US3549135A

US3549135A - Regenerative furnaces

Info

Publication number: US3549135A
Application number: US775740A
Authority: US
Inventors: Hubert Cecil Wynne; James Edward Hall; William Charlton
Original assignee: Pilkington Brothers Ltd
Current assignee: Pilkington Group Ltd
Priority date: 1967-11-14
Filing date: 1968-11-14
Publication date: 1970-12-22
Anticipated expiration: 1987-12-22
Also published as: GB1243185A

Description

United States Patent inventors Hubert Cecil Wynne St. Helens; James Edward Hall, Wigan; William Charlton, St. Heiens, England App]. No. 775,740 Filed Nov. 14, 1968 Patented Dec. 22, 1970 Assignee Pilkington Brothers Limited Liverpool, England a corporation of Great Britain Priority Nov. 14, 1967 Great Britain No. 5 1778/67 REGENERATIVE FURNACES 14 Claims, 5 Drawing Figs.

US. Cl. 263/15, 263/5 1 Int. Cl. ..F27d 17/00, F231 15/00 [50] Field ofSearch 263/15, 198, 51; 165/104 [56] References Cited UNITED STATES PATENTS 731,682 6/1903 Hillig 263/19B 745,689 12/1903 Tanner et al. 263/19B 2,612,364 9/1952 Peck et al. 263/19B 2,657,116 10/1953 Daniels 263/19B Primary Examiner-John J. Camby Attorney-Morrison, Kennedy & Campbell ABSTRACT: Regenerative heat exchange is effected in a regenerative furnace containing a bed of rubble; rubble is continually removed from the lower regions of the bed, is sieved and fed to the top of the bed so that fine materials are progressively moved down through the bed.

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' sumanfa HUBERT CECIL WYNNE JAMES EDWARD HALL and Inventor, WILLIAM CHARLTON A PATENTEDBEB22|970 A 3549135 "SHEET30F3- HUBERT CECIL WYNNE JAMES EDWARD HALL and lnvenlors WILLIAM CHARLTON REGENERATIVE FURNACES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improvements in regenerative furnaces, that is, furnaces having regenerative heat exchangers or regenerators whichrecover a proportion of the heat content of hot exhaust gases to preheat incoming combustion gas such as air or fuel gas.

In the glass industry regenerative furnaces are used which are side fired from a plurality of burners arranged down each side of the furnace chamber. The burners may be oil burners or producer gas burners, and each burner is associated with an air inlet port which can alternatively serve as an exhaust port. Firing is carried out alternately from each side of the furnace from ports on one, side acting as air or gas inlet ports to ports on the opposite side acting as exhaust ports. 2. Description of the-Prior Art In such a furnace, each port is connected to a regenerator, and each regenerator is alternately connected by an arrangement of ducting and a changeover valve to a supply of combustion gas, such as air or fuel gas and to an exhaust flue. Firing is carried out in alternate stages from each side of the furnace, and in each stage the ports along one side of the furnace act as exhaust ports so that the regenerators associated therewith are heated by exhaust gases, while the ports on the opposite side are receiving preheated incoming combustion gas from the regenerators associated therewith. The preheated gas allow a high combustion temperature to be maintained in the furnace, and returns to the furnace a large proportion of the heat taken from the furnace by the exhaust gases.

The regenerators each have a first gas inlet or outlet connected to an associated furnace port, and a second gas inlet or outlet connectable either to the combustion gas supply or to the exhaust flue. For simplicity. the said first gas inlet or outlet will be hereinafter referred to as the gas outlet ofthe regenerator.

In regenerative furnaces of the type described the regenerators having generally been of the so-called chequer-work" type, that is to say having an arrangement of refractory brickwork which provides a large number of gas passageways therethrough. Alternatively, however, the regenerator may comprise a bed of particulate material such as refractory brick rubble which is loosely packed so that the interstices thereof provide numerous gas passageways therethrough. The rubble bed rests on a grate disposed below the gas outlet of the regenerator so that exhaust gases passdownwardly through the bed in one stage of operation, and in the next stage combustion gas passes upwardly through the bed. Such a rubble bed, besides avoiding the cost of a chequer-work installation, also provides high efficiency of heat transfer with a comparatively small size of regenerator. The term rubble as used hereinafter refers to a particulate material capable of forming a heat retaining bed suitable for a regenerator.

A drawback of using rubble type regenerators for glass furnaces has been that some of the fine particulate batch material the rubble bed. As these materials accumulate the pressure drop through the, bed increases. The increasing pressure drop means that the flow of gases through the furnace is impeded,

SUMMARY According to the present invention a method of regenerative heat exchange of the type in which downwardly moving hot exhaust gases and upwardly moving incoming combustion gas pass alternately through a bed of rubble contained within a regenerator chamber, is characterized by continually removing rubble from the lower regions of the bed, sieving the removed rubble, and feeding to the top of the bed sieved rubble substantially free of fine materials at a rate equivalent to the rate of removal of rubble from the lower regions of the bed, so that fine materials such as are carried over from the furnace are moved progressively downwards through the bed and are finally eliminated from the bed with the removed rubble.

Since the exhaust gases are cooled while moving downwardly through the bed, while the incoming combustion gas is heated while moving upwardly, the lower regions of the bed are'always cooler than the top of the bed, so that the rubble removed therefrom is not subjected to high thermal shock, and the removal of the rubble and replacement by cool rubble does not unduly cool the bed.

The rubble is preferably removed by gravity from the lower regions of the bed at a rate which is regulated by movement of a regulator or regulators in the discharge path of the rubble.

The removal and replacement of the rubble can be carried out according to the invention at such frequency that a new bed surface is presented before the top surface has become significantly blocked by fine material migrating from the furnace. Furthermore, the disturbance caused to the rubble bed when some rubble is removed from the lowerregions of the bed tends to break up the top surface of the bed so that new gas passageways, and interstices blocked by migrating fine material, are opened up. After a cycle of rubble movement has been completed, the top part of the rubble bed is substantially free of fine materials and can therefore accept further migrating fine material from the furnace without becoming blocked, and the efficiency of operation of the regenerator is thereby restored.

The invention also comprehends a regenerative furnace which includes a pair of re generators each having a chamber communicating through a gas outlet with the furnace heating chamber, each regenerator chamber having a grate situated below the level of said gas outlet for supporting a bed of rubble and a rubble inlet aperture above the grate, the grate includes two inwardly and downwardly sloping portions, the lower edge of one portion being spaced from the surface of the other portion to provide an elongated discharge opening therebetween, and an obstruction is provided above the said discharge opening to ensure approximately uniform rubble flow through the bed.

One of the said downwardly sloping grate portions may extend downwardly below the said discharge opening to form a rubble deflector grating sloping towards a normally obturated aperture in the outer wall of the regenerator, this grating constituting a sieve for fine materials. This deflector grating may be associated with regulating means movable to regulate the movement of rubble through the discharge opening.

In another embodiment of the invention the regenerative furnace may include a pair of regenerators each having a v chamber communicating through a gas outlet with the furnace heating chamber, each regenerator chamber having a grate situated below the level of the said gas outlet for supporting a bed of rubble and a rubble inlet aperture above the grate, the grate is at least partially composed of bars which are rotatable about longitudinal axes, the openings between the bars being such as to allow rubble discharge therethrough when the bars are rotated.

In this type of regenerative furnace, the bars are preferably of circular cross section, such circular bars requiring less torque for rotation than bars of say square cross section. Such circular bars may be supported in seatings mounted in the walls of the regenerator chamber, and may have end portions extending beyond the external wall, each end portion carrying a lug for connection to a common link means for effecting simultaneous rotary oscillation of the bars.

In this type of regenerative furnace, the rubble may be caused to fall through the grate by oscillatory rotation of the parallel bars about their axes, the oscillation being effected through about 60. While this motion could-be effected continuously if at a sufficiently slow rate, it is found convenient to operate the bars intermittently at regular intervals, each operation allowing a proportion of the rubble bed to fall through the grate.

This type of furnace may also include a rubble deflector plate disposed beneath the rotatable bars of the grate, which deflector plate slopes downwardly towards a normally obturated aperture in the outer wall of the regenerator.

In either of said types of furnace, the deflector plate is preferably in the form of a grating adapted to pass fine material in the rubble removed from the lower regions of the bed. By

this means the rubble is subjected to a preliminary sieving operation before it is removed from the regenerator, the fine materials removed at this stage collecting in the bottom of the 1 regenerator for removal through separate access means.

The rubble removed from the lower regions of the bed in accordance with the present invention is at a temperature of from 200 C. to 400 C., as compared with the temperature at I the top of the bed which is about l350 C. Accordingly with the system so far described there is little thermal shock involved in removing the rubble from the bed, but the thermal shock involved in adding relatively cold rubble to the top of the bed is considerable. The thermal shock resulting from adding such cold rubble to the top of the bed may be reduced by the use of a modified regenerator wherein the inlet aperture 1 for rubble is above the level of the said gas outlet communicating with the furnace chamber. Such a modified regenerator preferably includes a header hopper mounted to extend above the regenerator chamber and at one side thereof remote from the gas outlet, the inlet aperture for the cold rubble being at or adjacent the top of said hopper. The lower end of the hopper may include a downwardly inclined passageway leading to a port in the external wall of the chamber located opposite the gas outlet and approximately at the level thereof.

- In a regenerator of this type, the rubble entering the chamber at one side thereof through the inclined passageway and the port forms an upper surface of rubble which slopes downwardly at its natural angle of repose, the level of said surface at the side of the chamber containing the gas outlet being below the level of said gas outlet. Furthermore the hopper may contain a column of rubble which is not directly exposed to exhaust gases, and which is cooler than the region of the bed closest to the gas outlet. In this manner the rubble may be heated gradually on moving down this column, and undue thermal shock is avoided both to the rubble and to the refractory lining of the regenerator.

In this last type of regenerator, the bed of rubble in the regenerator chamber is preferably supported on two rotary bar grates, one grate extending across the half of the chamber which is adjacent the gas outlet, and the other grate extending across the other half of the chamber which is remote from the gas outlet and at a level above that of the first grate, whereby the distribution of the gas flow through the rubble towards or away from its said sloping upper surface is as uniform as is practicable.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to. FIG. 1, this shows a furnace heating chamber 10 which is constituted by a floor l1, sidewalls 12, end walls not shown, and a roof 13. One end of the furnace is provided with an elongated mouth for the charging of batch material into the furnace, and the other end of the furnace is associated with discharge means for molten glass.

The surface level of the molten glass in the chamber 10 is indicated at S, and each of the sidewalls I2 above the level S is provided with a series of ports 14 two of which are shown, the ports on one side of the furnace being in alignment with ports on the opposite side. Each port 14 is associated with an oil burner 15, so disposed that atomized oil projected by the burner can rapidly be burnt by air supplied by the associated port 14. Each port 14 is associated with an oil burner 15, so disposed that atomized oil projected by the burner can rapidly be burnt by air supplied by the associated port 14. Each port alternatively serves as an exhaust port, so that when firing is carried out from one side of the chamber the ports on the other side allow for the escape of combustion products.

Each port 14 is connected by a short duct to the gas outlet 16 of the upper end of a regenerator 20, the lower end of which regenerator is provided with ducting leading to an air supply inlet 17 and to an exhaust flue 18. Each regenerator is associated with a changeover valve 19 adapted selectively to obturate either the air inlet duct 17 or the exhaust outlet duct leading to the flue 18. Since each-regenerator is identical, apart from being of opposite hand at the opposite sides of the furnace, only one regenerator 20 will now be described in detail.

The regenerator comprises a chamber 22 communicating in the upper part through gas outlet 16 with the furnace chamber 10, and having in the lower part a grate including two

portions

23 and 24 which between them support a bed of rubble 25 when the regenerator is operative. The rubble bed extends upwardly as shown from the

grate

23, 24, to just below the level of the gas outlet 16. The rubble is preferably of basic refractory such as olivine ore, serpentine, or magnesite rubble, and it has been found satisfactory to use a bed of about 60 inches height of rubble having a particle size between 2 inches and 3 inches.

Grate portions

23 and 24 are mounted on the opposite end walls of the regenerator chamber, the lower edge of grate portion 24 being spaced from the surface of the portion 23 to pro-. vide an elongated discharge opening 26 therebetween for the grate structure. The

grate portions

23 and 24 are set at an angle greater than the angle of repose of the rubble, and the opening 26 is of sufficient area to allow the rubble to flow therethrough under the influence of gravity, if the rubble is not otherwise restrained.

Below the opening 26, the grate portion 23 is extended to form a deflector grating sloping downwardly towards an aperture 30 in the outer wall 31 of the regenerator chamber 22. Near the outer wall 31, the deflector grating is extended by means of an apron 23a. The aperture 30 is normally 'obturated by an airtight sealing door 32. The aperture 30 is also associated with a regulator for controlling the movement of rubble through the opening 26, which regulator is shown generally at 35 and is described in further detail hereinafter with references to FIG. 2.

The apertures in the grating 23 are such that most rubbleis retained by this grating, but fine materials are allowed to'i-fall through the grating and in this sense the grating constitutesa sieve. Such fine materials may include those produced in the rubble by internal abrasion, as well as batch material deposited in the rubble and volatile materials condensed in the rubble after being carried over by the exhaust gases leaving the furnace. Such fine materials, on passing through the grating, fall to the bottom of the regenerator where they do notinterfere with gas flow through the regenerator and from where they can be removed through an access door 36.

With the arrangement as so far described, the flow of rubble through the bed would be very much more rapid at the center cessation of movement of the angle pieces in the two end walls 310 of the chamber 22 (one of which walls is indicated in FIG. 1), and this rigid obstruction has been found to ensure adequate rubble flow at points near the regenerator walls, sothat none of the rubble remains in the bed for an unduly long period.

In the upper part of the outer wall 31 of regenerator 20, and slightly above the level of the gas outlet 16, there is provided a rubble inlet aperture 38, through which rubble may be inserted onto the top of the rubble bed 25. This aperture 38 is normally closed by a door 39, but this door can be hoisted clear of the-aperture .38 by a hoist rope 39a passing over a pulley 39b.

; FIG. 2 shows in detail the regulator 35 indicated in FIG. 1.

As shown in FIG. 2, the regulator 35 is mounted on an angle piece 40 fixed to the upper edge of aperture 30, which angle piece also holds the mounting 32a, for pivoted door 32. The regulator 35 comprises two vertically depending angle brackets 41 (only one of which is shown) welded to the angle piece 40, each bracket 41 having a. series of four screw studs 42 set at intervals along its length. The upper. two studs 42 of each bracket 41 carry between them two horizontal angle pieces 43a and 43b, these angle pieces being secured on studs 42 by nuts 44. The lower two studs 42 of each bracket 41 carry simiiar'angle pieces 43c and 43d, but in this case these are retained on-therespective studs 42 by nuts 45 which are slackened away from the respective angle pieces to allow lateral movement thereof. By virtue of the slope of the apron 23a, the rubble discharge opening between the apron 23a and the angle piece 43d is varied by this lateral movement.

The space between the angle piece 43d and apron 23a is set at about 5% inches (minimum), and .with this gap the rubble flow is normally choked. To effect discharge of rubble from the lower regions of the bed the pivoted door 32 is opened, and a rake is inserted into the aperture 30 and manipulated in such a manner as to cause angle pieces 43c and 43d to move laterally to and fro In this manner the rubble is caused to flow gradually under gravity, at no risk tothoperator, since any 430 and 43d stops the rubble flow.

In operation, the furnace is fired alternately from each side of the furnace on a regular cycle, which is of several hours duration.

During the first stage of the cycle, oil is supplied to each of the left-hand side burners l5, and combustion air is drawn into the furnace chamber via the left-hand ducting 17, (the valve 19 being raised as shown in full lines in FIG. 1 to obturate the exhaust flue l8 atthis side), into the respective left-hand side regenerator chamber 22 and upwardly through the rubble end 25 thereof where the air is heated before passing through the left-hand side gas outlet 16 and .port 14 and into the fumace chamber 10.- I

The combustion products pass over the molten glass in the furnace chamber, and exit through the right-hand side ports 14 and respective outlets 16 into the regenerator chamber 22 on the opposite side of the furnace chamber. The hot exhaust gases pass from each outlet 16 downwardly through the respective rubble bed25 of right-hand side regenerator and the valves 19 are reversed in position as shown by dotted lines in FIG. 1. By these means' the direction of circulation of the gases through thefurnaceis reversed, so that the combustion air now supplied to the right-hand side port 14 is preheated in the right-hand side regenerator chamber 22 while the rubble bed 15 of the left-hand side regenerator is heated by the exhaust gases. The change from the first to the second stage is referred to as the changeover, and this cycle is repeated indefinitely according to well established practice in the working of glass melting tanks.

In each stage of the operation, fine particulate material and volatile compounds carried over from the furnace chamber tend to block the interstices of the bed, and, as described above, if no rectifying action is taken then the bed may become so blocked that the flow of gas through the furnace is seriously reduced. Moreover, on changeover accumulated dust may be blown into the melting chamber and may contaminate the glass.

In accordance with the invention, rubble including fine particles is continually removed, from the lower regions of the bed in the manner described above with reference to FIG. 2. The rubble removed from the bed of each regenerator is collected together and sieved to remove fine materials and rubble agglomerates therefrom, and the sieved rubble is hoisted up to the level of the rubble inlet aperture 38 and added to the top of the bed to maintain the bed height roughly constant.

In practice the removal and replacement of rubble is carried out continually, about 10 percent of the rubble bed being removed at each operation and being replaced by a similar quantity of sieved rubble. The frequency of the rubble removed is such that the complete bed is replaced over a period of about 40 days. The rubble removal is preferably carried out when the exhaust gases are passing through the bed,

so that dust created during this operation does not enter the furnace.

The rubble removed from the lower regions of the bed has a temperature of between 200 C. and 400 C. as compared to a temperature of about 1350 C. for the top of the bed. Accordingly the removal of rubble as described from the lower regions of the bed only involves a loss of heat small in comparison to the total heat of the bed. Nevertheless, the heat content of the rubble removed from the bed may be conserved by subjecting the rubble to a vertical cycle of movements, the cycle including at least one sieving stage, and the cycle time being such that the rubble retains most of its heat between the removal from and return to the bed. For this purpose each regenerator may be associated with a vertical conveying system, the lower end of which system is disposed adjacent to the rubble outlet aperture 30 in the' wall of the regenerator, and the upper end of which system is located adjacent to the inlet aperture 38, the conveying system having an outlet allowing movement of rubble from the conveying system onto the grate.

A particular form of vertical conveying system may comprise a vertically movable hopper with raising and lowering means, the hopper having an upper opening which is situated to receive the rubble from the rubble outlet aperture 30 when the hopper is in its lowermost position, and the hopper having a bottom plate sloping downwardly towards an outlet in the side of the hopper adjacent to the regenerator, which outlet is normally closed by a flap hinged along its lower edge, the arrangement of the flap being such that with the hopper in its uppermost position the rubble may slide by gravity from the hopper through the inlet aperture 38 along the top surface of the open hinged flap.

Referring now to FIGS. 3 and 4, these show an alternative type of grate for the regenerator. of FIGS. 1 and 2, and parts of this alternative type of regenerator which correspond to parts shown in FIGS. 1 and 2 are similarly referenced.

In this arrangement, a substantially horizontal grate 50 supports a bed of rubble 51 and is formed of a plurality of parallel bars 52 of circular cross section. The end portions of these bars 52 are of reduced cross section and are supported in seatings mounted in the inside and outside regenerator chamber walls 53 and 54-respectively, which seatings are clearance fits on the bar end portions so that the bars are rotatable thereon about their longitudinal axes.

The end portions of the bars 52 extend beyond the outside wall of the regenerator, where they are fitted with lugs 55, the

ends of which lugs are all connected to a common link means comprising a horizontal connector rod 56 as most clearly shown in FIG. 4. One of the lugs 55 is extended beyond the 1 rod 56 to constitute an actuating lever 55a and is pivotaliy attached (as indicated at 55b) to the piston rod of a fluid that a grate 50 so formed is capable, when stationary, of sup- ;porting a bed of rubble of graded particle sizes between 2 .inches and 3 inches, since the rubble particles form supporting arches between the bars. When however, the bars are oscillated, the rubble arches break down and rubble falls through the grate at asteady rate as long as the bars are oscillated. The

,rate of rubble flow is quite uniform across the whole grate, and

consequently all rubble can be cycled through the regenerator chamber 22 with a closely controlled residence time in the bed.

In this second type of regenerator, the oscillatable bars 52 themselves regulate the movement of rubble through the openings which are defined between the bars.

While bars of circular cross section have been found to be satisfactory, bars ofnoncircular cross section may also be used providing that the extra torque required to rotate such bars does not impose undue strain on the operating mechanism.

Below the grate 50 is a deflector grating 60 sloping downwardly towards an aperture 30 in the outer wall 54 of the regenerator chamber 22 and normally obturated by a flap "door 32. The grating 60 is in the form of a sieve sized so that the'majority of rubble falling through the grate 50 is deflected out of the regenerator chamber through aperture 30, but fine materials in the rubble can pass through the grating 60 to collect in the bottom of the regenerator until removed through an access door as shown at 36 in FIG. 1. In operation, the regenerators of the type illustrated in FIGS. 3 and 4 are used for heating incoming combustion air and for cooling exhaust gases in the manner already described with reference to, FIGS. 1 and 2.

' To prevent the rubble in bed 51 from becoming blocked by carryover from the furnace chamber, a proportion (say percent) of the rubble bed is intermittently removed from the lower regions thereof by rotatably oscillating the bars 52 for a short period of time. Rubble is thus caused to fall through the openings between the bars 52, and onto the deflector grating 60. Fine materials which have collected in the rubble can pass through the deflector grating 60 for eventual removal from the bottom of the regenerator as already described with reference ,of FIG. 1, but the majority of the rubble is removed for sieving and regrading in similar manner to the rubble which is removed from the regenerators of FIGS. 1 and 2.

The removal of rubble from the rubble bed has been described as an intermittent process, but with the type of regenerator shown in FIGS. 3 and 4 the rubble removal may be made virtually continuous by uninterrupted slow oscillation of the bars'52.

It will be appreciated that a grate for a regenerator according to the invention need not necessarily be wholly constituted by rotatable bars, and that for example a part of the grate may comprise a pair of sloping grate sections, each sloping down towards a central grate comprising oscillatable bars 52.

Apparatus as herein described with reference to FIGS. 1 and 2 and FIGS. 3 and 4 for removal of part of the rubble bed in rubble bed regenerators is advantageous as compared to renewal of the whole bed in a single operation, in that no interruption in working the melting operation in the furnace chamber is necessary and in that the grates in the chamber 22 are at no time directly exposed to hot exhaust gases. Also, the frequency and ease with which the rubble may be moved reduces the amount of rubble adhesion taking place within the regenerator.

FIG. 5 shows a modification to the regenerator as describe with reference to FIGS. 1 and 2. Since this modification only affects the upper part of the regenerator, the lower part thereof is identical to that of FIGS. 1 and 2 and like parts are similarly referenced.

The rubble inlet aperture 38 is similar to that of FIGS. 1 and 2 and is provided with the same door and

door operating mechanism

39, 39a and 39b as already described. In this modification however the inlet aperture 38 is well above the level of the gas outlet 16, and the upper part of the regenerator between the aperture 38 and outlet 16 includes aprojection in the form of a stepped portion of the inner wall extending towards the rubble bed. The projection 70 has an extremity 71 so disposed between the aperture 38 and the gas outlet 16 that the angle formed from the extremity 71 to the lower edge of outlet 16 is less steep than the angle of repose of the rubble. With this arrangement, any rubble passing from the rubble inlet aperture 38 onto the bed is prevented from enteringthe gas outlet 16, even when the rubble is piled up to the level of the aperture 38 as shown.

The advantages of the'use of a rubble bed as shown in FIG.

5, over that of FIGS. 1 and 2, are:

a. The hot exhaust gases leaving outlet 16 do not strike directly onto the opposite wall of the regenerator, so that the refractory lining in this region is preserved from rapid deterioration.

b. The rubble entering the bed is heated gradually before arriving at the hottest part of the bed opposite the gas outlet 16. This much reduces the thermal shock to the relatively cold rubble entering the bed, and also reduces the thermal shock to which the refractory lining of the regenerator is subjected when cold rubble is introduced.

c. The larger volume of rubble which can be used with'the arrangement shown in FIG. 5 improves the thermal efficiency of the heat recovery system. I

Since the modification described with reference to FIG. 5

only affects the upper part of the regenerator, this modification can equally well be applied to the regenerator shown in FIGS. 3 and 4.

We claim:

1. A method of regenerative heat exchange of the type in which downwardly moving hot exhaust gases and upwardly moving incoming combustion gas pass alternately through a bed of rubble contained within a regenerator chamber, characterized by feeding rubble from the lower regions of the bed into an extension chamber disposed beneath the regenerator chamber, the base of the extension chamber comprising a grating and acting as a sieve for the rubble fed thereto, removing from the extension chamber sieved rubble substantially free of fine materials and feeding said sieved rubble to the top of the bed, whereby fine materials such as are carried over from the furnace are moved progressively downwards through the bed, and then through the base of the extension chamber thereby being separated from the rubble.

2. A method according to claim 1, wherein the rubble is fed by gravity into the extension chamber at a rate which is controlled by movement of a regulator in the discharge path of the rubble.

3. A method according to claim 1, including the step of maintaining the rubble flow through the bed substantially uniform.

4. A regenerative furnace including a pair of regenerators each having a chamber communicating through a' gas outlet with the furnace heating chamber, and of the type in which downwardly moving hot exhaust gases 'and upwardly moving incoming combustion gas pass alternately through a bed of rubble contained within each regenerator chamber, each regenerator chamber having a grate situated below the level of said gas outlet for supporting a bed of rubble and a rubble inlet aperture above the grate, the grate forming a division between the regenerator chamber and an extension chamber and also at least one elongated discharge opening between the regenerator chamber and the extension chamber, and each extension chamber having a base comprising a grating to act as a sieve for the rubble fed thereto from the lower regions of the bed.

5. A regenerative furnace according to claim 4, wherein said grate includes two inwardly and downwardly sloping por tions, the lower edge of one portion being spaced from the surface of the other portion to provide said elongated discharge opening.

6. A regenerative furnace according to claim 5, wherein one of the said downwardly sloping grate portions extends downwardly below the said discharge opening to form said grating sloping towards a normally obturated aperture in the outer wall of the regenerator.

7. A regenerative furnace according to claim 6, wherein said grating is associated with regulating means movable to control the movement of rubble through the said discharge opening.

8. A regenerative furnace according to claim 7, wherein the regulating means includes a horizontally disposed member mounted for lateral movement, whereby the rubble discharge opening is varied in size to allow controlled discharge of the rubble therethrough.

9. A regenerative furnace for operation according to claim 4, in which the grate of each regenerator chamber is at least partially composed of bars which are rotatable about longitudinal axes, the openings between the bars being such as to allow rubble discharge therethrough into the extension chamber when the bars are rotated.

10. A regenerative furnace according to claim 9, wherein the bars are of circular cross section.

11. A regenerative furnace according to claim 10, wherein the circular bars are supported in seating mounted in the walls of the regenerator chamber.

12. A regenerative furnace according to claim 11, wherein the circular bars have end portions which extend beyond the external wall of the chamber each such end portion carrying a lug for connection to a common link means for effecting simultaneous rotary oscillation of the bars.

13. A regenerative furnace according to claim 4, wherein each regenerator includes a header hopper mounted to extend above the regenerator chamber and at one side thereof remote from the gas outlet, the inlet aperture of the regenerator being at or adjacent the top of said hopper.

14. A regenerative furnace according to claim 4, wherein an obstruction is provided in the regenerator chamber to ensure substantially uniform rubble flow through the bed.