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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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