US2517255A - Space heater - Google Patents

Description

Aug. 1, 1950 F. c. STREBE EI'AL SPACE HEATER 2 Sheets-Sheet 1 Filed April 9, 1946 jivl/f/vrops FkA/vk C. 579555.. M0005 6. EL/Aso/v.

FOR THE F P W Patented Aug. 1, 1950 UNITED STATES PATENT OFFICE SPACE HEATER Frank C. Strebe and Marcus 0. Eliason, Los Angelcs, Calif.

4 Claims.

Our invention relates in general to a method of and apparatus for heating fluids and, more specifically, to a method and apparatus which are particularly adapted for heating and circulating a fluid such as the air in a room. For convenience in disclosing our invention, the present specification is primarily concerned with an application of our method and apparatus to heating and circulating air, although it will be understood that the invention is susceptible to other applications which will be apparent to those skilled in the art and, consequently, is not to be limited to the specific application discussed herein.

A primary object of our invention is to provide a method of and apparatus for heating a body of fluid, such as air, with radiant energy by controlling and confining the radiant energy in such a manner that it repeatedly traverses the body of fluid until substantially all of the energy, or at least a substantial portion thereof, is absorbed by the body of fluid.

More specifically, the primary object of our invention is to provide a method of and apparatus for subjecting a body of fluid to thermal radiation from a source of radiant energy, and for controlling the radiation in such a manner that it repeatedly traverses the body of fluid by reflection from, absorption by, and re-radiation from other bodies until substantially all of the radiation is absorbed by the body of fluid. The radiant energy absorbed by the fluid in this manner increases the temperature thereof, and the resulting change in the density of the fluid causes the fluid to move by convection, another object of our invention being to provide a method of and apparatus for heating the fluid in such a manner that convection currents of extremely large magnitude are created, thereby insuring rapid circulation of the fluid.

The large convection currents created by our method and apparatus are particularly adapted for circulating the air in a room to provide a more uniform room temperature distribution than is obtainable with most conventional heating installations and methods, and are adapted for circulating the air to provide such a room temperature distribution in a much smaller interval of time, which is still another object of our invention.

The foregoing objects and advantages of our invention, together with various other objects, advantages, and features thereof which will be made evident hereinafter, may be realized by means of the exemplary embodiments which are illustrated in the accompanying drawings, and

which are described in detail hereinafter. Referring to the drawings, which are for illustrative purposes:

Fig. 1 is a vertical sectional view of a heater which embodies the fundamental principles of our invention;

Fig. 2 is a front elevational view of the heater shown in Fig. 1, parts of the heater being broken to reveal the internal structure thereof;

Figs. 3 and 4 are perspective views of heating units for use with the heater shown in Figs. 1 and 2;

Fig. 5 is a vertical sectional View of another heater which embodies the fundamental principles of our invention; and

Fig. 6 is an elevational view of the heater shown in Fig. 5 with parts of the heater broken away to reveal the internal structure thereof.

In disclosing our invention, it is convenient to consider in a general manner some of the characteristics of thermal radiation and the effect of various materials thereon. The thermal radiation emitted by any body represents a continuous distribution of energy which depends upon the characteristics of and the temperature of the body. According to the Stefan-Boltzmann law, the total radiation from any body is a function of the absoiute temperature of the body to the fourth power, and is a function of the emissivity of the body. The emissivity of the body is equal to the ratio of its radiating power to that of a perfect black body, a perfect black body being one that absorbs all of the radiant energy of any wave length which falls upon it.

When radiant energy falls upon any body, with the exception of a perfect black body, some of the energy is absorbed by the body and the remainder thereof is reflected thereby. The absorptivity of the body is the fraction of the radiant energy falling upon it that is absorbed, and the reflectivity of the body is the fraction of the radiant energy falling upon it that is reflected. In the case of a perfect black body, all of the radiant energy falling thereon is absorbed so that its absorptivity is equal to unity and its reflectivity is equal to zero.

Another characteristic of thermal radiation is that any body in thermal equilibrium at a given temperature emits the same kind of radiation, i. e., radiation of the same wave length, that it absorbs, and, according to Kirchhoffs law, the emissivity of the body is equal to its absorptivity for radiation of the same wave length. For practical purposes, Kirchhoifs law may be approximated by the statement that the absorptivity of any body is equal to its emissivity for any radiation in the invisible portions of the spectrum. In the case of a perfect black body, the emissivity and absorptivity thereof are equal regardless of the wave length of the radiation.

In actual practice, a perfect black body, i. e., a body having an emissivity and absorptivity of unity and a reflectivity of zero, is impossible to attain, although the characteristics of a perfect black bodymay be approximated quite closely by a heated enclosure having a blackened inner surface, or by an enclosure having an inner surface with a reflectivity which is equal to unity. For the purposes of the present specification and the claims, the term black body shall be defined as a body comprised of a material having an emissivity which approaches unity as compared to a perfect black body having an emissivity of exactly unity. Various materials have emissivities which approach unity, lampblack, for example,. having an emissivity of around 0.95. Various other non-metallic materials have emissivities ranging from 0.85 to 0.95, and some metallic materials, such as black iron, or iron having an oxidized surface, have emissivities ranging up to 0.90.

Bodies having low emissivity values may be employedas reflectors for thermal radiation since the absorptivity thereof is low. Polished metal surfaces have relatively low emissivities, a polished silver surface, for examplahaving an emissivity which ranges from 0.025 to 0.035 between temperatures of '70 to 1000 degrees Fahrenheit. Polished aluminum surfaces also make good reflectors, the emissivity thereof ranging from 0.05 to 0.075 in the same temperature range.

Our invention involves the utilization of some or all of the principles indicated by the foregoing general considerations of the characteristics of thermal radiation, and the manner in which these principles are related to our invention will become apparent hereinafter. In general, our invention involves a method of" and apparatus for heating a fluid, such as air, by subjecting the fluid to thermal radiation from a source of radiant, energy, and by controlling the thermal radiation in such a manner that more efficient heating of the fluid is attained than is possible with. most conventional methods. Certain practices of our invention involve the employment of reflecting, means for confining the thermal radiation by reflecting it back into the fluid being heated, and involve the employment of a black body for absorbing the thermal radiation and for re-radiating it into the fluid being heated, whereby the radiation repeatedly traverses the fluid being heated until. a substantial portion of the radiant energy is absorbed by the fluid.

Referring particularly to Figs. I and 2, the heater illustrated therein is adapted to be mounted in a recess in a wall (not shown) and may be termed a wall heater. The heater includes a housing Iii having an inlet II at the lower end thereof for cold air, and having an outlet 12 at the upper end thereof for heated air, the heated air being directed toward the outletby bafile means l3 so that the air flows through the heater as indicated by the arrows in. Fig. l.

The housing If) includes a platform I4 having a core or. black body mounted thereon, the black body having a passage l6 therethrough. The heater includes reflecting means comprising apair of reflectors l8 which are disposed on opposite sides of the black body I5, and which cooperate to define a heating space l9 therebetween.

Electrical heating means 20 for providing a source of radiant energy is mounted on the black body 15, and preferably comprises a plurality of spaced resistance heating elements or conductors 2i in series. In order to prevent any possibility of a short circuit resulting from unequal sagging of the conductors. 21 when heated, we prefer to employ tensioning means 22' for maintaining a predetermined spacing of the conductors.

Considering the structure of the wall heater shown in Figs. 1 and 2 in more detail, the housing it includes a rear wall 26, a pair of end walls 2'1, an upper wall 28, and a lower wall 2-9, the end walls and the upper and lower walls being formed to provide a peripheral flange 30 at the front of the housing. A front wall or plate 3! is secured. to the flange 0, and is pro vided with a plurality of openings at each end thereof to define gratings or grilles 32 and 33 for the inlet l! and outlet i2, respectively. When the heater is disposed in a suitable recess therefor in a wall (not shown), it may be secured to the wall by means of screws or the like which extend through the front plate 3! and flange 30.

The baffle means it includes bafiies 36 and 3'! whch divide the upper portion of the housing it into a plurality of passages for the heated air, and which deflect the air forwardly and. outwardly through the outlet l2 as indicated by the arrows in Fig. 1. The baffles 36 and 31 are provided with flanges 38 which may be spot welded, or otherwise secured to the end walls 21 of the housing In to support the baiiies.

The platform It is provided with a flange 40 which may be spot welded or otherwise secured to the rear wall 25 and end walls 2'! of the housing [0. The platform It is provided with a central opening t! which registers with the passage l5 through the black body l5, portions of the platform around the periphery of the opening ll being bent to provide tabs it which serve to position and retain the black body. As best shown in Fig. 1, the platform It is provided with a pair of rows of openings 43 therein to permit cold air from the inlet H to flow upwardly between the black body l5 and reflectors is as indicated by the arrows, only one opening 43 in each row being shown in the drawing. The platform I4 is also provided with a third row of openings l l which admit cold air into the space between the rear reflector l8 and the rear wall 26 of the housing it, only one of the openngs ts being shown in the drawings.

As best shown in Figs. 1 and 3, the black body I5 is preferably a pyramidal body of a generally trapezoidal cross section. The passage 56 through the black body l5 converges upwardly, as best shown in Fig. 1, so that the passage is of a generally triangular cross section, the inlet end i! of the passage being substantially larger than the outlet end 38 thereof for reasons to be discussed hereinafter.

Referring particularly to Fig. 3, which illustrates one construction of the black body l5, the black body includes a pair of trapezoidal supporting members or end members 50 of a suitable electrical insulating material, each end member being provided with a pair of grooves 51 therein which converge upwardly toward each other. The grooves 5| preferably extend downwardly from the upper ends of the end members 50 and terminate a short distance above the lower ends thereof for a reason to be discussed hereinafter.

The black body l5 includes a pair of lateral members or plates 52 which are inserted into the grooves 5| in the end members 50. The plates 52 are formed of a materal having a high absorptivity and emissivity so that the characteristics of the black body 5 approach those of a perfect black body. For example, the plates 52 may be formed of materials such as black iron, iron having an oxidized surface, black-anodized aluminum, or any suitable material having a high emissivity.

Another black body 55 is illustrated in Fig. 4, the configuration of the black body 55 being substantially the same as that of the black body IS. The black body 55 is also provided with an upwardly converging passage 56 therethrough which is substantially identical to the passage |6 through the black body l5. The construction of theblack body 55 differs from that of the black body I5 in that the former includes a pair of end members 5'! and a pair of lateral members 58 which formed in one piece to provide bite-- gral unit. The black body 55 is formed of an electrical insulating material having a high emissivity and absorptivity so that the characteristics thereof approach those of a perfect black body. The black body 55 may be formed of a material such as ceramic which is impregnated with lamp-- black, although any other suitable material may also be employed.

The reflectors l8 are provided with flanges 6| thereon which may be spot-welded, or otherwise secured to the end walls 21 of the housing It. The surfaces 62 of the reflectors l8 which bound the heating space H! are preferably provided with a finish such that the reflectivity thereof is relatively high so that substantially all of the radiant energy falling thereon is reflected back into the heating space as will be discussed in more detail hereinafter.

As best shown in Fig. 3, the end members 50 of the black body I5 are provided with a plurality of spaced notches 63 therein which receive the conductors the electrical heating means 25, the latter preferably including a single resistance wire 64 which may be wound around the black body l5 and disposed in the notches 63 in the manner illustrated to provide the spaced conductors or resistance heating elements 2|. The ends 55 of the wire 54 may be connected to a suitable source of electrical energy (not shown).

The electrical heating means 28 may be mounted on the black body 55 in a similar manner as shown in Fig. 4, the end members 51 being provided with a plurality of notches 56 therein to receive the wire 56. Fig. 4 also illustrates an.- other manner in which the wire 54 may be wound around the black body to provide the spaced conductors 2|], the ends 55 of the wire being connected to a suitable source of electrical energy (not shown).

In both cases, the notches 63 and 65 in the black bodies and 55 are non-uniformly spaced in such a manner that the spacing between the conductors is greater at the bottom of the black bodies than at the top thereof. Consequently, when current flows through the conductors 25, the upper portions of the black bodies l5 and 55 are heated to a higher temperature than the lower portions thereof for reasons to be discussed hereinafter.

As best shown in Figs. 1 and 3, the tensioning means 22 includes a pair of spacing members 69 of insulating material which are disposed on onposite sides of the black body 55 intermediate the end members 50, and which are slidable relative to the lateral members or plates 52, the spacing members having a plurality of notches 10 therein which receive the conductors 2| to maintain the spacing thereof. The spacing members 69 are urged transversely of the conductors 2| by resilient tensioning members or springs H which are attached to the lower wall 29 of the housing ID to maintain some tension in the conductors when heated by current flowing therein. The tensioning means 22 thus prevents unequal sagging of the conductors 2| when heated so that it is impossible for one of the conductors to contact an adjacent conductor, thereby eliminating any possibility of short circuiting the electrical heating means 20. The tensioning means 22 also eliminates any possibility of grounding the electrical heating means 20 by contact with the plates 52 if these are metallic. It will also be noted that since the lower ends of the grooves 5| which receive the plates 52 are spaced from the lower ends of the end members 50 as previously described, the plates 52 are electrically insulated from the housing in of the heater by the end members. This construction prevents grounding the electrical heating means 2|] to the housing I0 in the event that the wire 54 should break and contact the plates 52 when metallic plates are employed, thus eliminating any danger of shocking persons touching the heater, which is an important feature of our invention.

As best shown in Fig. 4, the electrical heating means 20 on the black body 55 may also be provided with tensioning means including a pair of spacing members 12 which are slidable relative to the lateral members 58 and which are provided with notches 13 therein for the conductors 2|, only one of the spacing members being shown in the drawings. The spacing members 32 may be urged transversely Of the conductors 2i by means of springs, for example, which are not shown in the drawings.

In considering the operation of the wall heater illustrated in Figs. 1 to 4, the description of the operation will be limited to a consideration of the black body l5, the characteristics of the black body 55 being substantially identical to those of the black body I5. When current flows through the electrical heating means 20, the resistance heating elements or conductors 2| become heated. Substantially all of the heat developed in the conductors 2| is radiated into the heating space I9 between the reflectors I8, although a small amount of the heat may be dissipated into the air in the heating space by convection, and a small amount may be dissipated by conduction through the end members 56 of the black body l5.

Thus, the electrical heating means 20 acts as a source of radiant energy, the thermal radiation emitted thereby being radiated into the heating space 59 largely in the form of infrared rays. Part of the radiant energy emitted by the electrical. heating means 2% falls on the black body i5 and is largely absorbed thereby because of the high absorptivity thereof, this absorbed radiation being re-radiated into the spaces between the black body and the reflectors |3, and into the passage i5 through the black body. Part of the radiant energy emitted by the electrical heating means 26, and part of the radiant energy which is re-radiated by the black body I5, falls upon the reflectors l8 and is largely reflected back into the heating space l9 because of. the high'reflectivity' of the surfaces 62' thereof. Since an enclosure having perfectly reflecting walls has characteristics approximating those of a *fect black body, as previously mentioned, the reflectors 13 also act as a black body to a certain extent, depending on the reflectivity of the surfaces 62 thereof, and on the extent to which the heating space iii is enclosed thereby.

The radiant energy emitted by the electrical heating means 2i! is repeatedly absorbed and reradiated by the black body l5, and is repeatedly reflected back into the heating space 19 by the reflectors ES. The net result of the repeated absorption, re-radiation and reflection of the radiant energy is that the energy repeatedly traverses the body of air in the heating space it until substantially all, or at least a substantial portion of the radiant energy emitted during any given time interval is. absorbed by the air in the heating space, thereby heating the air to a considerably higher temperature than is possible with conventional methods and apparatus.

As the temperature of the air in the heating space it increases, the density thereof decreases so that the air moves upwardly out of the heating space by convection in a continuous stream, some of the air moving upwardly through the passage it and some moving upwardly through the openings and through the spaces between the black body i5 and the reflectors 58. Since the conductors 2% are more closely spaced at the upper end of the heating space 19 than at the lower end thereof, the rate at which the temperature of the air increases is higher at the upper end of the heating space than at the lower end so that the rate at which the density of the air decreases is accelerated as the air reaches the upper portions of the heating space. Consequently, the velocity of the air flowing upwardly through the spaces between the black body l5 and the reflectors l8 and through the passage It increases rapidly as the air reaches the upper portions of the heating space i9. Moreover, the converging configuration of the passage it also serves to accelerate the air as it flows upwardly therein.

The increased heating rate in the upper portions of the heating i9 and the convergence of the passage i=3 accelerate the air stream to such an that the velocity of the air stream at the upper end of the heating space is extremely high. Consequently, the air stream, which is deflected outwardly the bafi'le means 13, extends into the room being eated for a considerable distance, thereby tending to heat the room in a substantially uniform manner. Moreover, the high velocity of the stream causes the air in the room to circulate through the heater rapidly so that the room is heated in a short interval of time, cold air being drawn into the heater through inlet 9 i to rep-lace the heated flowing through the outlet l2.

In effect, our invention provides a method of circulating air a relatively high temperature by forced convection instead of circulating air at a relatively low temperature by free or natural convection as the case with many conventional heaters, thereby providing more uniform heating in a shorter time interval. Thus, our invention employs a given quantity of heat more effectively and more efficiently than heaters which rely upon ree convection to circulate the air.

Referring to Fig. 1, it will be noted the baflle 3? is so positioned that it divides the stream of heated air leavirn the passage is through the black body [53. This construction causes the two portions of the stream of heated air leaving the passage it to flow past the upper ends of the spaces between the black body I5 and the refiectors E8 in such a manner that low pressure areas are created, the action of the baffle 31 being analogous to the action of a venturi. These low pressure areas further increase, the rate of flow of heated air through the spaces between the black body and the reflectors [8, which is another important feature of our invention.

It will be apparent that although the reflectivity of the reflectors it may approach unity, in actual practice the reflectivity thereof will be somewhat less than unity depending upon the material employed. Consequently, although the reflectors is largely confine the radiant energy in the heating space l9 so that substantially all of the energy is absorbed by the air therein as the energy repeatedly traverses the heating space, some of the radiant energy will be absorbed by the reflectors. The quantity absorbed in this manner will, of course, depend upon the absorptivity of the reflectors i8, which is preferably quite small.

Some of the radiant energy which is absorbed the reflectors is will be l e-radiated back into the heating space and some will be dissipated by conduction and convection. The remainder of the radie. it energy absorbed by the reflectors 3 will radiated into the spaces between the reflectors and the rear and front walls 26 and 35 of the housing. As will be apparent from the arrows in Fig. 1;, some of the cold air from the inlet i i will flow through the openings M into the space between the rear reflector it and the rear wall. of the housing it, thereby absorbing some of the energy which is radiated into this space by the rear reflector. The stream, of air flowing through the space between the rear Wall 2-6 and the rear reflector it also cools the walls 26, 2'5, and 23 of the housing it by convection so that the temperature thereof is only a few deabove room temperature. In actual tests of the wall heater illustrated with a 1000, watt electrical heating 253', thev temperature of the rear wall it was found to be of the order of magnitude of only 6 to 8 degrees Fahrenheit above room temperature. Consequently, it is unnecessary to insulate the housing Hi from the structure of the wall in which the heater is mounted, which is another important feature of our invention;

Although we prefer to: employ the electrical heating means til-as previously discussed, our invention is not necessarily limited thereto. For example, we may employ heating means such as the burner '55 shown in Figs. 1 and 2', the burner being connected to a source (not shown) of com.- bustibie fuel, such as any suitable liquid or gase ous fuel.

In general, the operation of the heater with the burner litin use will be similar to that discussed previously except that only part of. the heat produced by combustion at the burner will be transmitted to the air stream by radiation from the black body [5 and reflection from the reflectors E8, the remainder of the heat being transmitted to the air by mixing of the air and the products of combustion. The quantity of heat energy which is absorbed by the black body and rc-radiated therefrom in the manner previously described depends to a certain extent on the radiation characteristics, i. e., emissivity, of the products of combustion of the fuel at the burner 15. For high concentrations and high temperatures, the emissivity of gases and flames is quite high so that a large portion of the heat energy developed by combustion may be absorbed by the black body I5. The energy absorbed by the black body 15 by radiation, and by convection, is re-radiated therefrom and is reflected by the reflectors !8 until absorbed by the air in the heating space i9 in the manner previously described.

The embodiment of our invention which is illustrated in Figs. and 6 may be termed a floor heater, and is similar to the wall heater disclosed previously. Consequently, identical numerals have been employed to identify corresponding components.

One of the principal difierences between the wall heater shown in Figs. 1 and 2 and the floor heater shown in Figs. 5 and 6 resides in the configuration of the housing ID, the configuration of the housing of our floor heater being similar to that of conventional floor heaters. The housing H] of our floor heater is provided with supporting legs 80, and the cold air enters the housing through arches between the legs, flows upwardly through the heater as indicated by the arrows in Fig. 5, and out vertically through a grating or grille 8| which provides the outlet 12, the baffle means 13 of the wall heater omitted from the floor heater.

The platform I4 of the floor heater shown in Figs. 5 and 6 supports the black body 15, which may be of the type shown in Fig. 3, or the type shown in Fig. 4, for example. Another principal difference between the floor heater and the wall heater discussed previously resides in the employment of a pair of reflectors I8 on each side of the black body 15. The additional, or outer, reflectors l8 serve to reflect back into the heating space It any radiant energy which is absorbed and re-radiated outwardly by the inner reflectors, thereby further reducing the temperature of the walls of the housing I0. It will be understood that the double reflector arrangement illustrated in Fig. 5 may also be employed in the wall heater shown in Figs. 1 and 2 if desired. As best shown in Fig. 5, the platform 14 of the floor heater is provided with a pair of rows of openings 82, in addition to the rows of openings 43 described previously, so that cold air may flow into the spaces between the inner and outer reflectors l8, as well as into the spaces between the inner reflectors and the black body l5.

The heating means employed in the floor heater shown in Figs. 5 and 6 may be the electrical heating means 20 as previously described, or may be a burner such as the burner 15 (not shown in Figs. 5 and 6). If the electrical heating means is employed, the floor heater is preferably provided with the tensioning means 22 for the reasons discussed previously.

The operation of the floor heater shown in Figs. 5 and 6 is substantially identical to that of the wall heater shown in Figs. 1 and 2. Consequently, the operation of the floor heater will not be described separately.

Thus, our invention provides a method of and apparatus for heating a fluid with radiant heat energy by controlling and confining the radiant energy in such a manner that it repeatedly traverses the fiuid until a substantial portion of the heat energy is absorbed by the fluid. The heat energy absorbed by the fluid increases the temperature and decreases the density thereof so that the fluid circulates by convection, the convec- 10 tion currents created being of such magnitude that the fluid is circulated at a high rate. The structure of our heating apparatus is such that the fluid, in eiiect, circulates by forced convection as compared to many conventional heaters which rely upon free convection. It will be understood, of course, that additional means for circulating the fluid being heated may be employed if desired and that our invention is not to be limited to circulation by convection alone.

Although we have described our method of heating fluids and have disclosed apparatus for performing our method, it will be understood that various changes, modifications, and substitutions may be incorporated in the apparatus disclosed without departing from the spirit of our invention, and we hereby reserve the right to the protection offered by the full scope of our appended claims.

We claim as our invention:

1. In a heater of the character described, the combination of a housing; a first pair of generally vertical reflectors in said housing and defining an upwardly extending space therebetween; a second pair of generally vertical reflectors in said housing outwardly of and spaced from the respective reflectors of said first pair to provide a pair of upwardly extending air passages which are located outwardly of said space and which are separated therefrom by the reflectors of said first pair; a black body in said space and spaced inwardly of the reflectors of said first pair, said black body being provided with an upwardly eX-- tending passage therethrough; and heating means in said space for radiating heat energy thereinto, said heat energy being absorbed and reradiated by said black body repeatedly, and being reflected by said reflectors repeatedly so that said heat energy repeatedly traverses said space and said passages, whereby air in said space and in said passages absorbs substantially all of said heat energy and moves upwardly out of said space and said passages by convection.

2. A heater as defined in claim 1 wherein the reflectors of said first pair converge upwardly with respect to each other to render said space upwardly convergent.

3. A heater as set forth in claim 2 wherein said passage through said black body is upwardly convergent.

l. A heater as set forth in claim 3 wherein said heating means comprises elements for radiating more heat energy into the upper portions of said space than into the lower portions thereof.

FRANK C. STREBE. MARCUS C. ELIASON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,332,852 Levy Mar. 2, 1920 1,669,588 Burns et al May 15, 1928 1,831,820 Noble Nov. 17, 1931 1,902,074 Holinger Mar. 21, 1933 1,923,083 Fisher Aug. 22, 1933 2,070,129 Ireland Feb. 9, 1937 2,351,466 Weida June 13, 1944 2,442,900 McCormick June 8, 1948 FOREIGN PATENTS Number Country Date 317,773 Great Britain Aug. 21, 1929

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