It
wasn't that long ago when you could walk into any vacuum laboratory and
find a vapor diffusion pump on every system. Vapor diffusion pumps were
first conceived about 1915-16, and used mercury as the pumpi
ng
fluid. A decade later, experimenters found that some oils had high
boiling points and low vapor pressures and were good pumping fluids.
These oils were useful because they remained in the pump indefinitely and allowed lower pressures
to
be attained without the use of a cold trap. During W.W.II, and again
during the 1960's for the space effort, diffusion pumps went through
some significant design changes that in creased
their pumping speed, increased their ability to produce lower
pressures, and oils gave way to synthetic pumping fluids. Due to it's
simplicity, high performance, and low initial cost, the diffusion pump
remains the primary industrial high vacuum pumping mechanism.
Applications for this type of pump are found in R&D labs, coatings
facilities, manufacturing, and space simulation. When diffusion pumps
are used with the correct fluid, traps, and baffle, they can producepressures to approximately 2*10-10Torr.
Theory Of Operation
Diffusion
pumps are vapor jet pumps that work on the principle of momentum
transfer. This occurs when a heavy, high speed vapor molecule collides
with a gas molecule and moves it in a preferred direction through the
pump. The bottom of the pump contains an electric heater which is used
to heat the pumping fluid to it's boiling point, thus, producing the
apor. This must be done at a reduced pressure. This means that before
the diffusion pump is started, it must be "rough pumped" down to an acceptable pressure, typically 100 millitorr.
To do otherwise will result in no pumping action and possible damage to the pumping fluid.
Once boiling of the fluid has begun, the vapor is forced up the central
columns of the jet assembly. It then exits at each downward directed
jet in the form of a molecular curtain that impacts the pump body.
The
pump body is externally cooled so that the fluid will condense on its
inside surface and run back down into the boiler. Pump bodies are
typically water-cooled, but some are air-cooled. As gas molecules from
the system randomly enter the pump (molecular flow conditions) , they
encounter the top jet. Some of them are impacted and driven on to the
next jet. Subsequently, they reach the foreline where they are exhausted
to the atmosphere by the mechanical backing pump.
Compression Ratio
The
diffusion pump is similar in character to other compression pumps in
that it develops a relatively high exhaust pressure compared to the
inlet pressure. For most gases this compression ratio may be one million
to one (or greater). For example; for an inlet pressure of 2*10-7 Torr
and a foreline pressure of 2.0*10-1 Torr, the compression ratio would be
one million. As far as compression goes, in a mixture of gases, each
species may be pumped with different effects. It is possible to have
different maximum compression ratios and different flow rates for gases
having different molecular weights. For example, the compression ratio
for hydrogen will differ greatly from the compression ratio for argon
simply because their molecular weights are very different. Also, when
the pumped gas has a molecular weight differentfrom air the maximum
compression ratio
Refrence
Book: Coating fundamental and nanostructure analysis, jahanbakhsh mashaiekhy, iup,2015
Vacuum Technology 60A & 60B, Chapter 7: Oil Vapor Diffusion Pumps, Las Positas College