How Does a Roots Pump Work?

A Roots pump, also known as a Roots blower, is a type of positive displacement vacuum pump that uses two counter-rotating lobes to move gas.

Understanding the Working Principle

The fundamental operation of a Roots pump relies on the controlled movement of gas by meshing rotors within a fixed housing (stator). Unlike pumps that compress gas internally, a Roots pump captures a volume of gas at the inlet and transfers it to the exhaust port.

Here's a breakdown of the process, incorporating information from the provided reference:

Key Components

• Rotors: Typically figure-eight shaped lobes, two of which rotate in opposite directions. They are precisely machined and timed so they spin closely together but without touching.
• Stator: The stationary housing or body of the pump that encloses the rotors.
• Inlet Port: Where gas enters the pump.
• Exhaust Port: Where gas exits the pump.

Step-by-Step Operation

1. Gas Entry: Gas enters the pump through the inlet flange, which is located perpendicular to the rotating units.
2. Gas Isolation: As the rapidly rotating rotors (which are spinning in opposite directions) turn, they trap pockets of gas between themselves and the inner wall of the stator. This process effectively “isolates” a specific volume of gas from the inlet pressure.
3. Gas Transfer: The isolated gas volume is carried around the periphery of the rotor and casing towards the exhaust side.
4. Gas Expulsion: When the rotors open to the exhaust port, the trapped gas is released and mixed with the gas already present at the exhaust pressure. The compressed gas is then expelled via the exhaust port. Note that the compression itself occurs after the gas is transferred to the exhaust side due to the pressure difference, not within the working volume between the rotors.

This continuous cycle of trapping and transferring gas allows the Roots pump to move large volumes rapidly, making it effective at lower pressures.

Practical Insights

• Roots pumps are dry pumps, meaning they don't use oil within the pumping chamber itself, which is beneficial for many vacuum applications to avoid contamination.
• They are frequently used as "booster" pumps in combination with other vacuum pumps (like rotary vane or screw pumps) to increase pumping speed in the rough and medium vacuum ranges. They cannot pump down to very low pressures on their own as there's no internal compression mechanism significant enough to create a very high vacuum.
• The timing gears that connect the two rotors are crucial; they ensure the rotors spin synchronously without touching each other, which could cause damage.

Summary Table

To learn more about different types of vacuum technology, you might explore resources on positive displacement pumps like rotary vane pumps or screw pumps.