Molecular vacuum pump with a gas-cooled rotor

DETAILED DESCRIPTION This and other objects of the invention, which will become apparent hereinafter, are achieved by providing between the suction flange, which defines a gas inlet, and the gas outlet opening, an additional gas inlet for admitting a cooling gas which should have a thermal conductivity larger than that of the compressed gas.
An effective cooling of a molecular pump and, in particular, an effective heat transfer from the rotor to the stator in a molecular pump takes place when the rotor and stator parts have a large surface and are arranged close to each other.
Further, to avoid an adverse effect of cooling on the regular pumping process, the admitted amount of the cooling gas should be small in comparison with the pumped gas.
This requires the use of cooling gas with a high thermal conductivity.
Because the cooling gas, during the pumping process, can be seized in the pump and is also compressed, measures need be undertaken to prevent a noticeable increase of the temperature by the friction caused by the flow of the compressed cooling gas through the pump.
This requires that the inner friction of the cooling gas be small in comparison with the inner friction of the pumped gas.
In view of the foregoing requirements, first, the dependence of the thermal conductivity .
lambda.
and the inner friction .
eta.
on the molecular weight M should be considered.
Generally, the thermal conductivity .
lambda.
is proportional to 1.
sqroot.
M and the inner friction is proportional .
sqroot.
M.
Therefore, with a decreased molecular weight, the thermal conductivity increases while the inner friction decreases.
Thus, gases with a small molecular weight, e.
g.
, such as helium, are especially suitable for use as cooling gases.
The more so that in general, molecular pumps are used for delivery of gases having a high molecular weight.
The amount of the cooling gas need be so selected that a maximum amount of heat is transferred.
This takes place when a laminar flow region is reached.
The thermal conductivity increases from a molecular flow region to a laminar flow region with increase in pressure and then becomes independent on the pressure