VACUUM PUMPS AND SYSTEMS:

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d*

A REVIEW OF CURRENT PRACTICE

I
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STUART GILES
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HELIUM LEAK TESTING, INC. \+ 9 5 37-L

ABSTRACT

A review of the fundamental characteristics of the many types of vacuum pumps

and vacuum pumping systems may provide helpful perspective for the designer and
user. The optimum operating range, relative cost, performance limitations,
maintenance problems, system operating costs and similar subjects are discussed.
Experiences from the thin film deposition, chemical processing, material handling,
food processing and other industries as well as space simulation are used to
support the conclusion and recommendations. Large space simulation systems
have been and are discussed in detail by others at this, and other conferences
and are therefore omitted from this paper.

INTRODUCTION

There is no one "best way" to design and build a space or altitude simulation
vacuum pumping system but, as in any design project, an unbiased approach will
provide optimum, cost effective results. This paper is only a brief summary of
the characteristics of vacuum pumps and systems with some field experience
examples and the full knowledge that it may over simplify the problems.

Vacuum pumps are grouped under DISPLACEMENT PUMPS AND ENTRAINMENT PUMPS to
clarify the operating mode of each type. The former can run continuously as they
compress the pumped gas and discharge it to atmosphere or to a backing pump. The
latter traps and holds the pumped gas within the structure and therefore requires
occasional regeneration or replacement of the gas trapping elements.

DISPLACEMENT PUMPS

OIL SEALED ROTARY TYPES (Fig. 1 & 2)

I Vane and rotary piston sealed vacuum pumps are the most commonly used. Most
t pumps provide nearly constant throughput within their operating range.
I
I

95
Advantages i n c l u d e s i m p l i c i t y , r e l i a b i l i t y , and economy. O i l v a p o r d i s c h a r g e o u t
t h e v e n t , ' p a r t i c u l a r l y a t h i g h p r e s s u r e and o i l v a p o r b a c k s t r e a m i n g can be problems.
S p e c i a l c o n f i g u r a t i o n s o f t h i s t y p e o f pump a r e a v a i l a b l e f o r c o r r o s i v e g a s pumping,
h i g h w a t e r v a p o r l o a d s and o p e r a t i o n a t c o n t i n u o u s l y h i g h i n l e t r e s s u r e s which w i l l
o v e r l o a d t h e common o i l s e a l e d pump d e s i g n e d t o b l a n k o f f i n lo-! P a s c a l r a n g e o r
below.

ROOTS BLOWERS ( F i g . 5 )

Lobe t y p e b l o w e r s ( R o o t s ) a r e u s e f u l as m e c h a n i c a l b o o s t e r s when backed by an

o i l s e a l e d pump. The b l o w e r i s b y p a s s e d u n t i l a p r e s s u r e between 3 and 50 P a s c a l
( d e p e n d i n g on d e s i g n ) i s r e a c h e d a t which t i m e t h e b l o w e r becomes a b o o s t e r .
S i n g l e s t a g e u n i t s h a v e good t h r o u g h p u t t o 100 P a s c a l r a n g e and b l a n k - o f f i n t h e
low 0 . 1 r a n g e .

S t a g i n g can produce lower p r e s s u r e s . L u b r i c a t i o n i s e x t e r n a l t o t h e vacuum

pumping p o r t i o n of t h e pump so t h a t t h e Roots b l o w e r i s n o t a s o u r c e o f b a c k s t r e a m -
ing .
Roots b l o w e r s p r o v i d e t h e i n c r e a s e d pumping s p e e d o f t e n r e q u i r e d i n t h e
1 t o 40 P a s c a l r a n g e . They a r e a r u g g e d , s i m p l e m a c h i n e , r e a s o n a b l y r e s i s t a n t t o
d u s t and m o i s t u r e . B a c k s t r e a m i n g c o n t a m i n a t i o n i s n o t u s u a l l y a problem but c a n b e
i f t h e o i l s e a l e d b a c k i n g pump and t h e Roots b l o w e r a r e a l l o w e d t o r e a c h t h e
m o l e c u l a r f l o w p r e s s u r e r a n g e and t h e blower l o b e s become c o a t e d w i t h o i l from t h e
b a c k i n g pump. T h i s can b e p r e v e n t e d by a u t o m a t i c a l l y p u r g i n g t h e b a c k i n g pump w i t h
a d r y g a s t o m a i n t a i n v i s c o u s f l o w p r e s s u r e l e v e l i n t h e b a c k i n g pump o r by l i m i t -
i n g t h e p r e s s u r e by o t h e r means.

TURBOMOLECULAR PUMPS ( F i g . 6)

T h i s t y p e o f vacuum pump i s a l s o a b o o s t e r pump r e q u i r i n g a b a c k i n g pump.

Modern d e s i g n p e r m i t s t h i s pump t o b e mounted w i t h t h e i n l e t downward which r e d u c e s
t h e chance of p a r t i c u l a t e m a t t e r e n t e r i n g and damaging t h e h i g h s p e e d t u r b i n e
b l a d e s . B a c k s t r e a m i n g p o s s i b i l i t y from t h i s t y p e of pump i s v e r y r e m o t e , contamina-
t i o n o f o i l from t h e b a c k i n g pump, as d i s c u s s e d u n d e r Roots b l o w e r s , can produce
b a c k s t r e a m i n g . Normal m a i n t e n a n c e i n v o l v e s r e l u b r i c a t i o n o f t h e b e a r i n g s a f t e r
5,000 t o 1 0 , 0 0 0 h o u r s o f o p e r a t i o n . Another a d v a n t a g e o f t h e t u r b o pump i s t h a t
o v e r l o a d i n g ( h i g h p r e s s u r e s u c t i o n ) s i m p l y s l o w s t h e pump; Roots pumps o v e r h e a t and
d i f f u s i o n pumps b a c k s t r e a m e x c e s s i v e l y i f o v e r l o a d e d i n t h i s way.

The a v a i l a b l e s i z e s o f t h i s t y p e o f pump a r e l i m i t e d s o t h a t t h e y a r e n o t
s u i t a b l e f o r l a r g e vacuum s y s t e m g a s l o a d s . B a c k s t r e a m i n g of t h e m e c h a n i c a l b a c k i n g
pump o i l c a n be r e d u c e d by a p r o p e r l y i n s t a l l e d f o r e - l i n e t r a p . The problem o f
syphoning o i l from t h e b a c k i n g pump a f t e r s h u t down i n t o t h e t u r b o pump can b e
p r o t e c t e d a g a i n s t by i n s t a l l i n g an a u t o m a t i c u p - t o - a i r v a l v e .

96
 .
. . .
. . ..

..

fcl

ROTARY VANE PUMP

w i t h g a s - b a l l a s t FIG. 1
ROTARY P I S T O N PUMP
FIG. 2

slem -

Diffuser--7

rn
Discharge OMIT,,,"
WALKS

STEAM J E T EJECTOR PUMP LIQUID R I N G PUMP

FIG. 3 FIG. 4

97
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-
----..-.-

A D

C IJ

ROOTS T Y P E BLOWER TURBOMOLECULAR PUMP

FIG. 5 FIG. 6

I t'";" t
To w c u m syslcm

m1..:;. '.:
. :
f. :;. .-,.,TT"A

D I F F U S I O N PUMP MOLECULAR DRAG PUMP

FIG. 7 FIG. 8
DIFFUSION PUMPS (Fig. 7)

Oil diffusion pumps are most commonly used for industrial processing in
the10 to Pascal range. This type of pumping system i s the lowest first cost
for its pressure range, but cryogenic cold trapping must be used for processes
sensitive to backstreamed oil. Diffusion pumps are available in a wide range of
sizes.

Diffusion pumps are boosters and therefore require backing pumps which must
be protected against backstreaming or syphoning the lubrication oil into the
diffusion pump: this is the same problem as discussed under Roots pumps, except
that the fore-line (discharge) pressure must be maintained below its tolerable
level or the diffusion pump backstreaming will be excessive. It is advisable to
use the same oil in the diffusion pump and backing pump if practical.

ENTRAINMENT PUMPS

CRYOGENIC PUMPS (Fig. 9 )

Cryo vacuum pumping is accomplished by a condenser and adsorber at cryogenic

temperatures within or connected to the vacuum system. This type of pump is
available in medium to large capacity sizes for most high vacuum applications.
Because of its lack of contamination backstreaming and high pumping speed, it is
popular for space simulation, manufacturing of thin film electronic devices and
other sensative processes. Regeneration is required and the possibility of power
failure during operation should be considered in application design.

IONIZATION PUMPS (Fig. 11)

The ionization pump uses the Penning gage principle to collect gas within the
pump structure by ionization and gettering. These work best below the lo2 Pascal
range as pump life is relatively short at higher pressures. They find use in
materials research and microminiature circuitry fabrication as well as space
simulation. A primary advantage is that they provide a "dry vacuum environment"
as there is no oil or other contaminant in the system. The roughing system may
include a turbo or cry0 pump and a well trapped two stage mechanical pump or an
adsorption pump. Blank-off in the 10-9 Pascal range 'is practical. The lack of
traps and baffles, low power requiremens and lack of need for LN2 are operating
advantages particularly for long term life tests. A turbo or cry0 pump may be
necessary to pump the non-getterable gases in some' applications.

The ion pump has a finite capacity and is relatively.costly to rebuild when
saturated; it cannot be regenerated as can a cry0 or adsorption pump. Start-up
pressure i s critical and a used pump, if started at too high a pressure, may go
into a glow discharge mode and release previous pumped gasses which can contaminate
the chamber and the product under test or production.

99
 ORIGINAL F'AG.1.3 is
OF POOR QUALITY

////I\\\\\ 8OK CONDCNSlh'L

ARRAY

-RCi.ICT VALVC
A S S C M 0 LY
H Y D R 06 C N -vn P O I;.

COLD HCAD
R c G c H c R A TION
PURGC A D I P T C R

c R Y O ~ U PM
ROUGHI#G
VALVC CRYOGENIC PUMP TITANIUM SUBLIMATION SYSTEM
FIG. 9 FIG. 10

IONIZATION PUMP
PIG. 11

I ! SORPTION PUMP
FIG. 12

100
TITANIUM SUBLIMATION PUMP (Fig. 10)

Sublimation of titanium (or other active metal) onto a surface will getter
some gases providing high pumping speeds. This technique suppliments other
pumping methods. The titanium source is replaceable and the reacted deposits may
be mechanically removed. Combined with an ionization pump and the advantage of
requiring no backing pump make it a completely closed system. A small turbo pump
may be desireable to remove traces of non-getterable gases. No contamination
occurs on power failure and only a small pressure rise takes place in a well-built
system.

SORPTION PUMP (Fig. 12)

Silica gel and other gas sorption materials, may be used to evacuate a
chamber by chilling with liquid nitrogen or other refrigerant when the risk of
backstreaming from a mechanical pump must be avoided. They are only practical for
small systems, when cycle time is not a problem. Regeneration is frequently
required and time consuming.

OTHER TYPES OF VACUUM PUMPS

Centrifugal, diaphragm and reciprocating vacuum pumps are used for some
industrial processes but not applicable to high vacuum work.

Liquid ring pumps are commonly used for dewatering and deaerating chemicals
when solvents may be pumped. (Fig. 4 ) . The required water supply and disposal may
be a problem and the ultimate pressure is limited by the water vapor-pressure.

The steam jet ejector (Fig. 3 ) is one of the oldest devices for producing
vacuum in industrial applications. Staging of steam jets can produce ultimate
pressures in the low 10-3 Pascal range. Their primary application i s for large
degassing operations at 100to 1.0 Pascal. The principal advantage is rugged
simplicity: The most practical vacuum pump for pumping acids and caustics since
they can be fabricated from almost any metal or even ceramics. Costs are very
reasonable if "free" steam is available. If the steam generator, condenser,
cooling water and accessory equipment are considered, costs are comparable to
mechanical booster systems. This type is not safe for some applications as steam
flashback is possible.

The molecular drag pump (Fig. 8 ) uses a rotating helical seal to produce the
pumping action in shear instead of impingement as in the turbo pump. Now available
in a small size, it may be something for the future.

VACUUM PUMPING SYSTEMS

The performance of a vacuum pumping system depends not only on the pumps
employed but also on the piping design, the valving, the gauging and the controls.
The following discussion is designed to introduce the designer and user t o some of
the criteria which should be considered for the various types of vacuum pumping
systems. Each application of a system imposes special considerations and require-
ments.

101
 VACUUM PUMPING SYSTEM CASH FLOW ANALYSIS (Fig. 13 & 14)

The cumulative cash flow analysis curves show the net costs of owning and
operating four types of vacuum systems over a period of 10 years. The capital
investment, installation cost, utilities, maintenance, and operating costs,
depreciation and taxes are included. The required pumps and controls for a typical
1000 liter/second pumping stack without a vacuum chamber, roughing pumps and gaug-
ing were used to estimate the purchase cost. Inflation was assumed at 5% per year,
straight line depreciation over the 10 years to zero value is included, 40% tax
rate and expensed installation cost are also included.
This type of cash flow is the net cost of ownership after taxes and deprecia-
tion. The curves will vary from one installation to another as different
experience, type of operation and accounting methods dictate but shows the trends
for the diffusion, cryo, turbo and ion pump systems.

The ion pump is most favorable because it is only used below the lO-*Pascal
range. This and the fact that they are limited in pumping speed make their appli-
cation limited. The limited size of turbo pumps makes them impractical for large
systems.

The cost difference between the cry0 pump and diffusion pump helps explain the
popularity of the cry0 system in addition to its availability with very high pump-

I
When operating on a continuous basis ( 7 day, 3 shift) maintenance cost per
hour of operation is reduced due to less wear and tear during start-up. Continuous
operation yields 4.35 times the production per year compared to a standard 40 hour
week. Because of the reduced maintenance, net cost will increase only 4 times for
diffusion, 1.5 times for ion, etc. This shows the advantage of operating a pro-
duction system on a continuous basis.

The popularity of the cry0 pump is due to the net cost saving as well as per-
formance advantages despite the higher initial capital cost. The cost savings
shown for the cry0 pump over the diffusion with mechanical refrigeration over the
10 year period on a continuous basis yields a 236% return on the additional $2,205
cry0 system cost. The Internal Rate of Return Method was employed in this calcula-
tion.

DIFFUSION PUMP SYSTEMS

Figure 15 is a 20 port manufacturing system f o r evacuating, purging, baking,

I
backfilling, pinch-off and leak testing glass plasma display devices. These
devices are extremely sensative to contamination of any sort and reflect the effect
of impriorities by poor color and shortened life. The liquid nitrogen cold trapped
diffusion pump used in this system has proven to be very satisfactory by increasing
theexcellaratedlife test of the devices from 500 hours to 1200 hours when compared
to the previously employed vacuum processing system.

Various system features contribute to the cleanliness of this system:

102
 190
CUME CASH FLOW ANALYSIS I
170 - 24 HR/DAY OPERATION
Fig. 14 7-86 DALE SMET
150 -

TIME IN YEARS
50 r
CUME CASH FLOW ANALYSIS
8 HR/DAY OPERATION
45 - Fig. 13 7-86 DALE SMET

40 -

0
0
35 -
0
4

2 3
H 0 -

0 1 2 3 4 5 6 7 8 9 10
TIME IN YEARS

103
 --
I' I/

OPERATOR S I D E WITH OVEN R A I S E D

EVACUATE/BACK-FILLITEST SYST.

S E R V I C E S I D E SHOWING D I F F U S I O N PUMP,
LN COLD TRAP, ROUGHING & BACKING PUMPS

F I G . 15 PROCESSING SYSTEM FOR GLASS

PLASMA D I S P L A Y D E V I C E S
ORIGINAL PAGE IS
104 OF POOR QUALITY
 The devices are repetatively evacuated to 1 x Pascal and back-filled
with an inert gas. The evacuation of this inert gas purges the piping, cold trap
and vacuum pumps maintaining the oils in good condition. Each device is evacuated
through an 8 mm diameter tube and control valve which provides a relatively hiPh
gas velocity during initial evacuation,

The 300 degrees C oven is hydraulically raised and lowered s o that it can be
raised slowly after the heat cycle and not shock the hot devices with cold room
air. The baking process, while evacuating and purging, has been proven in many
processes to be the best way and sometimes the only way to assure complete cleanli-
ness of a device.

The diffusion pump and mechanical pumps in this system use the same type of
oil. This assures that no cross contamination of the pump oils. The user of this
system has banned all silicon oils and greases from his facility as they have been
found to be an impossible to remove contaminant.

A roughing pump and a backing pump are included in this system. Using one
pump for these two functions can cause excessive backstreaming of the diffusion
pump during the period when the foreline valve is closed and the mechanical pump is
used to rough the system.

Figure 16 shows a completely automated sputtering system for high production

of 5" x 5" substrates. The vacuum pump systems, one at each end of the 9 foot long
sputtering chamber, roughs out the entry and exit air locks and maintains the
sputtering chamber argon environment in the low 10-1 Pascal range. Oil sealed
rotary vane type pumps (not shown) and the two diffusion pumps are connected to
mechanically refrigerated cold traps. The diffusion pumps are maintained in the
10-4 Pascal range by throttling vanes in the suction.

Diffusion pumps are not considered good practice for this application and yet
three of these systems produced completely satisfactory titanium thin films for
many years on a 3 shift basis. The diffusion pump oil wasn't changed for the first
six years of operation and even then it showed little degradation!

This example is given to illustrate that a well designed system, properly

operated can give many years of service at low cost. The fact that these pumps
were constantly purged with argon probably explains this outstanding performance
record which contradicts the cost analysis previously presented.

CRYOGENIC PUMPING SYSTEM

Figure 17 illustrates a large portable cry0 pumping system with a rotary vane
pump and Roots blower for roughing. This system w a s designed to evacuate a group
of 34,000 liter chemical reactor chambers to 1 x l@ Pascal in less than one hour
through 30 feet of 10" pipe without hydrocarbon backstreaming. The system is moved
from one reactor to another with a motorized pallet jack and connected to the
reactor 10" suction line with a ring clamped, O-ring sealed flange. Pump exhaust,
nitrogen and compressed air supplies, power, and central control room computer
connections are all made with quick disconnect type fittings. No cooling water is
required.

105
 ORIGINAL PAC% IS
OF POOR QUALITY

FRONT VIEW: OPERATOR LOADS AND UNLOADS SUBSTRATES

REAR VIEW WITH ACCESS PANELS REMOVED

FIG. 16 DIFFUSION PUMPED SPUTTERING SYSTEM

106
 SYSTEM ASSEMBLY

FIG. 17 A CRYO, ROOTS & O I L SEALED

PORTABLE PUMPING SYSTEM

107
 ORIGINAI; PAC33 IS
OF POOR QUALITY
The 3" diameter rough pump and 6" diameter Roots pump connections assure that
viscous flow is maintained during pump down and a nitrogen purge into the Roots
pump suction after blank-off prevents backstreaming. An alumina ball filled trap
is added insurance against backstreaming and protects the mechanical pump against
the remote possibility that abrasive dust may be evacuated from the reactor. The
trap also protects the cry0 pump against possible hydrocarbon contamination during
regeneration evacuation. This type of trap is usually filled with silica gel but
because of its large size, the more shock resistant alumina was used. Silica gel
would shatter, settle, and create dust and small particles which might damage the
pumps and the settling would provide a bypass channel.

This cry0 system is the 2nd generation for the same application; the previous
four smaller systems are used to evacuate 1800 liter reactors. A cry0 pump system
design must protect the cryogenically cooled surfaces against the in-rust of warm
air at too high a cross-over pressure which would warm the cold condensing shrouds.
If this is allowed to happen, the shrouds will release the previously condensed
gases as the refrigeration capacity is small, require a long time to recover pump-
ing speed. The system shown in Figure 17 is able to cross-over at 15 Pasc'al because
it is connected to the large reactor by a conductance limiting suction line (10"
diameter x 30'). If the cry0 system were directly connected to the chamber, the
required cross-over pressure would be much lower.
OIL SEALED ROTARY PUMP SYSTEMS

There are probably more oil sealed pump systems in operation than all other
types combined when we consider the automated material handling, vacuum packaging
and degassing, vacuum hold down, hospital and laboratory, and other commercial and
industrial applications. (Don't forget your dentist).

Figure 18 ROTARY OIL SEAI.ED P

PUMP wITH IIROP--OUT

108
 Figure 18 shows a relatively simple method of protecting a vacuum pump in
many of these types of applications. The vacuum tank acts not only as a surge tank
but also as a drop-out tank for liquids and solid particles which may cause pump
maintenance problems. Machine shops using vacuum chucks induct cutting oils and
chips into the vacuum suction lines, the hospital operating room is another problem,
the electronic circuit board de-soldering and many other applications find this
type of vacuum system to be cost effective.

These types of central system applications impose other problems for the stand-
ard vacuum pumps. The suction pressure in the system must often be limited to
20" Hg for which specially designed oil sealed pumps are available. Critical
applications such as hospitals require redundant pumps and emergency power for re-
liability. Some applications employ a vacuum pressure switch on the tank or auto-
matic air bleed valves to control the tank at the desired system operating pressure.
Time delay of on-off control must be provided to prevent fast cycling the pump.

ROOTS PUMP SYSTEM

Figure 19 and 20 show an application of a rotary piston and Roots blower for
freeze drying (lyophilization) of food products. This is a severe application as
the pumps will be exposed to water vapor, food acid, caustics and oils; the re-
frigerated condensors are not 100% effective. In spite of this, the system shown
has been in operation for 15 years on a one 10 hour run, 5 day per week basis.
Each of the two rotary piston pumps and Roots blowers are overhauled approximately
every 15 months. A spare of each type is kept on hand so that production is not
interrupted during the pump overhaul.

A s this system sublimes up to 125 Kilograms of water per run, the problem of
condensed moisture in the oil sealed pumps is severe. After trying oil filtration
and other methods, it was found cost effective to change the oil after each run.
The rotary piston pump is the best choice for this application as it is most toler-
ant to dust, acid, water vapor and other contaminants.

CONCLUDING REMARKS

This discussion of vacuum pumps and vacuum systems is only a brief introduc-
tion to the subject. The designer and the user must make an in-depth study of the
many types of pumps and systems which are available to determine the most effective
and efficient for the process to be performed. A s indicated by the cash flow
analysis, the lowest first cost system may not be the best investment. It may be
necessary to conduct process tests on a pilot basis to help select the best pumping
system. There is no substitute for field experience to determine process criteria.
A search for experience reports in the same or similar industries will frequently
provide valuable data but do not accept "we have done it this way for 20 years and
therefore, it is the best way". Vacuum technology is moving ahead and better ways
may be available.

109
 ORIGINAL PRGS IS
OF POOR QUP.LITY

F I G . 19 FRONT VIEW SHOWING CHAMBER ACCESS AND ONE

ONE SET OF PUMPS

160 Hp CASCADE R E F R I G . SYSTEM

2 . 7 4 D I A . X 1.22M
_- --VACUUM CIIAMBER

--FOOD CARRIER

COLD P L A T E S
--VACUUM VALVE
/ROOTS PUMP

’,ROTARY PISTON
PUMP 81 L / s

FIG. 20 FOOD F R E E Z E DRYING SYSTEM SCHEMATIC

110
PICTURE CREDITS

The author wishes to thank the following publishers and manufacturers for use
of their drawings and photographs.

Fig. 1, 2, 6 3 Dushman, Saul and Lafferty, J. H., Editor,

Scientific Foundations of Vacuum Technique,
John Wiley & Sons, Inc., New York 2nd Edition, 1962

Fig. 4 Kinney Vacuum Unit of General Signal, Canton, MA,

Liquid Ring Vacuum Pumps, Bulletin 4105

Fig. 5 6 7 Guthrie, Andrew, Vacuum Technology, John Wiley & Sons,

Inc. , New York, 1965

Fig. 6 Leybold-Heraeus, Inc., Export, PA, Product and Vacuum

Technology Reference Book

Fig. 8 Alcatel Vacuum Products, Inc. , Hingham, MA,

New Products Introduction, 1986

Fig. 9 CTI-Cryogenics, Helix Technology Corp., Waltham, MA,

Cry0 Pump Regeneration - Simplified, 1980

Fig. 10 Varian Industrial Components Div., Santa Clara, CAY

Titanium Sublimation Pumps Bulletin, 1982

Fig. 11 & 12 Perkin-Elmer Vacuum Products Div., Ion and Sorption

Pumps Bulletins

Fig. 13 & 14 Smet, Dale, Cash Flow Analysis, Helium Leak Testing, Inc.,
Northridge, CA, 1986
Fig. 1 5 , 17, & 18 Helium Leak Testing, Inc., Northridge, CA, Specially
designed and manufactured systems.

Fig. 16 Vacuum Atmospheres Co., Hawthorne, CAY Specially

designed and manufactured system.

Fig. 19 & 20 Brewster Foods, Inc., Reseda, CAY Specially designed

system for in-house use.