US2603299A

US2603299A - Electrostatic charge protected

Info

Publication number: US2603299A
Authority: US
Publication date: 1952-07-15
Anticipated expiration: 1969-07-15

Description

D. MAPES `uly 15, 1952 ELECTROSTATIC CHARGE PROTECTED CARBON DIOXIDE" DISPENSING SHIELD Filed Aprii 9 INVENTOR. a/z/ Mapoes BY ATTORNEY and issued-to my assignee.

condenser.

Patented July l5, 1952 UNITED STATES PATENT oFFlcl-jf `ELECTROS'IATIC CHARGE PRQTECTED CARBON'DIOXIDE DISPENSING SHIELD A --Daniel Mapes, Wes/t CaldwelLN., J., assigner to Specialties Development Corporation, 'Belleville, N., J., incorporation ofgNew Jersey 1 AppiiationApril 9, 1951 s1ria1NoQ219.7906 j i claims. (ci. 169-11) The present invention relates to shields or A.

or horn disclosed in the co-pending application for vUnited States Letters Patent-of Marcel K. Newman, `Serial No. 743,739, led April 25, 1947, now Patent No.v 2,569,490, dated October` 2, 1951,

In the aforementioned application, it is stated that high l'pressure liquid and/or gaseous carbon dioxide, whendischarged-through a nozzle surrounded by a shield or horn, isconverted to a mixture of snow particles and gas.- The carbon dioxide; upon leaving thenozzle, expands rapidly from a 'relatively high pressure to5nearly atmospheric pressure to produce a considerable amount of snow, While the shield provides for further ex- `pansion of gaseous carbon dioxide to suiiiciently reduce its forward velocity upon discharge from lthe shield to prevent substantial entrainment of air.

The shieldgenerally is formed of dielectric material having an innitesimal electricalv conductivity which material can be said to be nonconductive in the sense that it isk safe for `use in the vicinity of high voltage installations. Due

y2 inner Wall of the shield from one end to.l the other; y

It is believed that, by reason of suchleakage conductivity of the applied surface, a substantial number of the electrons forming the charge ilow along the interior surface of the shield in the direction of the expanding carbon dioxide stream and recombine with the carbon dioxide snow cryStals'WhiCh may have lost one or more electrons Yupon initial impactwith the shield adjacent the-nozzle, therebyeffecting substantial ncutralization or dissipation of the charge generated to minimize charge accumulation. .It is also' believed that the generation of charges is minimized because the specic inductive capacity `(dielectric constant) of the applied surface approximates that of carbon dioxide snow crystals.

"Since a generally accepted theoryv is that one .of

the functions upon which the value of an electrostatic'cl-iargeV depends is the difference of 'the dielectric constants of two bodies coming 'incontact `with each other, two bodies ofA materials having like dielectric constants 4should generate a minimum charge.

' While shields constructed in accordance with the disclosure of the aforementioned application have been' proven to be usefuhit has now kbeen discovered that-such shields can be greatly f improved insofar asvtheir safe use is concerned.

to the nature of this material, it has been foundl that, where the shield is not grounded as in the use thereof with portableflre extinguishing apparatusuadapted to be carried bythe operator, electrostatic charges are. generated and yare accumulated bythe horn whichactsin a senseas a 4'llflese charges, when grounded through the operator, mayubject the operator to severe and disagreeable shocks..

j'Ihe aforementionedv application Vproposes `to ,protect the operator against such shocks'by providing a tubular shield or horn,inwhich.,car bony dioxide snow is formed, wherein theinner 'wall isprovided. with .a surface -of. a materialhaving-a surface resistance of a value to eiect dissipation of electrostatic-charges without groundying" the shield and still maintaining the shield sufliciently non-conductive for safe .use in the vicinity of high voltage installations. This. is accomplished by providing Suche Surface on the Accordingly, the primary object of the present invention is to provide such .shields whichl can be used'with even'` greater safety in the vicinity of high .voltage electrical Vinstallations underf-a-ny conditions of relative humidity without sacrificing the effectiveness and which .electrostatic charges aredlssipated but in fact making it possible `to increase the .charge dissipating effectiveness.

'Another object is to-provide such improved Vshields without increase in cost.

Ay furtherfobject is lto providesuch improved shields which can be readily manufactured.

Other and further objects of the rinvention will be obvious upon an understanding of the illustrative embodiment about to 'be' described, or will be indicated in the appendedfclaims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice. Y

.In accordance with the present inventionpthe foregoing objects are accomplished by providing a tubular shield having an open end and an inlet at its opposite end for introducing carbon dioxide under pressure, the shield being constructed of dielectric material which normally is non-conductive in the sense that it has an iniiinitesimal electrical conductivity and is safe for use in the vicinity of high voltage installations, and the shield being provided on its inner surface with a substantially continuous lamina of a material having a surface resistance of a value to effect dissipation of an electrostatic charge without grounding the inner wall; the present invention being characterized in that the lamina extends from adjacent the inlet to an annular zone spaced from the open end, but closer to the open end than to the inlet, to provide an inner wall surface band of dielectric material 'at the open end, whereby the electrostatic charge. dissipating lamina is provided inthe zone ofmostv rapid extension of the carbon 'dioxidebutfdoes not constitute a hazard when using the shield in the vicinity of high voltage installations under any conditions of relative humidity. Y

A preferred embodiment of the invention has been chosenfor purposes of illustration and description, and is shown in the accompanying drawing, forming a part -of the specification,

wherein: n v

Fig. 1 is an elevational View, illustrating a portable lire extinguisher provided with a shield or horn embodying the present invention.

gFig. 2 is an enlarged sectional view of the shield alone taken along the line 2-2 on Fig. 1, illustrating the inner wall of the shield and the location of the electrostatic charge dissipating lamina thereon. y

Fig.. 3k is a further enlarged fragmentary sectional view taken along the linel 3-3 on Fig. 2,

lthe rthickness of the lamina shown being 'exag gerated.

Referring to the drawing in detail andmore particularlyto Fig. 1 thereof, there is shown a portable fire extinguisherV comprising a container ID for storing carbon dioxide under pressure, a

discharge head or valve Il for releasing the carbon dioxide from the container, a handle l2 on the discharge head to facilitate carrying the extinguisher, a tubular discharge directing shield or horn I4 supported by a bracket l5 on the container when the extinguisher is not in use, and a exlble hose IB connecting the outletof the discharge head to the inlet end of the shield.

As shown more particularly in Fig. 2, the shield I4 has an open end |`1 constituting, an outlet for carbon dioxide discharged through the shield and has an inlet at its opposite end provided by a nozzle or orice member I9 formed with an opening for introducing carbon dioxide to be expanded in the shield. `The shield generally is formed of a resin or plastic material of the type having high dielectric strength and infinitesimal electrical conductivity. For example, as shown in Fig.

A3, the shield maybeformed of one or more layers of fabric impregnated and coated with aphenolformaldehyde-type resin. A handle 20 is provided at the inlet Vend of the shield tofacilitate manual handling thereof.

In accordance with the invention, as shown in Fig. 2, the inner wall surface ofthe shield is provided with a substantially continuouslaminaJ 2| gof anV electrostaticy charge dissipating material which extends from adjacent the nozzle l 9 to an annular zone 22 (shown in broken lines) near the open end, but spaced therefrom, to provide an inner wall surface band 24 of the non-conductive material of which the shield is formed. By so setting back the charge dissipating lamina, the lamina, which is more conductive than the shield material, does not constitute a hazard when using the shield in the vicinity of high voltage installations. Also, the band 24, by being less conductive than the lamina, increases the overall surface resistance of the inner wall surface measured from the open end l1 to the opposite end, this characteristic being an advantage as will be described hereinater. v

The lamina 2l may be applied asta coating of a material which when dried and/or cured has a surface resistance of about .5 to about 500 megohms per inch under any conditions of relative humidity. Such a material, as described in theY aforementioned application, generally comprises colloidal carbon such as carbon black and a vinyl-type resin, dissolved in suitable solvent or thinner which is driven off during the drying or curing of the coating. Ten parts by weight of carbon and resin may contain between 1 to 9 'weight of resin.

parts V,by weight of carbon and 9 to 1 parts by It will be appreciated that other compositions or materials having similar surface conductivities or resistances may be utilized in practicing the 'present invention. The thickness of the lamina or coating may be varied within suitable limits to vary the conductivity or surface resistance thereof. A coating having a thickness of about .0001 inches has been found to be effective.

Usually, the discharge rate of carbon vdioxide through the nozzle is increased with the size of the extinguisher, that is, the weight of the carbon dioxide stored in the container, and the overall length L of the shield is'increa'sed with the discharge rate to provide for suiiicientrexpansion of the carbon dioxidewithin the shield to reduce its forward velocity upon leaving the shield toa value which will not cause substantial' entrainment of air. It isgenerally believed that thelength of the zone in which electrostatic charges are generated is dependent upon the discharge rate `and the lineal velocity of'the snc-w particles adjacentthe nozzle and the velocity of the expanding gaseous carbon dioxide propelling these particles through the shield. Thus, it has been found desirable to provide the coatingin the zone where the majority of electrostatic charges are produced and forwardly thereof to conduct the charges pickedup by the shield back into the mixture of gaseous carbon dioxide and snow particles whereby the generated charges are substantially dissipated. Itis also believed that the value of the charge generated is not a function of the over-all length of the shield, hence the length Z of the zone in which the coating is provided is a function of the discharge rate rather than a` function of the length of the shield'.V

Based on observations, it is believed, for e'xample, that for a certain .discharge raterequiring a shield of about fifteen inches in lengthv` a coated zone having a length of about teninches is adapted to Aeffectively dissipate the charges; for discharge rates requiring atwenty inc h shield a fifteen inch coated zone is sufficient; andvfo'r a thirty inch shield a twenty inch coated zone is sufficient. This leaves considerable leeway in adjusting the width W of the uncoated band 24.

VIn practice, it has been found that thevoblength L of the shield, except possibly'in extremeinches to provide the optimum over-al1 resistance characteristics -while maintaining the, llength Z of the coated-zone of' a value tov effectively dissipate the generated charges. For

shields having a length of about thirty inches, the Width of the band may vary between one and ten inches to provide a wide range of over-all resistance characteristics. In most applications, it is desirable that the band have a width of at least about 29% of the over-al1 length of the shield to set back the coating a sufficient distance to guard against direct contact of live high voltage current conductors therewith.

In this manner, the lamina is provided in the zone in which the most violent and rapid expansion of carbon dioxide takes place and in which zone the major generation of electrostatic charges occurs, so that suchy charges are effectively dissipated. Since the forward end of the zone in which the lamina is provided is spaced from the open end of the shield, it has been found that the lamina may be slightly more conductive to enhance its charge dissipating eiectiveness without unduly reducing the over-all surface resistance of the shield measure from end to end and Without making the use of the shield in the vicinity of high voltage installations hazardous. Thus by utilizing the present invention, a shield may be provided with a lamina of high charge dissipating efectiveness and a safe over-all surface resistance, the exact Width of the band and the location of the lamina usually being governed by the length of the shield and the discharge characteristics of the nozzle i9 at the inlet.

In each instance, it is desirable that the lamina be suiiiciently conductive at low relative humidities where electrostatic charge generation is more pronounced to effectively dissipate such charges Vand yet be suiciently non-conductive to effectively resist high voltages under conditions oi extremely high relative humidity.

A shield having an over-all surface resistance of at least twenty-live megohms measured under conditions of extremely high relative humidity (that is, between about 70% and nearly 100%) is considered acceptable and safe for use according to present standards under any conditions of humidity in the vicinity of exposed conductors carrying electrical current at potentials or" about 125,000 volts. n

In order to illustrate the present invention more specically, a coating composition was prepared by dissolving approximately five parts by weight of vinyl acetate-vinyl chloride copolymer resin in ninety parts by weight of methyl isobutyl ketone and then dispei'sing therein ve parts by weight of colloidal carbon. Two clean shields constructed of the materials specified herein and having an over-all length of seventeen inches were masked at the open end to cover an annular band at the inner Wall surface thereof yhaving a width of about four and one-half inches.

With the nozzle removed and the inlet end closed, the composition was poured into the shields, While in upright position, up to the lower margin of the masked band. The composition was then withdrawn by opening the inlet end, and the shields were centrifuged while in a position to expel the excess coating through the inlet end opening and while being rotated about their longitudinal axes. The shields were placed in an oven for one hour in which a temperature of ,about225" F; wasgrnaintaned, whereupon the solventoffthe composition was driven off and a resinelike. iilm,c0ating or lamina was provided.

A,The maskingA 'material' was then' removed.

V`These shields were Vhumidirled 4 by supporting the'same over-water invia large crock and placing the Acroci: in fan oven for seventy-two hours in which. a temperature of about 90? was maintained to produce an extremely high relative humidity. The l resistance ofv the vshields was determined on galvan@meterV` equipmentl using, as

one electrode, the'inletopening of the shields,

and, as the other electrode, a spot at the open end of the shields. Contact to this spot was made by the use of a piece of metal foil which was clamped to the surface of the spot, the metal foil being approximately one-eighth of an inch from the very end of the shields. The over-al1 resistance of these shields was found to be as follows:

Shield Resistance lvfegohms N o. 1 28.0 No. 2 29. 0

Three shields were prepared and tested in the foregoing manner having an uncoated band of three inches in Width, and were found to have an average over-all resistance of 19.6 megohms. This demonstrates the effect of width of the band on over-all surface resistance.

Three shields were preparedand tested at relatively low humidity having an unc-cated band of four and one-half inches in width, and were found to have an average over-all resistance of about 500 megohms, thus demonstrating the effect of humidity on resistance.

All of these shields, upon being subjected to a carbon dioxide discharge test at low humidities, indicated that the coating as applied herein dissipated more than of the charge normally accumulated on uncoated shields, in spite of the relatively wide uncoated band at the open end of the shield.

From the foregoing description, it will be seen that the present invention provides a greatly `improved shield for dispensing carbon dioxide snow is formed having an open end and an inlet at the opposite end for introducing carbon dioxide under pressure, said shield being constructed of dielectric material which normally is nonconductive in the sense that it is safe for use in the vicinity of high voltage installations, and said shield being provided on its inner wall surface with a substantially continuous lamina of a Inaterial having a surface resistance of a value to eiect dissipation of an electrostatic charge without grounding the inner Wall, said lamina extending from adjacent said inlet to an annular zone spaced from said open end but closer to said open end than to said inlet to provide an inner wall surface band of dielectric material at said open end, whereby the electrostatic charge dissipating lamina does notconstitute a hazard when using said shield in the vicinity of high voltage installations, said shield having an overall surface resistance of at least twenty-five megohms measured from said open end to a point adjacent said inlet under conditions of extremely high relative humidity.

2. A shield according to claim 1, wherein said V10 band has a minimum width of about one inch. 3. A shield according to claim l, wherein said No'references cited.