GB1581648A - Gas fired radiant heater - Google Patents

Description

(54) GAS FIRED RADIANT HEATER (71) I, THOMAS MARSDEN SMITH, a Citizen of the United States of America, of P.O. Box C 94, 114 Villinger Avenue, Cinnaminson, State of New Jersey, 08077, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is concerned with radiant heaters.

The invention provides a gas-fired radiant heater comprising a porous refractory mat suitable for the passage of a gaseous combustion mixture therethrough, means engaging the mat and defining adjacent first and second zones for separately supplying gas to corresponding areas of the mat, means for supplying a gaseous combustion mixture to the first zone whereby in use the mixture passes through the thickness of the mat and burns where it emerges from the mat, and means for supplying a non-combustible gas to the second zone to prevent the combustion mixture at the first area of the mat from spreading through the mat to the second area.

Heaters according to the invention may be used for sealing metal tubes in a sheet, plication No. 7903219 (Serial No. 1,581,649).

as described and claimed in appending ap The mat of the heater of the invention may comprise interfelted ceramic fibers, a gaseous combustipn mixture being continually passed through the mat and burnt at the mat face from which it emerges. The combustion takes the form of a flame that extends over the entire area of the face from which the combustion mixture emerges, the flame length being very small so that the surface fibers at the flame are heated to red heat or hotter and form an essentially continuous wall of heat that makes a very effective heat radiator. By increasing or decreasing the rate of flow and/or composition of the combustion mixture, the temperature of the heated fibers can be controlled.

A number of Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a face view of an infrared heater according to the present invention; Figure 2 is a sectional detail view of the heater of Figure 1, taken along the line 2-2; Figure 3 is a plan view of a component that can be used in the making of the heater of Figures 1 and 2; Figure 4 is a detail view similar to that of Figure 1, showing some structural features suitable for the infrared heaters of the present invention; Figure 5 is a view similar to that of Figure 2 showing an optional method of constructing the heaters of the present invention; Figure 6 is a vertical sectional view partly diagrammatic of a heating arrangement pursuant to the present invention;; Figure 7 is a somewhat diagrammatic vertical sectional view of apparatus for sealing tubes in a sheet, using a heater in accordance with the present invention; Figure 8 is a similar view of a modified tube-sealing apparatus incorporating a heater in accordance with the present invention; Figure 9 is a sectional view of a gas-fired radiant heater according to another embodiment of the present invention; Figure 10 is a p]an view of an assembly of heaters each of the construction illustrated in Figure 9; Figure 11 is a sectional view similar to Figure 9 of a heater construction according to another embodiment of the present invention; Figure 12 is a plan view of the heater of Figure 11; Figure 12A is a plan view of a component of the structure of Figures 11 and 12;; Figure 13 is a broken-away plan view of a portion of a heater according to the present invention showing a detail feature; Figure 14 is a sectional view of the construction of Figure 13, taken along line 14--14; Figure 15 is a sectional view of a different heater construction pursuant to yet a further aspect of the present invention; Figures 16 and 16A are sectional views of a heater construction typifying yet another aspect of the present invention; and Figure 17 is a view of another embodiment of the present invention.

In one embodiment of the present invention a gas-fired radiant heater comprises a supported porous refractory panel through which a gaseous combustion mixture is passed and on the face of which the mixture is burned as it emerges. A narrow stream of relatively cold non-combustible gas is passed through the panel immediately adjacent the panel support as the foregoing burning takes place.

This non-combustible gas stream acts as a barrier which directs the combustible mixture through the refractory panel and minimizes leakage of combustible gases past the frame members that hold the panel. By acting as a combustible gas barrier, the noncombustible gas stream significantly reduces the importance of the seal shown in U.S.

Patents 3,785,763 and 3,824,064, greatly reducing burner assembly time and parts tolerance. The non-combustible gas stream also greatly reduces contact of the hot gaseous products, resulting from the combustion at the panels surface, on the frame members keeping them much cooler and reducing heat warpage.

The narrow stream of relatively cold gas is conveniently provided by holding the porous panel on the ledge carried by the combustion mixture plenum for the porous panel, and a slot extends along the ledge and is connected to a supply of the noncombustible gas.

Turning now to the drawings, the radiant heater of Figs. 1 and 2 has a porous refractory mat 10 held at its margins by upper frame members 21, 22, 23, 24, against a lower frame 30. Frame 30 has four lengths of tubular supports, two of which are shown in Fig. 2 at 31, 32, secured to the margins of a rectangular back plate 40 by welding, brazing, cementing as with epoxy or other cement, or otherwise joining in a gas-tight manner, indicated at 42. Back plate 40 and the four tubular supports thus define a plenum for the combustion mixture fed to the panel 10. A pipe connected can be welded through an aperture in the back plate in the standard manner for receiving a combustion mixture supply conduit, and a baffle a portion of which is shown at 44, can be fitted to help equalize the combustion mixture flow toward all portions of panel 10.

One or more lengths of the tubular support frame can also have a connector 53 welded through an aperture for the supply of air from a pump or a storage tank or the like. A slot 55 is also provided along the top wall 57 of the support frame for discharge of the air from the interior of the tubular support lengths through the margin of the porous refractory panel. The individual tubular supports are mitered together at the corners of the frame with the mitered joint sealed as by welding, brazing, cementing or otherwise securely joining, to keep combustion mixture from leaking out of the plenum as well as from being nonuniformly diluted with the air moving through the tubular supports.

The upper frame members 21, 22, 23, 24 are shown as angles each having an upper flange 61 that overlies a margin of the outer face of panel 10, and a depending flange 62 that is secured to a lower frame member, as by means of the screws 64. The screws can be threadedly received in the outer walls of the tubular support members, and can pass through openings in the flange 62. Such openings can be elongate in the direction perpendicular to the wall 57 if adjustability is to be provided for the spacing between wall 57 and flange 61.

The porous panel 10 permits the gaseous combustion mixture to freely pass through it so that pressures in the plenum need only be about 2 to 7 inches of water above the ambient atmosphere to provide very effective uniform combustion over substantially the entire outer face of panel 10. A similar air pressure in the interior of the tubular support will cause streams of air to pass through the margin or porous panel 10 and emerge from its outer face. The porous interfelted fibrous structure of the panel surprisingly does not permit much change in the width of the air stream moving through the panel, particularly when the pressure the propels the air stream from the tubular support is within an inch or two of water height with respect to the pressure that propels the gaseous combustion mixture from the plenum.

This is readily noted when the burner is in operation inasmuch as the outer surface of the panel glows red hot over its entire area except for a narrow and sharply defined band around its periphery and adjacent the outer frame members.

The frame members are thus kept much cooler than they would be without the marginal air stream, particularly where the burners are operated with the outer surface of their panels 10 positioned in a generally vertical plane, or positioned facing downwardly. In those positions the very hot gaseous products resulting from the combustion at the panel's surface. rise and flow over the frame members of the burners of Patents 3.785,763 and 3.824.064 to heat them un to high temperatures that can reach 1000"F in some cases. A marginal air stream however, used in the heater of Figure 1, acts as a barrier layer against the hot combustion products, keeping those hot gases from directly reaching the frame members in substantial volume.Marginal stream taken from the air at ambient temperatures and passing through a panel as much as 12 inch thick will generally keep the outer faces of 1/16 inch thick steel frame members several hundred degrees F below the temperatures corresponding frame members reach in the constructions of the above patents. The temperature of the frames will be even lower where the heater is used to heat objects that do not cause much reflection of the burner's radiating heat back to those frame members.

A further benefit of the construction of Figures 1 and 2 is that by minimizing contact of the hot combustion gases with the frame members and thus keeping them much cooler, emission of radiation from the frame members themselves is greatly reduced. The heaters can be positioned much closer to their targets, than prior art heaters and still permit minimizing damage to the target in the event of emergency shutdown. The porous refractory mat itself cools down very sharply when the fuel gas flow into the plenum is stopped and the air flow is maintained but the frame members of the prior art heaters take much longer to cool down.

Thus when using such heaters to heat a moving web of heat-sensitive material, the heaters are preferably arranged to generate much more heat than the web can tolerate should the web stop moving. With the prior art heaters the rate of cool-down for the frame members can become a critical factor that determines how close the prior art heater can be brought to the web without damaging the web in the event the web suddenly stops and the heaters cannot be mechanically pulled away from it. The heaters of Figs. 1 and 2 don't have to be pulled away and can therefore be installed in a less expensive manner. Their closer proximity to the target makes the heat trans fer to the target more efficient and enables the use of less fuel to achieve the desired results.

Moreover in some treatments such as the volatilizing of water from a target web, the most effective wave-lengths are between about 3.2 to about 3.6 microns, a range that is most efficiently produced at relatively low radiation temperatures. By moving the heaters closer to their targets, their radiation temperatures can be diminished to thus make more efficient use of the fuel energy and with less fuel, without decreasing the treatment effectiveness.

Heaters placed very close to targets may be desirably made to extend beyond the edges of the target to attain greater treatment uniformity. Each such extension can be approximately equal to the distance from the heater to the target, for good results.

Locating slot 55 immediately opposite the panel margins alongside the inner edges of the upper frame members 21, 22, 23, 24 helps guide the protective streams to the desired location. This guiding action is further improved by sealing the edges of the panel so that not much non-combustible gas can escape laterally. Fig. 2 illustrates a prior art edge sealing technique according to which a thin foil of aluminum 70, about 2 mils thick, is wrapped around each panel edge, and the lower face of the foil is sealed against wall 57 by a narrow line of sealant 72 such as a silicone rubber vulcanized in place.

When edge sealing of the mat of a heater in accordance with Figures 1 and 2 is desired, the sealant 72 can be of a material such as ordinary rubber or neoprene, that need not be resistant to high temperatures. However during normal operation of the burner, only air from the interior of the tubular support will tend to leak out from the margins of the mat. Such leakage is not dangerous nor is it extensive when sealant 72 is entirely omitted. Omitting the foil 70 can cause extensive air leakage unless the outer frame is a very close fit against the support frame.

The marginal air stream with or without leakage prevents the combustion mixture from leaking out of the edges of the mat.

The slot 55 does a very effective job when it is about 4 inch wide, although it can be as little as 1/16 inch or as much as -1- inch wide and still give good results. The width of protective gas stream emerging from the face of the mat is generally a little larger than the width of the slot, and changes in gas pressure vary this broadening effect. A desirable gas pressure in the tubular frame is one that approximately equals the pressure in the combustion mixture plenum.

The cooling and combustion-mixture-leakblocking effects of the marginal gas stream described above are also obtained when the discharge slot 55 is located further toward the outer face of the frames so that the gas discharged through the slot is directed partly or completely at frame flange 61. Most of the discharge gas will then move along the interior of panel 10 and escape just past the inner margin of that frame flange.

It is not essential to make the tubular support members gas-tight where they are threadedly engaged by screws 64. Even where relatively expensive inert gas is used rather than air, the leakage through such threaded connections is miniscule as compared with the discharge through slot 55. The threaded engagement can be sealed however, as by applying pipe-thread dope or the like to the mating threads before they are engaged.

Alternatively the connection between the outer and inner frames can be made as shown in U. S. Patents 3,785,763 and 3,824,064.

Instead of making tubular frame 30 of four separate lengths, it can be made from a single piece of formed sheet metal as illustrated in Fig. 3. An elongate strip of sheet metal twenty to fifty thousandths of an inch thick can be bent into the form illustrated by the sectional view in Fig. 2, or a standard metal tube of rectangular section can be milled to cut the slot 55 through one wall, and the resulting shape then subjected to mitering cuts 81, 82, 83, 84 and 85 as shown in Fig. 3. These cuts leave wall 88 intact and the mitered length then bent to form a one-piece tubular frame a corner of which is shown in Fig. 4. The inner edge of each corner is then welded, brazed, cemented or otherwise joined as at 89 to seal the entire height of that corner and the tubular frame is ready for similar joining to the back plate 40.

It is not necessary to seal the outer face 90 of the mitered joints, particularly if the joints are a close fit. A little extra leakage at those locations from the interior of the tubular frame does no particular harm.

However, that outer face can be sealed, especially if lateral leakage from the frame margin takes place.

The tubular frame need not extend inwardly of the slot 55, although it helps to have that frame provide an additional flat support 73 for the porous panel 10. Such support can be reduced to the thickness of the metal from which the tubular frame is made, as by suitably shaping the tube from which it is sliced, or by milling the slot 55 alongside the inner wall of the tubular frame.

Figure 5 shows another tubular frame construction of which may be incorporated into heaters in accordance with the present invention, which is simple to manufacture. Here a flat support 173 takes the place of support 73 and extends toward the center of - the plenum.

It is also helpful to seal the outer margin of panel 10 as by dipping it in or brushing on a hardenable liquid resin that hardens to a temperature-resistant solid. Solutions of silicone rubber, colloidal silica, and sodium silicate are examples of suitable hardenable materials. When this type of edge sealing is used, the aluminum foil is not needed.

In some installations the panel temperature is so hot are there is so much reflection of heat from the surfaces being heated by the heater, that aluminum can be damaged.

Other metals such as stainless steel can then be used for the sealing foil.

Fig. 6 shows a particularly effective heating arrangement for heat treatment of a moving web 100, such as textile drying and curing or paper processing, the direction of movement is shown by arrow 102. In this arrangement a series of burners 110 face the moving web adjacent each other on opposite sides of the web. Immediately facing each burner 110 is a re-radiator 120 having a very thin layer of heat-absorbing material such as oxidized stair!css steel 122, backed by a high temperatu.e insulator 124 such as refractory felt. The re-radiators are preferably substantially wider than the burners and in use the heat absorbing layer 122 absorbs substantial quantities of heat which penetrate web 100 so that the layer becomes quite hot and re-radiates heat back to the web 100.To improve the drying or gasremoving effect of the heat treatment process, intake and exhaust ducts 130 and 132, respectively introduce streams of poorly saturated air adjacent the location where the web approaches the burner, and withdraw more saturated air adjacent the locations where the web leaves the burner. To further improve the efficiency of this system, heat from the withdrawn air can be used to preheat the incoming poorly saturated air.

The present invention is not confined to use with mats 10 that are flat. Such panels can also be convex or concave such as when the infrared radiation they produce is to be specially oriented. Thus a concave mat does a very good job of concentrating such rays.

The mats are preferably formed by felting ceramic fibers on a screen surface, and that surface can be shaped to fit the desired mat configuration. A binder of some sort, such as starch or sodium silicate or the like can be mixed in small amounts with the fibers to set and help to hold the fibers to each other where they touch each other.

Figures 7 and 8, show apparatus for the sealing of metal joints as described and claimed in copending application no.

7903219 (Serial No. 1,581,649), such as the joints between metal heat exchanger tubes and metallic tube mounting sheets. The apparatus incorporates a heater in accordance with the invention.

Industry has need for relatively small allmetal heat exchangers, as for use in cooling oil that lubricates an internal combustion engine. Such a heat exchanger can have as many as several hundred heat exchange tubes connected between two sheets in a leak-proof manner. Leak-proof connections for this purpose are generally made by a fusible metal sealant whose melting takes place at a temperature well above the maximum operating temperature of the heat exchanger. While tin-lead solders can be used as sealants for operating temperatures near the normal boiling point of water when no great mechanical stresses are encountered, brazing alloys including the so-called silver solders are used for higher operating temperatures or higher stresses.Such leak-proof brazing of a quantity of relatively small tubes in a tube sheet has been an awkward industrial operation that takes substantial time to assure the heating of all joints to the desired sealing temperature, and generally requires patching to seal leaks resulting from uneven heating during the original sealing.

According to the method of application no. 7903219 (Serial No. 1,581,649) more rapid and more effective sealing is accomplished by holding an assembly of the heatexchange tubes each tube having one end in a sheet with the sheet in essentially horizontal position and carrying on its upper surface a quantity of fusible metallic sealant adequate to seal all tubes into the sheet, applying radiant heat downwardly on the sheet to heat it at least to the fusion point of the fusible sealant, and moving gases down from above the tube ends down through the tube ends during the heating to cause the heating to be more uniformly applied to the tubes so that the sealant rapidly seals all tubes to the sheet.

The heat for the fusion is applied in the apparatus of figures 7 and 8 by a ceramic fiber burner in accordance with the invention. Those burners generally have a ceramic fibre mat made of the ceramic fibers described in U.S. Patent 3,449,137, with the mat formation as described in U.S. Patent 3,787,194.

The most efficient heating results are obtained when the burner that supplies the heat envelopes the top and sides of the sheet in the tube-and-sheet assembly, as shown in Figure 7. The burner of Figure 7 is divided into sections that can be operated indepen dently to first heat the margin of the sheet in the sheet-and-tube assembly, and then heat the center. A particularly effective burner construction for this purpose uses a single porous ceramic fiber mat in the general shape of a hat with a shallow plenum divided by a wall into two parts, air or other incombustible gas being fed through one part when that part is not being operated while the other part is being operated. The porous margin of the mat can be sealed by a high-temperature-resistant impregnant like aqueous sodium silicate, and the sealed margin clamped in place.Sheets of soft material like aluminum foil can be interposed between the sealed margin and the clamping members.

The apparatus of Fig. 7 includes a table 10 movable up and down as indicated by the two-headed arrow 12, and a radiant heater 50 positioned above the table. The table carries on its upper surface a block 14 having a number of vertical passageways 16 corresponding to the number of tubes 18 to be assembled into a heat exchanger, and located in a corresponding pattern. The upper ends of the passageways 16 are enlarged as at 20 to receive and position the lowest portion of each tube. The lower ends of the passageways 16 open at the bottom of block 14 over a suction opening 22 in table 10.

A blower 24 is shown as carried by table 10 and as provided with a suction tube 26 connected as by flexible duct 28 to a mounting ring 30 secured around opening 22. A butterfly valve 32 can be fitted to the suction tube 26 to enable controlling of the suction applied to the bottom of block 14 when the blower 24 is operated. Also the suction tube 26 can be spaced as by webs 27 within a wider intake mouth 29, so that when the blower operates it sucks air in around the suction tube 26 as it sucks through tube 26.

Block 14 also carries a set of supports 34 encircling the tubes 18 and holding a tube sheet 36 in position at or near the tops of tubes 18. Supports 34 can be removably fitted in sockets 40 in block 14, and can have their lower portion cut away as at 42 to allow for the positioning of another tube sheet 37 on block 14.

Heater 50 has a porous ceramic fiber mat 52 in the general shape of a hat with a horizontal flange 54 by which it is mounted in place behind a face plate 56. The crown section of the hat shape consists of a cylindrical portion 58 a few inches in height and a hemispherical portion 60, and a relatively shallow plenum space 62 is provided around the crown by a housing 64 to which the face plate 56 is removably secured.

The plenum space is divided by a partition 66 that extends around the inside of the housing, into a lower generally annular plenum portion 68, and an upper hemispherical shelllike portion 70. Separate inlet nipples 71, 72 are provided on the housing for separately supplying combustion mixture to the separate plenum portions. In the illustrated embodiment the housing 64 is made of a lower cylindrical section 74 and an upper hemispherical section 76. Outwardly projecting flanges 78, 80 on these housing sections where they meet, serve as attachment structure for holding the entire housing together and also holding partition 66 in place. To this end a number of threaded flange bolts 82 project through aligned sets of openings in flanges 78, 80 and in partition 66, and nuts 84 threaded on these bolts secure these members together.The bolts are distributed around the housing and they also project downwardly for enough to provide securing means for the face plate 56 which is also provided with mounting openings aligned with the bolts. An extra set of nuts 86 threaded on the bolts secures the face plate in place.

The burner is constructed by first assem- bling the housing portions 66, 74, 76, then forcing the pre-formed and prepared mat in the assembly so that it firmly engages the inner lip of partition 66, and then securing the face plate. The partition lip can be turned up as shown at 67, to make a better seal against the mat.

An internally directed flange 88 at the lower end of lower housing section 74 is used to provide a ledge against which the mat flange 54 is held to help seal the edges of the mat against gas leakage. A cylindrical flange 90 is also shown as integral with and projecting up from the top of the face plate, to encircle the mat edges and closely fit around the lower edge of the housing. This helps hold the mat in position and strengthen the face plate. A central hole 92 in the face plate slightly larger than the mouth of the mat 52 permits the top of the tube-and-sheet assembly to be brought into the burner a short distance above the mouth of the mat, as well as the movement of gases out from and into the work space 94 enveloped by the mat.

The burner is operated with gaseous combustion mixtures, and it is accordingly helpful to seal all locations through which such a mixture can leak out from the burner.

Thus the joint between the housing members 64 and 66 as well as between 66 and 74, can be sealed by gasketing or as shown by painting these junctures with a liquid silicone that cures to a solid sealant. Also the margin of the mat flange 54 is shown as encircled by a sheet of aluminum foil 93 carefully folded around the upper, lower and edge faces 94, 95, 96, and sealed against ledge 88 by a sealant such as a self-curing liquid silicone rubber.

It is also helpful to fill the pores of the mat in the outer section of mat flange 54, as by impregnating that section with aqueous sodium silicate that dries in place or liquid silicone rubber that cures in place, as indicated at 97. Another desirable feature is to water cool the outer margin of the face plate, as by brazing water-cooling coils 98 to its lower surface.

In operation the apparatus of Fig. 7 has its table first fitted with the tubes and tube sheets as shown, although there will usually be many more tubes than indicated in the figure, and a quantity of powdered or granular fusible sealing material 99 spread over the upper sheet 36. The blower 24 is started and the table is raised to the position illustrated so that the upper sheet 36 has its upper surface and side edges enveloped by the bumer. Both sections of the burner are then started, followed by opening of suction control valve 32.When the tubes 18 are copper or brass with a wall thickness of about 30 mils, and the upper sheet 36 is of copper, brass or steel with a width of 8 inches and a wall thickness of about 90 mils, and the burner is burning about 130,000 B.T.U. per hour of combustion mixture, a copper-phosphorous or silver-copper-flux sealing braze will in less than about 1 minute be melted and will flow into and seal each tube to the sheet with a text-book seal, regardless of how many tubes there are. Care should be used when applying the fusible sealing material so that excess material does not plug any tubes, which would impede the flow of hot gas through the tube resulting in uneven heating.

To avoid overheating, the burner is shut off as soon as the sealing is completed, although the suction can be continued. Prolonging the suction helps cool down the heated assembly and thus further reduces surface oxidation.

If the suction is not used during the heating the heat-up of the sheet is not uniform and much more heat-up time is needed before all parts of the sheet are hot enough to melt the sealing material. By that time the outer portions of the sheet are greatly overheated and if not badly damaged can also become sealed to the supports 34 even if the upper ends of the supports are about W inch thick steel. On the other hand when the burnt combustion gases are sucked down the tubes at a speed as low as about 12 linear foot per second the heat-up becomes so uniform that the sealing of all the tubes is completed long before the upper ends of supports 34 get hot enough to seal.The portions of the sheet 36 touched by the supports 34 will not heat-up very rapidly, with or without the foregoing gas flow, and this will also tend to make the immediately adjacent portions of the sheet a little slow in heatingup so that for best results it is desirable to have the tubes at least about { of an inch away from all supports. Those supports can also carry special fittings that make their upper ends more massive for even greater thermal inertia, but the 38- inch spacing of the tubes from their tops is still enough.Where there is considerable hardware around the margin of the tube sheet it is helpful to start the lower section 58 of the burner 50 before starting the upper section 60, and to start the upper section a few seconds later after the margin of the sheet has absorbed sufficient heat to be well on its way to sealing temperature. To guard against misoperation air without fuel is blown through the upper portion 60 of the mat while the lower portion is burning and the upper portion is not burning. This practically equalizes the pressures on both sides of partition 66 and thus minimizes flow of combustible mixture to undesired locations where it can be unintentionally ignited.

Filling the mat pores at 97 also avoids localized collection of stagnant combustible mixture.

There is no practical upper limit to the speed with which the hot combustion gases are forced down the tubes. There is for example no need for gas-tight connections between the tubes and passageways 16; indeed as shown by the open gap between suction tube 26 and suction intake 27, it is helpful to have air leaks that draw unheated air into the blower along with the hot combustion gases and thus help guard against overheating of the blower.

The tubes 18 are themselves not very wide, generally less than a half inch in inside diameter, so that it is difficult to effect extremely rapid gas movement through them. Speeds of 20 feet per second are suitable.

The seals made in a fraction of a minute by this method are found to have far fewer flaws than seals made in two-and-a-half minutes without the use of the gas movement down the tubes. Moreover because of the much greater uniformity of the heat-up, the melting and flow of the sealing material is also more uniform so that less sealing material is needed. As compared to the quantities of sealing material ordinarily used in the prior art, about half as much is needed in the above method. Thus for joints in which the tubes have an outside diameter about 2 mils smaller than the diameters of the holes in the sheet, only about one gram of sealing material are needed for every square inch of sheet surface.

Fig. 8 shows a modified sealing arrangement. Here a burner 150 having a generally flat burner face 152 is used. This extends the heat-up time somewhat as compared to the construction of Fig. 7, and as a result wide assemblies may take as much as 50% more time to seal. However the sealing time is still far less than obtainable from the prior art.

The burner 150 of Fig. 8 is constructed in the manner described earlier (see Figures 1 to 6), preferably along the lines of Fig. 5, where the ceramic fiber mat has its margin merely fitted to a frame having an inert gas blow-through arrangement in which the inert gas thus blown through the margins of the mat acts as to seal those margins against combustible mixture leakage. No other margin sealing is then needed.

In the Fig. 8 arrangement tubes 118 are sealed to a sheet 136 while the tube-andsheet assembly is held within a tubular casing 119 which eventually forms the shell of the heat exchanger. In about a half minute such an assembly can be sealed following which the assembly is inverted so that the opposite end is similarly sealed, and the sheets are then later brazed or welded to the shell margins. Where the shell is steel of low wall thickness it can be sealed against the sheets at the same time as the tubes are sealed, preferably using the enveloping burner arrangement of Fig. 7.

While suction provides a convenient tech- nique for moving the hot burnt combustion gases through the tubes, they can also be forced through from above. Thus the burner of Fig. 8 can have its frame provided with a depending cylindrical extension that encircles the shell 119 and has an asbestos lining pad that closely engages the shell.

Operating the burner in such an arrangement causes the hot burnt combustion gases to be discharged downwardly through tubes 118 since they have essentially no other way to escape.

The heater can be operated continuously while sealing a succession of assemblies, but is preferably operated only for short intervals while the sealing metal is being melted and flows into place. Thus the heater can be completely shut off between sealing sequences, and ceramic fiber burners are particularly helpful in such intermittent operations inasmuch as they heat up and cool down in only a few seconds. For such intermittent operations it is also helpful to have the burner plenum of relatively small volume, preferably not over about 1-t inches deep.

In this way combustion gas can be intermittently fed to the plenum and rapidly reach the exit surface of the fiber mat where it is burned, so that the timing of the burner action is simplified.

An igniter such as a pilot light assembly or an electric spark ignitor can be fitted near the margin of the burner to assure that it lights up each time a combustion gas feed is initiated. A settable automatic switching sequencer can be used to time the gas feed to the different burner portions as well as the suction blower.

Instead of, or in addition to, moving the table up and down to bring the work to the burner, the burner can be moved toward and away from the table. In the construction of Fig. 8 no vertical movement is needed by the table or the burner.

An auxiliary heater can also be provided around and above the lower tube sheet 37 in the construction of Fig. 7 and operated to seal the lower tube ends into that sheet while the tube-and-sheet assembly is held in the illustrated position. Thus a layer of sealing mixture can be applied to the upper surface of the lower sheet and the auxiliary heater started even before the burner 50 is lit inasmuch as the heat-up of the lower sheet takes longer than that of the upper sheet.

Where the margin of a ceramic fiber mat has its pores well sealed, as by the silicone or sodium silicate or other alkali metal silicate impregnant, the mat margin can be clamped in place without wrapping the aluminum foil 93 around those edges. The aluminum foil or other gasketing can still be inserted between the mat margin and the plenum margin, or the silicone or alkali metal silicate can also be used to seal the mat edge to the plenum.

Turning now to Figures 9 to 19, yet further embodimiénts of the invention will be described. In particular, modified radiant heaters in accordance with the invention a particular type of igniter structure, a furnace, and a refractory panel will now be described.

A gas-fired radiant heater in this embodi ment of the invention has a porous refractory mat through the thickness of which a combustion mixture is passed and which is mounted by its edges, and which has a narrow stream of non-combustible gas such as air passed through the panel all along its edges, in an amount that without further help keeps the combustion mixture from escaping through the panel edges. The porous refractory mat can be flat, convex, concave, cup-shaped, hat-shaped, or have any other desired configuration.

An even greater simplification of the foregoing heater construction and operation is effected by squeezing the panel margins so that they are compressed at least about 10% from their uncompressed thickness, inasmuch as this simple expedient also helps reduce the escape of combustion mixture through the panel edges. When combining a marginal gas stream seal with the edge compression, very effective edge sealing is accomplished with only a fraction of the flow of noncombustible gas otherwise required. With either arrangement however, no other assistance is needed to seal against edge leakage of combustion mixture, and edge impregnation as well as edge wrapping of the panel is completely dispensed with.

Fig. 9 illustrates a heater 100 with the improved edge sealing. Heater 100 has a cup-shaped panel 102 of interfelted refractory fibres, such as described earlier, clamped by its edges around a support assemble 104 made of stainless steel or other metal members shaped from relatively thin stock, about inch inch thick. A central dish 106 has a floor 105 and inclined walls 107 with raised edges 108 against which the panel 102 is pressed to define a combustion mixture plenum 110. Outer face 103 of panel 102 is of rectangular shape, and so is plenum 110.

Secured to the outer margin of the floor 105 of dish 106 is a series of angles two of which are shown at 112, 114, defining a rectangular frame against which the edges 120 of panel 102 are fitted. These angles are illustrated as having horizontal webs 122 welded or brazed to the floor of dish 106, and vertical webs 124 that approach but do not quite reach the dish edges 108. The frame angles define with dish walls 107 an outer plenum 130 that encircles combustion mixture plenum 110 and has a discharge slot 132 that is engaged by the margin of panel 102. The frame members are mitered or otherwise interfitted at the corners of the frame to minimize, or completely seal the outer plenum against, leakage in those locations.Supply nipples 146, 148 are fitted in openings in the floor 105 ard one or more of the frame angles 112, to deliver, respectively, combustion mixture and non-combustible gas. Baffles such as the U-shaped deflector 116 can also be provided to help more uniformly distribute the incoming gases.

Inasmuch as air is generally the non-combustible gas that flows through plenum 130, a little leakage doesn't do any particular harm other than consume a little excess air.

Anchoring of panel 102 in place is shown as effected with the help of a series of four or more clamping angles 136, 138, clamping the panel edges 120 against the frame angles, with the help of screws 140 that penetrate through aligned openings in the angles and are threaded into self-locking nuts 142 mounted in webs 124 as by securing clips or welding. The screws which need be no thicker than about 3/lG inch, are readily pushed through the edges of the panel without seriously damaging the panel, and any damage that might promote gas leakage is more than compensated by drawing up the clamps sufficiently to compress the panel edges.Standard panels have a wall thickness of about 11 inches and an interfiber spacing such that more than half that thickness is fiber and binder, so that compressing the edges to reduce the overall thickness only about 10% sharply reduces the air space between fibers and greatly limits leakage.

However very effective panels of interfelted fibers can be made by needling a mat of such fibers without the help of binder.

Such needled panels can be extremely pliable, as compared to molded binder-containing mats that are stiff like boards, and can have their edges compressed down to as little as about 30% of their uncompressed thickness.

Even compressing such edges that are originally about one inch thick down to about { inch provides an extremely effective backup for the air seal.

For such pliable panels it is preferred that the edge compression be down to about half the original thickness, or less. If desired however a pliable panel can be stiffened over its edges alone, or over its entirely, as by impregnating it with a water solution of starch or the like. In such stiffened condition, the degree of edge compression can be reduced.

To reduce any effect that the compression may have in breaking panel fibers that are binder-impregnated, the panel edges to be compressed are first dipped in water or other solvent for the binder carried by the fibers. Such wetting makes the edges more readily deformable so that the compressing is easily effected without seriously stressing the clamping structures. To assure uni formity of compression of board-like panels, the screws 140 are no more than about 8 inches apart when the angles have the abovenoted wall thickness. Where the heaters are operated in confined spaces so that the clamping angles are subjected to considerable reflected heat, it is helpful to cut slots about six inches apart through the vertical webs of those angles, to allow for thermal expansion and contraction without distortion of the support.Such slots need only be about 20 mils wide, but can be omitted where the clamping angles do not engage each other at the corners of the frame so that expansion is possible at those comers.

A feature of the heater construction of Fig. 9 is that a plurality of such heaters can be juxtaposed to make an effectively continuous radiant heating assembly that covers an extended area. Thus individual heaters are conveniently made with rectangular heater faces about one foot by two feet in size, larger sizes of stiff board-like panels being somewhat awkward to manufacture because the molding and handling is more difficult. However by making the smaller sized panels so that their edges 120 are bent down at least about 90 degrees from the plane of the panel body, considering such edge as a flange bent down from a flat sheet, and locating the edge mountings so they are at least partially inboard of the outer face of that flange and not projecting beyond that face more than about 5 millimeters, they juxtapose in a very desirable manner as illustrated in Fig. 10.

In Fig. 10 an assembly 200 of individual heaters 100 is made with the adjacent faces of their panel edges 120 about 3 millimeters apart as indicated at 202. The margins of the panel faces 102 can be made so that they have an essentially zero radius of curvature where they bend into the edges 120, but it is sometimes simpler to make them with a radius of about X inch, and the foregoing 3 millimeter spacing of such rounded corners does not significantly detract from an effectively continuous heater surface junction, particularly where the combustion mixture is arranged to burn over the entire rounded corner. Increasing the spacing from about 3 millimeters to about 5 millimeters does make a significant discontinuity in the radiation uniformity but this can generally be tolerated.

While the clamping screws 140 are shown as having round heads and thus project out the furthest from the outer faces of the refractory panel edges, such projection is not a problem so long as it is not over the 5 millimeter limit noted above, or the preferred 3 millimeter limit. These screws can be in unsymmetrical locations along each edge, so that the screws on one heater are offset from the screws of an adjacently positioned heater, as also illustrated in Fig. 10, Indeed the round-head screws can be replaced by socket-head screws which project a trifle more but are easier to install during manufacture. Flat-head screws can alternatively be used with the screw openings in the clamping angles countersunk so that the screw heads do not project beyond those angles, if minimum or zero spacing 202 is desired.

The burner construction of Fig. 9 can also have panels of the pliable needled type described above. Such a pliable panel behaves very much like a blanket, and can have its edges folded and tucked in place between the side anchorage members. Because of their high pliability, the corners of such panels will squeeze into shape, although it may be helpful to cut away all excess corner material, and to even notch out some of the panel corners to make it easier to clamp these panels into place. It is preferred to confine any notching to portion of the corners covered by the anchorage members so as to reduce the leakage of gas at the notches.

It is not necessary to have the entire margin of each refractory panel 102 flanged over as at 120. Thus each of the panels in Fig. 10 has at least one margin that is not juxtaposed to another panel, and some have two such non-juxtaposed margins. Where only two panels are to be juxtaposed, each can have only one margin provided with a flanged-over edge 120, in which event the remaining three margins can have simple constructions as shown in the flat panel exemplifications in Figs. 11 and 12.

Very close juxtaposition can also be provided by molding or shaping juxtaposed edges 120 so that they are bent down more than 90 degrees from the horizontal as measured by the angle 150 in Fig. 9. A panel can thus be molded around a suitably shaped molding screen with as many as three of its four sides having flanged edges bent as much as 100 or 110 degrees measured at angle 150, and the thus molded panel can then be slipped sideways off the mold in the direction away from its fourth side. Where only one flanged edge margin is desired, it can be made when molding the panel, or by bending down the edge of a flat-molded panel, after that edge is softened by wetting.

The construction of Figs. 11 and 12 is one that is easily manufactured from readily available sheet metal, for flat heater panels.

It has a panel support which is a weldedtogether assembly of a rectangular plenum box 302 and a hollow-centered rectangular encircling plenum tube 304. Plenum box 302 is conveniently prepared by suitably notching out the corners of a rectangular sheet, then bending up the four wings thus formed, and welding the resulting corners gas tight. A hole can then be punched in the floor of the box to receive a PTM half close nipple 306 also welded on gas tight. A baffle 308 can also be spot welded over the hole to distribute the combustion mixture fed through it. If desired an extra tap 310 can also be provided at a second hole in the box floor, for a pressure gage or the like.

Tubular plenum 304 is easily made from sheet metal bent into the shape of a channel having a web 312, and unequal flanges 314, 316. The channel is cut into four lengths each of which is mitered and then welded together gas tight, if desired. The tubular plenum can then be affixed to the plenum box as by spot welding the flanges 316 to the floor of the box. A gas inlet 320 in the form of half a close nippel can be affixed to the tubular plenum, along with an extra tap 322 in the same manner as for the box plenum, and a baffle 324 can be fixed over inlet 320 by spot welding to either the outside of the box plenum or the inside of the tubular plenum.

A slot 330, preferably t inch wide, encircles the top of the box plenum. The refractory matrix is clamped in place by a clamping frame 342 of angular section as illustrated in Fig. 11 and having slits 344 cut in the web overlying the face of the panel as shown in Fig. 12. The slits can be about 8 inches apart and preferably l/,6 inch wide to take care of the most severe thermal conditions. The clamping frame is secured by screws 346 as in the construction of Fig.

9, although sheet metal screws can be used instead in either construction, in which event the nuts can be omitted and if desired locking washers fitted under the screw heads.

In severe thermal conditions, such as firing face down or when firing directly at opposing burners, it is desirable to insulate the clamping frame 342 from the radiated and convected heat by over-wrapping with a high temperature insulating material such as mineral fibers felted or needled in blanket form. Fig. 11 shows a fiber blanket 350, approximately inch thick, clamped and compressed between clamping frame 342 and refractory matrix 340 is wrapped around the clamping frame 342 and web 312 and secured to flange 316 by means of clamp 360 and sheet metal or other screws 362.

The fiber blanket 350 insulates the clamping frame from convected heat and its pure white color reflects some radiated energy from onposing burners making the system more efficient. In very high ambient operating conditions it may be desirable to completely wrap the non-radiant surfaces of burner 300 with the fiber blanket.

Fig. 12A shows the fiber blanket 350 as prepared for installation, having a tuck-in margin 370 which is inserted under the face of clamping frame 342.

In less severe applications it may be desirable just to cover the face of 342 and hold the blanket in place with the screws 346 and washers under their heads.

The radiant heaters of the present invention can be equipped with automatic igniters such as electric spark igniters or pilot lights.

Figs. 13 and 14 show a particularly desirable automatic igniter construction fitted into a heater of the type illustrated in Figs. 11 and 12. A standard combination 500 of spark rod 501, ground rod 502 and flame-checking rod 503 is mounted so that the rods are generally parallel to and about 1/ 16inch above the outer face 505 of the porous refractory panel 340. Below the opposite face of the panel underneath the rod assembly, the box plenum is provided with a partition 507 that isolates a chamber 509 from the remaining space in the box plenum, and the chamber is fitted with its own supply connector 511 to receive a separate combustion mixture.

The spark rod 501 and flame-checking rod 503 are each housed in two identical insulators 550 which go through aligned openings punched in the top flange 520 of the clamping frame 342 and in the flanges 316 and 314 of plenum 304 as shown in Fig. 6. Ground rod 502 is welded or brazed to flange 520. The ends of rods 501 and 503 by flange 316 are threaded to accept connector 542 which holds them in place and provides a ready connection for necessary wiring.

The construction of Figs. 13 and 14 is operated to start the burners using a safety check. A separate pilot combustion mixture is first started into chamber 509 and at the same time the spark rod is electrically energized to begin sparking. If the flame rod does not sense a flame within a short period of time, such as 10 to 30 seconds, the flow of combustion mixture can be automatically cut off and the starting sequence must then be manually recycled, preferably after the combustion mixture flow is checked as by purging chamber 509. When the starting sequence cause ignition of the separate combustion mixture, the flame-checking rod 503 senses the ignition and opens the valve that feeds the main combustion mixture into plenum 302 which is then ignited by the flame at chamber 509.

By using a small chamber 509 with a low BTU/hour input for the automatic ignition test, the danger of explosion at ignition is minimized. A chamber volume of about 100 cubic centimeters or less is very effective for this purpose.

The pilot combustion on the radiating surface of the panel contributes to the overall radiation.

The spacing of the rod assembly from the refractory panel is preferably kept very small so that the rods do not interfere with placing the radiating surface close to the material being radiated, such as a moving textile web that is being dried. Because the effectiveness of the heater increases when brought close to the material treated, the spacing of the panel from that material is sometimes arranged to be as little as two inches or even less.

Fig. 15 illustrates a radiant heater 700 of the present invention particularly adapted for the sealing of metal tubes in a metal sheet in accordance with the technique described earlier. Heater 700 has a dome-shaped holder 702 welded gas-tight to a support ring 704 that is shaped to fit and receive the brim 710 of a hat-shaped refractory ceramic panel 720. The crown portion 712 of the panel is thus held in spaced relation to the dome-shaped holder 702 to define a plenum 730 for the combustion mixture to be burned on the concave surface of the crown 712.

An inlet 732 and pressure gauge tap 734 are shown as fitted to the holder 702.

The brim of panel 720 is shown as clamped against support ring 704 by a clamping ring 706 which is bolted to an extension 708 of support ring 704 and is offset from it to form a cylindrical wall 740 that defines an annular plenum 750 for the non-combustible gas. If desired the offset can be made integral with the clamping ring so that support ring extension 708 can be in the general plane of the main portion of the support ring.

Alternatively wall 740 can be divided into upper and lower short cylinders separately integral with the separate rings. An inlet 760 and a pressure gauge tap 770 are also provided for the annular plenum.

The radiant heater 700 can directly replace the corresponding heater in Fig. 7, even though heater 700 has only one combustion zone. Non-combustible gas pumped into plenum 750 of heater 700 flows through the brim 710 of the porous refractory panel 720 and keeps the combustion mixture fed through plenum 730 from reaching the lowest portion of the internal surface of the panel where it is aligned with plenum 750. No external cooling coil or jacket is needed for the heater 700, inasmuch as the noncombustible gas emerging from the lower portion of the interior of the panel flows outwardly along the bottom of clamping ring 706 and keeps it as well as the associated metal parts sufficiently cool. Holder 702 as well as the remaining members that hold panel 720 can all be made of aluminum about 60 mils thick.

Heaters with the air seal construction are particularly suited for use in house hot air and/or hot water heating furnaces. The air seal effectively prevents diffusion of the combustion mixture to edge locations where it can burn at a low feed rate and thus gradually burn back deeply into the binder holding the refractory fibers, eventually creating a line of weakness at which an unneedled panel tends to readily break. Indeed the burn-back can sometimes burn back far enough to cause ignition within the mixture plenum itself, rendering the heater unsuited for continued operation. The edge seal construction provides a very long life for the refractory panel, and is also so simple that it is inexpensively constructed and thus more attractive for relatively small hometype equipment.

Figs. 16 and 16A show a hot air heat exchanger construction for house heating in accordance with the present invention. Here a cylindrical heat exchanger 800 has a hollow interior 802 in which is received a fibrous panel 804 also of generally cylindrical shape. The panel has an open end 806 clamped to a mounting plate 808 as by means of a rib 810 formed or welded on the plate and around which the panel end is squeezed by a split sheet metal strap 812 whose ends can be pulled together by a tightening screw 814.

Before the panel is fitted in place a partition disc 820, held on a tubular support 822 having an externally threaded extension, is mounted on mounting plate 808 which has a threaded aperture 826 that threadedly receives the threaded extension 824.

Partition disc 820 has its periphery located just above the edge of rib 810, to define a marginal slot 830 for discharge of a sealing gas stream through the marginal portion of the panel 804. An inlet nipple 832 provides for the delivery of the sealing gas stream to the sealing plenum 840 below partition disc 820. Extension 824 provides for the supply of combustion mixture to the plenum 850 above the partition disc.

Strap 812 is also shown as carrying a ring of outwardly extending ears 842 that help retain a mass of insulation packing 844 fitted around the open end of panel 804 when mounting plate 808 is brought into engagement with the mouth 846 of heat exchanger 800. Some of those ears are also perforated to receive an ignition and test assembly 860 shown in the form of a series of ceramic tubes 862 each having an enlarged head 865 and threaded into aligned openings in the mounting plate. Through the passageway in each ceramic tube there penetrates a rod 867 having a disc-shaped inner end 870 and staked as at 872 so that it is appropriately located with respect to the ceramic tube. A washer 874 can be slipped over each rod before it is inserted in the ceramic tube, to furnish better positional coaction with the tube and the staking.The outer edge of each rod can be threadedly engaged to a mounting tip 876.

The discs 870 of each rod are arranged so that they are in edge-to-edge opposition suitable for sparking and for flame detection, as described in connection with Figs. 13 and 14.

The outside of heat exchanger 800 can be located in the circulating air plenum of a standard house heater, or if desired in a water tank containing water to be heated.

This heat exchanger can be made of metal or even of glass, borosilicate glass being particularly suited when the heat exchanger is used to heat water. Water to be heated in this way can be colored with dyes for example, to better absorb radiant energy transmitted through a transparent heat exchanger.

Metal heat exchangers are desirably ribbed to increase their effective surface area and thus increase their heat transfer to surrounding air or the like.

An inert or reducing gas may be used to seal the combustion mixture on its way through the porous refractory panel. Thus the sealing gas can contribute to make the burnt combustion mixture provide an atmosphere of exceedingly low oxygen content, or even of strongly reducing ability as for example by reason of a significant hydrogen content.

Fig. 17 shows an annealing tunnel furnace 900 having upper and lower radiant heaters 902, 904 facing each other and held in fixed relation by side blocks 906 of thermal insulation. A wire mesh conveyor 908 is arranged to slide through the furnace interior to carry workpieces that are to be annealed or brazed. A strip curtain 910 closes off the entrance to the furnace, above the conveyor, the portion of the entrance below the conveyor being closed by a one-piece wall 912.

The heaters 902, 904 are operated in the manner described above, except that the sealing gas streams, indicated by arrows 920, can be cracked ammonia, or a propanenitrogen mixture, or pure propane or the like. With such sealing gases, it is preferable to adjust the combustion mixtures so that they have little or no surplus oxygen. The furnace interior then becomes a very effective reducing atmosphere that will prevent oxidation of the workpieces and even reduce any oxidation present on those pieces when they are introduced into the furnace. Notwithstanding the strongly reducing character of the furnace interior, the burning of the combustion mixture takes place very effectively to provide radiation at temperatures at least as high as red heat.

The needled ceramic fiber panels described above are conveniently manufactured in very long lengths, as long as 25 feet or even longer. Such panels are particularly suited for use with very long radiant heaters, for example heaters in which a ceramic fiber panel about fifteen feet long and about one foot wide, has its edges clamped against the face of an air seal plenum surrounding a rectangular combustion mixture plenum.

Angles compress and clamp the panel edges, being drawn against the air plenum face by screws that can be fitted with shoulders against which they can be tightened at relatively high torque with a minimum of attention.

A panel that is not stiffened with binder or the like, will belly out under the influence of the pressure in the plenum. This is not particularly harmful, and is in some respects desirable because it reduces the heat radiation from the face of the panel to the clamping angles.

The bellying action can be reduced by pretensioning the panel when it is mounted.

Another technique for stiffening a pliable panel is to needle it around a stiffener. In this construction a wide mesh metal screen is laid in between two layers of ceramic fibers, and a needling operation then performed to interfelt the two fiber layers.

No claim is made herein to a method of sealing metal tubes in a sheet as claimed in Application No. 7903219 (Serial No.

1,581,649).

Subject to the foregoing disclaimer, WHAT I CLAIM IS : 1. A gas-fired radiant heater comprising a porous refractory mat suitable for the passage of a gaseous combustion mixture therethrough, means engaging the mat and defining adjacent first and second zones for separately supplying gas to corresponding areas of the mat, means for supplying a gaseous combustion mixture to the first zone whereby in use the mixture passes through the thickness of the mat and burns where it emerges from the mat, and means for supplying a non-combustible gas to the second zone to prevent the combustion mixture at the first area of the mat from spreading through the mat to the second area.

2. A heater as claimed in claim 1, wherein the second zone is peripheral to the first zone so that, when the heater is in use, gas from the second zone is peripheral to gas passing through the mat from the first zone, and prevents gas from the first zone from spreading to the edges of the mat.

3. A heater as claimed in Icaim 1 or claim 2, wherein the mat is composed of felted refractory fibers.

4. A heater as claimed in claim 3, wherein the fibers are ceramic fibers.

5. A heater as claimed in any one of claims 1 to 4 wherein the mat is mounted at its edges in a frame.

6. A heater as claimed in claim 5, wherein the means defining the first and second zones include the frame and a backing member for the frame.

7. A heater as claimed in claim 5 or

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (32)

**WARNING** start of CLMS field may overlap end of DESC **. so that they are in edge-to-edge opposition suitable for sparking and for flame detection, as described in connection with Figs. 13 and 14. The outside of heat exchanger 800 can be located in the circulating air plenum of a standard house heater, or if desired in a water tank containing water to be heated. This heat exchanger can be made of metal or even of glass, borosilicate glass being particularly suited when the heat exchanger is used to heat water. Water to be heated in this way can be colored with dyes for example, to better absorb radiant energy transmitted through a transparent heat exchanger. Metal heat exchangers are desirably ribbed to increase their effective surface area and thus increase their heat transfer to surrounding air or the like. An inert or reducing gas may be used to seal the combustion mixture on its way through the porous refractory panel. Thus the sealing gas can contribute to make the burnt combustion mixture provide an atmosphere of exceedingly low oxygen content, or even of strongly reducing ability as for example by reason of a significant hydrogen content. Fig. 17 shows an annealing tunnel furnace 900 having upper and lower radiant heaters 902, 904 facing each other and held in fixed relation by side blocks 906 of thermal insulation. A wire mesh conveyor 908 is arranged to slide through the furnace interior to carry workpieces that are to be annealed or brazed. A strip curtain 910 closes off the entrance to the furnace, above the conveyor, the portion of the entrance below the conveyor being closed by a one-piece wall 912. The heaters 902, 904 are operated in the manner described above, except that the sealing gas streams, indicated by arrows 920, can be cracked ammonia, or a propanenitrogen mixture, or pure propane or the like. With such sealing gases, it is preferable to adjust the combustion mixtures so that they have little or no surplus oxygen. The furnace interior then becomes a very effective reducing atmosphere that will prevent oxidation of the workpieces and even reduce any oxidation present on those pieces when they are introduced into the furnace. Notwithstanding the strongly reducing character of the furnace interior, the burning of the combustion mixture takes place very effectively to provide radiation at temperatures at least as high as red heat. The needled ceramic fiber panels described above are conveniently manufactured in very long lengths, as long as 25 feet or even longer. Such panels are particularly suited for use with very long radiant heaters, for example heaters in which a ceramic fiber panel about fifteen feet long and about one foot wide, has its edges clamped against the face of an air seal plenum surrounding a rectangular combustion mixture plenum. Angles compress and clamp the panel edges, being drawn against the air plenum face by screws that can be fitted with shoulders against which they can be tightened at relatively high torque with a minimum of attention. A panel that is not stiffened with binder or the like, will belly out under the influence of the pressure in the plenum. This is not particularly harmful, and is in some respects desirable because it reduces the heat radiation from the face of the panel to the clamping angles. The bellying action can be reduced by pretensioning the panel when it is mounted. Another technique for stiffening a pliable panel is to needle it around a stiffener. In this construction a wide mesh metal screen is laid in between two layers of ceramic fibers, and a needling operation then performed to interfelt the two fiber layers. No claim is made herein to a method of sealing metal tubes in a sheet as claimed in Application No. 7903219 (Serial No. 1,581,649). Subject to the foregoing disclaimer, WHAT I CLAIM IS :

1. A gas-fired radiant heater comprising a porous refractory mat suitable for the passage of a gaseous combustion mixture therethrough, means engaging the mat and defining adjacent first and second zones for separately supplying gas to corresponding areas of the mat, means for supplying a gaseous combustion mixture to the first zone whereby in use the mixture passes through the thickness of the mat and burns where it emerges from the mat, and means for supplying a non-combustible gas to the second zone to prevent the combustion mixture at the first area of the mat from spreading through the mat to the second area.

2. A heater as claimed in claim 1, wherein the second zone is peripheral to the first zone so that, when the heater is in use, gas from the second zone is peripheral to gas passing through the mat from the first zone, and prevents gas from the first zone from spreading to the edges of the mat.

3. A heater as claimed in Icaim 1 or claim 2, wherein the mat is composed of felted refractory fibers.

4. A heater as claimed in claim 3, wherein the fibers are ceramic fibers.

5. A heater as claimed in any one of claims 1 to 4 wherein the mat is mounted at its edges in a frame.

6. A heater as claimed in claim 5, wherein the means defining the first and second zones include the frame and a backing member for the frame.

7. A heater as claimed in claim 5 or

claim 6, wherein the frame has its outer face covered with thermal insulation.

8. A heater as claimed in claim 7, wherein the thermal insulation is a thin layer of mineral felt.

9. A heater as claimed in any one of claims 1 to 8, including a plenum box, and wherein the means defining the second zone include channel members secured to the outside of the box to form a plenum ring.

10. A heater as claimed in claim 9, wherein the channel members are secured to the box by spot welds.

11. A heater as claimed in claim 9 or claim 10, wherein the channel members have flanges of unequal width, the wider flanges being secured to the face of the box opposite the mat, and the narrower flanges have their free edges spaced from the box to define a slot-like exit for the plenum ring.

12. A heater as claimed in any one of the preceding claims, wherein means defining the second zone include a tubular member around the periphery of the frame, which member has at least one slot in a portion thereof which abuts the mat and at least one gas supply conduit in another portion thereof.

13. A heater as claimed in claim 12, wherein the tubular member has a substantially rectangular cross section.

14. A heater as claimed in any one of claims 1 to 13, wherein the mat is substantially flat.

15. A heater as claimed in any one of claims 1 to 9, wherein the mat is substantially flat with a depending margin.

16. A heater as claimed in any one of claims 1 to 8, wherein the mat is convex, concave, cup-shaped or hat-shaped.

17. A heater as claimed in claim 16, wherein the mat is thimble-shaped and the first and second zones are, respectively the upper and lower portions of the thimble.

18. A heater as claimed in claim 2 or in any one of claims 3 to 16 when appendant thereto, wherein the edges of the panel are not provided with a physical edge seal to prevent gas escaping therethrough from the first zone.

19. A heater as claimed in any one of claims 1 to 18, wherein the edges of the mat are squeezed to reduce the mat thickness by at least about 10%.

20. A heater as claimed in any one of the preceding claims, wherein the porous refractory mat is generally rectangular and flat with an integrally formed flange along at least one of its edges, and the heater includes a support for the mat, and mounting elements securing the flange to the support, and mounting elements and support being at least partially inboard of the outer face of the flange and not projecting beyond that face more than about 5 millimeters.

21. A heater as claimed in claim 20, wherein the mounting elements and support do not project beyond the outer face of the flange more than about 3 millimeters.

22. A heater as claimed in claim 20 or claim 21, wherein the support includes the lips of a marginal gas-discharge slot that extends along the flange for discharging through the flange a non-combustible gas that prevents the combustion mixture from escaping through the flange when the heater is in use.

23. A heater as claimed in any one of claims 20 to 22, wherein a section of the flange has its thickness compressed by the mounting elements to recess the outer face of that section with respect to the body edge that carries the flange.

24. A gas-fired radiant heater as claimed in any one of the preceding claims, which includes a support for the mat, wherein the second zone is adapted to provide a narrow stream of relatively cold non-combustible gas passing through the mat immediately adjacent the mat support when the heater is in use, thereby to help cool the support.

25. A heater as claimed in any one of claims 1 to 19, wherein the mat is hat-shaped including a crown and a brim, and the heater includes a generally flat support annulus for the lower face of the brim, a clamping ring encircling the upper face of the brim, radially outward projections extending from the annulus and ring to define beyond the edge of the brim the second zone for non-combustible gas, the projections being secured together to clamp the brim between them, and a dome member secured to the clamping ring to cover the crown and define outside the crown the first zone for a gaseous combustion mixture, each zone having a gas inlet.

26. A heater as claimed in any one of the preceding claims, including means for supplying a gaseous combustion mixture to the second zone whereby each of the said areas of the mat is capable of functioning as a heat-radiating area.

27. A gas-fired radiant heater substantially as hereinbefore described with reference to and as illustrated in figures 1 to 4, figure 5, figure 7, figure 8, figure 9, figures 11, 12 and 12A, figures 13 and 14, figure 15, or figures 16 and 16A, of the accompanying drawings.

28. A row of radiant heaters presenting a substantially uniform radiating surface lengthwise of the row, each of the heaters being a heater as claimed in any one of the preceding claims.

29. A row as claimed in claim 28, wherein both mats at each juncture between successive heaters have the flange-and-mount edge as defined in claim 20 and 21, the individual radiating surfaces of the mats at each juncture being not more than at 1 inch apart.

30. A method of producing radiant heat which comprises supplying a gaseous combustible mixture to the first zone of a heater as claimed in any one of claims 1 to 26 and igniting the combustible mixture on the side of the mat opposite the first zone and supplying a relatively cold non-combustible gas to the second zone of said heater.

31. A method as claimed in claim 30, wherein the non-combustible gas is a reducing gas.

32. A method as claimed in claim 30 for operating a gas-fired radiant heater, substantially as hereinbefore described.