FIELD OF THE INVENTION:
The present invention relates to tunnel kilns having improved aerodynamic configuration of
the different zones of kiln, directed to achieve increased productivity and improved quality
at less cost with optimized control on the variable process parameters applied in
manufacturing ceramic wares in white ware industries. The invention is also directed to
achieving said improved product quality and productivity at decreased specific fuel
consumption in said tunnel kiln.
BACKGROUND ART:
It is well known in the field of ceramic ware manufacture in small/medium sector companies
with varying capacities, that the traditional coal-fired down draft kilns were made
inoperative under imposition of law for causing severe environmental pollution. Moreover,
the range of products manufactured in these ceramic/white ware industries such as the
kitchenware, sanitary ware and High/Low Tension electrical insulators etc., suffered from
disadvantages like the poor product quality, higher rate of rejection associated with higher
product cost, increasing cost of fuel (Coal) and low fuel efficiency of the conventional coal-
fired down draft kilns. These factors caused almost all the manufacturers of the ceramic
wares to switch over to either of Oil fired shuttle kilns or Tunnel kilns in a phased manner to
be competitive and sustain quality and economy in operations.
Most of tunnel kilns developed and installed in the recent years, comprising essentially, as
the name suggests, long tunnels with varying lengths of 32-50 meters. The materials are
charged into the kiln through one end of the tunnel by loading them on movable bogies. The
movable bogies enter from one end of the tunnel kiln, travel on track through the entire
stretch of the tunnel at specified speed and come out through the other end. Firing of the
ceramic products on the bogie is done usually at almost the middle of the length of tunnel in
a region called the Tiring zone'. The hot products of combustion move from the firing zone
towards the charging end. Cold air is sent to the cooling zone of the kiln through cold air
pipe and its branches, on the other side of the firing zone i.e. on the exit side of tunnel, for
cooling the material coming out from the firing zone and approaching the exit end of tunnel.
This cooling air peaks up heat from the hot ceramic wares and gets heated itself. This hot
air in turn, is utilized for combustion of fuel and also for pre-heating of the charge prior to
entering the firing zone.
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Conventional tunnel kilns are constructed by relying mainly on measurement and
dimensions of the kilns based on individual experience and observation of persons skilled in
the related art, without any documented drawings. The general feature of such kilns
comprise of only one blower to supply combustion air to burners in firing zone, cooling air
for the cooling zone and hot air to move flue gases in the preheating zone for drying of
ceramic wares. Oil burners, four in numbers are provided in the bottom level of the firing
zone receiving combustion air from the cooling zone as preheated air and the secondary air
supplies are partly or fully closed. The conventional kilns are provided with 3 or 4
thermocouples connected to sense the preheating, firing and cooling zone temperatures, the
indicators to read the temperatures being mounted on the panel at the charge end. No
other instruments are provided to control the kiln operating parameters. Supply of air to
burners and the air curtains is controlled manually by the operation of valve or by adjusting
damper position. The temperature control in the three zones of the kiln are achieved by
adjusting the supply of fuel and air to the burners in the firing zone and also the control of
air draft in the preheating and cooling zones by the air curtains.
Because of the absence of adequate control means in the conventional tunnel kilns already
in use, a number of disadvantages and limitations are experienced as follows:
a. Improper utilization of the hot gases or combustion products of cooling zone for
preheating of products prior to firing;
b. Considerable temperature difference observed along the vertical section of kiln and
thereby from top of stack of material on trolley to its bottom most layer, which
causes non-uniform heating in the preheating zone, inducing defects.
c. Improper stacking design of materials on trolley, causes insufficient draft available
for the exhaust of products of combustion/flue gases and thereby non-uniform
heating creating variability in temperature in different layers and also outside wall
and the central part of stack of material on trolley.
d. Improper design of heating zone and ineffective burner orientation causing
insufficient heating of materials/products on trolley.
e. Air curtains in inadequate numbers causes poor air flow/flue gas circulation affecting
resultantly the convective heat transfer.
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There was therefore a need in the operation and effectiveness of conventional tunnel kiln for
production of white wares in similar industry, to develop a new tunnel kiln that is capable on
one hand to eliminate the limitations of the existing kiln and there operation in the white
ware Industry, as stated above, and on the other hand provides better control of ceramic
ware manufacturing process using the tunnel kiln in less time with superior quality and
lesser rate of rejection by administering optimized control of parameters for operation in
said tunnel kilns, ensuring uniform heating and desired rate of flow of air/flue and fuel, at
less cost and less time with lesser specific fuel consumption.
OBJECTS OF THE INVENTION:
The basic object of the present invention is therefore, directed to developing an energy
efficient improved tunnel kiln for increased productivity and quality for industrial
manufacturing of white ware products in such kilns.
A further object of the present invention is directed to reduce the variation in temperature
of products on trolley traveling on tracks through the entire length of the kiln and reducing
the resulting defects and the rate of rejection.
A further object of the present invention is directed to obtaining an improved tunnel kiln
wherein means for optimized control of parameters are provided to ensure desired flow of
air and fuel to the burners and also control and maintain proper flow and draft of hot
air/flue gas so as to obtain lower specific fuel consumption.
A still further object of the present invention directed to an improved tunnel kiln with
modification of the firing region with selective profile of the heating/firing zones so as to
achieve better aerodynamics to establish desired flow of flue gas and uniform temperature
profile along any vertical section of kilns at these regions.
A still further object of the present invention directed to an improved tunnel kiln having the
configuration adapted to create air draft by selective provisioning of air-ejectors at the
preheating and the cooling zones of the tunnel length.
According to a further object of the present invention oil burners and thermocouples are
provided in selective number at selective locations to achieve desired uniform temperature
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profile in different zones of the kilns, indicated through desired number of indicator display
means.
SUMMARY OF THE INVENTION:
Thus according to the basic aspect of the present invention there is provided a tunnel kiln
adapted for improved aerodynamics and higher productivity comprising:
a preheating zone for pre heating of the charge;
a centrally disposed heating zone with improved aerodynamics and adapted for better
temperature distribution such as to favour effective heating avoiding direct heat of flame
over the stack;
a cooling zone ;
means adapted for circulation of products of combustion from latter half of said cooling zone
to said pre hating zone through plurality of hot air removal ports;
air ejector means for suction of hot gases in cooling zone and increasing the draft in the
kiln;
means for ensuring movement of flue gases downwards for better temperature uniformity;
and
means adapted for optimization of combustion regime through controlled flow of fuel and
combustion air.
A further aspect of the present invention directed to a tunnel kiln wherein said means
adapted for circulation of products of combustion from latter half of said cooling zone to said
pre heating zone comprises two horizontal hot air suction headers, each suction header
operatively connected to four number of pipes which in turn are connected to the kiln, said
horizontal headers connected to a main header though an inverted U shaped pipe assembly
operatively connected to a horizontal pipe reaching right upto the end of pre heating zone
where it is connected to an inverted U shaped pipe assembly which again connects to two
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numbers of horizontal headers disposed slightly above the top of the moving trolley and
lower portion of the products to be heated , said horizontal headers connected to the kiln
through set of smaller dimension pipes which favour supply of products of combustion (hot
gases) from the cooling zone to the pre heating zone.
A still further aspect of the present invention a tunnel kiln wherein said air suction pipelines
in said cooling zone and air delivery pipelines in said pre heating zone are provided with
valves/dampers adapted to regulate the suction or delivery of hot gases.
According to a still further aspect of the present invention directed to said tunnel kiln
wherein said means for ensuring movement of flue gases downwards comprise multi port
bottom flue outlets operatively connected to small dimension pipelines on either sides
connecting two horizontally laid pipelines at entry of the kiln , said horizontal pipes joined
together by an inverted U shaped pipe assembly and an air ejector means operatively
connecting said U shaped pipeline to a chimney means, for creating desired suction.
According to yet another aspect of the present invention directed to a tunnel kiln wherein
said selective disposition of burners in said burner means is provided with better
aerodynamics and temperature distribution facility comprising increased length of the
heating zone and by positioning of burners in a staggered relation at the two sides which
enable better heating and avoiding direct heating of flame over product.
According to yet another important aspect of said tunnel kiln of the present invention
wherein air-fuel ratio is optimized for optimum heat availability.
A further aspect of said tunnel kiln of the invention comprising means to improved air/flue
circulation for better convective heat transfer.
A still further aspect of the present invention directed to a tunnel kiln wherein the number
of suction points for the hot air in cooling zone comprise four on each side, number of hot
air delivery pipes in preheating zone comprise four on each side, number of suction pipes in
the beginning of preheating zone comprise four at each side.
According to another aspect of the present invention directed to said tunnel kiln wherein the
number of suction points for the hot air in cooling zone, number of hot air delivery pipes in
preheating zone and the number of suction pipes in the beginning of preheating zone
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selectively vary as per the kiln dimensions, capacity of burners, air blowers, production rate
and other factors and also the capacity of the blowers also vary depending upon the
application such as for provision of combustion air to burners, provision of air to air curtains
and air for ejectors.
The present invention and its objects and advantages are described in greater details with
reference to the following accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES:
Figure 1: is the illustration of the conventional tunnel kiln showing the different zones and
components in the kiln.
Figure 2: is the illustration of the improved tunnel kiln according to the present invention,
comprising the front View/Elevation and the Top view/Plan, with the details of the different
zones showing the multiple port arrangement of air suction and hot air delivery for
improved draft involving ejector means and the blowers for supplying combustion air.
Figure 3(a): is the illustration of the detailed arrangement of suction header through
multiple air removal port on either side of kiln, involving damper control and air-ejector
means for creating desired draft at the cooling zone of kiln.
Figure 3(b): is the illustration of the multiple port hot air delivery on either side of kiln at
selective height in the preheating zone ensuring uniform heating of stack on trolley.
Figure 3(c): is the illustration of the detailed arrangement of suction header through
multiple air exhaust port on either side of kiln, involving damper control and air-ejector at
chimney side near charging end of kiln.
Figure 4(a): is the illustration of the 4 inch size air Ejector mounted at the chimney end,
connected to the suction header.
Figure 4(b): is the illustration of the air Ejector at the cooling zone for Hot air Suction from
either side of kiln and exhaust to the hot air line providing adequate draft.
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Figure 5: is the illustration of the preferred Profile of shell and Ceramic fibre heat insulation
at the heating zone and selective location of burners for desired uniform cooling of materials
stack on trolley.
DETAILED DESCRIPTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS:
The existing tunnel kiln facilities for whiteware industries are found to be inefficient in
operation and fuel economy vis-a-vis the product quality due to reasons like the non-
uniform heating, inefficient burning of fuel and improper utilization of the flue gas heat
caused due to lack of adequate draft and control for the operating parameters including
temperature distribution/variations in different zones. The products produced by the
operation of these deficient kilns led to higher fuel consumption and fall in product quality
and rise in rate of rejection.
The improved configuration of the Tunnel kiln of the present invention is therefore
configured to favour eliminating the said limitations and ensure optimized control of
operating parameters in the preheating, firing and cooling zones of the tunnel kiln, reducing
cost of production by bringing down the rate of rejection and fuel consumption, leading to
higher furnace yield.
Reference is first invited to the accompanying Figure 1, wherein the features of the
conventional Tunnel kiln used in white ware industries are shown that has driven to modify
the existing design and develop the kiln with novel features as of the present invention. It is
observed that in the conventional tunnel kiln, general features comprise only one blower to
supply combustion air to burners in firing zone, cooling air for the cooling zone and hot air
to move flue gases in the preheating zone for drying of ceramic wares. Oil burners, two in
numbers are provided in the bottom level on sides and two numbers on the ends of the
firing zone, receiving combustion air from the cooling zone as preheated air and the
secondary air supplies to the nozzles are partly or fully closed. The conventional kilns are
provided with 3 or 4 thermocouples connected to sense the preheating, firing and cooling
zone temperatures, the indicators to read the temperatures being mounted on the panel at
the charge end. No other instruments are provided to control the kiln operating parameters.
Supply of air to burners and the air curtains is controlled manually by the operation of valve
or by adjusting damper position.
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The improvements over the conventional kiln has been achieved in the kiln of present
invention in operation and design configuration, in the following respects:
Circulation of hot air to preheating zone:
Reference is now invited to the accompanying Figure 2, that illustrates the innovative
features of a preferred embodiment of the tunnel kiln of the invention providing improved
system of uniform heating of stack and less fuel consumption with improved quality of
product , overcome the limitation of the conventional tunnel kiln by improving upon the
temperature distribution through out any vertical section ensuring uniform heating of stack
of materials on trolley and also desired draft and lower specific fuel consumption to achieve
economy and improved quality of product with reduced rejection. In the improved
configuration of tunnel kiln according to the present invention, the circulation of products of
combustion/flue gas produced in the firing zone from the later half to preheating zone of
cooling zone by adopting advantageous air ejector type draft creating suction head through
multiple air removal ports. This hot flue gas is utilized to pre-heat the material stack on
trolley at the entry side of tunnel, approaching the firing zone located at the central part. It
is shown clearly in the accompanying Figure 2, that the two horizontal air suction
headers(l) are installed on two sides of the tunnel at the exit side of cooling zone of tunnel,
each being connected with four numbers horizontal pipes(2) opening at four ports on the
tunnel wall. These two suction headers/horizontal pipes on the two sides of the tunnel are
connected to a main header (4) for hot air, running parallel to the longitudinal axis of kiln
upto the end of preheating zone, through an inverted 'U' shaped pipe assembly (3). The
details of connection of the pipes and headers to the multiple air removal ports Figure 3(a).
A similar multi-port arrangement, as illustrated in the accompanying Figur 3(b), comprising
of horizontal pair of pipes/delivery headers (5) and an 'U' shaped inverted connector are
operatively connected to the mainheader (4) at the start of preheating zone at the entry
side of the tunnel these two suction headers having a level slightly above the top height of
moving body of trolley(6) on track and bottom most part of stack on trolley to be heated.
These pipes are connected to a set of four numbers smaller diameter pipes (7), on either
side of the kiln, which in turn are connected to the port openings on the wall of the kiln.
These pipes supply the products of combustion (hot gases) from the cooling zone to the
preheating zone. The smaller diameter pipes in sets of four (2 & 7) at both cooling zone and
the preheating zone are provided with valves and dampers to regulate the suction or
delivery of hot gases. Such an arrangement for circulation of hot air/flue, provide desired
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draft for effective and uniform heating of the stack on trolley as well as efficient utilization
of the heat of hot flue gases.
Provision of Ejectors substituting the suction blowers:
The present invention makes use of neither of the exclusive conventional means of creating
draft for transporting the hot air or flue gas at desired flow rate through ducts by providing
natural draft by installing Chimney of required height at a favorable location to create the
desired suction pressure head or a fan or blower to create induced draft. Instead, it uses an
economic method for effective draft at desired level by creating suction head at the
preheating end and the cooling end by installing two numbers of air ejectors of suitable
capacity to favor uniform convective heat transfer and heating of stack. Accompanying,
Figure 2 shows the locations as A and B at the preheating and cooling ends of hot air/flue
duct respectively, and the ejector at the preheating side, 4 inches in size, is operatively
connected to facilitate draft for circulation as well as to force out the residual combustion
products of which effective heat has been extracted and utilized is exhausted through a
chimney, where as the ejector on the cooling side is utilized to create the necessary suction
head to suck in the hot air near the exit end of tunnel in to the main header line for hot
air/flue gas through the multiport horizontal suction headers. The details of the ejector and
its constructional features are illustrated in the accompanying Figures 4(a) and 4(b) and
their inter connectivity with the air removal ports and the air exhaust ports on the kiln walls
to the main hot air header line for sucking in air to create desired draft, through the kiln, on
either of preheating side and the cooling end side of kiln, has been illustrated in the
accompanying Figures 3(a) and 3(c).
The provisioning of the Ejectors have saved the complexity and cost involvement to create
equivalent air draft by erecting Chimney alone and also eliminating the Power requirement
or sensitivity in suction pressure selection in case of exclusive use of blower/fan for creating
said draft.
Improved configuration of Heating/Firing Zone:
The kiln of the present invention has a firing zone which is longer in length than
conventional one and four numbers of burners (8) have been installed in this zone, in
staggered position on side walls of firing zone of kiln as illustrated in the general
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arrangement drawing of kiln in accompanying Figure 2. The selective staggered positioning
of four burners helps achieving better temperature distribution and two numbers burners
installed on the end wall near cooling zone further helps in uniform heating and at the same
time avoiding direct heating of flame over the product. The kiln's section profile in the
heating/firing zone with ceramic fibre insulation lining on kiln wall has been configured to
ensure better aerodynamics for flow of hot air/flue gas favoring attaining uniform
temperature at all positions of material stack on trolley, as illustrated in accompanying
Figure 5, and thus reducing the rejection of ceramic ware.
Multi-port bottom outlets for flue to attain uniform temperature at all depths of stack:
The kiln of the present invention introduces multi-port bottom flue outlets to create desired
rate of circulation of hot air or flue through all zones of the kiln, as illustrated in
accompanying Figure 3(c) said ports being connected to a pair of horizontal suction header
pipes (10) placed on either side of kiln at almost the entry point and interconnected by an
inverted U tube assembly (12) to the main hot air header line at the chimney end. The
arrangement of the hot air suction headers are connected to the kiln by four numbers pipes
(10) on either side of kiln to the port openings. The location of exhaust ports are at a level
lower than the hot air delivery ports, ensuring uniform heating and desired rate of
circulation of hot air, for uniform heating of the materials on stack. An air ejector, 4 inch in
size is operatively connected to both the suction header and the chimney is used to create
the desired suction head for exhaust of the residual gases with lesser heat content than
conventional to be released in the atmosphere.
Optimizing the flow rates of fuel and preheated air for complete combustion of fuel:
The present invention provide for optimized control of the flow rates of fuel and air to the
burners, maintaining proper Air/Fuel ratio so as to favor complete combustion of the fuel
and proper thermal regime in the firing/heating zones, preventing possibility of unburnt fuel
due to lack of adequate air supply for complete combustion, and thus effecting saving in
specific fuel consumption. Products of combustion have been analyzed to ensure desired
air/fuel Ratio Optimized for the optimum heat availability.
The tunnel kiln for white ware industry according to the present invention further comprises
some additional features to provide uniform heating of material stack on trolley favoring
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superior quality, less rejection, lower specific fuel consumption and higher productivity/yield
in kiln operation,
a. The Air curtains utilized for control of heat flux inside the kiln (please provide
relevant functional detail to highlight the inventive merit of the present invention)
has been selectively numbered and positioned/spaced at the different zones of the
tunnel kiln from the roof top such as to favor improved air/flue circulation to achieve
desired rate of forced circulated convective heat transfer, providing preferred
thermal profile on any vertical section inside the furnace.
b. The number of port openings for the suction and delivery headers are selectively
maintained four in numbers in the preheating and cooling zone on either side of kiln
in an embodiment of the kiln of the invention, decided on the basis of kiln
dimension/section, capacity of burners and air blowers production rate and other
related factors, in order to attain desired rate of circulation/draft of hot air/flue in the
kiln.
c. While the preferred embodiment used two blowers to meet the total air
flow/pressure requirement in the devices/components in the entire system such as
the combustion air for burners in firing zone, provision of air to air curtains and air
for air ejectors. However, to save on the investment on equipment costs, one blower
with sufficient discharge volume and pressure has also been successfully tried up for
proper regulation of discharge and pressure serving different applications in the
same system.
This is thus possible by way of this invention to achieve a system of improved performance
of tunnel kilns for production of white ware, means for improved circulation of the hot air
and flue through the different zones of the furnace by adopting selective numbers of
multiple-port connection of suction and delivery headers at selective locations and preferred
temperature profile thereby obtain uniform heating of the materials stack on the trolley on
track moving through the different zones of the tunnel kiln, efficient burning of fuels
through selective number of burners and preferred fuel/air ratio to lower the specific fuel
consumption, lesser rejection of product, higher productivity and yield of the kiln.
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We claim:
1. A tunnel kiln adapted for improved aerodynamics and higher productivity comprising:
a preheating zone for pre heating of the charge;
a centrally disposed heating zone with improved aerodynamics and adapted for better
temperature distribution such as to favour effective heating avoiding direct heat of flame
over the stack of product;
a cooling zone ;
means adapted for circulation of products of combustion from latter half of said cooling zone
to said pre hating zone through plurality of hot air removal ports;
air ejector means for suction of hot gases in cooling zone and increasing the draft in the
kiln;
means for ensuring movement of flue gases downwards for better temperature uniformity;
and
means adapted for optimization of combustion regime through controlled flow of fuel and
combustion air.
2. A tunnel kiln as claimed in claim 1 wherein said means adapted for circulation of products
of combustion from latter half of said cooling zone to said pre heating zone comprises two
horizontal hot air suction headers, each suction header operatively connected to four
number of pipes which in turn are connected to the kiln, said horizontal headers connected
to a main header though an inverted U shaped pipe assembly operatively connected to a
horizontal pipe reaching right upto the end of pre heating zone where it is connected to an
inverted U shaped pipe assembly which again connects to two numbers of horizontal
headers disposed slightly above the top of the moving trolley and lower portion of the
products to be heated , said horizontal headers connected to the kiln through set of smaller
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dimension pipes which favour supply of products of combustion (hot gases) from the cooling
zone to the pre heating zone.
3. A tunnel kiln as claimed in claim 2 wherein said air suction pipelines in said cooling zone
and air delivery pipelines in said pre heating zone are provided with valves/dampers
adapted to regulate the suction or delivery of hot gases.
4. A tunnel kiln as claimed in anyone of claims 1 or 2 wherein said means for ensuring
movement of flue gases downwards comprise multi port bottom flue outlets operatively
connected to small dimension pipelines on either sides connecting two horizontally laid
pipelines at entry of the kiln , said horizontal pipes joined together by an inverted U shaped
pipe assembly and an air ejector means operatively connecting said U shaped pipeline to a
chimney means, for creating desired suction.
5. A tunnel kiln as claimed in anyone of claims 1 to 4 wherein said selective disposition of
burners in said burner means is provided with better aerodynamics and temperature
distribution facility comprising increased length of the heating zone and by positioning of
burners in a staggered relation at the two sides which enable better heating and avoiding
direct heating of flame over product.
6. A tunnel kiln as claimed in anyone of claims 1 to 5 wherein air-fuel ratio is optimized for
optimum heat availability.
7. A tunnel kiln as claimed in anyone of claims 1 to 6 comprising means to improved air/flue
circulation for better convective heat transfer.
8. A tunnel kiln as claimed in anyone of claims 1 to 7 wherein the number of suction points
for the hot air in cooling zone comprise four on each side, number of hot air delivery pipes
in preheating zone comprise four on each side, number of suction pipes in the beginning of
preheating zone comprise four at each side.
9. A tunnel kiln as claimed in anyone of claims 1 to 7 wherein the number of suction points
for the hot air in cooling zone, number of hot air delivery pipes in preheating zone and the
number of suction pipes in the beginning of preheating zone selectively vary as per the kiln
dimensions, capacity of burners, air blowers, production rate and other factors and also the
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capacity of the blowers also vary depending upon the application such as for provision of
combustion air to burners, provision of air to air curtains and air for ejectors.
10. A tunnel kiln adapted for improved aerodynamics and higher productivity substantially
as hereindescribed and illustrated with reference to the accompanying figures.

A tunnel kiln for white ware industries adapted for circulation of products of combustion
from latter half of its cooling zone to the pre heating zone comprising two horizontal hot air
suction headers, each suction header operatively connected to the kiln at cooling zone, said
horizontal headers connected to a main header though an inverted U shaped pipe assembly
to a horizontal pipe reaching right upto the end of pre heating zone where it is connected to
an inverted U shaped pipe assembly which again connects to two numbers of horizontal
headers disposed slightly above the top of the moving trolley and lower portion of the
products to be heated , such that uniform heating of product at all depth of stack on trolley
is ensured by favoured supply of products of combustion (hot gases) from the cooling zone
to the pre heating zone. Said tunnel kiln providing air ejector means and a chimney means,
for creating desired draft; air-fuel ratio optimized for optimum heat availability, means to
improve air/flue circulation for better convective heat transfer, enabling efficient heating
and avoiding direct heating of flame over product ensuring improved quality at less cost.