Vacuum degassing applications: removing gas from liquids and melts

How vacuum degassing works

Gas dissolves in a liquid in proportion to its partial pressure above the liquid, which is Henry’s law in its simplest form. Air-saturated water at atmospheric pressure holds a few tens of milligrams of dissolved gas per litre. Lower the pressure above it and the equilibrium concentration falls in proportion; the dissolved gas then moves towards the surface and leaves.

That movement is the slow part. Gas must diffuse through the liquid to an interface, form or enlarge bubbles, rise and break the surface. The degasser’s job is to make these distances short and the time long enough. In practice, three levers set performance:

• Driving force: the gap between the dissolved concentration and the equilibrium at the vacuum partial pressure.
• Interfacial area: thin films, sprays, droplets, cascades or agitation that expose more surface per kilogram.
• Residence time: how long each part of the product spends under vacuum at that surface.

Temperature affects all three. Warming usually reduces viscosity and gas solubility, which helps, but it also raises the product’s own vapour pressure and so the vapour load on the vacuum system.

Typical applications

Start with a measurable objective

“Degas the product” cannot be tested. A useful objective names the gas, the limit, the units and how it is measured. For example: dissolved oxygen in mg per litre at the outlet, measured with a stated instrument at a stated temperature; or no visible voids larger than a set size in a cast part; or water content in ppm by a stated titration method.

Keep separate ideas separate. Dissolved gas, visible bubbles, foam volume, volatile loss and what the vacuum exhaust contains are different measurements. A clear-looking resin can still hold dissolved air that comes out later when it warms; a foaming product can look worse while it is actually degassing well.

Choosing the contacting method

The method is set by the product and the throughput:

• Batch chamber: simple and flexible for small volumes of resin, silicone or slurry, but a deep layer of viscous product degasses slowly because bubbles have far to rise.
• Thin film or cascade: product flows over a cone, disc or plate, giving short diffusion distances for viscous liquids.
• Spray or nozzle: atomising into the vacuum gives very high area for low-viscosity liquids such as water or juice.
• Packed or tray column: for continuous water deaeration, with counter-current stripping by the liberated vapour.
• Vented extruder or devolatiliser: for polymer melts, where surface renewal by screws or rotors does the work.

Whatever the method, the vessel needs disengagement space above the liquid, so foam and droplets can collapse before reaching the vacuum line, and often a demister or separator before the pump.

Foam, entrainment and vapour load

Foam is the most common reason degassing systems underperform. When pressure is reduced quickly, bubbles expand and can fill the vessel and carry product into the vacuum line. Controls include reducing pressure in steps, feeding product as a thin stream rather than a pool, using a mechanical foam breaker, leaving enough freeboard, and, where the product allows it, a permitted anti-foam agent.

The vacuum system then has two loads. One is the non-condensable gas: the dissolved and entrained air or other gas released, plus any air leakage. The other is vapour from the product itself. Water at 20 °C has a saturation pressure of about 23 mbar, so a water deaerator working below that will boil, sending a large vapour flow to the pump. Products with volatile solvents or monomers behave the same way. A condenser between vessel and pump often removes most of that vapour, leaving the pump mainly non-condensable gas.

A simple throughput estimate

Take a hypothetical line degassing 2,000 kg per hour of a water-like liquid, initially saturated with air at about 25 mg per kg dissolved gas, operating at 50 mbar absolute and 20 °C. If the degasser removes most of that gas, the non-condensable load is about 50 g per hour of air. At 50 mbar and 20 °C, air occupies roughly 17 m³ per kg, so 0.05 kg per hour is under 1 m³ per hour at the vacuum. Add the design air leak, which may well be larger.

The liquid’s own vapour is a different matter. At 20 °C, water vapour pressure is about 23 mbar, below the 50 mbar working pressure, so the liquid does not boil, but vapour still saturates the gas leaving the vessel. Roughly half of the gas at the pump inlet by volume is then water vapour. If the operating pressure were reduced below 23 mbar, the liquid would boil, the vapour flow would rise sharply and the product would cool. The numbers show why vapour, not dissolved gas, often sizes the vacuum system.

Selecting the vacuum equipment

Commissioning and verification

1. Measure the inlet gas or moisture content of the product, so the removal can be proven against a real starting point.
2. Run at the design throughput and temperature, and record vessel pressure, pump inlet pressure and condenser temperatures.
3. Sample the product at the outlet with the stated method, allowing the sample to settle as specified.
4. Check the separator and vacuum line for product carry-over after each trial.
5. Repeat at the highest throughput and the warmest cooling water expected in summer.

Product-specific trials or a validated model are needed before anyone can promise a removal level. Treat supplier claims of a fixed removal percentage with care unless they relate to your product and conditions.

How Reddy Compressors helps

Degassing sits within a plant’s wider utilities. Our engineers support the parts of the system we know well:

• 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 vacuum duty, vapour load, condenser and pump data, and highlight where foam, vapour or leaks will limit performance.
• Sell, rent and service Roots boosters and blowers, and supply their spares, for vacuum and handling duties.
• Supply compressed air, dryers and filtration for valves, foam breakers, instruments and product transfer.
• Rent compressors for trials, shutdowns and temporary production, from our own fleet of 50–2,000 cfm and 2–30 bar machines.
• Identify spares from the nameplate, model, serial and old part, and confirm fit before quoting.

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. The degassing process design and product quality remain with the process owner and the degasser 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

• Product name, viscosity, temperature and throughput per hour or per batch.
• The gas to remove and the target, with test method.
• Current vessel type, operating pressure and any condenser or separator.
• Vacuum pump make, model and nameplate photo.
• Symptoms such as foaming, carry-over, pressure not reached or results out of specification.

Frequently asked questions

How does vacuum degassing remove air from a liquid?

Lowering the pressure above a liquid reduces how much gas it can hold in solution, so dissolved air comes out and rises to the surface as bubbles. Entrained bubbles expand and rise faster. The rate depends on how thin the liquid layer is, how long it stays under vacuum and its viscosity, so degassers spread the product into films, sprays or droplets.

What vacuum level is needed for degassing?

There is no single figure. The required level depends on the gas, the target content, the product’s temperature and its own vapour pressure. For water-based liquids, operating below the water’s saturation pressure makes them boil, which helps stripping but adds a large vapour load. Many resin and oil applications run at a few mbar to a few tens of mbar absolute.

Why does my resin foam over in the vacuum chamber?

Foaming happens when trapped air expands quickly as pressure drops and the bubbles cannot break fast enough. Reduce pressure in steps, use a container with plenty of headspace, pour a thinner layer or a thin stream, and warm the resin slightly if its specification allows to lower its viscosity. Releasing vacuum briefly helps collapse foam during the early stage.

Which vacuum pump is best for degassing water?

Water deaeration produces a lot of water vapour, so pumps that tolerate it are usually chosen. Liquid ring pumps handle wet gas well, and a condenser before a dry or oil-sealed pump can remove most of the vapour first. For large deaerators, steam ejectors are also common. The best choice depends on the pressure, flow and whether a seal liquid is acceptable.

Can vacuum degassing remove moisture from transformer oil?

Yes, vacuum treatment of heated oil in a thin film or spray removes both dissolved gas and moisture, which is why it is standard for insulating oil processing. The vacuum system must handle the water vapour, usually with a condenser or cold trap. The acceptance limits for gas and moisture content come from the transformer owner’s or equipment maker’s specification.

Is foam the same as degassing?

No. Foam shows that gas is leaving, but a large foam volume does not mean the product is fully degassed, and a calm surface does not prove it either. Dissolved gas can remain in a clear liquid and come out later. Judge the result by measuring the target gas or defect in the finished product with the agreed test method.

Related pages

• Specialist compression and vacuum engineering centre
• Vacuum condensable vapour handling
• Liquid ring vacuum seal liquid systems
• Vacuum ejectors and steam jet systems
• Vacuum Roots booster systems
• Vacuum packaging process systems