Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology
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2.4.6 Backstreaming
137
With
the use of
cryogenic traps
and
modern fluids, such
as
DC-705
or
Santo-
vac-5,
ultimate pressures
of
10"^^ torr and below can be achieved where contam-
inants from sources other than
the
vapor jet pump are controlled.
An increase
of
heat input will usually increase
the
pressure ratio obtainable
with
a
pump. However, depending
on
pump design, higher power
can
either
im-
prove
or
worsen
the
ultimate pressure. This depends
on the
ratio between fluid
thermal breakdown rate
and
the purification ability
of
the particular pump.
2.4.6
BACKSTREAMING
Any transport
of
the pumping fluid into
the
vacuum system may
be
called back-
streaming. The possibility
of
this backflow
is
perhaps
the
most undesirable char-
acteristic
of
vapor
jet
pumps.
It is in
this area that most misunderstanding
and
misinformation exists. Pump manufacturers usually report the backstreaming rate
at
the
inlet plane
of an
unbaffled pump. System designers
are
often concerned
about
the
steady-state backstreaming above liquid nitrogen traps.
The
users
are
often troubled
by
inadvertent high-pressure
air
inrush into
the
discharge
end of
the pump. The quantities
of
backstreaming fluid involved
in
these cases can range
over many orders
of
magnitude.
As
far as the
pump itself
is
concerned, there
can be
several sources
of
back-
streaming:
1.
The overdivergent flow
of
vapor from the
rim of
the upper nozzle
2.
Poorly sealed penetrations
at
the top nozzle
cap
3.
Intercollision
of
vapor molecules
in the
upper layer
of
the vapor stream from
the top nozzle
4.
Collisions between
gas and
vapor molecules, particularly
at
high
gas
loads
(10--"^
to
10-^ torr region)
5.
Boiling
of
the returning condensate just before
the
entry into
the
boiler (be-
tween the
jet
assembly
and the
pump wall), which sends fluid droplets
up-
ward through the vapor jet
6. Evaporation
of
condensed fluid from the pump wall
2.4.6.1 Primary
and
Secondary Backstreaming
All
the
items just listed that
can be
stopped
or
intercepted
by the
room tempera-
ture baffle may
be
called primaiy backstreaming. The reevaporation
of
the pump-
138
Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]
ing fluid from the baffle and the passage through the baffle may be called sec-
ondary backstreaming. The primary backstreaming can be effectively controlled
by the use of cold caps surrounding the top nozzle. Cold caps are water-cooled
baffles located above the top jets of a pump.
In systems with liquid nitrogen traps (barring accidents and high gas load op-
eration), the backstreaming level can be controlled at such a low level that con-
taminants from sources other than the vapor jet pump will predominate [9]. Prop-
erly operated and protected vapor jet pump systems can be considered to be free
of contamination from the pumping fluid for most applications.
2.4.6.2 Speed and Backstreaming
The backstreaming rates measured by the American Vacuum Society Recom-
mended Practice and Manufacturer's Specifications refer to the inlet plane of the
pump. It is important to understand that this rate quickly diminishes at some dis-
tance from the pump inlet. At a distance from inlet equivalent to two pump diam-
eters,
the rate is reduced typically 50 times (Figure 15). A 90-degree bend in the
inlet duct will act as a baffle and reduce backstreaming to the level of the natural
Rg.l5.
100 %
<
<
IAJ
<
m
100%
NET PUMPING SPEED
Reduction of backstreaming rate.
2.4.6 Backstreaming
139
Fig.
16.
100%
01
:(<c->)^
[
••''it'
=
L 1 2
T T
P
P
/
r
^
opaque
h Baffles
1 \ 1 i _i_
' ' ^ ' >^i
Pump 5/]
Inlet / j
^
... ^
/ Evaporation
/ Rate(DC-704) \
1
1
f \
f i
(DC-705)
50
Net Pumping Speed
100%
Qualitative relationship between conflicting requirements of minimum backstreaming and
maximum retaining pumping speed with baffles at ambient temperature.
evaporation rate of
the
fluid at the ambiance temperature. Without cryogenic traps
or other similar devices, this rate cannot be reduced much further.
Figure 16 shows the qualitative relationship between conflicting requirements
of minimum backstreaming and maximum retained pumping speed with baffles at
ambient temperature. If a series of elements that reduce backstreaming is intro-
duced above the pump, the backstreaming rate approaches the evaporation rate of
the fluid. It may be observed that in many applications, complete opaque baffles
are redundant. With efficient cold caps and low-vapor-pressure pumping fluids, a
system can be operated for a year or more with continuous liquid nitrogen cooling
of the trap. The condensed pumping fluid will be returned into the pump, and the
buildup on the trap will not be detrimental.
In normal practice, an optimum design for a vapor jet pump and trap combina-
tion can have nearly 40% net pumping speed and reduce backstreaming to less
than 1 X 10-^0 g/cm^ min (at the inlet plane of the trap). Values of this magni-
tude have been measured [10].
140 Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]
2.4.6.3 Surface Migration
Some pumping fluids may have a tendency to spread on metal surfaces as a thin
film. This spreading is discussed in the literature dealing with lubrication of small
instruments [11]. The spreading may be noticed sometimes as wetness on rough
(sand-blasted) surfaces. Modem low-vapor-pressure pumping fluids having sur-
face tensions above 30 dyne/cm do not spread on ordinary metal surfaces. They
are so-called autophobic liquids because they do not spread on their own mono-
layers covering a metal surface.
Surface diffusion is not likely to be a significant backstreaming mechanism on
surfaces at room temperature or below for pumping fluids with vapor pressures in
the
10 ~^
torr range and high surface tension.
2.4.6.4 Accidental Backstreaming
It is important to remember that malfunction and misoperation can destroy the
intentions of most intelligent designs. The most conmion causes of gross back-
streaming are accidental exposure of discharge side of the vapor jet pump to
pressure higher than the tolerable forepressure, high inlet pressure exceeding
maximum throughput capacity over long periods of time, incorrect startup, and
incorrect bakeout procedures.
BACKSTREAMING
RATE
VS.
PRESSURE
From lowest pressures to about 1 X 10""^ torr, the backstreaming rate appears to
be independent of pressure indicating that oil-gas collisions are not significant
in this region. Between
10 ""^
and
10 ~^
torr, a slight increase may be noticed [12].
Above the critical pressure point when the maximum throughput
is
reached (pump-
ing speed begins to decline), the backstreaming rate may rise markedly.
Measurements of the backstreaming rate at various inlet pressures made under
undesirable conditions are plotted in Figure 17. The tests shown were obtained
with a collecting surface in the immediate vicinity of the top nozzle. If the mea-
surements were made at the inlet plane as specified by the American Vacuum So-
ciety Recommended Practice, the rate would have been 10 or 100 times lower
(see Figure 15). It can be seen that the rate does not change significantly as long
as the inlet pressure is below about
10 "^
torr. This is the point where most vapor
jet pumps have an abrupt reduction in speed indicating that the top jet essentially
stops pumping.
The sudden increase of backstreaming at inlet pressures above 10"'^ torr points
2.4.6 Backstneaming
141
Rg. 17.
QC
O
2
<
en
o
<
0.1
10
h
L
r
^
I • . . 1 111
/
/
/
/
/
/
/
/
/
./]
/
I0~*
10"* I0~* 10"' K5"
INLET PRESSURE,Torr
Measurements of backstreaming rate at various inlet pressures.
out that a vapor jet pump should not be operated in this range for more than a
brief time. It is common practice in vacuum system operation to open the high-
vacuum valve after the chamber has been rough pumped to about 10"^ torr ex-
posing the pump to this pressure. Extended operation at high pressure can direct
unacceptably high amounts of oil into the vacuum system. (See Section 2.1.9 for
more information on crossover procedures.)
The higher-pressure conditions exist at the inlet to the pump briefly after the
high-vacuum valve is opened. Unless the vapor jet pump is severely overloaded,
the associated backstreaming peak normally lasts only a few seconds.
The same condition will exist when the pump is initially heated. In this case,
the duration is a few minutes and the amplitude a few times higher than the
steady-state backstreaming at lower pressures. The generalized picture is shown
in Figure 18. At pressures higher than 0.5 torr, depending on diameter and length
Fig. 18.
10
10'*
10"
PRESSURE (Torr)
Higher-pressure conditions that exist at the inlet to the pump,
10
142 Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]
of ducts, the pumping fluid vapor may be swept back into the pump. This can be
used to reduce initial backstreaming and prevent mechanical pump backstream-
ing by arranging a flow of air (or other gas) through a leak valve.
2.4.6.6 Mechanical Pump Effects
Another possibility for hydrocarbon contamination arises from the roughing
pump. A roughing pump should not be left connected to the system for long peri-
ods of time. After initial evacuation, the roughing valve should be closed and
pumping switched to the vapor jet pump.
In systems where only a mechanical pump is used for maintaining vacuum con-
ditions in the lO'-^-lO"^ torr range, sometimes it is advisable to pump periodi-
cally rather than continuously. A valve and a gauge are needed to start and stop
pumping whenever necessary. This prevents continuous back-diffusion of lubri-
cating oil and, in addition, keeps the oil cold, reducing its vapor pressure.
2.4.6.7 Pumping Fluid Loss
The pumping fluid level in a well-designed pump need not be precisely con-
trolled. Generally, 30% above and below normal level should be tolerable without
noticeable effects. When the level is too low, the boiling process may pass from
nucleated boiling to partial film boiling, which leads to overheating of the boiler
heating surface. If this condition is continued for an extended period of
time,
par-
ticularly for large pumps, it can cause distortions of the boiler
plate.
This, in turn,
may expose the center of the boiler plate above the liquid level and lead to further
overheating. The resulting poor contact between heaters and the boiler plate may
also overheat the heating elements and cause their failure.
If the liquid level is too high, the boiling process may foam the fluid and raise
its level as high as the foreline opening.
Excluding normal backstreaming, the fluid may be lost out of the pump in
several ways: prolonged operation above or near maximum throughput; acciden-
tal high pressure and high-velocity air flow through the pump in either direc-
tion; and evaporation of higher-vapor-pressure fluids due to incorrect temperature
distribution.
With relatively low gas loads, modem pump fluids, and correct system design
and operation, vapor jet pumps can be operated for many years without adding or
changing pumping fluid. Operation exceeding 10 years has been reported in the
2.4.6 Backstreaming
143
case of a particle accelerator [13]. Large pumps usually have means of monitor-
ing the fluid level. To reduce fluid loss, some pumps have built-in foreline baffles.
2.4.6.8 Fluid Films on LN Traps
An unusual mechanism of gross backstreaming has been observed in vapor jet
pump systems with liquid nitrogen traps. It occurs sometimes during or following
the charging of the trap with liquid nitrogen and can be recognized by the appear-
ance of small droplets upstream from the trap and by accompanying pressure
fluctuations. The phenomenon is produced by fracture of the frozen pumping
fluid film due to the unequal temperature expansion coefficients between it and
the metal surfaces on which it has been previously deposited. The elastic energy
stored in the film is sufficient to impart high velocity to the fragments resulting
from the fracture. The design can be improved by avoiding certain geometric
configurations that tend to accumulate heavy fluid films and produce highly
stressed films that are likely to fracture.
Liquid nitrogen traps generally should not be placed too close to unprotected
vapor jet pumps. Ambient or water-cooled baffles, partial baffles, or efficient cold
caps essentially remove primary backstreaming. Traps that have removable inter-
nal structures should be cleaned periodically. The frequency will depend on the
vapor pressure of the pumping fluid and possibly the degree of accumulation of
other condensables such as water vapor, assuming that the trap is continuously
kept refrigerated. A thin film (less than
10 ~^
nmi thickness) is not likely to frac-
ture.
In this connection, traps that have long liquid nitrogen holding time are par-
ticularly recommended.
If the trap has to be desorbed occasionally, such as during weekends, the re-
starting procedure needs some attention. It is good practice to keep the high-
vacuum valve closed during and after the trap-filling periods. In this manner, the
fractured frozen film particles and products of desorption can be prevented from
reaching the vacuum chamber. The valve opening should be delayed after recool-
ing the trap, to allow time for repumping of condensable matter.
With the combination of the design and procedural remedies mentioned, the
difficulties associated with the described phenomenon can be avoided. Figure 19
shows a record of a typical pressure variation due to this effect.
144
Fig.
19.
Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps)
PRESSURE
Record of typical pressure variation when trap is desorbed. Arrows show trap filling periods.
2.4.7
OTHER PERFORMANCE ASPECTS
2.4.7.1 Design Features
In their specifications, manufacturers of vapor jet pumps traditionally report only
pumping speed, ultimate pressure, forepressure tolerance (measured at the fore-
line),
throughput, and sometimes the backstreaming rate (referred to the inlet
plane of the pump). However, other performance aspects, listed next, can be used
as guides in selecting pumps for particular applications:
• Pumping speed per unit inlet area. This can be called speed efficiency, and
it is not likely to exceed 50% compared to a hypothetical pump with 100%
capture probability for molecules that cross the inlet plane into the pump.
• Power required to obtain a given maximum steady-state throughput without
overloading and without oversized forepumps.
2.4.7 Other Performance Aspects 145
• Ratio of maximum throughput and forepressure tolerance. This determines
the minimum required forepump speed that must be provided at the dis-
charge of the vapor jet pump (at its full load).
• Maximum pressure ratio for light
gases.
Of particular interest are helium and
hydrogen (pumping speed for helium and hydrogen, compared to the speed
for air, should be 1.2 times higher).
• Ratio of evaporation rate of pumping fluid at ambient temperature and the
actual pump hackstreaming without traps. This ratio is approaching unity in
some modern pumps that have efficient cold caps surrounding the top nozzle.
• Ratio of vapor pressure of working fluid at ambient temperature and ulti-
mate pressure obtained by pump without cryogenic traps. The target for this
ratio should also be near unity. To achieve this, the pump must have low
pumping fluid breakdown level and a high degree of fluid purification.
• Ratio of forepressure tolerance and boiler pressure. This ratio is usually
about 0.5. The significance of this figure is in keeping the fluid temperature
as low as possible to reduce thermal breakdown, while keeping the fore-
pressure tolerance as high as possible.
• Ratio of pump diameter and
height.
The height is normally minimized for the
sake of compactness, but some performance improvements could be realized
if pumps were allowed to be taller.
• Pressure stability in constant speed region. This can be expressed as a per-
cent variation referred to an average value. Pressure instability is more com-
mon in smaller pumps and with lighter gases.
• Sensitivity to heat input fluctuations. In addition to variations of heater power,
cooling water flow rate and temperature may have some significance.
The low degree of fluid breakdown and self-purification qualities of the pump
can
be
judged by the degree with which the ultimate pressure of the pump follows
the expected vapor pressure of the pumping fluid at room temperature. Figure 20
illustrates the performance of various fluids in a pump which must have a given
boiler pressure (for example, 0.5 torr). In modern pumps, the pressure of the oil
vapor inside
the
jet assembly may be about 1.5 torr. From Figure 20 it can be seen
that above the temperature rate of 250°C, the performance of the pump deterio-
rates,
as far as ultimate pressure is concerned. Generally, it is useless to employ
fluids of very low vapor pressure because of resulting excessive boiler temperature.
When using extremely low-vapor-pressure fluids (for example, Santovac-5), it
is sometimes necessary to adjust water-cooling rate or power to obtain optimum
performance. Some pumps may have to operate with more effective heat input,
some with less, depending whether they are below or above the valley of
the
curve
in Figure 20.
146
Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]
Fig.
20.
- 10 '
B 10^
15 10'
L OC-704
^
OS-138
XF-1051
1
S
150 200 250
300
350
Boiler Temperature pc
Illustration of performance of various fluids in a pump with a given boiler pressure.
2.4.7.2 Cold Caps
Cold caps surrounding the top nozzle of the pump reduce backstreaming rates 50
or more times [14-18]. In medium-sized pumps, they are usually made of copper
and cooled by radiation or conduction through supports contacting the water-
cooled pump wall. This allows the fluid drops from the cold cap to fall into the
pump, preventing evaporation into the regions outside the pump. Figure
21
shows
the evolution of cold cap design.
2.4.7.3 Pressure Stability
Pressure instabilities occasionally seen in vapor jet pump systems can originate
in the pump as well as from sources outside the pump. The sources outside the
pump are
1.
Gas bubbles from the elastomer seals
2.
Liquid dripping from the baffle
3.
High foreline pressure, light gases
4.
Throughput overload