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Powder & Bulk Solids, June 2018

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TechnicalExclusive 26 J u n e 2 0 1 8 R Rotary Airlock Valve Principals and Rotor Design Rotary airlock valves are a critical component in many material handling systems. From dust col- lectors to pneumatic conveying systems, rotary airlocks not only control the feed rate but also minimize air leakage between equipment above and below the valve with differing levels of pres- sure or vacuum. In dust collection systems the rotary airlock allows continuous product discharge without having to turn off the vacuum to dump the accumulated material. Rotary airlocks are vital in pneumatic conveying systems, allowing the in- troduction of material into a pressure or vacuum stream with minimal loss of air. Design & Construction Rotary airlock valves have a heavy duty cast hous- ing which consists of a body and two end covers, an internal rotating rotor, and a drive package. The body has an inlet and outlet that allows ma- terial to enter and exit the valve. The rotor has a shaft with multiple vanes, and the ends of the shaft extend out through the housing to external bearings supported by the end covers. Inside the housing, the vanes radiate out away from the shaft to the housing. The space between the rotor vanes creates pockets that pickup material at the inlet and as the rotor rotates, transport the pockets of material to the outlet. The empty pockets then rotate back to the inlet to pick up more material. The drive package consists of a gearbox and elec- tric motor and connects to the rotor shaft by a chain drive or may be directly coupled. The rotor rotates relatively slow, typical speeds are 22 rpm or less. So how does the rotary valve provide an airlock? While the valve does not create a 100% seal, the leakage rate is greatly reduced when properly designed. The body and end covers must be of heavy-duty design to withstand the pressure dif- ferential, and precisely machined to very tight tolerances. A raised face on the inside of the end cover fits tightly and accurately into the body bore to insure a good seal and proper alignment between the rotor and housing. The rotor needs a large shaft to prevent deflection and typically eight or more vanes. The rotor, too, is precisely machined to fit tightly inside the housing. Typical clearances between the rotor and housing are .004 to .006 in., or roughly the thickness of a strand of coarse hair. These tight tolerances minimize air leakage as they allow the rotor to rotate with minimal clearances. The tighter the clearances, the less air that can leak through. The geometry of the body design is also important, as the more vanes and pockets that are contained within the housing and not exposed to the throats – the greater the seal. Rotor Types & Selection Rotary airlock manufacturers will have several dif- ferent body styles, each that work best for certain applications. For this article, we are going to move on to the varying types of rotors and how to pick the best design based on the application. Proper ro- tor selection is a critical part of specifying a valve that will best suit the intended service, making a significant difference in the life of the valve and overall performance. The first consideration is whether to use an open end rotor or closed end (shrouded) rotor. Refer to the photos of each. When viewing the open rotor from the shaft end, you will see multiple vanes radi- ating out creating V's or U's, which are the pockets. The ends of the vanes are left open. If water was poured into the top pocket, it would simply run out the sides. By Paul Golden, Carolina Conveying Open end rotor

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