How to Select a Backing Pump for a Roots Vacuum Pump

Why a Roots Pump Needs a Backing Pump

A Roots vacuum pump has a distinct advantage: it delivers very high pumping speed in the low-pressure range—an edge that many other vacuum pump types simply do not offer. But the Roots pump also has a limitation—it cannot exhaust directly to atmosphere. It must be paired with another vacuum pump connected in series to operate properly.

In practice, the backing pump first pulls the system pressure down to a certain level, and only then does the Roots vacuum pump start. To put it simply, the backing pump handles the gas load from atmosphere down to the Roots pump's starting pressure, and the Roots pump then takes over to boost the vacuum level in the high-throughput range.

Consequences of Improper Selection

Choosing the wrong backing pump directly leads to Roots pump problems. The most common issues include:

• Backing pump speed is too small, causing excessive pressure differential across the Roots pump and rapid temperature rise

• Backing pump ultimate vacuum is insufficient, so the Roots pump never reaches its proper operating range

• Backing pump is not matched to the process gas, allowing condensable vapors to condense inside and damage the pump

So selecting a backing pump is not a matter of "just having one." It needs to be carefully matched to the operating conditions.

Three Factors to Consider in Selection

Process Gas Composition

This is the first thing to confirm. Does the gas stream contain water vapor? Corrosive components? Condensable gases? These directly determine what type of backing pump to choose.

If the process involves significant water vapor—such as in vacuum drying or distillation—a water ring vacuum pump is a good choice for the backing pump. A water ring vacuum pump naturally tolerates water vapor; when the gas compresses inside the pump, the vapor condenses directly into the seal liquid and is discharged without damaging the pump.

If the process gas is clean and contains no corrosive components, the selection range is much broader—oil-sealed rotary vane vacuum pumps or screw vacuum pumps can be considered.

Roots Pump Starting Pressure

Different Roots vacuum pump models have different starting pressure requirements. Some can start at higher pressures, while others require the backing pump to pull the system down to a much lower level before they can be switched on.

When selecting a backing pump, you must confirm that the pump's ultimate vacuum is at least one order of magnitude lower than the Roots pump's starting pressure. Otherwise, the pressure differential at start-up will be excessive, leading directly to overload and overheating.

Pumping Speed Matching

The backing pump's speed must be matched to the Roots pump's speed. A rough rule of thumb is that the backing pump speed should be roughly one-tenth to one-fifth of the Roots pump speed.

If the backing pump is too small, the Roots pump will draw in gas faster than the backing pump can discharge it. The discharge pressure rises, differential pressure increases, and overheating follows. If the backing pump is too large, it adds unnecessary equipment cost and higher energy consumption.

Backing Pump Type Comparison

Why Protection Measures Are Essential

Some users skip protection devices between the backing pump and the Roots pump to save cost. This approach is not advisable.

During start-up and operation, if the pressure differential across the Roots vacuum pump continuously exceeds the design value, the rotors will expand from heat and make contact, causing internal wear or even seizure.

Standard protection measures include:

• Installing a differential pressure switch across the Roots pump inlet and outlet—shuts down automatically if pressure differential exceeds the limit

• Setting a time-delay start to ensure the backing pump reaches the required vacuum level before the Roots pump starts

• Installing temperature sensors to monitor the Roots pump casing temperature

• Configuring a bypass line to allow some gas to bypass the Roots pump when differential pressure is too high

These measures should be considered at the same time as backing pump selection, not as an afterthought.

Recommended Configurations for Common Applications

Vacuum Drying

Drying processes generate gas streams with high water vapor content. The backing pump should prioritize a water ring vacuum pump. If drying temperatures are high and vapor loads are large, the seal liquid circuit also needs a cooling system to keep the liquid temperature from rising too much.

Vacuum Distillation

Distillation processes may involve organic solvent vapors. The choice of backing pump depends on the solvent type. If the solvent has a low saturation vapor pressure and tends to condense inside the pump, the best options are a water ring vacuum pump or a rotary vane pump equipped with a gas ballast device.

Vacuum Degassing

In metallurgical or plastics industry degassing, the gas stream is relatively clean. The main requirements for the backing pump are pumping speed and ultimate vacuum. For these conditions, rotary vane vacuum pumps or screw vacuum pumps are common choices.

Vacuum Packaging and Conveying

In food and pharmaceutical vacuum packaging and pneumatic conveying, the gas is clean with little moisture. Backing pump selection is more flexible. Small rotary vane pumps or dry vacuum pumps can meet the requirements.

Advice from a Manufacturer

As a Roots vacuum pump manufacturer, we see two main types of backing pump selection problems during field service: one is undersizing the pumping speed, and the other is completely ignoring process gas composition.

The first problem typically occurs because the purchaser only looked at the nameplate speed of the Roots pump without calculating the actual effective pumping speed of the backing pump at the Roots pump's starting pressure. The second problem is more common—specifying a standard oil-sealed pump as the backing pump for a process with water vapor. The oil emulsifies, the pump seizes, and the Roots pump gets damaged as well.

Our standard recommendation to customers is: write down the full process conditions before selecting a backing pump—gas composition, temperature, humidity, dust content. Then match the pump type and speed to those conditions. If you are unsure, having the manufacturer's technical team assist with selection is far more cost-effective than troubleshooting after a failure.

Conclusion

Selecting a backing pump for a Roots vacuum pump has no fixed formula, but one principle applies: the backing pump must provide sufficient effective pumping speed across the entire operating pressure range of the Roots pump, and it must tolerate the components present in the process gas. If you get four things right—gas composition, starting pressure, speed matching, and protection measures—most selection problems can be avoided.