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