US2985337A - Vented enclosure for electrical devices - Google Patents

Description

May 23, 1961 E. w. KLEINPETER VENTED ENCLOSURE FOR ELECTRICAL DEVICES Filed 001;. l, 1956 ls fzo FIG.

FIG.2

United States Patent VENTED ENCLOSURE FOR ELECTRICAL DEVICES Edward W. Kleinpeter, Baton Rouge, La., assignor to Efliyl Corporation, New York, N. a corporation of Delaware Filed Oct. 1, 1956, Set. No. 613,148

1 Claims. (Cl. 220-88) This invention relates to electrical devices. More specifically the invention relates to electrical devices of the category which employ repetitive contacting means and are intended for service in areas or locations having actual or potentially combustible atmospheric conditions. Further, the invention relates to new and improved electrical devices of the above general character adapted to operate safely in said conditions. I

Inasmuch as electrical devices which include make and break contact or switching means represent potential spark sources, the various safe practice authorities in industry have established cetrain categories of services, and associated with such categories, have defined requisite classes of electrical equipment suitable for employment therein.

The generally accepted definitions of hazardous locations are as defined in Article 500 of the National Electrical Code, published by the National Fire Protection Association, Boston 10, Mass. particularly concerned with, but not limited to, devices for use in class Ig rou-ps C and D locations. Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufi'icient to produce explosive or ignitible mixtures. This classis further sub-divided into two divisions, wherein division 1 refers to conditions in which hazardous concentrations of gases are present on such a frequent, semi-continuous, or continuous basis that the hazard might almost be considered a continuing existent hazard. Division 2, on the other hand, of this class refers to locations (a) wherein the explosive or flammable vapor or gas will normally be confined in closed containers or closed systems, or (b) wherein the hazardous material will normally be prevented, by mechanical ventilation, from accumulation to hazardous concentrations, or (c) locations adjacent to division 1 locations. It will be seen, then, that the two divisions of class I are difiering primarily in degree of frequency or probability of existence of the hazard. A third characterization factor of the definitions of hazardous locations are the Ham mable or explosive chemical which is or can be present in the atmosphere. Thus, this factor, referred to as group is as follows: I

Group Typical Components acetylene.

hydrogen.

ethyl ether vapors, ethylene, or cyclopropane.

gasoline, hexene, naphtha, benzene butene, propane, alcohol, acetone, benzol, lacquer solvent vapors, or natural gas.

The present invention is eral conditions.

enclosures, or oil immersed mechanisms, are normally.

In addition, the materials in groups .C and D are not explosive in as Wide a range of concentrations. It will be understood, however, that all members of the several groups are quite hazardous gases or vapors. Thus, the rate of pressure rise of a 10 volume percent acetylene in air mixture, on ignition at F., is as high as 11,000 pounds per square inch per second. For a 35 percent hydrogen system, the rate of pressure rise is 11,000 (group B). combustion (9 vol. percent) the rate is 8,300, and for propane (5 vol. percent) the rate is 2,500 (group D).

The foregoing services have been thought to demand certain special construction for electrical systems which are capable of sparking by reason of electrical contacting.

The electrical contacting which can give rise to' such' sparking can be switch type contactors, commutator brush contacts, or other repetitive contactors, either slidingor make and break in character. In the case of brush type specified, the term explosion proof means that the conwithstanding an explosion of a specified gas or vapor which may occure within it, and of preventing the ignition of the specified gas or vapor surrounding the enclosure by sparks, flashes or explosions of the gas or vapor within. It will be seen that the foregoing requirement imposes strict design characteristics upon a specific unit. Thus, for explosion proof equipment for an installation fora class I, division 2 location wherein propane is the hazardous gas, a motor or switch casing would have to contain a pressure (with a safety factor of about 4) of about 400 pounds per square inch. By contain is meant the prodnets of combustion cannot leak through any apertures ,or fissures at a rate suflicient to ignite a surrounding flammable atmosphere; These requirements impose a definite handicap on industry generally from the cost viewpoint, and also because explosion proof equipment has certain disadvantageous physical characteristics' .More specifically, explosion proof motors, switch gear or the like, in order to deserve this designation, are necessarily constructed with heavy casings. or enclosures are sturdy or heavy enough so that the explosion of a combustible gas mixture in the free space defined by such casings would generate a maximum pressure which is onefourth or less the allowable pressure sustainable by these enclosures. It should also be noted that explosion proof equipment generally has no significant gas or ventilation apertures. In other words, if an explosion proof motor is used, no provision is made for ventilation and for heat removal to maintain the motor temperature rise below the maximum level. The excess heat generated within the operating motor must then be first absorbed by the extra heavy casing and transferred veying equipment such as fork trucks or lift trucks which are conventionally powered by direct current supplied by storage batteries. It is obvious that space is at a premium with reference to equipment of this type and in addition the unnecessary use of weighty electrical motors or controllers limits the mobility of such'transport equipment.

Patented May-23, 1961 In the case of an ethylene Generally, these cases I have now developed a new and improved type of electrical equipment, or more specifically, enclosures for devices'having mechanisms for repetitive electrical contacting. The apparatus of the invention is beneficial in being compact and light in weight, but nevertheless suitable for service in the locations discussed. It is an object of this invention to provide such equipment wherein internal sparking does not give rise to a combustion or explosion hazard, but nevertheless, the equipment does not require explosion proof contruction, hermetically sealed mechanisms, oil immersion or similar precaution my design features. Other objects will be apparent hereinafter. I

In its broadest form the invention comprises electrical devices of the character described (that is, characterized by having spark inducing contacting elements), said devices including an enclosure for the operative portions, the, enclosure having at least one aperture therein for permitting relatively free flow of gas therethrough, to

conjunction with the enclosure body, or casing body, a

heat absorbing or dissipating system which apparently reduces the temperature of any escaping gases below the ignition temperature of a surrounding flammable system. Although the particular proportions will vary somewhat according to the specific service conditions, in substantially all instances, the last cited attribute is satisfied when the proportions of the linear peripheral dimensions of the apertures, in feet, to the actual area of the apertures,

are in the proportions of at least feet per 1 sq. ft. In certain preferred embodiments, described more fully hereinafter, this attribute or working ratio is effectively increased by a perforated back up plate or member. Back up members of this type actually exert dual functions. Not only do they effectively increase the effective peripheral lengthzarea ratio by replacing a single aperture with a plurality of smaller apertures, but they provide mechanical protection against external shock or blows, thus further benefiting the safety features of the apparatus.

A wide variety of foraminous metal plates are suitable for purposes of the present invention. In all instances, the foraminous plates are characterized by having tortuous, small, passages therethrough. Generally, these passages are small enough in cross section to prevent passage of particles of the order of above '0.001-in;. or 0.002-in. in diameter. In many cases, however, passages which permit passage only of particles smaller than 0.005-in. in diameter are perfectly satisfactory. In other words, the foraminae of the plates have effective diameters of from 0.00l-in. to about 0.005-in., the pre- Generally, plate thicknesses of about one-sixteenth to onefourth' inch are employed. When the thicker sections in this range are used, some minor increase in the proportions of plate area, to provide a unit area of effective vent, are required. Thus, if a one-fourth inch plate, having a porosity of about .50 percent, is used, the actual area to provide a square foot of effective vent area should be slightly over two square feet. When a thickness of one-sixteenth inch is employed, an actual area of two square feet is used to provide an effective vent area of one.

Figure 2, is an illustration of a preferred embodiment of a foraminous plate, with a back up plate; as an assembly.

In order to more readily illustrate the invention, the

following describes performance of a typical embodiment thereof under test conditions equivalent to operation in a defined hazardous atmosphere. The apparatus and test apparatus are illustrated by Figure l.

Referring to Fig. 1, a motor 11 is provided of the commutator or brush type, having a junction box 22 for receiving the power supply lines 23 for transmitting power 'from 'a supply source 24. The armature shaft bearing end assemblies 13 protrude from the end of the motor casing. An aperture 12 on the commutator end of the motor is visible. the motor housing and on the end plate of the enclosure.

All such apertures are covered by a. tight1y fastened foraminous porous metal plate. The ratio of the actual aperture area to internal free spacewas 56 sq.--feet to- 100' cubic foot. The foraminous plate was a sintered bronze plate (about 90-94 percent copper, the remainder being tin). The density of the plate was 0.16 pound per cubic inch. The plate provided tortuous passages having effective openings of about 0.0015 inch in diameter. The plate was designed to provide an effective vent ratio of about28 square feet per- 100 cubic feet of enclosed or internal free space, having aporosity of about From preceding explosion experiments, suchan effective vent ratio, for a 5 vol. percent propane-air 50 percent.

explosion, in a vented, generally cylindrical chamber, having a lengthzdiameter ratio of about 2.30, would be expected to generate a pressure of less than one pound per square inch.

The motor 11 isprovided with a specially fitted spark plug 14, projecting to the interior of the case 26. Lead lines 19, 25 provide for high voltage to the plug upon A line 17, having a control valve 18 therein, provides for feeding to the interior of the motor casing any de-- sired combustible gas mixture. A second vapor line 15 connectsfrom the interior of the motor casing to a pres-- sure gauge 16.

In operation, the gas combustible mixture, as defined more particularly hereafter, was introduced by passage through the feed line 17. After flow for sufficient period of time to assure thatthe entire interior of the casing free space was occupied by the combustiblemixture, a spark was applied by the plug 14, and the pressure riseengendered by the combustion within the casing was measured by a particularly sensitive manometer gauge 16.

A. series of test operations were performed. In this series oftests, a; mixture of, propane gas; and air, .having Other apertures were provided around 5 percent by volume of propane, was fed into the motor 11. The mixture was then ignited by the sparking plug 14. After each ignition, the flow of gases was continued for a sufficient period to purge the casing 26 of combustion products and replace with a fresh combustible mixture. In a series of some fifty operations as described, the maximum pressure recorded did not exceed about 0.01 pound per square inch. It is thus apparent that, using the apparatus of the present invention, a practically negligible pressure rise was encountered.

Following the above tests, a second series of operations were conducted in the same manner as those described above. However the motor 11 was entirely surrounded by a steel enclosure (not shown) so that upon ignition of the combustible gas mixture through the motor housing and the porous enclosure plates 12, the space defined by the exterior of the motor and the interior of the receptacle was also filled with the combustible gas mixtures. Prior to each ignition, this space was filled with a combustible gas mixture corresponding to the mixture fed to the interior of the motor 11. This series of tests was designed to determine the adequacy of the enclosure plate 12 to prevent ignition of the combustible gas mixture surrounding the exterior of the motor 11. A repetitive series of tests similar to those described above was made, and in no instance was there ignition of the gases on the exterior. In several of these operations, the flow of combustible gas was continued to the interior of the motor enclosure so as to sustain a steady flame. Even under such rigorous conditions there was no ignition of the flammable mixture outside the motor in the special test enclosure.

The foregoing series of operations were repeated using ethyl chloride as the flammable component. The results obtained were substantially identical.

In many instances, the invention is employed in converting general purpose equipment. In certain cases this requires modification of conventional cover plates which may be in physically exposed locations. Thus, a cover plate for a direct current controller may be exposed to a certain amount of external shock or rough handling. In such instances it is found highly desirable to employ composite structures, as illustrated in Figure 2 which shows a composite, arcuate cover plate or member for applying the principles of this embodiment.

Referring to Figure 2, a cover plate is shown, including a foraminous plate layer 31 and a cover plate member 32 coextensive in dimensions. A series of holes 33 around the boundary of both plates provide for fastening the assembly to the body of the enclosure (not shown) with cap screws. Suitable gaskets are usually provided under the rim of the plate 31 in making an installation. A series of regularly spaced holes 34, only in the cover plate member 32, provide the working apertures. These holes 34 are of relatively small sizeusually about onehalf to one inch in diameter. In designing an assembly of this type, the criteria of efiective vent area to enclose free volume, and the ratio of peripheral length to vent area is determined from the appropriate dimensions of the holes 34.

As already noted, in all cases the objects of the invention are attained by providing operative elements satisfying two criteria; viz., release to the exterior of the enclosure without 'generation of any appreciable pressure and concurrently, the prevention of any ignition of a comparable combustible atmosphere surrounding the embodiment of the invention. These criteria are satisfied by a combination of sever-a1 factors in certain degree. These factors include aperture area, provided 'will foraminous plates, wherein the efiective venting ratio is at least equivalent to 5 sq. feet to cubic feet of enclosed free space, and preferably a proportion of above 10 sq. ft.:100 cu. ft. The second important factor includes the peripheral length of the apertures compared to the area thereof, which, as already stated, should be in the proportions of at least 10 feet per square foot of each aperture.

As already discussed, the foraminous aperture closures should provide venting equivalent to at least 5 square feet per 100 cubic feet of free space. In determining the actual area of apertures required, the eifective area can be divided by the porosity of percent of void space, of the foraminous plate stock. In a typical example, then, for an enclosure defining 0.22 cubic feet of free space, the minimum effective venting area would be 1.58 square inches. Using a sintered, porous bronze plate of 50 percent porosity would then require a minimum actual aperture area of about 3.16 square inches. In most instances, it will be found practical and desirable to provide an efiective venting area of 10 square feetzlOO cu. ft., or in the present instance, an actual aperture area of 6.3 square inches.

The free space of any given enclosure can be determined by calculation. Alternatively, the free space can be filled with readily flowable particles such as BB shot, and the volume of the shot then measured.

The requisite length of periphery of the apertures is readily calculated. In a specific installation, the actual shapes and locations of the apertures are readily designed. Usually, in converting enclosures for motors according to the present invention, three or four elongated slot apertures around the motor casing, at the commutator end, will be used.

Having described the invention fully, what is desired to be claimed is:

An improved enclosure for electrical devices which are characterized by having spark inducing contactors, said enclosures being susceptible to having therein a combustible gaseous mixture including ethyl chloride, propane, or the like, and oxygen, while being surrounded by a similar mixture, the enclosure defining an enclosed free space in addition to the space occupied by the electrical device and comprising a metal casing having at least one opening therein, said opening being covered by an assembly including a foraminous metal plate and a metal cover plate contacting the outer surface of said foraminous metal plate and substantially coextensive therewith, the foraminous metal plate having a thickness of from A to M4 inch, the foraminae thereof having effective diameters of up to about 0.005 inch, the said cover plate having a plurality of apertures therein, whereby a plurality of apertures closed only by portions of the foraminous metal plate are established, the total of said apertures providing an efiective vent area of at least 5 square feet per 100 cubic feet of free space within said enclosure, and the periphery of each of said apertures being at least 10 feet per square foot of aperture area.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,702 Koehring Apr. 30, 1940 2,323,146 Manney June 29, 1943 2,618,540 Teti Nov. 18, 1952 2,667,554 Shores Jan. 26, 1954 2,801,768 Immel Dec. 20, 1954 2,810,631 Kanenbley Oct. 22, 1957 FOREIGN PATENTS 531,610 Great Britain Ian. 8, 1941

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