Abstract:
A combination including a diffusion pump and a reservoir for working fluid positioned remotely from the pump and connected to the diffusion pump via a valve and pipework.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to vacuum pumps of the type known as diffusion pumps.  
         BACKGROUND OF THE INVENTION  
         [0002]    Diffusion pumps are well known and widely used for the attainment of high and ultra high vacua. 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 vane vacuum pump, in conjunction with the diffusion pump itself.  
           [0003]    Diffusion pumps generally comprise 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.  
           [0004]    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.  
           [0005]    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 projected 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”.  
           [0006]    A heat source is provided in the base of the outer body, preferably in the form of an electric heater, and, prior to use of the pump, a working fluid is placed in the base of the body to a height above the lower edge of the chimney positioned within the body. The fluid is usually a low vapour pressure oil although some versions of diffusion pumps use mercury.  
           [0007]    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 previously described.  
           [0008]    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 within 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 high velocity (sometimes supersonic) and 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 (boiler) at the base of the body.  
           [0009]    With the diffusion pump turned on, gas molecules arriving at 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.  
           [0010]    Conventionally, as previously explained, the working fluid is held within the body of the diffusion pump and is consumed due to vapour losses. Provided that pump cooling is adequate the rate of these losses is determined by the mass flow through the pump. Generally, the working fluid needs replenishing from time to time.  
         OBJECT OF THE INVENTION  
         [0011]    It is an aim of the present invention to connect a reservoir of high volumetric capacity to the diffusion pump, the reservoir being positioned remotely from the pump body to minimise the thermal requirements of the pump that would result from having to keep a large volume of working fluid at boiling point continuously. With the reservoir connected to the pump, the pump can operate for longer periods at full throughput without oil maintenance, for example for a year or more.  
         SUMMARY OF THE INVENTION  
         [0012]    According to the present invention, a novel combination comprises a diffusion pump, a reservoir containing a working fluid spaced from the diffusion pump and pipework means for allowing the flow of the working fluid from the reservoir towards the diffusion pump.  
           [0013]    Preferably, the reservoir includes a tank for containing the working fluid, the tank being provided with a transparent window for viewing the level of the working fluid within the tank.  
           [0014]    In one embodiment, a valve, for example a solenoid valve is provided which is movable between a first closed position to prevent the flow of working fluid out from the reservoir to a second open position.  
           [0015]    The movement of the valve between said first and second positions may be controlled by a level monitoring sensor which includes a vertically mounted float and a reed switch and is connected to the diffusion pump via said pipework means.  
           [0016]    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 which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a cross section through a known diffusion pump;  
         [0018]    [0018]FIG. 2 is a perspective view of a high capacity oil reservoir ready for connection to a diffusion pump according to the present invention; and  
         [0019]    [0019]FIG. 3 is a diagrammatic sketch showing the high capacity reservoir of FIG. 2 operatively connected to a diffusion pump. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring first to FIG. 1, a known diffusion pump  1  comprises a hollow outer body  2  of generally circular cross section, a major portion of which is provided with cooling coils  4  for the circulation of cooling fluid, for example water. The body  2  includes a base  6  and vapour chimney  8  is positioned within the outer body  2  and extends upwardly from the base  6 . A working fluid  7  is located in the base of the chimney  8  and, as shown, an electric heater  10  is located immediately below the base  6 .  
         [0021]    As shown, the chimney is formed with a single jet stage  12 .  
         [0022]    The diffusion pump  1  also includes an outlet  14  for connection to a backing vacuum pump and an inlet  16  for connection to a chamber to be evacuated. As shown, the jet stage  12  is located adjacent the inlet  16 .  
         [0023]    In use, when the inlet  16  is attached to the chamber to be evacuated, the backing pump is turned on to reduce the pressure within the diffusion pump  1  and a coolant is passed through the coils  4 .  
         [0024]    Heating of the working fluid  7  by means of the heater  10  causes boiled vapour to rise up the chimney  8  in the manner described earlier and to emerge through the jet stage  12  and thereafter to fall downwardly in the general direction of the base  6  along the inner surface of the outer body  2 .  
         [0025]    According to the present invention, the diffusion pump  1  is connected to a high capacity reservoir.  
         [0026]    Referring now to FIGS. 2 and 3, the high capacity reservoir  20  comprises a tank  22  for working fluid connected to a support arm  24 . As shown, at its upper end there is a transparent window in the form of a glass disc  26  for viewing the level of working fluid in the tank  22 . At its lower end there is provided a normally closed valve  28  for example a solenoid valve. The actuation of the valve  28  is controlled by a level monitoring sensor, in this case a pot assembly  30  which contains a vertically mounted float  29  and a reed switch  31 . The pot assembly  30  is connected to the working fluid fill and drain ports of the diffusion pump body  2  via connections  32 ,  34 .  
         [0027]    In use, when the float  29  drops to a predetermined level as a result of the working fluid level dropping within the diffusion pump body  2 , the reed switch  31  is actuated and energises the solenoid valve  28 . This allows working fluid to pass from the tank  22 , through the solenoid valve  28  and the pot assembly  30  towards the diffusion pump  1  via connections  32 ,  34 .  
         [0028]    To avoid the reed switch  31  fluttering about its switching point, as its hysteresis is limited and the working fluid level oscillates slightly, there is an electronic timer circuit that delays re-closure of the solenoid valve  28 . This ensures that an adequate volume of working fluid is fed into the body  2  with minimum valve actuations.  
         [0029]    The tank  22  is sealed under vacuum so that the fresh working fluid entering the diffusion pump is fully degassed before entering the body  2 .  
         [0030]    In the design of this system generally, the monitoring pot assembly  30  senses the fluid level when the pump  1  is fully operational. When the pump is operational a head of fluid is maintained on the outside of the pump as a consequence of the pressure difference across the boiler stage. This tends to cause an increase in the height of the fluid level outside the jet stage  12 . This pressure difference is the driving force that generates the vapour jets. This head takes a certain period of time to become established when the pump is started from cold. In the warm up condition the fluid level can be low and could cause emptying of the tank  22 . However, this has been overcome by the inclusion of a thermal snap switch that causes the monitoring circuit to lag until the operational temperature of the pump base  6  has been reached.  
         [0031]    While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.