Dry claw vacuum pumps: how they work and how to select

How a claw vacuum pump works

Two rotors with hook-shaped “claws” rotate in opposite directions inside a figure-of-eight casing, kept in step by timing gears. As they turn, the claws open a space at the inlet port, trap a pocket of gas, compress it internally and push it out through the discharge port. Because the rotors never touch, there is no friction in the pumping chamber and no need for lubricant there.

• Contact-free pumping: small running clearances, set at the factory, separate the rotors from each other and the casing.
• Internal compression: unlike a simple Roots lobe, the claw geometry compresses the gas before discharge, which makes it efficient at higher pressure differences.
• Oil only in the gearbox: the timing gears and bearings are lubricated and separated from the pumping chamber by shaft seals.
• Air or water cooling carries away the heat of compression.

The same claw principle is also used in some oil-free compressors and blowers. On a vacuum duty, the pump typically works in the rough vacuum range, from atmospheric pressure down to around 100 mbar(a) or below, depending on the model. Very low pressures need other technologies, such as dry screw pumps or Roots boosters on a backing pump.

Where dry claw pumps are used

Dry claw versus other vacuum pumps

Vacuum terms you need when selecting

• Absolute pressure in mbar(a) is the clearest way to state vacuum. Atmospheric pressure is about 1,013 mbar(a) at sea level, and lower at inland sites.
• Gauge vacuum (for example “−0.8 bar” or “80% vacuum”) depends on the local atmosphere and should be converted to absolute.
• Pumping speed is the volume flow at the pump inlet, at the inlet pressure, usually in m³/h. It changes with pressure, so always read it from the curve at the operating pressure.
• Ultimate pressure is the lowest pressure the pump can reach with a blanked inlet. A pump should not be selected to run at its ultimate pressure.
• Throughput or gas load is pressure multiplied by volume flow, for example mbar·l/s. It reflects the mass of gas actually removed.

How to size a dry claw pump

1. Define the operating pressure at the process, in mbar(a), and the lowest pressure required.
2. Estimate the gas load: leakage into the system, gas released by the product, any purge gas and, for pump-down duties, the volume of the vessel or pipework.
3. Convert the gas load to actual volume flow at the operating pressure. Because volume flow at low pressure is much larger than the free-air equivalent, a small leak at atmosphere becomes a large volume at vacuum.
4. Allow for pipework losses (conductance) between the process and the pump; at moderate vacuum these are usually modest, but long or narrow lines reduce the pressure available at the pump.
5. Read the pump’s pumping-speed curve at the pump inlet pressure, not at its maximum rated value, and check the motor power at the most demanding point.
6. For pump-down duties, check the time: for a simple vessel with no gas load, t ≈ (V / S) × ln(P1 / P2), using average pumping speed S over the range.
7. Check temperature, vapour, dust and discharge conditions, then decide on standby and controls.

Example: a 2 m³ vessel pumped from 1,013 to 100 mbar(a) with an average effective speed of 300 m³/h. t ≈ (2 / 300) × ln(10.13) ≈ 0.0154 hours, about 55 seconds, before allowing for leakage and outgassing. If the process also releases gas, the real time will be longer and the pump must be checked at the steady operating pressure.

Vapour, condensation and liquids

Dry claw pumps are not designed to swallow liquid. Water vapour from drying, packaging moist food or wet processes can condense inside the pump or in the exhaust as the gas is compressed and then cools. Condensation can cause corrosion, deposits and, if solids form, rotor contact. Good practice includes:

• A condenser or knock-out pot upstream when vapour loads are significant.
• Running the pump warm enough, as the maker recommends, so vapour stays in the gas phase through the pump.
• Avoiding low points in suction lines where condensate can collect and be sucked in as a slug.
• Drain points on the exhaust, with the exhaust line sloping away from the pump.
• Warm-up before process gas is admitted and a purge or run-on period before shutdown, if recommended for the duty.

Where vapour loads are heavy or liquid carry-over is likely, a liquid ring pump or a dry screw pump designed for vapour may be the better choice.

Dust, sticky materials and corrosive gases

Dust is the most common cause of early claw pump problems in Indian plants. Fine dust from woodworking, flour, powders or conveying passes into the pump if inlet filtration is inadequate, builds up on the rotors and casing and can reduce clearances. Fit an inlet filter rated for the dust, with a differential-pressure gauge so it is changed before it chokes the pump. In hot, dusty summers, check filters more often.

Sticky, polymerising or resinous vapours can coat the rotors and are a poor fit for a dry claw pump without careful process control. Corrosive gases need compatible materials and coatings confirmed by the maker. Where gas is flammable, toxic or oxygen-enriched, the pump, motor and exhaust must be specified for that hazard.

Cooling, discharge and installation

Claw pumps convert their shaft power into heat. Air-cooled units need good ventilation; a pump in a closed, hot room in an Indian summer can overheat or shut down on temperature. Water-cooled units need the correct flow and water quality.

• Discharge back-pressure: long or undersized exhaust lines, silencers or treatment equipment raise back-pressure, increase power and temperature, and should be within the maker’s limit.
• Exhaust routing: pipe the exhaust outside or to treatment where the gas is unpleasant or hazardous.
• Noise: claw pumps can be noisy; use enclosures or silencers where required and check noise at the operator’s position.
• Foundation and piping: use flexible connectors and support pipework independently to avoid strain on the pump.
• Electrical: check supply voltage and quality, motor rating for ambient temperature and starting method.

Controls, turndown and energy

For variable demand, such as central vacuum or several packaging machines, a variable-speed drive can hold a set vacuum level while matching speed to demand, which uses less energy than running fixed speed with a bleed valve. Multiple pumps can be staged with a controller. Keep within the maker’s speed range and check cooling at low speed. Energy should be judged on kWh per unit of actual operation, measured with power logging, not on motor nameplate rating alone.

Maintenance and fault finding

Claw pumps need less routine maintenance than oil-lubricated vane pumps, but they are not maintenance-free. Typical tasks are gearbox oil changes at the maker’s interval, inlet filter cleaning or replacement, cooling fin or water circuit cleaning and checks of shaft seals, couplings and bearings.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand specialist in compressed air, blowers and related equipment, based at Tarnaka, Secunderabad, and serving industry since 2002. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. We sell new and used/refurbished dry claw and other vacuum pumps, rent them, service them and supply their spares, and we help you define the duty properly before you buy.

• We review your process vacuum level, gas load, vapour and dust conditions and compare dry claw, vane, liquid ring and screw options neutrally.
• We advise on filtration, condensers, exhaust routing and controls, and on combining vacuum with the plant’s compressed-air and blower systems.
• Our team of more than 20 service engineers and technicians works across Telangana and Andhra Pradesh, and we supply equipment and spares across India; spares are identified from the nameplate and old part, with fit confirmed before quoting.

What to send us

• Process description and the vacuum level needed, in mbar(a) or as a gauge reading with your location.
• Vessel or system volume and any required pump-down time.
• Details of vapour, dust or chemicals in the gas.
• Existing pump nameplate photo, running hours and fault history.
• Room conditions, cooling and electrical supply.

Frequently asked questions

Does a dry claw vacuum pump use oil?

Not in the pumping chamber. The claw rotors run with small clearances and never touch, so the gas being pumped does not contact oil. The timing gears and bearings are lubricated in a separate gearbox, sealed from the pumping chamber, and that gear oil needs changing at the maker’s interval. The discharge is therefore oil-free in normal operation, which suits food, packaging and clean workplaces.

Can a claw vacuum pump handle water vapour?

Only small amounts, and with care. Water vapour can condense inside the pump or exhaust as the gas is compressed and cools, causing corrosion and deposits. Run the pump at the temperature the maker recommends, use a condenser or separator upstream for larger vapour loads, avoid low points in suction lines and drain the exhaust. For heavy vapour duties, a liquid ring or vapour-rated dry screw pump is often better.

What vacuum level can a dry claw pump reach?

Dry claw pumps work in the rough vacuum range, typically from atmospheric pressure down to around 100 mbar(a) or somewhat lower, depending on the model and number of stages. Each pump has an ultimate pressure stated by its maker, but it should be selected to run comfortably above that, at the point on its curve where it delivers the needed volume flow without overheating.

How do I choose between a claw pump and a rotary vane vacuum pump?

Choose a claw pump where you want an oil-free discharge, low routine maintenance and steady duty in the rough vacuum range, such as packaging, CNC hold-down or central vacuum. An oil-lubricated vane pump can reach lower pressures and costs less to buy, but needs regular oil, filter and exhaust separator servicing and gives oil mist that must be controlled. Compare lifetime running and maintenance, not only price.

Why is my claw vacuum pump overheating?

Common causes are poor ventilation in a hot plant room, high discharge back-pressure from a long or blocked exhaust, running close to the ultimate pressure for long periods, blocked cooling fins or low cooling-water flow, and dust deposits inside the pump. Check room temperature, exhaust line and operating pressure first, then the cooling system and inlet filter condition before opening the pump.

How long does a pump-down take with a vacuum pump?

For a simple vessel with no leakage, time is about the vessel volume divided by the average pumping speed, multiplied by the natural log of the start pressure divided by the end pressure. A 2 m³ vessel pumped from 1,013 to 100 mbar(a) at 300 m³/h takes roughly one minute. Leaks, outgassing, vapour and pipe losses make the real time longer.

Related pages

• Vacuum pump suppliers in Hyderabad
• Vacuum systems
• Dry scroll vacuum pumps
• Vacuum condensable vapour handling
• Vacuum pump-down and vessel sizing
• Central vacuum systems
• Specialist compression and vacuum engineering centre