Working Principle of Roots Vacuum Pump

 

 

A Roots vacuum pump is a type of vacuum pump that operates without internal compression, typically having a low compression ratio, which makes it necessary to use a fore pump for high and medium vacuum applications.

The operation of a Roots pump relies on a pair of synchronized, counter-rotating lobed rotors that move gas through the pump chamber, effectively evacuating the vacuum space. The pump itself cannot handle gas discharge to the atmosphere and therefore requires a backing pump, such as an oil-sealed or water-ring pump, to achieve optimal performance.

Structure and Operation

The Roots vacuum pump features a pair of lobed rotors, which are synchronized to rotate at high speeds (up to 3450-4100 RPM). The lobes are designed in various shapes, including circular arcs, involute curves, and cycloids, with the involute curve being most commonly used due to its high volumetric efficiency and ease of manufacturing precision.

During operation, gas enters the pump chamber through the intake port and gets trapped in the space between the lobes and the pump casing (known as volume $V_0$). The trapped gas is then transported to the exhaust port. As the rotors continue to turn, the gas in $V_0$ does not undergo compression or expansion, but when the rotor lobe uncovers the exhaust port, the pressure difference causes a portion of the gas to flow back into the chamber, momentarily increasing the pressure before the gas is expelled from the pump.

The rotor surfaces and pump casing are separated by small clearances (typically 0.1–0.8 mm), which prevent contact and the need for lubrication, allowing for high-speed operation without significant wear.

Key Features

1. Wide Pressure Range and High Pumping Speed: Roots pumps maintain a consistent pumping speed across a wide range of pressures.
2. Symmetrical Rotors: The rotors are geometrically symmetrical, resulting in minimal vibration and smooth operation.
3. Low Friction Losses: With no need for internal lubrication, the pump experiences minimal friction, reducing the required driving power and enabling higher operational speeds.
4. Oil-Free Pumping: The absence of oil in the pump chamber minimizes the risk of oil vapor contaminating the vacuum system.
5. Simple and Compact Design: The pump has a straightforward structure with no internal compression or exhaust valves, making it robust and less sensitive to dust and water vapor in the gas being pumped.
6. Low Compression Ratio: The pump has a lower compression ratio, which is less effective when pumping lighter gases like hydrogen.
7. Complex Rotor Shape: The rotors' intricate curved surfaces pose challenges in manufacturing and inspection.

Applications

Roots vacuum pumps have seen rapid development in recent years and are widely used in industries such as metallurgy, petrochemicals, electrical engineering, and electronics. They are particularly effective in processes requiring high pumping speeds over a wide pressure range, with minimal contamination of the vacuum environment.

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Keywords: Roots Vacuum Pump, High-Speed Rotors, Vacuum Pump Maintenance, Industrial Applications, Oil-Free Vacuum Pumping, Vacuum Technology