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How does a diffusion pump work   

Diffusion pumps.

Diffusion pumps will operate only if the pressure is less than a few tenths of a millimeter of mercury, and they operate best with a "backing pressure" of a few hundredths of a millimeter of mercury. The necessary "backing pressure" is obtained by mechanical pumps.

The operation of a mercury diffusion pump is illustrated in Fig. 5. The pump shown here illustrates Langmuir's practical adaptation of Gaede's discovery of the principle of diffusion pumping. [20] The following explanation of its action applies as well to the action of oil diffusion pumps.

A stream of mercury vapor is obtained by heating liquid mercury in boiler B to a temperature of about 110 C.

The vapor stream which effuses from the attached chimney is indicated by arrows. This stream forms a partition between chamber N and chamber M. The vapor finally condenses on the water-cooled walls of chamber N and returns under the influence of gravity to the boiler as a liquid. Gas molecules in chamber N which diffuse into the vapor partition have a small chance of penetrating it and entering chamber M. Rather, it is more probable that they will be carried by the stream back into chamber N. However, gas molecules in M which diffuse into the vapor partition are carried along by molecular bombardment into N, where they are removed by the mechanical pump.

The pressure in N must exceed that in M by a factor of the order of 100 if the rate of diffusion is to be the same in both directions across the vapor partition. Where N is evacuated by an auxiliary diffusion pump instead of the mechanical pump, pressures of 10^-7 mm of mercury or lower can be obtained in a tight glass apparatus connected to M (provided mercury vapor is removed with a liquid air trap).

Mercury pumps have been studied by many investigators.[21] Figs. 6 to 12 are representative of the designs which have evolved as a result of these studies.

We will not discuss these pumps in detail, as we are mainly interested in this chapter in kinetic vacuum systems and oil diffusion pumps. With oil diffusion pumps it is not uncommon to have pumping speeds of some tens or hundreds of liters per second, whereas with mercury diffusion pumps the speeds are ordinarily only a fraction of a liter per second up to a few liters per second.

The use of oils as diffusion pump liquids:

There have been many attempts to find a substitute for mercury as a pumping medium, for the use of mercury has one considerable disadvantage, namely, its vapor pressure is so high that traps are required to prevent it from diffusing into the vacuum system and destroying the vacuum. These traps, having a high resistance to the flow of gas, choke the pump.

The only widely used substitutes for mercury are high vacuum oils. The oils used for this purpose are either especially refined petroleum oils of the naphthene type as developed by C. R. Burch,[22] or they are organic compounds such as butyl phthalate as developed by Hickman and Sanford [23] of the Eastman Kodak Laboratories. Recently, Hickman has recommended a new synthetic organic oil called Octoil, which is claimed to be superior to butyl phthalate.[24]

The type oil developed by Burch are manufactured under Metropolitan Vickers' patents under the trade name of Apiezon oil. [25] Similar oils are now available in this country which yield pressures below 10^-6 mm of mercury.[26]

Oil pumps have the advantage over mercury pumps that they do not require traps except in certain applications. Another advantage is that oil pumps may be fabricated either from steel or from brass and copper, whereas metal mercury pumps must be constructed of steel with welded joints. Brass and copper pumps can be assembled with soft solder, except for the boiler and chimney, where it is advisable to use silver solder.

Aside from the questions of traps and construction, the contrast between oil and mercury pumps is less distinct. Oil pumps without traps do not give quite as low a limiting pressure as trapped mercury pumps, although their speed may be many times greater. If traps are used, there is probably little difference between the limiting pressures attainable. Oil pumps have the advantage that a baked-out total obstruction charcoal tube at room temperature is as effective as a liquid air trap. However, the use of a total obstruction charcoal trap sacrifices the higher pumping speed of the oil pump.

It is not advisable to use a single oil pump. One should use at least two oil pumps in series. The second pump serves to keep the oil in the first purified. The limiting pressure is about tenfold lower when a second pump is used. Because mercury pumps will operate against a slightly higher back pressure than oil pumps, there are many cases in which a single mercury diffusion pump is adequate.

Oil diffusion pumps.

Oil diffusion pumps are like mercury diffusion pumps in several respects. They have the same functional elememts - a boiler to vaporize the oil and a chimney for conducting the vapor to the jet.

The two types of pumps are also similar in the way in which they function. The oil vapor is projected from the jet across the throat of the pump and condenses on the cooled walls which form the outer boundary of the throat; and the condensed oil drains from the condensing surface back into the boiler by gravity. The vapor jet may be arranged in several ways: It may be directed upward as in the up-jet mercury pump shown in Fig. 5, it may be directed downward as in the umbrella down-jet mercury pump shown in Fig. 6, or it may project laterally as shown in Fig. 7.

Although oil and mercury diffusion pumps have the same functional elements, they differ in the details of construction. The construction of oil diffusion pumps can be carried out in an ordinary machine shop. The important considerations for proper construction are outlined below:

1. The oil is decomposed slightly at the working temperatures of the boiler. This decomposition is accelerated by the higher temperature necessary when the cross section of the boiler is not large enough to afford an adequate surface from which to create vapor, or when the chimney and jet are not ample to deliver the required amount of vapor without an excessively high pressure drop.

2. Since oil has a low latent heat, the pump should be designed so that the heat required to maintain the working temperatrure of the chimney and jet is supplied by conduction from the heater rather than by condensation of oil vapor. Naturally, copper is the best material for constructing the chimney on account of its large heat conductivity.

3. The decomposition of the oil is catalyzed by copper and brass and not by nickel. Accordingly, all parts of the pump exposed to the hot oil should be nickel-plated.[27]

4. The amount of oil decomposed in a given time is proportional to the amount of oil present in the boiler. It is, therefore, advisable to have only a shallow layer of oil in the boiler.

5. At least two single-jet pumps in series should be used. Multiple-jet pumps are not recommended because of the difficulty of regulating the flow of vapor to the various jets and of supplying the necessary amount of vapor required by them without an excessive boiler temperature.

6. Throat clearances narrower than 1/8 inch are practical only for up-jet pumps. Condensed oil will bridge gaps of this narrowness m pumps of the down-jet type.

7. Backward evaporation of the oil into the pumping line should be restrained by the use of baffles.

8. Cold oil is a better solvent for many gases and vapors than hot oil. Accordingly, the condensed oil should be returned to the boiler at the maximum temperature possible. Otherwise, a certain amount of the exhaust gases and vapors dissolve in the condensed oil and contaminate it.

9. The use of electric heat for the boiler is advisable, since it is subject to more delicate control than gas heat. A Calrod heater unit, such as used in electric stoves, can be recoiled into a helix of 2 inches in outside diameter or as a flat spiral of smaller dimensions.

Figs. 13 to 18 illustrate several oil pumps which are currently popular. [28]The pump shown in Fig. 13, designed by Sloan, Thornton, and Jenkins, satisfies the requirements for good design outlined above and at the same time combines these features together with simplicity of construction. The following description of this pump is a quotation from a paper of Sloan, Thornton, and Jenkins.[29]

The Apiezon oil diffusion pump was originally developed by the Metropolitan Vickers Company in England for this very purpose of continuously exhausting radio tubes. The oil is sold commercially in this countrry.

Fig. 13 is typical of the simplified designs which have been widely adopted in this country. The outer shell 2" in diameter consists of a water-jacketed brass cylinder with a copper plate silver soldered into its bottom. In the cavity beneath the bottom plate is placed an electric heater which boils the Apiezon "B" oil at less than 200 C. in the chamber above. The oil vapor rises through the copper chimney and is deflected downward by a spun copper umbrella. The 5/16" clearance between the edge of the umbrella and the condensing wall is not critical, although an optimum exists for any specified set of pressures. Around the chimney is a glass heat shield, and a metal baffle plate to retard the rise of oil vapor from the roof of the boiler, but these can be omitted without serious consequences. The two baffles above the umbrella prevent the escape of oil vapor directly into the region being evacuated. The convenient baffle system shown here reduces the speed of the pump to less than half, so that its overall speed is only thirty liters per second. This is more than sufficient for these oscillator tubes, since the connecting system reduces the speed to less than ten liters per second. A pressure in the oscillators of 10^-5 mm is sufficient.

Incidentally, the same general design is also well suited to larger pumps of 4" and 6" diameter, for use with larger tubes. The speed of an oil pump can be greatly increased by enlarging the diameter of the overhead region which contains the baffles necessary to guard against escaping oil vapor. A 2-inch pump of such construction will have a pumping speed of about 30 liters/sec., or a speed factor slightly greater than 50 per cent.

If such a high speed is not needed, an up-jet pump may serve. Fig. 14 shows Hickman and Sanford's all-glass design of an up-jet pump.

Fig. 15 shows an all-metal upjet pump designed by Edwin McMillan [30]

With the boiler temperature adjusted to give maximum pumping speed, this pump will work at a rate of 4 liters/sec. against a backing pressure of 1/2 mm of mercury. If the boiler temperature is too high, the action of the pump will be erratic, since returning condensed oil interferes with the vapor jet.

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