Patent Publication Number: US-8991475-B2

Title: Checker brick with through passages for a hot blast stove

Description:
TECHNICAL FIELD 
     The present invention generally relates to a checker brick, in particular refractory checker bricks used for recovering heat in recuperators, in particular in hot blast stoves. 
     BACKGROUND 
     In the metallurgical industry, the preheating of air for blast furnaces is conventionally carried out in adjacent regenerative heaters known as hot blast stoves. These stoves generally consist, for a stove with internal combustion chamber, of a cylindrical refractory wall and an internal vertical partition wall partitioning the stove into a combustion chamber and a checker chamber containing checker bricks or, for a stove with external combustion chamber, of two cylindrical refractory lined chambers with a connection dome. Air and fuel is introduced through one or two openings into a so-called ceramic burner or metallic burner in the combustion chamber for burning and the resultant combustion gasses flow upwardly from the combustion chamber over to the combustion chamber downwardly through the checker work chamber until they are finally exhausted at the base of that chamber. As the combustion gasses pass though the checker work chamber containing a plurality of checker bricks, heat from the combustion gasses is transferred to the checker bricks and retained therein. Once the checker bricks have reached a sufficiently high temperature, the direction of fluid flow in the stove is reversed. A cold blast is introduced at the base of the checker work chamber and is fed through the checker work chamber, where the cold blast absorbs heat from the checker bricks and passes over the partition wall and through the combustion chamber, where it leaves the stove through a hot blast outlet in the shell of the stove to be fed to the blast furnace. 
     Many different designs and arrangements of checker bricks have been designed over the years. An example of such a checker brick design can e.g. be seen in U.S. Pat. No. 4,436,144, which describes a checker brick having an octagonal outside contour and a central through passage of tetragonal cross-section. Furthermore, this brick has a substantially uniform wall thickness. Such bricks are preferably stacked in layers and staggered relative to each other. This results in a stack of checker bricks with vertical passages being formed for the gasses. In order to facilitate stacking of the checker bricks, they are provided with raised portions at the top surface of the brick and with corresponding recesses at the bottom surface of the brick. 
     Another example of such a checker brick design can e.g. be seen in U.S. Pat. No. 2,017,763, wherein an essentially square checker brick is provided with a plurality of through passages, each through passage being formed by a rectangular part and a tapered part. Due to the plurality of through passages, partition walls are being formed between the through passages. Compared to U.S. Pat. No. 4,436,144, these partition walls contribute to an increased strength of the checker brick. The plurality of through passages also allow to increase the total contact surface between the gas and the checker brick, thereby increasing the heating surface for a better heat exchange. 
     Checker bricks similar to the one disclosed in U.S. Pat. No. 2,017,763 have been suggested, wherein the through passages have circular, square or hexagonal cross-section, the latter being particularly preferred because they allow partition walls of substantially uniform thickness. Checker bricks of hexagonal cross-section are also commercially known as checker bricks of the GSI type. 
     BRIEF SUMMARY 
     The invention provides a further improved checker brick with better thermodynamic performance 
     More particularly, the present invention proposes a checker brick, in particular for hot blast stove, the checker brick having a top surface and an opposite bottom surface, wherein a plurality of through passages extend from the top surface to the bottom surface for allowing fluids to circulate through the checker brick, partition walls being formed between neighbouring through passages. According to an aspect of the invention, the through passages have a cross-section based on a hexagonal shape having alternating convex and concave sides. This particular shape enables to increase the heating surface, i.e. the surface between the through passage and the checker brick, where heat transfer between the checker brick and the gas passing through the through passage occurs. With respect to hexagonal through passages, as e.g. present on the prior art checker bricks of the GSI type, the heating surface can be increased by approximately 40%. The reduced hydraulic diameter of the through passage leads to a bigger heat exchange coefficient. A nearly constant free cross-section is also achieved. A checker brick having through passages with such a cross-section hence has better thermodynamic performance. 
     Preferably, neighbouring through passages are arranged such that a concave side of one through passage faces a convex side of a neighbouring through passage. Neighbouring through passages are preferably arranged such that partition walls of substantially constant thickness are formed between neighbouring through passages. Substantially constant wall thickness allows a uniform heat transfer and, more importantly, a uniform heating up and cooling down of the partition walls themselves, thereby avoiding damages to the partition walls due to varying temperatures within the partition wall. 
     The concave sides can be formed with a curvature of a first radius; and the convex sides can be formed with a curvature of a second radius. The first radius can substantially correspond to the second radius. With the first and second radii being substantially the same, the convex f(tx+(1−t)y)&lt;f (x) +(1−t)f (y)  and concave f(tx+(1−t)y)&gt;tf (x) +(1−t)f (y)  sides of neighbouring checker bricks become complementary. 
     According to a preferred embodiment, the convex sides have two edge regions and a central region therebetween, wherein the concave sides are formed with a curvature of a first radius, the central regions of the convex sides are formed with a curvature of a second radius and the edge regions of the convex sides are formed with a curvature of a third radius, the third radius being smaller than the first and second radii. The third radius can e.g. be about half of the second radius. The smaller radius of the edge regions of the convex sides allows creating a smoother transition from the convex side to the concave side. 
     Advantageously, the through passages are tapered in a direction towards the top surface of the chequer brick. 
     Preferably, the chequer brick has substantially hexagonal cross-section, six side faces extending from the top surface to the bottom surface. 
     The side faces of the checker bricks are advantageously provided with channels having a cross-section corresponding to half the cross-section of a through passage; the channels being arranged in such a way that, when two neighbouring checker bricks are arranged side-by-side, the chambers of the side faces of the checker bricks form a through passage. The outer walls of the checker bricks hence also have an increased heating surface. Furthermore, additional through passages can be formed between two neighbouring checker bricks when arranged side-by-side. More importantly however, the outer walls of the checker bricks also have substantially constant thickness, just like the partition walls. Uniform heat transfer is hence also guaranteed in these outer walls. 
     According to a preferred embodiment of the invention, one of the top and bottom surfaces is provided with at least one raised portion, the other one of the top and bottom surfaces being provided with a corresponding at least one recess, the at least one raised portion and the at least one recess forming tongue and groove joints between stacked checker bricks. The at least one raised portion may comprise a central raised portion on the respective top or bottom surface. The central raised portion can have a cross-section with 3-fold rotational symmetry. The tongue and groove allows avoiding that checker bricks are incorrectly installed. Furthermore, the present tongue and groove configuration creates a bigger base area, which provides an improved creep-in-compression. As a consequence, checker bricks of lower quality material can be used to achieve comparable results, thereby reducing the costs of the checker bricks. The hot blast stove can be constructed smaller and lighter, which will reduce material cost and shorten erection time, without however reducing the performance of the hot blast stove. 
     Furthermore, the at least one raised portion preferably comprises peripheral raised portions in corner regions of the respective top or bottom surface, the peripheral raised portions being dimensioned and arranged so as to be complementary to peripheral raised portions of neighbouring checker bricks. The peripheral raised portions can be dimensioned and arranged so as to have a cross-section corresponding to the cross-section of the central raised portion. Central raised portions can interact with peripheral recesses, whereas peripheral raised portions can interact with central recesses. It follows that such a configuration of raised portions and recesses enables the staggered stacking of checker bricks. Due to the shape of the raised portions and recesses, it is ensured that the checker bricks are always correctly arranged. 
     It should also be noted that, in the present document, the term “concave” is to be understood to have the mathematical meaning of “strictly concave”, thereby excluding the straight line. Similarly, the term “convex” is to be understood to have the mathematical meaning of “strictly convex”, thereby excluding the straight line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more apparent from the following description of one not limiting embodiment with reference to the attached drawings, wherein the figures show: 
         FIG. 1 : a perspective view of a checker brick according to the invention; 
         FIG. 2 : a cross-section of a through passage of the checker brick of  FIG. 1 ; and 
         FIG. 3 : a top view on the top surface of the checker brick of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a checker brick  10  according to the invention. The checker brick  10  is of substantially hexagonal cross-section and has a top surface  12 , an opposite bottom surface  14  and six side faces  15  extending from the top surface  12  to the bottom surface  14 . The checker brick is provided with a plurality of through passages  16  extending from the top surface  12  to the bottom surface  14  for allowing fluids to circulate through the checker brick  10 , partition walls  18  being formed between neighbouring through passages  16 . The through passages  16  have a particular cross-section, which can be more closely described by referring to  FIG. 2 . 
       FIG. 2  illustrates the cross-section of a through passage  16 . This cross-section is based on a hexagonal shape, as represented by dotted lines  20 , wherein however the straight sides  22  of the hexagon have been transformed to alternating convex sides  24  and concave sides  26 . The concave sides  26  are formed with a curvature of a first radius r 1  and the convex sides  24  are generally formed with a curvature of a second radius r 2 . According to the particular embodiment shown in  FIG. 2 , the convex side  24  comprises two edge regions  28 ,  30  and a central region  32  therebetween, the central regions  32  of the convex sides  24  being formed with a curvature of a second radius r 2  and the edge regions  28 ,  30  of the convex sides  24  being formed with a curvature of a third radius r 3 , wherein the third radius r 3  is smaller than the second radius T 2 . Preferably the third radius r 3  is about half of the second radius r 2 . Furthermore, the first radius r 1  is advantageously substantially identical to the second radius r 2 . Advantageously, the radii are chosen such that there is a smooth transition between convex and concave sides  24 ,  26 . 
     The shape of the cross-section of the through passages  16  may also be described as being a closed organic shape having six inflection points, each of these inflection points lying on a corner of a hexagonal shape. 
       FIG. 3  shows a top view of the checker brick of  FIG. 1  wherein the arrangement of through passages  16  with respect to each other can clearly be seen. Neighbouring through passages  16 ,  16 ′,  16 ″ are arranged in such a way that a concave side  26  of one through passage faces a convex side  24  of a neighbouring through passage. Furthermore, the arrangement is such that partition walls  18  between neighbouring through passages  16 ,  16 ′,  16 ″ are of substantially constant thickness. 
     As can also be seen on  FIG. 3 , the side faces  15  of the checker brick  10  are provided with channels  34  having a cross-section corresponding to half the cross-section of a through passage  16 . These channels  34  are arranged such that, when two neighbouring checker bricks  10  are arranged side-by-side, the chambers  34  of the touching side faces  15  of neighbouring checker bricks  10  form a through passage  16 . 
     Although not seen on the figures, the through passages  16  are tapered in a direction towards the top surface  12  of the chequer brick  10 , i.e. the cross-section of the through passage  16  at the bottom surface  14  is bigger than the cross-section of the through passage  16  at the top surface  12 . 
     Tongue and groove joints are provided for improving the stacking capabilities of the checker bricks  10 . As seen in  FIGS. 1 and 3 , the top surface  12  of the checker brick  10  is provided with raised portions  36 , whereas the bottom surface  14  of the checker brick  10  is provided with corresponding recesses  38 . The hexagonal checker brick  10  of  FIG. 3  is shown to comprise a central raised portion  40  having a cross-section with 3-fold rotational symmetry wherein the central raised portion  40  is symmetrical about an x-axis, a y-axis, and a z-axis, each of the axes passing through the checker brick  10 , thereby ensuring correct orientation of the stacked checker bricks. This central raised portion  40  is arranged around a central through passage  16 , which is surrounded by six neighbouring through passages  16 . The central raised portion  40  has a generally triangular cross-section, wherein the corner regions of the triangle are rounded off to conform to the curvature of the concave sides  26  of the three neighbouring checker bricks having their concave sides  26  facing the central checker brick. 
     In addition to the central raised portion  40 , the hexagonal checker brick  10  of  FIG. 3  comprises peripheral raised portions  42  in corner regions  44  of the top surface  12 . The peripheral raised portions  42  have a cross-section corresponding to a third of the cross-section of a central raised portion  40  and are arranged such that, when three neighbouring checker bricks  10  are arranged side-by-side, the peripheral raised portions  42  of neighbouring checker bricks  10  form a raised portion corresponding to the central raised portion  40 . This allows correct orientation of the checker bricks stacked in a staggered configuration. As can be seen on  FIG. 1 , without however being described herein in detail, the bottom surface  14  of the checker brick  10  comprises a central recess and peripheral recesses. 
     It should also be noted that the raised portions  36  may also be provided on the bottom surface  14  if the recesses  38  are provided on the top surface  12 .