Sizing a rotary airlock valve correctly is critical for the efficiency of your pneumatic conveying system or dust collection process. An undersized valve leads to material backup, while an oversized valve can cause excessive air leakage and wasted energy.
Airlock Sizing Tool
Understanding Rotary Airlock Valve Sizing
Rotary airlock valves, also known as rotary feeders, are the "gatekeepers" of bulk material handling. Their primary job is to move material from one chamber to another while maintaining a pressure seal. To size them correctly, you must account for the physical properties of the material and the mechanical limitations of the valve.
Key Variables in Sizing
- Volumetric Capacity: The total volume of material you need to move per hour. Calculated as Mass Flow Rate / Bulk Density.
- Displacement: The internal volume of the valve pockets per single revolution. This is usually provided by the manufacturer in cubic feet per revolution (CFR).
- Efficiency: No valve fills 100%. Gravity-fed systems usually operate at 60-80% efficiency, while pneumatic systems may drop to 50% due to "blow-back" air resisting the material entry.
- RPM: The speed at which the rotor turns. For abrasive materials, lower RPMs (under 15) are preferred to reduce wear.
The Sizing Formula
The standard formula used in our calculator above is:
Required RPM = (Capacity in lbs/hr) / (Bulk Density in lbs/ft³ × Displacement in ft³/rev × Efficiency % × 60)
Factors That Affect Efficiency
When using the rotary airlock valve sizing calculator, the "Efficiency" field is often the most difficult to estimate. Consider these factors:
1. Material Flowability
Free-flowing materials like plastic pellets fill pockets quickly and evenly. Cohesive or sticky materials may leave "dead zones" in the pockets, significantly reducing the actual volume moved per revolution.
2. Differential Pressure
If there is a high pressure difference between the inlet and outlet, "leakage air" will flow upward through the returning empty pockets. This upward air velocity can prevent fine powders from falling into the pockets, dropping efficiency to as low as 30%.
3. Rotor Design
Closed-end rotors have less volume than open-end rotors. Additionally, the number of vanes (6, 8, or 10) affects how much material can enter and exit the valve during each cycle.
Maintenance and Longevity
Sizing isn't just about throughput; it's about the lifespan of the equipment. If you size a valve to run at 30+ RPM to meet capacity, and you are handling an abrasive material like sand or fly ash, you will be replacing the rotor and housing frequently. In such cases, it is often more cost-effective to select a larger valve running at a slower speed.