A kiln includes a hollow elongated housing which has an inlet and an outlet spaced from each other defining between them the interior of the housing designed to receive through the inlet the items to be treated. The housing includes a number of sections operative for treating the items as they gradually pass through the sections towards the outlet with different respective treating regimes. The sections downstream of the housing are interconnected with the section upstream of the housing so as to recover the heated gas from the downstream sections and supply the same into the upstream section in predetermined quantities and at a predetermined temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to kilns for baking items for example of 
ceramic or refractory material. 
More particularly the invention concerns arrangements for controlling the 
temperature within the kilns, for example roller kilns. 
It is known in the prior art to provide a roller kiln with a transport path 
for the items to be treated. This path may be constituted by a plurality 
of rotatable rollers extending transversely relative to the length of the 
kiln. The items transported through the kiln are subjected to different 
treating regimes in different treating sections of the kiln. Such sections 
may include one (or more) preheating and degassing sections, one (or more) 
baking sections, a forced cooling section, a natural cooling section, etc. 
It has been recognized that conventional kilns do not make it possible to 
change, namely to increase the speed of at least some treatment steps. It 
is to be understood that such a shortcoming results in unduly slow 
transportation of the items to be treated in the kiln or a greater than 
otherwise necessary length for the kiln, which in turn increases the cost 
of the kiln. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to overcome the 
shortcomings of prior art kilns. 
More particularly, it is an object of the present invention to provide a 
kiln with an arrangement which would render it possible to regulate the 
various treatment steps, that is to adjust the different steps to 
different treating regimes depending upon the particular kind of items to 
be treated thus increasing the productivity of the kiln. 
In pursuance of these objects and others which will become apparent 
hereafter, one feature of the present invention resides in providing a 
hollow elongated housing having an inlet and an outlet spaced from each 
other defining between them the interior of the housing designed to 
receive through said inlet the items to be treated. The housing includes 
at least one heating section and one cooling section located downstream 
relative to said heating section and serving to cool the items to be 
treated after the latter have left the heating section. There are provided 
first means for feeding a first medium into said cooling section for 
cooling the hot items exiting from the heating section. The first medium 
becomes heated as a result of contact with the hot items. The thus heated 
medium is withdrawn from the interior of the housing downstream of said 
cooling section. There are provided second feeding means for feeding at 
least a portion of the thusly withdrawn heated medium back into a 
predetermined part of the interior of the housing (for example upstream of 
said cooling section into a preheating section). 
The rest of the withdrawn heated medium is discharged to the atmosphere. It 
is to be mentioned that the portion of the heated medium which is fed back 
into the interior of the kiln can be varied in accordance with the desired 
temperature regime in the preheating section. The heated medium is 
supplied through a number of burners located upstream of said cooling 
section and spaced one from another. Before being fed into the interior of 
the housing the heated medium is mixed with a portion of fresh air in 
quantities. 
The housing is provided with a prekiln located before the inlet of the 
housing. Another advantageous feature of the present invention resides in 
providing means for withdrawing hot gas somewhere along the length of the 
kiln (preferably from a preheating section) and supplying the thus 
withdrawn hot gas into the prekiln. Usually, this preheating section is 
designed to remove vapors and gases from the items to be treated. The 
prekiln serves for preliminary heating of the items before they enter the 
preheating section of the kiln. 
The cooling means advantageously include a blower for supplying the medium 
and conduits connecting the blower with the interior of the cooling 
section. The conduits (preferably two: one above and one below the 
transport path) are provided with nozzles extending along the length of 
the cooling section and open onto the transport path so as to direct the 
medium onto the items to be cooled as they pass through the cooling 
section. Another advantageous feature of the present invention is that the 
conduits are adapted to rotate around their respective axes so as to vary 
the direction of the supplied medium onto the items to be treated in 
accordance with the desired cooling speed and the material of the items to 
be treated. 
The kiln in accordance with the present invention is further provided with 
means for controlling and adjusting the flow of the medium to be fed back 
into the upstream sections of the kiln. 
It is to be mentioned, that in prior-art fueled or electrically heated 
kilns, the heated gas created in the baking section is permitted to flow 
in the direction opposite to the direction of advancement of the items. 
This can create very serious disadvantages, for example affecting the 
temperature regime in the preheating section. This fact, results in unduly 
and uncontrollabe increases in the temperature of the preheating section. 
However, it is to be understood that the desired result, that is 
separating the items to be treated from gaseous residues (whether 
originally present or generated by organic impurities) can be obtained if 
the items are heated relatively slowly, that is in accordance with a 
predetermined regime. Usually, the highest permitted temperature for this 
regime is somewhere around 700.degree. C., that is the temperature above 
which there will start to form a gasproof film which will prevent any 
separation of the gases from the items. 
To avoid this undesired influence, it was a common practice with prior-art 
kilns to introduce into the preheating section a considerable quantity of 
ambient air. It has been recognized, however, that such a technique is not 
satisfactory with respect to the requirement for effective control of the 
temperature in the preheating section. Besides that, such a solution leads 
to the wasting of almost 50% of the total heat supplied into the kiln, 
which considerably increases the cost of operation of such a kiln. In 
order to eliminate this disadvantage of prior art kilns, the present 
invention teaches means for withdrawing the heated gas from the portions 
at the downstream part of the kiln and at least partially feeding said 
withdrawn hot gas back into the predetermined sections of the kiln 
(preferably at an upstream part of the transport path). 
This feature makes it possible to considerably decrease the fuel or 
electric power consumption of the kiln and to reduce the length of the 
preheating section, which is normally the longest of the sections of the 
kiln, which in turn makes it possible to further decrease the cost of the 
kiln and its maintenance. 
In the preferred embodiment of the present invention, the hot gas withdrawn 
from the baking section is enriched with the hot gas withdrawn from the 
cooling sections so as to naturally decrease the temperature of the thus 
created hot gas mixture before supplying such a mixture into the interior 
of the preheating section. 
It is to be mentioned that this arrangement is operative independently or 
in combination with any other forced cooling equipment mounted on the 
kiln. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings and first to FIGS. 1, 2 and 3 thereof, 
reference numeral 1 designates a roller kiln having a tunnel 2 with a 
transport path 3 for the items to be treated. The items are transported by 
a plurality of refractory rollers 4 extending horizontally and 
transversely relative to the length of the transport path 3. The rollers 4 
are individually driven. 
The items to be treated are introduced through an inlet 5 and are withdrawn 
from the kiln 1 through the outlet 8 of a natural cooling section 7. 
The kiln 1 is subdivided into a number of successive sections, each of them 
operating with a different respective treating regime. 
The sections includes: a short initial section d-1 wherein the hot gas is 
withdrawn and supplied in a direction opposite to that of the transport of 
the items; a second section d-2 wherein the items are preheated in order 
to degas the same from the vapor and smoke originating from the organic 
residues contained in the material of the items; a third section d-3 where 
the material is subjected to baking. In the section d-3 the temperature is 
raised as fast as possible and then maintained at a level sufficient to 
bake the material of the items. 
In a fourth section d-4 the items are subjected to a forced cooling. The 
hot air is withdrawn from the interior of the kiln 1 downstream of the 
cooling section d-4 in a fifth section d-5. The items are then transported 
through the tunnel 7 where they are subjected to natural cooling. In the 
end of the tunnel 7 there is provided a compressor 10 for supplying 
cooling air to effect further forced cooling of the items. The air is 
blown into the interior of the housing from above and below the items. 
The cooling equipment of the section d-4 in accordance with the present 
invention includes (see FIGS. 2, 4 and 5) a cooling device designated in 
toto by the reference numeral 13. The device 13 includes a blower 14 
supplying ambient air through a conduit 15 into the interior of the kiln 
1. The conduit 15 is connected with a conduit 17 which has two branches on 
each side of the kiln 1. The conduit 17 is connected with two transverse 
conduits 18 which extend beyond and above the transport path 3 of the 
items 19. The conduits 18 have a longitudinal slit 20, through which the 
cooling air is discharged into the interior of the kiln 1. The direction 
of the stream of the cooling air can be adjusted so as to vary the degree 
of heat transfer to the cooling air, thus varying the time required for 
cooling the items in correspondence with the actual composition of the 
material of the items to be cooled. The air flow supplied by blower 14 can 
be adjusted by a remote controlled throttle 21. The cooling air upon 
entering the section d-4 tends to become heated due to contact of the same 
with the walls of the section, the hot items to be cooled, etc. The thus 
heated air passes to the section d-5, where the heated air is withdrawn by 
means of a suction device shown in toto by reference numeral 22 (see FIGS. 
2, 6 and 7). The device 22 includes a blower 23 which serves to withdraw 
the heated gas from the interior of the section d-5 through a conduit 24 
located above the kiln 1. The conduit 24 is connected to conduits 25 which 
communicate with the interior of the section d-5 through the side walls of 
the kiln 1. 
The quantity of gas withdrawn is regulated by a remote controlled throttle 
21 for the cooling device 13 which controls the position of throttle 26 as 
a function of the position of the throttle 21. 
The fan 23 supplies the withdrawn gas through a conduit 27 partly into a 
discharge conduit 28 and partly through an elbow 29 and an adjustable 
air-lock 30, to a mixing arrangement 31 and further through a conduit 32 
into a blower 33. The conduit 28 can be connected to a chimney or to any 
other recovery equipment which will be discussed in detail later on. 
The blower 33 receives a portion of the withdrawn heated air and a portion 
of the fresh air supplied by the mixing arrangement 31, and supplies the 
thus formed mixture into the conduit 34 in the form of combustion air. The 
mixture is then supplied through the descending tubes 35 onto the burner 
distributed along the length of the section d-3. 
The conduit 34 is located along the section d-3 and the section d-2. The 
part of the conduit 34 which runs along the section d-2 has a 
cross-sectional dimension smaller than that running along the section d-3 
because of a diameter reduction 36. The section d-2 is supplied with the 
mixture through a plurality of tubes 37. 
The initial portion of the kiln, namely at least the whole section d-1 (or 
a part of section d-2) is provided with a withdrawing conduit 38, which is 
operative to withdraw therethrough by means of conduits 39 the hot gas and 
supply the same into a prekiln 9. It is to be noted that the portion of 
the conduit 38 extending along the section d-2 may be separated from the 
remaining portion of the conduit 38 by an air valve (not shown) which is 
operative to connect or block the conduit 38. The withdrawn gas from the 
section d-1 may be also partially transported to a chimney 42 and 
partially through a bypass 43 and an air valve 44 into the prekiln 9 which 
is operative for supplementary preheating of the items before they enter 
the kiln 1. 
The cooling device 13 and the suction arrangement 22 are interconnected 
with each other (see FIG. 2) so as to mutually influence each other with 
respect to the cooling air in the sections d-4 and d-5. 
The air in the blower 33 instead of being mixed separately with the air 
supplied by the blower 23, can be heated by a heat exchanger (not shown), 
before the air reaches the discharge tube 28 (which can also be connected 
directly to the chimney) so that the blower 33 supplies fresh air and 
recovers at least a part of the heat, which otherwise would be lost 
through the conduit 28. 
It is to be understood by now that at least a portion of the heat required 
for the upstream sections of the kiln is recovered from the kiln, instead 
of being supplied exclusively by the fuel or electrical heating equipment. 
Besides that, such an arrangement makes it possible to reach the required 
preheating and baking temperatures more easily and further to accelerate 
some treating steps, for example the preheating step. This fact leads to 
an increase in the productivity of the kiln, reducing the length of the 
kiln. This is very important with regard to reducing the cost of the kiln. 
The embodiment shown in FIGS. 1, 2 and 3 is further provided with an 
arrangement for connecting the downstream sections of the kiln 1 with the 
upstream sections, for example with the preheating section d-2 so as to 
supply this section with the gas withdrawn from the downstream sections 
(thence having a relatively low temperature) to thereby keep the 
temperature of the preheating section below the maximum permissible 
degassing temperature. This arrangement includes a member 46, for example 
a blower, designed for treating gas at high temperature. The blower 46 is 
connected to the discharge conduit 28 of the suction device 22 of section 
d-5 by means of a conduit 47 with an air valve 48. The blower 46 is 
furthermore connected to a manifold 49 provided with a plurality of 
conduits 50 which communicate with the interior of the kiln baking section 
d-3. 
The conduit 47 communicates with a conduit 56 through a valve 56'. The 
conduit 56 communicates through a plurality of tubes 57 with the interior 
of the cooling section 7. At least one tube 58 connects the conduit 56 
with an outlet 59 of the device 10--11--12. It is to be mentioned that 
prior art kilns discharge the air heated on this section completely to 
atmosphere. 
The outlet of the fan 46 is provided with a manifold 51 extending along the 
whole section d-2. The manifold 51 has a plurality of conduits 52 which 
connect the manifold 51 with the interior of the section d-2. Each conduit 
52 is provided with an air valve 53 operative to regulate the air flow 
through the respective conduits 52. 
Each conduit 52 enters the interior of the section d-2 in the space between 
two adjacent baffles 54. The baffles 54 are adjustably mounted on the 
upper wall of the kiln 1. By adjusting the height of the baffles 54 in the 
interior of the section d-2, it is possible to regulate the thickness of 
the gas layer embracing the items in the passage between the baffles 54 
and corresponding baffles 55 mounted on the lower wall of the kiln. In 
accordance with the present invention the two successive sections d-2 and 
d-3 are separated from each other by two upper and lower double baffles 54 
and 55. The two baffles 54 as well as the two baffles 55 define between 
each other a restricted chamber, into which it is possible to blow an air 
stream of appropriate temperature and pressure in order to avoid any 
mutual influence between the temperature regimes of these two neighboring 
sections. 
Thus the blower 46 receives the gases from the section d-3, the section d-5 
(through the air valve 48), the natural cooling section 7 and from the air 
compressor. 
The hot flue gases of the section d-3 mixed with the air of the sections 
d-5 and d-7, said gases containing air with all oxygen percentage, are 
supplied to the manifold 51 to be then distributed into the interior of 
the section d-2. In this way a hot gas mixture is formed which has 
recovered the greater portion of the heat that, otherwise, would be lost 
out of the kiln, said mixture being very rich of oxygen and being in the 
best conditions, to contribute to the heating of the section d-2, saving 
much heating power (fuel or electric power) and to create an ambience rich 
of oxygen facilitating the combustion of the organic residues contained in 
the materials of the items. 
It is clear that the hot gases in the section d-3 are prevented from 
directly entering the section d-2, thus the disadvantages discussed 
further above are eliminated. However, this hot gas is not wasted, but 
rather in controlled quantities fed back without exceeding the maximum 
permissible temperature in the section d-2. 
The arrangement of the double-baffle separation chamber makes it possible 
to considerably increase the temperature of the baking section without 
affecting the temperature regime of the preheating section. Thus, for 
example the temperature differential between two sections may even be 
greater than 500.degree. C. The same is true for the temperature 
differential between sections d-3 and d-4. 
The hot gas can also be recovered upon its exiting the chimney 42 and used 
for other purposes, for example for drying the items before glazing. 
Although, the blower 46 supplies the heat-containing air, it is still 
preferable to additionally heat the section d-2 by conventional means. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
constructions differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
kiln, it is not intended to be limited to the details shown, since various 
modifications and structural changes may be made without departing in any 
way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.