Note: Descriptions are shown in the official language in which they were submitted.
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5 LAMP FOR GENERATING HIGH POWER
ULTRAVIOLET RADIATION
Background of the Invention
This invention deals generally with the use of ultraviolet radiation
to sterilize liquids and more specifically with a high power excimer lamp
structure used to expose liquids to intense ultraviolet radiation to kill
bacteria, even when the liquids are essentially opaque to ultraviolet
radiation.
The exposure of liquids to ultraviolet radiation in order to sterilize
them by killing bacteria is a long established technique. Many patents
have been issued which are based on the ability of ultraviolet radiation to
destroy bacteria, and such devices are common enough to be in use in
many households and industries. Typically, such systems expose water to
ultraviolet radiation by passing the water through an enclosure in which it
is exposed to ultraviolet radiation.
One consideration which pervades all the prior art and is so well
25 accepted that it is rarely even mentioned is that the treatment of water by
exposure to ultraviolet radiation depends upon the water itself being
significantly transparent to the ultraviolet radiation. The penetration of
ultraviolet radiation through clear water typically may range from a few
inches to more than a foot. Without such transparency to ultraviolet
30 radiation, the purification of any liquid is very difficult because only
the
boundary of the liquid in actual contract with the source of radiation is
affected by the radiation. Without the liquid's transparency to ultraviolet
radiation, treatment requires very high intensity radiation and turbulence,
so that the bacteria in the liquid can be killed in a very short time during
35 which the turbulence assures that each portion of liquid is in direct
contact
with the ultraviolet radiation lamp.
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Hor~rever, achieving high intensity ultraviolet radiation is difficult.
_ Traditional high intensity mercury lamps actually generate such a broad
~spectcvm of radiation that the preferred wavelengths necessary to kill
bacteria
are only a small. park of the power output. Moreover, conventional mercury
lamps require an insulating sleeve over the lamp body which prevents direct
contact between the Iarnp surfaces and the fluid being treated. On the other
hand, excirnex Lamps, which have very narrow t~andwidths at the appropriate
wavelengths for sterilization of liquids, have been available only in
relatively
low power ratings. The historic lirxiita#ions on the ultraviolet output power
of
I O existing excimer lamps have been rooted in their geometry.
Exc;imer lamps are essentially gas filled enclosures which are subjected
to high voltage AC poorer by electrodes which are outside of, but in contact
with, the enetosure. 'Tla.e :lamp enclosures are constructed of a material
such as
. quartz, so that they are transparent to ultraviolet radiation. W'lxen AC
electrical bower is applit:d, the tamp acts as the dielectric o~ a capacitor
in
which the electrodes axe the plates of the capacitor, and, as in all
capacitors,
floe dielectric provides alt the impedance and uses all the power. For planar
lamps, this means two metal electrodes are located in contact with the quartz
gas filled planar envelope. Fox coaxial lamps, the envelope is usually formed
of concentric cylinders sealed together at the ends, with the gas fill between
the cylinders, The metal electrodes are then additional cylinders in contact
with the e~uter surface of the outer quartz cylinder and inner surface of the
inner cylinder.
Sir~ee existing lamps use one or more metal electrodes in contact with
2S the lamp c;nvelope, there is an inherent restraint on both lamp output and
on
lamp cooling. The electrode in contact with the outer surface of the lamp
envelope has typically been a mesh which is partially transpareztt to
ultraviolet
radiation ox a metal hlxn which is so thin that some ultraviolet radiation
passes
. through ~2. T~owevex, :for high power operation such electrodes must also be
capable of haadling high electrical currents. That requires that they have
r . ~ significant volume in cyrder to preve~at limiting the electrical current
or causing
resistance heating. This high current requirement eliminates thin films and
increases wire ttaickness in a mesh so greatly that the mesh blocks
significant
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amounts of the ultraviolet radiation output. It then becomes a diminishing
tradeoff in which the thick wire screens required for higher power levels
block
more of the ultraviolet radiation output which should be available for
treatment of the liquid. Furthermore, the same mesh with large wires
interferes
S with the cooling of the lamp surface, and when excimer lamps increase in
temperature lamp efficiency and life are adversely affected.
The result has been an impasse which has prevented the use of excimer
lamps in applications, such as the purification of opaque liquids, which
require
high ultraviolet radiation output.
Summary of the Invention
The present invention overcomes the dilemma caused by using mesh
electrodes when purifying liquids with high power ultraviolet radiation lamps
by completely eliminating all the metal electrodes in contact with the lamp
envelope. The excimer lamp of the invention is powered by high voltage AC,
but has no metallic electrodes within or in contact with the envelope.
The lamp is constructed in the form of two concentric quartz cylinders
sealed together at their ends with the excimer gas fill between the cylinders.
Cooling liquid is pumped through the central region inside the inner quartz
cylinder where an electrically conductive pipe that is not in contact with the
inner cylinder is used to supply this cooling liquid. Although it is not in
contact with the inner quartz cylinder, this central pipe also acts as the
high
voltage electrode. A cable attaches the central pipe to a high voltage AC
power source, but this high voltage electrode is electrically isolated from
the
source of cooling liquid by a suitably long Length of electrically insulated
tubing which also supplies the cooling liquid.
The entire lamp is enclosed within an outer metal cylindrical sheath
which is also not in contact with the quartz envelope, but is connected to the
return of the high voltage AC power source and is also grounded. The liquid
to be treated flows through the metal sheath and over the outside surface of
the
external envelope of the excimer lamp.
The electrical circuit is dependent on the fact that the power applied to
the lamp is alternating current, and, therefore, power can be transferred
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through capacitances. The two different liquid layers, cooling liquid inside
the
inner cylinder and treated liquid outside the outer cylinder, are the only
electrical power feeds to the lamp and, although they theoretically have some
conduction, they essentially act as capacitors to couple AC power to the
excimer lamp. These liquid filled capacitors have little power loss because
the
liquids have high dielectric constants. Therefore, the capacitors formed by
the
liquid, and also the capacitors formed by the walls of the quartz envelope,
result in impedances which are very much lower than that of the excimer gas
within the lamp. Thus, virtually all the power is delivered to and used by the
lamp.
Moreover, the liquid flowing within the central enclosure of the lamp
and the treated liquid on the outside of the lamp are near perfect coolants
for
the quartz lamp envelope. Since there are no electrodes contacting the quartz
envelope, the entire surface of the envelope is liquid cooled, and that liquid
I S can be temperature controlled to establish the most desirable temperature
for
the quartz envelope. This temperature control is a major factor in securing
long life operation for high power excimer lamps.
Finally, when the cooling liquid in the center of the lamp is selected to
be a clear liquid, it also permits ultraviolet radiation emitted from the
inner
envelope of the lamp to pass through the cooling clear liquid and the other
side
of the lamp and to still reach the treated liquid on the far side of the lamp.
In
such a configuration, and unlike the situation in the traditional lamp with
metal
electrodes, there are no solid or mesh electrodes to absorb any of the
ultraviolet radiation before it irradiates the liquid being treated.
The present invention thereby not only furnishes an ultraviolet
radiation generating excimer lamp with high efficiency and long life, but
there
is no reason to believe that there is any inherent limit on its power
capability.
Brief Description of the Drawings
FIG. 1 is a cross section view across the liquid flow path of the
excimer lamp of the preferred embodiment of the invention.
FIG. 2 is a simplified schematic diagram of the electrical and fluid
flow arrangement of the invention.
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Detailed Description of the Invention
FIG. 1 is a cross section view along the liquid flow path of excimer
lamp 10 of the preferred embodiment of the invention in which lamp 10 is
constructed from multiple concentric cylinders. The internal cylinder is
simple hollow metal pipe 12 through which liquid flows into volume 14 which
is located around pipe 12. Volume 14 is essentially the volume enclosed by
inner quartz cylinder 16 which is also one wall of excimer gas enclosure 18.
Cylindrical sleeve 15 is an extension of inner quartz cylinder 16, closes off
the
end of volume 14, and helps maintain the position of inner quartz cylinder 16.
Outer quartz cylinder 20 forms the outer wall of excimer gas enclosure 18.
End walls 22 and 24 join inner quartz cylinder 16 and outer quartz cylinder 20
to complete excimer gas enclosure 18 and to form an annular space which is
filled with excimer gas. End wall 24 is also extended to close off the end of
inner quartz cylinder 16, thus also closing off remote end 26 of inner volume
14.
The actual operation of excimer gas filled enclosure 18 is the same as
any conventional excimer lamp in that, when electrical energy is applied to
the
gas, micro-discharges within the gas generate ultraviolet radiation, with the
wavelength of the radiation determined by the particular gas within gas
enclosure 18.
The outermost cylinder is housing 28 and is held spaced away from
outer quartz cylinder by supports 30 and 32. Supports 30 and 32 are among the
several supports spaced around outer quartz cylinder 20 to center quartz
cylinders 16 and 20 within housing 28 while maintaining volume 34 between
housing 28 and outer quartz cylinder 20 open for the free flow of liquid
through volume 34. Volume 34 is closed off at one end by end plate 36 which
can either be an integral part of the cylinder of housing 28 as shown, or can
be
a removable cap bolted on in a manner similar to end plate 38 at the electrode
connection end of lamp 10. End plate 38 is, however, constructed of an
electrically insulating material such as plastic to electrically insulate
central
pipe 12 from housing 28. End plate 38 is held tight against plate 40 of
housing
28 by bolts 42 and sealed by conventional "O" ring 44.
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There are only two electrical connections to lamp 10. The high voltage
connection is cable 46 attached to central pipe 12 and the return voltage and
ground connection is a simple wire attached to housing 28. These connections
can be made by any conventional means such as nuts on studs welded to the
part to which the connection is made.
Liquid input and output connections are furnished for both cooling
water and the liquid to be treated. Central pipe 12 serves to supply cooling
water to volume 14. This cooling water flows out of pipe 12 near remote end
26 of volume 14, flows back along inner quartz cylinder 16 and sleeve 15, and
leaves lamp 10 through outlet pipe 50. The liquid being treated enters the
lamp through housing input pipe 52, flows along and around the outside of
outer quartz cylinder 20 as it is irradiated by the ultraviolet radiation
generated
by the excimer discharge within excimer gas enclosure 18, and exits the lamp
through housing outlet pipe 54.
In operation, as the treated liquid flows through lamp 10, the lamp
appears electrically as a series of five dielectrics between the electrical
inputs
formed by pipe 12 and housing 28. Beginning at pipe 12, which is the high
voltage connection and acts as one "plate" of the capacitor, the first
dielectric
is the cooling water within volume 14, the second dielectric is inner quartz
cylinder 16, the third dielectric is the excimer gas within volume 18, the
fourth
dielectric is outer quartz cylinder 20, and the fifth is the treated liquid
within
volume 34. Housing 28, which is grounded for safety and is the return for the
electrical power, acts as the other "plate" of the capacitor.
It is well understood that the impedance of any dielebtric of a capacitor
varies inversely with the dielectric constant of the material of the
dielectric, so
since water and quartz have high dielectric constants and the excimer gas has
a
low dielectric constant, the only high impedance in the series of dielectrics
is
the excimer gas. Thus, virtually all the electric power furnished by the power
source is supplied to the excimer gas, while the liquids and the quartz serve
essentially as connections to the dielectric of the excimer gas.
However, the liquids also serve another vital purpose. The liquids
flowing across inner quartz cylinder 16 and outer quartz cylinder 20 cool the
quartz walls of excimer gas enclosure 18 so that the excimer gas transfers its
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heat to ths: quartz walls and is also prevented from becoming overheated.
Cooling tb,e e~ci,mer laxn.p in this manner is vital to securiztg high
reliability
and long life for lamp 10.
FICi. 2 is a simplified schematic diagram of the electrical and fluid
haw arra~agement of t:lze invention which depicts the meazts by which two
liquid flov~r paths can be used in lamp 10 along with high voltage alternating
current power supply 60.
As previously d~:esc~ibed, lamp 1 U is fed cooling liquid through central
pipe 12, but central pipe I2 is also connected to high voltage power supply 60
by cable 4~6. Conventional wisdom suggests that the source of the cooling
liquid wood have to be at the same high voltage as central pipe 12 ur the
powez supply would be shorted out, but that is not actually the case,
If the cooling liqv.d feed path and retmu path to central pipe 12 are
long ertou~;h and the impedance of the cooling liquid high enough, such liquid
1 S flow paths will merely act as high impedances in parallel with the lamp,
and
the load They cause on ~:hc; power supply will be inconsequezttial. Fox
instance,
typical tap water has a. resistivity in the range of 20 to 200 micramho, and
therefore leas a resistance of 1 SO kilohm to 1.5 megohzau per foot when
flawiztg
in a .45 inch diameter plastic hose. It is then only necessary to determine
what
leakage current would be tolerable for power supply b0 and to make feed hose
62 for pip~a 12 arid retLUZ~ ktose 64 long ezzough to limit the leakage
current to
that value.
As shown in both FIG. 1 and in FIG. 2, housing 28 of lamp 10 is
actually electrically grounded, so there is na cortcexn, at all about any
voltage
being applied to it. Thus, treated liquid input pipe 66 and created liquid
outlet
pipe 68 can be connected to any required equipment and handle liquid of any
xesistivity. However, in most anticipated applicariozts event the liquid
beiztg
treated ha;: such a low conductivity that it would cause ono difficulty even
if it
were used as the cooling liquid within the central portion of the lamp. This
is
actually a possibility in some applications because it would eliminate the
need
for a separate liquid supply for the cooling liquid, and even if the cooling
liquid was opaque, the dimensions of the lamp could be desigzted to treat the
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cooling liquid as well. The preferred embodiment of the invention has been
operated with the following structure, conditions, and results.
Housing (28) - material - stainless steel
length - 110 cm
S Outer quartz cylinder (20) - wall thickness - 2.0 mm
length - 95 cm
Inner quartz cylinder ( 16) - wall thickness - 2.0 mm
length - 95 cm
Central liquid hoses (62)(64) - length - 1 meter
inside diameter - i .2 cm
Central pipe (12) - material- stainless steel
inside diameter - 1.5 cm length - 95 cm
Central liquid - tap water
Central liquid flow rate - 2 GPM
Treated liquid - 5% metal working fluid in water
Treated liquid flow rate - 150 GPM
Power supply (60) - S Kw
Excimer gas fill - Xenon/Bromine (dependent upon output
wavelength desired, as well established in the literature)
Ultraviolet radiation output - 300 mw per square cm.
Bacteria- kill rate - 3 log removal in 40 hours treating 600 gallons per
lamp
The preferred embodiment of the described ultraviolet radiation
generating lamp has operated in an industrial environment purifying opaque
machine cutting fluids, and has operated for more than 1000 hours at full
power output without failure.
It is to be understood that the forms of this invention as shown are
merely preferred embodiments. Various changes may be made in the function
and arrangement of parts; equivalent means may be substituted for those
illustrated and described; and certain features may be used independently from
others without departing from the spirit and scope of the invention as defined
in the following claims.
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For example, the configuration of lamp 10 need not be cylindrical,
although that is simpler to construct. The lamp could be constructed of
parallel planar sheets, in which case FIG. 1 would be a cross section view
across a portion of such a configuration. Moreover, materials other than metal
may be used for central pipe 12 and housing 28, as long as the materials are
electrically conductive, and walls 16 and 20 of gas volume 14 may be
constructed of materials other than quartz as long as the materials are
transparent to ultraviolet radiation of the wavelength generated by the lamp.
Furthermore, because most liquids have a dielectric constant greater than 10
and the invention is relatively independent of liquid conductivity, virtually
all
liquids are usable in this invention.
