Edwards EXT Turbomolecular Pumps and Controllers
Our range of EXT turbomolecular and compound molecular pumps
and EXC Controllers are state-of-the-art technology and provide
reliable, high and ultra high vacuum.
Key Performance Factors
A turbomolecular pump (TMP) is a multi-stage axial-flow turbine in
which high speed rotating blades provide compression by increasing
the probability of gas molecules moving in the pumping direction. The
turbomolecular pump is optimized for molecular flow conditions and
requires a suitably sized two stage rotary vane pump or an oil free
scroll pump to exhaust to atmosphere.
A compound molecular pump (CMP) is based on the concept of
combining bladed turbomolecular stages with molecular drag stages
on the same rotor. This design allows:
- High critical foreline pressures (typically up to 10 mbar)
- Options to use smaller backing pumps or dry diaphragm backing pumps
Pumping Speed (volume flow rate) is determined by the rotor
diameter, inlet flange size and rotational speed. The pumping speed
reduces at high inlet pressures to a value determined by the size of
the backing pump.
As the inlet pressure rises, the motor power dissipation and pump
temperature increase. Maximum continuous inlet pressure sets
the maximum throughput limit for steady state pumping and depends
on the cooling method used. Above this pressure, the rotational
speed of the pump reduces as temperature sensors limit the pump
power. With a water-cooled pump, the actual maximum throughput
depends on the size of the backing pump.
Quiescent Electrical Power is the nominal power dissipated by a
pump operating normally at full rotational speed and with low gas
throughput (inlet pressure below the 10-3 mbar range). During the
run-up time, or when operating at high gas throughput or above the
critical backing pressure, the pump power dissipation will rise and
approach the maximum power output for the turbo Controller used.
Critical backing pressure for conventional turbomolecular pumps is
approximately 0.1 to 0.2 mbar.
Compression Ratio is determined by the rotational speed, the
number of pump stages and the molecular weight of the pumped gas.
It is higher for heavier gases which explains why the suppression of
hydrocarbon backstreaming is so effective and why the ratio for
hydrogen is important for ultra high vacuum applications.
Ultimate Pressure measured according to Pneurop standards, is the
lowest pressure achieved in the test system, 48 hours after bakeout.
The system is backed only by a two-stage rotary vane pump.
Fluoroelastomer inlet seals are used with ISO-flanged pumps and
metal seals are used with CF-flanged pump models.
Bearing and Suspension Technologies
We use two basic technologies: magnetic bearings and mechanical
ceramic ball bearings.
Ceramic bearings, which are lubricated for life by either grease or oil,
have replaced conventional steel bearings. The silicon nitride
ceramic balls are lighter, harder and smoother than steel equivalents,
leading to longer life and lower vibration characteristics. Reliability is
increased because the ball and race materials are different, which
prevents micro pitting.
Magnetic bearings further increase reliability. Our EXT
turbomolecular pumps up to 540 l s-1 use a hybrid bearing
arrangement with a permanent magnet upper bearing and an oil
lubricated ceramic lower bearing.
We use two basic technologies:
- compound molecular and fluid dynamic (combining
turbomolecular, drag and fluid dynamic stages)
- compound molecular (combining turbomolecular and drag stages)
In addition, EXT pumps up to 540 l s-1 use monobloc rotors machined
from solid bar by computer controlled high speed milling machines.
This technology produces stable, rigid rotors and allows virtually
unlimited design flexibility for optimum vacuum performance.
EXT pumps use brushless d.c. motors and are available in 24 and 80
volt variants. For the 24 volt pumps the TIC line of controllers are
available with the added benefit of integrated instrument controllers.
For the 80 volt pumps you can choose from our EXC line of
controllers to optimize the performance and cost options for your
The Controllers incorporate a regenerative back-up supply which
provides power in the event of electrical supply failure to keep the
vent-valve closed for several minutes.
For maximum life and reliability in the exacting process conditions
encountered in semiconductor wafer processing applications, we
recommend that you use turbomolecular pumps from our Edwards
STP-C and STPH-C series. These Maglev pumps have magnetic
bearings and are ideal for these harsh duty applications.
The EXT pumps all have purge-ports which can be used to purge the
motor and bearing cavity with an inert gas (such as nitrogen). We
recommend that you purge the pump when you pump corrosive and
abrasive gas mixtures or those with an oxygen content over 20%. You
can use our PRX10 purge-restrictor to set the purge gas flow rate.
This typically adds up to 25 sccm to the total gas load and the backing
pump must be sized accordingly.
To maintain the cleanliness of your vacuum system, we recommend
that you vent a turbomolecular pump at or above half rotational
speed, when the rotor is still spinning fast enough to suppress any
backstreaming of hydrocarbons from the backing line.
The vent port on the EXT pump is part way up the rotor stack to
ensure maximum cleanliness even with fluoroelastomer sealed ventvalves.
Each pump is supplied with a manual vent-valve. If you use
this manual valve care must be taken not to open it too quickly,
especially if the system volume is small (typically less than the
approximate volume of the turbomolecular pump), because if the rate
of pressure rise is too high, the pump bearing life may be reduced.
In a small volume system, the rate of pressure rise will be greater than
in a large volume for a given vent flow rate, and it may be necessary
to restrict the vent gas flow. We offer the VRX range of vent restrictors
which you can fit to your EXT pump.
Since the rate of pressure rise cannot be accurately controlled by the
manual vent-valve, we recommend that, unless you fit a suitable VRX
restrictor to the vent port, you must wait until the turbomolecular pump
has slowed down to 50% speed, as indicated by the controller, before
you open the manual vent-valve.
The maximum rate of pressure rise varies by pump model, and the
Instruction Manual supplied with an EXT pump gives further guidance
on this, and the size of vent restrictor needed to meet the fastest
pressure rise allowed.
Control of the rate of venting is particularly important with pumps
using fully magnetic bearings, otherwise the safety bearings may be
The manual vent-valve can be replaced with a TAV solenoid valve
driven by the Controller to allow venting after a 2 second delay on
shut-off, or delaying vent until the rotational speed has dropped to
50%. The Controller can also control the TAV vent-valve in the event
of power or pump failure.
You can choose from two solenoid vent-valve options; the TAV5
which covers most auto-venting applications, and the TAV6 which
has a higher conductance than the TAV5 and is designed either for
use on larger chambers (typically with a volume greater than 10
liters), or when you want to use a two-stage venting procedure for the
fastest possible vent times.
For two-stage venting you need two TAV valves. By using the
appropriately restricted flow for the first stage vent-valve you can start
venting when the EXT pump is still at full rotational speed. Once the
pump has slowed to half rotational speed you can then introduce
higher flow rates from the second stage vent-valve.
An inlet-screen is fitted as standard to all EXT pumps. The inletscreen
prevents debris from falling into the pump-inlet. In addition, the
inlet-screen prevents you from coming into contact with the blades of
the pump when it is disconnected from your vacuum system.
EXT75DX and EXT255DX also have the facility for pulsed venting to
allow rapid yet controlled venting of a system.
For most applications, we recommend that you use forced-air cooling
with the appropriate ACX air-cooler connected to your EXT pump.
NB: high gas load, high backing pressure and rapid cycling require
However, if the ambient temperature is above 35 °C you must
water-cool the EXT. The barbed connectors on the water-cooler are
suitable for 6 mm internal diameter hose.
Water cooling reduces the running temperature of the pump motor
and bearings and is particularly recommended when you operate the
EXT with a continuous high throughput (that is, inlet pressure above
1 x 10-3 mbar) or when you bake the EXT pump to above 70 °C
(measured at the inlet flange).
Water cooling accessories need to be purchased separately.
Scope of Supply
For end users desiring front panel controls and indications we
suggest the following:
- TIC controller with EXT75DX and EXT255DX
- TIC controller with EXT406PX plus EXDC drive unit
- EXC300 controller with EXT556H and EXT to EXC cable
Each EXT pump is supplied with an inlet screen, elastomer or copper
gasket inlet seal (as appropriate) and manual vent valve. Where
required a water cooling accessory is available.
Turbo controllers require the appropriate mains cable to be selected