Vacuum condensable vapour handling: traps, temperatures and pumps

Why total vacuum pressure is not enough

A vacuum gauge reads the sum of the partial pressures of everything in the line: air that leaked in, gas released by the process, and vapours such as water, alcohols, toluene or acetone. Only the vapours can condense. Whether they do depends on one comparison: the vapour’s partial pressure against its saturation vapour pressure at the local wall temperature.

Water shows the point clearly. Its saturation pressure is about 6 mbar at 0 °C, about 12 mbar at 10 °C, about 23 mbar at 20 °C and about 42 mbar at 30 °C. A line running at 30 mbar absolute with mostly water vapour will stay dry on a pipe at 35 °C, condense on a pipe cooled to 20 °C by an air-conditioned room, and flood a pump inlet that sits a few degrees colder still. The gauge reads the same in all three places.

So the design starts with a map, not a pump catalogue. List the vapours, estimate their partial pressures in each operating state, and mark the temperature of each surface they meet: vessel wall, overhead line, condenser, trap, valve body, pump casing, exhaust pipe and silencer.

Where vapour should and should not condense

Every system has places where condensation is useful and places where it causes damage. The aim is to make the useful places capture almost all of it.

How condensers and cold traps are sized

A condenser ahead of the pump does two jobs. It removes most of the vapour load, so the pump handles mainly non-condensable gas, and it sets a ceiling on the vapour partial pressure leaving it, roughly equal to the saturation pressure at its coldest effective surface. Size it on these points:

• The vapour mass flow in each state, not just the average. Drying and distillation often start with a vapour peak several times the steady rate.
• The coolant temperature and approach. Cooling water from a tower in a Hyderabad summer can be 32–35 °C, which limits condensation at low pressure; chilled water, glycol or refrigeration may be needed.
• Non-condensable gas blanketing. Air collects at the cold surface and slows heat transfer, so the outlet must be at the cold end and the leak rate must be known.
• Pressure drop. A condenser that loses several mbar at low absolute pressure takes a large share of the available pressure difference.
• Liquid removal. Condensate must leave against vacuum through a barometric leg, a receiver that can be isolated and emptied, or a pump-out arrangement.

Cold traps using refrigeration or liquid nitrogen are used for small flows and deep vacuum. They freeze the vapour, so plan for regeneration: two traps in parallel, or a batch schedule that allows defrosting. A trap that is full of ice no longer protects the pump.

Controlling line and pump temperatures

Once the condenser has done its work, the remaining vapour must stay in the gas phase until it is clear of the pump. The practical tools are simple:

• Insulate and, where needed, heat-trace lines between the process and the condenser so they stay above the local dew point.
• Slope lines downward to the condenser or a drain pot. Avoid pockets and dead legs.
• Keep valve bodies, flexible connectors and instrument tappings warm; they are cold spots.
• For dry screw and claw pumps, follow the maker’s casing temperature guidance and any purge gas requirement. These pumps often run hot by design so that vapour passes through.
• For oil-sealed rotary vane or piston pumps, run with gas ballast open while vapour is present. The ballast air raises the non-condensable share during compression so the vapour reaches exhaust pressure before it reaches its saturation pressure.
• Warm the pump up before vapour arrives, and run it on ballast or purge after the process ends to clear what is left.

Matching pump type to the vapour

Avoid double counting. If the condenser removes most of the vapour, the pump is sized for non-condensable gas plus the vapour at the condenser outlet partial pressure, not for the whole process vapour. If the condenser is bypassed during start-up or regeneration, the pump sees the full load and must either cope or be protected.

A worked example: What changes when cooling-water temperature rises

Consider a hypothetical drying vessel releasing water vapour at 20 mbar absolute, with a small air leak. The condenser is fed with cooling water at 25 °C and runs at about 28 °C on its cold surface. Water’s saturation pressure at 28 °C is about 38 mbar, which is above the 20 mbar operating pressure, so the condenser cannot condense water at this pressure. Nearly all the vapour reaches the pump.

Change the coolant to chilled water at 7 °C with a surface around 10 °C. Saturation pressure falls to about 12 mbar. The condenser now captures most of the vapour and the gas leaving it is mainly air plus water vapour near 12 mbar partial pressure. The pump duty drops sharply, and it is the condenser and its drain that must now be sized for the peak.

The lesson: a summer rise in cooling-water temperature can move a working system into one that floods its pump. Check the worst-case coolant temperature, not the design brochure value.

Solvent vapours, materials and safety

Organic solvents bring three extra concerns. First, materials: elastomers, gaskets, pump coatings and oil must be compatible with the actual solvent mix, including traces. Second, flammability: compression in the pump raises temperature, and some vapour-air mixtures can ignite, so purge gas, temperature limits and any hazardous-area classification are decided by the process owner and the pump maker. Third, emissions: vapour that passes the pump leaves through the exhaust, so a post-pump condenser, scrubber or recovery unit may be needed under local pollution-control rules.

Corrosive vapours such as hydrochloric acid or ammonia need special pump materials and seal liquids. In these duties, follow the pump maker’s compatibility data for the exact chemical and concentration; do not extrapolate from water.

Commissioning and routine checks

1. Record the operating pressure, condenser inlet and outlet temperatures, coolant inlet and outlet temperatures and pump inlet pressure in each state.
2. Measure condensate collected per batch or per hour and compare it with the expected vapour load.
3. Check the pump oil or ring liquid for water and solvent after the first runs.
4. Feel or scan line surfaces for cold spots and look for liquid at low points and drains.
5. Run a leak-rate test with the process isolated, so non-condensable load is known.
6. Repeat the checks in the hottest month, when cooling capacity is lowest.

How Reddy Compressors helps

We look at the vapour problem as part of the whole utility, because the same plant usually depends on compressed air, cooling and blowers as well. Our engineers can:

• 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 vapour map, condenser temperatures and pump data, and point out where condensation is likely to end up.
• Sell, rent and service Roots blowers and boosters, screw blowers and side-channel blowers of all major makes, and supply their spares.
• Supply clean, dry compressed air or nitrogen for purge, gas ballast and instrumentation, with dryers chosen for the required dew point.
• Identify spare parts from the nameplate, model, serial number and old part, and confirm fit before quoting.
• Support start-up checks and log temperatures and pressures through the first batches.

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. For vapour handling, the process owner and the vacuum pump maker remain responsible for chemical compatibility and hazardous-area decisions; we help you collect and test the data they need.

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

• The vapour or solvent mix, with approximate composition and mass flow per hour or per batch.
• Operating pressure in each stage, in mbar absolute.
• Condenser or trap type, coolant and its summer temperature.
• Vacuum pump make, model and nameplate photo, and whether gas ballast or purge is fitted.
• A simple sketch of the line from vessel to exhaust with lengths, low points and drains.
• Symptoms: oil milky, pump flooding, pressure not reached, solvent smell at exhaust.

Frequently asked questions

Why does my vacuum pump oil turn milky?

Milky oil in an oil-sealed vacuum pump is almost always water or solvent that condensed inside the pump and emulsified with the oil. It happens when the pump runs cold, gas ballast is closed or the upstream condenser cannot cope. Change the oil, run the pump warm with ballast open while vapour is present, and check the condenser coolant temperature, which is often too warm in summer.

What is gas ballast on a vacuum pump?

Gas ballast is a small controlled bleed of air or dry gas into the compression stage of an oil-sealed pump. The extra non-condensable gas lets the mixture reach exhaust pressure before the vapour reaches its saturation pressure, so the vapour leaves as gas instead of condensing in the oil. It slightly raises the ultimate pressure and should be used whenever vapour is pumped.

Do I need a condenser before my vacuum pump?

You need one when the process releases enough vapour that the pump would otherwise have to carry it all. A condenser removes the bulk of the vapour at a controlled temperature, leaving the pump mainly non-condensable gas. It only works if its cold surface is below the vapour’s saturation temperature at the operating pressure, so check the coolant temperature against the pressure you need.

Can a liquid ring pump handle wet vapour?

Yes, liquid ring pumps tolerate wet gas and condensate well because the vapour simply condenses into the seal liquid. The limit is the seal liquid temperature: as it warms, its own vapour pressure rises and sets the lowest pressure the pump can reach. Near that limit the pump can cavitate, so seal liquid cooling and make-up need as much attention as the pump.

Why is condensate collecting in my vacuum line?

Condensate forms in a vacuum line wherever the wall is colder than the vapour’s dew point at that pressure. Typical causes are uninsulated pipe in an air-conditioned area, cold valve bodies, low points and dead legs. Slope the line back to the vessel or to a drain pot, insulate or heat-trace it, and make sure the condenser sits where you want the liquid to form.

How cold should a cold trap be?

The trap surface must be below the saturation temperature of the vapour at the partial pressure you want to leave in the gas. For water, that is about 0 °C for 6 mbar and about −20 °C for 1 mbar, with colder still for deeper vacuum. Allow for ice build-up, which insulates the surface, and plan a defrost or second trap for continuous duty.

Related pages

• Specialist compression and vacuum engineering centre
• Liquid ring vacuum seal liquid systems
• Vacuum leak and outgassing diagnosis
• Vacuum exhaust and contamination control
• Vacuum drying process systems
• Oil-sealed rotary piston vacuum pumps