What factors should be considered when configuring a backing pump for a Roots vacuum pump?
In industrial applications ranging from chemical processing and metallurgy to pharmaceutical drying and food packaging, the Roots Vacuum Pump has become a ubiquitous piece of equipment. Its ability to deliver high pumping speeds across medium to high vacuum ranges makes it indispensable. However, as any experienced engineer knows, a Roots Vacuum Pump cannot operate alone. It must be paired with a backing pump (also called a fore-vacuum pump) to function safely and efficiently. The selection of the appropriate backing pump is not a trivial decision; it directly impacts the system’s ultimate pressure, pumping speed, energy consumption, and reliability. Choosing incorrectly can lead to overheating, excessive power draw, premature rotor wear, or even catastrophic failure of the Roots Vacuum Pump.
So, what factors should be considered when configuring a backing pump for a Roots Vacuum Pump? This article provides a comprehensive answer, drawing on decades of industry best practices. We will examine three primary considerations: pre-evacuation time requirements, ultimate pressure targets, and the nature of the gases being pumped—including corrosivity and condensability. Additionally, we will discuss multistage Roots pumping systems where one Roots Vacuum Pump serves as the backing stage for another. By understanding these factors, engineers can design vacuum systems that are both cost-effective and operationally robust.
Factor 1: Pre-Evacuation (Roughing) Time Requirements
The first factor to evaluate is the time allowed for pre-evacuation—the period required to bring the vacuum chamber from atmospheric pressure down to the starting pressure of the Roots Vacuum Pump. This is a critical parameter because the backing pump alone must handle the entire gas load during this initial roughing phase.
Balancing pre-evacuation time with normal operation
Consider the duty cycle of your process. If the normal production or processing time (when the Roots Vacuum Pump is actively boosting) is substantially longer than the pre-evacuation time, you can select a relatively small backing pump. For example, in a continuous metallurgical degassing process, the system might run for hours at deep vacuum, with only a few minutes of roughing at the start. Here, a smaller backing pump will suffice, saving both capital cost and energy.
Conversely, if the vacuum chamber is large—such as in a space simulation chamber or a large freeze dryer—and the process demands a very rapid pump-down from atmosphere to the Roots Vacuum Pump’s allowable inlet pressure (typically ≤1,330 Pa), a much larger backing pump is necessary. A backing pump that is too small will prolong the roughing phase, reducing overall throughput and potentially causing condensation or oxidation of sensitive products.
The “dual backing pump” strategy
In some sophisticated Roots Vacuum Pump units, engineers employ two backing pumps: a large pump for rapid pre-evacuation and a smaller maintenance pump that takes over once the target vacuum is reached. The large pump is then shut off to save power. This approach is common in intermittent batch processes where quick turnaround is essential but steady-state gas load is low. When configuring such a system, the Roots Vacuum Pump must be equipped with appropriate valves and controls to isolate the large backing pump during the maintenance phase. While this adds complexity, it can significantly reduce electricity costs over the life of the equipment.
Practical guideline: For most industrial applications, a backing pump with a pumping speed between 1/5 and 1/2 of the Roots Vacuum Pump’s nominal speed provides a reasonable balance. However, if pre-evacuation time is critical, do not hesitate to size the backing pump larger—up to equal speed—but be aware that the Roots Vacuum Pump will then experience a higher compression ratio, which may require a bypass valve to limit temperature rise during startup.
Factor 2: Ultimate Pressure Requirements of the Roots Vacuum Pump System
The second and perhaps most commonly discussed factor is the required ultimate pressure (lowest achievable vacuum) of the entire Roots pumping system. The backing pump plays a decisive role in determining this limit because the Roots Vacuum Pump cannot create a vacuum deeper than the backing pump’s ultimate pressure multiplied by the Roots stage’s compression ratio.
Matching backing pump types to target vacuum levels
Industry experience has established clear mappings between ultimate pressure ranges and appropriate backing pump technologies:
For ultimate pressures down to 1×10⁻³ Pa to 1×10⁻² Pa (high vacuum):
A two-stage rotary vane oil-sealed mechanical pump or a two-stage sliding vane mechanical pump is typically required. These pumps can achieve blank-off pressures in the 10⁻² to 10⁻³ Pa range when properly maintained. When combined with a Roots Vacuum Pump, the system can reach 10⁻³ Pa or even lower, depending on the Roots stage’s compression characteristics. Such configurations are common in semiconductor coating, research vacuum systems, and advanced thin-film deposition.For ultimate pressures between 1×10⁻² Pa and 1×10⁻¹ Pa:
A single-stage oil-sealed mechanical pump (rotary vane or sliding vane) is usually sufficient. These pumps have ultimate pressures around 0.1 to 1 Pa, and the Roots Vacuum Pump boosts the combination into the 10⁻² Pa range. This is adequate for many industrial applications such as vacuum drying, impregnation, and metallurgical furnaces.For ultimate pressures between 133 Pa and 1,333 Pa (rough vacuum):
Here, the backing pump can be a reciprocating piston pump or a liquid ring vacuum pump. These pumps are robust, handle vapors well, and are economical for large volumes. However, they cannot achieve deep vacuum on their own. When paired with a Roots Vacuum Pump, the combination typically operates in the 100–1,000 Pa range, suitable for applications like vacuum filtration, conveying, and some chemical distillation processes.
Avoiding the compression ratio trap
One critical caution: When using a reciprocating or liquid ring pump as the backing stage for a Roots Vacuum Pump, the pumping speed of the backing pump should not exceed 1/2 to 1/4 of the Roots pump’s speed. Why? Because a backing pump that is too large will force the Roots Vacuum Pump to operate at an excessively high compression ratio (discharge pressure divided by inlet pressure). This high compression ratio generates intense heat at the discharge side, raising the pump’s temperature beyond safe limits—often exceeding 100°C and causing rotor expansion, seal damage, or oil coking. In severe cases, the Roots Vacuum Pump may seize entirely. Therefore, always consult the manufacturer’s allowable maximum pressure differential (typically 30–100 mbar for most Roots Vacuum Pumps) and size the backing pump such that this limit is not exceeded during normal operation.
Factor 3: Gas Composition – Corrosive and Condensable Components
The third factor is sometimes overlooked but can be the most damaging if ignored. The nature of the gases or vapors being pumped—specifically whether they contain corrosive chemicals or condensable steam/solvents—profoundly influences backing pump selection.
Handling corrosive gases
If the process involves corrosive gases such as chlorine, hydrogen chloride, sulfur dioxide, or acid vapors, an oil-sealed mechanical pump (rotary vane or sliding vane) is generally unsuitable. The corrosive agents will attack the pump’s internal metal surfaces, degrade the sealing oil, and quickly lead to pump failure. In such cases, alternative backing pump technologies should be considered:
Dry screw pumps: These have no oil in the pumping chamber and can be constructed with corrosion-resistant coatings (e.g., nickel or ceramic). They pair well with Roots Vacuum Pumps in aggressive chemical environments.
Liquid ring pumps with appropriate seal liquids: Using a compatible fluid (e.g., sulfuric acid for chlorine service or mineral oil for certain organics) can provide corrosion resistance.
Diaphragm pumps: For very small flows, but generally too small for backing a Roots Vacuum Pump in industrial scales.
Handling condensable vapors (steam, solvents)
Another common challenge is the presence of large amounts of condensable vapor—for example, water vapor in freeze drying or solvent vapor in chemical recovery. Standard oil-sealed mechanical pumps are poor at handling condensable gases because the vapor condenses inside the pump and mixes with the oil, causing emulsification. The oil turns milky, loses its lubricity, and can cause bearing failure. The remedy is twofold:
Use a backing pump with a gas ballast valve. A gas ballast introduces a small amount of dry air (or inert gas) into the compression chamber, preventing condensation by keeping the partial pressure of the vapor below its dew point. Most modern oil-sealed rotary vane pumps include this feature. However, the gas ballast slightly reduces ultimate vacuum.
If only trace amounts of condensable vapor are present, the same gas-balled oil-sealed pump is acceptable. But for high vapor loads, a liquid ring pump (using water or oil as the sealant) may be a better choice because it operates isothermally and can handle continuous vapor flow without emulsification.
When the gas contains micro amounts of condensable vapor, the Roots Vacuum Pump combined with a gas-balled oil-sealed backing pump is often the most economical solution. The Roots Vacuum Pump itself, being a dry pump (no internal compression), is less susceptible to vapor condensation, but the backing pump remains vulnerable. Some Roots Vacuum Pumps are available in gas-cooled or wet configurations that allow higher vapor tolerance, but conventional models still require a properly selected backing pump.
Multistage Roots Configurations: One Roots Pump Backing Another
For applications requiring very high pumping speeds at low inlet pressures (typically 1 to 100 Pa), a single Roots Vacuum Pump backed by a mechanical pump may not suffice. In these cases, engineers configure a three- or four-stage Roots pumping system, where one Roots Vacuum Pump serves as the backing pump for another Roots Vacuum Pump. The final stage (lowest pressure) is backed by a conventional mechanical pump, but intermediate stages are Roots units.
Pumping speed ratio guidelines
When stacking Roots Vacuum Pumps in series, the ratio of pumping speeds between stages is critical. Industry practice recommends a speed ratio of 2:1 to 5:1 between successive stages. For example, a system might have:
A large Roots Vacuum Pump (2,000 m³/h) as the first stage (closest to the chamber).
A medium Roots Vacuum Pump (800 m³/h) as the second stage.
A smaller Roots Vacuum Pump (300 m³/h) as the third stage.
A rotary vane backing pump (100 m³/h) as the final stage.
This progressive reduction in pumping speed matches the decreasing gas flow as pressure drops (due to constant mass flow but lower density). If the ratio is too high (e.g., 10:1), the downstream Roots Vacuum Pump will be overwhelmed and may overheat. If the ratio is too low (e.g., 1:1), the system becomes unnecessarily expensive without performance gains.
Additional considerations for multistage systems
In such configurations, each Roots Vacuum Pump requires its own bypass valve to manage differential pressure during startup. Also, interstage cooling may be necessary because gas heating accumulates across stages. These systems are common in large-scale vacuum furnaces, space simulation chambers, and particle accelerators.
Summary Table: Backing Pump Selection Guide for Roots Vacuum Pumps
Fast pre-evacuation, large chamber | Large backing pump (50-100% of Roots speed) | May need dual pumps (large for roughing, small for holding) |
Slow process, long hold time | Small backing pump (10-20% of Roots speed) | Energy-efficient |
Ultimate pressure ≤10⁻² Pa | Two-stage rotary vane or sliding vane | High vacuum capability |
Ultimate pressure 10⁻¹–10⁻² Pa | Single-stage oil-sealed mechanical pump | General industrial use |
Ultimate pressure 133–1333 Pa | Reciprocating or liquid ring pump | Rough vacuum, robust |
Corrosive gases | Dry screw or corrosion-resistant liquid ring | Avoid oil-sealed pumps |
High condensable vapor load | Liquid ring pump or gas-balled oil-sealed pump | Prevent emulsification |
Trace condensable vapor | Oil-sealed pump with gas ballast | Acceptable for low loads |
Very high pumping speed at 1–100 Pa | Multistage Roots (2–5:1 speed ratio per stage) | 3 or 4 stages typical |
Practical Tips for Implementation
When you have selected a backing pump based on the above factors, follow these additional steps to ensure a successful integration with your Roots Vacuum Pump:
Install a bypass valve between the Roots Vacuum Pump discharge and the backing pump inlet. This valve protects the Roots stage during differential pressure spikes.
Include a vacuum relief valve on the backing pump inlet to prevent the Roots Vacuum Pump from seeing atmospheric pressure if the backing pump stops unexpectedly.
Monitor interstage pressure with a gauge located between the Roots Vacuum Pump outlet and the backing pump inlet. This pressure should never exceed the Roots pump’s maximum allowable discharge pressure.
Provide adequate cooling – either air or water – for both pumps, especially when operating near the upper pressure limits.
Automate the startup sequence using a PLC: start backing pump → open valves → wait for pressure drop → start Roots Vacuum Pump. Many modern Roots Vacuum Pumps come with integrated controllers that handle this logic.
Conclusion: Thoughtful Configuration Yields Reliable Performance
Configuring a backing pump for a Roots Vacuum Pump is not a one-size-fits-all task. It requires careful analysis of pre-evacuation time, ultimate pressure requirements, and gas composition. A backing pump that is too small will prolong cycle times and may fail to reach the required vacuum. A pump that is too large—especially with coarse vacuum technologies like liquid ring pumps—can overheat and destroy the Roots Vacuum Pump due to excessive compression ratio. Corrosive or condensable gases demand specialized backing pump designs to avoid rapid degradation.
By following the guidelines presented here, engineers can design Roots pumping systems that are efficient, durable, and well-suited to their specific processes. The Roots Vacuum Pump is a remarkable technology, but its performance is only as good as the backing pump that supports it. Choose wisely, and your vacuum system will provide years of trouble-free service. Choose poorly, and you will face repeated breakdowns, high energy bills, and lost production. As with many engineering decisions, success lies in asking the right questions before making a purchase. We hope this article has equipped you with those questions.
