A vacuum pump removes gas molecules from a sealed volume so the pressure inside drops below atmosphere. The common types — rotary vane, diaphragm, scroll and water-ring circulating pumps — differ mainly in how deep a vacuum they reach and how well they tolerate solvent vapour. Your choice comes down to those two variables.
How Does a Vacuum Pump Work?
Every vacuum pump does one job: it moves gas molecules out of a closed volume faster than they leak back in, so the pressure inside falls below the air around it. Two specifications describe how well it does that job, and they drive almost every buying decision. Ultimate vacuum is how deep the pump can pull. Pumping speed is how much gas it moves per minute while getting there.
Most laboratory pumps are positive-displacement machines. They trap a pocket of gas at inlet pressure, squeeze it into a smaller space, and push it out to atmosphere — then repeat that cycle continuously. Rotary vane, diaphragm, scroll and water-ring pumps all work this way. They differ only in the moving part that traps the gas: an oiled rotor with sliding vanes, a flexing membrane, two interleaved spirals, or a ring of water.
A second mechanism family exists for very deep vacuum, where gas molecules are pushed along by momentum transfer instead of being trapped in a pocket. Those machines always work behind a roughing pump and rarely appear on a laboratory shopping list, so we will leave them there.
What matters when you choose is simpler: how deep you need to pull, and what the pump has to swallow on the way. Everything else — taps, tank size, voltage — comes after.
What Are the Main Vacuum Pump Types?
Rotary vane vacuum pump. An offset rotor with sliding vanes sweeps a crescent-shaped chamber, and a thin film of oil seals the clearances. Among common bench pumps it reaches the deepest vacuum and holds it well against small leaks. The oil is also its weak point. Solvent vapour condenses into the oil, thins it, and slowly wrecks the seal — so solvent duty means a cold trap, gas ballast, and regular oil changes.
Diaphragm pump. A membrane flexed by a crank moves gas through inlet and outlet valves. No oil anywhere, and chemically resistant head materials shrug off many solvent vapours. You trade away vacuum depth and pumping speed; the membrane itself is the wear part. For clean, moderate vacuum it is the lowest-maintenance option on the bench.
Scroll pump. Two interleaved spiral plates — one fixed, one orbiting — push gas from the edge of the plates to the center. Oil-free, and it pulls deeper than a diaphragm pump. The tip seals wear with running hours and eventually need replacement.
Water-ring (circulating water) pump. This is the type we build. An impeller spins a ring of water around the inside of the casing, and that ring acts as the piston: the chambers between the blades expand to draw gas in, then shrink to push it out. Solvent vapour that would destroy an oiled pump simply condenses into the ring water. The price you pay is vacuum depth — the ring water’s own vapour pressure sets the floor, and warmer water pulls a weaker vacuum.
A type list alone does not choose a pump. The next section does.
Which Two Variables Actually Decide the Choice?
After building and testing pumps for evaporation and filtration lines, we reduce selection to two questions.
First question: how deep a vacuum does your process need? Rough vacuum covers most laboratory work: evaporating solvents, vacuum filtration, degassing. Deep vacuum — molecular distillation, freeze drying, high-vacuum distillation — belongs to rotary vane pumps and beyond. If your process lives in deep vacuum, the water-ring branch of this article ends here.
Second question: what vapour will the pump swallow? Condensable solvent vapour is what kills oiled pumps. If your exhaust stream carries solvent, you either pick a pump that tolerates it — water-ring or diaphragm — or you protect a rotary vane pump with a cold trap and gas ballast and accept the oil maintenance.
The decision flow we run through with customers:
- Deep vacuum, clean or dry gas → rotary vane or scroll.
- Deep vacuum, solvent vapour → rotary vane with a cold trap, plus disciplined oil changes.
- Rough vacuum, solvent vapour → water-ring or diaphragm.
- Rough vacuum, several devices at once → a multi-tap water-ring pump.
- Rough vacuum, one benchtop device → a compact single-tap water-ring pump.
Most evaporation and filtration work lands in the bottom three branches, which is exactly where our production line sits.
How Does a Water-Ring (Circulating Water) Vacuum Pump Work?
Inside the casing, the impeller sits slightly off-center. As it spins, it throws water outward against the casing wall, forming a moving ring. The spaces between the impeller blades widen on one side — gas is pulled in through the suction port — and narrow on the other side, compressing the gas and pushing it out through the discharge. The water ring is the piston, the seal and the coolant in one part.
A tank recirculates the water, so the pump runs from its own reservoir instead of a drain line. Solvent vapour drawn in from a rotary evaporator condenses into that reservoir instead of contaminating oil or corroding vanes. That single property is why water-ring pumps own evaporation duty.
The same water also sets the vacuum floor: the pump cannot pull below the vapour pressure of its own ring water, and warmer water means a weaker vacuum. Keep the tank water cool and refreshed and the pump holds its rated vacuum; let it heat up through a long evaporation run and you will watch the vacuum gauge drift.
On our own line, the SHZ-D(III) benchtop circulating water vacuum pump is the unit we ship for rotary evaporators, reactors and filtration setups: 180 W of power, a 60 L/min circulation flow, and a 10 L/min pumping speed at the tap.
What Do Real Water-Ring Vacuum Pump Specs Look Like?
Type descriptions are easy to find; real numbers are not. Here are the two water-ring pumps we build, side by side:
| Spezifikation | SHZ-D(III) | SHZ-95B |
|---|---|---|
| Pump type | Water-ring circulating | Water-ring, 5 taps |
| Power | 180 W | 550 W |
| Flow rate | 60 L/min | 80 L/min |
| Suction capacity | 10 L/min (single tap) | 10 L/min per tap; 50 L/min total |
| Tank size | — | 50 L |
| Maximum vacuum | — | 0.098 MPa |
| Voltage | 220 V | 220 V or 110 V |
Read the suction row carefully. Per-tap speed is what each connected device actually feels; total capacity only matters if you genuinely run several devices at once. The SHZ-95B’s five taps at 10 L/min each let one pump serve a whole bench line, and its 50 L tank heats up far more slowly during long evaporations than a small reservoir would. The 0.098 MPa maximum vacuum tells you honestly where the water-ring working range ends.
Sizing follows the same logic. A benchtop rotary evaporator pairs naturally with a benchtop pump like the SHZ-D(III). Once you move to 5L-and-above equipment, vapour load and flask volume outgrow a single small tap — that is where the SHZ-95B vertical pump, built for 5L and above equipment earns its footprint, and it runs on either 220 V or 110 V supply. Keep tubing short and wide, because every narrow bend eats pumping speed you paid for, and remember the pump is only half the vapour strategy: the condenser and chiller carry most of the solvent load. We cover that balance in a separate guide on matching a rotary evaporator with its vacuum pump and chiller. Both parameter sheets also appear in our company catalog, and every unit passes the vacuum-hold test routine described on our quality and factory page before it ships.
When Is a Water-Ring Pump the Wrong Choice?
We would rather lose a sale than have a pump come back. Skip the water ring in these cases:
- Your process needs deep vacuum. Molecular distillation, freeze drying and high-vacuum distillation live below what a water ring can reach. That is rotary vane territory, and no multi-tap convenience changes the physics.
- Your solvent cannot touch water. Anything that reacts with water, or that you cannot allow into a water stream, belongs in a dry pump or behind a trap.
- You cannot handle the contaminated water. The ring water captures whatever the pump swallows. If disposing of solvent-laden water is a problem at your site, a diaphragm pump keeps the solvent in a condensate flask instead.
- You need oil-free and deep at the same time. That combination points to a scroll pump, not to water.
If none of those apply — and for rotary evaporation, filtration and reactor duty they usually do not — water-ring is the low-maintenance answer.
Fragen und Antworten
What is the difference between ultimate vacuum and pumping speed?
Ultimate vacuum is the deepest pressure the pump can reach; pumping speed is how fast it moves gas to get and stay there. A process that outgasses heavily needs speed more than depth. On our SHZ-95B, the figures are 0.098 MPa maximum vacuum and 10 L/min of suction per tap — depth and speed sit on separate lines of the sheet because they solve different problems.
Can a water-ring vacuum pump handle solvent vapour directly?
Yes — that is its main advantage. Solvent vapour condenses into the ring water instead of contaminating oil or attacking vanes. The trade-off is that the reservoir gradually becomes contaminated water, so refresh the tank regularly and treat the drained water as solvent waste.
How many devices can one pump run at the same time?
Count the taps and the per-tap suction. The SHZ-95B provides five taps at 10 L/min each, 50 L/min in total, so one pump can hold vacuum for several benchtop devices at once — provided each device’s own demand stays within what a single tap delivers.
Does a water-ring vacuum pump need oil changes?
No. There is no oil in the pumping chamber — the water ring does the sealing. Routine maintenance is refreshing the tank water, keeping the reservoir clean, and checking tubing and seals. That is the whole list.
Which Vacuum Pump Should You Start With?
If your work is benchtop rotary evaporation, filtration or reactor support on a single device, start with the SHZ-D(III) — 180 W, 10 L/min at the tap, nothing to maintain but water. If you run 5L-and-above equipment or several devices off one pump, the SHZ-95B’s five taps, 550 W and 50 L tank are the sensible step up. And if your process needs a deeper vacuum than the 0.098 MPa class a water ring reaches, stop here and look at rotary vane pumps instead — we would rather tell you that now.
You can compare both models in our vacuum pump range, and broader selection questions are answered on our FAQ page. Match the pump to your required ultimate vacuum and your vapour load, and the shortlist usually writes itself.