US2002465A - Slag drip and screen - Google Patents

Description

E. G. BAILEY ET AL May 21, 1935.

SLAG DRIP-AND SCREEN Original Filed Oct. 28, 1930 4 Sheets-Sheet INVENTORS vim 02B ATTORNEY May 21, 1935. E. G, BAILEY ET AL SLAG DRIP AND SCREEN Original Filed Oct. 23, 1930 4 Sheets-Sheet s INVENTORS a vbn 6.3 H MLZ J Iii 61$ AT I OR NEY Alli/ll/Il y 1935. E. G. BAILEY ET AL 2,002,465,

SLAG DRIP AND SCREEN Original Filed Oct. 28, 1950 4 Sheets- Sheet 4 INVENTORS 1 E'g'vl'n'a 6.19

ade wand-Lifer A o NEY Patented May 21, 1935 STATES SLAG DRIP AND SCREEN" Ervin G. Bailey, Easton, Pa., and Howard J.

Kerr, Westfield, N. J., assignors-to The cock & Wilcox Company, Bayonne,

Bab- J a corporation of New Jersey Application 27 Claims.

This invention relates to a steam generator of the high heat release type, wherein pulverized coal may be burned, and in which it is essential to adequately provide for the separation and removal of dust and slag from the gases of combustion.

An object of the invention is to provide, in a steam generator of the aforementioned type, a slag tap furnace equipped with a dust collector and gas agitating screen, of water tubes, and wherein pulverized coal is so directed into the furnace that .the gases of combustion contact with a pool of continuously molten slag whereby dust and slag are caught by the sticky surface thereof, and the gases turned by contact with said pool in a manner to cause them to pass over a group of water tubes constituting a slag and. dust collector, also arranged to serve as a gas agitating screen which removes additional slag and dust content of the gases before the gases reach the main tube bank.

Also, slag dripping means is provided on the tubes of the slag screen to catch and drip the slag off as it runs down the tubes.

A further feature resides in the provision, on the tube bank, of a slag collecting and dripping means on the lower tubes of the main bank, and which causes the slag to drip off at predetermined locations, thereby hindering the,building up of a large mass of slag on the refractory wall protecting the headers, and which would result in damage to the wall.

A still further feature is the particular structure of the tubes on which the slag is caught, and

which are provided with extended metal surfaces welded or otherwise secured or integrally joined to the tubes and which, by reason of these extendedinetal surfaces being hotter than the tube, maintain the slag thereon in a wet, sticky, dripping condition enhancing its ability to picl; up dust and separate out slag from the gases of combustion, while, at the same time, hindering any appreciable building up of the slag on the tubes, thereby hindering constriction of the spaces bea tween the tubes and thus affording a better passage for the gases in and around th'e'tubes.

Another phase of the invention is that, by reason of the cooler tube surfaces between the extended metal surfaces or fins, the least amount of slag held thereby forms concave annular surfaces between the flns of the tubes and the aggregate area of the slag wet concave surfaces presents a greater area of surface for the sepa-'" ration of slag and -dust from the gases of combustion than the aggregate area of the bare tube October 28,

Renewed April ,3, 1934 1930, Serial No. 491,682

surfaces, and in addition this action hinders bridging over of tubes by slag. V

Still another feature of the fins or extended metal surfaces on the tubes is that the fins or extended metal surfaces more thoroughly agitate the passing stream of gases than do plain tubes and which, in addition to promoting better combustion, by reason of greater agitation also re- I sults in more of the passing stream actually contacting with the slag wetted surfaces, consequently causing better separation of slag and dust and effecting cooperation between the slag wetted surfaces, the fins and the gas stream.

With the above and other features in view we will now describe, in connection with the accomj panying drawings, oneform of carrying the invention into effect, and wherein;

Fig. 1 is a cross section in a vertical plane through a steam generator incorporating the present invention. a 20 Fig. 2 is a sectional view on the line 2 -2 of Fig. 1, showing the arrangement of slag screen;

Fig. 3 is a sectional view, similar to Fig. 2, on the line 3-3 of Fig. 1;

Fig. 4 is a view, similar to on the line 44 of Fig. 1;

Fig. 5 is a fragmentary side elevation of the slag drip for the bottom row of the tube bank;

Fig. 6. is-a sectional view on the line 6-6 of Fig. 5; I

Fig. '7 is a sectional view on the line-'I-"I of Fig. 5;

Fig. 8 is a fragmentary ,side elevation of one type of slag drip tube;

Fig. 9 is a transverse section on the line 99 of Fig. 8;

Fig. 10 is a longitudinal fragmentary section of the type of tube shown in Fig. 8; and

Fig. 11 is a fragmentary view depicting what takes place in connection with the deposit of slag 0 on a tube having fins.

The steam generator, as shown herein for the purpose of illustrating the invention, comprises a boiler with a setting of the slag tap type adapted to maintain a pool of molten slag in the bottom, and which includes the steam and water drum A,

tube banks B and C and superheater D, having included in their circulatory system the radiant heat wall tubes E and the slag pit cooling tubes F, all of which are connected together for natural circulation.

There is also included in this circuit, two transverse rows of dust collector tubes G and H form- .ing a gas agitating and slag screen, said tubes being connected at the ends by the headers land Figs. 2 and s, but '25 2 for the lower row G and the headers 3 and 4 for the upper row H, the said tube rows G and H extending at an incline of about 32 degrees, as shown, and being located between the banks of tubes B and C and the slag pit I, so that they are disposed in the path of the gases travelling from the chamber J to and over the banks of tubes B and C, the fuel being fired through the pulverized coal burners 5.

The tubes comprising the rows G and H may be constructed in accordance with the details shown in Figs. 8, 9 and 10 and wherein the plain tube 6 has wound thereon and welded thereto the helical fin l which projects outwardly from the surface of the tube. If desirable this fin, at points along its length, may have additionally applied thereto by welding, as at8 and 9, extensions H] which form an additional surface which becomes hotter than the tube, projects beyond the normal extended metal fins, and forms a drip point for slag.

With reference to Figs. 2, 3 and 4 the arrangement of rows of tubes G and H shown with the tubes of each row divided into groups ll, [2, l3, I4, l5, IBand I! for the lower row G, and l8, l9,

- 20, 2|, 22, 23, 24 and 25 for the upper row H. It

will be noted that groups of tubes II to H inelusive each comprise 10 tubes arranged, in cross section, in a V-shape manner, and the tubes all having applied thereto metal extensions l,v thus providing a composite element which will act as a slag retaining screen, dust collector and gas agitator.

Cooperating with the row G is a similar row H of slag screen tubes and the groups of which are consecutively numbered from l8 to 26 inclusive. The upper row H is similarly arranged except for the end rows l8 and 25 which comprise only three tubes, but are cooperative with the groups IE! to 24 in that they serve to bafile the spaces between the groups II and ll of the lower row G and the walls, these tubes of the groups l8 to 25 are provided with helical fins 1 similar to that described in connection with thegroups of tubes in the lower row, and likewise have at intervals thereon the extensions ID for dripping slag.

It is to be noted in connection with these two rows of tubes that the upper row has some groups of tubes which are of inverted V.form in cross section, and some groups of tubes which are of natural V form in cross section, while tube groups 2| and 22 are flat in cross section, and that this irregular arrangement is illustrated for the purpose of indicating the latitude which may be resorted to in tube group arrangements in any one steam generator, and while the helical form of fin has been described, fins, such-as illustrated in Fig. 5, may also be used on these tubes.

In the operation of the steam generator with pulverized coal fired into the furnace thereof through the burner 5, there will be formed a large amount of slag which will flow on these tubes and, as the accumulation becomes heavy, will drop off andlodge in the slag tap pool I, and by reason of the peculiar characteristics of the exterior of the tubes, the extended metal surface formed by the helical or other type fins will be hotter than the tubes and therefore hinder slag from freezing to the tubes and allow it to.

Also, some slag forming material will be carried with the gases of combustion through this slag screen formed by the rows of tubes G and H and will be deposited along the bottom tubes of the tube bank C with the result that, due to dependent therefrom, possibly disintegrating this wall, or pulling it out of place due to the sheer weight of the'accumulated mass, but, in order to prevent this, the present invention applies thereto the same principle as is used in connection withthe slag screen groups H to 25, but with a slight modification; that is to say, there is applied to the upper portion of these tubes,

the upwardly extending lips 26 which only partially surround the tube on the upper side, as shown in Fig. 7, for a portion of the length of the tube suflicient to extend beyond the brick work K, another portion of the tube immediately therebeyond having applied thereto the fins 27 completely surrounding the tube and which thus forms a ledge at the point 28, causing the slag accumulated by the tube to drip off before reaching the brick work K and thus precluding the possibility of collection of a large amount of slag at any one point on the brick work K, thereby saving the brick work from this strain.

While we have described fins on the lower row of tubes C as welded in place around the tube, it is nevertheless obvious that these fins could be applied in the same manner as the fins have been applied to the tubes of the groups II to 25 by afiixing thereto, either welded or the like, a helical fin for replacing the fins 21 and a halfhelical portion thereof for the fins 26.

As previously described the steam generator wardly to pass through and around the groups of tubes G and H, a'large portion of the slag and dust being first deposited in the slag pool I by reason of the wet, sticky condition thereof,

some of the remaining dust and slag being deposited on and dripped from the slag screen tubes G and H, and the balance being removed and dripped by the row of tubes C of the main tube bank so that the gases passing through the tubes B are pretty clear, having been scrubbed of dust and slag and thoroughly mixed by the contacts arranged as described.

With reference to Fig. 11, it is tobe noted that the slight coating of slag which does adhere to the tube, by reasonof the portions of the tube between the fins being colder than the fins,

causes an annular concave surface to form which is always wet and thus provides, in the aggre-- gate, a larger slag wetted surface than is the case in the aggregate of the bare tube portions, and thereby causes the separation of more slag and dust than would otherwise be the case, and, in addition, hinders bridging over of adjacent tubes with a slag deposit.

What we claim and desire to secure by Letters Patent is- 1. In a steam "generator, a furnace, a water tube boiler positioned beyond the furnace relative to gas fiow, means for burning a slag forming fuel in said furnace, and a water tube screen forming a combined gas agitating and slag collecting screen between the furnace and the boiler, the tubes thereof having transverse metal projections hotter than the body of the tubes whereby slag directed there-against is hindered from freezing thereon and consequently runs oif, said projections being arranged along the tubes to prevent the accumulation of slag at positions near the ends of the tubes, the water tubes'of the screen consisting of a single thickness ofv metal with a cooling fluid contacting their inner surfaces and with the metallic extensions welded or otherwise integrally joined with the metal of the tubes on their outer surfaces.

'2. In a steam generator, a slag'tap furnace and a water tube boiler, means for burning a slag forming fuel in the furnace, a continuously maintained molten slag pool toward which coal and air are projected and turned away therefrom whereby slag and dust are causd to separate by contact with the sticky molten surface thereof, and a water tube screen in the boiler circulation circuit and between the slag pool and boiler, the tubes thereofhaving transverse metal projections hotter than the tubes themselves to main slag in'a sticky wet dripping condition accumulating dust and to hinder the collection of a quantity of slag sufiicient to constrict the gas passages between the tubes, the individual tubes having said projections spaced from each other along the lengths of the tubes, the water tubes of the screen consisting of a single thickness of metal with a cooling iiuid contacting their inner surfaces and with the metallic extensions welded for otherwise integrally joined with the metal of the tubes on their outer sur faces.

3. In a steam generator, a slag tap furnace constructed to maintain a pool ofslag atthe bottom thereof and a water tube boiler, a pulverized coal burner for said furnace, a continuously maintained molten slag pool toward which coal and air from said burner are projected and turned away therefrom whereby slag and dust are caused to separate by the sticky molten surface thereof and a water tube screen in the boiler circulation circuit and between the slag pool and the boiler, the tubes thereof having extended helical metal surfaces hotter than the tubes themselves to maintain slag in a sticky wet dripping condition accumulating dust and hindering collection of a quantity of slag sufiicient to constrict the gas passages between the tubes.

4. In a steam generator, a furnace, a boiler above the same, a pulverized coal burner projecting coal into said furnace, and a water tube screen forming a combined gas agitating and slag screen extending across said furnace between the burner and boiler, and welded metallic projections providing extended helical surfaces on said tubes to provide a surface hotter than the tube body whereby the slag contacting with the surfaces is hindered against freezing and consequently drips off.

5.'In a steam generator, a furnace, a boiler above said furnace, a pulverlzedcoal burner projecting coal into said furnace, and slag removing water tubes in the boiler circuit and extending across said furnace in'the upward path of the products of combustion and arranged in rows, the tubes of one row being in staggered relation to the tubes of another row to form agas agitating and slag collecting screen, and fins welded or otherwise integrally joined with said ,tubespresenting surfaces hotter than the body of the tubes where by the slag collected by the tubes is maintained in a wet, sticky condition capable of removing dust from the gases of combustion and dripping ofi into a slag pool, each finned tube having a plurality of said fins spaced from each other along its length.

6. In a steam generator, a furnace and combustion chamber, a boiler thereabove, means for introducing a mixture of slag forming fuel and air into said combustion chamber in a direction to cause slag to be collected at the bottom thereof and maintained molten, and a water tube screen comprising rows of. water tubes in the combustion chamber between said means and boiler and forming a combined gas agitating and slag col-* lecting screen, the tubes of each row being grouped with the groups arranged in staggered relation wfth respect to the groups of adjacent rows, and metallic means welded to or otherwise made integral with the tubes presenting hotter surfaces than the body of the tubes and whereby slag collected thereon is maintained in a molten condition.

7. In a steam generator, a furnace and combustion chamber, a boiler thereabove, means introducing a mixture of slag forming coal and air into said combustion chamber, in a direction to cause slag to be collected atthe bottom thereof and maintained in molten condition, and water tubes located in rows between the burners and boiler, the tubes of each row being grouped, with the groups of adjacent rows arranged in staggered relation whereby the gases passing therethrough are agitated and mixed and slag deposited on the tubes, and helical means on said tubes presenting hotter surfaces than the body of the tubes whereby slag collected by the tubes is maintained molten and allowed to drip off.

8. In a steam generator, a furnace and combustion chamber, a boiler thereabove, means for introducing a mixture of slag forming coal and air in said combustion chamber in. a direction to cause slag to be collected at the bottom thereof and maintain the surface of the slag pool in molten condition, and a screen of. water tubes located in rows between the combustion chamber and the boiler, the tubes of each row being grouped, with the groups arranged in staggered relation with respect to the groups in adjacent rows, and extended helical, surfaces relative tothe tubes and hotter than the surfaces thereof for collecting and dripping the slag coating.

9. In a steam generator, a furnace constructed I to maintain a pool of slag at the bottom thereof and a boiler, the furnace having a continuously molten slag bottom, and water tubes in the boiler circuit extending across the furnace intermediate the bottom and the boiler, said tubes arranged in groups for 'collectingslag, slag collecting fins on said tubes extending from the surface thereof and bined gas agitating and slag collecting screen be- 7 tween the furnace and the boiler, the tubes thereof each having a plurality of spacedtransverse metal projections arranged along the length of the tube, hotter than the body of the tubes whereby slag directed there-against is hindered from freezing thereon and consequently runs off, and

fins on some of the tubes of the boiler to collect and drip any slag deposited thereon.

11. In a steam generator, a slag tap furnace constructed to maintain a pool of slag at the bottom thereof, a water tube boiler, a pulverized coal burner, a continuously maintained molten slag pool toward which coal and air are projected and turned away therefrom whereby slag and dust are caused to separate by contact with the sticky molten surface thereof, a water tube screen in the boiler circulation circuit and between the slag pool and boiler, the tubes thereof each having a plurality of spaced transverse metal projections arranged along the length of the tube hotter than the tubes themselves to maintain slag in a sicky wet dripping condition accumulating dust and hindering collection of a quantity of slag suificient to constrict the gas passages between the tubes, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon.

12. In a steam generator, a slag tap furnace constructed to maintain a pool of slag at the bottom thereof and a water tube boiler, a pulverized coal burner, a continuously maintained molten slag pool toward which coal and air are projected and turned away therefrom whereby slag and dust are caused to separate by the sticky molten surface thereof, a water tube screen in the boiler circulation circuit and between the slag pool and the boiler, the tubes thereof having extended helical metal surfaces hotter than the tubes themselves to maintain slag in a sticky wet dripping condition accumulating dust and hindering collection of a quantity of slag sufficient to constrict the gas passages between the tubes, and

fins on some of the tubes of the boiler to collect and drip any slag deposited thereon.

13. In a steam generator, a furnace, a boiler above the same, a pulverized coal burner projecting coal into said furnace, a water tube screen forming a combined gas agitating and slag screen extending across said furnace between the burners and boiler, extended helical metal surfaces on said tubes to provide a surface hotter than the tube body whereby the slag contacting with the surfaces is hindered against freezing and consequently drips off, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon. v

14. In a steam generator, a furnace, a boiler above said furnace, a pulverized coal burner projecting coal into said furnace, slag removing water tubes in the'boiler circuit and extending across said furnace in the upward path of the products of combustion and arranged in rows, the tubes of one row being in staggered relation to the tubes of another row to form a gas agitating and slag collecting screen, fins on said tubes presenting surfaces hotter than the body of the tubes whereby the slag collected by the tubes is maintained in a wet, sticky condition capable of removing dust from the gases of combustion and dripping oif into a slag pool, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon, the fins on the slag screen tubes and the boiler tubes being arranged transversely of the tubes with a plurality of spaced fins arranged along the lengths of the individual tubes.

15. In a steam generator, a furnace and combustion chamber, a boiler thereabove, means for introducing a mixture of coal and air into said combustion chamber in a direction to cause slag to be collected at the bottom thereof and maintained molten, a water tube screen comprising rows of water tubes in the combustion chamber between said means and boiler and forming a combined gas agitating and slag collecting screen, the tubes of each row being grouped, with the groups arranged in staggered relation with respect to the groups of adjacent rows, means on the tubes presenting'hotter surfaces than the body of the tubes and whereby slag collected thereon is maintained in a molten condition, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon- 16. In asteam generator, a furnace and combustion chamber, a boiler thereabove, means introducing a mixture of coal and air into said combustion chamber, in a direction to cause slag to be collected at the bottom thereof and maintained in molten condition, water tubes located in rows between said means and the boiler, the tubes of each row being grouped, with the groups arranged in staggered relation with respect to the groups of adjacent rows whereby the gases passing therethrough are agitated and mixed and slag deposited on the tubes, helical means on said tubes presenting hotter surfaces than the body of the tubes whereby slag collected by the tubes is maintained molten and allowed to drip off into the pool, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon.

1'7. In a steam generator, a furnace and combustion chamber, a boiler thereabove, means for introducing a mixture of coal and air in said combustion chamber in a direction to "cause slag'to be collected at the bottom thereof and maintain the surface of the slag pool in molten condition, a screen of water'tubes located in rows between the combustion chamber and the boiler, the tubes of each row being grouped, with the groups arranged in staggered relation with respect to the groups of adjacent rows, extended helical surfaces relative to the tubes and hotter than the surfaces thereof for collecting and dripping the slag coating, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon.

18. In a steam generator, a furnace constructed to maintains. pool of slag at the bottom thereof and a boiler, the furnace having a continuously molten slag bottom, Water tubes in the boiler circuit extending across the furnace intermediate the bottom and the boiler, said tubes arranged in groups for collecting slag, slag collecting fins on said tubes extending from the surface thereof and consequently maintaining a hotter temperature than the body of the tubes whereby the slag thereon is always molten, the tubes being arranged in groups in each row, with the groups of adjacent rows alternately concave and convex, and fins on some of the tubes of the boiler to collect and drip any slag deposited thereon.

19. A water tube for boilers having transverse metal projections therealong reaching a temperature higher than the temperature of the main body of the tube whereby any slag forming thereon is maintained in a molten condition, and drip point extensions on the said projections at positions directly beneath the tube whereby slag drip is regulated.

20. The method of operating a furnace constructed to maintain a molten slag pool in the bottom thereof and fired by slag forming pulverized coal, the combustion of which heats a water tube boiler provided with slag drip projections arranged transversely of and integral with some of the tubes, which method comprises operating a burner to maintain slag dripping from the tubes and their projections, and projecting the .elements of combustion towards the surface of the molten slag pool where they are then turned upwardly through the dripping slag zone whereby the molten sticky condition of the slag removes dust from the elements of combustion.

21. In fluid heat exchange apparatus, a furnace, means for burning fuel in the furnace under slag forming conditions, a horizontally inclined bank of fluid heat exchange tubes extending across a gas pass leading from the furnace, additional fluid heat exchange tubes extending across the gas pass in front of said bank of tubes relative to the flow of furnace gases, and spaced metallic bodies arranged transversely of said additional tubes along their sides presented towards oncoming furnace gases and spaced along the lengths of the tubes, said bodies acting to prevent accumulation of slag in large masses near the lower ends of the tubes and to cause slag to be maintained in a molten condition thereon whereby the collected slag drips from the additional tubes over extended portions of their lengths and acts as a dust collector extended across the gas pass in front of said bank of tubes.

22. In fluid heat exchange apparatus, a furnace, means for burning fuel in the furnace under slag forming conditions, a bank of fluid heat exchange tubes extending across a gas pass leading from the furnace, additional fluid heat exchange tubes extending across the gas pass in front of said bank of tubes relative to the flow of furnace gases and forming a screen, spaced metallic bodies arranged transversely of said ad-, ditional tubes along their sides presented towards oncoming furnace gases-and distributed along substantial-portions of the lengths of those tubes, and refractory material disposed between the metallic bodies, said bodies and the interposed refractory acting to prevent the accumulation of slag in large mases n the tubes and to cause slag to be maintained in a molten condition thereon whereby collected slag drips from the additional tubes over extended portions of their lengths and the additional tubes act as a dust collector, the water tubes of the screen consisting of a single thickness of metal with a cooling fluid contacting their inner surfaces and with the metallic extensions welded or otherwise integrally joined with the metal of the tubes on their outer surfaces.

23. In a steam boiler installation, a bank of steam generating tubes, a furnace having a combustion chamber beneath said tubes, means for burning fuel in the furnace, spaced screen tubes interposed relative to the fuel burner and the steam generating tubes, spaced metallic projections integral with the metal of the screen tubes and arranged around those tubes throughout substantial portions of their lengths, and refractory material between the projections and around the screen tubes, the water tubes of the screen consisting of a single thickness of metal said tubes, means for burning a slag forming fuel in the furnace, spaced screen tubes interposed relative to the fuel burner and the steam generating tubes, spaced metallic projections integral with the metal of the screen tubes and arranged along those tubes throughout substantial portions of their lengths, and refractory covering on the screen tubes and between the projections and facing oncoming furnace gases, the screen tubes with their covering and metallic projections constituting a relatively permanent sticky ash and dust collecting screen while the projections provide a distributed support for the covering and maintain its adherence to the tubes.

25. In fluid heat exchange apparatus, a furnace, a fluid circulation system, a bank of tubes connected into the fluid circulation system and extending across the path of the furnace gases, furnace screen tubes in groups below said bank with the tubes of the groups arranged obliquely to the general path of gas flow so as to substantially limit direct heat radiation from the lower part of the furnace to oblique radiation towards the walls of the furnace below said bank of tubes, said oblique arrangement of the tubes in the groups being in planes transverse to the tubes, means for connecting the screen tubes into fluid circulation, means for burning a slag forming fuel below the screen tubes, and spaced metallic extensions welded to the screen tubes over substantial portions of their lengths so as to promote the adherence of slag to the screen tubes and promote the maintenance of such slag in a sticky molten condition to the end that the screen tubes may act as dust collectors, the screen tubes consisting of single layer hollow metallic cylinders with a cooling fluid contacting with the cylinder at its inner surface and the metallic extensions integrally joined with the metal at its outer surface.

26. In fluid heat exchange apparatus,a furnace, a fluid circulation system, a bank of fluid heat exchange tubes extending across the path of gases in the furnace and connected into the fluid circulation of said system, means for burning pulverlzed coal under slag forming conditions, furnace screen tubes positioned between the burning means and said bank of tubes and arranged in spaced groups with the tubes of the groups in formations oblique to the general path of gas flow so as to change the course of the furnace gases and present oblique wide dust collecting surfaces, said formations being oblique inplanes transversely related to the tubes, means for connecting the screen tubes into fluid circulation, and separate metallic extensions welded to the screen tubes over substantial portions of their lengths so asto collect and maintain slag in a sticky molten condition thereon, the screen tubes in the individual groups being spaced more closely than the groups.

27. In fluid heat exchange apparatus, a furnace, means for burning a slag forming fuel in the furnace, a bank of fluid heat exchange tubes extending across the path of the gases from the furnace, furnace screen tubes extending across the path of the gases forwardly of said bank of tubes relative to gas flow, and dust collectors eccentrically mounted upon the screen tubes, the dust collectors including spaced transverse metallic extensions distributed over substantial portions of the lengths of the tubes. 1

' ERVIN G. BAILEY. HOWARD J KERR.

CERTIFICATE OF CORRECTION.

Patent No. 2, 002, 465, a May 21 1935.

ERVING. BAILEY, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, first column, line 24, claim 2, for "main" read maintained; and line 33, for "for" read or;

, same page, second column, line 14, claim 6, after "grouped" insert a comma; I and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 26th day of November, A. D. 1935.

Leslie Frazer (Seal) Acting Commissioner of Patents.

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