Vacuum Pump for a Vacuum Oven: What Vacuum Level, Which Pump, and What Vacuum Ovens Are Used For
HCEM Engineering TeamShare
A vacuum oven dries or degasses material at a lower temperature than an ordinary oven, because lowering the pressure lowers the boiling point: at 100 mbar absolute, water boils at about 46 °C instead of 100 °C. The pump you pair with it should reach a pressure below the vapour pressure of the liquid at your shelf temperature, move the vapour the batch gives off, and survive that vapour.
For water-based drying, an oil-free piston pump reaching 33 to 93 mbar absolute is often enough, provided a condenser stops the vapour first. Solvents with low vapour pressure, and anything needing a few mbar or less, call for another pump type. HCEM figures below come from the spec tables on our product pages.
What is a vacuum oven, and how does it work?
A vacuum oven is a sealed, heated chamber connected to a vacuum pump. The chamber is usually stainless steel with heated shelves, a door with a gasket and a viewing window, a vacuum gauge, and two valves: one to the pump and one to let air or nitrogen back in at the end of the cycle.
It works on two effects:
- Lower boiling point. A liquid boils when its vapour pressure equals the pressure around it. Pump the chamber down and water, or a solvent, evaporates quickly at a temperature the product can tolerate.
- Little or no oxygen. At 50 mbar absolute the chamber holds about 5% of the air it held at atmospheric pressure, so oxidation and discolouration slow down. Backfilling with nitrogen removes even more.
Without air there is almost no convection, so heat reaches the product by conduction from the shelves and radiation from the walls. Spread it thinly on trays in good contact with the shelf: a deep pile dries on top while the core stays wet.
What is a vacuum oven used for?
Vacuum ovens are used wherever a material must lose moisture, solvent or trapped gas without being overheated or oxidised:
- Laboratory drying of heat-sensitive samples, precipitates, filter cakes and powders to constant weight.
- Removing residual solvent from powders, coatings and polymers after synthesis or washing.
- Degassing resins, silicones and potting compounds, so bubbles leave before the material cures.
- Electronics: baking moisture out of components and assemblies before soldering, potting or coating.
- Small-batch industrial drying of food ingredients, pharmaceutical intermediates and fine chemicals that a hot-air oven would damage.
Large electrode-drying lines in battery plants have their own requirements; our guide to vacuum pumps for lithium battery drying rooms covers that case. Freeze drying is a different process again: the product is frozen and the ice sublimes, usually below 1 mbar, deeper than an oil-free piston pump reaches.
Vacuum oven or hot-air drying oven: what is the difference?
| Hot-air (convection) drying oven | Vacuum oven | |
|---|---|---|
| Pressure | Atmospheric, about 1013 mbar | From about 200 mbar down to a few mbar absolute, lower with deep-vacuum pumps |
| Water boils at | 100 °C | About 46 °C at 100 mbar, 33 °C at 50 mbar |
| Heat transfer | Moving hot air | Shelf contact and radiation |
| Oxygen around the product | Full air | A few per cent of normal, or none with a nitrogen backfill |
| Solvent handling | Vapour leaves through the exhaust into the room or a fume duct | Vapour is drawn into the pump line, so a condenser can collect it before the pump |
| Best for | Robust materials, large batches, low cost | Heat-sensitive or oxidisable materials, solvents, degassing |
What vacuum level does a vacuum oven need?
Work backwards from the liquid and the temperature the product can take. To boil water off at a given shelf temperature, the chamber must be below water's vapour pressure at that temperature. Values for water, with the gauge reading against a standard 1013 mbar atmosphere:
| Water boils at | Absolute pressure | Gauge reading |
|---|---|---|
| 100 °C | 1013 mbar | 0 kPa |
| 80 °C | 473 mbar | −54 kPa |
| 70 °C | 311 mbar | −70 kPa |
| 60 °C | 199 mbar | −81 kPa |
| 50 °C | 123 mbar | −89 kPa |
| 45 °C | 96 mbar | −92 kPa |
| 40 °C | 74 mbar | −94 kPa |
| 30 °C | 42 mbar | −97 kPa |
| 26 °C | 33 mbar | −98 kPa |
| 20 °C | 23 mbar | −99 kPa |
Read the table against the pump. A standard oil-free piston pump with an ultimate of −92 kPa (about 93 mbar) lets water boil at about 45 °C. A high-vacuum version reaching −98 kPa (about 33 mbar) takes that down to about 26 °C. Drying does not need boiling, since a wet surface evaporates below its boiling point too, but working well below the vapour pressure is what makes a vacuum oven fast.
Worked example: a product may not exceed 55 °C, and the shelf runs at 50 °C. Water's vapour pressure at 50 °C is 123 mbar, so the chamber should sit clearly below that, for example at 60 to 80 mbar. That rules out a standard −92 kPa pump, which stops at about 93 mbar and slows to almost nothing well before it, and points to a high-vacuum version. Gauge readings shift with the weather and altitude, so set targets in absolute pressure; ultimate pressure vs working vacuum explains why a pump should not be specified to work at its ultimate.
Which type of vacuum pump suits a vacuum oven?
Four pump types are common on vacuum ovens. The right one depends on the pressure you need and what the vapour is. How each mechanism works is covered in how vacuum pumps work; this table is about the oven.
| Pump type | Typical ultimate | Good for in a vacuum oven | Watch out for |
|---|---|---|---|
| Oil-free piston | About 33 to 150 mbar abs | Water-based drying at 30 to 60 °C, degassing, oxygen-free storage; no oil to contaminate or change | Rough vacuum only; condensing vapour in the head; wetted materials not chosen for aggressive solvents |
| Chemistry diaphragm (PTFE-faced) | About 100 mbar (one stage) to a few mbar (several stages) | Solvent drying in labs, with chemically resistant wetted parts | Lower flow per size and price; diaphragms and valves wear |
| Oil-sealed rotary vane | Below 1 mbar | Deep drying, solvents with low vapour pressure, final traces of moisture | Vapour dilutes and acidifies the oil; frequent oil changes; oil mist at the exhaust |
| Dry scroll | Well below 1 mbar | Clean deep vacuum without oil | Higher price; tip seals wear, faster with dust and condensate |
Solvents show where each type stops. At 20 °C acetone has a vapour pressure of about 246 mbar, methanol 128 mbar, ethanol 59 mbar, isopropanol 44 mbar and toluene 29 mbar. On a warm shelf, all five evaporate readily within the reach of a high-vacuum piston pump, once their compatibility with the pump is checked. Dimethylformamide (about 3.5 mbar) and dimethyl sulfoxide (about 0.6 mbar) need a deeper pump, or much more heat.
Can an oil-free piston pump handle water and solvent vapour?
It can move them, but it should not be asked to swallow them. Here are the limits.
- Vapour condenses inside the pump. The pump compresses gas from oven pressure up to atmospheric. Water vapour drawn in at 100 mbar is squeezed about ten times, to near 1,000 mbar, far above its saturation pressure even in a warm pump head (about 200 mbar at 60 °C), so it turns to liquid in the cylinder, valves and outlet. Liquid on the valve reeds and the dry piston seal shortens their life and can corrode metal parts.
- Solvent compatibility is not assumed. Seals, valve reeds and the cylinder coating were chosen for air. Before a solvent passes through the pump, ask us for the wetted materials of the exact model, and check them against the solvent. Our inlet filtration and media compatibility guide lists what to request.
- Flammable vapour is a design question. If the vapour can form an explosive mixture, the oven, pump, motor and exhaust need an explosion-protection assessment under the rules of your market (DSEAR in the UK, ATEX in the EU). Do not assume a standard pump is suitable.
The practical answer is a condenser or cold trap between the oven and the pump. It turns the vapour back into liquid, so the pump sees mostly air.
Worked example: a condenser at 5 °C. Water's vapour pressure at 5 °C is about 8.7 mbar, so the condenser holds the vapour at the pump inlet near that partial pressure. With the oven at 100 mbar, vapour then makes up about 8.7 ÷ 100 = 9% of what reaches the pump, instead of nearly all of it. The condenser must also remove the heat: for 300 g of water an hour, at about 2,570 kJ/kg (vapour at 50 °C condensed and cooled to 5 °C), that is 0.3 × 2,570 = 771 kJ/h, or 771 ÷ 3.6 = 214 W of cooling, before any margin. For solvents with a low freezing point, a colder trap (for example −20 °C or below) catches more; drain or empty it before it fills.
How do you size a vacuum pump for a vacuum oven?
Check three loads, each in absolute pressure: pumping the empty chamber down, carrying the vapour the batch gives off, and keeping up with leaks.
1. Pump-down time
Use the pump-down formula with the pump's ultimate pressure pu: t = (V ÷ S) × ln[(p₀ − pu) ÷ (p₁ − pu)], with V the chamber plus line volume, S the free-air flow, p₀ the start pressure (1013 mbar) and p₁ the target. Worked example, a 50 L oven taken to 100 mbar:
- Standard HC580A (datasheet: 120 L/min, −92 kPa, about 93 mbar): (50 ÷ 120) × ln(920 ÷ 7) = 0.417 × 4.88 = 2.0 min.
- High-vacuum HC580A (60 L/min, −98 kPa, about 33 mbar): (50 ÷ 60) × ln(980 ÷ 67) = 0.833 × 2.68 = 2.2 min.
Almost the same time, but the standard pump is nearly stalled at 100 mbar: its net speed there is 120 × (1 − 93 ÷ 100) = 8.4 L/min, against 60 × (1 − 33 ÷ 100) = 40 L/min for the high-vacuum version. For an oven, speed at the working pressure matters more than free-air flow. The evacuation time guide derives the formula and shows how hoses slow it down; use the CFM to L/min converter if your oven or pump data is in CFM, and see what CFM means for the flow definitions.
2. Vapour load
Convert the water the batch gives off into a gas throughput with the ideal gas law. Worked example: a batch releases 300 g of water in its busiest hour, with the vapour at about 50 °C.
- Moles: 300 ÷ 18.0 = 16.7 mol/h.
- Throughput: Q = n × R × T = 16.67 × 8.314 × 323 = 44,760 Pa·m³/h. Since 1 Pa·m³ = 10 mbar·L, that is 447,600 mbar·L/h, or 7,460 mbar·L/min.
- At 100 mbar this is 7,460 ÷ 100 = 74.6 L/min of vapour, more than the high-vacuum HC580A's 40 L/min at that pressure.
Without a condenser the oven pressure rises until the pump keeps up. With the pump's throughput as S × (p − pu), the balance is 60 × (p − 33) = 7,460, so p = 33 + 124 = 157 mbar. That is above water's vapour pressure at the 50 °C shelf (123 mbar), so in practice the batch cannot give off 300 g an hour: evaporation slows until it matches what the pump removes, and the cycle runs longer. A high-vacuum HC1500A (110 L/min, −98 kPa) would hold 33 + 7,460 ÷ 110 = 101 mbar. Either way, all 300 g an hour pass through the pump and condense inside it. With the 5 °C condenser from the example above, the pump handles mostly air, and the high-vacuum HC580A can hold the oven below 100 mbar, as long as the condenser keeps up. The condenser is cheaper than the larger pump and protects it.
3. Leaks and gas released by the oven
Door gaskets, valves, feedthroughs and the window seal all leak a little. Measure it with a rate-of-rise test on the empty, cold oven: pump down, close the pump valve, and time the rise. Worked example: a 50 L oven rises from 50 to 56 mbar in 10 minutes, so Q = V × Δp ÷ Δt = 50 × 6 ÷ 600 = 0.5 mbar·L/s, or 30 mbar·L/min. On the high-vacuum HC580A that raises the lowest pressure by about Q ÷ S = 30 ÷ 60 = 0.5 mbar: no problem. If the same test with a warm, wet batch rises ten times faster, the difference is vapour, not a leak. Our leak rate acceptance test guide covers the limits, and how to test a vacuum pump shows how to check the pump on its own.
How do HCEM's oil-free vacuum pumps compare for a vacuum oven?
Five of our AC oil-free piston vacuum pumps, from the spec tables on their product pages. Each comes in a standard and a high-vacuum version; for a vacuum oven, the high-vacuum version is usually the one to shortlist. Flows (free-air), power and noise are at 220 V/50 Hz, with the 110 V/60 Hz figure in brackets where it differs. The 50 L column is the pump-down to 100 mbar from the formula above, without hoses or leaks.
| Model | Standard version | High-vacuum version | Net speed at 100 mbar (high-vacuum) | 50 L to 100 mbar (high-vacuum) | Max. power | Noise | Net weight |
|---|---|---|---|---|---|---|---|
| HC100A | 45 (50) L/min, −90 kPa | 22.5 (25) L/min, −98 kPa | 15 L/min | 6.0 min | ≤110 W (≤150 W) | ≤50 dB(A) (≤51) | 3.4 kg |
| HC280A* | 90 L/min, −92 kPa | 45 L/min, −98 kPa | 30 L/min | 3.0 min | ≤180 W (≤220 W) | ≤51 dB(A) (≤54) | 6.0 kg |
| HC580A | 120 L/min, −92 kPa | 60 L/min, −98 kPa | 40 L/min | 2.2 min | ≤340 W | ≤60 dB(A) (≤61) | 10.1 kg |
| HC680A | 165 (170) L/min, −92 kPa | 82.5 (85) L/min, −98 kPa | 55 L/min | 1.6 min | ≤340 W | ≤60 dB(A) (≤61) | 9.9 kg |
| HC1500A | 210 (250) L/min, −92 kPa | 110 (130) L/min, −98 kPa | 74 L/min | 1.2 min | ≤700 W (≤750 W) | ≤69 dB(A) (≤71) | 17.5 kg |
* The HC280A's parameter table gives 90 L/min for the standard version, while its product title says 95 L/min; we use the parameter-table figure here. See the datasheet on the product page, and ask us to confirm before you design it in.
For a small lab oven of 20 to 30 L the HC100A's 3.4 kg and ≤50 dB(A) suit a bench; the HC580A and HC680A cover most 50 L ovens, with the HC680A giving about 38% more flow at the same ≤340 W. The HC1500A suits larger chambers or high vapour loads, still with a condenser in front of it. Below about 30 mbar, none of these is the right pump. Browse all oil-free vacuum pumps for DC and other sizes.
What should a vacuum oven maker put in the pump RFQ?
- Chamber: volume including lines, and the oven's rated ultimate pressure.
- Target: working pressure in absolute mbar, and the pump-down time you promise your customer.
- Process: water or which solvents, the peak evaporation rate, and whether a condenser or cold trap is fitted and at what temperature.
- Duty: cycle length, whether the pump runs throughout or cycles with a pressure controller, running hours per year.
- Installation: where the pump sits, the ambient temperature around it, cooling air, mounting orientation, port positions.
- Electrical and commercial: supply voltage and frequency, noise target in dB(A) at a stated distance, annual volume and samples.
We build custom voltages from 50 units and can change the inlet and outlet direction; our OEM and ODM page lists the other options.
Common questions
Can I use any vacuum pump with a vacuum oven?
Only if it reaches a pressure below the vapour pressure of your liquid at the shelf temperature, with margin, and tolerates the vapour. A pump built for clean, dry air may reach the pressure but not survive months of water or solvent vapour. Match the pump to the process, and fit a condenser.
Do I need an oil-free pump for a vacuum oven?
Not always. An oil-sealed rotary vane pump reaches deeper vacuum, but vapour mixes with its oil, which then needs changing often. An oil-free pump avoids oil changes and oil mist, and suits water-based drying in the rough vacuum range; for deep vacuum without oil, a scroll or multi-stage diaphragm pump is the alternative.
Should the pump run throughout the drying cycle?
Usually yes, while the batch still gives off vapour, with a controller and valve holding the setpoint. Check the duty rating on the datasheet, and keep the pump away from the oven's heaters.
If you are choosing a pump for a vacuum oven or a drying system you build, send us the chamber volume, target pressure and time, the liquid you are removing, your condenser plan and supply voltage, and our engineers will suggest a model and the changes it needs.