How does a diffusion pump work?
Diffusion pumps consist basically (see Fig. 2.44) of a pump body (3) with a cooled wall (4) and a three, four or five-stage nozzle system (A - D). The oil serving as pump fluid is in the boiler (2) and is vaporized from here by electrical heating (1). The pump fluid vapor streams through the riser tubes and emerges with supersonic speed from the ring-shaped nozzles (A - D). Thereafter the jet so-formed widens like an umbrella and reaches the wall where condensation of the pump fluid occurs. The liquid condensate flows downward as a thin film along the wall and finally returns into the boiler. Because of this spreading of the jet, the vapor density is relatively low. The diffusion of air or any pumped gases (or vapors) into the jet is so rapid that despite its high velocity the jet becomes virtually completely saturated with the pumped medium. Therefore, over a wide pressure range diffusion pumps have a high pumping speed. This is practically constant over the entire working region of the diffusion pump (≤ 10-3 mbar) because the air at these low pressures cannot influence the jet, so its course remains undisturbed. At higher inlet pressures, the course of the jet is altered. As a result, the pumping speed decreases until, at about 10-1 mbar, it becomes immeasurably small.
- Pump body
- Cooling coil
- High vacuum flange
- Gas molecules
- Vapor jet
- Backing vacuum connection
- A, B, C, D Nozzles
The forevacuum pressure also influences the vapor jet and becomes detrimental if its value exceeds a certain critical limit. This limit is called maximum backing pressure or critical forepressure. The capacity of the chosen backing pump must be such that the amount of gas discharged from the diffusion pump is pumped off without building up a backing pressure that is near the maximum backing pressure or even exceeding it.
The attainable ultimate pressure depends on the construction of the pump, the vapor pressure of the pump fluid used, the maximum possible condensation of the pump fluid, and the cleanliness of the vessel. Moreover, backstreaming of the pump fluid into the vessel should be reduced as far as possible by suitable baffles or cold traps.
In oil diffusion pumps it is necessary for the pump fluid to be degassed before it is returned to the boiler. On heating of the pump oil, decomposition products can arise in the pump. Contamination from the vessel can get into the pump or be contained in the pump in the first place. These constituents of the pump fluid can significantly worsen the ultimate pressure attainable by a diffusion pump, if they are not kept away from the vessel. Therefore, the pump fluid must be freed of these impurities and from absorbed gases.
This is the function of the degassing section, through which the circulating oil passes shortly before re-entry into the boiler. In the degassing section, the most volatile impurities escape. Degassing is obtained by the carefully controlled temperature distribution in the pump. The condensed pump fluid, which runs down the cooled walls as a thin film, is raised to a temperature of about 266°F (130°C) below the lowest diffusion stage, to allow the volatile components to evaporate and be removed by the backing pump. Therefore, the re-evaporating pump fluid consists of only the less volatile components of the pump oil.
The magnitude of the specific pumping speed S of a diffusion pump - that is, the pumping speed per unit of area of the actual inlet surface - depends on several parameters, including the position and dimensions of the high vacuum stage, the velocity of the pump fluid vapor, and the mean molecular velocity c- of the gas being pumped (see equation 1.17). With the aid of the kinetic theory of gases, the maximum attainable specific pumping speed at room temperature on pumping air is calculated to Smax = 11.6 l · s-1 · cm-2. This is the specific (molecular) flow conductance of the intake area of the pump, resembling an aperture of the same surface area (see equation 1.30). Quite generally, diffusion pumps have a higher pumping speed for lighter gases compared to heavier gases.
To characterize the effectiveness of a diffusion pump, the so-called HO factor is defined. This is the ratio of the actually obtained specific pumping speed to the theoretical maximum possible specific pumping speed. In the case of diffusion pumps from Leybold optimum values are attained (of 0.3 for the smallest and up to 0.55 for the larger pumps).
The various oil diffusion pumps manufactured by Leybold differ in the following design features (see Fig. 2.45).
- Forevacuum connection flange
- Centering ring with forevacuum baffle
- Nozzle assembly
- Coolant line
- High-vacuum connection flange
- Cold cap baffle
- Pump housing
- Electrical junction box
- Sheet metal cladding for heating
In these pumps an evaporation process for the pump fluid which is essentially free of bursts is attained by the exceptional heater design resulting in a highly constant pumping speed over time. The heater is of the internal type and consists of heating cartridges into which tubes with soldered on thermal conductivity panels are introduced. The tubes made of stainless steel are welded horizontally into the pump’s body and are located above the oil level. The thermal conductivity panels made of copper are only in part immersed in the pump fluid. Those parts of the thermal conductivity panels are so rated that the pump fluid can evaporate intensively but without any retardation of boiling. Those parts of the thermal conductivity panels above the oil level supply additional energy to the vapor. Owing to the special design of the heating system, the heater cartridges may be exchanged also while the pump is still hot.
The DIP pumps are equipped with a jet stack in a four-stage-nozzle design and are suitable for pumping in a pressure range of 10-2 to 10-8 mbar.
The DIJ series features a further improved design for applications, where a high pumping speed in combination with high gas-throughputs in a pressure range of 5x10-1 to 10-7 mbar is needed. The heater design with conductivity panels was taken from the DIP series, but further improved. Instead of a tubed design, where heater cartridges are introduced in stainless steel tubes, a flanged design is provided with DIJ pumps. The heater cartridges are securely and leak-tight mounted in the heater vessel and directly immersed in the pump fluid. This design provides a further improved heat-up of the pump fluid as well as a simplified maintainability. The jet stack includes an additional ejector stage, which leads to a higher forevacuum pressure stability and an increased gas-throughput. As the Diffusion pump principle is based on heating oil, these pumps deal with one major issue. Approximately 80% of the energy brought in the pump will be emitted to the environment. The DIJ series is equipped with an insulation jacket around the heater vessel, which isolates it from the surrounding and leads to an improved heat-up time and energy consumption.
The suitable pump fluids for oil diffusion pumps are mineral oils and silicone oils. Severe demands are placed on such oils which are met only by special fluids. The properties of these, such as vapor pressure, thermal and chemical resistance, particularly against air, determine the choice of oil to be used in a given type of pump or to attain a given ultimate vacuum. The vapor pressure of the oils used in vapor pumps is lower than that of mercury. Organic pump fluids are more sensitive in operation than mercury, because the oils can be decomposed by long-term admission of air. Silicone oils, however, withstand longer lasting frequent admissions of air into the operational pump.
The typical mineral oil Leybold offers for Diffusion Pumps is LVO500. This mineral oil has fractions of a high quality base product (see our catalog) distilled with particular care. LVO 500 is our standard diffusion pump oil for applications in a high vacuum with a good thermal stability.
For the optimal performance, Leybold offers LVO521 (see our catalog), a high-purity silicone oil solution that contains a special silicon to help you to get the best performance from your pump in high and ultra-high vacuum applications. It has a high thermal stability and is highly resistant to oxidation and decomposition.
For oil vapor jet pumps, Leybold offers the LVO540 (see our catalog), a special hydrocarbon oil. It has an extended oil lifetime and improved temperature stability, thermally and chemically highly resistant and excels through a high degree of oxidation resistance. It delivers the essential high pumping speed of the vapor jet pumps in the medium vacuum range.
The heater power that is continuously supplied for vaporizing the pump fluid in fluid entrainment pumps must be dissipated by efficient cooling. The energy required for pumping the gases and vapors is minimal. The outside walls of the casing of diffusion pumps are cooled, generally with water. Smaller oil diffusion pumps can, however, also be cooled with an air stream because a low wall temperature is not so decisive to the efficiency as for mercury diffusion pumps. Oil diffusion pumps can operate well with wall temperatures of 86°F (30°C), whereas the walls of mercury diffusion pumps must be cooled to 59°F (15°C). To protect the pumps from the danger of failure of the cooling water - insofar as the cooling-water coil is not controlled by thermally operated protective switching - a water circulation monitor should be installed in the cooling water circuit; hence, evaporation of the pump fluid from the pump walls is avoided.
Mercury can be used as a pump fluid. It is a chemical element that during vaporization neither decomposes nor becomes strongly oxidized when air is admitted. However, at room temperature it has a comparatively high vapor pressure of 10-3 mbar. If lower ultimate total pressures are to be reached, cold traps with liquid nitrogen are needed. With their aid, ultimate total pressures of 10-10 mbar can be obtained with mercury diffusion pumps. Because mercury is toxic, as already mentioned, and because it presents a hazard to the environment, it is nowadays hardly ever used as a pump fluid.