Absolute Vacuum present:
Turbomolecular pumps - general info

Edwards^® Vacuum - Turbo pumps and accessories

EXT turbomolecular pumps

---

Turbomolecular pump controllers

---

EXT accessories

---

EXPT combined outfits

---

STP magnetically levitated turbomolecular pumps and accessories

---

Edwards^® turbo molecular pumps (turbo vacuum pumps) overview, information and definitions

Turbomolecular pumps and controllers

• Multi-stage axial-flow turbine: High speed rotating blades initiate compression by increasing the chance of gas molecules moving in the direction of pumping
• Turbomolecular pump are optimised for molecular flow conditions and need an appropriately sized 2 stage rotary vane pump or an oil free scroll pump exhausting to the atmosphere.
  

• Combed bladed turbomolecular stages with molecular drag stages on the same rotor
  
• High critical foreline pressures (typically up to 10 mbar) 
• Option to employ smaller backing pumps or dry diaphragm backing pumps
  

• Dictated by the rotor diameter, inlet flange size and rotational speed
• Pumping speed reduces at high inlet pressures, depending on the backing pump

• When inlet pressure increases, both motor power dissipation and pump temperature rise
• Maximum perpetual inlet pressure dictates the maximum throughput limit for stable pumping and is dependent on the methods of cooling
  
• Beyond this pressure, pump rotational speed is reduced as the pump is limited by sensors responsible for temperature
• With water-cooled vacuum pumps, the actual maximum throughput is dictated by the size of the backing pump

• Nominal power expended by a vacuum pump operating at full speed with low gas throughput - below 10–3 mbar
  
• In the course of run-up time/during operation at a high gas throughput/above critical backing pressure, a vacuum pump's power dissipation will increase and draw near to the maximum power output for the controller
  
• Critical backing pressure for conventional turbomolecular pumps is approx 0.1 to 0.2 mbar.

• Determined by rotational speed
• The number of pump stages
• Molecular weight of pumped gas.
  
  Compression is greater for heavier gases and is responsible for the requirement for effective suppression, and the reason why the ratio for hydrogen is key for ultra high vacuum installations.

Bearing and suspension technologies

• Magnetic bearings
• Mechanical ceramic ball bearings.
  
• Ceramic bearings, which are lubricated for life by either grease or oil, have recently replaced steel bearings.
  
• Silicon nitride ceramic balls are lighter, harder and smoother than steel counterparts, giving longer life and lower vibration
• Reliability is increased because the ball and race materials differ, Preventing micro pitting.
  
• Magnetic bearings further increase reliability.
  

Rotor technologies

• Conventional full stack turbomolecular (typically 12 stages)
  
• Compound molecular (combining turbomolecular and drag stages)
• Stable and rigid rotors for optimum vacuum performance.

Motor technology

• Brushless d.c. motors are common, and are usually available in 24 and 80 volt versions
• Ref' Edwards^® 24 volt pumps:Edwards^® TIC line of controllers are available with integrated instrument controllers.
  
• Ref' Edwards^®80 volt pumps the Edwards^® EXC line of controllers get the best out of performance
• Controllers often incorporate a regenerative back-up supply providing power in the case of electrical supply failure, keeping the ventvalve closed for several minutes.

Corrosive applications

• Maximum life and reliability in the harsh process environment encountered in semiconductor wafer processing applications
• Turbomolecular pumps from the Edwards^® STP-C and STPH-C series serve this application well.
• These Maglev pumps carry magnetic bearings which perform well in harsh conditions
  

Purge port

• Many turbo vacuum pumps have purge ports used to purge the motor/bearing cavity with an inert gas.
  
• It is common practice to purge the pump when corrosive and/or abrasive gas mixtures, or gas mixtures contain oxygen content over 20% are pumped
• The Edwards^® PRX10 purge-restrictor is recommended to set the purge gas flow rate
  

Venting

• It is recommended - in order to maintain cleanliness of vacuum system, that a turbomolecular pump is vented at or above half rotational speed. In other words during the period when rotor speed is quick enough to reduce backstreaming of hydrocarbons from the backing line
• The vent ports on modern vacuum pumps are often part way up the rotor stack to ensure maximum cleanliness even with fluoroelastomer sealed vent-valves. Each Edwards^® pump is usually supplied with a manual vent-valve. Care must be taken not to open this manual vent valves too quickly, vacuum pump bearing life may be affected
• Small volume vacuum systems: Rate of pressure rise will be pronounced over that of a large volume for any vent flow rate, hence the possibly necessity to restrict the vent gas flow. Edwards^® offer their VRX range of vent restrictors for Edwards^® EXT pumps