The present invention relates to vacuum pumps of the type known as
Diffusion pumps are well known and widely used for the attainment
of high and ultra high vacuum. When used with modern working fluids and accessories,
they can produce pressures approaching 10-10 mbar. The pumps are generally
incapable of exhausting directly to the atmosphere and require the use of a backing
pump, commonly an oil sealed rotary vacuum pump, in conjunction with the diffusion
On page 114 of "Modern Vacuum Practice" by Nigel S Harris and published
by McGrow-Hill International (UK) Limited, there is illustrated the basic elements
of a diffusion pump which generally comprises a substantially cylindrical outer
body which is cooled by, for example, coils helically wound around the outside
of the body through which cooling water can be circulated or alternatively by air
cooled fins attached to the outside of the body.
Within the outer body is positioned a hollow "chimney" sitting at,
or close to, the base of the outer body and which tapers (continuously or, more
usually, in stages) upwardly from the base. The chimney is generally contained
within the outer body and is positioned substantially concentrically therein.
Across the top of the chimney but not in contact therewith is a top
cap having a generally circular portion of somewhat larger diameter than the top
of the chimney and positioned symmetrically with regard to the chimney and having
a downwardly projecting annular side portion whose lower edge is somewhat beneath
the upper edge of the chimney. The top cap is therefore substantially an inverted
"cup shaped" component positioned about the top of the chimney with a circular
or annular passageway therebetween. The top cap and the adjacent portion of the
chimney defining the annular passageway is known as a 'jet stage'.
A heater is provided adjacent the base of the outer body and, prior
to use of the pump, a working oil is placed in the base of the body to a height
above the lower edge of the chimney positioned within the body. The oil is usually
a low vapour pressure oil although some versions of diffusion pumps use mercury.
A pump inlet is positioned in the outer body above the top cap and
an outlet in the side of the body towards the base of the chimney but above the
oil level. The outlet is normally connected to a backing pump as described above.
In use of pumps of this type, the backing (rotary) pump is turned
on and left running continuously, a pressure of at least 0.1 mbar being required
on the exhaust side of the diffusion pump. The cooling system (water or air) for
the outer body is turned on and the oil can now be heated by the heater for, for
example, fifteen to twenty minutes, when it begins to boil. Hot vapour rises up
the chimney and forms (aided by the taper) a relatively high oil pressure at the
top of the chimney. The vapour is then urged through the passageway between the
chimney and the top cap, the jet stage, to an area of much lower pressure and creates
an annular vapour jet. This jet is designed to move at a velocity which is supersonic
and which impinges on the inside surface of the cooled outer body where the vapour
condenses and condensed oil flows down the inside wall of the outer body and returns
to the oil reservoir at the base of the body.
With the diffusion pump turned on, gas molecules being pumped in to
the inlet of the diffusion pump are likely to collide with the much heavier oil
vapour molecules and be provided with a velocity component which will direct the
gas molecules towards the outlet of the diffusion pump where they will be subsequently
removed from the diffusion pump via the backing pump. A pressure difference is
thereby established across the continuously flowing vapour jet.
It is an aim of the present invention to provide a diffusion pump
with an integral oil reservoir with a view to increasing the interval between services.
According to the present invention, a diffusion pump comprises a hollow
outer body including a base, an outlet for connection to a backing pump, an inlet
for communication with a chamber to be evacuated, a sump within the base for containing
a working fluid and means for heating the working fluid when present in the sump,
characterised in that a reservoir for storing the working fluid is integral with
the hollow outer body.
An embodiment of the invention will now be described, by way of example
reference being made to the Figures of the accompanying diagrammatic drawings in
- Figure 1 is a cross section through a diagrammatic representation of a typical
known diffusion pump, and
- Figure 2 is a view partly in cross section of a diffusion pump incorporating
an integral oil reservoir according to the present invention.
As shown in Figure 1, a diffusion pump 1 comprises a hollow outer
body 2 of circular cross-section, a major portion of which is provided with cooling
coils 4. The body 2 includes a base 6 and a vapour chimney 8 is positioned within
the outer body 2 such that its base portion 10 fits accurately within the base
6 creating an annular gap connecting to the lower portion of the base. A working
fluid is located in the base portion 10 of the chimney 8 and a heater 18 is located
adjacent the base 6.
As shown, the chimney 8 is formed with a jet stage 12.
The diffusion pump 1 also includes an outlet 16 for connection to
a backing pump and an inlet 20 to which (by means not shown) a chamber/compartment
to be evacuated is attached. The inlet 20 as shown is located adjacent the jet
Referring now to Figure 2 which illustrates a diffusion pump 1 provided
with a reservoir 30 for the working fluid arranged integral with the hollow body
2. As shown, the base 10 of the chimney 8 is formed as a sump 32 for the working
fluid which in use, is heated by the heater 18 to cause vapour to rise up the chimney
8 in a manner known per se and to emerge through the jet stage 12. The fluid,
thereafter falls downwards, in the general direction of the sump 32 along the inner
surface of the body 2.
According to the present invention, the reservoir 30 for the working
fluid is formed integral with the hollow body 2 and comprises two cylindrical surfaces
34,36 concentric with the pump axis between which reserve oil/working fluid is
stored. As shown, the reservoir 30 communicates with the sump 32 by means of a
small bore duct 38.
A particular advantage of the reservoir 30 is that according to its
dimensions and those of the sump 32 it can increase the service interval of the
diffusion pumps by up to a factor of six.
In one embodiment only one sixth of the total working fluid charge
in a diffusion pump was used at any one time, the remaining five sixths being retained
in the reservoir 30. This means that the heat up and cool down times for the pump
were very low.
As shown a filler 40 is arranged to enable the reservoir 30 to be
topped up when appropriate.