US2727175A - Electric discharge lamp - Google Patents

Description

Dec. 13, 1955 Filed May 27, 1950 VAPOR PRESSURE IN MILL/METERS OFMEPCUQY w. T. ANDERSON, JR 2,727,175

ELECTRIC DISCHARGE LAMP 2 Sheets-Sheet l VAPOR PRESSURE IN MILL/METERS OFMERCURY RELATIVE INTEN5/TY 147 2537/].

\ a I i l 1 160 150 250 230 360 3E0 DEGREES FAHRENHE/T .HVVENTUR.

WlLLIA TANDERSONJR.

A TTOIQNEY Dec. 13, 1955 W. T. ANDERSON, JR

ELECTRIC DISCHARGE LAMP 2 Sheets-Sheet 2 Filed May 27, 1950 INVENTOR WILLIAM .ANDERSONIJR.

BY M

A TTORNEY United States Patent Ofiice 2,727,175 Patented Dec. 13, 1955 ELECTRIC DISCHARGE LAlVlP William T. Anderson, -Jr., Maplewood, N. J., assiguor to Hanovia Chemical and Mfg.'Company, Newark, N. 3., a corporation ofrNew Jersey Application May27, 1950, Serial No. 164,664

'1 Claim. (CL 313-225) This invention relates to mercury vapor electric discharge devices generally and more particularly to such devices for the production of ultraviolet radiations used for the destruction of bacteria, viruses, mold spores, and other cells, and for photochemical reactions responsive to wavelengths of ultraviolet radiations shorter than 2600 Angstroms.

Mercury vapor electric discharge lamps commonly used as the source of ultraviolet radiations for germicidal and'photochemical reactions, produce an abundance of ultraviolet rays but they vary greatly in output due to fluctuations in the electric power supply, and the ambient temperature, and their output continually decreases as a result of increasing opacity of the vitreous envelope. In many ofthese applications, it is necessary to maintain this output at.a predetermined value. For example, in the application of ultraviolet irradiation for the sterilization of blood .products for human use, such as blood plasma, blood serum vaccines, and the like, the ultraviolet ray intensity must be sufficient to kill all undesirable organisms in the blood product, but must be maintained belowtthat value which will. produce harmful antibodies in the product.

it is known in the art that the output, 1. e. intensity, of the radiations emitted by an ultraviolet ray lamp depends on the pressure of the mercury vapor in the envelope, that this pressure depends upon the temperature of the envelope, and that this temperature depends in part upon the-wattage supplied from the source of electric power. It is also known that slight fluctuations in the temperature of the envelope cause substantial variations in the vapor pressure of the ,mercury resulting in correspondingly largefiuctuations in intensity.

In irradation processes of the type herein contemplated which require the application of a steady intensity, it has been customary to employ elaborate and complicated control devices to maintain the wattage input at an even value. Also, control of the temperature of the surrounding atmosphere hassbeen necessary in order to prevent changes in the vapor pressure of mercury within the lamp envelope.

Improved control of the intensity of ultraviolet ray lamps has been had heretofore by limiting the amount of mercury in the lamp to a quantity that will be entirely vaporized before the lamp attains its operating temperature. in devices of this type, variations in the pressure of mercury vapordue to changes in temperature from fluctuations in wattage and ambient temperatures follow the gas law that pressure is directly proportional to absolute temperatures, and accordingly changes that result from the efiect of these fluctuations on the vapor pressure of HICICUI'], i.-e. vapor'in the presence of liquid, are not encountered. Although improved intensity control has been attained from the use of these lamps that have a limited quantity of mercury, they are not entirely satisfactory for irradiation processes of the type herein contemplated wherein variations in intensity must be reduced to a minimum.

It is an object of this invention to provide a new and improved gaseouselectric discharge device for the production of ultraviolet radiations for destroying bacteria, viruses, mold spores, and the like, and for photochemical reactions respomive to wavelengths shorter than 2600 Angstroms. Another object is to operate devices of the above type so that their output will remain practically constant and the etfect of fluctuations that are normal to commercial electric power supplies and that occur in the temperature of the ambient atmosphere will be negligible. A further object is the provision of a vapor electric discharge device that is economical to manufacture andto operate without employing wattage and temperaturecontrol means. These and other objects and advantages which will later appear, are accomplished by the method and apparatus hereinafter described and illustrated in the accompanying drawing, forming a parthereof in which:

Figure 1 is a graph showing how the intensity of a lamp varies as the pressure. of the mercury vapor within the lamp is increased,

Figure 2 is aigraph indicating the relation between temperatureand the vapor pressure of mercury within the lamp,

Figurejiis an elevation of apparatus which has been used in practicing the invention,

Figure 4 isanend view of the apparatus shown in Figure 3,

Figure 5 is an elevation of another embodiment of apparatus according to the invention.

[The present invention is founded on my discovery that the'intensity of radiations of wavelengths shorter than '2600 An stroms remain practically constant during a limited period in the overall change that may be produced in the pressure of mercury vapor within a lamp envelope. This limited range extends between the points A and B shown on the curve of Figure 1 representing the relation between the intensity of radiation at 2537 Angstroms, which was selected as a wavelength typical of radiations shorter than .2600 Angstroms, and the pressure of mercury vapor in a gaseous electric discharge lamp operated at, constant wattage input. The range of pressures of mercury vapor that are covered by the curve was provided by applying heat from an external source to a lamp having an excess or" liquid mercury present within the envelope.

It is seen from this curve that as the pressure of mercury vapor increases, the relative intensity of ultra-violet radiations also increases until a maximum intensity is reached corresponding to the portion of the curve at C. Thereafter, a further increase in pressure results in a decrease in intensity, which continues at a relatively constant rate except for that portion of the curve that lies between the points A and B.

To operate a lamp at any desired intensity along this curve, it is necessary .to maintain its temperature at a value that will develop the pressure of vapor which corresponds to the desired intensity. Since the rise in temperature of a lamp results from an increase in the energy input and since the lamp exchanges heat to the surrounding atmosphere at a rate that is proportional to its surface area, control of the lamp temperature may be efiectedby passing a sufficient amount of electric power intothe lamp to oppose the cooling-oil effect or" the lamp surface so that an equilibrium temperature is finally reached. This relation of electric power to surface area is herein referred to as power loadingwhich represents the total power in watts consumed within the lamp envelope divided by the surface area.

Prior to this invention, mercury vapor lamps forthe production of ultraviolet radiations shorter than 2600 Angstroms were commonly operated so that the pressure of mercury vapor was less than 0.01 millimeter to obtain maximum electrical eificiency for ambient temperatures of about 70 F., whereby the power loading characteristics of these lamps are such that their range of operation corresponds to the portion of the curve that lies below C. Changes in the operating temperatures of these lamps resulting from fluctuations in the supply voltage and variations in the ambient temperature cause marked changes in ultraviolet output which have no practical significance for common applications, but which must be controlled by special constant voltage devices when these lamps are used for critical applications such as the complete nontoxic sterilization of blood and blood products for human use.

For the purposes of this invention, constancy of ultraviolet ray output is more important than the attainment of maximum electrical efiiciency. It is to this end that the design of the electric discharge devices of this invention has been directed.

Referring again to the curveof Figure 1, it is seen that when the vapor pressure of mercury is increased considerably beyond that pressure which will produce maximum electrical efficiency in intensity of the lamp, a break in the curve will be reached, represented by the portion between points A and B, wherein an increase in vapor pressure of from about 4 millimeters to about millimeters will produce only a negligible change in the relative intensity of the lamp. This range is substantially greater than that needed to absorb all of the changes in vapor pressure that will result from the normal fluctuations encountered in commercial line potentials and from the usual variations that occur in the temperatures of the surrounding atmosphere.

The envelopes of the lamps of this invention are specially dimensioned with respect to the power consumed therein so that the lamps will operate in room temperature atmospheres and at a power loading that produces the mercury vapor pressures which lie in the range between the points A and B. The optimum range of vapor pressures, for reasons which will presently appear, is from about 5.5 mm. to about 7 mm., corresponding to points D and E.

The lamps of this invention are advantageously provided with a quantity of mercury that is sufficient to have mercury present in the liquid state preferably up to the top limit of the operation range herein contemplated, i. e. there should be only enough mercury in the lamp so that it will be entirely vaporized when the vapor pressure in the envelope reaches a value of about 10 mm. The amount of mercury needed to provide a vapor pressure of 10 mm. is 0.00098 gram per cubic inch of the space within the lamp envelope, and to obtain a vapor pressure of 4 mm. of mercury, it is necesary that there be present at least 0.00039 gram per cubic inch of the internal volume of the envelope.

Mercury in amounts within the above stated range is provided in order to enhance the intensity regulation characteristics of the lamp, which characteristics are apparent from a further reference to the curve shown in Figure 1. In a lamp which operates below the portion C, an increased fluctuation in the supply voltage with its corresponding increase in power to the lamp will cause an increase in lamp temperature resulting in an increase of mercury pressure and hence a higher output of ultraviolet radiations.

Contrasted with these results are those obtained by operation of the devices of this invention. When operating a lamp at a power loading that produces a vapor pressure of from about 5.5 mm. to about 7 mm., and there occurs a fluctuation in the supply voltages that increases the power to the lamp, the resulting increase in mercury pressure will cause a decrease in output of ultraviolet radiations as shown by the portion of the curve above point E. The increase in mercury pressure provides for less eflicient operation of the lamp and thereby 4 automatically compensates for the increase in power so that little change occurs in ultraviolet output. If then a subsequent variation in supply voltage reduces the power to the lamp below the normal value, the lamp operates at lower mercury pressure, corresponding to the portion of the curve below point D, which compensates for the lowered power input by providing for operation at improved efiiciency whereby the ultraviolet output again remains substantially constant.

The lamps of this invention will produce the desired intensity, i. e. between points A and B, when the mean temperature of the enclosing envelope is maintained between about 320 F. to about 363 F. To operate the lamps within the optimum range between points D and E, it is necessary to maintain a mean temperature within this range. By the term mean temperature of the enclosing envelope is meant the integrated average of all the temperatures within the envelope, which temperatures may be measured by known calorimetric methods.

The above stated temperatures were determined from the curve shown in Figure 2, wherein vapor pressures of mercury are plotted against temperatures in degrees Fahrenheit. This curve represents the conditions when an excess of liquid mercury is present. The desired mercury vapor pressures astound from the curve of Figure l are marked oil? on the curve of Figure 2 by the points a, a, e and b which indicate along the temperature ordinate the ranges of temperatures that are required in order to maintain the desired vapor in F. pressures (320, 335, 347 and 363, respectively.

Lamps according to this invention will maintain these temperatures when they are operated at a power loading of up to 3.5 watts per square inch of inner surface of the envelope. When the ambient air is at temperatures of from 40 F. to F., a power loading of from 3.0 to 2.5 will be required in order that the vapor pressure of mercury within the envelope will be about 7 mm. of mercury pressure. The watts required will be lower if the ambient air temperature is higher, as for example, at 101 F. a power loading of 2.4 is required to develop a vapor pressure of 7 mm., and a temperature of 143 F. will require a power loading of 2.0. At an ambient air temperature of 345 F., the required wattage is zero, and accordingly, it is not possible to operate the lamp of this invention in surroundings above this temperature, unless provisions are made to cool the lamp as with jackets containing a heat exchange fluid. Similarly, if ambient air temperature decreases, higher power loading is needed. For example, at 32 F., the power loading is 3.1, and at 0 F., the power loading is 3.5.

As is known in the art, sterlilization of liquids is efiective only when a number of correlated factors are carefully controlled. These factors include wavelength of the radiations, time of exposure, thickness of the liquid layer, and intensity of ultraviolet radiations. For reactions of the type herein contemplated, ultraviolet energy having a substantial part of its wave lengths shorter than 2600 Angstroms is required, and low pressure mercury lamps are used for this purpose. Many designs of equipment are known for controlling the time of exposure and thickness of the liquid layer factors.

For controlling intensity as provided by this invention, the lamp may be designed in many difllerent ways, two designs being shown, by way of illustration only and not of limitation, in Figures 3, 4, and 5 of the drawings, in which, unless otherwise indicated, corresponding elements are similarly numbered.

The lamp 1 in Figure 3 is in the form of two connected grids 2 and 3 between which extends a flat cell 4, through which the liquid to be treated is caused to flow. The cell 4 is formed of an ultraviolet transmissive material, such as, for example, fused quartz, and its thickness is preferably limited so that all particles of the liquid being treated are penetrated by the rays from the lamp. The lamp is provided with any desired type of electrodes, as

shown at 5, known to the art, such as cold cathode, hot cathode, activated metal cathode, metal on metal cathode, selfheating cathode, in a size adapted to properly carry the electric current. The lamp has a filling of rare gas, such as argon, argon-neon, xenon, and krypton at a pressure of, for example, between 1 and 50 millimeters for starting, and mercury preferably in an amount up to 0.00098 gram per cubic inch of volume.

The modification shown in Figure 5 is in the form of a tapered spiral and is provided with electrodes and filling similar to that described for Figure 3. The electrode chambers 7 are enclosed in jackets 8 which are interconnected by the conduit 9 so that cooling fluid may be passed through the jackets to assist in controlling the temperature of the lamp.

The above'lamps ofier important advantages for fields of application Where constancy of intensity of ultraviolet energy in Wavelengths shorter than 2600 Angstroms is essential such as in hospitals and in blood banks where ultraviolet is used for the sterilization of blood serum, plasma, etc., without the production of toxicity and where experienced physicists and engineers are not available to service, check and maintain accessory constant intensity controls of a phototube-electronic design.

In the foregoing, I have given specific values which enable anyone skilled in the art to practice the invention, but I desire to have it understod that my invention is not limited to these values as they can be varied with respect to each other provided always that the desired results set forth herein are produced.

What I claim is:

An electric discharge device comprising a sealed envelope containing at least a pair of electrodes, a filling of rare gas and mercury in an amount within the range of from 0.00039 to 0.00098 gram per cubic inch of space within said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 1,339,675 Snelling May 11, 1920 1,929,910 Zecher Oct. 10, 1933 2,054,846 Zillger Sept. 22, 1936 2,473,833 Stutsman June 21, 1949 2,478,119 Mitchel Aug. 2, 1949 2,485,398 Mandl Oct. 18, 1949 2,491,858 Hehenkamp Dec. 20, 1949

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