Oil Diffusion Pumps
Operating Principle of Fluid
Entrainment Vacuum Pumps
The main components of diffusion pumps, the operation of which
relies on vapor-phase pump fluids are:
Cooled pump body with intake and exhaust ports
System of nozzles
Pump boiler
In the case of diffusion pumps a pump fluid
contained in a boiler is heated to such an extent that it is
vaporized. The vapor is then forced through nozzles within the pump.
The nozzles are generally designed in such a way, that they
accelerate the vapor to a speed exceeding the
speed of sound (Laval nozzles), thus creating a high speed vapor
jet. The vapor is then deflected by the nozzles at a specific angle
onto the pump body. The pump body is cooled, so that the vaporized
pump fluid condenses and is returned back to the boiler as a liquid.
The
pumping action of diffusion pumps and fluid entrainment pumps in
general is based on the transporting capacity of the vapor jet.
The gas which is to be pumped is compressed sufficiently at the
forevacuum port so that it can be pumped out by a backing pump.
Applications and Accessories
General
Diffusion Pumps
Compared to other fluid entrainment pumps the density of the vapor in
the boiler and in the vapor jet is fairly low so that the gas
molecules may almost completely diffuse into the vapor jet.
Thus most of the molecules which enter the vapor jet are also pumped
out.
For this reason, the pumping speed of diffusion pumps is extremely
high with respect to the in-take area and constant – starting at an
inlet pressure of approximately 10-3 mbar (0.75 x 10-3 Torr) down to
very low pressures – as within the pressure range the vapor jet is
not influenced
in any way by the pressure within the vacuum vessel.
Operating
Oil Diffusion Pumps
Forevacuum
In all cases diffusion pumps require a sufficiently sized backing
pump (see Technical Data). The size and type of forevacuum pump
depends on the operating conditions and the quantities of gas which
are to be pumped.
Continuous operation at operating pressures above 10-4 mbar
(0.75 x 10-4 Torr) – large quantities of gas.
Continuous operation at operating pressures below 10-4 mbar
(0.75 x 10-4 Torr) – smaller quantities of gas.
In applications which rely on diffusion pumps, the vacuum chamber must
be connected via a valve (3) and a roughing line directly to the
backing pump. This is done so that the vacuum chamber may be
preevacuated by the backing pump down to a pressure where the
diffusion pump can take over. Until the high vacuum valve (4) opens,
both diffusion pump and pump fluid are preserved. Before venting the
vacuum chamber the forevacuum valve (2) and the high vacuum valve
(4) must be closed, whereby the diffusion pump remains in the ready
status.
Pumping Speed
The pumping speed of any pump is equivalent to the volume throughput
through the intake opening of a pump. In the case of diffusion pumps
the pumping speed for lighter gases is higher compared to heavier
gases.
Backstreaming of the Pump Fluid
Undesirable backstreaming of molecules from the pump fluid is caused
by the effect that some
molecules are able to leave the vapor jet and thus do not arrive at
the cooled pump body. Because of collisions between each other and
due to reflection at the pump body, these molecules are then able to
move in the direction of the vacuum chamber.
For DIP pumps the backstreaming effect amounts only to a few µg per
cm2 of intake area per minute. Backstreaming may be almost
completely suppressed by including a cold cap baffle or an
additional Astrotorus baffle.
Backstreaming of Oil in the Case of DiffusionPumps
Pump without baffle approx. 1 x 10-2 mg x cm-2 x min-1
Pump with cold cap baffle approx. 1 x 10-3 mg x cm-2 x
min-1
Pump with Astrotorus baffle (T = 10 °C (50 °F)) approx.
1 x 10-5 mg x cm-2 x min-1
The values stated were measured at the ultimate pressure and
apply to DIFFELEN normal. When using DC 705 the values may
improve on average by one order of magnitude.
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