FIELD OF INVENTION:
The present invention relates to a tunnel kiln for the whiteware industry and in, particular, to a tunnel kiln adapted for reduced heat loss through the refractory lining wall, effective utilization of hot flue in the furnace and achieve uniform heating of the stock in trolleys treated in the kiln. The tunnel kiln would achieve better kiln efficiency with respect to productivity associated with a kiln operation and economy in consumption of energy/fuel used for heating of ceramic ware. The kiln of the invention is further adapted to avoid stagnant heat flux and improve upon the heat radiation from the portion of wall and roof in the heating zone for effective heating of stock, with less rejection/losses and is thus directed to favour higher productivity making the use of tunnel kiln applications cost-effective and widely acceptable.
BACKGROUND ART:
The ceramic ware manufacturers in the small and medium sector are fairly large, in numbers and scale of production, in India. They mostly occupy the share of white ware covering the range of typical products including kitchen ware, crockery, sanitary ware and LT/HT electrical insulators etc. It is conventionally well known to carry out firing of these ceramic wares following a heating operation for these white wares carried out in tunnel kilns with length varying from 30-50 meters. For such purposes these ceramic materials/products are systematically stacked on movable trolley and charged into the kilns for heating/firing. Trolleys loaded with those materials enter through one end of the kiln and comes out through the other end of the kiln. The firing or heating zone is located at about halfway of the length of the tunnel kiln and spans over about 16-20 ft where the ceramic products are subjected to firing at a temperature of 1260°C by burning HSD through 2 to 4 burners located on inner walls at this zone and the products of combustion from the firing zone move toward the discharge end of the kiln. Cold air is sent into the cooling zone of
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the kiln near the other end, for cooling the hot ceramic products. This air, after picking up heat from the hot ceramic ware is utilized for combustion of the fuel and preheating of the charge. For a daily production of 2-3 tons of ceramic ware, the consumption of HSD fuel oil is of the order of about 600-700 litres per day in the existing system of kiln operation and refractory lining. Conventionally Ceramic fibre module of density 160 Kg/m3 and thickness of 300mm are used as lining material in the heating zone.
The thickness of lining varies in different zones depending on the temperature of the flue and stock. However, the gap between trolley and the ceramic lining is only 75mm in the preheating and cooling zones of the kiln. Shell temperature of the tunnel kiln in the existing system, in the heating zone is in the range of 140-160°C and that in the preheating and cooling zone are 90-110°C, which are very high and found to result in huge heat losses. Lining in the preheating and cooling zones are with ceramic fibre module of varying thickness. It is found that as the gap between trolley and the lining is only 75mm at these zones, excess pressure is created in the gap which does not allow the flue to traverse uniformly throughout the kiln and only moves through the upper portion of the trolley and top free portion of the kiln. Such non-uniform heating makes one or two bottom layer/s of the material stock loaded on trolley inside the kiln, defective due to under heating and lead to a rejection/ loss of production of the order of 10-15%. In the existing system of tunnel kilns, the heating zone is having a configuration with rectangular section and flat roof, this leads to a stagnant heat flux at the two top ends and less radiation from that portion of wall and roof. All these factors contribute to less effective heating of the ceramic products stacked on the trolley and moved to the firing zone in the existing tunnel kilns.
Need therefore exists in the art to provide for tunnel kilns with heating/firing provisions which would avoid the afore discussed limitations of conventional kilns presently in use. Also, considering the heat loss encountered in conventional kilns there is also the need to economize the use of such tunnel kilns such as to favour its wide scale and effective application and use.
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OBJECTS OF THE INVENTION:
It is thus the basic object of the present invention to provide for tunnel kilns for whiteware industry which would involve selective refractory lining for, on one hand, effective heat insulation such that heat loss through the walls is minimum and on the other hand achieve effective utilization of the hot flue in the furnace and thereby favour kiln efficiency with respect to productivity and energy consumption.
A further object of the invention is directed to provide for tunnel kiln arrangement which would favour the traversal of the flue gas uniformly throughout the kiln and in the process favour uniform heating of the stock in the kiln and avoid production losses.
Another object of the present invention is directed to selective use of an efficient and rational thermal insulation lining pattern on inside wall of tunnel kiln to reduce the existing high level of shell wall outer surface temperatures of 140°C-160°C at the firing zone and 90°C -110°C at the preheat and cooling zone by about 30°C at different zones of the tunnel kiln in conventional heating process of the ceramic ware, and to reduce the associated quantum of conductive and convective heat losses through the shell wall.
It is also an important object of the present invention to provide selective ceramic fibre module with higher density and selective reduced thickness than the conventional type of the ceramic module now in use such as to improve heat insulation and radiation properties, and also favour the required flue to traverse uniformly throughout the kiln for uniform stock hearting.
It is yet another object of the present invention to provide for a selective inside configuration of the firing zone of the tunnel kilns for heating of stocks such as ceramic ware, to favor avoiding stagnancy of heat flux and achieving improved radiation of heat from these surfaces to achieve efficient heating of the stock.
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It is yet another important object of the present invention to reduce the specific fuel consumption i. e. of HSD oil used as fuel for burning in the firing zone through the burners located in this zone to heat the ceramic ware stock loaded on movable trolley and thus resulting in energy and cost saving in kiln operation.
Yet another object of the present invention is to increase the kiln productivity by reducing the existing percent rate of production loss due to non-uniform heating of the stock of ceramic ware loaded on trolley, specially due to incomplete heating of the lower one or two layer/s of stock.
A further object of the present invention is to reduce the cycle time of heating and finishing operation in tunnel kilns for ceramic ware by favorably altering the trolley travel speed consequent upon efficient firing and heat transfer mechanism achieved in the designed system/ lining pattern.
SUMMARY OF THE INVENTION:
Thus according to the basic aspect of the present invention there is provided a tunnel kiln for whiteware industry comprising :
a refractory lined tunnel adapted for having a loading end and a discharge end with an intermediate heating/firing zone for said firing;
heating/firing zone comprising a selective ceramic fibre module lining of upto about 300mm such as to provide a gap there between the refractory lining and the stock carrying trolleys of atleast about 150mm such as to favour reducing heat loss through the walls ,effective utilization of hot flue in the furnace and uniform heating of the stock in trolleys.
According to a preferred aspect of the present invention there is provided a tunnel kiln for whiteware industry comprising :
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a refractory lined tunnel adapted for having a loading end and a discharge end with an intermediate heating/firing zone for said firing;
heating/firing zone comprising a substantially semi circular shaped roof top and selective ceramic fibre module lining of upto about 300mm such as to favour reducing heat loss through the walls ,effective utilization of hot flue in the furnace and uniform heating of the stock in trolleys.
According to another aspect of the present invention there is provided a tunnel kiln for whiteware industry comprising :
a tunnel having a loading end and a discharge end with an intermediate heating/firing zone for said firing;
refractory lining of selective ceramic fibre module and thickness provided along the entire internal walls of the kiln with said firing zone comprising a atleast an arch shaped rounded corners joining the vertical side walls to the tunnel roof and selective ceramic fibre module lining of upto about 300mm such as to thereby provide for reducing heat loss through the walls, effective utilization of hot flue in the furnace and uniform heating of the stock in trolleys.
Importantly, in the above disclosed tunnel kiln for whiteware industry of the invention, the said heating zone is provided with selective refractory lining comprising ceramic fibre module having density 190 kg/m3 and thickness 300 mm in wall and 225mm in roof.
In accordance with a preferred aspect the tunnel kiln for whiteware industry of the invention comprises pre-heating zone having a refractory lining comprising ceramic fibre module of density 160 kg/m3 and thickness 225mm and 200mm and said cooling zone comprises ceramic fibre module of density 160 kg/m3 and thickness 225mm and 200mm.
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Advantageously, by way of the above selective lining is directed to provide appropriate gap between the lining and the stock on movable trolley at the different zones of the tunnel kiln to favor for smooth flow of products of combustion and air without increase in pressure in gap and efficient utilization of the flue heat by the stock.
Thus the invention achieves the basic aspect of the present invention to provide an improved lining pattern of the ceramic fibre module and selective design configuration of the tunnel kiln for effective and uniform heating of ceramic white ware stock on movable trolley.
Also, by way of providing said selective lining on inside wall through out tunnel kiln it is possible to reduce the existing high level of shell wall outer surface
temperatures of 140-160°C at the firing zone and 90°C -110°C at preheating and

cooling zone of kiln, by about 30°C at respective zones of the tunnel kiln.
Importantly, the present invention provides for a refractory lining pattern wherein said lining is made of ceramic fibre module having selective insulation and heat radiation properties and is provided on inside wall through out tunnel kiln provided with appropriate thickness in the different zones, such that the heat loss is minimized.
Also, in the tunnel kiln for whiteware industry of the invention the same preferably comprises ceramic module lining having (a) AI2O3 33% min.,SiO2 52% max. and ZrO2 14% min. and (b) physical characteristics comprising mean fibre diameter 3.5u max.,Shot Content (+355u) of 10% max., Linear Shrinkage at 1400°C/20Hrs. of 3.5% max., Module Density of 190Kg/m3 and thermal conductivity, w/m°k at 1000°C of 0.19.
In accordance with another aspect of the present invention it is possible to adapt the flat roof top of conventional kiln for said substantially semi-circular roof top in the heating zone in the tunnel in accordance with the invention. For such purposes
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in the conventional flat top about 600mm from wall and about 600mm from roof at one side are cut and adapted to form the arch shaped roof top in said heating zone.
Advantageously, the tunnel kiln for whiteware industry of the invention as above is adapted for shell temperature in firing zone to 110-130°C and in the preheating and cooling zone in the range of 60-80°C.Futher the tunnel kiln achieves increased productivity of 25-30%, trolley pushing cycle time reduction by about 30% and reduced specific oil consumption by 15-20%.
The invention further achieves the associated quantum of conductive and convective heat losses through the shell wall to favor achieving specific fuel consumption (of HSD fuel) by 15-20 % and reduce related costs.
Also as disclosed above the said lining of ceramic fibre module is preferably
obtained of selective properties including:
withstanding temperatures of about 1400°C with grade of density 190 kg/m3
having: the chemical analysis on wt % basis as AI2O3-33% min., SiO2-52% max,
ZrO2-14% min. ; and
Thus the selective Physical characteristics of the ceramic fibre module comprises as
Mean fibre dia-3.5u max, Shot content(+355u)-10%max,linear shrinkage at
1400°C/20hrs-3.5%max, Module density-190 kg/m3, Thermal Conductivity at
1000°C-0.19 w/m°k.
According to another aspect of the present invention the said lining of ceramic fibre module involving selective higher density of about 190 kg/m3 and improved thermal insulation and radiation properties is directed to favor an energy efficient process for uniform heating of stock on movable trolley inside the tunnel kiln with increase in productivity by about 25-30%.
According to yet another aspect of the present invention the inside configuration/contour of the firing zone of the tunnel kilns for heating of ceramic ware stock, has been changed from the existing rectangular section with flat roof to
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a configuration having semicircular profile at top of firing zone, to favor avoiding stagnancy of heat flux and demonstrating improved radiation of heat from these surfaces to achieve efficient heating of the stock.
According to yet another aspect of the present invention, the selective lining pattern and kiln contour configuration at the firing zone is directed to reduce the cycle time of operation by about 30%by favorably reducing the total trolley travel time through the tunnel kiln.
The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to the following non-limiting exemplary illustrations and the accompanying figures wherein:
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES:
Figure 1: illustrates a conventional tunnel kiln presently in use in the whiteware industry; and
Figure 2: is an illustration of a modified tunnel kiln obtained in accordance with the present invention.
THE DETAILED DESCRIPTION OF THE INVENTION IN RELATION TO THE ACCOMPANYING FIGURES:
Reference is first invited to the accompanying Figure 1, wherein an existing design of the tunnel kiln for the purpose of firing/heating of ceramic white ware is illustrated. As already discussed hereinbefore, white ceramic wares of a variety of types and applications are usually heated in a tunnel kiln of the configuration as shown in the illustrative figure.
The said white wares cover the range of typical products including kitchen ware, crockery, sanitary ware and LT/HT electrical insulators etc. The heating operation for these white wares is carried out in tunnel kilns of the type shown in Figure 1 with length varying from 30-50 meters. For such purpose these ceramic
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materials/products are systematically stacked on movable trolley and charged into the kilns for heating/firing. Trolleys loaded with the stocks thus enter through one end of the kiln and are released through the other end of the kiln. The trolley usually travels on rail track of suitable gauge, being hauled by powered winch or like device, for smooth jerk free travel at a controlled speed to favour uniform heating of stock pile loaded on the trolley.
The firing or heating zone is located at about halfway of the length of the tunnel kiln and spans over about 16-20 ft where the ceramic products are subjected to firing at a temperature of 1260°C by burning HSD through 2 to 4 burners located on inner walls at this zone and the products of combustion from the firing zone move toward the charging end of the kiln. The said figure 1 showing the existing configuration of the tunnel kiln, illustrates the constructional features, especially the inner space of a kiln in two views of the kiln-the Top view and a sectional End-view. The top view shows the existing tunnel kiln having a total length of about 35 meters, wherein the firing or heating zone (HZ) is located at about half way at 12.192 meters away from the entry (for movable trolley) end of kiln and is covering a length of about 4.877 meters and there after a length of 18.288 meters for cooling zone (CZ) exist, till the tunnel end being the exit point for the movable trolley. The top view of the kiln also shows an uniform tunnel width of 700mm at the preheating zone (PZ) at entry side and cooling zone (CZ) at the exit side, where as the heating/firing zone is having a width of 600mm more on either side, over and above the 700mm width at entry/exit side, with respect to the longitudinal central axis of the kiln, making a total of 1900mm at this zone. The existing kiln has a flat horizontal roof of an uniform height of 1550mm through out the entire length. This is shown in the end sectional view in the same figure. Cold air is sent into the cooling zone (CZ) of the kiln near the other end, for cooling the hot ceramic products. This air, after picking up heat from the hot ceramic ware is utilized for combustion of the fuel and preheating of the charge. For a daily production of 2-3 tons of ceramic ware, the consumption of HSD fuel oil is of the order of about 600-700 litres per day in the existing system of kiln operation and refractory lining pattern. Importantly, in such existing lining patterns, Ceramic fibre module of density 160 Kg/m3 are used as heat insulating material on the inner
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wall at different zones of tunnel kiln while the thickness of lining is 300mm in different zones. In particular, the thickness of lining is 300mm through out the furnace and the gap between trolley and the ceramic lining with such thickness is only about 75mm in the preheating and cooling zones respectively of the kiln.
In particular, the refractory lining used under such conventional tunnel kiln embodiment is as detailed hereunder:
TABLE-1
Chemical Analysis,Wt. (%): AI2O3- 33,min; SiO2- 52,max; ZrO2-14,min.
Physical Characteristics: Mean Fibre Dia.-3.5 u max; Shot Content ( + 355u)-10% max;
Linear Shrinkage at 14000C/20Hrs- 3.5% max
Module Density - 160 Kg/m3
Thermal Conductivity,w/m°k at 1000°C-0.22
However, with the use of said lining pattern as detailed above, the insulation was found to be inadequate exhibiting poor energy efficiency in terms of high fuel consumption and huge heat loss through kiln walls. Shell temperature of the tunnel kiln in the existing system, in the heating zone was found to be in the range of 140°C-160°C and that in the preheating and cooling zone are 90°C -110°C, which are very high and resulting in huge heat losses.
Also as further apparent from Figure 1, in such existing system of tunnel kilns, the heating zone is having a configuration with rectangular section of inside space and flat roof, as specifically illustrated in the end-view of the said figure. This leads to a stagnant heat flux at the two top right-angled edges in this zone, and less radiation from that portion of wall and roof. All these factors contribute to less effective heating of the ceramic products stacked on the trolley and moved to the firing zone
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in the existing tunnel kilns. Moreover, lining in the preheating and cooling zones are with ceramic fibre module of varying thickness. As the gap between trolley and the lining is only 75mm at these zones, excess pressure is created in the gap which does not allow the flue to traverse uniformly throughout the kiln and only moves through the upper portion of the trolley and top free portion of the kiln. Such non-uniform heating makes one or two bottom layer/s of the material stock loaded on trolley inside the kiln, defective due to under heating and lead to a rejection/ loss of production of the order of 10-15%.
It is to take care of the above noted disadvantages and complexities of the
conventional tunnel kilns as above that the present invention is directed to by way
of providing for the selective modification in the refractory lining and the
configuration of the kilns and in the process eliminate the deficiencies and
limitations of the existing systems of firing ceramic white wares. :
Reference is now invited to the accompanying Figure 2, wherein the modified tunnel kiln in accordance with the present invention is illustrated. As shown in said figure the tunnel kiln of this invention is distinguished by way of the selective refractory lining of a reduced thickness and also the modification in the roof top configuration of the tunnel funnel for better and improved kiln performance and efficiency. The modifications in the heating/firing zone (HZ) of the kiln have been illustrated. However, the configuration of the preheating zone (PZ) and Cooling zone (CZ) are maintained as usual except for the change in selective thickness of refractory lining as compared to the conventional practice. Importantly, the end view in figure 2 illustrates the modification of the roof portion of the heating zone in the present tunnel kiln of the invention. As clearly apparent from the said figure, the conversion of the right angled comer edges of the firing zone into arch shape nearly semicircular section can be achieved by cutting off 600mm each from the top of two sides and edges of flat roof of the firing zone of kiln shell and forming them in arch shape and joining. Such a modified construction of the tunnel roof is directed to eliminate the stagnancy in heat flux at the top two edges adjacent to roof at the heating zone and to increase the uniform heat radiation from inner lined surface of the portion of the kiln, as compared to that of existing kiln operation.
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According to yet another important aspect of the present invention, an improved lining pattern with better ceramic fibre module of selective thickness and properties was incorporated to reduce the fuel consumption and heating cycle time and hence the cost of operation and also to improve productivity by reducing rejection/loss of production achieved through installing the lining pattern according to this invention. The selective refractory lining used in the tunnel kiln of Figure 2 is as detailed hereunder in Table-2:
TABLE-2
Chemical Analysis,Wt. (%): AI2O3- 33,min; SiO2- 52,max; ZrO2-14,min.
Physical Characteristics: Mean Fibre Dia.-3.5 u max; Shot Content (+355u)-10% max;
Linear Shrinkage at 1400°C/20Hrs- 3.5% max
Module Density - 190 Kg/m3
Thermal Conductivity,w/m°k at 1000°C-0.19
As evident from the above cited properties of the selective grade of the insulating ceramic fibre module the same has a higher density 190 kg/m3 and lower thermal conductivity of 0.19 w/m°k as compared to the conventionally used lining, making it a better heat insulator and as such providing means for reducing the thickness of insulation lining in the present tunnel kiln to about 225mm in place of existing 300mm in the kiln. Significantly, by way of such changes in the tunnel kiln, the shell wall temperatures were brought down to 110-130°C at the heating zone and to 60-80°C at the preheating and cooling zone, by introducing the present lining pattern, favouring reduction of specific fuel(HSD Oil) consumption by about 15-20%. The above illustrated tunnel kiln in accordance with the present invention also enabled use of appropriate thickness calculated on the basis of conductive heat loss through the shell wall and optimum gap between the lining and the stock of
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ceramic ware on trolley to favour uniform heating of stock by smooth flow of heat flux of the flue. As a result of these improvements in thermal behaviour inside the kiln, the cycle time of heating or the total trolley travel time inside the kiln could be reduced by about 30% with an associated increase in the overall productivity by 25-30%.
It is thus possible for the present invention to provide for an improved lining pattern of refractory material made of ceramic fibre module of selective properties and calculated thickness as the layer of insulation at different zones of the tunnel kiln, combined with selective design/shape modification of the roof top facilitating improved heat flux and better radiation behaviour and thereby uniform heating of ceramic ware stock loaded on movable trolley is achieved in an energy efficient manner and with higher productivity in the kiln, making the invention capable of having wide industrial applications.
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WE CLAIM:
1. A tunnel kiln for whiteware industry comprising:
a refractory lined tunnel adapted for having a loading end and a discharge end with an intermediate heating/firing zone for said firing;
heating/firing zone comprising a selective ceramic fibre module lining of upto about 300mm such as to provide a gap there between the refractory lining and the stock carrying trolleys of atleast about 150mm such as to favour reducing heat loss through the walls, effective utilization of hot flue in the furnace and uniform heating of the stock on trolleys.
2. A tunnel kiln for whiteware industry comprising :
a refractory lined tunnel adapted for having a loading end and a discharge end with an intermediate heating/firing zone for said firing;
heating/firing zone comprising a substantially semi circular shaped rooftop and selective ceramic fibre module lining of thickness upto about 300mm such as to favour reducing heat loss through the walls ,effective utilization of hot flue in the furnace and uniform heating of the stock in trolleys.
3. A tunnel kiln for whiteware industry comprising :
a tunnel having a loading end and a discharge end with an intermediate heating/firing zone for said firing;
refractory lining of selective ceramic fibre module and thickness provided along the entire internal walls of the kiln with said firing zone comprising a atleast an arch shaped rounded corners joining the vertical side walls to the tunnel roof and selective ceramic fibre module lining of upto about 300mm such as to thereby
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provide for reducing heat loss through the walls, effective utilization of hot flue in the furnace and uniform heating of the stock in trolleys.
4. A tunnel kiln for whiteware industry as claimed in anyone of claims 1 to 3
wherein the said heating zone is provided with selective refractory lining comprising
ceramic fibre module having density 190 kg/m3 and thickness 300mm in walls and
225mm in roof.
5. A tunnel kiln for whiteware industry as claimed in anyone of claims 1 to 4
comprising pre-heating zone having a refractory lining comprising ceramic fibre
module of density 160 kg/m3 and thicknesses 225mm and 200mm and said cooling
zone comprises ceramic fibre module of density 160 kg/m3 and thicknesses 225mm
and 200mm.
6. A tunnel kiln for whiteware industry as claimed in anyone of claims 1 to 5
comprising ceramic module lining having (a) AI2O3 33% min.,SiO2 52% max. and
ZrO2 14% min. and (b) physical characteristics comprising mean fibre diameter
3.5u max., Shot Content (+355u) of 10% max., Linear Shrinkage at 1400°C/20Hrs.
of 3.5% max., Module Density of 190Kg/m3 and thermal conductivity w/m°k at
1000°Cof 0.19.
7. A tunnel kiln for whiteware industry as claimed in anyone of claims 1 to 6
wherein the flat roof top of conventional kiln is adapted for said substantially semi
circular rooftop in the heating zone.
8. A tunnel kiln for whiteware industry as claimed in claim 7 wherein in the
conventional flat top about 600mm from wall and about 600mm from roof at one
side are cut and adapted to form the arch shaped roof top in said heating zone.
9. A tunnel kiln for whiteware industry as claimed in anyone of claims 1 to 8
adapted for shell temperature in firing zone to 110-130°C and in the preheating and
cooling zone in the range of 60-80°C.
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10. A tunnel kiln for whiteware industry as claimed in anyone of claims 1 to 9
adapted for increased productivity of 25-30%, trolley pushing cycle time reduction
by about 30% and reduced specific oil consumption by 15-20%.
11. A tunnel kiln for whireware industry such as for firing of ceramic wares and
the like substantially as herein described and illustrated with reference to the
accompanying figures.


Dated this day of 20th November, 2006

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A tunnel kiln for heating ceramic white ware involving selective use of refractory lining of ceramic fibre module adapted to keep the steady state conductive heat loss through wall at minimum, has been provided to reduce the shell wall temperature by about 30°C compared to existing level in conventional operation, at all the zones e.g. firing zone (HZ), preheating zone (PZ) and the cooling zone (CZ) of the kiln. The roof configuration of the heating zone is also selectively converted from flat top with rectangular section to almost semicircular section at top portion to favor uniform heat flux and improved radiation property of the lining surface that facilitated uniform heating of stock of ceramic wares on trolley inside the kiln. The said use of selective lining pattern and kiln roof modification favoured achieving lower fuel consumption and higher productivity of kiln.