Though used widely within many industries and known by multiple names, the rotary airlock feeders used to handle bulk materials are descended from a rather odd source: an implement first used for musical instruments. Invented in the early part of the 19th century by Boston resident Nathan Adams and later patented in 1835 by Joseph Riedl, the rotary valve is the ancestor of the rotary airlock feeders now used in factories globally. First used for brass instruments like tubas, trumpets, trombones, flugelhorns and horns, rotary valves are now integral for many material handling applications.
Within both pressurized and gravity-fed conveying systems, rotary airlock feeders – also known as rotary airlock valves, rotary valves, rotary feeders, or sometimes even rotary airlock valve feeders – can work as an airlock, a feeder or a combination of both. Operating as an airlock, it keeps air from entering the system to create a pressure differential that assists with conveying material. It can otherwise act as a simple feeding mechanism to control flow rates of bulk solid materials without causing a difference in air pressure. However, rotary airlock feeders generally refer to devices used both as an airlock and a feeder.
How Rotary Airlock Feeders Are Used in Industry
The rotary airlock feeders used in the industry provide a means for transferring material from storage containers into bulk material conveying systems. These rotary valves are relatively compact and among the simpler types of machinery utilized for bulk material processing. The only moving piece – other than the drive – is the multi-vane rotor that revolves within the rotary airlock feeder’s housing.
Rotary airlock feeders act to not only control the flow of product – at either a fixed or variable rate – but are also used to maintain an air pressure differential between an implement’s inlet and outlet when used as an airlock. However, along with these two primary functions, there are other purposes for its inclusion in material processing systems, such as isolating a particular process. Additionally, rotary airlock feeders can prevent or mitigate damage from conflagrations and explosions.
Rotary airlock feeders function to:
- Control the volume of material within a pneumatic conveying system.
- Introduce bulk solids into a processing system or remove them from cyclones, dust collectors or other such collection devices.
- Provide gravity discharge for cyclone dust collectors, filters, precision feeders and other rotary valves used for different purposes.
- Transfer bulk materials from one container or atmosphere to another without significant loss of air pressure.
- Transport precise amounts of material from bins, hoppers, silos and other containers within a system.
Though all rotary airlock valve feeders look very similar and have the same basic parts, their application within a processing system will depend on the industry in which they’re used. How big a rotary airlock feeder needs to be will often depend upon the purpose for which they’re used. Understanding these applications can help in choosing the best size for the application.
Basic Construction of Rotary Airlock Feeders & How They Work
Regardless of what they’re used for, all rotary airlock feeders have similar features. Basic components include a rotor equipped with vanes mounted on the rotor’s shaft, packing seals and bearings, along with an inlet and outlet within endplates that form the cylindric housing cavity of the rotary airlock valve.
As the rotor shaft turns inside the housing, the space between the vanes attached to the shaft acts as rotating pockets within the rotary airlock. The valve feeders transfer material entering from the top pockets through the inlet, where it rotates with the vanes, exiting either through the outlet or underside of the machine. The vanes and pockets they form turn continuously during processing, moving material as the rotor spins.
Used as a rotary airlock valve, feeders must be sufficiently leakproof to achieve uniform particle size distribution. This requires shaft seals that prevent both air (or other gas) and material from leaking from the device. Variation occurs with different types of rotors and the number of vanes of a rotary airlock valve. The feeders have larger pockets with fewer vanes, while they can better seal between inlet and outlet with more vanes.
Degassing Rotary Airlock Feeders
In pneumatic conveying systems, rotary airlock feeders are also used to dose material, which results in a significant amount of pressurized air or gases going upstream. This is primarily due to the rotating valve and the leaking between the rotor and stator. For the system to function optimally, a small hopper with an attached filter is often connected to the system to vent pressurized air or gases so that the product keeps flowing through the rotary airlock.
Options for Rotary Airlock Feeders
Numerous options can be added to optimize rotary airlock feeders for an application.
- Beveled rotor vanes: Able to keep drag to a minimum to increase operating efficiency, these vanes are used for processing finer material that can get stuck between the rotor tips and housing; they’re machined at an angle of 15° to narrow the area in which product can land.
- Closed-end rotors: By reducing wear on end plates and shaft seals, these rotors help decrease air leakage while also containing free-flowing material that tends to cause an excess of air; these rotors seal the whole pocket on each vane, as they’re designed to have the same diameter as the vane tips.
- Drop-through adaptors: This allows a blow-through rotary airlock feeder to be adapted to work as a more robust drop-through model so it can better withstand higher-pressure air as it moves material through the rotor pocket.
- Inlet baffles: When overly large particles or produce get stuck or clipped between the housing and rotor tips, inlet baffles help remove material from the vane tips before it enters the housing, limit material from filling the vane pockets’ back halves and decrease degradation of more friable materials.
- Removable wear bars: Made from materials like brass and certain other metallic alloys, polytetrafluoroethylene (aka Teflon) and synthetic rubbers like neoprene, removable wear bars make replacing portions of rotary airlock feeders’ rotors easier and quicker.
- Shallow pocket rotors: Used to handle more viscous products that tend to pack at the bottom of narrow pockets, these rotors reduce capacity so that when rotations per minute (RPM) are higher there is still an accurate and continuous flow feeding the system.
- V-plow: Used to extend the life of the housing and rotor, these implements help prevent jamming and are often used for applications involving pellets or wood.
Types of Rotary Airlock Feeders for Various Applications
There exist two main categories for rotary airlock feeders, blow-through models and the more common drop-through models, both with their pros and cons. The former is often used for such things as cocoa powder, flour and milk powder is stickier and more cohesive. In contrast, the latter is used for materials like alumina, coffee beans, grain, granulated fertilizers, powders, rice and sugar that flow more easily. These two categories additionally can be divided into various types of rotary airlock feeders, which differ regarding accessibility for maintenance, complexity of construction, cost, feed rate capacity, operational simplicity and sensitivity to the rate of feeding.
Blow-Through Rotary Airlock Feeders
With a blow-through rotary airlock valve, feeders are used to convey a wide range of material pneumatically, including for the chemical, flour milling, food processing, pharmaceutical and plastics industries. With a low profile and large capacity, these rotary valve feeders normally come with replaceable shaft seals, outboard bearings and 8 or 10-vane open-ended rotors.
Drop-Through Rotary Airlock Feeders
For more rugged applications, drop-through rotary airlock feeders are used. These require an outward bearing style that works better for abrasive products and applications where contaminants are an issue. For this type of rotary airlock valve, feeders with outward bearings are engineered to withstand systems that utilize high-pressure pneumatic conveying, while also enabling a more effective seal when exposed to higher temperatures. These devices are useful in applications where wear requires more than just a simple dust-collecting valve.
Angle of Repose Rotary Airlock Feeders
Able to work in harmony with material as it naturally feeds into the system, an angle of repose rotary valve operates at variable speeds, using an articulable gate and rotating member to control flow volume. Its operation is based on the bulk material’s fundamental angle of repose. The articulable gate’s positioning and rotational speed consistently feed material through the rotary airlock valve, feeding material gently without limiting material flow. While it maintains a constant flow of material, it also lessens dust generation, product degradation and surge loading.
Easy-Clean Rotary Airlock Feeders
A design that can be quickly disassembled and reassembled allows production facilities to minimize downtime for a rotary airlock valve. Feeders that enable this can be more easily cleaned, inspected, maintained and sanitized, often without the need for any tools. These types of rotary airlock feeders are used for applications where long shutdowns are prohibitively costly and cross contamination can result in major issues. Easy-clean models are used in industrial bakeries and for batching mixing systems where various colored resins are processed that require regular cleaning during changeovers. Industries that utilize these types of rotary airlock feeders include the chemical, dairy, food processing, paint, pharmaceutical, plastic and powder coating sectors.
Knife Gate Rotary Airlock Feeders
These rotary airlock feeders are designed to isolate material for applications involving suspended solids and as simple on-off feeders. Used to cut oversized material that could cause a blockage that prevents the rotor from moving, knife gate rotary valves are used in applications involving abrasive, corrosive and viscous media. This would include handling of slurries.
Rotary Airlock Feeders from Prater
Prater Industries makes several distinct models of rotary airlock feeders that can meet an assortment of challenging conditions and applications. We manufacture our rotary valves to optimize our customers’ production, making them extremely reliable and robust so that they require as little maintenance as possible. Add to this Prater’s penchant for customizing our already cutting-edge products in our test lab to best meet our customer’s needs, and there’s almost no bulk material processing solution for which our rotary airlock feeders can’t be effective.
Prater’s rotary airlock/feeders include:
- Abrasion-resistant rotary airlock valve: Feeders/airlocks made to handle highly abrasive and corrosive materials, this design includes an iron housing lined with ceramic to provide better performance.
- Prater Blow-thru rotary airlock valve: Feeders/airlocks specifically made for feeding into pneumatic conveying systems for applications where free-flowing product risks blocking rotor vane pockets.
- Prater Dust Collector Series of rotary airlock valves: Feeders/airlocks that offer an economical choice when used generally to feed into cyclones, dust collectors or hoppers, this design offers standard features that normally are only found with heavy-duty rotary airlock feeders.
- Heavy-duty PAV series rotary airlock valve: Feeders/airlocks primarily used to transfer free-flowing dry powders against pressure, these rotary airlock feeders are used to feed into pneumatic conveying systems.
- QTA (on rails) rotary airlock valve: Feeders/airlocks with the rotor on rails to allow quick disassembly and reassembly, the rails of this unit offer considerably more capacity than similar machines from competitors while also protecting the rotor from damage due to dropping.
- QTA rotary airlock valve: Feeders/airlocks designed for quick disassembly and reassembly without the need for tools are useful for applications where frequent cleaning is necessary or downtime eats considerably into profit.
Regardless of the industry, Prater’s engineers can assist with choosing the best rotary valve to suit your processing system’s needs, whether it’s for use as an airlock, feeder or both. To learn more about our rotary airlock feeders and other material handling equipment, contact the experts at Prater today.