Many people see it as just another piece of equipment within a production line, but there’s more to a rotary airlock than meets the eye. Simply constructed, it plays a vital role in conveying and material handling and can additionally be modified for just about any industry or application.
Considerations for Rotary Airlock Configurations
A rotary airlock comprises housing and end plates encompassing a multi-vane rotor turning on a shaft. The housing is open at the top and bottom, allowing the material to fill and discharge the pockets formed by adjoining vanes on the rotor. These pockets are continually filling at the top and discharging at the bottom, allowing for continuous displacement of product from one vessel to another while maintaining an air seal. The airlock's configuration can vary based on many factors, including its basic function, the properties of the material handled, and the types of equipment above and below it.
Other Elements of Rotary Airlock Configuration
A rotary airlock’s configuration is first based on how it will be used. In short, a rotary airlock can be configured to maintain a pressure differential across its housing, to precisely control the flow rate of bulk solids, or both. When selecting the best rotary airlock, configurations should consider the type, size, functionality, and other factors that make it highly efficient and long-lasting for its given application.
Factors that concern rotary airlock configuration include:
- Drop Through or Blow Thru: A drop-thru airlock uses a housing with a conventional discharge flange that mates to other equipment, where product discharges or drops thru this opening by gravity to other components in the system, such as another vessel or convey line (via transitions). With a blow thru configuration, pneumatic convey air passes directly thru the sides of the housing (and rotor) to “wash” the pockets empty. The two styles offer specific advantages but are not interchangeable.
- Construction: Materials used to construct the drive, housing, rotor, and other accessories should match requirements for the product and production environment. This is especially important when the rotary airlock is located in an area vulnerable to weather conditions, requires frequent washing, or when hygiene is a priority for construction. It's also crucial to select wear coatings that will provide a suitable life expectancy when handling abrasive materials.
- Cleanability: When it comes to cleanability, consider how often it will be cleaned, as less expensive airlocks tend to be difficult to take apart. For applications that require frequent cleaning or maintenance, it’s best to use a configuration that allows quick disassembly, with certain easy-to-clean models featuring rails to make maintenance and cleaning even easier.
- Fire and explosion protection: Valves that need to isolate a fire or explosion must meet specific design criteria specified by the National Fire Protection Association (NFPA) in the United States or ATEX (short for “atmosphères explosibles”) in Europe.
- Volumetric capacity: When a rotary airlock must precisely control the feed rate, they’re sized according to swept volume, which is the product throughput of one rotor revolution. Typically this is measured in either liters or cubic feet. They can also be optioned with special “reduced” or “split pocket” rotors, providing a more even flow at higher revolutions per minute.
When determining the needed size for a rotary airlock, configurational aspects like flange and inlet opening size should also be considered, as when acting simply as an airlock, the size of these openings should override volumetric capacity.
Material Considerations for Rotary Airlock Configuration
The type of material being processed is one of the most important factors when considering a rotary airlock. Configurations vary depending on the type of material being processed and conveyed. For example, with very fine powders like carbon black, particles will become completely airborne if the airlock turns too fast, making processing inefficient. Narrowing down the necessary options for processing certain materials will help determine the best rotary airlock. Such mechanical equipment's configurations can also differ considerably depending on the material being processed.
As material processing is about adding value, it’s important to consider the end product. For example, the rotary airlock configuration for granulated sugar production would need to differ from that for powdered or confectionary sugar. In pharmaceutical applications, both active and inactive ingredients require blending, so a rotary airlock could be used to feed from mixing vessels or containers upstream.
There are several considerations when it comes to sizing or configuring a rotary airlock. Configurations differ based on a material’s properties. For example, some lighter and finer powders won’t have the same bulk density as aggregates or other abrasive materials. The airlock’s size should be selected to displace the right amount of material based on its density and flow requirement.
Material properties that should be considered include:
- Bulk density during processing
- Coefficient of sliding friction
- Material density
- Moisture levels
- Particle size, distribution, and shape
Regarding abrasive materials, the product fineness, hardness, loading factor, and pressure/ temperature all affect the best types of wear options to choose from. For finer products, those of lower hardness value, or applications where the airlock will see minimal loading, a chrome coating may be suitable where in an application handling coarser, harder materials under a head load may call for more durable options like tungsten or ceramic. Protective coatings are recommended for highly abrasive materials to lengthen the life of rotary airlocks and other processing equipment.
Feed Considerations for Rotary Airlock Configuration
The way the material is conveyed also figures into rotary airlock configuration, with one of the most common uses involving the safe introduction of bulk materials into conveying systems. Though they aren’t completely airtight, rotary airlock configurations often have tolerances of 4-6 thousandths of an inch (101.6-152.4 micrometers) between the valve housing and rotating vanes. This allows airlocks to maintain a pressure differential, keeping air within the pneumatic systems to which they couple.
Some feed issues involving rotary airlock configurations include:
- Speeds above 22 RPM (rotations per minute) aren’t typically practical because they do not offer enough residence time for pockets to fill completely. Therefore each pocket is filled to a lower percentage than if run at a lower RPM, offsetting most, if not all, the gain expected.
- Flow rate may be decreased due to stress on the rotor’s tips when excessive material load is applied to the airlock, resulting in added wear on components.
- Conveying problems may result when rotary airlocks are configured to run too slowly, a problem known as “slugging” of material.
- Buildup of product or foreign substances can cause the airlock’s rotors to lock up, and even cause the motor to stall.
Proper venting is critical to achieving consistent flow and displacement when the airlock feeds a pressure convey line. As pockets empty into the conveying system, they are pressurized with air and returned to the inlet. If not treated, this air, combined with a portion naturally leaking around the rotor, can disrupt the flow of bulk solids in the bin, reducing displacement efficiency or, worse, causing an arch or bridge in the hopper above. Proper venting techniques can be as simple as diverting the air in empty pockets thru a vent in the body to more elaborate inlet vents that divert empty pocket air and natural leakage away from the flow of material.
In all cases, the approach taken will direct this “bypass” air around the flow of material to a suitable place, sometimes back into the vessel above, but above the fill point of material.
The rotor's speed can be adjusted for systems that utilize a variable frequency drive (VFD). This is particularly useful for applications where the product (or products) density varies. It is also practical when the airlock feeds a critical component downstream.
Understanding the process and product temperature is imperative to building the valve properly. An elevated temperature will cause the parts to expand. Uncompensated expansion can lead to the airlock squealing or the rotor seizing inside the housing all-together. When the process temperature is properly specified, the airlock rotor can be adjusted for an appropriate thermal expansion.
Extremely low or high operating temperatures may also play into the materials chosen to construct the valve (e.g., high temp grease for bearings), the elimination of temperature-limited plastic components in favor of metal, or materials that are thermally stable under either extreme.
Ask Prater About Rotary Airlock Configuration
Prater Industries helps customers consider all the factors necessary in picking the right rotary airlock. Airlocks can be configured to a customer’s unique needs, ensuring the design is optimal for the system for which it’s used and the materials being processed. To get started, look at Prater’s size recommendation tool, which will help determine the best rotary airlock for your application. Our team of rotary airlock experts can also help in choosing the most efficient design, explaining their reasoning behind any recommendations. For more information about Prater’s products and services, we invite you to contact us today.