Note: Descriptions are shown in the official language in which they were submitted.
133~9 ~:
This application is a divisional appiication of
Canadian patent application 561,916`filed March 18, 1988
Technical Field
The present invention is directed to an improved
gas laser apparatus, method of lasing gas and a turbine
type compressor therefor. More particularly, the
invention relates to an improved fast axial flow gas
laser apparatus wherein gases are circulated in a
closed loop through the laser tube at speeds
approaching the speed of sound in the laser gas.
Backaround Art
In known fast axial flow gas lasers the gas is
generally moved through the laser by use of a Roots
blower. A Roots blower is characterized by the use of
two rotors which are moved in synchronism in relation
to one another by means of gears and a driving motor.
Typically, the rotors rotate at a speed of 3,600 rpm.
A gas laser with a single stage Roots blower or pump
operating at this speed is disadvantageous for certain
laser applications because the output of the blower has
a 240 Hertz fluctuation in its discharge pressure.
This pulsing or reverberation in the pressure output of
the blower results in a corresponding fluctuation
or instability in the laser discharge which, in turn,
causes instability in the power output of the laser.
Such fluctuations in the power output of the laser are
unacceptable in many applications, e.g., fine
engraving, cutting, welding, etc. If the discharge
flow of the Roots blower is dampened to avoid these
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1331~ ~
pressure fluctuations, there is a reduction in the
useful output power or efficiency of the blower and the
associated gas laser. The vibration also associated
with the Roots blower can be disadvantageously
transmitted to the laser itself to actually vibrate the
output laser beam.
The gears and bearings normally employed in the
typical Roots blower require lubrication. The grease
or oil used for this purpose poses a serious problem
for the operation of both the blower and the associated
laser. That is, if this oil leaks into the pumping
chamber of the Roots blower, it can break down and be
deposited on the lobes of the blower impellers, closing
the small gaps between the lobes and adjacent pump
housing and causing the lobes to seize in the pump
housing. Oil leaked into the laser gas will be
vaporized in the laser tube thereby effecting discharge
stability. Oil vapor deposited on the optical elements
of the laser results in deterioration of the laser
performance and reduces the life of the optics. To
prevent these occurrences, vacuum chambers are provided
in Roots blowers adjacent the pumping chamber. A
higher vacuum is maintained in the vacuum chambers than
in the pumping chamber, so that any oil leaking from
the gears and bearings will be drawn into these higher
vacuum chambers and not the pumping chamber to thereby
maintain the integrity of the blower and laser.
The provision of such protective, higher vacuum
chambers adds to the cost of the blower and
contamination can still occur in the event of failure
of the higher vacuum chamber.
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- ~31~9
The Roots blower is also disadvantageous because
of its considerable size and weight. Further, it
requires frequent servicing of lip seals provided
therein about the rotary shaft of the blower. Every
800 to l,OOo hours of operation a trained technican
must shut down the operation of the blower to service
the lip seals. The down time associated with this type
of service as well as the labor costs of the trained
technician increase the cost of the related manufac-
turing operation.
Disclosure of Invention
An object of the present invention is to providean improved gas laser apparatus, method of lasing gas
and a turbine type compressor therefor which avoid the
aforementioned disadvantages of known gas laser
apparatus and methods employing the conventional Roots
blower. More particularly, an object of the present
invention is to provide a gas laser apparatus which has
a continuous, stable discharge and power output, so
that fine engraving, cutting, welding, etc., can
be performed.
A further ob;ect of the invention is to provide an
improved gas laser apparatus, method of lasing gas and
a turbine type compressor therefor wherein the problems
of lubricant contamination of the laser gas in the
compressor are eliminated without the provision of
special higher vacuum chambers adjacent the compressor.
An additional object of the invention is to
provide an improved gas laser apparatus, method of
lasing gas and a turbine type compressor therefor,
wherein the size and weight of the compressor are
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reduced as compared with the conventional Roots blower
and wherein the compressor has a long life and does not
require the frequent servicing of lip seals required by a
Roots blower.
These and other objects of the invention are
attained by the gas laser apparatus of the invention
which comprises means defining a flow path for a laser
gas, means for exciting gas flowing in the apparatus to
cause the gas to lase, and a compressor for flowing gas
along the flow path, wherein the compressor is a
regenerative compressor having a head coefficient of at
least 0.8 and is capable of operating with a pressure
ratio sufficient to flow the base along at least a
portion of the flow path at a speed of at least half the -
speed of sound in the laser gas with an inlet pressure to
the compressor of less than one-third atmospheric
pressure. The means defining the flow path preferably
forms an at least essentially closed loop for
recirculating gas through the laser apparatus. The
closed loop includes the compressor.
The specific pressure ratio pr of the
compressor depends on the gas or mixture of gases used
with the laser apparatus and the mixing ratio of the
components of the gas mixture. With a gas mixture of
helium, nitrogen and carbon dioxide according to a
disclosed embodiment, the compressor is capable of
operating with a pressure ratio of at least 1.5:1 with an
inlet pressure of between 50 and 100 torr, for example,
and a mass flow through the compressor on the order of
several hundred cubic feet per minute or more. ~-
In the disclosed, preferred embodiment of the
invention, the turbine type compressor flow flowing gas
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through the laser tube is a regenerative compressor
which comprises an impeller rotatable about an axis and
having generally radial blades extending from at or
near the tip of the impeller inward a distance of no
more than about 50% of the radius of the impeller.
Axially opposite the blades of the impeller is a
means defining a stationary annular passage with an
inlet and an outlet being provided for communicating
the gas to and from the passage for peripheral flow in
the passage in the direction of rotation of the
impeller. A dam is provided blocking the annular
passage between the inlet and outlet. The dam has
close clearance over the impeller. In the disclosed
embodiment, the regenerative compressor is a two stage
compressor with the respective stages being located
on opposite sides of a single impeller. An intercooler
is provided between the first and second stages of the
compressor for cooling gas compressed in the first
stage before it enters the second stage for further
compression. The intercooler includes a heat exchanger
formed with a plurality of concentric tubes where the
passages between adjacent tubes respectively convey the
gas and a coolant fluid for heat exchange to cool the
gas as it passes through the heat exchanger. Both
sides of the compressor houslng and also the radlally
outer or circumferential surface thereof are also
cooled by circulating coolant through passages provided
therein so that the gas is cooled as it is being
compressed. This is particularly important for
effective cooling and efficient compressor operation
especially with the very low pressures at which the
compressor operates. Approximately 50% of the required
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33~0~
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gas cooling to remove the heat of compression is taken
out by the cooling incorporated in the compressor
housing. This gives the effect of interstage cooling
- which results in increased compression efficiency.
The impeller of the regenerative compressor is
rotatably supported on a drive shaft of the compressor
at a first location along the shaft. The impeller is
rotated about the longitudinal axis of the drive shaft
at a high speed such that the circumferential speed of
the impeller is a substantial fraction of or near the
sonic speed for the laser gas. In this way the
acceleration of the gas in the compressor approaches
the speed of sound in the gas in the compressor at
the tip of the impeller blades thereby minimizing
friction losses. In the disclosed embodiment, the
speed of rotation is about 10,000 rpm.
Lubricated bearing means rotatably support the
drive shaft at at least a second location along the
shaft spaced from the first location. A positive
pressure fluid seal means is provided for preventing
lubricant from the bearing means from moving along the
drive shaft to the impeller and contaminating the laser
gas. The fluid seal means includes a mating ring
~ sealingly attached to the drive shaft between the
first and second locations for rotation with the shaft,
a pair of annular, spaced stationary members located
about the shaft and presenting respective sliding faces
for contacting respective opposite sides of the mating
ring, and means for directing a fluid or buffer qas
under a pressure slightly above the pressure of the gas
in the compressor, between the sliding faces and the
mating ring and along the drive shaft during operation
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~ 1331~9 -
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of the compressor to prevent lubricant from the bearing
means from moving along the drive shaft to the
impeller. In the preferred embodiment of the
invention, the buffer gas, which i8 non-contaminating
S with respect to the laser gas, is permitted to migrate
into the laser gas to prevent leakage of atmospheric
gas into the laser gas and to serve as a make-up gas
for losses of the laser gas. Preferably, the buffer
gas is the same kind of gas used in the laser.
The drive shaft and the impeller in the disclosed
embodiment of the compressor are rotated at a speed of
about 10,000 rpm and because in each stage of the com-
pressor the impeller carries characteristically 30
blades thereon, the pressure of the compressor dis-
lS charge is continuous and stable. Therefore, a more
continuous, stable laser discharge and laser power
output can be produced. The size and weight of the
regenèrative compressor of the invention are also less
than those o~ tha typical ~oot~ blowar whlcll pormlt n
reduction in the size and weight of the gas laser
apparatus. The regenerative compressor also has a
relatively long life and needs only infrequent
servicing. Gas, magnetic, ball or roller bearings can
be used in the compressor without fear of contamination
of the laser because of the special sealing arrangement
of the invention. Further, the requirements for
two rotors and gears as in Roots blowers are avoided
with the regenerative compressor of the invention.
!: ' The method of lasing gas in a fast axial flow gas
laser according to the invention comprises the steps of
compressing a gas in a regenera~ive compressor
operating with a head coefficient of at least 0.8 and
.
1~9
with a pressure ratio sufficient to flow the gas along at
least a portion of a flow path for the gas in the laser at a
S speed of at least half the speed of sound in the gas,
conveying gas compressed by the compressor along the flow
path for said gas in the laser and exciting said gas to
cause it to lase. The gas is recirculated through the laser
in a closed loop. The compressor forms part of the closed
loop flow path for the gas. Preferably the gas is moved at
speeds which approach or even exceed the speed of sound
along at least a portion of the flow path in the laser. The
method further includes the steps of cooling the gas both
before, during and after compression in the regenerative
compressor as it moves through the closed loop of the
apparatus, and positively sealing the lubricant in the
compressor against movement into the laser gas by means of a
pressurized fluid seal. The pressurized fluid of the seal
is permitted to move into the gas being compressed to make
up for lost laser gas.
In accordance with an embodiment of the invention,
a turbine type compressor is comprised of an impeller
rotatable about an axis and having a plurality of blades
thereon for compressing gas, apparatus defining a passage
opposite the blades of the impeller with an inlet and an
outlet being provided in the passage for communicating gas
to be compressed to and from the passage, apparatus for
rotating the impeller and the blades thereon to compress the
gas, and wherein the compressor is capable of operating with
a pressure ratio sufficient to flow gas at a speed of at
least half the speed of sound in the gas in a fast axial
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- 8a -
flow laser with inlet pressures to the compressor of less
than one-third atmospheric pressure.
S In accordance with an embodiment of the
invention, a two stage, double sided, regenerative
turbine type compressor is comprised of an impeller
rotatable about an axis and having a plurality of blades
thereon for forming first and second stages of the
compressor on respective sides of the impeller for
compressing gas, apparatus defining a passage opposite
the blades of the impeller with an inlet and an outlet
being provided in the passage for communicating gas to be
compressed to and from the passage, apparatus for
rotating the impeller and the blades thereon to compress
the gas, wherein the compressor has a pressure or head
coefficient of at least 0.8 and is capable of operating
with a pressure ratio for flowing gas at a speed of at
least half the speed of sound in the gas in a fast axial
flow laser with inlet pressures to the compressor of less
than one-third atmospheric pressure, wherein an
intercooler is provided between the first and second
stages of the compressor for cooling gas compressed in
the first stage before it enters the second stage for
further compression, and wherein the apparatus defining a
passage opposite the blades includes apparatus for
cooling essentially the entire surface of both sides of
the compressor and also the radially outer or
circumferential surface thereof.
In accordance with another embodiment, a
regenerative turbine type compressor is comprised of an
impeller rotatable about an axis and having a plurality
of blades thereon for compressing gas, apparatus defining
a passage opposite the blades of the impeller with an
inlet and an outlet be:ing provided in the passage for
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- 8b -
communicating gas to be compressed to and from the
S passage, apparatus for rotating the impeller and the
blades thereon to compress the gas, wherein the
compressor has a pressure or head coefficient of at least
0.8 and is capable of operating with a pressure ratio for
flowing gas at the speed of at least half the speed of
lo sound in the gas in a fast axial flow laser with inlet
pressures to the compressor of less than one-third
atmospheric pressure, wherein the blades of the impeller
are generally radial blades extending from at or near the
tip of the impeller inward a maximum distance of about
50% of the radius of the impeller, wherein the passage is
a stationary annular passage for peripheral flow of gas
in the direction of rotation of the impeller, wherein the
apparatus for rotatably driving the impeller comprises a
drive shaft upon which the impeller is mounted for
rotation at a first location bearing apparatus for
rotatably supporting the drive shaft at a second location
on the drive shaft, and a positive pressure fluid seal
apparatus located intermediate the first and second
locations for preventing contamination from the bearing
apparatus from moving along the drive shaft in the
direction of the impeller, and wherein the fluid seal
apparatus includes a mating ring sealingly attached to
~ the drive shaft at a location between the first and
; second locations for rotation with the shaft, a pair of
spaced stationary members presenting respective sliding
faces adjacent respective opposite sides of the mating
ring, and apparatus for directing a fluid under a
pressure above the pressure of gas in the compressor
between the sliding faces and the mating ring and along
the shaft during operation of the compressor to prevent
contamination from the bearing apparatus from moving
~long the drive shaft to the impeller.
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In accordance with another embodiment, a
regenerative turbine type compressor is comprised of an
impeller rotatable about an axis and having a plurality
of blades thereon for compressing gas, apparatus defining
a passage opposite the blades of the impeller with an
inlet and outlet being provided in the passage for
communicating gas to be compressed to and from the
passage, apparatus for rotating the impeller and the
blades thereon to compress the gas, and wherein the
compressor has a pressure or head coefficient of at least
0.8 and is capable of operating with a pressure ratio for
flowing gas at a speed of at least half the speed of
IS sound in the gas in a fast axial flow laser with inlet
pressures to the compressor of less than one-third
atmospheric pressure, wherein the blades of the impeller
of the regenerative compressor are generally radial
blades extending from at or near the tip of the impeller
inward a maximum distance of about 50% of the radius of
the impeller, wherein the passage is a stationary annular
passage for peripheral flow of gas in the direction of
rotation of the impeller, wherein the apparatus for
rotatably driving the impeller comprises a drive shaft
upon which the impeller is mounted for rotation at a
first location, bearing apparatus for rotatably
supporting the drive shaft at a second location on the
drive shaft, and a positive pressure fluid seal apparatus
located intermediate the first and second locations for
preventing contamination from the bearing apparatus from
moving along the drive shaft in the direction of the
impeller, and wherein the fluid seal apparatus is located
adjacent an impeller housing of the impeller and the
positive pressure fluid of the seal is a gas which is
non-contaminating with respect to the laser gas and which
iB permitted to migrate }nto the passage as a make-up as
for the gas to be compressed and to thereby prevent the
ambient atmosphere from leading into the gas.
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- 8d -
These and other objects, features and
advantages of the invention will become more apparent
S from the following description when taken in connection
with the accompanying drawings which show, for purposes
of illustration only, several embodiments according to -
the invention. ..
Brief Description of the Drawings ~ -~
Fig. 1 is a schematic illustration of a gas
laser apparatus according to the invention
Fig. 2 is a schematic, cross-sectional view
taken along the line II-II of a single sided regenerative
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- 9 -
133~9
compressor with circular flow channel as shown in
Fig. 3:
Fig. 3 is a side elevational view, partially in
cross-section, of the right side of the compressor of
5 Fig. 2;
Fig. 4 is a cross-sectional view of a portion of
the regenerative compressor of Figs. 2 and 3 taken
along the line IV-IV and Fig. 2;
Fig. 5 is a schematic view, partially in
cross-section, of a double-sided regenerative
compressor with rectangular flow channel;
Fig. 6 is a, partially in cross-section, side view
of a two stage, double sided regenerative compressor
according to a preferred embodiment of the invention;
Fig. 7 is an end view of the compressor of Fig. 6
taken from the right side of Fig. 6~
Fig. 8 is a cross-sectional view of the first
stage inlet of the compressor taken along the line
VIII-VIII in Fig. 7;
Fig. 9 is an end view of the compressor taken from
the left side of the compressor shown in Fig. 6;
Fig. lO is a cross-sectional view of the second
stage inlet taken along the line X-X in Fig. 9;
Fig. 11 is a cross-sectional view taken along line
XI-XI in Fig. 6 and showing an end view of the heat
exchanser or intercooler thereof;
Fig. 12 is a cross-sectional view taken along line
XII-XII in Fig. 6 and showing the bearing housing with
coolant water distribution holes therein;
Fig. 13 is an æide view of the outer side of the
first stage volute of the compressor of Fig. 6;
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Fig. 14 is a cross-sectional view of the gas inlet
taken along the line XIV-XIV in Fig. 13:
Fig. 15 is a cross-sectional view taken along the
line XV-XV in Fig. 13 in showing the gas outlet:
5Fig. 16 is a cross-sectional view of the first
stage of Fig. 13 taken along the line XVI-XVI;
Fig. 17 is an side view of the inner side of the
first stage volute;
Fig. 18 is a cross-sectional view of a portion of
10the flow passage in the first stage volute taken along
the line XVIII-XVIII in Fig. 17;
Fig. 19 is a cross-sectional view taken along the
line XIX-XIX in Fig. 17;
Fig. 20 is a cross-sectional view of the flow
15passage taken along the line XX-XX in Fig. 17;
Fig. 21 is a side view of the outer side of the
second stage volute of the compressor of Fig. 6;
Fig. 22 is a cross-sectional view of the second
stage volute in Fig. 21 taken along the line XXII-XXII:
20Fig. 23 is an side view of the outer side of the
-~ first stage water manifold of the compressor of Fig. 6;
Fig. 24 is a cross-sectional view of the water
manifold of Fig. 23 taken along the line XXIV-XXIV~
Fig. 25 is a cross-sectional view of the gas
25outlet in the water manifold of Fig. 23 taken along the
line XXV-XXV;
Fig. 26 is a cross-sectional view of the opening
for gas inlet in the water manifold of Fig. 23 taken
along the line XXVI-XXVI;
30Fig. 27 is a cross-sectional view of the gas inlet
taken along the line XXVII-XXVII in Fig. 23:
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-- 11 --
Fig. 28 is an inner side view of the water
manifold first stage of Fig. 23 and illustrating the
water passage relief therein; ~;
Fig. 29 is a side view of the outer side of the
second stage water manifold: -
Fig. 30 is a cross-sectional view of the water
manifold of Fig. 29 taken along the line XXX-XXX; ;
Fig. 31 is a side view of the inner side of the -
water manifold of Fig. 29 and illustrating the water
passage relief therein;
Fig. 32 is a side view, partially in cross-section ~
of the shaft of the compressor of Fig. 6; --
Fig. 33 is a right end view of the shaft of
Fig. 32 showing the polygonal configuration thereof;
Fig. 34 is a cross-sectional view along the
longitudinal axis of the bearing housing of the -
compressor of Fig. 6;
Fig. 35 is a right end view of the bearing housing
of Fig. 34;
Fig. 36 is a left end view of the bearing housing -;
of Fig. 34;
Fig. 37 is a side view of the impeller of the
compressor of Fig. 6;
Fig. 38 is a cross-sectional view through a
portion of the impeller of Fig. 37 taken along the line
38-38; ~ ;
; Fig. 39 is a cross-sectional view of the lmpeller
of Fig. 37 taken along the line XXXIX-XXXlX;
, ~ - Fig. 40 is a view of the configuration of the --
shaft receiving opening of the impeller shown in
Fig. 39 taken in the direction of arrow A; and --
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- 12 -
Fig. 41 is a side view, partially in cross
section, of a portion of the pressure fluid seal in the
compressor of Fig. 6.
Best Mode for Carrvina Out The Invention
Referring now to the drawings, a gas laser
apparatus 1 according to the invention is schematically
illustrated in Fig. 1. The apparatus comprises a laser
tube or other structure 2 defining a flow path for the
fast axial flow of a laser gas, at least two electrodes
3 and 4 arranged for electrically exciting gas flowing
in the apparatus to cause the gas to lase in the laser
tube, and a turbine type compressor 5 for flowing gas
through the laser tube. The compressor has a pressure
or head coefficient of at least 0.8 and is capable of
operating with a pressure ratio sufficient to flow the
gas along at least a portion of the flow path in the
laser at a speed of at least half the speed of sound in
the laser gas. The laser gas can be a gas mixture of
approximately 80% helium and approximately 20% nitrogen
(small amounts of carbon dioxide gas included). The
laser tube 2 and compressor 5 form part of an essen~
~- tially closed loop 6 for recirculating gas through the
gas laser and the compressor. Heat exchangers 7 and 8
are also provided in the essentially closed loop 6 on
respective sides of the turbine compressor for cooling
t`"', ;~ the circulating gas flowing to and from the turbine
compressor 5. A vacuum pump 9 is placed in fluid
communication with the closed loop 6 for maintaining
the necessary low pressure, for example a pressure
within the range of 50 to 200 torr, required for the
operation of the gas laser. A gas mixture tank 10 is
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- 13 -
also provided in selective communication with the
closed loop 6 for supplying and replacing the gas in
the loop to be lased. The replacing of laser gas in
the loop is also achieved by the positive fluid
pressure seal in the compressor during operation of the
compressor as discussed below.
The pressure or head coefficient ~ of the compres-
sor is at least 0.8 per stage and is defined by the
expression:
~ = g x head
.
u2 . ~
:
where g = 32.2 (ft. per sec.2)
U = Impeller Tip Speed of the Compressor
(Ft. perSec.)
r--1
Head = t ~ r ~ :
Feet
~ .
r = Ratio Spec. Heats
R = Gas constant (Ft/F)
Tl = Inlet Temp. (R) -
Pl = Inlet Press. (psia)
P2 = Outlet Press. (psia) ~- -
with the compressor of the invention the pressure of
the gas can be built up higher while operating the
compressor at a lower speed as compared with a single
, stage centrifugal compressor, for example, where the
head coefficients are only about 0.5-0.7. With lower
operating speeds of the compressor of the invention,
the centrifugal forces and therefore the stresses are
-- 14 -
much less than at higher speeds, which leads to
longer life and less maintenance as compared with a
single stage centrifugal compressor. According to the
preferred embodiment of the invention, the compressor
is a two stage regenerative compressor where each stage
has a head coefficient of about 3Ø
As noted above, the gas laser apparatus 1 is a
fast axial flow laser wherein the gas is moved through
the laser tube 2 or at least through a portion thereof
at speeds at least one half the speed of sound in the
laser gas and preferably at speeds approaching the
speed of sound. The velocity of sound vs for a gas is
vs = nRT where n is the isentropic exponent of the gas
mixture, R is the individual gas constant and T is the
lS absolute temperature. For the above mixture of helium,
nitrogen and carbon dioxide vs equals about 560 meters
per second, and for pure helium vs is 1200 meters per
second. The high speeds of the gas in the laser tube 2
can be achieved by passing the gas from the compressor
2~ 5 through a nozzle immediately upstream of the laser
tube 2 which serves to accelerate the qas. As an
example, a laser tube having a diameter of 18 mm can be
supplied with gas from the turbine compressor through a
conduit having a diameter of 19 mm with a nozzle having
a 9 mm diameter opening therein placed in the flow path
immediately upstream of the laser tube 2 to substan-
tially increase the speed of the gas flow to at least
one half of the sonic speed and preferably to a speed
-~ approaching or even exceeding the sonic speed for the
gas. The nozzle increases the pressure drop of the
moving gas in the closed loop 6 beyond that caused by
~ the laser tube, so that the pressure ratio of the
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- 15 -
outlet pressure of the compressor to the inlet pressure
of the compressor is, for example, 1.7:1 with an
inlet pressure of 90 torr to the compressor and a mass
flow through the compressor of about 300 cu. ft. per
minute using a gas mixture of approximately 80% helium
and approximately 20~ nitrogen with a small amount of
carbon dioxide. More generally, the pressure ratio pr
of the compressor depends on the mixture of gases used
with the laser and the mixing ratio of the components
of the gas mixture. The pressure ratio pr is expressed
by pr= (2/n+l)) exp (n/n-l) where n is the isentropic
exponent of the gas mixture. The pressure ratio
influences the mass flow density (kg per s and per m2)
which is the limiting factor. Flow rates of 300 cfm or
more are achieved in the turbine compressor of the
invention while the size of the compressor is small to
fit within the cabinet of a commercial gas laser.
The expression "laser tube" used herein is not
intended to be limited to a straight tube as shown
2~ herein but encompasses other geometries as well such as
a tapered tube, for example. The type of excitation of
the laser gas can also vary. For example, direct
current electrical excitation, radio frequency
excitation, and/or other forms of pumping such as
; 25 chemical and thermal pumping could be used.
The turbine type compressor 5 according to a
preferred embodiment of the invention which has the
aforementioned operating characteristics of head
, coefficient, pressure ratio, inlet pressure and mass
flow is preferably a regenerative compressor as noted
above. It has been found that the use of a
regenerative compressor in the gas laser apparatus
- 16 -
'
enables the compressor to have fewer stages of
compression and to be operated at lower speeds to
achieve the desired pressure ratio, as compared with a
centrifugal compressor, for example, and therefore,
provides a less complex machine having higher
reliability, lower initial cost and a longer operating
life. The compressor can also be relatively compact
for use in commercial lasers as noted above. As
shown in Figures 2-4, the regenerative compressor 11
comprises an impeller 12 rotatable about an axis A-A
and having generally radial blades 13 extending from at
or near the tip of the impeller inward a maximum
distance oE about 50% of the radius of the impeller.
Axially opposite the blades 13 of the impeller is a
volute 14 of the impeller housing which defines a
stationary annular passage 15 with an inlet 16 and an
outlet 17 being provided for communicating the gas to
be compressed to the passage 15 and the gas compressed
therein from the passage. A peripheral flow of
2Q the gas occurs in the passage in a direction of
rotation of the impeller, arrow B in Fig. 3. A dam or
stripper 18 is provided to block the annular passage 15
between the inlet and outlet. The dam has a close
clearance over the impeller 12. The regenerative
compressor illustrated in Figs. 2-4 is a single-sided
compressor with circular flow channel. A two stage
regenerative compressor is illustrated in Fig. 5
wherein it is seen that the first and second stages of
the compressor are located on respective sides of the
single impeller 19. Gas enters the annular passage
through the inlet 20 for compression by the blades on
one siàe of the impeller 19 and then passes through a
~ 3 ~ ~ O Ll ~
-- 17 --
rectangular flow channel 22 to the second stage where
it is compressed by the blades on the second side of
the impeller 19 after which it is moved through the
outlet 21.
According to the preferred form of the invention,
the turbine type compressor is a two stage,
double-sided regenerative compressor as shown in detail
in Figs. 6-41. The compressor 23 has an impeller 24
with basically radial blades 25 extending from at or
near the tip of the impeller inwardly a distance of no
more than about 50% of the radius of the impeller, see
particularly Figs. 37-40. More specifically, each side
of the impeller is provided with characteristically 30
blades which are spaced and located mid-way between
blades on the opposite side. The blades are inclined
forwardly at 40 + 5 for necessary compression of
the low density laser gas. The desired degree of
inclination of the blades depends on the qas mixture
and the sonic velocity thereof and, therefore, it could
vary. The impeller is formed of cast aluminum alloy
A356.0 temper T6, or other suitable material. A
central bore 26 therein is machined to have an opening
in the form of a polygon as shown in Fig. 40. The
impeller is mounted for rotation upon a correspondingly
shaped end 27 of a drive shaft 28 as shown in Figs. 32
and 33. The impeller is retained on the end 27 of
the drive shaft 28 by a ring 29 and fastener 30
depicted in Fig. 6. The rounded lobes of the polygon
shaped end 27 of the drive shaft 28 and the
complementarily shaped bore in the impeller 24
distribute the load or stresses around the shaft
and eliminate the need for a key or spline.
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~.331~9 ~ :
The drive shaft 28 is rotatably supported within a
bearing housing 31, see Figs. 34-36, by means of high
speed bearings 32 and 33. The drive shaft 28 is
rotated at high speed, about 10,000 rpm, specifically
9,908 rpm., by a motor 34 through a driving connection
comprising toothed pulleys 35 and 36 on the output
shaft of the motor and free end of the drive shaft 28,
respectively, and a high torque drive belt 37 extending
about the pulleys. Direct drive could also be used.
The desired rotational speed can be calculated for a
given impeller diameter and sonic speed for the laser
gas knowing that the circumferential speed of the
impeller should be a substantial fraction of or near
the sonic speed to avoid friction losses. The diameter
of the impeller is approximately 13 inches in the
disclosed embodiment. A nut 38 is provided on the
drive shaft 28 to preload the shaft with bearings and
balance the system, so that its critical speed is
higher than the operating speed by a factor of 1.4
or more. Grease fittings 39 and 40 are formed in the
bearing housing 31 for greasing the bearings 31 and 32,
respectively. Alternatively, instead of lubricated
bearings, it is possible to use other types of bearings
such as gas bearings. A water inlet 41, water outlet
42 and interconnecting water passages 43 are provided
in the bearing housing for cooling the housing,
bearings and shaft.
The impeller 24 is rotated at high speed within a
stationary, water cooled impeller housing 44.
Stationary first and second stage volutes 45 and 46 are
sealingly connected to the impeller housing 44 by means
of fasteners 47 as shown in Fig. 6. Water manifolds 47
,~ ,.
~ L 3 ~ 9 ~
-- 19 --
and 48 are provided on the outer end surfaces of the
- first and second stage volutes, respectively for
cooling the compressor. Cooling water is circulated
from an inlet 49 in the second stage water manifold 48,
see Fig. 9, through an annular channel Sl in the
surface of the manifold 48 adjacent the second stage
volute 46. A plurality of webs 52 spaced radially
about the manifold 48 in the passage 51 as shown in
Figs. 30 and 31 permit the water to pass therethrough
while maintaining relative spacing between the manifold
and the adjacent volute. A continuous web 53 to one
side of the water inlet 49 forces the cooling water to
pass completely around the annular passage 51 to an
outlet or passage for conveying the water through the
adjacent volute, impeller housing 44, and first stage
volute 45 to the annular ,cooling passage 54 in the
first stage water manifold 47. The incoming cooling
water is similarly circulated preferably through the
annular passage 54 adjacent the first stage volute for
cooling the same and then exits through an outlet 55.
The compressor operates at very low pressures at
discussed above. This makes it more difficult to cool
the compressor because of the low density of gas
passing through the compressor. By cooling essentially
the entire surface of both sides of the compressor and
also the radially outer or circumferential surface
thereof in the manner described, satisfactory cooling
is obtained. Approximately 50% of the reguired gas
cooling to remove the heat of compression is taken out
by the cooling incorporated in the compressor
housings. This gives the effect of interstage cooling
which results in increased compression efficiency.
~ ~ 3 ~
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Additional interstage cooling is accomplished in an
intercooler 61 as discussed below.
Each of the first and second stage volutes 45 and
46 is formed with an annular passage, identified as 56
S and 57, respectively, see Figs. 13-22. The passage 56
of the first stage volute 45 has an inlet 58 for the
laser gas to be compressed. A dam 59 blocks the
annular passage between the inlet and outlet 60 from
the passage 56. The dam has a close clearance over the
impeller so as to require the gases from the inlet to
flow through the full length of the passage 56 to the
outlet. The gas passing through the annular passage
56 from the inlet to the outlet is subjected to
compression during its interaction with the blades of
the adjacent, rotating impeller. The compressor can be
considered, in effect, a multi-stage centrifugal
because the flow in the channel enters the blading
passages of the impeller at the inner diameter thereof
and due to centrifugal action, flows out at the tip at
a higher tangential velocity. It then imparts momentum
to the remaining gas in the stationary channel which is
traveling at an average velocity of less than that of
the impeller. The gas then flows radially inward and
re-enters the impeller at the inner diameter. The
pressure build-up around the periphery between the
inlet and the outlet is sustained by the momentum
exchange.
Compressed gas leaving the outlet 60 in the first
stage volute 45 is conveyed through the intercooler 61,
refer to Figs. 6 and 11. The intercooler includes a
heat exchanger formed with a plurality of concentric
tubes 62, 63, 64 and 65. Cooling water is circulated
.-~,.
~331~
- 21 -
within the tube 62 and between the tubes 63 and 64,
while the compressed laser gas is passed between the
tubes 62 and 63 and also between 64 and 65 for heat
exchange with the coolant. Water is circulated
through the intercooler for cooling by means of inlet
66 and outlet 67.
Laser gas cooled in the intercooler 61 is conveyed
to the inlet 68 of the second stage of the compressor
where it moves through the annular passage 57 therein,
is compressed and then passes through the second stage
outlet 69. Compressed gas from the compressor 23 is
then circulated through heat exchanger 8 where it is
cooled before moving in the closed loop 6 through the
gas laser 2. The gas exiting the laser 2 passes
through heat exchanger 7 before returning to the two
stagè, double-sided regenerative compressor 23.
As an example of the operating parameters or
capabilities of the compressor, a He-N2-C02 laser gas
received from the heat exchanger 7 and operating as gas
in laser 2 is cooled to 90F as it enters the inlet of
the first stage of the compressor 23 at a pressure of
1.764 psi actual (approximately 91 torr). The gas is
flowing at a rate of 302 actual cubic feet per minute.
After being compressed in the first stage of the
compressor, the gas leaving the outlet 60 is at a
pressure of 2.562 psi actual, a temperature of 272F,
and flowing at a rate of 276 actual cubic feet per
minute. As a result of cooling in the intercooler 61,
the gas entering the inlet of the second stage of the
compressor is at a temperature of 185F, a pressure of
2.434 psi actual, and flows at the rate of 256 actual
cubic feet per minute. The second stage of the
13~1~49
compressor operates to further increase the pressure of
the laser gas to 3.355 psi actual at the outlet of the
second stage, while the temperature thereof is 332F
and the flow rate 228 actual cubic feet per minute.
The compressed gas is then passed through the heat
exchanger 8 where its temperature is dropped from 332F
to approximately 90F for passage through the gas laser
2. The pressure drop through the gas laser is
substantial, on the order of 52 to 60 torr, because of
the desired mass flow density of the gas and the high
speed of the gas in the laser. As noted above, the
speed of the gases is stepped up by way of a nozzle to
at least one half the speed of sound, and preferably to
a speed substantially greater than this and approaching
the speed of sound (560 meters per second) in the
laser gas. Of course, other operating parameters are
possible with the compressor and laser of the invention
as will be apoparent to the skilled artisan.
The pressure output of the turbine compressor of
the invention has a much higher frequency pulse and
lower amplitude than that of the conventional Roots
blower. Laser discharge stability and laser power
output with the gas laser apparatus of the invention
are therefore substantially more uniform or stable as
compared with the conventional gas laser apparatus
employing a Roots blower. The size and weight of
the turbine compressor of the invention are also less
as compared with the roots blower and the magnitude of
vibration is reduced.
To avoid contamination of the laser gas with any
lubricants for the bearings 32 and 33, according to the
invention a fluid seal 70 is provided for preventing
~ ~ 3 ~ 9
- 23 -
lubricant, from the bearings from moving along the
drive shaft in the direction of the impeller. The
fluid seal 70 includes a tungsten carbide mating ring
71 sealingly attached to the drive shaft 28 at a
location between the bearing 23 and the impeller 24 for
rotation with the shaft. A pair of annular, spaced,
stationary carbon members 72 and 73 with low friction
sliding faces are positioned on respective sides of
the mating ring and are yieldably biased against the
mating ring by means of springs 74 and 75. The mating
ring 71 has a central bore with a diameter of less than
.OOl inch clearance over the drive shaft 28, so that it
can be pushed over the shaft. An O-ring 76 located in
an annular groove in the shaft beneath the mating ring
71 acts to resist relative rotation between the mating
ring and the shaft. The faces of the ring 71 adjacent
the members 72 and 73 each have a spiral groove formed
therein which extend from a location radially outward
of the opposed forces of members 72 and 73 to a
location between opposed contacting faces of the ring
71 and members 72 and 73. During rotation of the drive
shaft 28 and ring 71 thereon the spiral grooves in the
faces of the ring 71 pump gas from within the seal
housing 77 between the ring 71 and members 72 and 73 to
cause the members 72 and 73 to move away from the ring
~ 71 by a small distance, typically 50 x 1O-6 inch. The
I members 72 and 73 move away from the ring 71 against
the bias of springs 74 and 75. The stationary
ring-shaped members 72 and 73 are sealingly supported
within the housing 77 of the fluid seal 70 as shown in
Fig. 1. O-rings 78 and 79 assist in sealing. A buffer
gas, which may be l:he same gas as used in the laser, i-
,
. ~
~ ~ 3 3 ~
- 24 -
supplied to the fluid seal 70 through an inlet 80. The
buffer gas is at a pressure slightly higher, several
psi higher, for example, than the gas within the
compressor, closed loop 6 and laser tube 2. The flow
rate of the buffer gas is low, for example, one cubic
foot per minute or less. This small amount of buffer
gas is allowed to bleed into the housing 77 in the
area of the mating ring and adjacent sliding faces of
the members 72 and 73 and is forced into gap 81 between
the impeller and the second stage volute of the
compressor and the gap 82 between the housing 77 of the
fluid seal 70 and the drive shaft 28 during rotation of
the shaft and ring 71 by the aforementioned viscous
shear type pumping action of the grooves in the ring
moving the members 72 and 73 away from the ring. The
flow of buffer gas into the laser through gap 81
reduces the laser make-up gas which is necessary and
prevents inflow of atmospheric gas through the gap 81
which would contaminate the laser gas. The positive
gas pressure in gap 82 also prevent flow of lubricant
from the bearing along the shaft in the direction of
the impeller. Higher gas flow rates could be used to
provide more make-up gas to the laser, if desired. No
~ lip seals are needed around the shaft. The buffer gas
passes through the bearings, but doesn't carry any
lubricant with it. As a result of this arrangement,
the bearings of the invention have at least a B10
rating and do not require freguent servicing to replace
lip seals as in Roots blowers. This is accomplished
while avoiding the problems of contamination of the
laser gas by any bearing lubricant.
: .:
~ ~! 3 ~
- 25 -
From the above, it is seen that the method of
lasing gas in a fast axial flow laser according to the
invention comprises the steps of compressing a gas in a
turbine type compressor operating with a pressure ratio
sufficient to flow the gas along at least a portion of
the gas flow path in the laser at a speed of at least
half the speed of sound in the laser gas with a
compressor inlet pressure which may be S0 to 100 torr
or even less, conveying compressed gas from the
compressor along the flow path and exciting the gas
to cause it to lase. The method includes recirculating
the gas through the compressor and the laser tube in a
closed loop. The gas is preferably conveyed through at
least a portion of the laser tube at a speed which
approaches the speed of sound, namely 560 meters per
second in the aforementioned gas. The turbine type
compressor is operated at a speed of about 10,000 rpm
as noted above and is preferably a compressor having a --
pressure or head coefficient equal to or greater than
0.8. More specifically, according to the preferred
embodiment of the invention, the compressor is a - ~
two stage, double-sided regenerative compressor, ~ -
wherein each stage of the compressor has a pressure ~-~
coefficient of about 3. The gas is cooled between
stages, as well as being cooled during compression and
before and after compression in its path through the
closed loop of the gas laser apparatus. In addition,
the method includes the step of positively sealing
! . ` lubricant in the compresRor a~Ain~t movement `to the
laser gas by means of a fluid seal and providing laser
make-up gas to the laser with the seal. The impeller ~ ~-
of the compressor is rotated about its axis on the
~ ~L331~9
- 26 -
drive shaft at a speed such that the circumferential
speed of the impeller is a substantial friction of or
near the sonic speed of the gas.
While we have shown and described only several em-
bodiments in accordance with the invention, it is
understood that the same is not limited thereto, but is
susceptible of numerous changes and modifications as
known to those skilled in the art. Therefore, we do
not wish to be limited to the detail shown and
described herein, but intend to cover all such changes
and modifications as are encompassed by the scope of
the appended claims.
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