The EXT 255H, EXT 255Hi and EXT 255HVi pumps are compound vacuum pumps that combine a turbomolecular pump and a drag pump on a single shaft. The turbomolecular pump is made from a multi-stage axial-flow turbine which is optimised for operation in molecular flow conditions. The drag pump is made from pumping channels formed by helical grooves. The drag pump is designed for operation at higher pressures than conventional turbomolecular pumps.
The light alloy rotor is machined from one piece to form the angled blades of the turbomolecular pump. The rotor blades rotate between angled stator blades which are supported by spacer rings. The rotor and stator blades have an open structure at the pump inlet, and a more closed structure at the outlet. This configuration gives an optimum combination of pumping speed and compression when pumping gases of both high and low molecular weight. For the blades to be effective, their speed must be close to the thermal velocity of the gas. The rotor therefore rotates at up to 60000 r min-1. Gas from the inlet is compressed through each of the bladed stages of the turbomolecular pump and transferred to the drag pump. In the drag pump stages, the gas is further compressed as it is pumped around the channels formed by the helical grooves. The rotating assembly is supported at the inlet end by a frictionless magnetic bearing and by a precision ceramic ball bearing at the outlet end. The ball bearing is lubricated from an oil reservoir and wick mechanism.
All of the pumps are supplied with an inlet-screen fitted in the bore of the inlet-flange. The EXT 255Hi pumps are also supplied with an inlet-strainer which fits in the interstage-port. The inlet-screen and inlet-strainer protect the pump against damage which would be caused by debris which enters the pump. The inlet-screen also protects you from the sharp blades in the pump.
All of the pumps have a vent-port you can use to vent the pump and your vacuum system to atmospheric pressure. The vent-port introduces vent gas part way up the pump rotor; this ensures maximum cleanliness. The pump is supplied with a manual vent-valve fitted to the vent-port.
All of the pumps have a purge-port in the motor and bearing housing chamber. In order to protect the bearing lubricant from the effects of high oxygen concentrations, you can introduce an inert purge gas through the purge-port. You can fit an optional vent-port adaptor and purge restrictor to the purge-port to control the flow rate of the purge gas and to filter the gas supply.
