Electric lamp and discharge devices – With gas or vapor – Having particular electrode structure
Reexamination Certificate
1998-06-24
2001-04-17
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Having particular electrode structure
C313S030000, C313S022000, C313S035000
Reexamination Certificate
active
06218780
ABSTRACT:
TECHNICAL FIELD
The invention proceeds from a high-pressure discharge lamp with a cooled electrode in accordance with the preamble of claim
1
. At issue here, in particular, are high-power mercury high-pressure discharge lamps, but also other metal vapour lamps, in particular metal halide lamps as well as inert gas high-pressure discharge lamps, in particular xenon high-pressure lamps.
PRIOR ART
U.S. Pat. No. 3,636,401 has already disclosed a high-pressure discharge lamp with a liquid-cooled electrode, in which the electrode shaft is a tube in which a cooling liquid circulates. An inner tube of small diameter, in which the cooling liquid is transported to the tip of the electrode, is concentrically surrounded by an outer tube of larger diameter, in which the cooling liquid flows back again.
It was recognized very early that the application of liquid-cooled electrodes, particularly in the case of metal-vapour lamps (mercury high-pressure discharge lamps), and possibly also in the case of metal halide lamps and inert gas high-pressure lamps, requires careful design of the electrode so that the temperature at the electrode head does not become too high. In the case of metal-containing lamps, on the other hand, the temperature at the electrode shaft is not permitted to become too low (because of the risk of condensation). U.S. Pat. No. 3,412,275 describes an electrode in which at the shaft the wall of the outer tube is so thin that the lamp current, which flows via the outer tube acting as electrode shaft, gives rise to additional resistance heating. In addition, the shaft tube feeding the cooling water is lined on the inside with a material of low thermal conductivity (ceramic, quartz). The electrode head is cooled in this way and, on the other hand, the cooling effect in the shaft region of the electrode is limited such that no undesired condensation of mercury can take place. The electrodes are sealed by means of a transitional glass seal with Kovar cups, the seal having a constriction for centring the electrode shaft which, however, does not seal the seal region situated therebehind in a vacuum-tight fashion. Part of the filling therefore diffuses into the region of the transitional glass seal. The disadvantage is the high energy consumption owing to the resistance heating and the low thermostability of such a seal. Furthermore, there is a risk of the formation of cracks and fissures in the region of the seal, with the result that cooling water can come into contact with hot points and start to boil.
DESCRIPTION OF THE INVENTION
It is the object of the present invention to provide a high-pressure discharge lamp in accordance with the preamble of claim
1
which is very powerful and permits a high radiant flux.
This object is achieved by means of the characterizing features of claim
1
. Particularly advantageous refinements are to be found in the dependent claims.
In principle, the present invention can be applied to inert gas high-pressure discharge lamps, but is chiefly suitable for mercury-containing lamps, in particular. The present invention may be used particularly advantageously for lamps with a short electrode spacing (a few millimeters up to a few centimeters) (so-called short-arc lamps). Mercury short-arc lamps are limited in their power density, because the fusing and vaporization of the electrode material sets a limit on the maximum achievable power density in the discharge arc. The present invention is particularly important for DC lamps, since here the anode is heated particularly intensely (distinctly more intensely than the cathode). However, it can also be used with AC lamps.
The simultaneous requirement for a high radiant flux and small spectral line widths of the mercury lines (in particular the i-line at 365 nm) can be satisfied only with a high current density in the discharge arc. The anode is particularly intensely heated by the work of electrons captured there.
By virtue of the liquid cooling of electrodes, it is possible to realize substantially more powerful lamps (up to more than 10,000 W) than when use is made of conventional electrodes, whose cooling is based on emission and convection.
It is particularly to be borne in mind in the case of mercury high-pressure lamps that the temperature is not permitted to be below the condensation temperature of the mercury at any point in the interior of the discharge vessel. In the present invention, this problem is solved by a particularly effective thermal insulation of the feeding and return of the coolant.
This is achieved by virtue of the fact that the cooling tube system, comprising a feed tube and return tube, is insulated with the aid of an external enveloping tube. Located between the enveloping tube and cooling tube system is an interspace which is evacuated or filled with a thermally insulating medium.
By comparison with U.S. Pat. No. 3,412,275, this solution is simpler, cheaper and more effective. The point is that instead of a watertight inner lining resistant to high temperatures, use is now made of an external enveloping tube which is simpler to produce and to process. Said solution exhibits a better insulating effect. Moreover, there is no need for resistance heating (caused by the lamp current), since the insulation is so effective owing to the enveloping tube that is sufficient on its own reliably to prevent condensation of the filling (mercury). A minimum temperature of approximately 300° C. is thereby ensured for the surface of all the parts in the lamp interior, even if a cooling tube system with substantially cooler coolants (a typical temperature being 20-40° C.) is located in the electrode shaft. The temperature of the coolant can be at most approximately 120° C., since there is a risk of bursting above that temperature. Below 20° C. there is the risk of condensation of atmospheric moisture. Operation with antifreeze-containing water as coolant is possible with xenon lamps down to −40° C.
In detail, the high-pressure discharge lamp according to the invention has a discharge vessel and two electrodes arranged therein. The electrodes respectively comprise a shaft and a head, the shaft being sealed in a vacuum-tight fashion in each case in an end region of the discharge vessel. At least one electrode (the anode in the case of DC lamps, in particular) is cooled by virtue of the fact that its shaft contains a tube system in which a liquid or a gas circulates. Said shaft tube is surrounded at a spacing by an additional enveloping tube, the interspace between the enveloping tube and shaft tube being fitted with a means of thermal insulation.
The means of thermal insulation is advantageously a vacuum or a medium of low thermal conductivity, in particular a suitable gas filling, for example, argon or nitrogen. Additionally or alternatively, a medium, such as mineral wool or ceramic felt, which reduces the convective heat transport is inserted into the interspace of the enveloping tube.
The enveloping tube itself advantageously consists of molybdenum, since because of its high melting point said material can be processed effectively with quartz glass (silica glass) and, moreover, has a high resistance to possible aggressive or corrosive filling constituents (sodium vapour, metal halide). However, other materials such as, for example, niobium, copper (possibly coated), tantalum or nickel or their alloys can also be used. The particular advantage of molybdenum is, however, that it does not form a compound (amalgam) with mercury.
In one embodiment, the enveloping tube consists, at least predominently of hard glass or silica glass. In a particularly preferred embodiment, the enveloping tube is partially formed by the end region of the discharge vessel. The connection between the enveloping tube and the shaft is advantageously then performed by a molybdenum cap seal or transitional glass seal. The principle of a seal with molybdenum caps is disclosed, however, in U.S. Pat. No. 3,685,475 and DE-A 2 236 973. The technique using Kovar cups and transitional glasses is described, for example, in U.S
Ehrlichmann Dietmar
Franke Dieter
McNeil William H.
Patel Nimeshkumar D.
Patent-Truehand-Gesellschaft fuer elektrische Gluelampen mbH
Smith Michael J.
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