Why a Roots Vacuum Pump Cannot Operate Alone

Many users assume that a Roots vacuum pump can be started like a standard fan or blower—just turn it on and let it run. That is not the case. The design and operating principle of a Roots pump prevent it from discharging directly to atmosphere. This limitation often causes confusion and has led to numerous equipment failures in the field.

To understand why, it helps to first clarify the fundamental difference between a Roots pump and an ordinary fan.

Why It Cannot Discharge Directly to Atmosphere

A Roots vacuum pump is a positive displacement pump with no internal compression. A standard fan or compressor compresses gas before discharging it, so the discharge pressure can exceed the inlet pressure. A Roots pump works differently—the rotors simply transfer gas from the inlet to the discharge side without compressing it.

This "transfer" process introduces a critical limitation: the pressure differential between the discharge and inlet must be kept within a certain range.

If the discharge is open directly to atmosphere while the inlet connects to a vacuum vessel, the pressure differential becomes extremely high. This creates two immediate problems:

• Motor overload – the motor struggles to overcome the pressure differential, current draw spikes, and the motor trips or burns out

• Rotor seizure – some operators try to fix the overload by installing a larger motor, but this only makes things worse. Heat generated by the pressure differential builds up inside the pump chamber, causing the rotor clearances—typically only 0.1 to 0.8 millimeters—to shrink rapidly due to thermal expansion, eventually locking the rotors and destroying the pump

This is not a manufacturing defect; it is a direct consequence of the pump‘s operating principle. Therefore, a Roots pump must be backed by a fore pump that reduces the system pressure to an acceptable level before the Roots pump starts.

Available Backing Pump Options

A backing pump for a Roots pump must be capable of discharging directly to atmosphere. Common options include:

• Liquid-ring vacuum pump – uses liquid (usually water) as the sealing medium, tolerant of vapors and light dust

• Rotary vane pump – oil-sealed mechanical pump, compact with good ultimate vacuum, but sensitive to water vapor

• Piston-type slide valve pump – oil-sealed mechanical pump, robust with large displacement, also prone to oil contamination

• Reciprocating piston pump – simple and durable, but large, noisy, and increasingly rare

Each type has its place. The oil-sealed pumps (vane and slide valve) offer higher ultimate vacuum and work well with dry, clean gases. But when water vapor is present, it condenses in the oil, causing emulsification and performance loss. Liquid-ring pumps have a lower ultimate vacuum but handle condensable vapors and dusty gases far better.

Comparison of Backing Pump Types

The table shows that if the gas stream is clean and dry, an oil-sealed pump can achieve a better ultimate vacuum. But if water vapor, organic solvents, or light particulates are present, the liquid-ring pump offers significantly better tolerance.

Why Liquid-Ring Pumps Are Often the Better Choice

Under typical operating conditions, using a liquid ring vacuum pump as the backing pump offers distinct advantages over other types, for two main reasons:

First, liquid-ring pumps handle condensable vapors effectively. Distillation, drying, and evaporation processes generate gas streams that often contain water vapor or organic solvents. With oil-sealed pumps, these vapors condense into the oil, causing emulsification, loss of lubrication, and reduced pumping efficiency. The liquid-ring pump uses water (or another compatible fluid) as its operating medium, so condensed vapors simply mix with the working fluid and are discharged, creating no performance penalty.

Second, liquid-ring pumps are ideal where oil contamination is unacceptable. In chemical, pharmaceutical, and food processing applications, oil vapor migrating into the vacuum system can compromise product quality or create safety hazards. Liquid-ring pumps do not rely on oil for sealing, so there is no oil vapor carryover.

These two conditions—high vapor content and oil sensitivity—are extremely common in industrial settings, which explains why liquid-ring pumps are the preferred backing pump for Roots systems in many industries.

Common Applications for Liquid-Ring Backing Pumps

• Chemical processing: vacuum distillation, solvent recovery

• Pharmaceutical manufacturing: vacuum drying, evaporation concentration

• Food processing: dehydration, freeze-drying

• Power generation: condenser evacuation, turbine sealing

• Environmental: wastewater aeration, gas scrubbing

Key Considerations When Sizing a Liquid-Ring Backing Pump

Selecting a liquid-ring pump to back a Roots vacuum pump requires more than simply matching nameplate flow ratings. Several practical factors must be addressed:

Sealing fluid temperature and flow rate
The pump uses liquid to form the sealing ring. Higher fluid temperature means higher vapor pressure and lower ultimate vacuum. During summer months when cooling water is warmer, the system‘s ultimate pressure will degrade. This is a characteristic of all liquid-ring pumps and must be allowed for during selection.

Cavitation risk
At very low inlet pressures, cavitation occurs inside the pump, causing significant noise and vibration. Over time, cavitation erodes the impeller. For high-vacuum applications, a cavitation protection system may be necessary, or the operating parameters may need adjustment.

Sealing fluid circulation system
If the gas stream contains corrosive components or significant amounts of condensables, the sealing fluid requires cooling and filtration for recirculation. Without proper treatment, the fluid accelerates corrosion and reduces pumping efficiency.

Materials of construction
When handling corrosive gases, wetted parts of the liquid-ring pump must be constructed from stainless steel or special alloys. This is often overlooked at the selection stage, but it has a direct impact on service life.

Accounting for these factors at the design stage prevents many field problems down the line.

Problems Caused by Improper Configuration

When a Roots pump and its backing pump are mismatched, a range of issues emerge:

• Insufficient backing pump displacement – the gas load from the Roots pump exceeds the backing pump’s capacity, discharge pressure rises, pressure differential exceeds limits, and motor overload trips

• Backing pump ultimate vacuum too high – the system’s ultimate vacuum is limited by the backing pump. No matter how good the Roots pump is, it cannot go beyond what the backing pump can achieve

• Wrong pump type – using an oil-sealed pump for a vapor-laden process leads to rapid oil degradation, frequent maintenance, and performance loss

• Incompatible materials – using a cast-iron liquid-ring pump for corrosive gases results in rapid corrosion of the impeller and casing, drastically shortening life

• Poor sealing fluid system design – the working fluid is not cooled, temperature climbs, and ultimate pressure degrades progressively

These issues are almost never due to a single failed component. They are configuration errors from the start.

What Proper Configuration Delivers

A well-matched Roots–liquid-ring system performs very differently in the field:

• Smooth startup – the backing pump pulls down to the set pressure, the Roots pump engages without overload tripping

• Stable operation – pressure differential stays within limits, temperatures are controlled, noise levels normal

• Low maintenance – the liquid-ring pump only needs periodic fluid changes, avoiding the frequent oil changes required by oil-sealed pumps

• Achievable vacuum – system pressure remains within process requirements, not drifting upward over time

• Extended life – rotor clearances stay within specification; no thermal expansion seizures

Why Choosing the Right Manufacturer Matters

The technical capability of a roots vacuum pump manufacturer is largely reflected in how well it understands backing pump matching. The same Roots pump, paired with different backing pumps and different numbers of stages, yields completely different performance curves.

An experienced industrial vacuum pump manufacturer will make recommendations based on the process gas composition, target vacuum level, operating load, and other factors—rather than simply selling a pump off the shelf. For systems handling corrosive, flammable, or toxic gases, material selection, seal design, and explosion protection all come into play.

This explains why more international clients prefer suppliers who can deliver complete vacuum system solutions rather than assembling components from multiple vendors. A well-matched system runs smoothly, requires less maintenance, and lasts longer. A poorly matched system causes problems from the day of commissioning.

A long-term relationship with a reliable roots vacuum pump supplier also ensures ongoing technical support and genuine spare parts availability. For continuous chemical or pharmaceutical production, this value far exceeds the upfront equipment cost difference.

Conclusion

The fact that a Roots vacuum pump cannot discharge directly to atmosphere makes backing pump selection an essential step in designing any vacuum system. The Roots pump must not be started until the backing pump has reduced the system pressure to the allowable limit. The liquid ring vacuum pump has distinct advantages for handling condensable vapors and avoiding oil contamination, making it the most common backing pump choice for Roots applications.

Choosing between oil-sealed pumps, liquid-ring pumps, single-stage or multi-stage configurations is not about comparing price or ultimate vacuum numbers alone. It requires a thorough assessment of the actual gas composition and operating conditions. Getting this right prevents many of the common field issues—overload, seizure, and failure to meet vacuum specifications—that otherwise plague poorly configured systems.