DK147247B - MERCURY OIL LAMP FOR CONTINUOUS OPERATION - Google Patents

Description

1 147247

The present invention relates to a continuous operation mercury vapor lamp in a defined enclosure with a quartz cap sealed at each end and a radiant conductor projecting from each end as well as heat sinks located at each end.

A significant improvement in photopolymerization processing is provided when the chemical coating to be cured is covered by an inert atmosphere while subjected to ultraviolet radiation. The main source of ultraviolet energy is a conventional mercury vapor lamp. Mercury vapor lamps are relatively inexpensive and relatively effective 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 subjecting the surface to irradiation, it is necessary to place the mercury lamps in a confined enclosure with the unit providing the inactive atmosphere.

However, when one or more mercury vapor lamps, particularly high-power lamps, are placed in the confined enclosure of the unit to provide the inactive atmosphere, sufficient heat is radiated to cause the ambient temperature of the enclosure to rise significantly. The elevated temperature of the surroundings will in turn accelerate the failure of the lamps. Such failure has been attributed to deterioration of the conductive elements of the lamps, and especially oxidation of the molybdenum strips, which are fused in the quartz sheath at the opposite ends of the lamp and extend inward into the lamp to the tungsten electrodes. Since the device for providing the inactive atmosphere is designed and arranged to feed inactive gas into the bounded enclosure, it would only be natural to assume that the design can be changed so that the inactive gas provides the additional function of cooling the ends of the mercury vapor lamps. .

This would then be analogous to other known mercury lamp systems in which air is passed over and around the ends of the mercury lamps to allow cooling. Nevertheless, this requires providing sufficient cooling in this way not only a relatively large gas flow but an indefinite flow which will vary with the number of lamps used and their effect and will not necessarily provide uniform cooling. Accordingly, although possible, it is incompatible with efficient design of the device to provide inert atmosphere. Furthermore, the requirement of a high gas flow is a serious economic disadvantage which can prove detrimental to the commercial viability of a photo curing plant which depends on such flow. In fact, considerable research efforts have been made to design an inert gas coverage plant as disclosed in United States Patent No. 3,807,052, where the required inert atmosphere is obtained using a very low flow of inert gas and is one of the main reasons why photopolymerization in an inert atmosphere has become commercially acceptable. The above-mentioned patent specification discloses a device for providing an inactive atmosphere containing an enclosure with a treatment chamber which receives the radiation source, such as a number of mercury vapor lamps, and an entrance and exit tunnel exiting the treatment chamber. The patent states the importance of the geometric design and placement of the inert gas injector and its orientation in the device to achieve dynamic provision of the inert atmosphere at low gas flows and emphasizes the importance of a non-turbulent flow of inert gas throughout the enclosure.

Theoretically, it was found that the problem of overheating at the sealed ends of the lamp could be prevented inside the lamp by coupling the electrically conductive elements at the opposite ends of the lamp to a heat exchange medium outside the lamp. Although this coupling can be performed in a number of ways, it is most preferred by a direct heat-conducting coupling. This method enables the direct heat transfer from the molybdenum elements which are most susceptible to degradation by oxidation when the lamp is operated under elevated temperature conditions without affecting the operating characteristics of the lamp. It has been shown that by heat conducting coupling in a predetermined manner, not only sealing failure is prevented, but the lamp is made essentially independent of the external temperature conditions.

The characteristic of the mercury vapor lamp according to the invention is (a) that each conductor is connected to the heat sink to provide a path for the heat conduit from the sealed end, (b) that the heat sink is maintained at a temperature below 10 ° C by a coolant which and (c) that the heat sinks are disposed in such close proximity to, but free from, the sealed ends that the sealed ends are kept at a temperature below 40 ° C.

In such an arrangement for cooling the mercury vapor lamp, there is only one connection between the sealed tube and each heater, namely the protruding conductors, and the heat sinks are disposed in close proximity to the ends of the sealed tube, leaving no space thereby will be transmitted by heat conduction elsewhere than by the protruding conductors. However, the gap is small enough to protect the seal from oxidation.

3 147247

If, as stated, the heater has a temperature below 10 ° C and the gap is small enough to keep the sealing temperature below 40 ° C, the desired result is obtained.

Compared to known designs, the invention provides that quartz-metal seal failures prevent the lamp from being made substantially independent of the ambient thermal conditions, avoiding the use of a cooling gas which would be required in large quantities, and which does not provide a uniform temperature, and that the quartz-metal seal is protected from the effect of radiation, thereby reducing the requirements for cooling.

The invention will now be explained in more detail with reference to the drawing, in which fig. 1 shows a longitudinal section in one embodiment of a combination of a mercury vapor lamp and terminal unit according to the invention; 2 is a view similar to that of FIG. 1, but with the apparatus rotated 90 ° about a horizontal axis relative to the position shown in FIG. 1, FIG. 3 is a greatly enlarged view of one of the sealed ends of the lamp shown in FIG. 1 and 2 and FIG. 4 is a schematic representation of the preferred apparatus for providing inactive atmosphere and containing the mercury vapor lamp and terminal unit shown in FIG. 1 and 2.

The apparatus of the present invention is shown in FIG.

1 and 2, and comprises a conventional mercury vapor lamp 1o and a pair of terminal units 12 and 14 located near each end of the lamp 1o.

The mercury vapor lamp lo is a commercially available 2.2 kW lamp manufactured by Sylvania Electric Products Inc. and designated No. H2200T4 / 24Q. Although a medium pressure lamp is shown here, the present invention is not limited to this lamp. In fact, any conventional mercury vapor lamp of any design and specification can be used.

The shown mercury vapor lamp Io is sealed at each end 3o and 32 to form therebetween an arc quartz 16 of quartz which surrounds a pair of tungsten electrodes 18 and 2o located at the opposite ends of the lamp Io. The electrode 18 is connected at the sealed end 30 to a current conductor 22 over an intermediate strip 24. The sealed connection between the conductive elements 4 147247 consisting of the tungsten electrode 18, the intermediate strip 24 and the current conductor 22 is more clearly shown in FIG. 3. Likewise, the electrode 20 at the sealed end 32 of the lamp 1o is connected to a conductor 26 over an intermediate material strip 28. Each material strip 24 and 28 consists of a tough material which is electrically conductive and has a high melting point, such as molybdenum. The sealed ends 30 and 32 are formed after the lamp is filled with argon gas and a small amount of mercury using conventional methods such as e.g. by mechanically compressing each end of the lamp 1, into intimate contact around the conductive strips 24 and 28, respectively. By compressing the ends to form a seal, the geometric shape of the end becomes substantially rectangular. Alternatively, the ends of the lamp 1o may be vacuum-drawn to form an elongated neck which intimately encloses the molybdenum strips. It is also common to encapsulate the sealed ends 3o and 32 in ceramic material 35. In addition, a conductive metal sheath can be placed over the sealed ends 3o and 32 of the lamp and be used as a replacement for the external power lines 22 and 26, respectively.

The terminal units 12 and 14 shown in FIG. 1 and 2 are preferably similar in structure and consist essentially of a blank shown in the form of a block which is thermally and preferably also electrically coupled to the molybdenum strips 24 and 28 of the lamp 1o. Although the terminal units 12 and 14 are primarily intended to act as heat sinks, they also serve to shield the sealed ends 30 and 32 from radiation, as explained in more detail below, and to transmit the alternating current to the tungsten electrodes 18 and 20.

The physical design and materials of each terminal unit 12 and 14 are not critical to the invention, provided that they act sufficiently as heat sinks to remove sufficient heat from the inner conductive elements of the lamp 1o and in particular from the molybdenum strips 24 and 28 for to prevent the failure of the lamp. To meet this requirement, each terminal unit must be made of reasonably good thermally conductive material and be of such design as to enable efficient heat transfer. In particular, it is preferred to use current conductors 22 and 26 as the intermediate heat transfer agent from molybdenum strips 24 and 28 to terminal units 12 and 12, respectively. 14. The physical location of each terminal unit 12 and 14 at each sealed end 3o and 32 of lamp 1o, respectively, is critical, since if there is too long a distance between them, the necessary cooling to avoid oxidation at the sealed ends will not be achieved. .

It has been found that the arrangement of the terminal units 12 and 14 must be so close to the sealed ends 30 and 32 that the melting temperature is kept below 40 ° C and preferably not above 35 ° C. In order to keep the melting temperature at this level, it has also been found necessary to ensure that the temperature of the terminal units 12 and 14 is kept at a temperature which is considerably lower than 35 ° C and preferably not more than approx. loo ° C. To meet this latter requirement, the terminal units 12 and 14 must be cooled with a coolant such as water. The degree of this cooling will determine the heat propulsion.

The preferred structural arrangement for each terminal unit 12 and 14 shown in FIG. 1 and 2, comprises a two-part structure, each section 5o and 52 respectively consisting of a material which is thermally and preferably also electrically conductive, such as copper or aluminum. The two sections 5o and 52 are intended to be coupled by means of bolts 54 so as to fit around one end of the lamp 1o. Each section 5o and 52 has bores when the two sections are connected to form two concentrically recessed portions 56 and 58, wherein the ceramic sheath 35 at each sealed end 3o and 32 of the lamp lo is placed before clamping the sections, and a bore through which the conductor is inserted at each end. It is also desirable to maintain a fairly narrow tolerance to achieve good surface contact between the sealed ends 3o and 32 at their end faces with the current conductors 22 and 26 and sections 5o and 52 of each terminal unit 12 and 14. A passage 6o is disposed in the upper section of each unit to pass cooling water from a source not shown through each terminal unit. The passageway 6o must be located sufficiently close to the conductive current conductors 22 and 26 to maintain an effective heat transfer gradient between the conductive strips 24 and 28 and the terminal units.

Alternating current is supplied to lamp Io from a power source 62 connected across terminal units 12 and 14. Alternatively, alternating current may be supplied directly over current conductors 22 and 26, in which case the terminal units need not be electrically conductive.

The recessed portions 56 and 58, in which the sealed ends of the lamp are disposed, preferably have a sufficient length to form a radiation shield around the molybdenum strips 24 and 28, respectively. be necessary to maintain the melting temperature below 40 ° C.

The preferred plant utilizing a plurality of lamps lO as an ultraviolet radiation source in an inactive enclosure is shown schematically in FIG. 4. A product P is intended to pass through the inactive enclosure 7o at a predetermined speed. The enclosure 1e contains a radiation chamber 72 which accommodates a plurality of lamps 1e, of which only one is shown, and an input and output tunnel 74 and 76, respectively. Each lamp 1e is as shown in FIG. 1 located in a terminal unit 12 and 14, respectively, which are again attached to the chamber 72.

A reflector 78 partially covers the lamp lO to direct transmitted light towards the passing product P. The cooling water passing through the terminal units 12 and 14 can also be used to cool the reflector 78. The atmosphere of the enclosure lO is controlled solely by the passage of inert gas. which is supplied from a pressure chamber 8o through an inert gas injector 82 into the envelope 1o. The process by which the inert gas can be supplied at a predetermined low flow rate to maintain an inert atmosphere over the migratory product is disclosed in U.S. Patent Application No. 461,378 of April 16, 1974, entitled Process for Generating an Inactive Atmosphere over a wandering product. A non-turbulent flow of inert gas is maintained throughout the enclosure.

Following the teachings of the present invention so that by conducting heat from the electrically conductive elements of each of the lamps 1o, a significant degree of thermal independence is obtained between each of the lamps 1o and the ambient temperature of the optically enclosed radiation chamber 72.