CA1047096A - Method and apparatus for operating a mercury vapor lamp - Google Patents

Abstract

METHOD AND APPARATUS FOR OPERATING
A MERCURY VAPOR LAMP

ABSTRACT OF THE INVENTION

A mercury vapor lamp is rendered substantially independent of the thermal environment by connecting each sealed end to a heat sink which is maintained at below 100°C and located sufficiently close to the sealed ends to hold the seal temperature to below 400°C.

S P E C I F I C A T I O N

Description

~,16 10~70~6 Thls lnvention relates in general to mercury vapor lamps ~or use in photocuring applicatlon.
BACKGROUND OF THE INVENTION
A significant improvement in photopolymerization processlng i~ realized when the chemical coating to be cured is blanketed by an inert atmosphere during exposure to ultraviolet radiation. The principle source of ultra-vlolet energy ls a conventional mercury vapor lamp. Mer~
cury vapor lamps are relatively inexpensive and relatively efficient as generators of electromagnetic radiation in ;~
the ultraviolet wavelength range.
In order to simultaneously provide a protective atmosphere at the surface of the coating while the surface is undergoing irradiation, it ls necessary to house the mercury lamps in a confined enclosure in common with the inerting assembly. However, when one or more mercury vapor lamps, partlcularly lamp~ of high wattage, are located in the confined enclosure of the inerting assem-bly, sufficient heat is radiated to cause the ambient temperature of the enclosure to rise considerably. The elevated tAermal envlronment will in turn precipitate failure of the lamps. Such failure has been attributed to deterioration of the conducting element~ within the -~
lamps and more specifically to oxldation of the molybdenum strips whlch are sealed to the quartz envelop at the opposlte ends of the lamp and extend internally of the lamp to the tungsten electrodes. Slnce the inerting assembly is designed to pass inert gas into the con~ined

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9~1f' ~ C~47096 enclosure~ it would be only natural to assume that the design can be modified so that the inert gas provides the additional function of cooling the ends of the rner-cury vapor lamps. This would then be analogous to other known mercury lamp systems which pass air over and around the ends of the mercury lamps to provide cooling. Nevertheless to provide adequate cooling in this manner requires not only a relatlvely large flow of gas but an inde~inite flow whlch will vary with the number of lamps used and their wattage characteristic and may not necessarily provide uniform cooling. Hence, although ~easible, it is not compatible with efficient inerting assembly design. Furthermore the requirement o~ high gas flow is a serious economic disadvantage which might prove fatal to the commercial viability of a photocuring system dependent upon such flow. In ~act, conslderable research effort has been expended t~ design an inert gas blanketing system as taught in U.S. Patent No. 3,~07,052 entitled Apparatus for Irradiation of a Moving Product ln which the required lner~ atmosphere is satisfied using a very low flow of inert gas and is one o~ the princlple reasons why photopolymerization in an inert atmosphere has become commercially acceptable.
The aforernentioned patent discloses an inerting assem-bly including an enclosure having a treatment chamber, which houses the source of radiation,such as a plurality o~ mercury vapor lamps, and an lnlet and exit tunnel extending from the treatment chamber. ~he dlsclosure '~616 teaches the importance Or the geometry and location Or the lnert ga~ inJector and its orientation withln the assembly to achieve dynamic lnerting at low flows and emphasizes the importance of a non-turbulent ~low of inert gas throughout the enclosure.
It was theorlzed that the problem of overheating at the sealed ends o~ the lamp could be prevented from within the lamp by coupling the electrically conductive elements at the opposlte ends of the lamp to a heat ex-change medium external to khe lamp. Although this coupling can be carried out in a number o~ ways the most pre~erred is through a direct conductive coupling. This technlque permit~ the direct transfer o~ heat ~rom the molybdenum elements most susceptible to oxldation deterioration when the lamp ls operating within an elevated thermal envlron-ment without affecting the operating characteristlcs o~ the lamp. It has been shown that by conductive coupling in a predetermined manner not only i~ seal failure prevented but the lamp is rendered substantially independent of the ex-ternal thermal envlronment.
SUMMARY OF THE INVENTION
In accordance wlth the present invention lt was discovered that accelerated fallure of a mercury vapor lamp due to its conrlnement in an optically enclosed space re~ulting in a high temperature environment can be over-come by conductlvely coupling the lnternal electrlcally conductive elements of the lamp to an externally located heat slnk and controlllng the temperature of the heat sink ' ~

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BO a~ to maintain a "seal temperature" of below 400C at the opposite ends of the lamp. The expresslon seal temperature 1~ hereinafter de~lned a~ the temperature at the sealed lnter~ace between the molybdenum conduc-tive strlp and the 3urrounding ~uartz at the end~ of the lampO
Accordingly, one aspect o~ the present inven-tion is dlrected to a method for continuously operatlng a hlgh wattage mercury vapor lamp ln a con~ined enc~osure.
with ~aid lamp having an elongated quartz envelop sealed .. ..
at each end; the lmprovement comprising the steps of:
^onnectin~ each sealed end of said lamp to a thermally conductive heat slnk;
maintalning sald heat ~ink at a temperature o~ below 10~C; and locating said heat sink in such physical proxlmity to the s~aled ends of said quart2 envelop ~o that the seal temperature is held to below 400C.
Another a~pect of the present lnvention ls directed to a method for continuou~ly operatlng a plural-lty of hlgh wattage mercury vapor lamps ln combination a~ a ~ource o~ ultraviolet radiation ln a con~lned enclo-~ure wlthin wh~ch a non-turbulent inert gas atmosphere ls maintained, each of ~aid lamps having an elongated quartz envelop sealed at each end about an electrlcally conduc-tive molybdenum strip; comprislng the ~teps o~:
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connecting said electrically conductlve molybdenum strip at the end3 o~ each lamp ln a serles circuit relationship wlth a ~ource Or alternating current;
intercoupling a thermally and electric-ally conductive heat ~ink into the serie~ circuit of each lamp re~pec-tively;
maintainlng the heat sink at a tempera-ture of below about 100~; and :~
placlng the heat ~ink ln each circuit ln such phy~lcal proximlty to 3ald ~ealed ends 80 that the temperature of the molybdenum strlp at the sealed end of each lamp i~ malntained at :;
below 400~C. ^ ~:Yet another aspect o~ the present invention ls directed to an improved mercury vapor lamp ~or generating ~:
ultravlolet light lncludlng an elongabed quartz envelop :
surrounding two tung ten electrode~ located at opposite end~ of the envelop, with each end sealed in lnt~mate contact about an electrically conductive molybdenum ~trip coupled to one o~ sald electrode and an electrlcal con-ductor extendlng from each sealed end, the improvement ~;
comp~lsin~: terminal mean~ located ad~acent each opposite end o~ the envelop and being ln electrical and thermal contact with each of said electrical conductors, means -:-for electrlcally coupllng ~aid terminal means acro~ a .

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~ource of alternatlng current and meanR for pa~ing a cooling liquid through ~ald termlnal means, A ~urther aspect o~ the present invention i9 directed to the combination of a mercury vapor lamp having an elongated quartz envelop ~urroundlng t~o tung-sten electrodes located at opposite end~ of the envelop with each end sealed in intimate contact about an elec-trically conductive molybdenum strip coupllng one of said electrodes, and an electrical conductor extending from each sealed end~ and a means ~or electrically coupling ~a~d conductor to a source of alternatlng current, sald means being located ad~acent a sealed end and being o~ relatively high thermal conductivity, and conduit means located contiguou~ to said coupllng mean~ an~ adapted for pa~sage o~ a coollng liquid.
AccordlnglyJ it is the sole ob~ect o~ the pre~-ent invention to provide a mercury vapor lamp which can operate ln an optlcally conPinëd space substantially independent o~ its surrounding thermal environment.
Other advantages o~ the present lnvention wlll become apparent ~rom the followlng detailed description o~ the pre~erred embodiment when taken in con~unctlon wlth the accompanylng drawings realizlng that variatlons ~ ~-and modl~icatlons may be made to the preferred embodiment wlthout departlng from the spirit and scope of the novel concept of the di2closure.

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BRIEF DESCRIPTION OF TXE DRAWIN~S
Figure 1 i~ a longitudlnal ~ectional view lllus-trating the combined mercury vapor lamp and terminal assem-bly of the present invention;
Figure 2 ig a slmllar view to that o~ Figure 1 with the apparatus rotated ninety degrees (90) about a horizontal axis ~rom the position shown in Flgure 1;
Figure 3 is a substantlally enlarged view of one of the sealed ends o~ the lamp ~hown in Flgures 1 and 2;
and Figure 4 ls a diagrammatlc lllustration of the pre~erred inertlng apparatus housing the mercury vapor lamp and terminal a~sembly hown in Figures 1 and 2.
DETAILED DESCRIPTION OF THE INVENTION
The apparatu~ o~ the present invsntlon ls shown ln Figure~ 1 and 2~inclusive, and includes ln combinatlon a conventlonal mercury vapor lamp 10 and a pair of terminal assemblies 12 and 14 located adJacent each end Or the lamp 10 re~pectively.
The mercury vapor lamp 10 i~ a typical commer~
cially available 2.2 kw lamp manu~actured by Sylvania Electric Products Inc. and commercially ldentl~ied by the No. H2200~4/24Q. Although a medium pres~ure lamp of the ~ ;;
partlcular con~iguratlon shown 1~ illustrated ln the drawings, the pre~ent lnvention ls not to be consbrued ~
a~ limlted thereto. In ~act, any conventional mercury ~ -vapor lamp may be u~ed o~ any conflguratlon and ~pec-lf`lcatisn, ~0 ~7 ~

The illu~trated mercury vapor lamp 10 i~ sealed at each end 30 and 32 definlng therebetween an arc envelop 16 of ~used quartz which surrounds a pair of tungsten electrode~ 18 and 20 located at the opposite ends of the lamp lO. The electrode 18 i5 connected at the sealed end 30 ko the power conductor 22 through an intermediate strip of material 24. The sealed interconnection between the conducting elements consisting o~ the tungsten electrode 1~, the intermedia~e ~trip 24, and the power lead 22 i~
Ishown more clearly in ~lgure 3. Likewisej electrode 20 is connected at the sealed end 32 o~ lamp 10 to the power conductor 26 through an intermediate strip o~ material 28.
Each ~trip of material 24 and 28 is compo ed of a ductile materlal which i9 electrically conductive and has a high melting point such as molybdenum. The sealed ends 30 and 32 are rormed,a~ter the lamp ls filled with argon gaæ and a ~mall amount of mercury, us~ng conventlonal method~ ;
æuch as, ~or example, by mechanically compressing each end o~ the lamp lO into intimate contact about the conducting strips 24 and 28 re~pectively. By compressing the enda ~;
together to ~orm a ~eal, the end geometry becomes substan-tially rectangular. Alternatively, the ends o~ the lamp 10 may be vacuum drawn ~orming an extended neck which in-timately enclose~ the molybdenum strips. It i~ also con-ventional to encap~ulate the ~ealed end~ 30 and 32 respec-tively in ceramic material 35. Moreover, a metal7ic con-ductlve cap may be placed over the ~ealed ends 30 and 32 ,., . . ~
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~047096 of lamp lO and u~ed a~ a ~ubstltute for the external power leads 22 and 26 respectlvely.
The terminal a~semblles 12 and 14, shown in Figures 1 and 2 inclusive, are prs~erably ldentlcal in structure with each es~entially represented by a member shown in the ~orm o~ a block whlch ls thermally and pref-erably elect~ically coupled to the molybdenum strips 24 and 2~ respectively of lamp 10. The terminal as~emblies 12 and 14 although primarlly lntended a~ heat sinks also ~unction to shleld the sealed ends 30 and 32 ~rom radiation as will be explained in more detail hereafter a~ well as -~ -to transmit the alternating current to the tungsten elec-trode~ 18 and 20.
The physical configuration and materials o~
construction ~or each terminal assembly 12 and 14 is not critlcal to the invention provided they adequately per~orm the ~unction o~ a heat slnk 90 as to drive su~
~lcient heat ~rom the internal conductive elements o~ -the lamp lO, and more particularly ~rom the molybdenum strips 24 and 28, to prevent ~allure. To ful~ill thls requirement each terminal assembly must be constructed o~ a reasonably good the~mally conductive ma~erial and be o~ a con~lguratlon so as to permlt efflcient tran~er o~
heat. It is particularly pre~erred to use the power con-ductors 22 and 26 to ~unctlon as the lntermediate heat tran~fer medium from the molybdenum strlps 24 and 28 to the terminal as~emblles 12 and 14 respectively. The phy~ical placement of each terminal assembly 12 and 14 , ... .
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to each sealed end 30 and 32 respectively of lamp 10 ls critical ln that lf there is too ~ar a separation the necessary cooling to overcome oxldation at the sealed endc wlll not be achleved. It ha~ been found that the locatlon o~ the termlnal assemblles 12 and 14 must be in such close proximity to the sealed ends 30 and 32 to keep the seal temperature below 400C and pre~erably not above 350C. To maintaln the seal temperature at this level it has also been found necessary to lnsure that the temp-erature o~ the termlnal as~emblies 12 and 14 be held to a temperature significantly lower than 350C and prererably not above about 100C. To sati~fy the latter requirement the terminal assemblles 12 and 14 must be cooled with a cooling liquid such as water. The sxtent of such cooling wlll determine the heat drlvlng ~orce.
~he prererred structural arrangement ~or each terminal a~sembly 12 and 14 ac shown in ~igures 1 and 2 com~rises a two section constructlon with each seckion 50 and 52 respectively composed of a material which is thermally and pre~erably electrically conductlve such as copper or aluminum. The two sectlons 50 and 52 are ln-tended to be coupled by bolts 54 so as to ~it about an end o~ the lamp 10. Each section 50 and 52 includes drillings for formlng, when the two sections are combined, two con-centrically reces~ed portions 56 and 5~, into which the ceramlc co~ering 35 at each ~ealed end 30 and 32 o~ lamp 10 is ~eated before ~irmly securlng the ~ectlons, and a bore through whlch the power conductor at each end is ~ ~'7~ 6 entrained. It is also desirable to hold a fairly close tolerance in order to achleve good surface contact, between the sealed ends 30 and 32, at their terminatlons with the power conductors 22 and 26 respectively, and the :
sectlons 50 and 52 Or each terminal assembly 12 and 14 reRpectively. A passageway 60 is located ln the upper section of each assembly for passing the cooling water ~rom a source (not shown) through each terminal assembly.
The pas~ageway 60 should be located suf~iciently close to the conductlng power lead3 22 and 26 to maintain an effec~
tive heat transfer gradlent between the.conductive strlps -~
24 and 2~ and the terminal assemblies. ~-AC power is applied to the lamp lO ~rom a source .
62 connected acros~ the terminal assemblies 12 and 14. ;~
Alternatively, AC power may be applied directly across the ;~
power conductors 22 and 26 in which case the terminal ;~
assemblles nee~ not be electrically conductive. .
. The recessed portions 56 and 58 into whlch the sealed ends of the lamp are seatod are pre~erably of a length su~ficient to form a radiatlon shield about the molybdenum ~trips 24 and 2~ respectively. Shielding the :
molybdenum strips from reflectlve radiat.ion limits the extent o~ internal coolln~ of the lamp 10 that would :
otherwise be necessary to maintaln the seal temperature at below 400C.
~e pre~erred system in which a plurality o~

lampR lO are used as a source o~ ultraviolet radiation in an inert enclosure 1~ schematically iIlustrated in . ~

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96~6 47C1~6 Figure 3~ A product P is intended to pass through the inert enclosure 70 at a predetermined speed. The enclos-ure 70 includes a radiation chamber 72 which houses a plurality of lamps 10, only one of which is shown, and an inlet and exit tunnel 74 and 76 respectively. Each lamp 10 is seated as shown in Figure 1 in a terminal assembly 12 and 14 respectively which is in turn affixed to the chamber 72. A reflector 78 partially covers the lamp 10 for redirecting radiated light toward the passing product P. The cooling water passing through the terminal assemblies 12 and 14 may also be used to cool the reflector 78. The atmosphere within the enclosure 70 is exclu-sively controlled by the passage of inert gas fed from the ~ ;
plenum chamber 80 through the inert gas injector 82 into the enclosure 70. The method by which the inert gas may be fed at a predetermined low flow rate to maintain an inert atmosphere above the moving product is taught in U.S. Patent No. 3,936,950 and entitled Method of Inerting The Atmosphere Above A Moving Product. A non-turbulent flow of inert gas is malntained throughout the enclosure.
By following the teachings of the present invention so as to conductively transfer heat out from the elec-tr~cally conducting elements within each of the lamps 10 a substantial degree of thermal independerlce is achieved between each of the lamps 10 and the ambient temperature within the optically enclosed radiatlon chamber 72.

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Claims (6)

I claim:

1. A method for continuously operating at least one mercury vapor UV lamp in any gaseous environment existing in a confined optical enclosure; said lamp consisting of a single elongated quartz envelope, a quartz to metal seal at each end and an exposed terminal at each end; the improvement comprising the steps of:
a. providing at least one thermally conductive heat sink independent of said lamp and locating said heat sink so that it does not contact said single elongated quartz envelope;
b. mechanically connecting only each exposed terminal of said lamp to said thermally conductive heat sink;
c. maintaining said heat sink at a temperature below 100°C by passing a cooling liquid therethrough;
d. and locating said heat sink in such close physical proximity to said quartz to metal seal so that the seal temperature is held below 400°C.

2. A method according to claim 1 wherein the cooling liquid is water.

3. A method as defined in claim 1 wherein heat sink is composed of material selected from the class consisting of copper, aluminum and alloys thereof.

4. Apparatus for photopolymerizing chemical coatings comprising an enclosure having a radiation chamber therein;
an inlet tunnel to said radiation chamber and an outlet tunnel from said radiation chamber; a plurality of terminal assemblies mounted in said radiation chamber; a plurality of mercury vapor lamps having sealed ends and exposed power terminals mounted in said terminal assemblies so that the exposed power terminals only are connected to the terminal assembly;
each of said terminal assemblies having water passages therein for carrying cooling water sufficiently close to the power terminals of said mercury vapor lamp to keep said sealed ends below a temperature of 400°C.

5. Apparatus according to claim 4 including a plenum chamber communicating with said inlet tunnel through a gas injector for maintaining a non-turbulent flow of inert gas throughout said enclosure.

6. In a mercury vapor lamp for generating ultraviolet light consisting of a single elongated quartz envelope surrounding two tungsten electrodes located at opposite ends of the envelope, a quartz to metal seal at each end, said metal in the quartz to metal seal comprising a molybdenum strip) and an exposed terminal at each end, the improvement comprising: terminal means located adjacent each opposite end of said envelope in close physical proximity thereto but in contact only with said exposed terminal; means for electrically coupling said terminal means across a source of alternating current and means for passing a cooling liquid through said terminal means at a predetermined rate so that the temperature of the molybdenum strip is held below 400°C.

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