WO2021173023A1

WO2021173023A1 - Checker-brick, construction method for a structure formed of a plurality of checker-bricks and the structure thereof

Info

Publication number: WO2021173023A1
Application number: PCT/RO2020/050001
Authority: WO
Inventors: Doru Tatar
Original assignee: Doru Tatar
Priority date: 2020-02-24
Filing date: 2020-02-24
Publication date: 2021-09-02

Abstract

The invention describes a checker brick (1) comprising: - a body (2) with an upper base (3), a lower base (3') and six lateral side faces (5.1, 5.2, 5.3, 5.4, 5.5, 5.6); - 7 vertical through passages (4) with internal walls (4.3) and one first end opening (4.1) and a second end opening (4.2); - the passages (4) being arranged in parallel rows with the side faces (5.1, 5.2, 5.3, 5.4, 5.5, 5.6), - one lateral channel (6) located on each side face (5.1, 5.2, 5.3, 5.4, 5.5, 5.6), at the intersection of two of the rows with passages (4); - each channel (6), having the area of a cross-section equal to half of the area of a cross-section of a passage (4); - one corner channel (7), placed instead of each side edge of the body (2), connecting the two bases (3, 3'); - each channel (7) representing one end of a row with passages (4) and has an area of a cross section equal to one third of the area of a cross section of a passage (4); - the upper base (3) has the area larger than the area of the lower base (3') and the first opening (4.1) of each passage (4) in the plane of the base (3) has the area smaller than the area of the second opening (4.2) of each passage (4) in the base plane (3') characterized in that, - each passage (4) is connected to the adjacent passage (4) on two directions of parallel rows from the total of three directions in the plane of the bases (3, 3') and, depending on the position of the passage (4), to the adjacent channel (6, 7) by means of at least one horizontal channel (8, 8'), placed such as to allow the flow of fluids both vertically and horizontally through themselves and through the passages (4) and the channels (6, 7) and - the walls (4.3) of the passages (4), the side faces (5.1, 5.2, 5.3, 5.4, 5.5, 5.6) and the channels (6, 7) have the same angle of inclination from about 0.85° to 4°, measured with respect to the central vertical axis of the body (2) and - the area of a cross-section in the plane of a base (3, 3') of each passage (4), channel (6, 7) allows stacking of several bricks (1) by aligning the centers of the longitudinal axis of symmetry of at least two passages (4), at least two channels (6, 7) or of one passage (4) and of one channel (6, 7) such that each passage (4) and each channel (6, 7) with a large area of a cross-section in the plane of a base (3, 3') is stacked on a passage (4), a channel (6, 7) with a small area of a cross-section in the plane of another base (3, 3'); each passage (4) and each channel (6, 7) with a small area of a cross section is stacked on a passage (4), a channel (6, 7) with a large area of a cross section of at least two stacked bricks (1 ). This invention also presents a structure (11) with a plurality of such bricks (1) and a method for constructing a structure (11).

Description

CHECKER-BRICK, CONSTRUCTION METHOD FOR A STRUCTURE FORMED OF A PLURALITY OF CHECKER-BRICKS AND THE STRUCTURE THEREOF

DESCRIPTION

[0001] The present invention relates to a checker brick, to a construction method for a structure formed of a plurality of such checker bricks and to a structure thereof. This type of checker brick is preferably used for the construction of regenerative heat exchangers (also known as intermittent heat exchangers), and especially for the construction of Cowper air preheaters.

BACKGROUND ART

[0002] The regenerators of such Cowper air preheaters are used in metallurgy to preheat the air introduced into furnaces. Inside the furnace, it is necessary to achieve a very high temperature which is needed for melting the iron ore, thus the air introduced into the furnace must have a temperature as high as possible, usually between 1200-1350 °C. The installation providing the air to be introduced into the furnace must withstand these high temperatures and must be able to provide high flow rates. These installations are made up of three main parts, namely: the combustion chamber, the regenerator chamber and the dome. The Cowper air preheater dome connects the combustion chamber with the regenerator chamber. The regenerator chamber is built in the form of a massive structure that may have the appearance of a right circular cylinder or an elliptical cylinder, formed of a plurality of checker bricks that are put in place without use of binders. The structure of such bricks is provided with continuous vertical through passages, both between the adjacent bricks as well as in the body of the bricks and distributed evenly throughout the entire volume of the structure and throughout its height. Just like the furnace who is always running, the regenerator chamber is used continuously, in two alternating phases, namely: the flue gases heating the bricks, flow downwards through the continuous vertical through passages and the air introduced into the furnace takes over the heat generated by the bricks in an ascending way.

[0003] The checker brick is a brick made of a special material (called refractory material), of several structural types depending on the working temperature, able to withstand high temperatures (higher than 1450°C) without changing their structure and/or chemical composition and having a high capacity for heat storage and also for the thermal transmission with the gases it interacts with. In the past, checker bricks of usual form (rectangular or tronconical) were used. Currently, a special shaped bricks are used having a high efficiency and which reduce pressure losses. The materials used in the manufacturing process of said checker bricks have as main component: Aluminum Oxide (also known as Alumina) (Al₂0₃) or Forsterite (also known as White Olivine) (Mg₂Si0₄). The chemical composition of the checker brick is usually the following: Aluminum Oxide (Alumina) or Forsterite (White Olivine) - between 30% and 60%, Silicon Dioxide or Silica - between 30% and 50%, Ferric Oxide - between 1 % and 3% and other components - 7%. The silica-alumina checker brick, currently used, has in its composition: Aluminum and Silicon Oxides in different proportions, according to the required refractory index, with some important restrictions for different impurities, such as Iron Oxides, because the presence of more than 1 .5% Iron in the composition of checker bricks in the dense silica- alumina bricks from the middle third of the brick structure, can cause the brick material to fail due to the creep phenomenon.

This silico-aluminous material is used for the bricks present inside the regenerator structure at 2/3 of the height of the regenerator structure, towards the base of this structure. At the upper third of the height of the regenerator structure, silica bricks, are used which contains exclusively Silicon Oxides due to the almost non-existent expansion coefficient of the brick material at temperatures above 700°C.

[0004] There are known checker bricks used for the construction of a structure for Cowper type regenerators as those described in the Patent specification number FR 1096652. These bricks are made of a refractory material containing as main components Aluminum Oxide and Silicon Dioxide and the brick body has substantially the shape of a regular hexagonal prism. Each vertical contour wall of the brick body is provided with a vertical channel, opened up to the outside of the brick body. Two of these channels have polygonal cross sections (preferably V2 of a regular hexagon) and the remaining four channels have semicircle cross sections, in a plane parallel to the bases of the regular hexagonal prism. Each vertical channel divides the vertical contour wall of the brick body on which it is provided in two equal parts. The vertical edges of the brick body are replaced by a vertical channel, opened up to the outside of the brick body and having polygonal cross sections, preferably one-third of the regular hexagon, in a plane parallel to the bases of the brick body having the form of a regular hexagonal prism. The inside of the brick body is provided with 7 vertical through passages, having substantially the shape of a regular hexagonal prism and/or of a cylinder; the vertical through passages connect the two bases of the prismatic body of the brick. The vertical through passages are arranged on three rows, parallel to the six vertical contour walls of the brick body and having the center of their longitudinal axis of symmetry at the intersection point of 6 equilateral triangles out of the total of 24 equilateral triangles which separate each base of the brick body.

[0005] The sizes of the hexagonal sections of the vertical through passages correspond to the similar sizes of the circular sections of the vertical through passages, in the sense that the regular hexagon can be inscribed in the corresponding circle. According to FR1096652, the hexagonal prismatic body of the brick comprises:

- Three vertical through passages with hexagonal section, equal and arranged on the same row,

- Four vertical through passages with circular section, equal and arranged on two parallel rows and adjacent to the row containing the three vertical through passages with hexagonal section,

- Four equal vertical peripheral channels open to the outside of the brick body with a semi circular section,

- Two equal vertical peripheral channels open to the outside of the brick body with a semi- hexagonal section, as well as

- Six also equal vertical peripheral channels open to the outside of the brick body with a one third of a hexagonal section. The radii of the half circles as well as the sides of the hexagon halves and of the thirds of the hexagon of the sections of the peripheral vertical channels are equal to those of the sections of the vertical circular or hexagonal through passages.

[0006] Three of the six vertical through passages surrounding the central vertical through passage are adjacent and inter-axially oriented at 120° with respect to the central vertical through passage, have three semitoroidal - shaped grooves at the surface of the lower base of the hexagonal prismatic body of the brick, and at the surface of the upper base of the hexagonal prismatic body of the brick, three semitoroidal-shaped protrusions which form joints of tongue and groove type with the grooves corresponding to another refractory brick stacked above and/or below.

[0007] According to FR1096652, the checker bricks are stacked so that the rows comprising the vertical hexagonal through passages of a brick are placed above the rows or below the rows containing the vertical through passages with circular section or peripheral vertical channels with semicircular section so that the longitudinal symmetry axes of the through passages will be parallel and will follow the same direction with that of the through passages/vertical peripheral channels from the next row of stacked bricks. Thus, vertical columns are created for the circulation of gas and/or air inside the brick structure of Cowper-type air preheaters. These vertical columns have, from one brick row to another, alternately-arranged, through passages with a large section and through passages with a small section.

[0008] It is known that the main concern in the technical field of checker bricks for Cowper- type air preheaters is the improvement and intensification of the heat transfer. This is based in particular on increasing the overall heat exchange coefficient. The change of the geometry of the heat exchange surface is accompanied by the increase of the local heat exchange coefficient and consequently of the overall heat exchange coefficient. A recognized method of intensifying the heat transfer in the case of fluids is to ensure a turbulent flow. In this case, the thermal resistance is concentrated in the boundary layer from the vicinity of the surface of the column wall where the gas and/or air circulation takes place. The achievement of a turbulent flow requires the disturbance of thermal resistance, for example, by reducing the hydraulic diameter of the column. This creates a helical fluid flow and favors the mixing of fluids in the cross section.

[0009] The disadvantages of the refractory bricks presented in FR1096652 lies in the fact that the vertical gas/air circulation within the structure formed by these rows of bricks does not have a turbulent regime if the temperature is high. This affects the heat transfer at high temperatures right in the area where the heat is more intense. The alternating between the hexagonal and the cylindrical section of channels/through passages from a brick row to another is intended to obtain a turbulent effect, since the hydraulic diameter of the section changes periodically. In this way an additional disadvantage is created, because the surface of the hexagon inscribed in the circle is quite close in value to that of the circle, therefore the difference of the thermal transfer surface is not very large and consequently, the difference in volume between the respective channels is not large.

[0010] Another disadvantage of these bricks is that their thermal transfer surface is limited to the sum of the surfaces of the vertical channels/through passages. The need to adopt different values for the surfaces, at different levels of the bricks rows along the height of the structure, makes that the only solution to this problem to be the decrease or increase of the total surface of the vertical channels/through passages. The variations cannot compensate the needs because of changes in the general structure porosity and implicitly, the bricks lose or gain weight, thus affecting the refractory mass of thermal accumulation, a very important fact in the thermal calculations.

[0011] Another disadvantage is that the corresponding protrusions and grooves, forming together a "slot and feather" or "mother-father" or "tongue and groove" joint, are unfortunately chosen, as significant percentages of the bricks' support surface are decreased. This fact must be compensated by the use of high performance refractory materials and therefore more expensive.

[0012] In addition, in the midsection, i.e. in the area of 60% -75% of the height of the checker bricks structure, where it is subjected to high temperatures and where the compression in the refractory material is high, there is a risk of deformation of bricks and the appearance of the creep phenomenon i.e. the continuous and slow variation of the irreversible stresses and deformations of a material subjected to continuous and lasting stresses. Consequently, the creep in structures working under load at high temperatures, such as the checker bricks structures of Cowper-type air preheaters is an important factor to be taken into consideration when designing a structure.

[0013] Another disadvantage of these bricks is that all channels for the vertical gas circulation are independent. They do not communicate horizontally between them and thus, any obturation of a channel forces the gases to produce additional thermal loads in the neighboring channels. This results in the differentiation of temperatures in the brick body. Thus, different expansion and contraction stresses are created and therefore brick cracking or even breaking may occur in time.

[0014] Another disadvantage of the refractory brick structure according to FR1096652 is that all its vertical channels obtained by the staggered overlap of the individual vertical channels of each brick are continuous, parallel and isolated from each other from the top to its base. This leads to the danger of blocking an entire channel if this disadvantage occurs at the level of a single brick. Thus, tens of kilograms of refractory material are removed from the heat transfer when the obturation takes place even in one place along its height. The disadvantage also extends to the fact that the trapped gases are forced to flow through the adjacent channels, creating important thermal imbalances, both in the other normally operated channels, as well as in the channels without circulation. These cause differentiated expansions in very close volumes of refractory material, which crack and crumble the structure in time. Thus, the obturation becomes progressive, involving the adjacent channels and increasing in time. In a few years, the structure may lose its permeability, requiring its replacement.

[0015] Another disadvantage of this structure, according to FR1096652, is that the operation is carried out in a non-uniform regime, because the differences in the flow rates with which the hot air enters the regenerator from the dome of the Cowper-type preheater remain the same until the hot air exits from the structure, at the bottom of the structure. This fact is due to the swirls created by the gas circulation in the dome when coming from the combustion chamber. The uneven flow intake on the upper surface of the structure overloads a part of it, while the thermal load of another part is below the level of estimated calculations. The non-uniformities of gas circulation in the structure lead to the reduction of the generator efficiency and causing its progressive deterioration or clogging. Because of the aforementioned reasons, during the dismantling of the old structures, indentations in the refractory material of the bricks were observed on the horizontal contact surfaces between the rows of bricks at the top of the structure. These indentations occur in the form of a straight line from one vertical channel to the adjacent one and are caused by the erosion of the hot gases over time (at temperatures above 1000°C) that dig their channel to ensure their transfer from one vertical channel to the next one, less stressed.

[0016] Another disadvantage of the operation of this structure is related to the insulation of its vertical channels with respect to the offset of several centimeters wide between the structure and the wall of the chamber with a circular outline in which it is located. This space ensure also the continuous circulation of hot gases, but with higher speeds and pressures, because this space is common throughout the circumference of the structure; consequently, there are no pressure losses during the gas circulation nor communications with the vertical channels of the bricks structure that might take up and equalize the impact, or to produce the effect of turbulence of air/gas circulating, as it happens to some extent inside the structure, due to the alternating of hexagon-circle sections along the height of its vertical channels. The thermal transfer produced during the gas circulation through this offset is reduced and so, the high temperatures reach the base of the structure, endangering its stability due to thermal loads not covered during the structure design calculations.

[0017] Another disadvantage of this type of structure is that the attempt of swirling the flow of gas by changing the profile of the vertical channels wall from circle to hexagon and vice versa, from a horizontal layer of bricks to the next layer of bricks is limited by the small difference of surface and hydraulic diameter between said channels profile. The hexagon perimeter, compared to the circumference of a circle equal as surface, is about 5% larger. In this case, the hexagon is inscribed in a circle from a dimensional point of view, so that the difference in perimeter, i.e. the thermal transfer surface, between said two types of profiles becomes even smaller. The pressure shocks caused by these differences are insufficient to cause a significant swirling motion.

[0018] Another disadvantage of these structures is represented by the lack of flexibility concerning the modification of its constructive and functional characteristics required by stresses and working regimes at different height levels of the structure. Thus, at the top, where the process of fixation and stabilization of bricks within the structure is important due to the swirls in the dome of the Cowper-type recuperator and the lack of a higher loading pressure to hold them in position, the "tongue and groove" joint may prove to be insufficient. In addition, in the same area, the high temperatures cause an intense heat transfer and a larger refractory mass is required to take over the enthalpy of the exhaust gas. The only solution is to reduce the surface of the vertical channels section, which causes an increase in the gas flow rate, forcing the laminar flow circulation, thus, detrimental to an optimal thermal transfer. By contrast, the refractory mass is insufficiently used at the lower part of the structure, where the heat transfer is reduced due to the low temperature. Consequently, there is a need for an increased thermal transfer surface, but this can only be achieved by increasing the sizes of vertical channels, which leads to the reduction of the structure support surface and the effective increase of the compression efforts in the refractory material. [0019] Other bricks for building the structure of Cowper-type air preheaters are presented in the Patent specification No. US 5924477/20.07.1999. These bricks consist of a body made of refractory material in the shape of a regular hexagonal truncated pyramid, similar to that described in the previous FR patent regarding the placement of the peripheral vertical channels and the through passages. The differences between the brick described in the patent specification FR1096652 and that described in the patent specification No. US 5924477 are given by:

- the shape of the cross-section of the through passages (in the Patent No. US 5924477 there are only regular hexagons - Figures 1 and 3);

- the shape of the brick body and of the through passages (in the Patent No. US 5924477, the lower base of the brick body, shaped like a regular hexagonal truncated pyramid, has the area smaller than the area of the upper base of the brick body so that the side walls of the body have an angle of inclination measured in relation to the central vertical axis of the body and the through passages narrow down towards the lower base of the brick body with the same angle of inclination measured in relation to the central vertical axis of the brick body. The value of this angle of inclination is not specified in the Patent No. US 5924477, but specifies that it is "more pronounced" - abstract, paragraphs [0005], [0008], [0009], [0011], claims 1 and 7 and Figs. 2 and 3 (reference sign 9));

- the existence of three protrusions provided on the surface of the upper base of the brick body (in the Patent No. US 5924477), these protrusions are located radially from the center of the brick body towards three of the edges of the hexagonal upper base of the brick body so that they are oriented axially, with an angle of 120° measured between these protrusions. In the patent FR1096652, the tongue - groove joints are replaced by a completely flat surface on the lower base of the brick body and said three protrusions on the surface of the upper base of the brick body - paragraphs [0008], [0009], Fig. 1- 2). [0020] A structure for Cowper-type air preheaters, consisting of stacked rows of bricks according to US 5924477, has between the side walls of the adjacent bricks a triangular space in cross section into a plane perpendicular to the bases of the bricks body. This space allows the air/gas circulation along the lateral walls of the bricks as well as in a vertical direction at the level of lower and upper bases of the bricks bodies of the structure. The bricks are stacked according to the "three-on-one" and "one-on-three" rule.

[0021] The disadvantages of these bricks and structures according to US 5924477, which although may allow the horizontal communication between some vertical channels and can thus be considered as technically superior compared to the structures with isolated vertical channels, consist in the fact that they have a relatively smaller refractory mass of heat storage related to the unit of built-in volume due to the pronounced angle of inclination. This situation can be compensated in case of a new Cowper-type recuperator, by designing additional horizontal rows of bricks in the structure, but cannot be compensated when replacing the existing structures with isolated vertical channels, where the concerned volume is limited to the value of the previous structure.

[0022] Another disadvantage of this type of brick and structure is that the support surface of each brick is significantly decreased by the presence in the same plane of the continuous network of protrusions belonging to the lower bricks row. This leads to a decrease in the area of the support surface and, implicitly, to the development of significant compression efforts, thus requiring the use of a refractory material with a higher compressive strength. The fact that stacking the horizontal rows of bricks can be made in more than one variant represents a threat because a wrong choice may isolate over great heights, three of the through passages of each brick, even over the full height of the structure. Even in the situation of adopting a correct solution of stacking the bricks rows, the integration in the compensatory horizontal circulation of flows and pressures of all the vertical channels is carried out only once at every three consecutive horizontal rows. On a horizontal plane, the temperature equalization rate between the vertical channels of the entire structure is low. The turbulence of circulation in the vertical channels is reduced, and only for less than half of the channels of each horizontal row of bricks. The turbulence does not even exist where the flow rates of the adjacent channels are identical, because the gases have no reason to flow horizontally from one channel to the other through the network of triangular spaces between bricks. This implicitly affects the optimum value of the efficiency of the heat exchanger's operation. The surface of thermal contact between bricks and gases and, implicitly of the structure they form, is limited to the surface of the walls of all vertical channels, as well as the walls of the peripheral horizontal triangular channels, but in case of the latter, only where it is favored by pressure differences between the adjacent vertical channels. These are relatively short as length and, at the level of any horizontal row, serve only less than half of all the vertical channels of the respective row. The value of the functional efficiency of the heat-exchanger structure is not optimal, although it is superior to the one obtained by the structure with isolated vertical channels.

[0023] Another disadvantage of these bricks and structures is that they do no ensure an adequate uniformity of the pressure and the gas flow rates on the horizontal between the vertical channels and do no increase the efficiency of fixing and stabilizing the bricks in the structure, although they allow a greater design flexibility concerning the adaptation to the different thermal requirements of the structure on the upper side compared to the lower one. At the bottom of the structure, the need to increase the surface of the heat transfer is limited only to the increase of the surface of the vertical channels, of the bases and of the side walls of the brick body, where horizontal pressure differences are present in the structure.

PRESENTATION OF THE TECHNICAL PROBLEM

[0024] Therefore, the object of the present invention is to eliminate the disadvantages presented above by providing a refractory brick for Cowper-type air preheaters with optimum heat transfer surfaces according to the variable working conditions at different heights in a structure of such refractory bricks, ensuring a efficient turbulence of the gas circulation through such a structure, regardless of the working temperature, horizontal leveling of flows and pressures at the level of each bricks row and providing a variable support surface to the bricks in the structure according to the mechanical stresses to which they are subjected at different working temperatures depending on the height of the structure. They are among the most important technical aspects. Furthermore, the use of this type of refractory brick in a structure for the construction of Cowper-type air preheaters ensures a better fastening and stabilization of bricks in the structure, regardless of their position in a horizontal or vertical plan. [0025] Another objective of the present invention is to present a method for the construction of such a structure for Cowper-type air preheaters which eliminates the disadvantages presented above.

[0026] In order to attain these objectives, the present invention describes a refractory brick for Cowper-type air preheaters, according to claim 1 , which comprises:

- a body having substantially the shape of a truncated pyramid, with an upper base, a lower base and six lateral side faces, wherein any cross section in a plane parallel to the bases is substantially a regular hexagon;

- 7 vertical through passages inside said body, having substantially the shape of a truncated pyramid defining inner walls of the through passages and having a first end opening and a second end opening; said vertical through passages connecting said two bases of said body;

- the vertical through passages being arranged in many rows parallel to said six lateral side faces, and having the center of their longitudinal axis of symmetry at the intersection point of the apexes of 6 equilateral triangles out of the total of 24 equilateral triangles which partition a base of said body;

- one side channel placed on each of said lateral side faces, at the intersection of two of the rows with through passages and which connects the two bases;

- each side channel having in its cross-section the area equal to half of the cross- sectional area of a through passage;

- one corner channel, placed instead of each side edge of said body and which connects the two bases;

- each corner channel representing an end of a row with through passages and having in its cross-section the area equal to one third of the cross-sectional area of a through passage;

- the body upper base having the area larger than the area of the body lower base and the first end opening of each through passage situated in the plane of the body upper base has the area, smaller than the area of the second end opening of each through passage situated in the plane of the body lower base characterized in that,

- at the level of the surfaces of the upper and lower base, on two directions of said parallel rows out of the total of three directions in the plane of the bases, each through passage is connected to each of at least one adjacent through passage and, depending on the position of the through passage, to the neighboring channel by means of at least one horizontal channel placed so as to allow the fluids to flow both vertically and horizontally through these horizontal channels, as well as through the vertical through passages, through the side channels and through the corner channels and in that

- the inner walls of said 7 vertical through passages, the lateral side faces and said channels have the same angle of inclination measured with respect to the body central vertical axis of approximately 0.85° ÷ 4° and in that

- the cross-sectional area in the plane of a base of each vertical through passage, respectively of each channel, allows the stacking of many refractory bricks by aligning the centers of the longitudinal axis of symmetry of at least two through passages, respectively of at least two channels or of one through passage and one channel such that each through passage, respectively each channel with a cross section with a large area in the plane of a base will be stacked on a through passage, respectively a channel with a cross section with a small area in the plane of another base, respectively each through passage, respectively each channel with a cross section with a small area will be stacked on a through passage, respectively on a channel with a cross section with a large area of at least two stacked refractory bricks.

[0027] This invention also relates to a structure for the construction of Cowper-type air preheaters, according to claim 12, having a plurality of rows of stacked refractory bricks and having identical bodies on the same row of bricks according to any one of claims 1-11 , the rows of bricks having a plurality of bricks placed horizontally side by side and maintaining the parallel orientation of the three directions in the plane of the bases for each brick so that, by placing side by side at least two refractory bricks, their lateral sides form through passages in the form of a substantially triangular prism having two bases in the form of an isosceles triangle and two out of the total of three lateral side faces of said prism being two of the lateral side faces of the two adjacent bricks, and the channels of these at least two adjacent refractory bricks form vertical through passages by complementing each other characterized in that these refractory bricks are stacked in a three-on-one and one-on-three formation and respectively four-on-one and one-on-four formation and in that in three-on-one and one-on-three formation, the number of tongue and groove joints between at least two stacked refractory bricks is at least three and at most seventeen and in four-on-one and one-on-four formation, the number of tongue and groove joints between at least two stacked refractory bricks is at least four and at most sixteen.

[0028] The present invention also relates to a method according to claim 14, for constructing a structure according to claims 12 or 13, wherein a plurality of rows of refractory bricks having identical bodies on the same row of bricks, the rows being made by placing side by side the lateral side faces of a plurality of checker bricks and by keeping the parallel orientation of the three directions in the plane of the bases for each brick; the bricks are stacked in three-on-one and one-on-three formation, respectively four-on-one and one-on-four formation characterized in that it comprises the following successive steps: a) placing a first row of refractory bricks, according to claims 1 -6, at the base of the structure; b) placing above the first row of bricks, a second row of refractory bricks according to claims 1 -6 which comprise at the upper base of each brick at least three and at most four protrusions according to claims 7-9; c) placing above the row of bricks previously placed, the following rows of refractory bricks according to claims 7-11 , corresponding to a zone of 25% -60% of the structure height, in which the number of tongue and groove joints between at least two stacked checker bricks is at most four and the number of horizontal channels decreases to at most one horizontal channel, up to the rows of bricks that correspond to a zone of 60% -75% of the structure height; d) placing the following rows of bricks according to claims 7-11 , which correspond to a zone of 75% -100% of the structure height, above the row of bricks which corresponds to the zone of 60% -75% of the structure height, wherein the number of horizontal channels increases up to at most four horizontal channels in a zone of 95% -100% of the structure height and the number of tongue and groove joints between at least two stacked refractory bricks increases along the height of the structure up to a maximum of sixteen for the four-on-one and one-on-four formation respectively up to a maximum of seventeen for the three-on-one and one-on-three formation, on the row of bricks corresponding to the zone of 95% -100% of the structure height and wherein the placement of the rows of bricks according to the successive steps from a) to d) takes place in compliance with the condition that: the cross-sectional area in the plane of a base of each vertical through passage, respectively of each channel shall allow the stacking of many refractory bricks by aligning the centers of the longitudinal axis of symmetry of at least two through passages, respectively of at least two channels or of a through passage and a channel such that each through passage, respectively each channel with a cross- section with a large area in the plane of a base is stacked on a through passage respectively on a channel with a cross-section having a small area in the plane of another base, and each through passage and each channel having a cross-section with a small area, is stacked on a through passage, respectively on a channel having a cross-section with a large area, of at least two stacked refractory bricks, in said structure.

[0029] According to the present invention, the solutions have the following advantages with regard to the technical solutions known in the state of the art:

- increase of the thermal transfer surface meant to improve and intensify the thermal transfer within the structure of refractory bricks;

- obtaining a strong swirling effect of gas circulation through all vertical channels of the structure, on each row of bricks, according to the needs dictated by the structure working temperature in different areas/zones along its height;

- optimal thermal transfer through:

- combining the air/gas horizontal circulation network through the horizontal channels with the air/gas vertical circulation network through the vertical channels (through passages), at each row of bricks, together with providing the differentiation of the air/gas flow directions in this horizontal network from a row of bricks to another row;

- the sequence of cross sections with different areas of the structure vertical channels from one row of through passages/channels to another row, in the body of the stacked bricks ensures the swirling of air/gas circulation and the horizontal uniformity of the flows and pressures of these gases in the structure;

- an optimum thermal load of the refractory mass and a virtually unlimited reliability of this structure during its operation, without changing the technical performance over time by adopting a method of constructing a structure according to the invention, structure whose technical characteristics may differ substantially in different areas/zones along its height thereof; - the small value (0.85° ÷ 4°) of the angle of inclination measured from the central vertical axis of the structure checker bricks body ensures a maximum support surface for the bricks; thus, a peripheral horizontal channel is created, common to all the bricks in each row of bricks, ensuring an optimal horizontal peripheral circulation meant to equalize the air/gas flows and pressures as well as the air/gas swirling in these areas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other objectives, advantages and characteristics of the invention shall be presented in the following description of the embodiments, which do not restrict the purpose and extent of this patent application, accompanied by drawings in which:

Fig. 1 provides a perspective view of a checker brick that is located in the first row of a brick structure according to an embodiment of the present invention;

Fig. 1a provides a cross-section view A-A through the checker brick body of Fig. 1 ;

Fig. 1 b provides a detail view of a vertical through passage having the shape of a regular hexagon in cross-section;

Fig. 2 is a top or bottom view of an upper or lower base of the checker brick, according to an embodiment of the present invention;

Fig. 3.1 is a detail view in a cross-section of a tongue and groove joint according to an embodiment of the present invention in which a protrusion and a funnel-type groove is provided;

Fig. 3.2 is a detail view in a cross-section of a tongue and groove joint according to another embodiment of the present invention;

Fig. 3.3 is a detail view in a cross-section of a funnel-type groove according to an embodiment of the present invention;

Fig. 3.4 is a detail view in a cross-section of a horizontal channel according to an embodiment of the present invention;

Fig. 4 is a top or bottom view of the first row of checker bricks from the base of a structure according to an embodiment of the present invention;

Fig. 5 is a perspective view of some rows of refractory bricks without protrusions and having a complete network of horizontal channels, of a structure of bricks stacked in three- on-one and one-on-three formation, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The Figures 1 - 5 present a checker brick (1 ) for Cowper-type air preheaters having a body (2) of refractory material, in the form of a substantially truncated pyramid with an upper base (3), a lower base (3’) and six lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6), wherein any cross section in a plane parallel to the bases (3, 3') being a substantially regular hexagon. Preferably, the side lenght of the regular hexagon which represents the top base (3) of the brick (1) body (2) has a length from 80 to 150 mm and the distance measured in a plane perpendicular to the two bases (3, 3’) is preferably from 50 to 200 mm. This distance is also called the "height" of the brick (1). In the context of this invention, the terms "upper" and "lower" should be understood as extremities of a body. The "upper" base may be the flat surface that is up or above, at a distance from the "lower" base that may be the flat surface below or underneath, at the same distance from the "upper" base on which the body rests. These "upper" and "lower" bases can be reversed in the sense that the "lower base" can become the "upper base" and the "upper base" can become the "lower base" of the body.

[0032] In the context of this invention, "truncated pyramid" can be understood as the section between two parallel planes which cut off a polyhedron. The polyhedron can be a pyramid with a polygonal surface (called "base") and a peak. The polygonal base is a flat, closed geometrical shape, consisting of a finite number of straight line segments, called sides. Depending on the number of sides, the base can be: a triangle (3 sides), a quadrilateral (4 sides), a pentagon (5 sides), a hexagon (6 sides), even a circle (which can be regarded as a polygon with a high number of sides).

[0033] Each base (3, 3’) of the brick (1) body (2) is formed as a compact group of 24 equilateral triangles. The centers of the longitudinal axes of symmetry of seven vertical through passages (4) inside the body (2) are provided at the point of intersection of the apexes of 6 equilateral triangles out of the total of 24 equilateral triangles which partition each base (3, 3’); these through passages (4) make the connection between the two bases (3, 3’) of the brick (1) body (2). The shape of the through passages (4) is a substantially truncated pyramid, as defined in the context of this invention, thus defining inner walls (4.3) of the through passages (4) and having a first end opening (4.1) and a second end opening (4.2).

[0034] The vertical through passages (4) are arranged in many rows parallel to the six lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) of the brick (1 ) body (2), in three directions in the plane of the bases (3, 3'). Preferably, the shape of the cross-section of the through passages (4) is a substantially regular hexagon and/or circle. The radius of the circle will be preferably chosen in the range from 12 to 25 mm.

[0035] The area of the upper base (3) of the body (2) is greater than the area of the lower base (3') of the body (2) and the area of the first end opening (4.1) of each through passage (4) located in the plane of the upper base (3) of the body (2) is smaller than the area of the second end opening (4.2) of each through passage (4) located in the plane of the lower base (3') of the body (2).

[0036] The brick (1) comprises also a side channel (6), open to the outside of the brick (1) and which connects the two bases (3, 3') and is provided on each of the lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) of the body (2), at the intersection of two of the rows with through passages (4). Each side channel (6) has a cross sectional area equal to half of the cross sectional area of a through passage (4) arranged in the row of through passages (4) identical in shape and size, on which direction is placed, in the plane of the bases (3, 3’). [0037] Instead of each side edge of the body (2), a corner channel (7) open to the outside of the brick (1) is provided, making the connection between the two bases (3, 3’) of the body (2). Each corner channel (7) represents the extremity of a row with through passages (4) and each channel (7) has a cross sectional area equal to one third of the cross sectional area of a through passage (4) arranged in the row of through passages (4) identical in shape and size, on which direction is placed, in the plane of the bases (3, 3’). [0038] On two directions of said parallel rows, on which the vertical through passages (4) are arranged, out of the total of three directions in the plane of the bases (3, 3’), at the level of the surfaces of the latter, each through passage (4) is connected to at least one adjacent through passage (4) and, depending on the position of the through passage (4), at the adjacent channel (6, 7), on one or both bases (3, 3') by means of at least one horizontal channel (8, 8’). These horizontal channels (8, 8') are positioned such as to allow the flow of fluids both vertically and horizontally through themselves and as well as through the vertical through passages (4), through the side channels (6) and through the corner channels (7). In a preferred example of this invention, the horizontal channels (8, 8') have a cross section in a plane perpendicular to the longitudinal axis of the channel (8, 8'), preferably in the form of an isosceles triangle with the base located on the plane of an upper (3) or lower (3') base, preferably with a length from 3 to 7 mm and with the height corresponding to the base of the isosceles triangle preferably from 10% to 80% more preffered from 20% to 80% of the distance measured between the two bases (3, 3').

[0039] Preferably, on the third direction of the parallel rows in the plane of the bases (3, 3'), where there are no horizontal channels (8, 8') provided, the through passages (4) are provided having the shape (for example hexagon or circle) and the cross-sectional area of the same type respectively the same numerical value and the channels (6, 7) that are in the extension of the row of through passages (4) with the shape of the cross-section identical as type with that of the said through passages (4). The channels (6, 7) have, preferably, the cross-sectional area in the plane of the bases (3, 3’) half of, respectively one-third of the cross-sectional area of the through passages (4) with the shape and the cross-sectional area of the same type, respectively the same numerical value and placed on the same row with these channels (6, 7).

[0040] Preferably, the cross-sectional area of the through passages (4) and of the channels (6, 7) arranged on the same row and in the third direction of the parallel rows in the plane of the bases (3, 3') alternates from one row to another adjacent row. In a preferred example of this invention, a row of through passages (4) and channels (6, 7) having the shape of the cross-section of the same type, for example a regular hexagon with equal sides lenght will have through passages (4) and channels (6, 7) in the adjacent parallel row/rows, with the same type of the shape of the cross section, for example a circle with equal radii. Thus, a row will have larger areas of the cross-sections in the plane of the bases (3, 3’) of the through passages (4) and of the channels (6, 7), preferably by 10% to 40% compared to the areas of the cross-sections in the plane of the bases (3, 3') of the through passages (4) and the channels (6, 7) that are on the adjacent parallel row.

In the example presented, the area of the hexagonal cross-sections will be preferably from 10% to 40% larger than the area of the adjacent circular sections.

[0041] The cross-sectional area in the plane of a base (3, 3’) of each vertical through passage (4), respectively of each channel (6, 7) will allow the stacking of at least two refractory bricks (1) by aligning the centers of the longitudinal axis of symmetry of at least two through passages (4), respectively of at least two channels (6, 7) or of a through passage (4) and of a channel (6, 7) such that each through passage (4) and each channel (6 , 7) with a cross section with a large area in the plane of a base (3, 3') will be mounted over a through passage (4) respectively a channel (6, 7) with a cross section having a small area in the plane of another base (3, 3’).

Also, each through passage (4) and each channel (6, 7) with a small cross section area will be mounted over a through passage (4), respectively a channel (6, 7) with a large cross section area. This results in an efficient swirling of the air/gas circulation from a row of through passages (4) and/or channels (6, 7) to another row, through the vertical channels of the structure (11 ) of stacked bricks (1 ), by means of abrupt passage of the air/gas circulation from one type of cross-section to another type (eg. hexagon and circle), as well as of a reversal of the direction of gas movement through the horizontal channels (8, 8') of each row of through passages (4) and/or channels (6, 7), from a vertical channel with a cross-section of one type to a vertical channel with another type of cross-section. Thus, uniformity in the horizontal plane of flows and pressures of these gases is also obtained in the structure (11). In this case, the thermal transfer in the structure (11) is optimal.

[0042] The inner walls (4.3) of the 7 vertical through passages (4), the lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) and the channels (6, 7) have the same angle of inclination measured in relation to the central vertical axis of the body (2) and is from about 0.85° to about 4° (fig. 1a). This minimum angle of inclination ensures a refractory mass and a maximum support surface for each brick (1) on each horizontal row of bricks (1 ) of the structure (11 ).

[0043] The construction of each horizontal row of bricks (1 ) of the structure (11 ) is carried out by placing side by side at least two refractory bricks (1) with a single type of brick (1) body (2) and keeping the parallel orientation of the three directions in the plane of the bases (3, 3') for each brick (1 ). By placing side by side the lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) of each brick (1 ) body (2), through passages are formed, each in the form of a substantially triangular prism (12) due to the inclination angle from about 0.85° to 4°; the through passage having two bases in the form of an isosceles triangle and two out of the total of three lateral side faces of the prism (12) being two of the lateral side faces (5.1 ,

5.2, 5.3, 5.4, 5.5, 5.6) of the two adjacent bricks (1) (fig. 5). The channels (6, 7) of these at least two adjacent checker bricks (1) form in their turn vertical through passages by complementing each other. Through this placing side by side process, each body (2) of the bricks (1) will be surrounded by six other identical bodies (2). This ensures the optimal equalization of peripheral circulation of flows and pressures as well as the air/gas swirling throughout the entire structure (11 ) of bricks (1 ).

[0044] Preferably, one of an upper base (3) and a lower base (3') of the body (2) is provided on its surface with at least three and at most seventeen protrusions (9) instead of the horizontal channels (8, 8') and the other between an upper base (3) and a lower base (3') of the body (2) is provided, at the level of its surface, with at least three and at most seventeen correspondent grooves (10). These protrusions (9) and grooves (10) form tongue and groove joints with the protrusions (9) and/or the corresponding grooves (10) belonging to another stacked refractory brick (1) placed above and/or below (fig. 3.1 , 3.2). The placement and stabilization of each brick (1 ) in the structure (11) is made by a variable number of such pairs of protrusions (9) and/or grooves (10), according to the technical needs of the different zones and heights of the structure (11 ). One or more protrusions (9) can swap their place with the corresponding grooves (10), each located on one of the two bases (3, 3').

[0045] In a preferred example of this invention, the corresponding protrusions (9) and/or grooves (10) are preferably arranged in an uniform manner in the vicinity of the regular hexagon side lenghts forming the contour of the bases (3, 3') because in the case of carving the body (2) of brick (1 ) for stacking at the edge of the row of bricks (1 ), close to the curved walls of the room in which the structure (11 ) is mounted, the part that will be support for the upper row of bricks (1 ) must contain at least one protrusion (9) and/or groove (10) for stabilizing in the row the respective body fragment (2).

The checker bricks (1 ) are stacked by means of tongue and groove joints, based on the rule of structure staggering, in three-on-one and one-on-three, respectively four-on-one and one-on- four. It is specified that in a three-on-one and one-on-three formation, in order to ensure a minimum correct fixation, each of at least three protrusions (9) of a body (2) from a row of bricks (1 ) must correspond to a groove (10) belonging to each of the 3 bodies (2) that rests on it. Similarly, in a four-on-one and one-on-four formation, each from a minimum number of 4 protrusions (9) of a brick (1 ) body (2) from a row of bricks (1 ), must correspond to a groove (10) belonging to each of the 4 bodies (2) that rests on it. [0046] In a three-on-one and one-on-three formation, the number of tongue and groove joints between at least two stacked checker bricks (1) is at least three and at most seventeen. In a four-on-one and one-on-four formation, the number of tongue and groove joints between at least two stacked checker bricks (1) is at least four and at most sixteen. Theoretically, even it’s not technically advisable for structures (11) with high heights, the minimum number of protrusions (9) can be reduced to two for all 4 patterns of stacking in formation, situation in which a minimum condition for fixation and stabilization of the structure (11 ) can be obtained. The aim of this invention is not intended to describe the two-on-one and one-on-two formation.

[0047] Preferably, the number of tongue and groove joints between at least two stacked bricks (1) increases vertically, starting from the second row of bricks (1 ) located at the base of the structure (11 ) to the top of the structure (11 ), depending on the stacking pattern in the formation. Thus, for the four-on-one and one-on-four formation, the number of joints increases from at least four to at most sixteen and for the three-on-one and one- on-three increases from at least three up to a maximum of seventeen.

[0048] In a preferred example of this invention (Figs. 3.1-3.2), the protrusions (9) have the shape of its cross-section, in a plane perpendicular to the longitudinal axis of symmetry of a protrusion (9), preferably concave, semi-elliptical, semi-circular or polygonal with the base width of the cross section measured in the plane of a base (3, 3') comprised preferably from 10 to 14 mm and the height comprised preferably from 5 to 10 mm. The grooves (10) have the shape of its cross-section, in a plane perpendicular to the axis of longitudinal symmetry of a groove (10), preferably convex, semi-elliptical, semi-circular or polygonal with the width of the groove opening (10) measured in the plane of a base (3, 3') mentioned above, preferably from 14 to 22 mm and the depth preferably from 5% to 25% of the distance measured between the two bases (3, 3'). In another embodiment of this invention, the grooves (10) have the shape of the cross-section, in a plane perpendicular to the longitudinal axis of symmetry of a groove (10), preferably in a funnel-type shape, this shape resulting from the combination of the groove shape (10) presented above and of the shape of a horizontal channel (8, 8'), by the vertical extension of the groove (10) with a horizontal channel (8, 8'), inside the body (2), as presented in figs. 3.1 , 3.3.

[0049] In addition to their role of fixation and stabilization, the grooves (10), together with the corresponding protrusions (9), have also a communication role through the channels thus formed. The surface of the cross-section of a pair formed by a protrusion (9) and a corresponding groove (10) provides after installation a sufficient free space for the circulation of air/gas in the structure (11) of bricks (1). This free space can be comparable to the space provided by the communication between channels (6, 7) ensured by a horizontal channel (8, 8'). In addition, the grooves walls (10), like the walls of a horizontal channel (8, 8'), also contribute to the increase of the heat transfer surface of the body (2), because the air/gases are forced to circulate through these grooves (10)/horizontal channels (8, 8'), due to the differences in flow and pressure between the adjacent channels, which have different shapes and sizes. In addition, a funnel-type groove (10) increases, when necessary, its heat transfer surface. The swirling caused by the air/gas flow circulating through the horizontal channels (8, 8') and/or grooves (10) will penetrate perpendicularly the air/gas flow circulating through the vertical channels of the structure (11) thus increasing the heat transfer.

[0050] In order to assemble the structure (11), the bricks (1 ) are placed side by side horizontally, in rows of stacked checker bricks (1) and keeping the parallel orientation of the three directions in the plane of the bases (3, 3') for each brick (1) (Figures 4 and 5). The bricks (1 ) have identical bodies (2) on the same horizontal row of bricks (1 ). These rows of bricks (1) overlap successively in a staggered manner, so that each brick (1) rests partially on more bricks (1) and also constitutes a partial support on other bricks (1). A horizontal row of bricks (1 ) has an approximately circular surface (with a diameter that can vary preferably from 5 m to 15 m) or an oval surface. An offset, with a width approximately equal in value to the value of the average diameter of the vertical through passages (4) is provided between each row of bricks (1) and the curved walls of the room in which the structure is mounted (11 ).

[0051] The problem of stabilizing the horizontal row of bricks (1) involves the analyze of last rows of bricks (1) in the structure (11 ), on the one hand, and the analyze of the row contour adaptation to the curvature of the walls of the room in which the structure (11 ) is mounted, on the other hand. At the upper part of the structure (11 ), meaning in the zone of 95% -100% of the height of the structure (11), the hot gases penetrate the upper rows of bricks (1) in the form of swirls due to the influence of the dome shape of the room in which the structure (11 ) is mounted and of the passage of gases into the dome. The relatively high lateral pressures of gases on the bricks (1) in the structure (11 ), pressures that can be found only here, can destabilize the last rows of bricks (1), which decrease progressively and move vertically into the dome space in order to break these swirls. This zone of 95% -100% of the height of the structure (11 ) must be made up of bodies (2) with a maximum number of tongue and groove joints, so that the fixation and stabilization of these rows will be guaranteed by the accentuated interlocking of the fixation elements. [0052] Below the upper extreme zone, at the periphery of each row of bricks (1), in order to produce the necessary curvature needed to approach the walls of the room in which the structure is mounted (11 ), fragments of bodies (2) of bricks (1) from which contour parts are missing, but never the whole contour are used. For this reason, the distribution of the protrusions (9) and/or the grooves (10) of the bodies (2) and of the body (2) fragments from the edge of the brick row (1) must be made as uniform as possible, near the side lenghts of the regular hexagon that forms the contour of bases (3, 3').

[0053] The establishment of the construction method for the structure (11) involves primarily the study of working zones height with their implied priorities caused by the variation in the structure (11 ) on its height of the working temperature, of the swirling, of the heat transfer intensity, of the refractory mass needed and of the structure support surface (11).

The number of rows of bricks (1 ) in a structure (11 ) may be from 100 to 200 ... to 600 rows, depending on the height of the structure (11) and of the height of a brick (1).

[0054] The maximum temperatures in the structure (11 ) are located at the top of the structure (11 ), i.e. in the zone of 95% -100% of its height, where hot gases penetrate. These temperatures can reach a value of up to 1550° C. The minimum temperatures in the structure (11 ) are located at its base, where the evacuation of hot gases takes place.

These temperatures vary from 200 °C to 350 °C. The internal compression stresses in the structure (11 ) have a reverse relationship with the temperature. These are minimum at the top of the structure (11) and maximum at the bottom of the structure (11). The bricks (1) requirements for the support surface are similar to those for pressures, with one exception: in the zone located at approximately 60% of the height of the structure (11 ), where the compression is not high, but the temperatures are still high, because in this zone is the danger of deforming the bricks by creep phenomenon. The heat transfer of the enthalpy of hot gases to the bricks (1 ) refractory material is directly proportional to the working temperature. This heat transfer is very intense at the top of the structure (11 ), therefore, in that zone, there is a very high need for a refractory mass which can take up the heat. Consequently, the size of the channels will decrease, even if the heat transfer surface will decrease in this way. However, the heat transfer can be helped by a higher coefficient of gas swirling using horizontal channels (8, 8') and/or grooves (10). However, these should not be deep, because they may implicitly affect the refractory mass.

[0055] By contrast, the low temperatures at the base of the structure (11 ) make the heat transfer due to the temperature to be less intense. It can be compensated by swirling and large heat transfer surfaces. These are obtained with the help of large channels and horizontal channels (8, 8')/grooves (10) as numerous and deep as possible. However, the latter should not be in the maximum number possible because the support surface is thus dangerously reduced. Thus the compression stresses in the refractory material of the bricks (1), already increased at the base of the structure (11), become even higher. The problem is solved to some extent due to the fact that the compressive strength of the refractory material is better at low temperatures.

[0056] Particular attention is paid to swirls. At the base of a structure (11), where the hydraulic diameters of the channels are large, the temperature and the speed of gas flowing through the channels is small, the swirling appears easier. This is very important because the swirling flow produces an increase of over 20% of heat transfer intensity compared to the laminar flow regime.

[0057] The creation of the gas circulation network is very important for the construction method of the structure (11 ) for Cowper-type air preheaters. First, the swirling must be maintained and increased in the upper half of the structure (11 ), where high speeds and the reduced hydraulic diameter of channels, in combination with high temperatures, shall transform the swirling flow into a laminar flow. This phenomenon is counteracted in two stages:

- the first is related to the gas circulation through the horizontal channels (8, 8'), which creates the disturbance of the laminar flow at the level of each channel, of each brick (1), through the horizontal shocks during the passage of gases from the channels with a cross- section of a type towards channels with a cross-section of another type (for example channels with different sections in shape and value of area such as hexagon or circle);

- the second is related to the fact that vertically, from one row to another row, a channel with a cross-section of one type is followed by a channel with a cross-section of another type and thus the direction of horizontal shocks is changed from one row to another.

In addition, the succession of different cross sections of the channels in a vertical direction creates in itself an additional swirling. This succession is also the reason for which the structure (11 ) can be achieved only in the four patterns of stacking the bricks (1) mentioned above, i.e. three-on-one and one-on-three, four-on-one and one-on-four formations.

[0058] In view of the optimization of the construction method of a structure (11), the choice of the suitable type of brick (1 ) body (2) will be carried out by dividing the structure (11 ) into 5 different zones, according to the requirements previously analyzed: the zone of 0- 25% of the height of the structure (11), the zone of 25% -60% of the height of the structure (11 ), the zone of 60% -75% of the height of the structure (11 ), the zone of 75% -95% of the height of the structure (11 ) and the zone of 95% -100% of the height of the structure (11 ). [0059] In the zone of 95% -100% of the height of the structure (11 ), the main requirement is to settle and stabilize the bricks (1) of these rows. The body (2) of the bricks (1) in this zone will have a maximum number of tongue and groove joints, i.e. 16, respectively 17 depending on the stacking pattern of bricks (1), in a four-on-one formation and one-on- four, respectively a three-on-one and one-on-three formation. A maximum number of 4, respectively 3 horizontal channels (8, 8') are required in this zone to ensure an efficient swirling. The large number of grooves (10) and horizontal channels (8, 8') shall have no impact on the lower support surface of the stacked bricks (1) because the compressive stresses of the weight of the structure (11 ) are reduced in this zone. The horizontal channels (8, 8'), the grooves (10) and the projections (9) will have the minimum height and width, so as to avoid the drastic reduction of the body (2) mass. This is necessary to store as much heat as possible in the zone of 95% -100% of the height of the structure (11) where large heat exchange surfaces are not required due to the intense heat transfer, characteristic of high temperatures.

[0060] In the zone of 75% - 95% of the height of the structure (11), many tongue and groove joints are no longer needed. So, their number will be gradually reduced from 16 to 4, respectively from 17 to 3, preferably one protrusion (9) at every few rows of bricks (1) or even from one row to another, in order to avoid the creation of large differences between the support surfaces of bricks (1) from one row to another, by replacing the grooves (10). The elimination of a protrusion (9) from 16 to 4 or from 17 to 3 from one row of bricks (1) to another shall be carried out gradually so that the latter will be preferably located near the regular hexagon side lenghts forming the outline of the bases (3, 3') and at a distance of 3 successive lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6), one from the other.

[0061] Referring to the zone of 60% - 75% of the height of the structure (11), we can say that we approach the so-called "critical zone" of the structure (11) in which the weight of the stacked bricks (1 ) above this zone increases the compression force in the refractory material of the structure (11) and the temperatures are still high. In this zone, the horizontal channels (8, 8 ') from the surfaces of the bases (3, 3') shall be gradually eliminated to the minimum number of a horizontal channel (8, 8') on the row/rows of bricks

(I ) which correspond to the zone of 60% of the height of the structure (11 ), called the "critical zone". This channel together with the 3 or 4 grooves (10) shall ensure the air/gas swirling inside the vertical through passages (4) and of the channels (6, 7) with a single connection between the channels with different cross sections.

[0062] In the zone of 25% - 60% of the height of the structure (11), the problem of swirling decreases in terms of importance due to the reduction of the temperature in the structure

(I I) towards the zone of 25% of its height. The compression in the refractory material and the need for large heat transfer surfaces are raising. In this zone, the number of tongue and groove joints between at least two stacked refractory bricks (1) is at most four and the number of horizontal channels (8, 8') increases to a maximum of 16. It is recommended that these horizontal channels (8, 8') to have a width as large as possible, the protrusions (9) to be as high as possible and the grooves (10) to have the greatest possible width and depth because usually the refractory mass of the structure (11) in this zone is not used efficiently.

[0063] A maximum number of horizontal channels (8, 8') on the surfaces of the bases (3, 3') is reached at the limit of 25% of the height of the structure (11) and may have an average depth. Preferably, the grooves (10) may be in the form of a funnel, as mentioned above.

[0064] The adaptation to the maximum compressive stresses in the structure (11) begins in the zone of 0% -25% of the height of the structure (11 ). Thus, in the vicinity of the base of the structure (11 ), more precisely on the second row of bricks (1 ) of the structure (11 ), refractory bricks (1) having a body (2) provided with at most 3 or 4 protrusions (9) at the level of the surface of the upper base (3) and at most 3 or 4 grooves (10) at the level of the lower base surface (3') and at most 17 horizontal channels (8, 8') shall be used to ensure the required heat exchange surface. Even if the surface of the lower base (3') of each brick (1 ) in the second row of the structure (11 ) has a complete network of grooves (10) and horizontal channels (8, 8'), the support surface of each brick (1) is not affected due to the fact that the compressive strength of the refractory material is maximum in this zone of the structure (11 ). The working temperature is below 400 °C in this zone.

[0065] The first row of bricks (1) of the structure (11) placed side by side comprises refractory bricks (1) with a maximum number of horizontal channels (8, 8'), preferably twenty and without tongue and groove joints (fig. 5). This brick (1) body (2) meets the technical conditions required for the transition from the supporting refractory cast iron structure to the structure (11) of the recuperator, taking into account that the offset between the row of bricks (1) and the curved walls of the room, in which the structure (11) is mounted, is not necessary.

[0066] Given the new type of refractory brick (1), the new type of structure (11) and the construction method of a structure (11) according to this invention, it is possible to assemble/build a mixed structure consisting of a classic structure (at the lower part of the mixed structure) and a new structure (11 ), according to the present invention (at the upper part of the mixed structure) by means of an adaptive refractory brick (as shown, for example in patent R0122218B1). This mixed structure can be assembled using the new bricks (1) according to the present invention and classic checker bricks such that the lower base (3') of a new brick (1) is compatible (allows stacking of bricks in the structure) with the upper base of a classic checker brick.

[0067] The scope of the present invention is not limited to the specific embodiments described herein. In addition, as results from the description and accompanying figures, there are various other modifications of this invention for a skilled person, in addition to the embodiments presented here, which also fall within the scope of the claims. Furthermore, various documents from the state of art are mentioned in the description, the content of which is presented in their entirety by reference.

Claims

1 . Checker brick (1 ) for Cowper- air preheaters comprising:

- a body (2) having substantially the shape of a truncated pyramid, with an upper base (3), a lower base (3') and six lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6), wherein any cross section in a plane parallel to the bases (3, 3') is substantially a regular hexagon;

- 7 vertical through passages (4) inside said body (2), having substantially the shape of a truncated pyramid defining inner walls (4.3) of said through passages (4) and having a first end opening (4.1 ) and a second end opening (4.2); said vertical through passages (4) connecting said two bases (3, 3') of said body (2);

- the vertical through passages (4) being arranged in many rows parallel to said six lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) and having the center of their longitudinal axis of symmetry at the intersection point of the apexes of 6 equilateral triangles out of the total of 24 equilateral triangles which partition a base (3, 3') of said body (2);

- one side channel (6) placed on each of said lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6), at the intersection of two of the rows with through passages (4) and which connects the two bases (3, 3');

- each side channel (6), having in its cross-section the area equal to half of the cross-sectional area of a through passage (4);

- one corner channel (7) placed instead of each side edge of said body (2) and which connects the two bases (3, 3');

- each corner channel (7) representing an end of a row with through passages (4) and having in its cross-section the area equal to one third of the cross-sectional area of a through passage (4);

- the upper base (3) of the body (2) having the area larger than the area of the lower base (3’) of the body (2) and the first end opening (4.1) of each through passage (4) located in the plane of the upper base (3) ) of the body (2) has the area smaller than the area of the second end opening (4.2) of each through passage (4) located in the plane of the lower base (3’) of the body (2) characterized in that,

- at the level of the surfaces of the upper base (3) and of the lower base (3'), on two directions of said parallel rows out of the total of three directions in the plane of the bases (3, 3'), each through passage (4) is connected to at least one adjacent through passage (4) and, depending on the position of the through passage (4), to the adjacent channel (6, 7) by means of at least one horizontal channel (8, 8') placed so as to allow the fluids to flow both vertically and horizontally through themselves, as well as through the vertical through passages (4), through the side channels (6) and through the corner channels (7) and in that

- the inner walls (4.3) of said 7 vertical through passages (4), the lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) and said channels (6, 7) have the same angle of inclination measured with respect to the central vertical axis of the body (2) from about 0.85° to about 4° and in that

- the area of a cross-section in the plane of a base (3, 3') of each vertical through passage (4), respectively of each channel (6, 7) allows the stacking of many checker bricks (1) by aligning the centers of the longitudinal axis of symmetry of at least two through passages (4), respectively of at least two channels (6, 7) or of one through passage (4) and of one channel (6, 7) such that each through passage (4) and each channel (6, 7) with a large area of a cross section in the plane of a base (3, 3') will be stacked on a through passage (4) respectively on a channel (6, 7) with a small area of a cross section in the plane of another base (3, 3'), respectively each through passage (4) and each channel (6, 7) with a small area of a cross section will be stacked on a through passage (4), respectively on a channel (6, 7) with a large area of a cross section of at least two stacked checker bricks (1).

2. Checker brick (1 ) according to claim 1 , characterized in that the through passages (4) with the same shape type and the same type of area, respectively the same numerical value of an area of a cross-section and the channels (6, 7) that are an extension of the row of said through passages (4) and which have the same shape type of the area of a cross- section as the shape of said through passages (4) are arranged on the third direction of said parallel rows in the plane of the bases (3, 3') where no horizontal channels (8, 8') are provided and said channels (6, 7) have the area of the cross-section in the plane of the bases (3, 3') ½, respectively ½ of the cross-sectional area of the through passages (4) with the same shape type and the same type of area, respectively the same numerical value of an area of a cross-section and located on the same row with said channels (6, 7).

3. Checker brick (1 ) according to claim 2, characterized in that the area of a cross- section of said through passages (4) and of said channels (6, 7) which are located on the same row and on the third direction of said parallel rows in the plane of the bases (3, 3') alternates from one row to another such that one row will have larger areas of the cross- sections in the plane of the bases (3, 3') of the through passages (4) and of the channels (6, 7), preferably by 10% to 40% compared to the areas of the cross-sections in the plane of the bases (3, 3 ') of the through passages (4) and of the channels (6, 7) located on the adjacent parallel row.

4. Checker brick (1 ) according to the preceding claims, characterized in that the shape of the cross-section of the through passages (4) is substantially a regular hexagon and/or a circle.

5. Checker brick (1 ) according to claim 4, characterized in that the radius of the circle shall preferably be selected in the range from 12 to 25 mm.

6. Checker brick (1 ) according to the preceding claims characterized in that the horizontal channels (8, 8') have a cross section in a plane perpendicular to the longitudinal axis of symmetry of said channel (8, 8'), whose shape is preferably of an isosceles triangle with its base in the plane of a said base (3, 3'), preferably from 3 to 7 mm and with its height corresponding to the base of the isosceles triangle preferably from 10% to 80%, more preferably from 20% to 80% of the distance measured between the two bases (3, 3').

7. Checker brick (1 ) according to the preceding claims characterized in that one of an upper base (3) and a lower base (3') of the body (2) is preferably provided with at least three and at most seventeen protrusions (9) instead of the horizontal channels (8, 8') and the other of an upper base (3) and a lower base (3') of the body (2) is preferably provided with at least three and at most seventeen correspondent grooves (10), said protrusions (9) and grooves (10) forming tongue and groove joints with the corresponding protrusions (9) and/or grooves (10) belonging to another checker brick (1) stacked above and/or below.

8. Checker brick (1 ) according to claim 7 characterized in that the corresponding protrusions (9) and/or grooves (10) are preferably arranged in a uniform manner, near the side lenghts of the regular hexagon forming the outline of said bases (3, 3').

9. Checker brick (1 ) according to claim 7 or 8 characterized in that the cross section of the protrusions (9) in a plane perpendicular to the longitudinal axis of symmetry of a protrusion (9) has preferably a concave, semi-elliptical, semi-circular or polygonal shape with the width of the base of the cross section measured in the plane of a said base (3, 3') preferably comprised from 10 to 14 mm and the height preferably comprised from 5 to 10 mm.

10. Checker brick (1) according to claims 7-9 characterized in that the cross section of the grooves (10) in a plane perpendicular to the axis of longitudinal symmetry of a groove (10) has preferably a convex, semi-elliptical, semi-circular or polygonal shape with the width of the opening of a groove (10) measured in the plane of a said base (3, 3') preferably comprised from 14 to 22 mm and the depth preferably comprised from 5% to 25% of the distance measured between those two bases (3, 3').

11 . Checker brick (1) according to claim 10 characterized in that the cross-section of the grooves (10) in a plane perpendicular to the axis of longitudinal symmetry of a groove (10) has preferably a funnel-type shape, this shape being the result of the combination of the shape of a groove (10) according to claim 10 and of the shape of a horizontal channel (8, 8') according to claim 6 by extending said groove (10) with said horizontal channel (8, 8') vertically inside the body (2).

12. Structure (11) for the construction of Cowper-type air preheaters, comprising a plurality of rows of stacked checker bricks (1 ) having identical bodies (2) on the same row of bricks (1) according to any preceding claim, the rows of bricks (1) having a plurality of adjacent bricks (1) placed side by side horizontally and keeping the parallel orientation of the three directions in the plane of the bases (3, 3') for each brick (1) such that by placing side by side at least two refractory bricks (1), their lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) form through passages in the form of a substantially triangular prism (12) having two bases in the form of an isosceles triangle and two of the total of three lateral side faces of said prism (12) being two of the lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) of the two adjacent bricks (1), and the channels (6, 7) of said at least two adjacent refractory bricks (1) form together by complementing each other, vertical through passages characterized in that these checker bricks (1) are stacked in three-on-one and one-on-three formation and four- on-one and one-on-four formation respectively, and in that in three-on-one and one-on-three formation, the number of tongue and groove joints between at least two stacked checker bricks (1) is at least three and at most seventeen, and in four-on-one and one-on-four formation, the number of tongue and groove joints between at least two stacked refractory bricks (1) is at least four and at most sixteen.

13. Structure (11) according to claim 12 characterized in that the number of tongue and groove joints between at least two stacked bricks (1) increases vertically, starting from the second row located at the base of said structure (11) up to the top of the structure (11), from at least four to at most sixteen for the four-on-one and one-on-four formation and at least three to at most seventeen for the three-on-one and one-on-three formation respectively.

14. Method for constructing a structure (11) according to claims 12 or 13, wherein a plurality of rows of checker bricks (1 ) having identical bodies (2) on the same row of bricks (1), the rows being made by placing side by side the lateral side faces (5.1 , 5.2, 5.3, 5.4, 5.5, 5.6) of a plurality of checker bricks (1) and by keeping the parallel orientation of said three directions in the plane of the bases (3, 3') for each brick (1 ); the bricks (1) are stacked in three-on-one and one-on-three formation and four-on-one and one-on-four formation respectively, characterized in that it comprises the following successive steps: a) placing a first row of refractory bricks (1 ) according to claims 1 -6, at the base of the structure (11); b) placing above the first row of bricks (1 ), a second row of checker bricks (1 ) according to claims 1-6 having at the level of the upper base (3) of each brick (1) at least three and at most four protrusions (9) according to claims 7-9; c) placing above the row of bricks (1) previously placed of the following rows of checker bricks (1 ) according to claims 7-11 , which correspond to the zone of 25% -60% of the height of the structure (11), wherein the number of tongue and groove joints between at least two stacked checker bricks (1) is at most four and the number of horizontal channels (8, 8') decrease to no more than one horizontal channel (8, 8'), up to the row of bricks (1) corresponding to the zone of 60% -75% of the height of the structure (11); d) placing the following rows of bricks (1 ) according to claims 7-11 corresponding to the zone of 75% -100% of the height of the structure (11 ), above the row of bricks (1 ) which corresponds to the zone of 60% -75% of the height of structure (11 ), in which the number of horizontal channels (8, 8') increases to a maximum of four horizontal channels (8, 8') in the zone of 95% -100% of the height of the structure (11 ) and the number of tongue and groove joints between at least two stacked checker bricks (1 ) increases along the height of the structure (11) up to a maximum of sixteen for the four-on-one and a one- on-four formation, respectively up to a maximum of seventeen for the three-on-one and one-on-three formation on the row of bricks (1) that corresponds to the zone of 95% -100% of the height of the structure (11 ) and wherein the placement of the rows of bricks (1) according to the successive steps from a) to d) takes place in compliance with the condition that: the area of a cross section in the plane of a base (3, 3') of each vertical through passage (4), respectively of each channel (6, 7), allows the stacking of a plurality of checker bricks (1) by aligning the centers of the longitudinal axis of symmetry of at least two through passages (4), respectively of at least two channels (6, 7) or of one through passage (4) and of one channel (6, 7) such that each through passage (4) and each channel (6, 7) with a large area of a cross section in the plane of a base (3, 3') is stacked on a through passage (4) respectively on a channel (6, 7) with a small area of a cross section in the plane of another base (3, 3'), respectively, each through passage (4) and each channel (6, 7) with a small area of a cross section is stacked on a through passage (4), respectively on a channel (6, 7) with a large area of a cross section of at least two stacked refractory bricks (1 ) in said structure (11 ).

15. Method for constructing a structure (11) according to claim 14, wherein during the step b), the bricks (1) used for construction preferably comprise:

- horizontal channels (8, 8') according to claim 6, preferably with the base of the isosceles triangle from 5 to 7 mm and the height corresponding to the base of the isosceles triangle preferably from 10% to 80% of the distance measured between said two bases (3, 3') and

- at most four protrusions (9) according to claim 9, preferably with a height of 10 mm.

16. Method for constructing a structure (11) according to claim 14, wherein during the step c), the bricks (1) used for construction preferably comprise horizontal channels (8, 8') and at most four protrusions (9) according to claim 15 and additionally:

- at most four grooves (10) according to claim 10, preferably with the width of a groove (10) opening measured in the plane of a base (3, 3') of 22 mm and the depth of 25% of the distance measured between said two bases (3, 3').

17. Method for constructing a structure (11) according to claims 14-16, wherein during the step d), the bricks (1) used for construction preferably comprise:

- at most four horizontal channels (8, 8') according to claim 6, preferably with the base of the isosceles triangle from 3 to 5 mm and the height corresponding to the base of the isosceles triangle preferably of 20% of the measured distance between said two bases (3, 3'),

- at most seventeen protrusions (9) according to claim 9, preferably with a height of 5 mm, and

- at most seventeen grooves (10) according to claim 10, preferably with the width of a groove (10) opening measured in the plane of a base (3, 3') of 14 mm and the depth of 5% of the distance measured between said two bases (3, 3').

18. Method for constructing a structure (11) according to claims 14-17, wherein preferably step b) is followed by an additional step b') during which the next rows of checker bricks (1) are placed on the second row of bricks (1) according to claims 7-11 , up to the row of bricks (1) which corresponds to the zone of 25% of the height of the structure (11) and wherein preferably bricks (1 ) having at most four grooves (10) are used, according to claim 11