US2400719A - Apparatus for filtering gas - Google Patents

Description

May 21, 1946. W. E. sTAcKHoUsE APPARATUS FOR FILTERING GAS Filed OC.. 28, 1939 Patente/d May 21, 1946 UNITEDA STATES PATENT oFFlcE APPARATUS FOR FIIKTERING GAS Wilton E. Stackhouse, Springfield, Pa.,'ass'lgnor to The United Gas Improvement Company, a corporation of Pennsylvania ,Application October 28, 1939, Serial No. 301,678

-7 Claims. (Cl. 183-45) This application is a continuation in part of and replaces my copending application Serial Number 190,371; filed February 14, 1938, the latter having since been formally abandoned in favor of this application.

The invention pertainsmore 4particularly to apparatus for treating gas- The invention pertains more particularly to apparatus for thetreatment of combustible gas containing impurities suspended therein.

The invention pertains still more particularly to apparatus for filtering combustible gas containing particles of vapor phase gum.

L In the distribution and utilization of manufactured combustible gas, such as coal gas, coke oven gas, oil gas, carburetted water gas and the like, considerable trouble is encountered due to the presence` in the gas of particles of gum (the socalled vapor phase gum"). These particles are generally formed in suspension by )reactions between minute concentrations of oxides of nitrogen and certain ,hydrocarbons found in the gas. For a general discussion of vapor phase gum. reference may be had to Industrial and Engineering Chemistry, 26, 947 and 1028 (1934).

The gum particles are very small. They range upward from the threshold of visibility with the aid of the ultramicroscope to particles of a diameter of 3 mu, though those larger than 1 mu tend to settle out of the gas.

The formation of the particles takes place throughout the distribution system as long as there remain any nitric oxides present in the gas.

'I'hese gum`particles block the small orices of the control valves of low rate appliance burners, such as the needle valves of gas range and water heater pilots, and needle valves controlling the ow of gas to gas refrigerator burners. The consequent stoppage of the valves causes extinction of the burners associated therewith so that the vapor phase gum problem, generally, constitutes not only a nuisance and a source of expense, but

also in the case of automatic gas appliances, a possible source of danger to life and property.

i To avoid pilot trouble it is necessary to remove these gum particles completely, as only an extremely small quantity of gum is required to block the needle valves of commonly used pilot burners, such for instance as the lighter known commercially as the Ruta lighter. which in heavily gummed gas may be extinguished in four hours by as little as 0.000025 gram of gum. This is of the order of 1 pound of gum for 18 million pilot valves. The small quantity of gum upon the valve promotes the further deposition of dirt, iron oxide, grease, and fat, which in the absence of `gum rarely cause pilot trouble in manufactured gas systems.

It is, therefore, necessary to remove the gum completely from the gas. 'That is, the gas entering .the pilot burner for combustion should show no vapor phase particles visible in the Tyndall beam (i, e., by ultramicroscopic examination) and microscopic examination of valveneedles and seats should reveal no gum deposits thereon.

The ideal solutiony would be to remove at least one of the gum-formingconstituents (e. g., theoxides of nitrogen) from the freshly generated gas at the gas plant. `But While this is done to long as any nitric oxides remain `in the gas, it is not enough to treat'the gas merely at the gas plant. The gas must lbe treated at the appliances as Well, to remove any'last traces o1' vapor phase gum particles before they can enter the pilot valves. l

Due to the low pressure of the gas at the appliances, the drop in pressure across any iilter for the removal of gum must be very low. This is because pressures in gas services and piping to domestic appliances are quite low as compared with pressures frequently employed in transmission mains1 For instance, transmission main pressures may be as high as 100 pounds per square inch, but distribution main pressures are of the order of from 3 to V8 inches lof water column. With such low pressures only a very low pressure drop through any lter may be tolerated. A pressure drop of 1 inch of water column is all that may be allowed in most cases.

Another problem oi. extreme importance in this eld is that of obtaining a reasonable filter life. For gum filtration purposes it is conceivable that a diaphragm might be chosen or a fibrous material sufiiciently compressed to substantially coml pletely prevent the passage of even the smallest gum particles while at the same time permitting the flow of gas. that filters of this type are necessarily very large to permit a reasonable ow of gas. Furthermore, such filters are of very short useful life. This is because the gum particles build up rapidly on the face of the filter to form a lacquer-like lm which rapidly becomes substantially impervious to the gas.

To overcome the latter difculty and in accordance with my invention, I arrange a brous mass having the required specificity, in a manner such that the interstices between the bers are sul- However, experience has shown I face conditions by turbulence, by Brownian movement, and in the case of thelarger particles by v gravitational fall.

' dro However, the interstices between the iibersmust not be so large as to permit the gas to flow therethrough without the required turbulence'.

The necessary surface conditions for good filtration are shared by very few fibrous materials, among which is the material known as rock wool.

gum particles is approximately a the density ranges given.

Since `the permissible maximum pressure drop is one inch of water column, a depth of 1% inches is sufdcient when the density of rock wool is 3. grams per cubic inch and a depth of one inch l' when the density is 5 grams per cubic inch.

lV have discovered that these requirements may be met by arranging the rock wool iibers, usually by compression, in :d lter masses of deiinite density characteristics.

T'he term "density" is used here and throughout the speciiica-tion and in the claims to mean apparent density as distinguished from absolute density, apparent density being equal to the weight in grams of a given iibrous mass divided by the volume in cubic inches of the space occupied byit. Because' oi variations within the material itself this density is obviously average.

I have discovered that in the case of rock wool a minimum density of 3 grams per cubic inch provides interstices sufiicien'tly small for my purposes provided the iilter path is sufficiently deep, the latter being a consideration which will be hereinafter treated in connection with pressure p. I have also discovered that in the case of rock wool a maximum density oi 16 grams per cubic inch provides interstices approximately just sufiiciently large to permit' the desired penetration by the gum particles of the brous mass.

A maximum range in densities therefore, assuming o i' course, 'that the fibers are not too variable in distribution and size is from 3' grams per cubic inch to 16 grams per cubic inch. A preferred range is between 5 grams per cubic inch and 14 grams per cubic inch, since such range is less susceptible to possible wide variations.

It will be recognized'that for -a xed rate of gas flow 'the'depth of penetration of gum par-A ticles will be greatest in the case of a lter mass having a density o1 3 grams per cubic inch and will be least in the case of a filter mass having a density oi.' 16 grams per cubic inch. Good aver- The depth of penetration is to some extent dependent upon the rate of gas iiow, it being natural to assume that the higher the rate of gas iiow the deeper the penetration.

The rate of gas flow, however, through any filter mass suitable for lmy purposes is limitedy by the permissiblemaximum pressure drop of one inch of water column.

Accordingly, I have found that when employing rock wool having a density of from 16 grams per cubic inch d own to about 13 or 14 grams per cubic inch, the minimum filter mass depth is preferably' 0.25 inch and that the filter mass If desired, greaterdepths may .be employed with a corresponding decrease in gas iiow with any given pressure drop which should not great- 1yv exceed one inch of water column.

J Depths lower than 0.25 inch shouldbe avoided.

The above observations with respect to depth are necessarily general in view of variations in rock-wool. Thus; a depth of 0.25-inch is generally suitable with the'14 gram material up to a. pressure drop of one inch of water column.' With material of considerably lower density a depth as low as 0.25 inch is suitable up to pressure drops somewhat lower than one inch of water column. Conceivably the 3 gram material might be used in depths of the order of 0.25 inch with very low pressure drops, that is, withv very low rates of gas iiow.

Even though rock wool as commercially pro duced at the present time varies considerably in. fiber diameter and general uniformity, not only in different lots but also 'within the same lot which naturally reects vitself in diiferencesin the sizes and arrangement of interstices when compressed to my range densities, highly satisfactory filtration of the gas may be obtained provided the iilter masses are arranged to meet the following test:

The test gas is passed through a given iilter mass having a density within any range above given at a rate to produce a pressure drop across the iilter mass of one inch of water column. Under such circumstances, to insure against passage of gum, the iilter mass should be of a depth to restrict the ilow in cubic feet per hour of the test gas to below specific gravityof Vtest gas per square 'inch of filter cross-section. Upon simpliication this mathematical expression becomes 3.1 divided by the square root of the speciiic gravity of the test gas. To insure good capacity with lter compactness, the iilter mass is preferably not made of a depth suiiicient to restrict the i'low in cubic feet per hour of test gas to below A specific gravity of test gas Upon simplification this latter mathematical expression becomes 1.3 divided by the square root of the specific gravity of the test gas.

When using a test gas having a specific gravity of 0.60 the limits become 1.7 and 4 .0 cubic feet per hour per square inch of lter cross section.

Examples of gases which may be employed as a test gas are any of the manufactured gases, or atmospheric air. Carburetted water gas is an example of a gas which may be made with a specific gravity of 0.60..

Filters meeting this test, which is not based onA filtering efficiency but merely on gas ow, will eiliciently lter gum particles from the gas at all rates of gas iiow provided the pressure drop through the iilter does no rl; substantially'exceed one inch of water column and provided the iilter within mass is of a density within any of the a'bove ranges.

Accordingly, it is readily seen that the given values are essentially critical as particularly de'- iininga filter which will cause the effective removal of -vapor phase gum particles from manu- /factured gas.

'I'he inventionwill be further described in connection with the attached figures which form a part of this specification and which are chosen for illustration and in which: y

Figure 1 shows a vertical cross-section of a filter for domestic gas range pilots.

Figure 2 shows a horizontal cross-section along provided with pipe,threads as indicated at 5.`

The interior bore of member I is threaded at 6 to engage the screw 4plug .1 whichI is adapted to be screwed into the bore and to seat in gas tight manner against the end/of member I as indicated at 8. The screw plug is provided with the passage 9 which is furnished with pipe threads as shown at I0, and which communicates with the filter chamber. II and I2 are spiders provided with rims as I3 and arms as I4 and adapted to furnish support for screens I5 and I6. II is a iilter pad of compressed rock wool. Fork convenience in` assembly and connection, the member I may -be provided with wrench grips, as at I8, and the head of the screw plug 1 may be formed as a nut. ,y

All metal parts are preferably of a material or materials which do not corrode in the presence of manufactured gas. As an example, the filter chamber member I, the screw plug l, and spiders II and I2 may be of brass which preferably has a copper content not greater than 68% and still more preferably not substantially greater than 63%, and screens I5 and I5 may be of stainless steel. S16-mesh screen is very satisfactory, although other suitable mesh screen may bey employed. Y.

The interior bore is preferably accurately dimensioned in diameter and length, as is also the screw plug. The screws and spiders are preferably accurately formed. in thickness. Such accurate dimensioning facilitates the securing of the desired rock wool arrangement in assembling the filter, as in such case a predetermined quantity of rock wool may bemerely compressed to the desired cross-sectional area, depth and density.

In so assembling ythe filter, the spider I l is positioned against the seat 2 and the screen I5 placed abutting the spider. A'weighed or otherwise previously determined quantity of rock wool is then placed in the filter chamber and the screen I8 and spider I2 positionedon the other side of it. The screw plug I is then introduced and tightened until it seats in gas tight contactl with the end of member I as at 8, compressing the rock wool within the chamber.

The lter pad, if desired, may be stamped, punched or otherwise formed from a, rock wool blanket of the required. thickness to give the desired weight of rock wool, or a f' lurality of blankets may be superimposed "'-to give the required thickness" for the purposesindicated. In this Acase it is convenient to have the diameter of the filter pad slightly larger than the diameter of the inner bore of the filter chamber member .I to insure a tight fit and thus avoid possibilities of channeling about the wall of said inner bore. As an example, the filter pad may have a diameter 116 inch larger.

Rock wool blankets such as used commercially for! heat insulation purposes may bel employed, with or without the customary binding materials, if desired, particularly if irregularities in fiber distribution and fiber size are not too great, and the blanket is not too full of the usual beads which at present -are apparently unavoidably formed during the manufacture of rock wool commercially.

Accurate dimensioning of the parts predetermines the cross section, density and depth of the rock Wool after compression. The quantity of rock Wool selected by weighing or punching from a mass of predetermined thickness is thatv which will give the density appropriate to the predetermined depth after compression to the 'predetermined volume.

the screens and prevent their bulging While transmitting the pressure for compressing the filter pad and further serve to provide gas spaces which distribute the gas across the area of the pad.

As an example, a filter similar to that shown in Figure z 1 having a chamber bore of 0.75 inch diameter and an overall chamber length of 0.75 inch is provided with a screw plug, screens and spiders dimensioned to provide a space for rock wool 0.50 inch deep after compression. The lter is filled with 1.8 grams of rock wool and, after assembly and installation, will filter heavily gummed gas, and' is suitable for filtering, for example 0.75 cubic feet of gas per hour, without exceeding a pressureY drop of one inch of water column through the filter. The density of the rock wool after compression is approximately 8.15 grams per cubic inch.

. The degree of compression of rock Wool from a form typical of that used commercially for other purposes is shown by the following.. Filter pads for filters of the size and construction of the above example are formed commercially by punching from a blanket [nass six inches in thickness. These punched pads when compressed in the lter chamber to a depth of 1/2 inch have a density of approximately 8 grams per cubic inch.

Referring to Figures 3 and 4.

These figures illustrate a larger filter differing in some details from that in Figures 1 and 2.

20 indicates a filter chamber member having an interior bore, providing the filter chamber. Member 20 is provided with the lug 2I having the passage 22 communicating with the filter chamb`er. Lug 2i is provided with the pipe threads 23.

The member 20 is provided with lthreads 24, adapted to engage the threads of the cover 25. The cover 25 is provided with the passage 26,

the chamber member .20.

compressed rock wool.

As in Figures 1 and 2,.the parts are preferably accurately dimensioned to provide a space for compressed rock wool of accurately predetermined area and depth to facilitate the securing of the proper density and depth in the compressed filter pad.

.'I'he assembly of the apparatus of Figures 3 and 4 is similar tothat described in connection with Figures l1 and 2, except for the changes due to the fact that the spiders are integral with the filter chamber member and cover, and that a gasket is employed. As in Figures l and 2, a convenient method of assembly includes the compression of 28 and 29 are screens similar to those of substantially uniform and desired' lthickness. This method better insures uniform packing than introducing the rock wool in balls or tufts, al-

though the latter method may be employed satisfactorily especially if care is taken to avoid undue unevenness in density.

a weighed quantity` of rock wool, or a punching hour, without exceeding a pressure drop of one inch of water column. The density of the rock wool in the above example when compressed is approximately 8.15 grams per cubic inch.

In al1 cases care should be exercised when inserting the rock wool into a filter chamber to avoid unduefunevenness in density throughout the body of the material, and to avoid channeling at the side Walls.

The rock wool employed may be of the type used for heat insulation. Preferably, it is free from relatively excessive quantities of beads customarily formed during the manufacture of such material. Rock wool is usually made from slag and is ordinarily composed of` oxides of calcium, magnesium, aluminum, iron and silicon. As an example, the fibers of rock wool may have average diameters between 3 and 4 microns,v with' some fibers ranging between l and 8 microns and may be of'all lengths up to 80 millimeters. In

the case of blankets, an organic binder is fref quently employed to cause the fibers to adhere. Typical samples of commercial rock Wool gave the following analyses.

Sample 1 Sample 2 Percent Percent SiO, 3l 40 A1105 14 l5 Ca0 29 27 MgO l5 l5 impurities. l1 3 Uniform packing of the rock wool is highly desirable and the filter pads are preferably formed by cutting or punching out discs from a blanket or superimposed blankets of rock wool It is to be understood that after extended use \,the pressure drop through the filter will increase due to the collection of gum in the interstices of the lter element which may eventually necessitate renewal; The life of' the element, of course,- depends somewhat on the quantity of gum in thel gas it is required to fllter'. 'I'he characteristics of the filter pad made in accordance with this invention, however, permit considerable penetration in depth by the gum particles, so that filtration is not confined to a'narrow zone on the entrance `side of the element, a factor influential in securing a low pressure drop along with a long filter element life. `The lter will, in most cases, out/live the appliance to which it is attached. This is-oi` greatest importance commercially since the filter after being installed will in by far the larger number of cases require no servicing `what soever.

Theapparatus of the invention Ahas been vdescribed particularly in connection with the filtering of Vapor phase gum particles from gas. It

may, however, have use in other applications.v

For instance, it is an excellent filter for dust laden combustible gas. The gures illustrate forms of the invention chosen for illustration.

It is to be understood that the foregoing is by way of illustration and that changes, omissions, additions, substitutions, and/or modications might be made Within the scope of the claims without departing from thespirit of the invention. I

I claim: l 1. A gas filter adapted to remove from a gas suspended particles of ultramicroscopic dimensions and of less than 1 mu in diameter, comprising a. filter casing having inlet and outlet openings, and a filter element of rock wool arranged in said casing for the flow of said gas therethrough, said filter element having a depth in the direction of gas iiow of at least 0.25 inch and said rock wool having a density between 3 grams per cubic inch' and 16 grams per cubic inch.

2. A gas filter adapted t0 remove extremely minute vapor phase gum particles of less than 1 mu in diameter suspendedin combustible gas flowing to a low rate appliance burner, comprising a filter casing having inlet and outlet openings, and a filter element of rock wool arranged in said casing for the flow of said gas therethrough, said filter element having a depth in the direction of gas flow of at least 0.25 inch and said rock wool having a. density between 5 grams per cubic inch and 14- grams per cubic inch.

3. A gas filter adapted to remove from a gas suspended particles of ultramicroscopic dimensions and of less than 1 mu in diameter, comprising a filter casing having inlet and outlet openings, and a filter element of rock wool arranged in said casing for the flow of said gas therethrough, said rock wool having a density between 3 grams per cubic inch and 16 grams per cubic inch, and said filter element having a depth in the direction of gas flow such that when'subjected to test with' a test gas flowing through said lter element at a rate sufficient to cause a pressure drop of one inch of water column the flow of said test gas per square inch of filter element cross-section takenat right angles to the direction of gas flow is restricted in cubic feet per hour to below 3.1 divided by the square root of the speciiic gravity of the test gas.

4. A gas nlter adapted to remove extremely minute vapor phase gum particles of less than 1 mu in diameter suspended in combustible gas flowing to a low rate appliance burner, comprising a filter casing having inlet and outlet openings, and a filter element of rock wool arranged in said casing for the ow of said combustible gas therethrough, said rock wool having a density between grams per cubic inch and 14 grams per cubic inch, and said filter element having a depth in the direction 0i' gas flow such that when sub- .iected to test with a test gas flowing through said filter element at a rate suiiicient to cause a pressure drop of onev inch of water column the flow of said test gas per square inch' of iilter element cross-section taken at right angles to the direction of gas :dow is restricted in cubic feet per hour to below 3.1 divided by the square root of the specic gravity of the test gas.

5. A gas filter adapted to remove extremely minute vapor phase gum particles of less than 1 mu in diameter suspended in combustible gas iiowing to a low rate appliance burner, Comprising a filter casing having inlet and outlet openings, and a filter element of rock wool arranged in said casing for the flow of said combustible gas therethrough, said rock wool having a density between 3 grams per cubic inch' and 10 grams per cubic inch, and said filter element having a depth in the direction of gas now such that when subjected to test with a test gas flowing through said lter element at a rate sunicient to cause a pressure drop of one inch of water column the flow of said test gas per square inch of lter element crosssection taken at right angles to th'e direction of gas now is restricted in cubic feet per hour to below 3.1 divided by the square root of the speciiic gravity of the test gas.

6. A gas filter for the removal of extremely minute vapor phase gum particles of less than 1 mu 'in diameter from combustible gas owing to a low rate appliance burner, comprising a iilter casing having inlet and outlet openings, and a lter element of rock wool in said casing between said inlet and outlet openings and in close contact with the side walls thereof, said rock wool having a density between 3 grams per cubic inch and 16 grams per cubic inch, and said lter element having a depth between said inlet and outlet openings such that when subjected to test with a test gas owing through said ltex' element at a rate sufficient to cause a pressure drop of one inch of water column the now of said test gas per square inch of filter element cross section is restricted in cubic feet per hour to below 3.1 divided by the square root of the specic gravity of the test gas, said filter element being of insunicient depth to restrict the iiow in cubic feet per hour of said test gas per square inch of filter element cross section to below 1.3 divided by the square root of the specific gravity of the test gas.

7. A gas iilter adapted to remove from combustible gas owing to a low rate appliance burner vapor phase gum particles of ultramicroscopic dimensions and of less than 1 mu in diameter, comprising a lter casing having inlet and outlet Openings, and a filterv element of rock wool arranged in said casing for the iiow of combustible gas therethrough, the bers of said rock wool being relatively evenly distributed, said lter element having a depth in the direction of gas now of at least 0.25 inch and said rock wool having a density between 3 grams per cubic inch and 16 grams per cubic inch to provide interstices between said iibers sufficiently large to permit said gum particles to penetrate into the body of said lter element but insuiiciently large to prevent substantially complete removal of' said gum particles from said gas.

WLTON E. STACKHOUSE.

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