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Oil Diffusion Pumps-DIP Series

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.
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. 1. Continuous operation at operating pressures above 10-4 mbar (0.75 x 10-4 Torr) – large quantities of gas. 2. 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 pre-evacuated 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.
Operating Oil Diffusion Pumps - 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.
Operating Oil Diffusion Pumps - 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 Diffusion Pumps - 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 have been measured at an intake pressure of < 1x10-4 mbar and apply to DIFFELEN normal. When using DC 705 the values may improve on average by one order of magnitude.
Operating Oil Diffusion Pumps - Attainable Ultimate Pressure
The attainable ultimate pressure for a particular vacuum system depends not only on the type and pumping speed rating of the diffusion pump, but also on the vapor pressure of the pump fluid, shape and temperature of the baffle, leaks at connecting flanges or welded joints and the condition of the surfaces within the vacuum chamber. When excluding all effects which contribute to an increase in pressure within the vacuum chamber due to leaks and contamination of the vacuum chamber walls, it will be possible to attain the ultimate pressures stated in the table “Attainable Ultimate Pressures with Oil Diffusion Pumps (DIP)” given in section “General”. In practice the following combination has been found to work very well when needing a low vacuum free of oil vapors. - Water-cooled cold cap baffle as a integral part of the diffusion pump together with a water-cooled Astrotorus baffle which may be installed as an additional component on the high vacuum flange of the diffusion pump.
Operating Oil Diffusion Pumps - Sealing Methods
For ultimate pressures down to 10-8 mbar (0.75 x 10-8 Torr) bakeout temperatures of up to 150 °C (302 °F) are sufficient. FPM (FPM = Fluor caoutchouc, temperature resistant up to 150 °C (302 °F)) sealing rings or ultra sealing rings made of aluminum must be used. In order to prevent pressure variations, ultra sealing rings must be used in the connections, between diffusion pump and baffle. Ultimate pressures below 10-8 mbar (0.75 x 10-8 Torr) require bakeout temperatures up to 400 °C (752 °F). However, it is only necessary to bake out the vacuum chamber to 400 °C (752 °F) and to maintain a temperature gradient across the baffle or the cold trap so that a temperature of 150 °C (302 °F) is not exceeded at the intake flange of the pump. In this way, it is still acceptable to use FPM sealing rings or ultra sealing rings made of aluminum.
Operating Oil Diffusion Pumps - Cooling
The cooling water temperature should not exceed 25 °C (77 °F) at the intake and 30 °C (86 °F) at the discharge, otherwise sufficient condensation of the pump fluid cannot be ensured. When connecting the cooling system of the pump and the baffle in series, the cooling water must always be made to flow through the baffle first and then through the diffusion pump, because the attainable ultimate pressure in the vacuum chamber depends strongly on the condensation temperature of the pump fluid in the baffle.