Roots Vacuum Pump for Paper Mills: Steady Vacuum at High Speed

Here is why it is important to select the right roots vacuum pump for paper mills and tissue paper converting. At 350 metres per minute, a tissue folding line has no tolerance for a wandering vacuum. The suction belt that grips each sheet and carries it through the fold plates is holding on by kilopascals, not by anything you can see. When the vacuum sags, even briefly, the sheet lifts off the belt, the fold misses, and the web snaps. The line stops, and stays stopped for the minutes it takes to clear the break, rethread the paper, and climb back to speed.

This article is about the one property that matters most when you run tissue converting at high speed: steady vacuum. Not peak vacuum. Not deepest vacuum. Steady vacuum, held through filter loading, reel changes, and grade switches. A roots vacuum pump delivers that steadiness by design. Positive displacement means vacuum follows rotor speed, and a variable-frequency drive means the speed can be trimmed to hold a setpoint. On a paper mill floor, that is the difference between a converting hall that runs to schedule and one that bleeds tonnage in four-minute stops.

Where a roots vacuum pump works in a paper mill

Before the drive and the silencer, understand the duty. A tissue converting line uses vacuum to do mechanical work: it transports the sheet. Napkin folders, facial tissue lines, and paper towel machines all grip the web against a suction belt and pull it through the fold geometry. Every fold on a finished napkin was held in place by vacuum somewhere upstream. That is a different job from wet-end dewatering, where vacuum boxes and suction rolls pull water out of the pulp web before the dryer. Wet-end duty is high volume and shallow vacuum, usually a separate and larger pump.

Converting vacuum and wet-end vacuum are not the same specification. This article is about converting, because that is where vacuum stability most directly protects uptime, and where a roots vacuum pump earns its keep.

What a high-speed tissue line demands from its vacuum pump

The vacuum duty on a converting line is set by three numbers that move independently across a shift.

Line speed. The faster the web runs, the more suction the belt needs to hold it. At 200 m/min, the grip requirement is one thing; at 350 m/min it is another. The vacuum level is not a fixed target. It tracks speed.

Machine count. A hall runs multiple folding machines in parallel, often four to six, all drawing from a common vacuum header. When one machine cycles, a reel change or a jam clear, the change in demand ripples through the header and reaches the others.

Grade and dust. Lighter facial grades need deeper vacuum to hold the web than napkin stock. And every line throws paper dust into the air, which gradually loads the inlet filter and changes the suction the belt actually sees.

A fixed-speed vacuum pump answers all three the same way: run flat out and bleed off the excess. It holds a steady speed, not a steady vacuum. As the filter loads or a reel changes, the suction at the belt drifts, and the drift is what breaks the sheet.

The cost of unstable vacuum on a tissue converting line

The visible cost of a web break is the lost tonnage during the stop. The hidden cost is larger. Operators learn the line is only safe below a certain speed, so they throttle the whole hall down and run below design throughput all day, every day. That lost capacity never shows up on a single report, but it is the difference between a mill that meets its delivery dates and one that misses them.

The wrong fix is to oversize the pump. An oversized fixed-speed pump still cannot react in milliseconds, and it wastes energy into the bargain. The right fix is a vacuum source that actively holds a setpoint, and that is what the positive-displacement roots mechanism, driven by a variable-frequency drive, does.

Why positive displacement keeps a roots vacuum pump steady

A roots vacuum pump is a positive-displacement machine. Two three-lobe rotors counter-rotate inside a figure-eight housing, synchronized by timing gears so they never touch. Each lobe traps a pocket of gas and carries it from inlet to outlet, a fixed volume per revolution. Because the geometry is fixed, delivered volume tracks rotor speed almost linearly: turn the rotors faster and you get more suction, predictably. (The full mechanism, including timing gears, non-contact clearance, and the internal slip that sets the ultimate vacuum, is covered in our roots vacuum pump working principle explainer.)

That linear speed-to-vacuum relationship is the whole point. It means a variable-frequency drive can command a precise vacuum setpoint by trimming rotor speed, rather than throttling a fixed-speed pump. When the inlet filter loads with paper dust and the suction at the belt starts to sag, the drive sees the drift and speeds the rotors up a fraction to hold the setpoint. When a reel changes and demand drops, the drive trims speed back down. The vacuum at the belt stays inside a tight band.

The rotors run non-contact, with clearance in the tenths-of-a-millimetre range, and the timing gears and bearings sit outside the gas path. There is no sealing oil in the vacuum stream, which on a converting line means no oil carryover onto the tissue and no contamination rejects.

Internal slip, the small back-leakage through the rotor clearances, is what caps a single-stage roots pump at roughly minus 45 to minus 50 kPa. That ceiling is not a limitation for tissue converting. It is the pump’s natural operating band. Napkin lines run around minus 30 to minus 35 kPa, and lighter facial grades need margin toward minus 45 kPa. You are working the pump where it is most efficient, not pushing it to its slip limit.

The Slovdson LC roots vacuum pump: PMSM + VFD in constant-vacuum mode

The rotors are only half the machine. The Slovdson LC unit turns them with a rare-earth permanent-magnet synchronous motor, a PMSM, governed by a variable-frequency drive, and it runs in constant-vacuum mode. Set the target vacuum level, and the drive holds it by adjusting rotor speed in real time. That is the practical payoff of positive displacement.

The drive reacts in milliseconds, which is what a high-speed line needs. A reel change on one of six folding machines produces a demand transient that a throttled fixed-speed pump cannot catch; the VFD absorbs it before the other five machines notice. The result is that belt suction stays steady, the sheet stays gripped, and the line stays running.

One detail worth knowing. Slovdson builds in Ganzhou, Jiangxi, the heavy-rare-earth producing region of China, and the permanent magnets in the PMSM are sourced locally. That is not a marketing line. It is the reason the motor holds above 93% efficiency at part load instead of burning fixed magnetising current the way an induction motor does, and part load is where a converting line lives most of its running hours. On the 37 kW unit, rated torque is 186 Nm at 80.3 A on a 380 to 400 V, 50 Hz supply.

More videos for energy-saving roots blower & vacuum pump on the Slovdson YouTube Channel.

Low-noise roots vacuum pump design: the four-chamber silencer

A converting hall stacks several folding machines, their vacuum sources, and the operators who run them, all in one room. Noise is a compliance number and a fatigue number, and on a 24/7 line it is a retention number.

The LC unit ships with a four-chamber resonant silencer and a six-sided sound enclosure in an oversize cabinet, 1500 by 1350 mm. The result on the SLDS-3700LC is 74 to 84 dB at one metre with the silencer fitted. Against a conventional 37 kW roots vacuum pump of the same rating, that is 8 to 13 dB quieter. A 10 dB drop reads as roughly half the perceived loudness to a person standing next to it, so this is not a rounding improvement. It is the difference between a hall where you shout and a hall where you talk.

The package is also built for the hall itself. Inlet filtration is sized for the paper-dust load, and the VFD speed trend flags when the element needs changing- no special tools, no guesswork. If you have ever stood next to an unsilenced roots unit on a tissue line, you know the sound. The four-chamber silencer is why the LC cabinet is larger than the unit it replaces, and it is a worthwhile trade.

Energy-saving roots vacuum pump: a tissue hall in numbers

Against a fixed-speed roots unit of the same rating, the PMSM plus VFD host cuts power consumption by 30 to 65%. The mechanism is simple. At part load, the PMSM stays above 93% efficiency while a fixed-speed induction motor sags toward 85%, and it draws its magnetising current whether you need the torque or not.

Here is what that means in a real hall. Take a tissue hall running five folding lines, 18 hours a day, 320 days a year:

• A conventional 37 kW fixed-speed vacuum pump draws about 213,000 kWh a year in that duty.
• The 37 kW LC vacuum pump in constant-vacuum mode trims that by 35 to 55%, roughly $9,000 to $14,100 per unit per year at typical industrial tariffs.

But the energy saving is not the headline. The headline is the sheets that did not break because the vacuum did not sag. A single avoided web break a day at 350 m/min is recovered tonnage that shows up on the delivery schedule, not on the electricity bill. The 37 kW unit is our most commonly specified model for tissue exactly because it covers the four-to-six-machine hall with headroom, and the 45 kW LC steps up for six-to-eight machines or line speeds above 350 m/min.

Sizing a roots vacuum pump by line count and speed

Specify by the duty point, not by habit. For a converting line, the duty point is set by machine count, line speed, and the vacuum the grade demands:

• 3 to 4 folding machines: 30 kW class
• 4 to 6 folding machines, napkin or facial, 200 to 300 m/min: 37 kW, the tissue sweet spot
• 6 to 8 machines, or line speed above 350 m/min: 45 kW class
• Single-line or compact operations: 15 to 22 kW class

The 37 kW covers the four-to-six-machine hall most tissue converters run, with vacuum depth to minus 45 kPa for lighter facial grades. The 37 kW product page carries the full specification, and the LC energy-saving lineup lists the complete range, including the smaller 15, 22, and 30 kW classes and larger custom builds for integrated mills. The platform does not stop at 45 kW: custom units are engineered to the duty point at 90 kW, 132 kW, and beyond.

Specifying a roots vacuum pump for your paper mill

A roots vacuum pump is specified for a paper mill because it holds vacuum steady at high speed. Positive displacement makes delivered volume follow rotor speed; the VFD closes the loop in constant-vacuum mode; the PMSM keeps the motor efficient at the partial loads where a converting line actually runs. Sized to the duty point- four to six machines for the 37 kW, larger halls for the 45 kW- the unit protects uptime at the exact moment a fast line has no margin for a sag. It also runs 8 to 13 dB quieter and cuts the energy bill 30 to 65%, and both of those matter over the unit’s working life even after the first day of steady vacuum has done its job.

Send us your line speed, machine count, and grade mix, and our technical team will return a duty-point recommendation with a payback calculation. Contact us for a specification and quotation.

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