US3476368A

US3476368A - High temperature kiln

Info

Publication number: US3476368A
Application number: US686129A
Authority: US
Inventors: Hiroshi Saiki
Original assignee: Denver Fire Clay Co
Current assignee: Denver Fire Clay Co
Priority date: 1967-11-28
Filing date: 1967-11-28
Publication date: 1969-11-04
Anticipated expiration: 1986-11-04

Description

3 Sheets-Sheet l HIROSHI SAIKI s R. U ml N N m m v T T A m s 0 m/% H% Y B n. 1

HIGH TEMPERATURE KILN Nov. 4, 1969 Filed Nov. 28, 19s? Nov. 4, 1969 HIROSHI SAIKI HIGH TEMPERATURE KILN 3 Sheets-Sheet 2 Filed Nov. 28, 1967 INVENTOR.

SA I KI HIROSHI ATTORNEY Nov. 4, 1969 HIROSHI SAIKI 3,476,368

HIGH TEMPERATURE KILN Filed Nov. 28, 1967 3 Sheets-Sheet '5 INVENTOR. HIROSHI SAIKI AT TORNE Y United States Patent 3,476,368 HIGH TEMPERATURE KlLN Hiroshi Saiki, Golden, Colo., assignor to The Denver Fire Clay Company, Denver, Colo., a corporation of Delaware Filed Nov. 28, 1967, Ser. No. 686,129 Int. Cl. F231 15/04; F23c /08 US. Cl. 263-20 19 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a novel and improved kiln, and more particularly relates to a high temperature kiln, such as, those for heating and firing ware.

In high temperature kilns presently designed for heating ceramic ware to temperatures in excess of 3000 F., it has been difiicult to effect the controls necessary to uniformly heat the kiln and ceramic Ware to the high temperature level within a reasonable time period, since the requirements and conditions for most efficient heating and insulation are materially different at the lower temperatures up to 2400 F. and intermediate temperatures from 2400 F. to 3000 F. than at the higher temperature level. For instance, in order to maintain a uniform temperature in shuttle kilns, relatively thick refractory walls have been utilized and have necessitated slow, gradual heating into the maximum range and excessively high fuel consumption in maintaining thermal equilibrium across the walls.

In heating the ware to the high temperatures required, it has been the practice in the past to preheat the combustion air or fuel/ air mixture prior to introduction into the kiln. Definite limitations are imposed on the extent of preheating due to the danger of preignition; and the means devised for preheating the air has not afforded close accurate control over the degree of preheating desired. Thus preheating should be held to a minimum in the lower temperature ranges but rapidly increased to a maximum in the higher temperature ranges in order to increase the maximum temperature attainable while substantially reducing fuel consumption. At the same time it is desirable that the means employed for circulating and preheating the combustion air also function as lightweight insulative wall coverings in order to reduce the refractory wall thickness and enable the use of lighter re fractory materials; and further, to so position the circulating means in relation to the refractory walls as to provide for maximum insulation therebetween in the lower and intermediate temperature ranges.

It is therefore an object of the present invention to provide a high temperature kiln in which combustion air is circulated externally of the kilnwalls for closely controlled, adjustable preheating of the air over a wide temperature range in such a way as to minimize heat losses and fuel consumption as well as to accelerate the rate of heating and increase the maximum temperature level of the kiln; and in association therewith to provide relatively lightweight refractory walls of reduced thickness having a reduced heat storage factor thereby minimizing the time required to attain thermal equilibrium across the wall sections.

It is a further object of the present invention to prosection shown in FIGURE'S.

Referring in detail to the drawings, there is schemat Patented Nov. 4, 1969 ice 2 l vide in a kiln for novel and improved nozzle mix burner. sections stationed at spaced intervals along the kiln walls being characterized by mixing preheated combustion air and-fuel with close control of thefuel ignition point and sensitive temperature controlfrom ambient to temperatures in excess of 3000 F., and wherein the nozzle mix burner sections are conformable for use with different selected fuel gases which may be preheated without danger of preignition and safely mixed with the preheated air upon introduction into the kiln.

It is a still further object of the present invention to provide for a novel and improved high temperature kiln in which the outer plenum chambers are so constructed and arranged externally of the kiln roof and wall sections as to effect close accurate control over the circulation and preheating of combustion air over a wide temperature range while reducing the heat storage factor in the kiln walls, and are operative to heat and cool the walls externally in order to further increase the heating. and cooling rates of the wall sections.

The present invention contemplates the use of relatively broad flat plenum chambers in outer spaced relation to the refractory roof and wall sections of the kiln, the plenum chambers being controllable to accurately regulate the extent of preheating of the combustion air as the temperature is increased from the low to the high temperature ranges. The plenums are insulated and in cooperation with an insulating air space left between the plenums and the walls define relatively lightweight insulated wall sections which will permit substantial reduction in the refractory wall thickness. In this relation, the insulating space will afford improved insulation in the lower heat ranges against preheating of the air but which at the higher temperature ranges will have minimum insulating effect.

Nozzle mix burner sections are stationed at intervals along the lower portions of the walls which can be operated with preheated air in excess of stoichiometric requirements so as to reduce flame temperatures and hot spots, to increase the flame propagation rate, and to raise the maximum temperature level of the kiln in a rapid, dependable manner.

The above and other objects of the present invention will become more readily understood and appreciated from a consideration of the following detailed description when taken together with the accompanying drawings, in which:

FIGURE 1 is a somewhat sche rr'iatic view in perspec-' tive and illustrating the relative disposition and arrangement between the plenum chambers and outer wall sections of the kiln. f

FIGURE 2 is a CI'OSS-SGCtiOIIaI'ViCW of the preferred form of invention shown in FIGURE'3.

FIGURE 3 is a perspective view with portions broken form of the present invention.

FIGURE 4 is an enlarged view in detail of the upper side walls and roof sections of the'preferred form.

FIGURE 5 is a fragmentary view in detail the nozzle mix burner sections; and

FIGURE 6 is an end view of away to illustrate the kiln wall construction in a preferred ically shown in FIGURE 1 the method and means for circulating combustion air from'a blower 10 into separate preheat plenum systems 12- and 13 which are located at" the back and along either side of" a kiln 14. The air is delivered by the blower 10 through a main duct 15 having a butterfly blast valve 16 to regulate the flow of aii through .an air header 17'into theplenum systems. Asa preliminary to describing in detail the preferred form of kiln construction, the construction and operation of the of one of the nozzle mix burner ceramic ware through a door or other opening in the front end, not shown, of the kilnoA series of burner sections 24 are located at spaced intervals along the lower part of the sidewall sections 19, each burner section being supplied with fuel from a common supply line 26 through a gas line 27, the latter being suitably provided with a stop cock 28, gas regulator 29 and limiting orifice valve 30 leading into each burner section. Also shown is an impulse line 32 running from the gas regulator 29 to the plenum supply chamber, to be hereinafter described, leading into each burner section.

In the schematic showing of FIGURE 1, each of the

plenum systems

12 and 13 is correspondingly formed with a relatively broad flat plenum 33 along the back wall of the kiln communicating through a metallic flexible hose 34 with an upper adjustable crown plenum 35 which traverses thesubstantial length of the top wall section 22. Each crown plenum distributes the air downwardly through metallic flexible hoses 36 provided with flow regulating butterfly valves 37 into the upper end of relatively broad flat plenums 38 where the air flows into the burner section for combustion as the air mixes with the gas at this point. The combustion gases pass under the ware in the kiln, upwardly along the sides and through the ware, then are exhausted through flues located at spaced intervals along the roof section.

In a manner to be shown and described in detail with reference to the preferred form of invention the

plenums

33, 35 and 38 are positioned in outer spaced relation to the external surfaces of the kiln whereby to form a space therebetween which has insulative value below radiant temperatures but which decreases as the main mode of heat transfer shifts from convection to radiation. As a result, insulation is provided at lower temperatures where air preheat is unnecessary and decreases as the requirement for air preheat increases in the higher temperature ranges. In this same connection, the upper crown plenums 35 are adjustably mounted for swinging movement toward and away from the roof section; thus, at the lower kiln temperatures and under conditions of low temperature control the crown plenums may be raised away from the exhaust gas stream in order to minimize air preheating. Under increasing temperatures the crown plenums may be lowered into the path of the exhaust gas stream through the roof section. Further, each of the crown and sidewall plenums is provided with a stack 42 having a butterfly valve 43 to adjustably control the pressure and temperature of air supplied through each crown and sidewall plenum to the burner section.

In heating ceramic ware, air is directed by the combustion blower through the main duct into each of the preheat plenum systems 12-and 13 where the pressurized air is caused to flow .upwardlythrough the back wall plenum 33 into the end of the crown plenum 35 and downwardly through the sidewall plenum 38 into the burner section. In heating the ceramic ware at low kiln temperatures, the crown plenums35, are raised to eliminate preheating of the combustion air so that in the lower temperature range the plenums act merely as passageways for the air and as lightweight insulation for the kiln. As stated, the air space between the plenums and outer surfaces of the kiln will serve as, added insulative value in the lower kiln temperature ranges.

At higher kiln temperatures, the crown plenums 35 are lowered into the path of the exhaust gas stream flowing upwardlyfrom the kiln through openings in the roof section. The broad flatv configuration of the plenums will permit rapid preheating of the air prior to its downward passagethrough .the sidewall plenums 38. and in turn the sidewall plenprns as they are heated w ll r n f r h at is e one: has at tbs rs rsswry walls as well as to the other outer insulative layers protecting the skin. The pressure and temperature of the air can be closely controlled by regulating the disposition of. the crown plenums in the exhaust stream as well as by individually regulating the side stacks 52 communicating with the sidewall plenums.

Now considering in more detail the preferred form ofinvention as shown in FIGURES 2 to 6 and with particular reference to FIGURES 2 and 3, the kiln 14 takes the form of a shuttle kiln having a hinged door member- 50 which serves as a closure for the opening 50' in the front end wall of the kiln. The kiln is reinforced and supported by a steel frame including horizontally extending I-shaped beam members 51 and standards 52; beneath the floor section of the kiln. Vertical beam members 53 are arranged at spaced intervals along opposite sides of the kiln outwardly of the plenum sections together with an upper rectangular beam structure 54, and a refractory battle 55 extends the length of the kiln between the upper crown plenums.

The ware to be heated may be placed upon a waresupporting car 58 having flanged wheels 59 movable along guide rails or tracks 60 running longitudinally of the kiln through the central opening in the floor section. In a conventional manner, opposite sides of the waresupporting car 58 may be provided with laterally projecting ribs complimentary to channels or grooves 65 in the floor section along the sides of the opening. Additionally, the lower side edges of the car each have a downwardly projecting flange 66 which extends into a sand seal 68 positioned in a trough 69, a pair of troughs being provided in inwardly spaced relation at the upper ends of the standards 52 on opposite sides of the floor sections. An air seal, not shown, may be positioned above the sand seal to prevent the flow or circulation of hot gases from the kiln into the space above the sand seal.

In the construction of the sidewalls 19 of the kiln, as best seen from FIGURE 2, each comprises an inner brick wall section 72 composed of a lightweight refractory material, which extends continuously along either side of the kiln with double brick wall sections 75 at each corner of the kiln and outer brick wall sections 75 extending vertically at spaced intervals along either side from the outer edge of the floor section to the outer wall section 76 to reinforcing opposite sides of the roof section 22. Here the roof section 22 is in the form of an arch composed of lightweight refractory i brick and provided with flue openings 78, and the upper wall sections 76 are similarly composed of lightweight brick which span the length of the kiln between the corner sections 74. Although not shown, the 'back Wall 20 of the plenum similarlyincludes an outer wall section 75 centrally located between the back wall plenums 33.

Each of the back and

sidewall plenums

33 and 38 is correspondingly comprised of a generally rectangular, thin-walled metallic shell which is dimensioned to occupy the entire space formed intermediately between the outer sidewall sections and between a section 75 and a corner section 74. Specifically, each plenum includes relatively narrow side and end walls 80 joined by rectangular inner and

outer panels

81 and 82, respectively, the inner panel including an inwardly directed offset portion 83 at its lower end 24 in order to increase the crosssectional area of the plenum at the burner section. The outer panel 82 of each plenum is covered with a lightweight insulating material, such as, mulitple layers 85 and 86 of rigid fiber insulation. Preferably, each plenum chamber is mounted to leave a clearance space 88 between the inner panel and the outer hot face lining or skin of the inner wall section 72 which as stated defines an inv, sulating space at lower temperature ranges. Here,'each plenum is mounted in place between the outer wall sections with a series of ducts 90 extending upwardly through openings in the wall sections 76 for connection to the flexible hoses 36 leadingv from the upper crown plenums. In turn, each exhaust stack 42 has a lower horizontal duct portion extending horizontally through the thickness of the outer insulating

layers

85 and 86 into a T-shaped fitting 92, the lower open end 92 of the fitting being adapted to draw relatively cool air into the exhaust stack for mixture with the relatively hot air discharged from the plenum when the butterfly valve 43 is open.

The upper crown plenums 35 are each formed in the same manner as the back and sidewall plenums, except that the crown plenums are dimensioned to cover each side of the upper roof span and to extend the length of the kiln. Further, the side of each crown plenum has an outwardly and downwardly inclined extension 35' provided with openings therein to receive the upper ends of the flexible hoses 36. In the space between the upper crown plenums and the roof section, angularly extending flame deflector plates 94 are positioned in spaced relation above the flue openings 78, and limited openings 95 are provided in each deflector plate to permit the hot exhaust gas stream to pass upwardly therethrough into contact with the inner wall panelv of the crown plenum. After passing across the-inner'wall of the crown plenums 35 the gases are free to escape around the inner edges, in the direction of the arrows as shown in FIGURE 2.

Each of the crown plenums is adjustsable toward and away from the roof section 18 about the lower ends of the flexible hose connections into the sidewall plenums between the elevated position shown on the left and the lowered position shown on the right in FIGURE 2. This adjustment may be suitably accomplished by means of a winch, not shown, having pulleys and cable lines 97 and connected to eyes 98 at the inner free ends of the plenums. If desired, slidably adjustable supporting plates 99 may be positioned outwardly of each of the flexible hoses to loosely retain insulating material in place along the outsides of the hoses.

The air passing downwardly into each of the sidewall plenums is introduced into the burner section 24. Each burner section includes a burner block 100 mounted in the inner refractory wall sections 72, and the burner block is provided with an inwardly divergent, conical recess 102 communicating with a series of orifices 104. The orifices 104 are defined by ceramic tubes concentrically mounted in a burner plate 105 for introduction of air from the plenum into the kiln. A ceramic fuel supply tube 106 extends through the burner plate centrally of the air supply orifices 104 to introduce fuel into the cavity 102 for mixture and ignition with the air supplied through the orifices 104. The gas supply tube extends rearwardly through an outer concentric pipe member 107, the latter being mounted in the side plenum and having at its outer end an elbow-shaped fitting'108 provided with an observation port 109. As described in connection with FIGURE 1, gas is supplied from the main fuel line 26 through each individual gas supply line 27, each line being connected to the fittings 108 leading into the burner sections. Fuel supply may be closely regulated in relation to the amount of air supplied through the side plenums to provide optimum ignition and flame conditions at each burner section. Of particular importance is that the nozzle mix burner enables close accurate control of the combustion process in the low temperature range, and the burners may further be operated with a supply of air directed through the concentrically arranged orifices in excess of stoichiometric requirements in order to reduce the flame temperature and localized heating in the vicinity of the burner sections.

In operation, pressurized combustion air is directed upwardly through the back wall plenums to enter the rear ends of the crown plenums through the flexible hose members 34. The crown plenums distribute the air 'to the sidewall plenums 38, the air flow being regulated by adjustment of the butterfly valves 37 in the hose connectors 36 between the "crown and sidewall plenums. In passing downwardly into the sidewall plenums the air is directed into the nozzle mix burner blocks through the concentrically arranged orifices 104. The gas is introduced through the central fuel supply tube 102, combustion occurring in the burner block and beyond as the air mixes with gas at this point. The combustion gases pass under and through the ware in the ware-supporting section and pass upwardly along the kiln walls to exhaust through the tines in the roof section.

In the lower temperature ranges, the upper' crown plenums are raised, as shown on the lefthand side of FIGURE 2, to avoid preheating of the air, and in this relation the crown plenums merely act as passageways for the combustion air. Further, in the lower temperature range the air space between the plenums and hot face linings of the refractory wall sections serve as insulating spaces below radiant temperatures; however the insulative value of the spaces will decrease as the heat transfer shifts from convection to radiation in the higher temperature range. For high temperatures or accelerated heating, the crown plenums are lowered into the hot exhaust gas streams passing upwardly through the flues to preheat the air prior to passage into the sidewall plenums. The heated combustion air will enable higher flame temperatures and proportionately decrease the fuel consumption. In turn, the higher flame temperatures will result in higher kiln temperatures and accelerated heating rates, and can produce oxidizing flames by providing excess air through the burner sections to more rapidly reach kiln temperatures in excess of 3000 F. In this connection, the high temperature combustion air is safely handled in the burner sections which are entirely of re'- fractory material, and the gas and air are mixed in the burner nozzles where ignition temperatures and flame propogation rates are of minor significance. Moreover, the flame propagation rate is materialy increased by mixing the air and gas in this manner, the effect being to shorten flame length particularly by use of excess air.

As the kiln is heated into the high temperature range and the insulative value of the air space between the plenums and kiln walls decreases, the air is preheated further by radiant heat tranfer from the hot face linings of the kiln walls through the side and back wall plenums. At the same time, the plenums function as lightweight insulating mediums along the kiln walls and is of particular value in reducing the outer skin temperature without adding appreciable weight in the form of thicker refractory walls. Moreover, the plenum system materially reduces the heat storage factor by reducing the static mass and results in substantially less time in reaching thermal equilibrium both in heating and cooling of the kiln, since the preheat temperature of air in the plenum system can be closely regulated. Thus, heat may be added'by preheating the combustion air in the crown plenums and the hot sidewall plenums will then transfer heat to the outside of the inner refractory wall sections, or hot face lining, as well as to the outer insulative layer protecting the plenums. The converse is true when cooling is desired since the crown plenums may be elevated away from the exhaust gas streams and the cooler air circulated through the plenums will operate to more rapidly reduce the temperature along the outer surfaces of the kiln walls.

It will be appreciated that the pressure and temperature of the air can be regulated for the system as awhole through the butterfly blast valve at the outlet of the blower. The pressure and temperature is also controlled by the degree of immersion of the crown plenums in the exhaust gas stream and further by the air bleedoif stacks provided for each of the sidewall and crown plenums.

- Summarizing the many advantages and features of the high temperature kiln of the presentinvention, closely controllable preheat airtemperatures and pressures are obtained throughthe utilization ofthe crown plenums which can be adjusted to intercept the flo'w'of' exhaust gases from the kiln to regulate the preheat temperature in cooperation with flow regulating means for the sys tern and for each individual plenum chamber to regulate the pressure and temperature of the air' supplied to each burner section. The plenum system materially reduces the thickness of the refractory Walls and permits utilization of lighter refractory materials thereby reducing the heat storage factor and permitting faster heating and cooling rates of the kiln walls. The plenums function to heat and cool the outer refractory wall surfaces to permit accelerated heating and cooling rates, and the' air spaces between the kiln walls and plenums insulate the plenums from the kiln walls in the lower temperature ranges. Moreover the nozzle mix burner sections'perrriit closer regulation and control of the flame characteristics and heating of the kiln throughout the entire temperature range. Low temperature control is afforded byemploying excess air for 'flame quenching and high tem perature control is afforded through the use of burner blocks composed of refractory material which serve to contain the high temperature combustion process. The system devised therefore results in a recuperative cycle in which waste heat from the exhaust gas is employed to preheat the air, the preheated air permitting higher flame temperatures to be generated and the higher exhaust gas temperatures accelerating the rate of preheating which in turn further increases the flame temperature elevation rate.

While there has been described a preferred form of the present invention it is to be understood that the plenum system and burner section described may be utilized independently or in cooperation with one another in kilns other than that described in the preferred form, and for example, in the manner broadly described with reference to the kiln construction shown in FIG- URE 1. It is further to be understood that various modifications and changes may be made in the specific construction and arrangement of elements comprising the present invention as well. as the specific kiln construction without departing from the spirit and scope thereof as defined by the appended claims.

What is claimed is:

1. In a kiln having refractory roof and wall sections defining an enclosure for heating and firing ware, the combination comprising:

relatively broad plenum sections substantially covering the roof and wall sections of said kiln, burner sections disposed at spaced intervals incommunication with the lower ends of the wall plenums, means for circulating air through the roof plenums and downwardly through the wall plenums into said burner sections for admixture and combustion with fuel introduced into said burner sections, and. means associated with each of said plenum sections to adjustably control the pressure and temperature of air flowing through each plenum section. I

2. In a kiln according to claim 1, said plenum sec tions being disposed in outer spaced relationto the-roof and wall sections of said kiln. a p

3. In a kiln according to claim 1, each of said plenum sections being provided with an outer lightweight insulating layer of a composition other than the composition of said refractory wall and roof sections.

4. In a kiln according to claim 1, further including means adjustably supporting said roofplenum sections for movement toward and away from heat transfer relation to the roof section of said kiln.

5. In a kiln according to claim 1, the roof plenum .sections being defined by a pair of plenums extending length? wise of the roof section in side-by-side relation, and means adjustably mounting each of said roof plenumsections for movement toward and away from heat transfer relation to said roof section.

6. In a kiln according to claim 5, wherein flue open- 8 ings are provided in the roof section'of said kiln:bneath said plenum sections, and flame deflector plates are interposed between the flue openings and the-inner surfaces of saidroof plenum sections.

7. In a kiln according to claim 1, said. wall plenum sections being disposed in external recess portions along the refractory walls of said kiln, eachof said wall plenums being in the form of a generally rectangular thin-walled shell having an insulating .layer along the external surface of said shell, and

. flexible air ducts interconnecting said roof plenum tions and said wall plenum sections. 8.'In a kiln accordingzto claim 7, said means for adjustably controlling the" pressure and temperatureiin'said roof and wall plenums being defined by exhaust stacks each including a valve member being adjustable to regulate the-flow of air from each plenum section through each respective stack.

9. In a kiln according to claim 8, each of said exhaust stacks being further characterized by having a relatively cool air inlet communicating with the vent to cool the" combustion air withdrawn from the wall plenums.

10. In a kiln according to claim 1, said burner sec? tions. each having a refractory burner block disposed in the refractory wall of said kiln, a fuel supply line extending through each burner block and air supply orifices communicating with the plenum wall sections and being concentrically disposed about said fuel supply line.

11. In a kiln according to claim 10, said burner sections each being characterized by having a conical-shaped recessed portion diverging inwardly from the inner ends of said fuel supply line and fuel supply orifices for mix ing and combustion of said fuel and air.

12. In a kiln having refractory roof and wall sections defining an enclosure for heating and firing ware with a combustible mixture of fuel and air supplied through burner sections located at spaced intervals along the walls of the kiln, and flue openings in the roof section of the kiln for discharging the gaseous products of combustion,

a plenum system for supplying preheated combustion air to each of said burner sections, said plenum system comprising relatively broad, thin-Walled plenum sections being disposed in side-by-side relation to substantially cover the refractory roof section of the kiln, and

means for adjustably supporting said roof plenum sec tions for movement toward and away from the path of 'thegaseous products of combustion discharged iihlrou-gh the flue openings in the roof section of the in. I" x 13. In a kiln according to claim 12, each of said plenum sections being provided with an outer lightweight insulating layer, and

means for circulating combustion air through said roof plenum sections including flexible hose connections leading into and away from one side of each of said plenum sections. 14. In a kiln according to claim 12, said roof plenum sections being defined by a pair of plenums extending lengthwise of the roof section in side-by-side relation, said flexible hose connections being disposed along the outsidesof each of said plenum section and said adjustable supporting means being operative to angularly raise and lower each of said plenum sections about said flexible hose connections.

4 15. In a kiln according to claim 14, said roof plenum sections being further characterized by having adjustable exhaust stacks to regulate the pressure and temperature of preheated air 'in each of said plenum sections.

16. In a kiln according to claim 14, further including plenum sections disposed along the sidewalls of the kiln in communication with said burner sections, and said flexible hose connections leading away from said roof plenum sections being connected to each of said wall plenum sections for circulation of preheated combustion secair from said roof plenum sections into said burner sections.

17. In a kiln according to claim 14, further including flame deflector plates being interposed in spaced relation between the flue openings in said roof sections and the inner surfaces of said roof plenum sections.

18. In a kiln having refractory floor, roof and wall sections dfining a common enclosure for heating and firing ware,

a series of burner sections being located at spaced intervals in the walls of said kiln above said floor section,

each of said burner sections having a burner block composed of refractory material mounted in the wall of said kiln, a fuel supply line extending centrally through each burner block, air supply means providing a source of combustion air for each of said burner sections including ceramic tubes defining air supply orifices being concentrically arranged about said fuel supply line in each burner block with a conical-shaped recessed portion in each burner block diverging inwardly from the inner ends of said fuel supply line and air supply orifices, and

said air supply means including plenum sections disposed along the external surfaces of said kilnfor preheating the combustion air delivered into each of said burner sections, and means associated with each plenum section to adjustably control the pressure and temperature of preheated air supplied to said burner sections.

19. In a kiln according to claim 18, said plenum sections including sidewall plenums inserted in recessed portions in the sides and back of said kiln, upper crown plenums in outer spaced relation to the roof section of said kiln, and fiexiblehoses interconnecting the crown plenums and adjacent sidewall plenums.

References Cited UNITED STATES PATENTS 1,864,087 6/1932 Moomaw 263-20 3,229,746 1/ 1966 Wagner et al. l227 X 3,241,823 3/1966 Pentek 26319 EDWARD G. FAVORS, Primary Examiner US. Cl. X.R. 431-178