Roots Vacuum Boosters: How to Extend Your Vacuum System Performance

When a single vacuum pump cannot achieve the required combination of vacuum level and pumping speed, a Roots vacuum booster is the standard solution. By combining a Roots booster with a backing pump, engineers can dramatically extend the performance envelope of their vacuum system — achieving deeper vacuum, higher throughput, or both.

This guide explains how Roots boosters work, why they cannot operate alone, and how to size a booster system correctly.

What Is a Roots Vacuum Pump?

A Roots vacuum pump (also called a Roots blower or Roots booster) is a positive displacement machine that uses two figure-8-shaped rotors that counter-rotate inside a precision housing. The rotors do not touch each other or the housing — they operate with a small clearance, relying on tight tolerances rather than contact sealing.

This non-contact design gives Roots pumps their defining characteristics: very high pumping speed, low ultimate pressure capability when backed, and the ability to handle large gas flows at low pressures.

Why Roots Pumps Cannot Operate Alone

The Roots pump's non-contact design means it has a relatively high compression ratio limit — typically 10:1 to 15:1. If the pump tries to compress gas against a closed discharge (i.e., with no backing pump), the gas simply leaks back through the clearances between the rotors and housing.

This means a Roots pump must always discharge into a backing pump (also called a fore pump) that maintains the discharge pressure low enough for the Roots pump to operate effectively. The backing pump handles the final compression from the Roots pump discharge pressure to atmospheric pressure.

How the Roots + Backing Pump Combination Works

The combination works as follows:

1. Backing pump evacuates the system from atmospheric pressure down to the Roots pump's starting pressure (typically 10–50 mbar)
2. Roots booster starts once the system pressure drops below its maximum differential pressure rating
3. Roots booster rapidly pumps large volumes of gas from the process chamber, discharging into the backing pump
4. Backing pump handles the gas from the Roots booster discharge and exhausts to atmosphere

The result is a system that combines the high pumping speed of the Roots booster at low pressures with the atmospheric discharge capability of the backing pump.

Performance Benefits of Adding a Roots Booster

Adding a Roots booster to an existing backing pump system provides two key benefits:

Deeper Ultimate Vacuum

A liquid ring pump alone might achieve 33 mbar ultimate vacuum. With a Roots booster, the same system can reach 1–2 mbar. With two stages of Roots boosters, pressures below 0.1 mbar are achievable.

Higher Pumping Speed at Low Pressures

At low pressures, the volumetric efficiency of backing pumps decreases significantly. A Roots booster maintains high volumetric efficiency down to much lower pressures, dramatically increasing the effective pumping speed of the system at process pressure.

For example, a liquid ring pump with a pumping speed of 500 m³/h at 100 mbar might only achieve 100 m³/h at 10 mbar. Adding a Roots booster with a 2,000 m³/h displacement can increase the effective system pumping speed at 10 mbar to 800–1,000 m³/h.

Roots Booster Sizing: Key Parameters

Compression Ratio

The compression ratio of a Roots booster is the ratio of its discharge pressure to its inlet pressure. Most Roots boosters are rated for a maximum compression ratio of 10:1 to 15:1 at their maximum differential pressure.

Operating above the maximum compression ratio causes excessive gas backflow through the rotor clearances, reducing efficiency and potentially overheating the pump.

Speed Ratio (Booster to Backing Pump)

The speed ratio is the ratio of the Roots booster's displacement to the backing pump's displacement. Common speed ratios range from 3:1 to 10:1.

A higher speed ratio gives more pumping speed benefit but requires the backing pump to handle a larger gas load. The optimal speed ratio depends on the required vacuum level and the shape of the pumping speed curve needed.

Starting Pressure

Roots boosters should not be started at atmospheric pressure — the high differential pressure would overload the motor and potentially damage the pump. Most Roots boosters are started when the system pressure drops below 50–100 mbar, either manually or via a pressure switch.

Some Roots boosters are equipped with a bypass valve that allows them to start at atmospheric pressure by bypassing gas around the pump until the differential pressure drops to a safe level.

Multi-Stage Roots Booster Systems

For applications requiring very deep vacuum (below 0.1 mbar), two or more stages of Roots boosters can be used in series:

• First stage Roots booster: Handles the high-flow, medium-vacuum range (10–100 mbar)
• Second stage Roots booster: Handles the low-flow, deep-vacuum range (0.1–10 mbar)
• Backing pump: Handles final compression to atmosphere

Each stage is sized to operate within its compression ratio limit, with the backing pump handling the final stage.

Typical Backing Pump Options

Liquid Ring Pumps

The most common backing pump for Roots boosters in industrial applications. Liquid ring pumps handle wet gas, condensable vapors, and corrosive gases well. The combination of Roots booster + liquid ring pump is the standard configuration for chemical, pharmaceutical, and food processing vacuum systems.

Rotary Vane Pumps

Used as backing pumps for Roots boosters in applications requiring deeper ultimate vacuum. The combination of Roots booster + two-stage rotary vane pump can achieve ultimate pressures below 0.01 mbar.

Dry Screw Pumps

Increasingly used as backing pumps for Roots boosters in semiconductor and pharmaceutical applications where oil-free vacuum is required. The combination provides deep, clean vacuum with excellent corrosive gas handling.

Key Applications for Roots Booster Systems

Chemical Distillation and Evaporation

Large-scale vacuum distillation columns require high pumping speeds at pressures of 1–50 mbar. Roots booster + liquid ring pump systems are the standard configuration.

Freeze Drying (Lyophilization)

Pharmaceutical and food freeze dryers require vacuum levels of 0.1–1 mbar with high pumping speed to handle water vapor sublimation. Multi-stage Roots booster systems are commonly used.

Vacuum Metallurgy

Vacuum induction melting and vacuum arc remelting furnaces require deep vacuum (below 0.1 mbar) with high pumping speed. Multi-stage Roots + rotary vane or dry screw systems are standard.

Transformer Drying

Power transformers are dried under vacuum before oil filling. The large volume of moisture to be removed requires high pumping speed at low pressure — ideal for Roots booster systems.

Vacuum Impregnation

Impregnation of electrical windings, porous castings, and composite materials requires deep vacuum to remove trapped air before resin injection. Roots booster systems provide the required vacuum level and pumping speed.

Maintenance Considerations

Roots boosters are relatively low-maintenance machines due to their non-contact design:

• Oil changes: Roots boosters use oil only in the timing gear and bearing housings, not in the compression chamber. Oil change intervals are typically 4,000–8,000 hours
• Clearance checks: Rotor-to-housing and rotor-to-rotor clearances should be checked during major overhauls (typically every 20,000–40,000 hours)
• Seal inspection: Shaft seals should be inspected annually and replaced if leaking

Conclusion

Roots vacuum boosters are the most cost-effective way to extend the performance of an existing vacuum system — achieving deeper vacuum, higher pumping speed, or both. When properly sized and matched to the backing pump, a Roots booster system provides reliable, long-service-life performance across a wide range of industrial applications.

Zhongtian Pumps manufactures a complete range of Roots vacuum pumps and vacuum boosters from 150 m³/h to 70,000 m³/h displacement, designed for use with liquid ring, rotary vane, and dry screw backing pumps. Contact our engineering team for system sizing assistance.

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