How Do Vacuum Pumps Work? Principles, Types and Spec Sheets Explained
HCEM Engineering TeamShare
A vacuum pump works by trapping gas from a sealed space, compressing it and pushing it out to the atmosphere, over and over. Each cycle removes part of the gas that is left, so the pressure inside falls quickly at first and then more slowly as fewer molecules remain. The pump does not "suck": it removes gas, and the atmosphere outside does the pushing.
This guide explains what happens inside a pump, how the main types differ, and how to read the two numbers that matter most on a spec sheet: flow and ultimate vacuum. The worked examples use datasheet values from our own oil-free piston vacuum pumps.
What does a vacuum pump actually do?
It lowers the pressure in a closed volume (a chamber, a tank, a line or a suction cup) below the pressure of the surrounding air. At sea level the air presses at about 101.3 kPa (1013 mbar). A "vacuum" is any pressure below that.
Vacuum is written in two ways, and mixing them up is the most common mistake on a datasheet:
- Gauge pressure is measured against the atmosphere, so it is negative: −92 kPa means 92 kPa below atmospheric.
- Absolute pressure is measured against a perfect vacuum, so it is always positive: −92 kPa gauge is about 9.3 kPa (93 mbar) absolute.
| Gauge | Absolute | What it means in practice |
|---|---|---|
| 0 kPa | 1013 mbar | Atmospheric pressure, no vacuum |
| −50 kPa | about 510 mbar | Enough for many suction cups lifting smooth, light parts |
| −86 kPa | about 150 mbar | Typical of compact DC pumps |
| −92 kPa | about 93 mbar | Typical of a standard oil-free piston pump |
| −98 kPa | about 33 mbar | High-vacuum versions of oil-free piston pumps |
Most industrial work (pick and place, CNC workholding, packaging, laboratory filtration, medical suction) happens in this "rough vacuum" range, between atmospheric pressure and about 1 mbar.
How does a vacuum pump work, step by step?
Almost every pump you will meet in a factory or a lab is a positive displacement pump. It repeats the same four steps, whether the moving part is a piston, a diaphragm, a vane or a scroll:
- Intake. A space inside the pump opens up. Gas from the chamber flows into it through the inlet.
- Isolation. The space is sealed off from the inlet, trapping a pocket of gas.
- Compression. The space shrinks, raising the pressure of the trapped gas until it is slightly above atmospheric.
- Exhaust. The gas is pushed out through the outlet, and the cycle starts again.
In a piston pump, for example, the piston moves down and the inlet valve opens (intake). The piston moves up, the inlet valve closes (isolation), the gas is compressed, and the outlet valve opens to release it (exhaust). In an oil-free piston pump the piston seals against the cylinder with a self-lubricating ring instead of oil, so nothing but air leaves the exhaust.
The pump stops making progress when it can no longer compress the little gas left in the chamber above atmospheric pressure, or when gas leaking back past the seals equals what it removes. That pressure is its ultimate vacuum.
What are the main types of vacuum pump?
The types differ in how they trap and compress the gas, and that decides how deep they go, how clean they run and how much care they need. The ultimate pressures below are typical; each model's datasheet is what counts.
| Type | How it moves gas | Typical ultimate pressure | Good for | Watch out for |
|---|---|---|---|---|
| Oil-free piston | Piston and valves, dry piston ring | about 30–150 mbar abs (−98 to −86 kPa) | Pick and place, CNC tables, packaging, medical and lab suction | Pulsating flow; rings wear over time |
| Diaphragm | Flexing membrane and valves | about 100 mbar (one stage) down to a few mbar (several stages) | Lab filtration, chemistry, gas sampling | Lower flow per size |
| Oil-sealed rotary vane | Vanes in an oil-filled rotor housing | below 1 mbar | Freeze drying, degassing, deeper vacuum | Oil changes, oil mist at the exhaust |
| Dry rotary vane | Self-lubricating vanes, no oil | about 100–150 mbar | Continuous duty in printing and packaging | Vane wear and dust |
| Scroll | Two interleaved spirals | well below 1 mbar, dry | Clean deeper vacuum in labs and instruments | Higher price; tip seals wear |
| Liquid ring | Rotating ring of water as the seal | about 30 mbar, limited by the water's vapour pressure | Wet or dirty gas, process plants | Water use, size |
| Turbomolecular | High-speed blades push molecules | high and ultra-high vacuum | Semiconductor, analytical instruments | Needs a backing pump |
Oil-free or oil-lubricated: what changes?
Oil seals the gaps inside a pump very well, so oil-sealed pumps reach deeper vacuum. The price is maintenance and contamination: the oil has to be changed, it can carry back into the chamber, and the exhaust carries an oil mist that needs a filter.
Oil-free (dry) pumps give up some depth in exchange for clean exhaust and almost no routine maintenance. That matters wherever oil would end up somewhere it must not be: food and medical devices, labs, electronics, cleanrooms, and machines sold to end users who will never change pump oil. If your process runs at −90 kPa or so, an oil-free pump is usually the simpler choice.
How do you read a vacuum pump spec sheet?
Six numbers tell you most of what you need:
- Free-air flow (L/min or cfm; 1 cfm = 28.3 L/min) is how much gas the pump moves at atmospheric pressure. It is the best case: the real flow falls as the vacuum gets deeper.
- Ultimate vacuum is the deepest pressure the pump reaches with the inlet closed. Check whether it is given as gauge or absolute.
- Power (W) and voltage (AC mains or 12/24/48 V DC for battery and mobile equipment).
- Noise in dB(A), measured at a stated distance.
- Duty cycle: continuous duty, or intermittent with rest periods.
- Weight and size, which decide whether it fits inside your machine.
Here is how those numbers look across five of our oil-free piston vacuum pumps, from datasheet values. Several models come in a standard and a high-vacuum version: the high-vacuum version reaches deeper but moves less air.
| Model | Supply | Free-air flow | Ultimate vacuum | Power | Noise | Weight |
|---|---|---|---|---|---|---|
| HC30 | DC | 12 L/min (0.43 cfm) | −92 kPa | ≤50 W | ≤50 dB(A) | 0.55 kg |
| HC80D | 12/24/48 V DC | 28 L/min (0.99 cfm), or 12 L/min in the high-vacuum version | −86 kPa, or −95 kPa | ≤70 W | ≤48 dB(A) | 1.5 kg |
| HC580A | 220 V/50 Hz or 110 V/60 Hz | 120 L/min (4.24 cfm), or 60 L/min | −92 kPa, or −98 kPa | ≤340 W | ≤60 dB(A) | 10.1 kg |
| HC680A2-24L | 220 V/50 Hz or 110 V/60 Hz, with 24 L tank | 165 L/min (5.83 cfm), or 82.5 L/min | −92 kPa, or −98 kPa | ≤340 W | ≤60 dB(A) | 9.9 kg |
| HC1500A | 220 V/50 Hz or 110 V/60 Hz | 210 L/min (7.42 cfm), or 110 L/min | −92 kPa, or −98 kPa | ≤700 W | ≤69 dB(A) | 17.5 kg |
Read the table the way you would read any datasheet: the HC80D's DC supply and 1.5 kg suit portable and battery-powered equipment, while the HC1500A trades weight and noise for flow on a large CNC bed or packaging line. To convert flows between units, use our CFM to L/min converter.
Why does flow drop as the vacuum gets deeper?
On every stroke the pump traps the same volume, but at lower pressure that volume holds fewer molecules. At −92 kPa a litre of chamber gas contains less than a tenth of the air it held at atmospheric pressure, so each stroke removes less. That is why a pump rated at 120 L/min free-air flow empties a tank fast at first and then slows down near its ultimate vacuum.
For a first estimate of pump-down time, use t = (V ÷ S) × ln(p₀ ÷ p₁), where V is the volume, S is the pumping speed and p₀ and p₁ are the start and target absolute pressures. A 24 L tank pumped by 120 L/min from 1013 mbar to 100 mbar gives (24 ÷ 120) × ln(10.1) ≈ 0.46 min, about 28 seconds. Treat that as the lower limit: the real speed is below free-air flow at depth, and every hose, fitting and leak adds time. Size with margin, and test on the real system.
Which pump suits which job?
| Application | Usual vacuum | Common choice |
|---|---|---|
| Pick and place, robot grippers, suction cups | −60 to −90 kPa | Oil-free piston or dry vane |
| CNC and woodworking vacuum tables | −80 to −92 kPa, high flow | Oil-free piston (several in parallel) or dry vane |
| Food packaging and tray sealing | −90 to −98 kPa | Oil-free piston, or oil-sealed vane for very deep packs |
| Medical suction and aspirators | −60 to −92 kPa, quiet | Oil-free piston or diaphragm |
| Laboratory filtration and evaporation | −80 to −98 kPa | Diaphragm or oil-free piston |
| Freeze drying, deep degassing | below 1 mbar | Oil-sealed rotary vane or scroll |
For a full list of what to put in an enquiry, see our vacuum pump specification checklist for OEM buyers. Application guides go deeper on CNC workholding, food tray sealing and laboratory instruments.
Common questions
Does a vacuum pump suck air?
Not really. It removes gas from the chamber; the higher pressure outside then pushes things in, which is what you see as suction.
Can a vacuum pump reach a perfect vacuum?
No. Every pump has an ultimate pressure set by leakage past its seals, the gas its own materials release and, in wet pumps, the vapour pressure of the sealing fluid. Deeper vacuum takes a different type of pump, or several in series.
What is the difference between a vacuum pump and a compressor?
Both move gas with the same mechanisms. A compressor takes air at atmospheric pressure and delivers it at higher pressure; a vacuum pump takes gas from below atmospheric pressure and delivers it to the atmosphere. Some oil-free piston units are built in both versions.
Browse our oil-free vacuum pumps, or send us your flow, vacuum level, supply voltage and duty cycle and our engineers will suggest a model, including OEM versions for your machine.