NL2021438B1

NL2021438B1 - Internal lining system

Info

Publication number: NL2021438B1
Application number: NL2021438A
Authority: NL
Inventors: Van Der Woude Bastiaan; De Kreij Albert
Original assignee: Hadek Protective Systems B V
Priority date: 2018-08-08
Filing date: 2018-08-08
Publication date: 2020-02-17
Also published as: WO2020032792A1

Abstract

_ 16 _ ABSTRACT A building element (40) for use in an internal lining system of a chimney, preferably made from closed cell foamed borosilicate glass, comprises a front face (42) and a back face (44) 5 and side faces (46, 48, 50, 52). The front face (42) has a height (Z) and a width (X) and is spaced apart by a depth (Y) from the parallel back face (44). At least one of the side faces (46) has a first recess (54) for accommodating an adhesive, allowing to construct an internal lining system without horizontal joints of adhesive at the front faces. 10 Fig. 3

Description

Technical Field

The present invention relates to an internal lining system for use in an industrial chimney, for instance designed for operating at wet stack conditions. In addition it relates to the construction elements used for the improved lining system.

Background Art

Nowadays many coal-fired utility power plants employ flue gas technologies. In most wet stack operations flue gas enters the stack directly from the flue gas plant. A “wet stack” is a chimney, stack, or flue that exhausts water saturated flue gas downstream from a wetscrubbing process, such as a wet flue gas desulfurization (WFGD) system. Most recently designed and constructed WFGD systems have installed wet stacks. Although the technology is relatively mature, there are a number of technical issues that utilities must address to achieve a successful installation. The Revised Wet Stack Design Guide, final report 1026742, Copyright © 2012 Electric Power Research Institute, Inc., (hereafter the EPRI Guide) is still the guide on wet stack design, whether the installation is new or retrofit.

From the EPRI Guide it is known that the design of ducts and stacks for wet operation must address several issues that were not present in unscrubbed or reheated gas stack designs. One of the important issues to consider in the design of a wet stack system is the gas velocity in the chimney. A relevant issue is whether the gas velocity will result in droplet reentrainment from the internal lining applied to the inner surface of a chimney. The liquid on the lining surface is produced by deposition and condensation. Its flow in the form of droplets, film or rivulets is governed by gravitational, surface-tension, and gas-shear forces. As the droplets accumulate, they are pulled downward by gravity, whereas the gas drags the liquid in the same direction as the flow of the gas. When the force from the gas reaches or exceeds the forces of gravity and surface tension, the liquid is sheared from the ductwork or liner walls. Liquid then re-enters or is re-entrained back into the gas stream and is carried out of the stack. When this occurs, the gas velocity is referred to as the critical reentrainment velocity. Re-entrainment is the most frequent source of stack liquid discharge (SLD), also known as rainout or acid-mist fallout, of liquid droplets in the vicinity of the stack.

It is known from the EPRI Guide that surface discontinuities and protrusions, such as weld seams, fiberglass-reinforced plastic (FRP) joints, and joints of mortar or mastic in internal

-2linings may disrupt gas and liquid flow locally, causing re-entrainment. As a result, liquid reentrainment will be in the form of large droplets (300-6000 pm), that will be discharged at the top of the stack. Droplets of this size will impact ground-level surfaces in the vicinity of the wet stack installation because they will not be able to evaporate before reaching the ground. This is a significant problem.

The liquid-film flow over the internal lining is a function of the gas-shear and gravitational forces, which are acting in opposite directions to each other. For most internal lining surfaces, in which gas velocities are below 19.8 m/s (65 ft/sec), gravitational forces dominate, and the liquid film will flow downward. At velocities between 21.3 and 27.4 m/s (70 and 90 ft/s), the gravitational and shear forces have approximately the same magnitude, and the forces are balanced. In this range, the liquid film on the internal lining will generally be stagnant on the wall and will not move in either direction. At velocities above 27.4 m/s (90 ft/s), the gas-shear forces dominate, and the liquid film will start to flow vertically toward the stack outlet. This velocity point is called the flow-reversal velocity. It is therefore common to operate at maximum values of the gas velocity below the critical re-entrainment velocity, e.g. 18.5 m/s.

The observations described above apply to the ideal case of a smooth wetting surface. In reality, the surfaces of the internal lining are anything but smooth. The most common construction elements for use as internal lining comprise silicate glass blocks, in particular borosilicate blocks (e.g., Pennguard® blocks made from closed cell foam of borosilicate glass). Horizontal adhesive (mastic) joints are found in typical linings made with (boro)silicate blocks. These disturbances are referred to as lining-wall discontinuities. From the EPRI Guide it is known that when the liquid film flows over a horizontal discontinuity, there is a potential for the upward- flowing flue gas to get under the liquid, resulting in the formation of droplets. As mentioned above, if the gas velocity is high enough, a portion of these droplets will be re-entrained back into the gas flow and will exit the lining and stack as SLD.

The currently recommended lining-gas velocities for several lining materials are presented in Table 2-1 of the EPRI Guide. The recommended values also provide the plant some margin to account for increases in the flue gas flow rate as a result of changes in fuel source, increases in plant efficiency, and/or future increases in plant output. For borosilicate blocks the recommended stack-liner velocity for wet operation is 18.3 m/s (60 ft/s). This recommendation takes into account the significant increase in the effective surface area afforded by the closed-cell surface structure of the material and the resulting increased surface-tension forces holding the liquid to the material.

-3It is an object of the present invention to raise the critical re-entrainment velocity.

Summary of the invention

Accordingly, the invention provides a building element for use in an internal lining system of a stack, wherein the building block comprises a front face and a back face and side faces, wherein the front face has a height (Z) and a width (X);

the front face and back face are parallel and a distance apart, defining a depth (Y) of the building element; and at least one of the side faces has a first recess that is parallel to the width direction and covering at least partially the width (X) of the building element; and parallel to the depth direction covering a depth (y1), wherein (y1) is smaller than the depth (Y) of the building element, such that the recess does not extend to the front face.

A building element according to the invention has a front face, which during use comes into contact with the flue gas that is being discharged. The back face opposite the front face is typically attached to an adhesive membrane that is provided on the inner surface of the chimney. The building element generally has a rectangular shape, such that the element has four side faces perpendicular to the front and back faces.

In this specification the building element, in particular the front face, has a width (X) and a height (Z). The depth (Y) determines the distance in the depth direction between the front and back face.

At least one side face, during use a horizontal side face, typically the bottom side face, is provided with a first recess, that covers at least partially the width of the building element and extends over a part of the depth of the building element. The recess can be filled with a suitable adhesive, such as glue, mastic or mortar, for building the internal lining system without horizontal joints of the adhesive at the front faces of the building elements being present.

The recess does not extend over the full depth (Y) of the lining block. Thus, a certain difference between the length (y1) and the entire depth (Y) is necessary, which defines the unmodified front face of the building element. When building elements, or rather rows of building elements are placed on top of each other, the unmodified front faces may be placed

-4seamlessly at least in horizontal direction on each other. As a result, there is no visible horizontal joint of adhesive at the front faces.

Surprisingly it has been found that the absence of such horizontal joints of adhesive at the surface of the internal lining system allows to increase the gas velocity without the occurrence of liquid re-entrainment in the flue gas in a wet stack operation. Thus the critical re-entrainment velocity in the invention is higher than in a prior art chimney provided with an internal lining system of closed cell borosilicate glass blocks having horizontal joints of adhesive.

The invention offers an increased safety margin towards SLD at the same recommended gas-liner velocity in a prior art chimney. The increased critical re-entrainment velocity allows a higher volume of the flue gas through a chimney without risking SLD. The invention also enables increasing the capacity of existing chimneys with a given diameter, as well as higher capacities at small diameter stacks.

According to a second aspect the invention relates to an internal lining system comprising an arrangement such as a wall, of building elements as defined above, wherein the building elements are attached with their back face to the inner surface of a chimney, the at least one side face having the first recess of each building element is oriented horizontally, wherein an adhesive is provided in the first recess. In other words, an internal lining system according to the invention comprises an arrangement of building elements adhesively adhered together without horizontal discontinuities of adhesive at the surface contacting the flue gas, of the internal lining system.

The internal lining system may be used to protect the inner wall of a chimney. More in particular, the invention also provides an industrial chimney comprising the above internal lining system attached to the inner wall of the chimney, preferably a chimney for wet stack operation.

Finally, the invention concerns a process for refurbishing an existing chimney with a fresh internal lining system for the purpose of increasing the critical re-entrainment velocity.

Brief description of drawings

Fig. 1 is a schematic representation of a conventional lining block;

Fig. 2 is a schematic representation of a conventional internal lining system made from lining blocks of Fig. 1;

-5Fig. 3 is a schematic representation of an embodiment of the building element according to the invention;

Fig. 4 is a schematic representation of an embodiment of the internal lining system prepared from building elements of Fig. 3;

Fig. 5 is a schematic representation of an industrial chimney for wet stack operation; and Fig. 6-8 show various different embodiments of a building element according to the invention.

Description of embodiments

The internal lining system is typically constructed starting with a first row of building elements according to the invention, e.g. resting on a bottom element having a flat horizontal top face, such as a floor or plinth. An internal lining system is constructed by adding subsequent rows of building elements on top of the first and each subsequent row. In the prior art these rows are connected by horizontal glue joints over the full depth of the lining blocks, as mentioned in the EPRI Guide.

An advantage of the building elements according to the invention is that the recesses in the building elements allow the rows of building elements to be placed on top of each other without horizontal joints of adhesive between the front faces of adjacent building elements, as the adhesive needed to form the joint is provided and contained within the recesses in the building elements. As a result, an internal lining system according to the invention can be built without horizontal joints made of adhesives at the front faces, thereby reducing these kind of discontinuities. As a result the critical re-entrainment velocity is increased, allowing to increase the (recommended) gas-liner velocity, e.g. from 18.3 m/s to about 21 m/s (without further modifications to the chimney design).

In a preferred embodiment of the building element according to the invention the first recess extends over the full width of the building element, allowing to apply adhesive over the full width and thus to establish an adhesive bond between over the full width of the stacked building elements. In other words upon use, the recess opens into a horizontal side face and both adjacent upstanding side faces. In a further preferred embodiment the first recess extends up to the back face in view of easiness of production and simple design. In this embodiment the recess opens into the horizontal side face, the back face and two adjacent upstanding side faces.

Typically the building elements will be arranged in the internal lining system such that the recesses are in the bottom side faces.

-6In yet another embodiment the building element comprises a second recess in a side face opposite the side face having the first recess, thus upon use in both the bottom side face and the top side face. Preferably the first and second recess are line symmetrical with respect to a centre line of the building element, which makes the proper arrangement of the building elements less complicated and which also makes it easier to apply the appropriate amount of adhesive in the recesses. If identical recesses are provided in both bottom and top face, then the craftsmen building the internal lining wall from the building elements have less to worry about the orientation of the building element, other than back face to the inner chimney surface. Moreover, the recesses may be smaller, as the combined space from the recesses of the adjacent building elements may provide sufficient volume and surface area for adequate adhesive bonding to and protection of the substrate material of the industrial chimney.

In another preferred embodiment the building element has a protrusion having a shape complementary to and slightly smaller than the first recess, in a side face opposite the side face having the first recess. Typically, the protrusion fitting in the recess leaving free a small gap for the adhesive, is at the top side face of the building element.

In the internal lining system of building elements according to the invention vertical joints of the adhesive may be present. Vertical joints have been found to have only a negligible effect on the critical re-entrainment velocity compared to horizontal discontinuities.

In order to avoid the presence of vertical joints of adhesive at the front faces as well, a building element is provided with a further recess in one or both of the side faces, which, upon use, are the vertical side faces, which further recess is parallel to the height direction and covers at least partially the height of the building element and is parallel to the width direction covering a length x2, which is smaller than the width X. In such a way both horizontal and vertical discontinuities derived from adhesive joints at the front faces can be avoided.

Preferably the building element is made from silicate glass such as foamed silicate glass, more preferably closed cell foamed borosilicate glass.

The building elements according to the invention are shaped with a parallel front and back face. Preferably the front face has rectangular shape. The building elements may have conventional dimensions similar to those of the known borosilicate glass blocks, typically ((X xZxY) in cm) 15.2x22.9x5.1 (6”x9”x2”) or 15.2x22.9x3.8 (6”x9“x 1.5”) in size. In

-7building elements having these dimensions typically the length of the recess in the depth direction is 1 cm or more less than the depth (Y) of the building element.

Dimensions similar (taking into account the absence of horizontal joints) to those of known building elements offer the advantage that the building elements according to the invention can be easily substituted for the lining blocks presently used. Nonetheless, building elements greater or smaller in height or greater or smaller in width may be used. The depth of the building elements is typically determined by the desired protection of the chimney substrate material and the required strength of the internal lining system.

The shape of the recess is not essential. Thus, the recess may be formed by a prismshaped stroke cut away over the entire width of one or both faces of the building element. However, the cut-off may also have a rectangular shape or cylindrical shape. Moreover, the building elements may have conjugal shaped bottom and top faces, in other words cooperating male and female parts, provided a gap with a suitable volume is present for the adhesive.

The building elements of the present invention may be provided by the processes presently used for preparing lining blocks of foamed silicate glass. Either they may be prepared directly, by using suitable moulds, or they may be cut, sawn or sanded to the desired shape, in particular the recess. Excess material may be re-used to form new lining blocks.

The invention is illustrated herein below by the attached drawing, wherein:

Fig. 1 is a schematic representation of a conventional lining block;

Fig. 3 is a schematic representation of an embodiment of the building element according to the invention;

In Fig. 1 a conventional lining block 10 is shown diagrammatically. The lining block 10 has a rectangular shape, with a front face 12, a back face 14 and four side faces 16, 18, 20 and 22 respectively. The front face 12 and back face 14 have the same dimensions, that is to

-8say height (Z) and width (X). The front face 12 and back face 14 are a distance (Y) apart, also called the depth of the building element.

In Fig. 2 a common internal lining system 30 is shown diagrammatically. Shown is a part of the inner surface 32 of the upstanding wall 34 of an industrial chimney. The wall or shell 34 may be steel, concrete, ceramic brick or any other material. In this embodiment the inner surface is first treated with a primer. The use of a primer is preferred, but not always needed. The lining blocks 10 are applied with their back face 14 to the inner surface 32 using an adhesive membrane 36. Adhesive from the adhesive membrane 36 extends between adjacent lining blocks 10, creating vertical adhesive joints and horizontal adhesive joints 38. These horizontal joints 38 are discontinuities in the face of the internal lining system that comes into contact with the flue gas flowing through the chimney.

In Fig. 3 a rectangular embodiment of a building element 40 according to the present invention is shown diagrammatically having a front face 42, a back face 44, a bottom side face 46, an opposite top side face 48 and upright side faces 50 and 52 respectively. In this embodiment the bottom side face 46 is provided with a shallow recess 54. The recess 54 is rectangular shaped and extends over the full width (X) with a depth (y1) and a height (z1), such that it opens in the back face 44, upstanding side faces 50 and 52, as well as bottom side face 46.

In Fig. 4 an internal lining system 60 manufactured from the building elements of Fig. 3 is shown diagrammatically. Similar to Fig. 2, an adhesive membrane 36 is applied to the inner surface 32 of a chimney. As can be seen, the recesses 54 are filled with part of the adhesive membrane 36, while at the front faces 42 the building elements are stacked seamlessly. Since the adhesive is contained within the recesses 54, at the front faces 42 horizontal adhesive joints are absent. Adhesive from the adhesive membrane 36 extends between adjacent building elements, creating vertical adhesive joints(not shown).

In Fig. 5 an embodiment of a wet stack 70 is shown diagrammatically. The upright wet stack 70 comprises a shell 72, provided with an inner lining system 60 according to the invention. The shell 72 delimits an upstanding duct 74 for flue gas. An inlet 76 for introducing flue gas derived from an industrial plant 78, such as a (coal-fired) power plant provided with a wet desulphurisation system 80, is positioned at a lower part of the duct 74. Typically a false floor 86 is positioned in the duct 74. A rear deflection plate 88 may be positioned at the inner wall 90 of the shell 8 opposite the inlet 76. The lower row of building elements of the internal lining system 60 may rest on a horizontal part of deflection plate

-9Fig. 6 shows another embodiment of a building element 40 according to the invention having a first recess 54 in the bottom side face 46, while the top side face 48 is provided with a protrusion 100, The shape of the protrusion 100 matches that of the recess 54. Its dimensions are slightly smaller allowing a small gap for adhesive between protrusion 100 and recess 54 of adjacent building elements.

Fig. 7 shows yet another embodiment of a building element 40 having a first recess 54 in the bottom side face 46, wherein also the left upstanding side face 50 is provided with a further recess 102 allowing to avoid vertical adhesive joints being present at the flue gas contacting surface of the internal lining system constructed from these building elements.

Fig. 8 is still another embodiments of a building element 40 having a first recess 54 in the bottom side face 46, wherein the top side face 48 has a second recess 104 similar to first recess 54.The first and second recess 54, 104 are line symmetrical to the centre line of the building element.

The present invention may be applied in new industrial chimneys for wet stack operation, during repair of internal lining systems in existing chimneys for wet stack operation and when chimneys are retrofit with an internal lining system to be used for wet stack operation. As indicated herein before, the chimney provided with an internal lining system of the present invention may be operated at a gas velocity higher than the currently recommended one.

In order to illustrate the effectiveness of the internal lining system according to the invention, model experiments have been carried out to determine the liquid re-entrainment behaviour.

Example

Test panels, representing an internal lining system, were constructed, using a mastic membrane, from conventional Pennguard® borosilicate blocks of 38 mm thick, 152.4 mm wide and 228.6 mm tall, and from building elements according to the invention made from the same material and having similar dimensions. The test panel made of conventional blocks had a commonly staggered pattern, such that the short edges of the blocks were installed horizontally and the long edges were installed vertically. The vertical seams were staggered. The mastic material in the joints was scraped during installation such that the mastic recessed slightly away from the front faces of the blocks. The radial tolerance of construction was less than 3mm.

- 10The building elements according to the invention were cut from rectangular borosilicate blocks to form a rabbet joint (i.e. recess and protrusion; compare Fig. 6) and installed in the same staggered way as the conventional blocks, except that no horizontal adhesive joints were present at the front faces (flue gas side). This test panel was constructed with a radial tolerance of 0.8 mm (1/32 inch) with maximum vertical gaps of 1.6 mm (1/16 inch).

The test panels as manufactured were observed to have minimal mastic smearing and minimal radial protrusions.

Each panel oriented vertically was then evaluated at several gas flow conditions ranging from 13.7 m/s (45 ft/s) to 25.9 m/s (85 ft/s) in increments of 1.5 m/s (5 ft/s) in a vertical wind tunnel test facility to determine the performance of the panel with respect to liquid flow, drainage and re-entrainment from the surfaces of the panel.

Liquid was sprayed onto the front faces of the blocks and elements using a high flow spray nozzle to simulate wet stack operation, wherein the internal lining surface will always be wet due to condensation of water vapour from the saturated flue gas. Once the front faces were uniformly wetted a second low flow nozzle was used to inject smaller amounts of water onto specific areas of interest.

At each tested gas flow velocity visual observations were made concerning the:

1) Direction of liquid motion on the surface and over the mastic joints,

2) Observations of the liquid surface appearance as a function of velocity, and

3) Entrainment of liquid from the borosilicate block surfaces or from joints between blocks.

The below Tables 1 and 2 summarize the test results.

- 11 Table 1. Test observations conventional internal lining system

Gas flow velocity (m/s)	Observations
13.7	Liquid flows freely down across both the Pennguard® block faces and the mastic membrane joints.
15.2	Liquid flows freely down across both the block faces and the mastic membrane joints. Small liquid bars are beginning to form at some of the horizontal mastic joints.
16.8	Small liquid bars are forming at most of the horizontal mastic joints. Liquid is easily draining across the horizontal mastic joints and down the face of the borosilicate blocks.
18.3	Larger liquid bars are forming at the horizontal mastic joints. The liquid drainage is very good on the surfaces of the borosilicate blocks. No reentrainment was observed.
19.8	There is still good liquid drainage on the surfaces of the borosilicate blocks but the liquid bars are getting larger at the horizontal mastic joints. Minimal re-entrainment was observed.
21.3	Liquid bars are getting noticeably larger and waves can be seen on their surfaces with some re-entrainment. Liquid is still able to drain across the horizontal mastic joints. More re-entrainment was observed.
22.9	Liquid bars are getting noticeably larger and waves can be seen on their surfaces. Significant liquid re-entrainment at the liquid bars. The liquid film on the block surface is still flowing down.
24.4	The panel is entering the flooding regime where the liquid film thickness increases such that the liquid on the gas-liquid interface flows up and the liquid on the block surface flows downward. Significant reentrainment was observed at numerous locations.

- 12 Table 2. Test observations internal lining panel according to the invention

Gas flow velocity (m/s)	Observations
13.7	Liquid flows freely down across the block faces. The liquid flows uninterruptedly across the thin horizontal joints (i.e. no distinction in drainage pattern versus the block face).
15.2	Liquid flows freely down across the block faces. The liquid flows uninterruptedly across the thin horizontal joints. One small liquid bar was observed at a small horizontal discontinuity but the liquid still flowed freely down across this location.
16.8	Liquid flows freely down across the block faces. The liquid flows uninterruptedly across the thin horizontal joints. One small liquid bar was observed at a small horizontal discontinuity but the liquid still flowed freely down across this location.
18.3	Liquid flows freely down across the block faces. Small liquid bars formed at some of the small horizontal discontinuities but the liquid still flowed freely down across these locations.
19.8	Liquid flows freely down across the block faces. Larger liquid bars formed at some of the small horizontal discontinuities but the liquid still flowed freely down across these locations.
21.3	Liquid flows freely down across the block faces. Larger liquid bars formed at some of the small horizontal discontinuities but the liquid still flowed freely down across these locations. Waves are starting to form in the liquid bars.
22.9	Liquid flows freely down across the block faces although at a slower velocity. Larger liquid bars formed at more of the small horizontal discontinuities but the liquid still flowed freely down across these locations. Waves are prevalent in the liquid bars. Sporadic reentrainment was observed.
24.4	The panel is entering the flooding regime where the liquid film thickness increases such that the liquid on the gas-liquid interface flows up and the liquid on the block surface flows downward. Re-entrainment was observed at several locations.

- 13As is apparent from these observations the gas flow velocity in the internal lining system according to the invention can be higher than in the conventional lining system. Instead of

18.3 m/s the recommended gas flow velocity could be raised to at least 21.3 m/s.

Claims

CONCLUSIONS

An industrial chimney interior lining system (60) comprising an arrangement of adhesively attached structural members (40), the structural element (40) having a front face (42) and a back face (44) and side faces (46, 48, 50, 52), wherein the front face (42) has a height (Z) and a width (X);

the front face (42) and back face (44) are parallel and spaced, defining a depth (Y); and at least one of the side surfaces (46) has a first recess (54) running parallel to the width direction and at least partially covering the width (X) of the construction element (40); and parallel to the depth direction covers a depth (y1), wherein (y1) is less than the depth (Y) of the construction element (40), so that the recess does not extend to the front face (42), at least one side face (46) with the first floor (54) of each structural element (40) is oriented horizontally, an adhesive being provided in the first floor (54), without horizontal joints of the adhesive to the front surfaces (42) of the construction elements (40) are present.

The internal cladding system of claim 1, wherein the first floor (54) extends the full width (X).

An interior cladding system according to claim 1 or 2, wherein the first floor (54) extends to the rear surface (44).

Internal cladding system according to any one of claims 1 to 3, wherein the construction element (40) has a second recess (104) in a side face (48) opposite the side face (46) with the first recess (54).

Internal cladding system according to any one of claims 1-3, wherein the construction element (40) has a projection (100) of a shape complementary to and smaller than the first floor (54), in a side face (48). opposite the side face (46) with the first floor (54).

Internal cladding system according to any one of the preceding claims, wherein a further recess (102) is provided in one or both of the side surfaces (50, 52), which in use are the vertical side surfaces, which further recess (102) is parallel to the

- height direction and at least partially covers the height (Z) of the construction element (40); and parallel to the width direction covers a width (x2), wherein (x2) is smaller than the width (X) of the construction element (40).

Internal coating system according to any of the preceding claims, wherein the construction elements (40) are made of borosilicate glass, preferably closed cell foam borosilicate glass.

Use of the internal lining system according to any of the preceding claims to protect the wall (32) of a chimney (70).

An industrial chimney (70) comprising the internal cladding system (60) according to any of the preceding claims 1-7 attached to the inner surface (32) of the chimney (70), preferably a chimney for wet operation of the chimney .

A method for renovating an existing chimney with a new internal cladding system according to any of the preceding claims 1-7 for the purpose of increasing the critical readmission rate.

A construction element (40) for use in an internal chimney lining system (60), the construction block (40) comprising a front face (42) and a back face (44) and side faces (46, 48, 50, 52) wherein the front face (42) has a height (Z) and a width (X);

the front face (42) and back face (44) are parallel and spaced, defining a depth (Y); and at least one of the side surfaces (46) has a first recess (54) running parallel to the width direction and at least partially covering the width (X) of the construction element (40); and parallel to the depth direction covers a depth (y1), where (y1) is less than the depth (Y) of the construction element (40), so that the depression (54) extends to the rear face (44) and not to the front face (42) extends.