US2895544A - Radiant wall furnace - Google Patents

Description

\ J. R. PARSONS RADIANT WALL FURNACE Filed July 19, 1954 July 2 1, 1959- H W 5. t! h l M M j w a 1W FLUE United States Patent RADIANT WALL FURNACE Joseph R. Parsons, Park Forest, 11]., assignor to Chicago Fire Brick Company, Chicago, 111., a corporation of Illinois Application July '19, 1954, Serial No. 444,317

3 Claims. (Cl. 1584.5)

This invention relates to a radiant wall furnace apparatus and a method of operation of a furnace designed to burn fluid fuel.

The apparatus and method utilize combustion of the surface combustion type wherein fluid fuel is burned on or in the vicinity of refractory surfaces. Such refractory surfaces are elevated to radiant heating temperature and heat is radiated therefrom to the heating zone of the furnace and to opposing refractory surfaces which in turn radiate heat to the heating zone and to the originating surfaces.

The contemplated fuel may be a gas or a liquid, such as oil, which is vaporized prior to or during combustion. The term fluid as here used comprehends fuels of both the gaseous and liquid types.

Radiant fluid burners, particularly radiant gas burners of a type sometimes called porous diaphragm burners, have been known since prior to 1900 and have been subject to periodic experiment and efforts at commercialization. Attempts to devise practical and efiicient radiant gas burners for more than limited and specialized applications have met with failure for various reasons, although certain advantages possessed by burners of this character are recognized.

One of the reasons for lack of success appears to be inadequacies of the refractory material used in the porous diaphragms. Another reason appears to be that it was considered necessary in most instances to subject the diaphragm refractories to prefiring. This requirement imposes sen'ous size, shape and strength limitations on the diaphragms.

Another reason for generally unsuccessful results with porous diaphragm burners was the tendency of such burners to back-fire or flash-back. The fact that the prior porous diaphragms were thin in section contributed to the backfire problem.

.For these and other reasons the porous diaphragm burner is little used today. However, the more general principle of surface combustion is still in use in limited applications. An example of currently used surface combustion is found in the Selas burner wherein a small, prefired refractory is washed with a burning mixture of gas and air. High temperatures are developed on the surface of the refractory and useful heat is radiated from such surface.

One object of this invention is to provide an improved apparatus and method for utilizing surface combustion as developed by a porous diaphragm burner.

Another object of the invention is to provide an improved porous refractory material for use in the apparatus. One type of improved porous refractory material suitable for use in this invention forms the subject matter of my copending US. patent application for Refractory Plastic Composition, filed January 15, 1954 under Serial No. 404,313, now abandoned. Other types will be described hereinafter. I

In general, the porous refractory material used in the ice invention comprises a plastic mass of properly sized aggregate which can be troweled, cast, blown, or gunned mto place. This plastic mass makes possible the construction of a monolithic, homogeneous wall of anyreasonable shape and size. The plastic mass includes a quick-setting binder which enables a new or repaired wall to be placed in service immediately.

Another object of the invention is to provide a surface combustion type of furnace which requires only a comparatively small combustion space to insure complete combustion and efficient utilization of the fuel used. Most furnaces, generally speaking, rely on convection currents to convey heat from a combustion zone to the heating zone of the furnace. In many instances part of the combustion zone and part of the heating zone comprise the same space. In such furnaces a substantial amount of space within the furnace walls must be provided, and thefurnace walls are correspondingly extensive in area. Thus: the construction and maintenance costs of such furnaces: are correspondingly high.

The present furnace provides complete combustion virtually on the wall surfaces (minimum space) and relies primarily on radiation to convey heat from the combustion zones to the heating zone. Thus substantially all the space within the furnace Walls comprises heating zone, and the furnace may be made as small as is consistent with the space required in the heating zone.

Another object is to provide a furnace of this character and a method of operation which has high efliciency characteristics. One high efliciency characteristic is established by the fact that combustion is complete. Another high efiiciency characteristic is established by the fact that there are no lateral heat losses through the walls of the furnace and there is no appreciable heat retained in the discharged products of combustion. Another efliciency characteristic may be designated by the term radiating efficiency. This efficiency means the ratio of the heat actually radiated within the furnace to the total calorific value of the gas burned. As will be seen, the radiating efliciency of the present furnace is particularly high because most of the heat developed is made available as radiant heat.

Another object of the invention is to provide a method of furnace operation whereby fuel enters a furnace more or less uniformly through a porous refractory wall of substantial area and combustion occurs in the vicinity of (on, just above, or just below) the Wall surface. The products of combustion are discharged more or less uniformly through another porous refractory wall of substantial area. The surface of the discharge wall acquires most of the heat contained in the products of combustion and thus is elevated to radiant heating temperature. Any heat in the products of combustion not acquired by the surface of the discharge wall is acquired by the wall behind the surface where it is available for preheating incoming fuel if and when furnace operation is reversed and that wall is used as a burner.

Another object is to provide a furnace apparatus deapparatus and method of operation characterized by conditions which impart extended life to the refractories used in the furnace walls.

In one aspect of the invention the apparatus and method utilize low pressures in the furnace interior. Where this is the case the furnace walls experience minimum penetration by destructive agents, suchas furnace gases, slags and fluxes, contained within the furnace. Thus deterioration of the furnace walls is reduced to a minimum.

In another aspect of the invention the pressure within the furnace, while comparatively low, has a value substantially in excess of atmospheric pressure. This pressure, here termed back pressure, is developed by the gas expansion which occurs during combustion and by the provision of restricted outlet openings in the furnace. It has been found that a mild back pressure cooperates to insure uniform combustion on the porous wall surface and it also makes possible the use of fuel supply rates greatly in excess of any heretofore found practical in radiant wall burners. The high fuel supply rates permit the development of temperatures correspondingly greater than those previously provided by radiant wall furnaces.

Other objects, advantages and details of the invention, both in the apparatus and method, will be apparent as the description proceeds, reference being had to the accompanying drawing wherein one suggested form of apparatus is shown. It is to be understood that the description and drawing are illustrative only and that the scope of the invention is to be measured by the appended claims.

In the drawing:

Fig. 1 is a view in side elevation, partly in section, of a furnace illustrating both the apparatus and method aspects of the invention, and

Fig. 2 is a sectional view on line 22 of Fig. 1.

The embodiment of the invention shown in the drawing illustrates the invention in various aspects both as to the apparatus and method, as will be seen from the following description and the appended claims.

Referring to the drawing, the illustrated form of apparatus (pot type furnace) includes a pail-shaped housing 5 having a side wall 6 and a bottom 7. Housing 5 is substantially impervious to the passage of fluids except at the open end and at lateral openings presently to be referred to. A suitable support or base 8 may be provided for housing 5.

Housing wall 6 is interiorally lined with a layer of refractory material of substantial porosity and thickness. Refractory material 10 is described in general above, and various satisfactory specific materials are disclosed in the aforesaid copending application, Serial No. 404,313. The refractory material comprises a refractory aggregate of suitable material and size which is converted to a plastic state, installed in place and then set by a suitable setting agent.

Refractory aggregate found particularly suitable for use in the invention comprises crushed firebrick, chrome, mullite, alumina, perlite, vermiculite or refractory fiber, such as rock wool or the like. The aggregate may consist of any one of the foregoing or it may include a combination of some or all of them, depending on the particular characteristics desired in the resultant product. Refractory fibers, for example, are used when extreme light weight is desired.

The selected aggregate is screened to exclude most, if not all, of the fines so that the subsequently formed mass will have substantial porosity. The precise maximum and minimum size of the particles depends on the specific characteristics desired. For example, good results have been experienced with crushed firebrick of a size which passes a. screen and is retained on a 35 screen. Also, good results have been obtained with crushed firebrick which passes through a 7 screen and is retained on a 10 mesh screen. Similarly, aggregate which passes through a 10 mesh screen and is retained on a 20 mesh mesh screen has been found satisfactory. In all cases a minor portion of fines (material passing the smaller of the two screens) can be tolerated, but the portion should be less than 25% to insure adequate porosity.

The refractory aggregate is converted to a plastic mass by the addition of a carrier containing a binder and a plasticizer such as kaolin, the latter preferably being air floated. The carrier may comprise hydrated sodium silicate (sodium silicate containing combined water). Wa-

ter preferably is present in such quantity as to provide substantial saturation. Other satisfactory additives are bentonite, ground fluxes, calcium 'aluminate, cements, phosphates, feldspar and potassium silicate.

A highly satisfactory plastic mass is provided by 50 pounds of crushed firebn'ck, 8 pounds of hydrated sodium silicate and 7 pounds of air floated kaolin. This plastic mass, because of the absence of fines, has high porosity and it may be troweled, rammed, cast, blown, vibrated or gunned into place.

Panels or walls of this plastic mass readily can be constructed to any desired shape and size after which the binder is quickly set by passing through the pores of the mass a dehydrating agent, or some material which reacts with sodium silicate to give a silica gel. Carbon dioxide gas gives particularly satisfactory results as it acts both as a dehydrating agent and as a chemical reactant. Ammonia gas, hot air, alcohol vapors and liquid alcohols, such as ethyl or methyl alcohols containing less than 10% water, may also be used to set the binder. The resultant panel or wall is monolithic and sufficiently strong to be self-sustaining in all reasonable sizes and resistant to damage from the high temperatures developed during use.

As shown in the drawing, porous refractory material 10 completely lines the side wall 6 and bottom 7 of housing 5. The interior wall and bottom surfaces of material 10 define a heating zone 12 extending longitudinal- 1y of the housing. The refractory surface at the bottom or inner end of zone 12 may be referred to as hearth 13.

The lined housing 5 is provided with a removable door or cover 15 through which the furnace may be charged with material to be heated or burned. Cover 15 may have the same cross-sectional shape as housing 5. It comprises a top 16, side wall 17 and porous refractory material 18 filling the space between the wall and top. Material 18 preferably is the same as the material 10 which lines housing 5.

A suitable actuator, such as a ring 20 secured centrally to the top of cover 15, cooperates with a hoisting device (not shown) for raising and lowering the cover. Studs 22 on cover 15 engage cooperating receptacles 23 secured near the upper edge of housing 5 to insure proper alignment between cover 15 and housing 5.

Preferably the refractory material 18 in cover 15 lies flush against the annular top surface of the refractory material 10 in housing 5 as shown in the sectionalized portion of Fig. 1.

In one aspect of the invention the porous refractory material 10 in housing 5 and the porous refractory material 18 in cover 15 are divided into two separate sections by diametrically extending partition walls 25 and 26 respectively. The wall 25 in housing 5 is centrally cut away so as not to extend into heating zone 12, as best shown in Fig. The walls 25 and 26 are in alignment and thus divide the refractory materials 10 and 18 into two symmetrical portions, a right hand portion and a left hand portion, looking at the figures.

Referring to the right hand side of Figs. 1 and 2, one end of a pipe 30 extends through an opening 31 in housing Wall 6. Pipe 30 is welded or otherwise suitably secured to the housing wall. The other end of pipe 30 communicates with two branch pipes, a branch pipe 33 and a branch pipe 34.

Branch pipe 33 is a supply inlet for the air and fluid fuel mixture burned in the furnace while branch pipe 34 is an outlet for the products of combustion. The two branch pipes operate in alternation, as will be seen. Branch pipe 34 leads to a pump 35, diagrammatically shown, and the pump 35 discharges to a suitable flue. Depending on various conditions, pump 35 may or may not be used during furnace operation.

Valves 37 and 38, respectively, are provided in the branch pipes 33 and 34. As will be seen, valves 37 and 38 are "alternately opened and closed to provide revers ible furnace operation.

The left hand side of the furnace has a pipe 40 which is similar to pipe 30. Pipe 40 leads to branch pipes 41 and 42 which have valves 43 and 44. The pipes and valves on the left hand furnace side have the same con nections and functions as those on the right hand furnace side, the branch pipe 42 leading to a pump 45 which discharges to a flue.

The above described furnace is designed for reversible operation. The right hand side operates as a burner for the first time period, say ten or fifteen minutes, while the left hand side functions during this time period as a source of radiant heat and as an exhauster for the products of combustion. During the succeeding time period of like duration the left hand side operates as a burner while the right hand side functions as a source of radiant heat and as an exhauster for the products of combustion.

To initiate the aforesaid reversible operation, inlet valve 37 on the right hand side is opened and exhauster valve 38 is closed. On the left hand side inlet valve 43 is closed and exhauster valve 44 is opened. A mixture of air and gas or liquid fuel enters the furnace under suitable pressure through pipe 30. The mixture diffuses more or less uniformly throughout porous refractory material and 18 in the right hand side of the furnace. The mixture flows through the porous refractory material to the bounding surfaces of heating zone 12 on the right hand side of partition walls 25 and 26. The mixture is ignited in any suitable manner to initiate surface combustion. In a matter of seconds combustion of extreme uniformity occurs in the vicinity of the aforesaid surfaces. These surfaces quickly are elevated to radiant heating temperature by the heat developed during combustion and by heat radiated back from the surfaces of the Wall and ends on the left hand side of partition walls 25 and 26, as will be seen. Materials in the furnace thus immediately receive heat radiation from these walls and the materials likewise are elevated to working temperature.

The products of the surface combustion, essentially hot gases, travel through heating zone 12. to the surfaces of the zone in the left hand side of the furnace. The exhauster pump 45 connected to branch pipe 42 functions if necessary to impress a comparatively low pressure on the left hand urfaces to establish the aforesaid travel direction. The products of combustion passing from the right hand side of the furnace to the left hand side convey heat by convection to the material in the furnace, thereby imparting useful heat to the material other than that provided by radiation.

The high temperature products of combustion are forced or drawn into the surfaces of heating zone 12 on the left hand side of the furnace. As these gases enter the porous refractory material on the left hand side of the furnace, the heat contained therein is transferred to the surfaces of the material. Almost all of the contained heat is given to these surfaces and to the refractory material lying imrnediately below the surfaces. The surfaces, therefore, are heated to radiant heating temperature in a short period of time and the products of combustion which exit through pipe 40 and branch pipe 42 are substantially free of heat. Thus there are no appreciable heat losses in the flue during the time period of operation in the described direction.

After ten or fifteen minutes operation as above described, the porous refractory material 10 and 18 on the left hand side of the furnace becomes more or less heated throughout. The highest temperatures, of course, are in those portions of the material nearest heating zone 12, while the material farther distant from the zone 12 is at lower temperature. However, at this time the porous refractory material is unable to extract substantially all the heat from the outgoing products of combustion with the result that sensible heat would pass to the flue if operation in the first direction were continued.

At this time the below-theesurface major part of the porous refractory material 10 and 18 in the right hand (burning) side of the furnace is at low temperature, the temperature of this material being about the same as that of the incoming air and fluid mixture 'which travels radially inward through the material and thus prevents any appreciable flow of heat in a radially outwand direction. The high temperatures of the material in the right hand side of the furnace are confined essentially to the surfaces vvhich define heating zone 12. Thus, when the furnace is burning on the right hand Side there are no lateral heat losses from that side. This is evidenced by the fact that the right hand side of housing wall 6 always is cool to the touch during right hand operation.

At this point, operation of the furnace is reversed by closing right hand inlet valve 37, opening right hand exhauster valve 38, opening left hand inlet valve 43 and closing left hand exhauster valve 44.

The air and fluid mixture now enters the furnace through branch pipe 41 and pipe 40. The mixture diffuses more or less uniformly throughout the porous refractory material 10 and 18 in the left hand side of the furnace. In so doing the mixture is preheated by the above mentioned heat Which is present in the material. The preheated mixture is discharged from the surfaces bounding heating zone 12 on the left hand side of the furnace and Surface combustion of extreme uniformity occurs in the vicinity of those surfaces.

The products of combustion travel across heating zone 12, and convey heat to the materials in the space by convection. Thereafter the products of combustion are forced or drawn into the surfaces which bound zone 12 on the right hand side. These surfaces and the low temperature porous refractory material immediately below the surfaces extract the remaining heat contained in the products of combustion and thus are elevated to or maintained at radiant heating temperature. The low temperature products of combustion exit through pipe 30 and branch pipe 34 to a suitable flue.

Furnace operation continues in this direction for ten or fifteen minutes until such time as the temperature begins to rise in the right hand radially outward portions of the porous refractory material 10 and 18 and in the discharged products of combustion. At this time furnace operation is reversed by changing the valves 37, 38, 43 and 44 to their alternative positions.

During a period of operation in one direction or the other the incoming air and fluid mixture utilizes for preheating all the heat contained in the porous refractory material of the incoming side of the furnace so that the material, except for the interior surface portions, are at low temperature at the end of the time period for operation in that direction. Thus, throughout successive cycles of operation the entire exterior of housing wall 6 is at low temperature, thus evidencing complete absence of lateral heat losses.

Tests have established that surface combustion radiating efiiciency is a maximum when gas is burned (without appreciable back pressure) at the approximate rate of 6 B.t.u. per sq. in. per minute on a surface combustion Wall. Therefore, this rate of fuel supply is preferred when maximum radiating efficiency is desired in the absence of mild back pressure. Inasmuch as 6 B.t.u. per sq. in. per minute is a comparatively low B.t.u. release, it will be seen that a radiant wall must have substantial area to produce high furnace temperatures. As previously mentioned, the plastic, quick-setting, porous refractory material of this invention is capable of use over large areas of regular or irregular shape. Thus the invention enables the construction of large furnaces which are capable of high temperatures and which yet do not exceed the burning rate necessary for maximum radiating efiiciency.

When temperatures in the higher ranges are desired the furnace is operated at fuel supply rates substantially in excess of 6 B.t.u. per sq. in. per minute and the exit openings of the furnace are restricted to aid in developing a back pressure. Desired restriction may be provided by a comparatively low porosity wall, not using a discharge pump, or by partially closing the discharge valve.

In prior surface combustion burners of the porous diaphragm type the desire to obtain radiant heat from the burning surface required the burners to operate without excess air. Operation under this condition gave rise to a carbon build-up on the burner surface and a consequent rapid destruction of the porous diaphragm. In this invention it is preferred to operate with excess air so as to insure complete combustion and avoid the aforesaid carbon build-up.

It is possible to use excess air satisfactorily with this invention because it is not necesary to develop radiant heating temperatures on the burning surfaces directly from the burning gases alone. Rather, the hot products of combustion are forced or drawn to opposing surfaces which remove heat from the products of combustion. These surfaces are thereby elevated to radiant heating temperatures and they in turn radiate heat back to the burning surfaces to elevate the latter to radiant heating temperatures.

The surface combustion provided by the above described porous refractory material is characterized by extreme uniformity. The uniformity of burning appears greater than reasonably can be justified by the uniformity in porosity of the refractory material. It is concluded, therefore, that porosity uniformity of the refractory material is not particularly critical insofar as uniform burning is concerned. Rather, it is believed that the expansion of gases during combustion, which is of the order of 6:1, is mainly responsible for the uniform burning. This severe gas expansion, as previously mentioned, produces a uniform back pressure on or in the vicinity of the surface of the refractory material and this uniform back pressure insures uniform fuel flow and hence uniform burning. It has been found that maximum uniformity of burning is achieved when the pressure of the incoming gas and air mixture is maintained at a comparatively low value, i.e. the pressure of the mixture at the burning surface of the refractory material falls in the range of 0.05 to 0.80 inch of water.

Since the burning surfaces are raised to incandescent temperature, the uniform incandescence of the surfaces demonstrates that burning occurs uniformly. There is a complete absence of hot spots (areas of different incandescence). Even if a wall is cracked (thereby providing a passage of low resistance to the flow of air and fuel mixture), the burning which occurs at the face of the crack is substantially the same as that which occurs over the rest of the surface.

By making the porous refractory walls comparatively thick (:1 large radial dimension in the example shown) the furnace has been found to operate completely free of back-fire diiliculties. Thick walls also are relied on for insulation properties and for the aforesaid preheating function which aids in the elimination of heat losses.

The radiating efficiency of a furnace as above described is at least double that of a furnace wherein no comparable means are provided to extract heat in usable form from the products of combustion. During furnace operation in either direction all surfaces of heating zone 12 are maintained at radiant heating temperature. Heat is radiated back and forth between opposing walls, thus heating said walls to radiant temperature as well as material placed in the furnace.

.To summarize the method aspect of the invention, the method comprises the steps of feeding a mixture of air and fuel to a furnace interior uniformly through a first furnace wall portion of substantial area, completely burning the mixture in the immediate vicinity of the Wall portion whereby the surface of the wall portion is heated toward radiant heating temperature. The products of combustion are discharged uniformly through a second furnace wall portion of substantial area whereby the interior surface of the second wall portion is heated to radiant heating temperature. The second wall portion radiates heat back to the first wall portion to bring that portion to radiant heating temperature. On discharge from the second wall portion the products of combustion are substantially free of heat.

The method also comprehends the additional step of periodically reversing the operation of the furnace whereby the respective wall portions alternately are burners and exhausters.

To summarize the apparatus aspect of the invention, the furnace comprises a structure enclosing a heating zone, the structure having an extensive portion consisting of refractory material of substantial porosity and thickness. A fuel mixture of air and fluid is fed into the heating zone and burned. The products of combustion are exhausted through a portion of the porous refractory material whereby the inner surface of that portion extracts heat from the outgoing products of combustion and radiates such heat to the heating zone.

The apparatus also comprehends a furnace as above described wherein the fuel mixture enters the heating zone by passing through a porous refractory wall of substantial area. The fuel burns in accordance with surface combustion principles in the vicinity of the surface of such wall. In this case it usually is desirable to restrict the furnace outlets to such an extent that a back pressure is developed in the furnace.

In addition the apparatus invention contemplates a furnace having porous refractory walls and constructed in a more or less symmetrical manner to provide reversible operation whereby heat losses are minimized under conditions of prolonged operation.

Where the term air is used in the specification, claims and drawing it is intended to comprehend any combustion supporting gas such as other mixtures containing oxygen, pure oxygen, or the like.

From the above description it is thought that the construction and advantages of my invention will be readily apparent to those skilled in the art. Various changes in detail may be made without departing from the spirit or losing the advantages of the invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

l. A radiant wall furnace comprising a fuel impervious housing, refractory material of substantial porosity and thickness lining said housing and defining a heating zone, said refractory material being made up of bonded refractory aggregate wherein substantially all grain sizes fall in the range of inch to inch, said porous refractory material being partitioned by fuel impervious means into at least two separate portions, means cooperating with said two separate portions whereby first one portion and then the other is supplied interiorly with a mixture of air and fluid fuel and whereby each portion, when not being so supplied, has a low pressure applied to the interior thereof, and means providing access to said heating zone.

2. A radiant wall furnace comprising a fuel impervious housing, refractory material of substantial porosity and thickness lining said housing and defining a heating zone, said refractory material being made up of bonded refractory aggregate wherein substantially all grain sizes fall in the range of inch to inch, said porous refractory material being partitioned by fuel impervious means into at least two separate portions, a first pipe extending through said housing and communicating with one portion of porous refractory material, a second pipe extending through said housing and communicating with aneating with each of two portions of said porous refractory material, one pipe of each pair being connected to a source of air and fluid fuel mixture and the other pipe of each pair being connected to a low pressure generator, and an other portion of said porous refractory material, each pipe 5 adjustable valve disposed in each of said pipes.

having a pair of branch pipes connected thereto, one branch pipe of each pair being connected to a source of air and fluid fuel mixture and the other branch pipe of each pair being connected to a low pressure generator, and an adjustable valve disposed in each of said branch pipes.

3. A radiant wall furnace comprising a fuel impervious housing, refractory material of substantial porosity and thickness lining said housing and defining a heating zone, said refractory material being made up of bonded refractory aggregate wherein substantially all grain sizes tall in the range of 55 inch to ,5 inch, said porous refractory material being partitioned by fuel impervious means into at least two separate portions, a pair of pipes communi- References Cited in the file of this patent UNITED STATES PATENTS 166,977 Eustis Aug. 24, 1875 580,020 Thomson Apr. 6, 1897 1,213,470 Finlay Jan. 23, 1917 1,222,922 Bone et a1 Apr. 17, 1917 1,238,011 Ellis Aug. 21, 1917 1,561,389 Wollers Nov. 10, 1925 2,421,744 Daniels et a1 June 10, 1947 FOREIGN PATENTS 575,064 Great Britain Feb. 1, 1946

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