Introduction to the backing pump for selecting Roots vacuum pumps
A Roots vacuum pump is a positive displacement rotary pump that offers high pumping speeds at low inlet pressures, making it an indispensable component in countless industrial applications—from chemical processing and pharmaceutical manufacturing to metallurgy, semiconductor production, and aerospace simulation. However, one fundamental characteristic of Roots vacuum pumps must be understood from the outset: a Roots vacuum pump cannot be used alone.
Unlike pumps that can discharge directly to atmosphere, a Roots vacuum pump has a limited compression ratio and cannot operate against atmospheric pressure. It must be combined with a backing pump (also called a fore pump or primary pump) to form a complete vacuum unit. The backing pump serves to reduce the discharge pressure of the Roots vacuum pump to a level at which it can operate safely and efficiently.
Selecting the correct backing pump is therefore one of the most critical decisions in designing a Roots vacuum pump system. The pumping speed, type, and configuration of the backing pump directly determine the ultimate vacuum, pumping speed, and overall reliability of the entire Roots vacuum pump unit. This article provides a comprehensive introduction to the principles and practices of selecting a backing pump for Roots vacuum pumps, covering compression ratio calculations, pumping speed matching, ultimate pressure requirements, gas composition considerations, and pre-evacuation time trade-offs.
Understanding the Compression Ratio – The Foundation of Backing Pump Selection
What Is the Compression Ratio?
The compression ratio of a Roots vacuum pump is defined as the ratio of the pump's discharge pressure to its inlet pressure. Because the Roots vacuum pump has a limited compression ratio, the backing pump must be capable of reducing the system pressure to a level within the Roots vacuum pump's operating range.
The maximum compression ratio of a Roots vacuum pump varies significantly with the discharge pressure:
Under high discharge pressure, the maximum compression ratio is only about 3:1.
At discharge pressures between 1 and 10 mbar (1 mbar = 0.75 Torr), the maximum compression ratio can reach 50:1 or even higher.
This variation means that the selection of a backing pump for Roots vacuum pumps must be based on the long-term working pressure range of the system.
The Relationship Between Compression Ratio and Pumping Speed
The compression ratio of a Roots vacuum pump is inversely proportional to the pumping speed ratio between the Roots vacuum pump and the backing pump. In other words:
K = S_Roots / S_Backing
Where:
K = Compression ratio
S_Roots = Pumping speed of the Roots vacuum pump
S_Backing = Pumping speed of the backing pump
This fundamental relationship is the key to understanding how to select a backing pump for Roots vacuum pumps. A higher compression ratio requires a smaller backing pump relative to the Roots vacuum pump; a lower compression ratio requires a larger backing pump.
Practical Example – Calculating the Required Backing Pump Size
To illustrate the selection process, consider the following example, which is widely cited in industry literature:
Application requirement: A Roots vacuum pump unit must provide a pumping speed of 600 liters per second at an inlet pressure of 1 mbar, operating continuously over long periods. What size backing pump should be selected?
Step 1 – Assume a backing pump size: Start by assuming a backing pump with a pumping speed of 30 liters per second.
Step 2 – Calculate the compression ratio: K = 600 / 30 = 20
Step 3 – Determine the maximum discharge pressure: From the compression ratio curve (or performance data), a compression ratio of K = 20 corresponds to a maximum discharge pressure of approximately 30 mbar.
Step 4 – Determine the permissible inlet pressure: When using a 30 L/s backing pump, the Roots vacuum pump can operate reliably at inlet pressures below 1.5 mbar.
Step 5 – Compare with the requirement: The required working vacuum is 1 mbar, which is below 1.5 mbar. Therefore, a 30 liters per second backing pump can satisfy the pumping requirements of this vacuum system.
Conclusion: If the required working pressure had been above 1.5 mbar, a larger backing pump would have been necessary.
General Principles for Backing Pump Sizing
The 1/2 to 1/4 Rule
When a reciprocating or liquid ring rough vacuum pump is used as the backing pump for a Roots vacuum pump, a general rule applies: the pumping speed of the backing pump should be 1/2 to 1/4 of that of the Roots vacuum pump.
Why this range?
If the backing pump is too large (exceeding 1/2 of the Roots vacuum pump's speed), the compression ratio becomes too large, causing the Roots vacuum pump to operate at excessively high discharge temperatures.
If the backing pump is too small (less than 1/4 of the Roots vacuum pump's speed), the Roots vacuum pump may not achieve the required ultimate vacuum or may experience unstable operation.
Typical Gradation (Pumping Speed Ratio)
In the vacuum industry, the ratio of pumping speeds between a Roots vacuum pump and its backing pump is referred to as the gradation.
Typical gradations range between 1:2 and 1:8.
General selection range: The compression ratio is typically selected between 2 and 10.
Low vacuum applications: For operation in the low vacuum range, a smaller displacement ratio (2:1 to 4:1) is recommended.
Medium or high vacuum applications: For operation in the medium or high vacuum range, a larger displacement ratio (4:1 to 10:1) should be preferred
Ultimate Pressure Requirements – Matching the Backing Pump to the Vacuum Level
The required ultimate vacuum of the Roots vacuum pump unit is another critical factor in backing pump selection. Different backing pump types are capable of achieving different ultimate vacuum levels:
High Vacuum Requirements (1×10⁻³ Pa to 1×10⁻² Pa)
When the Roots vacuum pump unit must achieve ultimate pressures in the range of 1×10⁻³ Pa to 1×10⁻² Pa, a two-stage rotary vane oil-sealed mechanical pump or a two-stage slide valve mechanical pump should be selected as the backing pump.
These pumps provide the deep vacuum capability required for demanding applications such as semiconductor manufacturing, vacuum coating, and research vacuum systems.
Medium Vacuum Requirements (1×10⁻² Pa to 1×10⁻¹ Pa)
For ultimate pressure requirements between 1×10⁻² Pa and 1×10⁻¹ Pa, a single-stage oil-sealed mechanical pump can serve as the backing pump. This configuration is common in general industrial vacuum applications such as vacuum drying, impregnation, and packaging.
Rough Vacuum Requirements (133 Pa to 1,333 Pa)
When the required ultimate pressure falls in the range of 133 to 1,333 Pa, a reciprocating vacuum pump or a liquid ring vacuum pump can be used as the backing pump.
Important note: When a liquid ring or reciprocating pump is used as the backing pump for a Roots vacuum pump, the backing pump's speed must be limited to 1/2 to 1/4 of the Roots vacuum pump's speed. Exceeding this ratio will cause the compression ratio to become too large, resulting in excessively high discharge temperatures and potential damage to the Roots vacuum pump.
Pre-Evacuation Time – Balancing Speed and Efficiency
Beyond steady-state operation, the pre-evacuation (roughing) time requirement is an important factor in backing pump selection for Roots vacuum pumps.
When a Smaller Backing Pump Is Sufficient
If the normal working time of the Roots vacuum pump unit is much longer than the pre-evacuation time, a smaller backing pump can be selected. The slower pre-evacuation is acceptable because it represents only a small fraction of the total operating cycle.
When a Larger Backing Pump Is Required
If the vacuum chamber is large or if the process requires very rapid pre-evacuation, a larger backing pump must be used. The higher pumping speed during the roughing phase reduces the time required to reach the Roots vacuum pump's starting pressure.
The Two-Pump Strategy for Power Savings
Sometimes, to better utilize the efficiency of the Roots vacuum pump unit, the system is equipped with two backing pumps:
A large pre-evacuation pump that is used only during the roughing phase.
A smaller holding pump that maintains the vacuum during normal operation.
After the system reaches normal operating vacuum, the large pump is shut down, and the smaller pump continues to maintain the vacuum. This strategy saves power while still providing rapid pump-down when needed.
Gas Composition – Corrosive and Condensable Gases
The composition of the gas being pumped is a critical consideration in selecting a backing pump for Roots vacuum pumps.
Corrosive Gases
If the pumped gas contains corrosive media, oil-sealed mechanical pumps cannot be used as the backing pump. The corrosive gases will attack the pump's internal components, degrade the sealing oil, and lead to rapid pump failure.
In such cases, alternative backing pump technologies must be considered:
Liquid ring vacuum pumps (using compatible sealing liquids)
Dry screw pumps (with corrosion-resistant coatings)
Reciprocating pumps (with appropriate materials)
Condensable Vapors
Oil-sealed mechanical pumps are not suitable for pumping gases containing large quantities of condensable vapor (such as steam or organic solvents). When condensable vapor enters an oil-sealed pump, it condenses and mixes with the sealing oil, causing rapid emulsification that destroys the oil's lubricating properties.
Solutions include:
For large quantities of condensable vapor: Use a liquid ring vacuum pump or other pump type that can handle wet gases.
For trace amounts of condensable vapor: Use an oil-sealed mechanical pump equipped with a gas ballast device. The gas ballast introduces air into the compression chamber, preventing condensation by keeping the vapor partial pressure below its dew point.
The Water Ring Option
For applications involving both corrosive gases and condensable vapors, the liquid ring vacuum pump is often the preferred backing pump choice for Roots vacuum pumps. When a water ring pump is paired with a Roots vacuum pump, the working liquid can be water or organic solvents (such as methanol, ethanol, xylene, or acetone). The liquid ring pump provides:
Tolerance to liquid carryover
Ability to handle corrosive gases (with appropriate materials)
Resistance to vapor condensation issues
Multi-Stage Roots Vacuum Pump Configurations
For applications requiring high pumping speeds at low inlet pressures (in the range of 1 to 100 Pa), a single Roots vacuum pump with a backing pump may not be sufficient.
Three- and Four-Stage Units
To achieve high pumping speeds at low inlet pressures, a Roots vacuum pump can be used as the backing pump for another Roots vacuum pump, forming a three-stage or four-stage unit.
In these multi-stage configurations:
The first Roots vacuum pump (closest to the vacuum chamber) provides the primary pumping speed.
The second Roots vacuum pump serves as an intermediate stage.
The third Roots vacuum pump (or a conventional backing pump) handles the discharge.
Pumping Speed Ratios in Multi-Stage Units
In multi-stage Roots vacuum pump units, the pumping speed ratio between successive Roots vacuum pumps is generally between 2 and 5.
This progressive reduction in pumping speed matches the decreasing gas flow as pressure drops (due to constant mass flow but lower density), ensuring each stage operates efficiently within its design range.
Summary Table – Backing Pump Selection Guide for Roots Vacuum Pumps
Ultimate vacuum: 1×10⁻³–1×10⁻² Pa | Two-stage rotary vane or slide valve pump | High vacuum capability |
Ultimate vacuum: 1×10⁻²–1×10⁻¹ Pa | Single-stage oil-sealed mechanical pump | General industrial use |
Ultimate vacuum: 133–1,333 Pa | Reciprocating or liquid ring pump | Speed = 1/2–1/4 of Roots pump |
Fast pre-evacuation, large chamber | Larger backing pump | May use dual pumps strategy |
Slow process, long hold time | Smaller backing pump | Energy-efficient |
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 |
High pumping speed at 1–100 Pa | Multi-stage Roots (2–5:1 speed ratio per stage) | 3 or 4 stages typical |
Conclusion – A Systematic Approach to Backing Pump Selection
The selection of a backing pump for Roots vacuum pumps is not a one-size-fits-all decision. It requires careful analysis of multiple interrelated factors:
Compression ratio requirements – determined by the working pressure range of the Roots vacuum pump. The compression ratio is inversely proportional to the pumping speed ratio between the Roots vacuum pump and the backing pump.
Ultimate pressure requirements – dictate the type of backing pump needed: two-stage oil-sealed pumps for high vacuum, single-stage pumps for medium vacuum, and liquid ring or reciprocating pumps for rough vacuum.
Pre-evacuation time – larger chambers or faster cycle times require larger backing pumps; longer process times allow smaller pumps.
Gas composition – corrosive gases and condensable vapors may rule out oil-sealed mechanical pumps in favor of liquid ring or dry pumps.
Multi-stage configurations – for very low inlet pressures, multiple Roots vacuum pumps can be connected in series with speed ratios of 2 to 5 between stages.
By following the principles and examples outlined in this article, engineers and plant managers can confidently select a backing pump that matches their Roots vacuum pump's characteristics and application requirements, ensuring reliable, efficient, and cost-effective vacuum system performance.
