In a vacuum pump unit combining a Roots pump and a rotary vane pump, proper filter selection is critical for system stability and protection against particulate contamination. However, a common mistake is to add the flow rates of both pumps or to use the flow rate of the main pump (Roots) as the sole criterion for all filters. It will cost more.
1. Flow Rate Benchmark: Rotary Vane Pump as Lower Limit, Roots Pump Connection as Dominant Factor
Theoretically, the filter's flow rate benchmark should be based on the pumping speed of the rotary vane pump. However, because the inlet filter is installed directly at the inlet port of the Roots pump, an overly small filter that exactly matches the rotary vane pump's displacement cannot be used. The reason is that the inlet port of a Roots pump is significantly larger than the exhaust port of a rotary vane pump. Selecting a filter strictly based on the smaller flow rate would result in a tiny connection that cannot mate with the Roots pump's large-diameter flange (e.g., DN100 or DN160). Forcing such a connection with reducers would create enormous flow resistance, severely limiting the Roots pump's inlet efficiency or even preventing it from starting.
Therefore, the correct compromise strategy is: Take the continuous pumping speed of the rotary vane pump as the lower flow limit, while using the connection size of the Roots pump's inlet port as the dominant selection criterion. In practice, you should select a filter whose connection directly fits the Roots pump flange (requiring no or only a very short reducer), and whose rated flow rate is as close as possible to the rotary vane pump's pumping speed. If a conflict arises, prioritize connection compatibility, allowing the filter's rated flow to be moderately larger than the rotary vane pump's speed. This "connection-first, flow-compromise" approach protects the Roots pump with adequate precision without undermining the unit's pumping performance through excessive damping.
2. Inlet Filter Selection: Balancing Precision and Connection
Filter Precision: Regardless of the flow compromise, the filter precision of the inlet filter must be strictly selected according to the Roots pump's requirements (typically 1-3μm) to capture micron-sized dust and protect its precision rotors. The rotor clearances in a Roots pump are minimal, making them highly sensitive to particles. This requirement remains unchanged by adjustments to flow rate or connection size.
Connection Matching: The connection size must match the inlet port of the Roots pump. Typically, Roots pumps feature large-diameter vacuum flanges such as KF, ISO, or DN standards (e.g., DN100, DN160). This is the primary constraint in the selection process.
3.Exhaust Filter Selection: The Key to After-Treatment
The exhaust filter is typically installed at the discharge port of the rotary vane pump. Its primary function is to separate oil mist, enabling environmentally friendly emission or oil recovery.
Flow and Connection: No compromise is needed here. The selection must strictly follow the rotary vane pump's own exhaust flow and connection size. The exhaust port of a rotary vane pump is usually a small-threaded connection (e.g., G3/4″, G1″). The exhaust filter connects directly to this port.
Functional Consideration: Since rotary vane pumps carry significant oil mist in their exhaust, the filter should have an efficient coalescing function (efficiency ≥99.9%). It is also advisable to equip it with an oil return line to guide the filtered oil back to the pump for reuse, reducing maintenance costs.
Abandon the simplistic idea of flow summation. Take the rotary vane pump—the pump that actually exhausts the gas—as the lower flow limit, while using the Roots pump's large-diameter connection as the dominant selection criterion. This approach achieves the optimal balance between protection and performance.
Post time: Jun-05-2026
