OF MATERIAL 30 5 10 15 20 25 30 35 40 45 50 BULK WT. OF MATERIALS IN LBS. PER CU. FT. Spark-Resistant Construction Described below are three classifications as defined by AMCA Standards for spark resistant construction. Type Construction Details “A” All parts of the fan in contact with the air or gas being handled shall be made of nonferrous material. Steps must also be taken to ensure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components. “B” The fan shall have a nonferrous impeller and nonferrous ring about the opening through which the shaft passes. Ferrous hubs, shafts, and hardware are allowed provided con-struction is such that a shift of impeller or shaft will not permit two ferrous parts of the fan to rub or strike. Steps must also be taken to ensure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components. “C” The fan shall be so constructed that a shift of the impeller or shaft will not permit two ferrous parts of the fan to rub or strike. NOTES: 1. No bearings, drive components or electrical devices shall be placed in the gas stream unless they are constructed or enclosed in such a manner that failure of that component cannot ignite the surrounding gas stream. 2. The user shall electrically ground all fan parts. 3. For this standard, nonferrous material shall be any material with less than 5% iron or any other material with demonstrated ability to be spark resistant. 4. The use of aluminum or aluminum alloys in the presence of steel that has been allowed to rust requires special consideration. Research, by the U.S. Bureau of Mines and others, has shown that aluminum impellers rubbing on rust steel may cause high intensity sparking. The use of the above standard in no way implies a guarantee of safety for any level of spark resistance. Spark resistant construction also does not protect against ignition of explosive gases caused by catastrophic failure or from any airstream material that may be present in a system. 11 Aerovent Bulletin 705
Page 1 and 2: ® The Industrial Choice. MH INDUST
Page 3 and 4: Wheel Design BW - Back Plate Radial
Page 5 and 6: Accessories Shaft Seal Four types o
Page 7 and 8: Optional Construction Heat Fans MH
Page 9: Performance Corrections For Tempera
Page 13 and 14: Performance Data Standard Duty Size
Page 47 and 48: Construction Specifications Standar
Page 49 and 50: Dimensional Data Standard Duty Arra
Page 51 and 52: Dimensional Data Standard Duty Arra
Page 53 and 54: Typical Specifications - Type MH-AW
Page 55 and 56: Typical Specifications - Type MH-OW

MH INDUSTRIAL EXHAUST FANS - Aerovent

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