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Diffusion Pump Basics
Diffusion pumps are used where throughput for heavy gas
loads is important. The diffusion pumps begin to work at
3
approximately 10
Torr after a mechanical backing
pump has exhausted most of the air in the system.
Figure 1 HS-20 Diffusion Pump
There are no moving parts in a diffusion pump, the heart
of which is the multistage jet assembly, a group of con-
centric cylinders that are capped to leave small openings
through which vapor can be deflected down and out
toward the pump walls. A cold cap, mounted on top of
the jet assembly helps keep pump fluid vapor out of the
evacuation chamber. The pumps are water-cooled.
The vacuum fluid heater is mounted at the bottom of the
pump body. The pumps also have a fill and drain assem-
bly, and thermal protection switches. The inlet is at the
top, and the exhaust is through the foreline.
High Throughput Diffusion Pumps
11
Pump Operation
The diffusion pump works by heating the pump fluid to its
boiling point. The vapors travel upward inside the jet
assembly and are accelerated out and downward through
the jet nozzles toward the cool outer walls of the pump,
where the vapor condenses back into a fluid. As the vapor
passes the inlet, it picks up elements of the gas to be
exhausted and carries them to the ejector and out of the
pump via the foreline. The pump's ability to reach low
pressures is governed in part by the inlet size. The gas
migrates by thermal motion and is captured and expelled,
thus lowering the pressure in the evacuation vessel.
Large diffusion pumps achieve a vacuum using a
five-stage jet assembly consisting of four diffusion stages
and one ejector stage. The cold cap and body are water
cooled. Optional stainless steel Quick Cool coils quickly
suppress boiling by cooling the heater; they require an
independently valved water supply.
Operating Characteristics
The Operating Characteristics of the large diffusion
pumps are given in Table 4 on page 12. The graphs in
Figure 2 through Figure 5 on page 14 show air speed and
throughput as a function of inlet pressure. The dimensions
of the inlet flange that connects to the system to be evacu-
ated are given in Table 6 on page 15 through Table 12 on
page 18
NOTE
The data in Table 4 on page 12 refers to
pumps charged with DC-704 diffusion
pump fluid.
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