Series application of Roots vacuum pumps
Roots vacuum pumps have become indispensable across a wide range of industrial sectors—from paper manufacturing and chemical processing to metallurgy, pharmaceuticals, and food production. Their ability to deliver high pumping speeds in the medium-to-high vacuum range makes them the preferred choice for countless applications. However, there are situations where the pumping capacity of a single Roots vacuum pump simply cannot meet process requirements.
In production environments—particularly in paper manufacturing where Roots vacuum pumps are used extensively for the wire section, vacuum couch rolls, vacuum presses, and felt dewatering—the output of the Roots vacuum pump directly affects paper production volume and quality. When flow rate drops or vacuum level decreases, paper moisture content fluctuates, leading to roll sticking, pressing marks, and other paper defects. When a single Roots vacuum pump cannot achieve the required vacuum level, the solution is often to connect two Roots vacuum pumps in series.
This article provides a comprehensive examination of the series application of Roots vacuum pumps, covering the fundamental principles of tandem configuration, proper sizing and selection, efficiency considerations, practical operational safeguards, and the critical issue of white water backflow prevention.
Why Series Connection of Roots Vacuum Pumps Is Necessary
The Efficiency Curve Limitation
Every Roots vacuum pump has an optimal operating range where it delivers its highest efficiency. For most Roots vacuum pumps, the high-efficiency operating zone lies between -300 mmHg and -400 mmHg. When the required vacuum level exceeds -500 mmHg, a single Roots vacuum pump often struggles to achieve the necessary vacuum.
This is not a flaw in the Roots vacuum pump design—it is a fundamental characteristic of positive displacement vacuum technology. As the pressure decreases (vacuum increases), the compression ratio required of the Roots vacuum pump increases, and internal leakage (backflow) through the rotor clearances becomes more significant. At very low pressures, the effective pumping speed of a single Roots vacuum pump drops dramatically.
The Series Solution
By connecting two Roots vacuum pumps in series, the total compression ratio is distributed across both stages. The first Roots vacuum pump (front pump) serves as a pre-evacuation stage, reducing the pressure from the rough vacuum level down to an intermediate pressure. The second Roots vacuum pump (rear pump) then further compresses the gas to achieve the final required vacuum level.
This configuration allows the system to achieve vacuum levels that would be impossible with a single Roots vacuum pump while maintaining each pump within its efficient operating range.
Proper Sizing – The 1.5 to 2 Times Rule
The Pumping Speed Ratio
The key to successful series operation of Roots vacuum pumps lies in proper sizing. The pumping speed of the rear pump must be 1.5 to 2 times greater than the pumping speed of the front pump. This ratio is not arbitrary—it is based on the fundamental physics of gas flow in series-connected vacuum systems.
When two Roots vacuum pumps are connected in series, the gas flow rate is the same through both pumps, but the volumetric flow at the inlet of the rear pump is compressed (lower volume) compared to the inlet of the front pump. Therefore, the rear pump must have a larger displacement to handle the same mass flow at the lower pressure.
The consequence of incorrect sizing: If the rear pump is not sufficiently larger than the front pump, the system will not achieve the expected vacuum level, and the rear pump may overload or overheat. If the rear pump is too large relative to the front pump, the system becomes unnecessarily expensive and may experience instability.
Different Types and Sizes
International practice in series application of Roots vacuum pumps goes beyond simply using two pumps of the same type with different capacities. Many overseas installations use different types of pumps in series. The principle remains the same: the front pump has smaller capacity, and the rear pump has larger capacity.
This approach allows engineers to optimize the system for specific process requirements—for example, using a pump with tighter clearances and higher compression ratio for the front stage and a pump with larger displacement and higher throughput for the rear stage.
Coaxial Drive Configuration
A particularly elegant design approach for series-connected Roots vacuum pumps is coaxial transmission. In this configuration, both Roots vacuum pumps are mounted on the same shaft and driven by a single motor. The benefits include:
Coaxial compensation: The load is distributed evenly across both pumps
Small current fluctuation range: The motor current remains stable during operation
Convenient operation: Single motor control simplifies the system
This design is particularly common in overseas installations and represents the state of the art in series-connected Roots vacuum pump technology.
Efficiency Considerations in Series-Connected Roots Vacuum Pumps
Understanding Zero-Flow Compression Ratio (K₀)
To fully appreciate the performance of series-connected Roots vacuum pumps, it is helpful to understand a key technical parameter: the zero-flow compression ratio (K₀) . This is defined as the ratio of the pump's discharge pressure to its inlet pressure when the inlet is sealed (zero flow condition). K₀ depends on rotor geometry, internal clearances, and the type of gas being pumped.
The efficiency of a Roots vacuum pump in a series configuration can be expressed as:
Efficiency = K₀ / (K₀ + S_ratio - 1)
Where S_ratio is the pumping speed ratio between the Roots vacuum pump and the backing pump.
Practical Efficiency Guidelines
When the S_ratio equals 1, the Roots vacuum pump efficiency equals 1 (100%). However, as the S_ratio increases, efficiency decreases.
For practical applications, when K₀ is greater than 20 and the S_ratio is less than 10, the efficiency of the Roots vacuum pump can exceed 70%. This is generally considered the minimum acceptable efficiency for industrial applications.
Ultimate Vacuum Estimation
The ultimate vacuum of a series-connected Roots vacuum pump system can be estimated by dividing the backing pump's ultimate pressure by the compression ratio. However, in practice, the actual achievable ultimate vacuum is typically 3 to 5 times higher than this theoretical value.
For example:
With a single-stage rotary vane pump as the backing pump, the theoretical ultimate vacuum is approximately 50 Pa / 35 = 1.4 Pa, but the practical achievable vacuum is around 5 Pa
With a water-sealed liquid ring pump as the backing pump, the theoretical ultimate vacuum is approximately 3,300 Pa / 15 = 220 Pa, but the practical achievable vacuum is around 660 Pa
Practical Operational Safeguards – The White Water Backflow Problem
Understanding the Risk
One of the most critical operational concerns when using series-connected Roots vacuum pumps in paper manufacturing—and one that is frequently overlooked—is the white water backflow phenomenon.
When a series-connected Roots vacuum pump unit is shut down, the pipeline often contains significant amounts of white water (the water and fiber mixture from the papermaking process) that can flow back into the pump chamber—particularly into the rear pump.
Why this is especially dangerous: The inlet of the rear pump is typically located at the bottom of the pump body. Water that flows back accumulates in the pump chamber and cannot drain out naturally. In some cases, the water level can be more than 250 mm above the pump rotor.
The Consequence of Ignoring the Problem
If the Roots vacuum pump is started with water accumulated in the pump chamber, the pumping medium changes from gas to liquid. The motor will attempt to pump water, which is incompressible, leading to:
Motor overload – the motor draws excessive current
Circuit breaker tripping – the overload protection activates
Potential motor burnout – if the overload protection fails
Rotor damage – the incompressible water can cause hydraulic lock
As one industry source succinctly states: "If started under such conditions, the pumping medium changes from gas to water, and the motor will inevitably overload and trip".
The Solution – Check Valves and Drainage
The solution to the white water backflow problem is straightforward and effective:
Install a check valve (non-return valve) on the inlet pipeline. This prevents white water from flowing back into the Roots vacuum pump when the unit is shut down.
Drain accumulated water promptly after shutdown. Even with a check valve, some water may still enter the system. Operators should establish a procedure to check for and remove any accumulated water before restarting the Roots vacuum pump.
Additional Safeguards
Beyond check valves and drainage, consider these additional safeguards for series-connected Roots vacuum pumps:
Startup sequence verification: Ensure the correct startup sequence is followed—backing pump first, then the front Roots vacuum pump, then the rear Roots vacuum pump.
Pressure monitoring: Install pressure gauges between stages to monitor the pressure distribution and detect any abnormalities.
Temperature monitoring: Monitor the temperature of each Roots vacuum pump to prevent overheating from excessive pressure differential.
Automatic control systems: Modern Roots vacuum pump systems can be equipped with automatic controls that start and stop the pumps based on pressure readings, providing overload protection.
Multi-Stage Configurations for Higher Vacuum
Beyond Two Pumps
For applications requiring even higher vacuum levels, more than two Roots vacuum pumps can be connected in series. Multi-stage Roots vacuum pump configurations are common in applications requiring ultimate pressures as low as 1 Pa.
The general principle is that the number of stages is determined by the required working vacuum level, and the ultimate vacuum of the system must be half to one order of magnitude higher than the working vacuum.
Combination with Different Backing Pump Types
Roots vacuum pumps in series can be combined with various types of backing pumps, depending on the application requirements:
Rotary vane pumps – for general applications requiring high ultimate vacuum
Liquid ring pumps – particularly advantageous when pumping large quantities of condensable vapors
Dry pumps – for applications where oil contamination cannot be tolerated
Slide valve pumps – for heavy-duty industrial applications
When a liquid ring pump is used as the backing pump for a series-connected Roots vacuum pump unit, it offers a significant advantage: the ability to pump large quantities of condensable vapors, especially when oil-sealed mechanical pumps would suffer from oil degradation or when the vacuum system cannot tolerate oil contamination.
Application Examples of Series-Connected Roots Vacuum Pumps
Paper Manufacturing
In paper mills, Roots vacuum pumps are used at critical points throughout the production process: the wire section, vacuum couch rolls, vacuum presses, and felt dewatering. Series-connected Roots vacuum pumps are employed when the required vacuum level exceeds the capability of a single pump.
Vacuum Distillation and Chemical Processing
In chemical and pharmaceutical industries, series-connected Roots vacuum pumps are used for vacuum distillation, vacuum drying, and other processes requiring medium-to-high vacuum levels. The ability to achieve higher vacuum with greater pumping speed makes series configurations ideal for these applications.
Metallurgy and Vacuum Coating
In vacuum metallurgy and coating applications, series-connected Roots vacuum pumps provide the high pumping speeds needed for rapid evacuation of large chambers. Multi-stage configurations can achieve the low pressures required for processes such as vacuum melting, degassing, and sputtering.
Power Generation and Deaeration
In thermal power plants, series-connected Roots vacuum pump units are increasingly used for condenser air extraction and deaeration, offering faster startup and significant energy savings compared to traditional steam ejector systems
Shandong Zhangqiu Blower Co., Ltd. – Expertise in Roots Vacuum Pump Series Applications
Shandong Zhangqiu Blower Co., Ltd. (often referred to as "Zhanggu" or "SDZG"), founded in 1968, has accumulated over 50 years of experience in the design, production, and manufacturing of industrial blowers and vacuum equipment. The company is a modern enterprise integrating product design, production, and sales, with multiple holding subsidiaries, joint ventures, and a wholly-owned subsidiary in the United States.
The company's ZJP series Roots vacuum pumps are positive displacement pumps that use a pair of figure-eight rotors rotating in opposite directions within the casing. Key features include:
High pumping speed at low inlet pressure – the defining advantage of Roots vacuum pumps
Automatic overload protection – built-in safety features protect the pump from damage
Series connection capability – designed for integration with backing pumps in series configurations
As the company's technical documentation emphasizes: "ZJP series Roots vacuum pumps cannot be used alone; they must be connected in series with a backing pump". This fundamental requirement is the foundation of all series applications of Roots vacuum pumps.
Shandong Zhangqiu Blower Co., Ltd. offers comprehensive technical support for series applications of Roots vacuum pumps, including:
System design consultation – helping customers select the right pump sizes and configurations
Installation guidance – ensuring proper piping, check valve installation, and drainage
Operational training – teaching operators the correct startup sequence and maintenance procedures
After-sales support – providing spare parts, troubleshooting, and repair services
The company's deep expertise in Roots vacuum pump technology and commitment to customer success make it a trusted partner for industrial facilities requiring reliable series-connected Roots vacuum pump solutions.
Conclusion – Series Connection Unlocks the Full Potential of Roots Vacuum Pumps
The series application of Roots vacuum pumps is a proven and effective solution for achieving vacuum levels beyond the capability of a single pump. By connecting two Roots vacuum pumps in series—with the rear pump having a pumping speed 1.5 to 2 times greater than the front pump—industrial users can achieve vacuum levels exceeding -500 mmHg while maintaining each pump within its efficient operating range.
Key takeaways for successful series application of Roots vacuum pumps:
Proper sizing is essential – The rear pump must have 1.5 to 2 times the pumping speed of the front pump to achieve the required vacuum level.
Coaxial drive offers advantages – Mounting both Roots vacuum pumps on the same shaft provides coaxial compensation, stable current draw, and convenient operation.
Different types can be combined – International practice shows that different types of Roots vacuum pumps can be used in series, as long as the front pump has smaller capacity and the rear pump has larger capacity.
White water backflow must be prevented – In paper manufacturing applications, install check valves on inlet pipelines and drain accumulated water promptly after shutdown to prevent motor overload and pump damage.
Multi-stage configurations are available – For higher vacuum requirements, three or more Roots vacuum pumps can be connected in series, with the number of stages determined by the required working vacuum.
By following these principles and working with experienced manufacturers like Shandong Zhangqiu Blower Co., Ltd., industrial users can unlock the full potential of Roots vacuum pumps—achieving the vacuum levels, pumping speeds, and operational reliability that modern production processes demand. The series application of Roots vacuum pumps is not just a technical solution; it is a pathway to higher quality, greater efficiency, and more competitive production.
