How to Connect Vacuum Pumps in Series?

Connecting vacuum pumps in series involves directing the discharge (outlet) of one vacuum pump to the suction (inlet) of the next pump in the sequence. This configuration is primarily used to achieve a lower ultimate vacuum pressure and/or to increase pumping speed at lower pressure ranges.

Understanding Series Connection for Vacuum Pumps

To connect vacuum pumps in series, the gas flow progresses from the primary vacuum system through a series of pumps, each further compressing the gas. As described in a relevant demonstration, the process involves a sequential flow: "It's close so goes to the yellow. Line open so that it flows. And then it will go to the suction of this pump. Three. So finally from pump one pump two pump. Three. So the final suction." This highlights that the outlet of an preceding pump (e.g., pump one's discharge) connects to the inlet (suction) of the subsequent pump (e.g., pump two), and so on, until the final pump discharges to the atmosphere or a foreline.

This setup typically involves:

• Primary Pump (Roughing Pump): Connects directly to the vacuum chamber or system to perform initial evacuation.
• Secondary Pump (Booster/High Vacuum Pump): Its inlet connects to the primary pump's discharge. This pump is designed to operate at lower pressures and further reduce the system pressure.
• Tertiary Pumps (Additional Stages): If needed, more pumps can be added in series, with each subsequent pump's inlet connected to the discharge of the previous pump.

Why Connect Vacuum Pumps in Series?

Connecting vacuum pumps in series offers several key advantages:

• Achieve Lower Ultimate Pressure: By staging pumps, each pump handles a smaller pressure differential, allowing the final pump to reach a significantly lower vacuum level than a single pump could achieve alone.
• Increase Pumping Speed at Low Pressures: Booster pumps, often used in series with roughing pumps, significantly increase the volumetric pumping speed in the transitional and high vacuum ranges.
• Handle Higher Throughput: For applications requiring rapid evacuation of large volumes or continuous processing, series connection can provide the necessary pumping capacity.
• Optimize Efficiency: Different types of pumps are efficient at different pressure ranges. Combining them in series allows each pump to operate within its optimal performance window.

Key Steps and Considerations for Series Connection

Successfully connecting vacuum pumps in series requires careful planning and execution:

• 1. Pump Compatibility:

  • Ensure the discharge pressure of the preceding pump is within the acceptable operating inlet pressure range of the next pump in the series.
  • Match pump types (e.g., a rotary vane pump often serves as a backing pump for a turbomolecular or diffusion pump).

• 2. Proper Plumbing:

  • Use short and wide connecting lines between pumps to minimize flow impedance and maximize effective pumping speed.
  • Ensure all connections are leak-tight to prevent atmospheric gas from entering the vacuum path.
  • Use appropriate vacuum fittings (e.g., KF, ISO, ConFlat).

• 3. Isolation and Bypass Valves:

  • Isolation valves are crucial to isolate individual pumps for maintenance or to protect them during system cycling.
  • Bypass lines with valves can allow the system to rough down directly through the primary pump before engaging the higher vacuum stages, preventing unnecessary wear on high-vacuum pumps.

• 4. Vacuum Gauges:

  • Install vacuum gauges at various points in the system (e.g., chamber, between pump stages, foreline) to monitor pressure and ensure proper operation.

• 5. Cooling and Venting:

  • Ensure adequate cooling for pumps, especially high-vacuum pumps that can generate significant heat.
  • Provide proper venting mechanisms for pump oil or exhaust.

• 6. Operational Sequence:

  • Always start pumps in the correct sequence (e.g., start the backing pump first, then the high vacuum pump).
  • Vent the system in reverse order.

Example Series Configuration

Here's a common example of a two-stage vacuum system:

In this setup, the rotary vane pump pulls the initial vacuum, and its discharge is connected to the inlet of the turbomolecular pump. The turbomolecular pump then further reduces the pressure, discharging into the foreline created by the rotary vane pump.