How vacuum drying works
A liquid boils when its vapour pressure equals the surrounding pressure. Water boils at 100 °C at atmospheric pressure, at about 46 °C at 100 mbar absolute and at about 21 °C at 25 mbar. By lowering the pressure, a vacuum dryer evaporates water or solvent at temperatures that do not harm heat-sensitive products such as pharmaceutical intermediates, food ingredients, herbal extracts, fine chemicals and plastics granules.
Evaporation needs energy: roughly 2,400 kJ for each kilogram of water at these temperatures. Under vacuum there is very little gas to carry heat by convection, so heat must reach the product by conduction from heated shelves, jackets or agitators, or by radiation or microwave in special dryers. This is why heat transfer, not pumping, normally sets the drying time.
The three stages of a drying cycle
| Stage | What happens | What limits it | Vacuum duty |
|---|---|---|---|
| Evacuation | Air is removed from the chamber and product voids | Pump speed at high pressure; product disturbance | Mostly air, rapidly falling pressure |
| Constant-rate drying | Surface liquid evaporates as fast as heat arrives | Heat transfer to the product | Peak vapour flow; condenser carries most of it |
| Falling-rate drying | Liquid must diffuse from inside particles | Internal mass transfer | Low vapour flow; deeper vacuum and longer time |
The peak vapour rate in the constant-rate stage can be several times the average over the whole cycle. A vacuum system sized on the average will lose pressure at the start, the product will warm, and some products will overheat or form a skin that slows later drying.
Types of vacuum dryer
- Vacuum tray or shelf dryers: product spread on heated shelves; flexible for small batches and multiple products.
- Rotary cone and double-cone dryers: tumbling vessels with heated jackets, common for pharmaceutical powders and granules.
- Agitated nutsche filter dryers and paddle dryers: filtration and drying in one vessel, with agitation renewing the heat-transfer surface.
- Vacuum band and drum dryers: continuous drying of pastes and extracts.
- Freeze dryers (lyophilisers): product is frozen and ice sublimes under deep vacuum, used for injectable drugs, biologicals and high-value foods.
- Vacuum hopper dryers for plastics: remove moisture from hygroscopic granules before moulding.
Sizing the heat and vapour load
Start with the product: batch mass, initial and final liquid content, and the basis used (wet basis, dry basis or loss on drying). These are not interchangeable, and confusing them is a common error.
Take a hypothetical batch of 500 kg of wet cake with 20 percent water on a wet basis, dried to 1 percent. Dry solids are 400 kg. Final mass is about 404 kg, so about 96 kg of water must be removed. If the cycle is planned for 8 hours, the average evaporation rate is 12 kg per hour, needing about 8 kW of heat to the product. If the first two hours remove half the water, the peak rate is about 24 kg per hour, needing about 16 kW plus heat losses.
At 50 mbar, where water boils at about 33 °C, one kilogram of water vapour occupies roughly 28 m³. A peak of 24 kg per hour therefore represents nearly 700 m³ per hour of vapour at the chamber. No reasonably sized pump should carry that; the condenser must capture it.
Condensers and cold traps
The condenser converts the vapour back to liquid between the chamber and the pump. Its cold surface must be well below the saturation temperature at the operating pressure: to condense water at 50 mbar (saturation about 33 °C) a surface at 10–15 °C works well; at 10 mbar (saturation about 7 °C) chilled water is not enough and glycol or refrigeration is needed. Cooling-tower water in summer, often 32–35 °C in Telangana and Andhra Pradesh, cannot condense water at low pressures.
Freeze dryers use refrigerated condensers well below −40 °C, because ice at −20 °C has a vapour pressure of only about 1 mbar and at −40 °C about 0.13 mbar. The condenser must hold the ice from the whole batch and be defrosted between cycles.
Drain condensate from the receiver regularly, and check that solvent recovered at the condenser matches the expected quantity.
Selecting the vacuum pump
| Pump type | Strengths for drying | Limitations |
|---|---|---|
| Liquid ring | Tolerates wet vapour and fine dust; robust | Seal liquid temperature limits pressure, often to tens of mbar; seal liquid becomes contaminated with solvent |
| Oil-sealed rotary vane | Deep vacuum at modest cost for small dryers | Needs gas ballast and frequent oil changes on wet duty; powders damage it |
| Dry screw | Clean, oil-free and solvent-tolerant; recovers solvent at discharge | Higher purchase cost; needs purge and deposit control |
| Roots booster with backing pump | Fast pump-down and deep vacuum in the falling-rate stage | Must be protected from overload at high pressure |
Protect any pump from product dust with a filter or cyclone, and from liquid with a knock-out pot. The pump size is set mainly by the evacuation time, the non-condensable load (leaks and released gas) and the deepest pressure needed at the end.
Deciding when the product is dry
The endpoint must be measured, not assumed. Useful signals include:
- A pressure-rise test: isolate the chamber briefly and watch how fast pressure rises; a slow rise means little vapour is still being released.
- Product temperature approaching the heating-surface temperature, showing evaporative cooling has ended.
- Condensate collection slowing to near zero.
- A sample tested by the agreed method, such as loss on drying, Karl Fischer water content or residual solvent analysis.
Sample carefully. A surface sample can pass while the interior is still wet, and hygroscopic products can pick up moisture from humid air during sampling, especially in the monsoon.
Common problems and fixes
| Symptom | Likely causes | First checks |
|---|---|---|
| Pressure rises at the start of drying | Peak vapour beyond condenser capacity; warm coolant | Coolant temperature; condenser outlet temperature; drain blocked |
| Long drying time | Poor heat transfer; product layer too thick; low heating temperature | Jacket or shelf temperature; tray loading; agitation |
| Pump oil milky or ring liquid hot | Vapour passing the condenser | Condenser performance; gas ballast; seal liquid cooling |
| Deep vacuum not reached at the end | Leaks; outgassing; pump worn | Pressure-rise leak test with chamber empty; pump blank-off |
| Powder in the vacuum line | Pressure dropped too quickly; filter failed | Evacuation rate; filter element |
How Reddy Compressors helps
Vacuum dryers depend on compressed air, nitrogen and blowers as much as on the vacuum pump, and these are the areas we support:
- Sell new and used/refurbished vacuum pumps (rotary vane, liquid ring, dry screw, dry claw, scroll and rotary piston), rent them, service them and supply their spares.
- Review your batch data, condenser temperatures and pump curves, and show whether heat, condenser or pump is the limit.
- Sell, rent and service compressors of all major brands for instrument air, pneumatic valves, filter cleaning and product transfer, with dryers and filtration to the required ISO 8573-1 class.
- Sell, rent and service Roots boosters and blowers, and supply their spares.
- Sell, rent and service nitrogen and booster compressors, and supply their spares, for vacuum break and inerting on solvent dryers.
- Supply oil-free compressors, new or used, for pharmaceutical and food plants, with rental cover during shutdowns or breakdowns.
We are an independent multi-brand specialist working since 2002, not an authorised distributor of any pump or compressor maker. We have tie-ups with all major and reputed brands. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India, with project-based site support elsewhere where it is feasible. Drying recipes, validation and product quality remain with the process owner and the dryer maker.
Reddy Compressors, Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017. Phone +91 99590 60718 or +91 92468 88812; email reddycompressors@gmail.com.
What to send us
- Dryer type, chamber volume and batch size.
- Product, solvent or water content at start and end, and the basis used.
- Heating medium and temperature, and condenser coolant and its summer temperature.
- Vacuum pump make, model and nameplate photo, and operating pressure.
- Cycle time now and the problem you see.
- Compressed air and nitrogen demand for the dryer.
Frequently asked questions
Why is my vacuum dryer taking so long to dry?
Most slow drying is caused by poor heat transfer rather than weak vacuum. Check the heating-surface temperature, the depth of product on trays, agitation and fouling on jackets. If pressure rises at the start, the condenser may be overloaded or its coolant too warm. In the final stage, internal diffusion is slow by nature, so a deeper vacuum and gentle heat help.
At what temperature does water boil under vacuum?
Water boils at about 100 °C at atmospheric pressure, about 46 °C at 100 mbar absolute, about 33 °C at 50 mbar, about 21 °C at 25 mbar and about 7 °C at 10 mbar. Below about 6 mbar, it freezes before it boils, which is the basis of freeze drying, where ice sublimes directly into vapour.
Do I need a condenser on a vacuum dryer?
For almost any dryer removing more than small amounts of water or solvent, yes. The vapour volume under vacuum is enormous, often hundreds of cubic metres per hour at the chamber, and no practical pump can carry it. The condenser captures the vapour as liquid, protects the pump and lets solvent be recovered. Its coolant must be cold enough for the operating pressure.
Which vacuum pump is best for a pharmaceutical dryer?
Dry screw pumps are widely used on pharmaceutical dryers because they handle solvent vapour, keep the exhaust free of oil and allow solvent recovery. Liquid ring pumps remain common where rough vacuum is enough. A Roots booster can be added for faster pump-down and deeper final vacuum. The final choice depends on solvent, pressure, cleaning needs and validation.
How do I know when vacuum drying is complete?
Use more than one signal. A pressure-rise test shows whether vapour is still being released, product temperature approaching the heating temperature shows evaporation has slowed, and condensate collection falling to near zero confirms it. Then test a representative sample by the agreed method, such as loss on drying or residual solvent analysis, before discharging.
Why does my vacuum dryer lose vacuum at the start of the cycle?
At the start, surface moisture evaporates fastest, so vapour flow is at its peak. If the condenser cannot capture it, because it is too small, fouled or fed with warm coolant, the vapour reaches the pump and pressure rises. Ramp up heating more gently, improve coolant temperature or condenser capacity, and check the condensate drain is not blocked.
Related pages
Calculators and worked examples
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Vacuum dryer water removal, heat and vapour load
From the batch and moisture contents, work out the water to remove, the average and peak evaporation rate, the heat the product needs, the vapour volume at the chamber, and whether the condenser surface is cold enough.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Wet batch mass (kg): 500
- Initial moisture, wet basis (%): 20
- Final moisture, wet basis (%): 1
- Planned cycle time (h): 8
- Share of the water removed in the peak period (%): 50
- Length of the peak period (h): 2
- Latent heat of the liquid (kJ/kg): 2400
- Chamber pressure (mbar a): 50
- Vapour temperature at the chamber (°C): 30
- Condenser cold-surface temperature (°C): 10
Result
- Dry solids: 400 kg
- Final batch mass: 404 kg
- Water to remove: 96 kg
- Average evaporation rate: 11.99 kg/h
- Average heat to the product: 8 kW, plus heat losses
- Peak evaporation rate: 23.99 kg/h (2 × the average)
- Peak heat to the product: 15.99 kW, plus heat losses
- Vapour specific volume at the chamber: 27.98 m³/kg (water vapour)
- Peak vapour volume flow at the chamber: 671 m³/h (average 336 m³/h)
- Boiling temperature of water at 50 mbar a: 32.9 °C
- Note: Vapour temperature below the boiling temperature pure water vapour at this pressure is at least 32.9 °C; the product evaporates rather than boils, and the volume figure is within a few percent
- Condenser surface: saturation pressure: 12.26 mbar a at 10 °C
- Condenser check: Condenses 22.9 K below the boiling temperature; about 12.26 mbar of water vapour is left in the gas
Method. Wet-basis moisture: dry solids = M × (1 − w₀); final mass = dry ÷ (1 − w₁); water = M − final. Average rate = water ÷ cycle time; peak rate = water × share removed in the peak period ÷ its length. Heat (kW) = rate (kg/h) × latent heat (kJ/kg) ÷ 3,600, plus heat losses. Vapour specific volume = 461.5 × (t + 273.15) ÷ (p × 100) m³/kg (about 28 m³/kg at 50 mbar, 30 °C). Boiling temperature of water at the chamber pressure from the Magnus formula (about 33 °C at 50 mbar, 7 °C at 10 mbar). A condenser surface condenses water only if its saturation pressure is below the chamber pressure; the vapour left in the gas is about the saturation pressure at its coldest surface (over ice below 0 °C). Dry-basis or loss-on-drying figures must be converted to wet basis first.
What to take from it. Size the condenser and its coolant for the peak rate, not the average; the pump then only handles the non-condensable gas and the vapour the condenser leaves behind.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Water vapour condensation check
Compare the water vapour partial pressure in a vacuum line with the saturation pressure at a surface it meets, find the dew point, and the cold-trap temperature for a target partial pressure.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Water vapour partial pressure, or operating pressure if the gas is mostly vapour (mbar a): 20
- Surface temperature: condenser, trap, pipe wall or pump casing (°C): 28
- Water vapour partial pressure to leave in the gas, for cold-trap sizing (mbar a): 1
Result
- Saturation pressure at the surface: 37.711 mbar a at 28 °C
- At this surface: Stays vapour partial pressure 20 mbar is not above 37.711 mbar
- Dew point of the vapour: 17.5 °C: surfaces colder than this collect liquid
- Water vapour left in the gas after this surface: 20 mbar a (about the saturation pressure at the coldest surface)
- Surface temperature needed to leave 1 mbar: -20.3 °C or colder (ice forms: plan defrosting or a second trap)
Method. Water condenses on a surface when its partial pressure is above the saturation pressure at the surface temperature. Saturation pressure (Magnus formula): over water 6.112 × exp(17.62 t ÷ (243.12 + t)) mbar; over ice, below 0 °C, 6.112 × exp(22.46 t ÷ (272.62 + t)). About 6 mbar at 0 °C, 12 at 10 °C, 23 at 20 °C, 38 at 28 °C, 42 at 30 °C; ice about 1 mbar at −20 °C. The inverse gives the dew (or frost) point: the highest surface temperature that condenses the vapour. Gas leaving a condenser carries about the saturation pressure at its coldest surface. The gauge reads total pressure: where air or other gas is present, enter the water vapour partial pressure. Water only: for solvents use their own vapour-pressure data. Range of the formula about −65 to +100 °C.
What to take from it. Check the worst-case (summer) coolant temperature: a condenser surface a few degrees too warm lets the whole vapour load through to the pump.