GB2077902A - Inserts for ovens or boilers - Google Patents

Abstract

An insert 6 is provided for improving the construction of and reducing soot produced by a liquid fuel atomizing burner of an oven or boiler. The insert comprises a tube 7 having an internal diameter dR which is larger than the external diameter DM of the burner mixing tube 14. Annular reversing discs 10, 11 are mounted at the tube ends and have central apertures 12 of diameter dU smaller than the internal diameter of the tube. The external diameters DU are larger than the external diameter DR of the tube. Openings 20 are provided between the disc surfaces 18 and the faces 19 of the tube ends 8, 9. The tube and the discs are made of ceramic material with a high thermal shock stability. <IMAGE>

Description

SPECIFICATION Improvements in or relating to inserts for ovens or boilers The present invention relates to inserts for ovens or boilers which are equipped with atomizing burners for liquid fuel, such as heating oil.

Inserts of the kind described above are provided to improve the combustion and therefore to improve the efficiency of heating systems. A known insert comprises a spiral or basket of iron which is placed onto the flame jet of an oil burner in the combustion chamber of the boiler. An insert of this kind can in some circumstances result in a slight increase of the C0z content and therefore effect improved combustion. However, since the insert is made to glow and must be operated with excess oxygen in the interests of good combustion of the burners, it follows that such an insert is destroyed by corrosion after a short operating time and is thus rendered useless.

The German Gebrauchsmuster 7428 571 or the German Offenlefungsschrift 24 40 451 discloses means for introducing inserts in the form of thermal storage blocks of cast iron into the combustion chamber by means of which the shutdown phases of the burners are prolonged and the thermal balance can be improved.

According to one aspect of the invention, there is provided an insert for an oven or boiler provided with an atomizing burner for liquid fuel, comprising a tube having an internal diameter which is greater than the external diameter of a mixing tube of the burner, an annular reversing disc mounted at each tube end with a central aperture whose diameter is smaller than the internal diameter of the tube and whose external diameter is greater than the external diameter of the tube, open apertures being provided between the internal surface of each reversing disc nearest to the tube and end face of each tube end, the tube and the reversing discs comprising ceramic material with a high thermal shock stability.

It is thus possible to achieve a distinct improvement of combustion with a substantial reduction of the soot coefficient to zero or in very poor firing systems at least to acceptable values involving an increase of the CO2 proportion to at least 1 4 to 1 5% while avoiding the presence of hydrocarbons in the exhaust gas.

Without adopting any additional measures, these steps result in an immediate improvement of the exhaust gas values to permissible and up to maximum values of the insert is introduced into the combustion chamber so that the flame enters the tube immediately after leaving the burner. This can achieve an improvement of energy yield by between 10 and 20% so that a larger amount of heat is generated or, for example, a smaller nozzle can be inserted into the burner for the same heat requirement.

It is possible to arrange the insert so that destruction of the individual components due to numerous high thermal shock stresses and high thermal loading and therefore destruction of the entire insert can be avoided. This may be achieved by constructing the tube so as to expand a barrel configuration from the tube end to the middle.

This construction of the tube surprisingly leads to an approximately indentical glow temperature in operation over the entire tube length so that thermal stresses are substantially avoided.

Moreover, a further improvement of the combustion operation by a higher CO2 yield is also obtained.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a part side view of the boiler in section with an insert constituting a preferred embodiment of the invention; Figure 2 is a side view of an insert with a conically converging tube; Figure 3 is a view of the insert according to Figure 2 from the flame inlet side with a half-cut reversing disc; Figure 4 shows an insert from the side along the section A-B of Figure 5; Figure 5 shows the insert according to Figure 4 along the section D-E; Figure 5a shows a part view X and Figure 5b shows the view C-D of Figure 5; Figure 6 shows another embodiment of an insert along the section F-G of Figure 7;; Figure 6a shows the section K-K of Figure 6; Figure 7 shows the view according to the section H-J of Figure 6; Figure 7a shows the section L-L of Figure 7; Figure 8 shows a sectional side view of an insert constituting a preferred embodiment of the invention; Figure 9 shows a sectional side view of an insert comprising two tube members; Figure 10 is a view of one end face of the insert according to the view Y; Figure 11 shows a sectional side view of the insert along the sectional lines M-N of Figure 12; Figure 12 shows a cross-section along the sectional lines O--P of Figure 13; Figure 1 3 is a sectional side view;; Figure 14 shows a side view of an insert constituting a preferred embodiment of the invention along the section R-S of Figure 15; Figure 1 4a shows a part view of the crosssection through the top tube portion with a longitudinal notch; Figure 1 4b is a part view of the cross-section of the top tube portion with a continuous longitudinal slit; Figure 1 5 is an internal view of a reversing disc; Figure 16 is a plan view of a reversing disc with a notch; Figure 1 6a shows the enlarged circular part of Figure 16; and Figure 1 6b shows part of a plan view of a reversing disc with a continuous slit.

The numeral 1 in Figure 1 refers to part of a boiler as seen in lateral section. The combustion chamber 2 is at the bottom and is provided at the rear wall with a heat-resistant ceramic lining 3. A hot water duct 4 with smoke gas flues 5 forms the upper closure.

The combustion chamber 2 is provided with an insert 6 according to the invention comprising a tube 7 of ceramic material with a high thermal shock stability, closed at each tube end 8 and 9 with an annular reversing disc 10 or 11 respectively. The reversing discs 10, 11 also consist of ceramic material with a high thermal shock stability. They have a central aperture 12 whose diameter du is smaller than the internal diameter ds of the reversing discs 10, 11 is greater than the external diameter DR of the adjoining tube ends 8 or 9 of the tube 7.

The insert 6 is placed in the combustion chamber 2 at a short distance from an atomizing burner 13 or from the mixing tube 14 thereof and is disposed at a height, for example by means of supporting bars 1 5, so that the central axis 1 6 of the insert 6 coincides with the axis 1 6' of the burner nozzle 17 and of the mixing tube 14, at least approximately.

The internal diameter du of the aperture 12 of the reversing discs 10, 11 is at least as large as the internal diameter dM of the mixing tube 14.

However conveniently it is at least 10 to 100%, more particularly 25 to 80% greater than that of the mixing tube 14.

Open apertures 20 are disposed between the internal surfaces 1 8 of the reversing discs 10, 11 and the end faces 19 of the tube ends 8, 9. The said apertures are formed by the tube ends 8, 9 being constructed in the manner of a dog clutch, i.e. they have recesses 21 and lugs 22 in an alternating configuration.

The method of operation of the insert according to the invention can be explained as follows: After the burner flame is ignited, it passes through the aperture 12 of the adjacent reversing disc 10 into the tube 7. Within the said tube it covers the internal wall thereof and heats the latter to a temperature of approximately 8000C.

This encourages combustion and increases the flame temperature. Ungasified fuel particles contained in the flame are thus partially gasified and burnt or the fuel particles strike the internal tube wall where they are instantly gasified and the gas then burnt.

The combustion process is further assisted, more particularly by the reversing discs 10 and 1 The flame expands in the field 7. On exiting from the aperture 12 of the reversing disc 1 1, part of the exhaust gases in the flame are built up through the region of the reversing disc 11 which projects inwardly beyond the internal wall of the tube and such gases and flame are passed outwardly through the open apertures 20. A negative pressure is generated on the other side of the reversing disc 10 due to the higher flow velocity of the flame in the region of the aperture 12 of the said disc. The negative pressure ensures that gases are drawn from the combustion chamber 2 from the outside, more particularly through the open aperture of the reversing disc 10.Gases, which are ejected from the reversing disc 11, move along the external tube wall to the reversing disc 10 so that a large proportion of the gases exiting through the open apertures 20 of the reversing disc 11 are once again drawn into the tube 7 where they are subjected to after-burning.

This results in a higher flame temperature which in turn improves combustion.

Tests have shown that when an insert 6 according to the invention is inserted into a boiler which is heated by an atomizing oil burner, having a soot coefficient of 6 and a CO2 exhaust gas content of less than 10%, the soot coefficient can be improved to 2 and the CO2 content can be improved to 14.5% without adopting any additional steps. Previously unacceptable values have thereby risen to almost optimum values.

Furthermore, the exhaust gas temperature was increased from 240"C to 2600C. It was possible to reduce the burner nozzle from 0.75 to 0.65 to give the same heat output. Furthermore, the shutdown phase has lengthened by 10 minutes. This is due on the one hand to the higher temperatures and possibly to the thermal storage effect of the insert 6 since the insert continues to boil for approximately 1 minute. This does not however prolong the switched on period but rather shortens it.

An inwardly extending edge speed 25, which results in improved reversing of the circulating gases, is provided on each of the internal edges 23 and the internal edges 24 of the reversing discs 10, 11 in order to improve circulation.

In addition to the recesses 21, the open apertures 20 can also be formed by ports 26 which are provided in the tube 7 as indicated in Fig. 2.

The tube 7 in Fig. 2 is constructed in the shape of the surface of a truncated cone. When the flame or exhaust gases exit, this results in back pressure through which the gases are thrust outwardly through the open apertures 20 and/or the ports 26.

The reversing discs 10, 11 can be attached to the tube 7 by mechanical means and/or by ceramic means, for example, by cementing or by means of a slurry. Mechanical fastening in Fig. 2 is performed by three tie rods 27 which pull the reversing discs 10, 11 against the tube 7. Another mechanical fastening can be obtained by a kind of bayonet closure, the corresponding extensions and grooves being integrally formed on the tube 7 and on the reversing discs 10, 11.

According to one advantageous embodiment of the invention, extensions 28, which function as base 29 for the insert 6, are integrally formed on the reversing discs.

To ensure perfect coordination of the reversing discs 10, 11 with the tube 7 the latter and the reversing discs 1 0, 11 are provided with centring aids in the form of bores or indentations 30 and centring pins 31 as shown in Figs.4,5 and Sb.

Support strips 32 are formed on the tube 7 to ensure good stability of the insert 6.

Sloping ribs 33 which generate improved vorticity of the combustion gases and in some circumstances result in better combustion, can also be provided on the internal edges 24 of the reversing discs 10,11.

Figs. 6 and 7 show an embodiment of an insert 6 for which steps were taken or means were provided to maintain a different distance between the reversing discs 10 or 11 respectively, depending on their arrangement of the particular side of the tube 7, which is conically constructed in this case. This enables one and the same reversing discs 10, 11 to be employed for both functions, so that only one forming tool is required. This is achieved in that the indentations 30 or 30' for receiving centring pins 31 or 31' are provided at different planes. This enables the apertures 1 2 on the flame entry side to be made larger than on the exit side. Each of Figs. 6a and 7a show a portion of the centring means along the sections K-K or L-M respectively.

Ceramic material for the inserts can comprise blends based on fire clay containing approximately 3050% by weight of clay; silicon carbide containing approximately 3050% by weight of clay; alumina containing approximately 30 to 60% by weight of clay and where appropriate slight quantities of, for example, 5 to 20% by weight of zirconium silicate and/or zirconium dioxide; or pure zirconium dioxide with approximately 30 to 60% by weight of clay or mixtures of the above mentioned materials. It can be advantageous to add slight proportions of, for example, 0.5 to 7% by weight of lithium as this substantially improves the thermal shock stability of ceramic material.

Fig. 8 shows the insert 6 with a slightly conically constructed tube 7 which, in relation to the conical part 34 has a relatively short cylindrical part 35 on the side of the incoming flame. The reversing discs 10 and 11 are stressed to the tube 7 by means of three tie rods 27 which are symmetrically distributed over the circumference. To this end indentations 36 are disposed in corresponding offset configuration on the external surfaces of the reversing discs 10, 11.

The tie rod 27 engages by means of a hook 38 in the indentations 36 of the reverse disc 11. A stressing screw 40, more particularly a wing screw, can be screwed into a bent lug 39, which is integrally formed on the opposite side of the tie rod 7. The said wing screw can co-operate with the indentation 36 of the reversing disc 10 so that the three components, namely reversing discs 10 and 11 and the tube 7 can be stressed to form one structural unit.

Conveniently, the indentations 36 are provided with sloping support surfaces 41 the slope of which corresponds at least approximately to the angle of the hook 38. The hooks 38 as well as the stressing screws 40 advantageously co-operate therewith so that the tie rod 27 is pulled to the middle of the tube 7 on being stressed. One indentation 36 is shown in enlarged form in the view "X".

A groove 42, in which the appropriate tie rod 27 is located and advantageously does not project beyond, is conveniently provided at the edge 23 of each of the reversing discs 10, 11 in the region of the indentations 36. This ensures that the correct angular position of the reversing discs 10, 11 relative to each other is maintained during assembly.

If the tube 7 is to have an accurate angular position with respect to the indentations 36 it is possible for support ribs 43 to be formed on the tube 7 in accordance with the distribution of the indentations 36 and the stretched part 45 of the tie rod 27 can bear on the plane support surface 44.

Advantageously, the tie rod 27 consists of flat strip material and the grooves 42 as well as the support surface 44 is adapted thereto.

As illustrated in Fig. 9, the insert 6 can conveniently comprise two advantageously identical and slightly conical tube parts 7' and 7" in order to achieve optimum combustion properties and the smaller apertures of such parts abut against each other. This results in a nozzle effect which is presumably responsible for the improvements, more particularly in preventing the formation of soot.

Advantageously, two identical reversing discs 10, 11 are used so that only two tools are required to produce the insert 6 comprising the four parts.

In this case, the open apertures 20 are of equal size on both sides.

In this case, the tube parts 7', 7" can also be provided with support ribs 43 whcih are provided with support surfaces 44 for the tie rods 27 or the stretch part 45 thereof. All four components, namely the tube parts 7' and 7" and the reversing discs 10, 11 are thereby centrally stressed relatively to each other in the correct angular position during assembly.

Fig. 10 shows the reversing discs in the view "Y" of Fig. 2. The distribution of the grooves 42 can be clearly seen as well as the inwardly projecting centring pins 31 and the projections 28, which may be required and function as support feet 29.

Fig. 11 shows a U-shaped support frame, designated with the numeral 46 and functioning to support and fix the insert 6. The insert 6 can comprise refractory ceramic material which can conveniently also have a good thermal conductivity, more particularly silicon carbide or a mixture of silicon carbide with up to 70% by weight of a ceramic blend comprising 35 to 70% by weight of Six,, 20 to 40% by weight of At203; 7 to 14% by weight of MgO, 0.5 to 2% by weight of Fe203; and where appropriate up to 1% by weight of each of CaO, Na20 and TiO2.

According to one advantageous embodiment of the invention, the tube 7 comprises two tube sections 7' and 7", for example two truncated cones. The latter can be produced by a simple press forming operation.

The left-hand tube section 7' forms an expansion zone 48 for the flame in that the inlet region 49 of the said zone expands slightly towards the interior and the adjoining reversing zone 50 is almost or entirely cylindrical and its diameter is substantially greater than that of the mixing tube 14, for example by being nearly twice as large. The adjoining tube section 7" which converges conically or in barrel configuration towards the exit, forms a compression zone 51 for the flame The diameter at the end 9 of the tube section 7" is conveniently smaller than the diameter at the inlet of the tube section 7'. This results in a high flame concentration at that place and therefore results in intensive mixing of the gas components and a very good combustion which is free of soot.

The high efficiency of the insert 6 can be explained by the fact that the flame velocity is reduced in the expansion zone so that fuel droplets which have not yet been gasified are exposed for a longer time to the flame temperature and to the temperature radiated by the tube 7 and are completely gasified when they reach the exit. This already increases the flame temperature because improved combustion is taking place. The process is assisted by recirculation of part of the flame gases from the exit through the open apertures 20 between the tube end 9 and the inside of the reversing disc 11 along the external wall of the tube 7 or of the tube portions 7' and 7" accompanied by the suction action through the open apertures 20 between the inlet of the tube portion 7' and the inside of the reversing disc 10.This causes hot gas to be supplied to the combustion and thus further assists the process of gasifying the fine fuel droplets.

In experiments carried out on inserts for oil burners of heating systems in detached and semidetached houses it was found that a range of 8 to 120, more particularly 100 was an optimum angle for the tube section 7' in the inlet region 49 and the angle of the tube portion 7" was found to be in a range of approximately 10 to 1 50, more particularly 120. It has been found advantageous for the compression zone 51 to be as long as possible, i.e. for compression to take place without suddenness as this would result in a large back pressure effect which has less favourable results.

On the other hand, the structural length of the insert is defined by the length of the boiler spaces.

A sufficient amount of free space must be maintained between the exit of the insert 6 and the boiler rear wall in the interests of firing technology and for aerodynamic reasons. It is therefore advisable for the first tube section 7' to be made shorter than the second tube section 7".

It is particularly advantageous if the diameter of the edge 8 on the inlet side of the tube 7 orthe first tube portion 7' corresponds approximately to the diameter of the middle third 52 of the ring of the reversing disc 10. It has been found that this results in a high negative pressure and therefore in a very good suction effect which ensures good recirculation.

The insert 6 bears on two members 53 of a Ushaped support frame 46 or piate or tool individual members which are fixedly connected to each other by means of at least one U bar 54. Upwardly projecting lugs 55 are provided at the ends of the members 53 and an inwardly oriented bendable tongue 56 is provided on a lug 55 at each member 53. The tongue is provided for the longitudinal fixing of the insert 6 even if the inserts 6 are of different length. Longitudinal grooves 57 into which part of the members 57 engage, are provided in the reversing discs 10, 11 to prevent rotation of the inserts 6.

According to one further embodiment of the invention the tube 7, in this case comprising two approximately conical tube portions 7' and 7", is provided with a longitudinal notch 58 which advantageously extends over the entire external surface as can be seen by reference to the embodiment of the tube portion 7' illustrated in Fig. 1 4a. This step produces a place of preferred frangibility and forms a crack under the alternating thermal stress which occurs in this case. The tube or the tube portions 7', 7" can therefore expand and contract without producing any further stress cracks. A continuous longitudinal cut 59 can also be provided in place of the longitudinal notch 58 and this also avoids stress cracks.This possibility of an embodiment is illustrated for the tube portion 7" in Fig. 146.

To prevent stress cracks in the reversing discs 10, 11 these can also be provided with a notch 60 and where appropriate with a second notch 61, disposed on the opposite side. This notch or the notches form a place of preferred frangibility which comes into action under alternating temperature stresses. The notch or notches 60, 61 are conveniently arranged radially for reasons of manufacture. An embodiment of this kind is provided in the left-hand reversing disc 10 and is shown in Figs. 15, 1 6 and 16a. In some circumstances a crack 62 is produced in operation as shown in Fig. 16a. A continuous parting slit 63, which divides the discs 10, 11 can be provided in place of the notch or notches 60 and 61 as shown in the right-hand reversing disc 11 of Fig. 14 and Fig. 166.

As illustrated in Figs. 11 to 13, a fixing part 65, consisting of a metal strip or metal bar and constructed as retaining bracket is placed or inverted at the top as retaining device for retaining and fixing the insert 6 in the oven or boiler and the downwardly and inwardly bend ends 66 of the fixing part thrust against the reversing disc outside 67, where appropriate with slight prestress or bear thereon without pressure or grip around the said outside with slight clearance. The retaining bar 65 is situated in each of a groove 68 of the reversing discs 10,11.

The entire insert 6 bears loosely on a support frame 46 comprising two members 53 which are disposed at a distance from each other which is less than the diameter of the insert 6. The members 53 can be part of a plate which is bent in U-configuration. Conveniently, however, they are produced as individual components and are connected to each other by means of at least one U-bar 54 which is narrow in relation to the length of the insert 6. Advantageously, the connection is obtained by spot welding.

The top edges 69 of the members 53 are adapted to the contour of the insert 6 and conveniently they are cut free at the edges of the insert parts 7', 7", 10, 11 or at the parting places of adjoining insert parts 7', 7", 10, 11 as indicated by the positions 70 (Fig. 11). The remaining top edge 69, which serves for support purposes, is conveniently constructed so that a multipoint support or linear support is made possible for each individual component 7', 7", 10, 11.

The reversing discs 1 0, 11 are provided with two longitudinal grooves 57 into each of which a member 53 is able to engage. This provides the insert 6 with an accurate position which is immovable in the circumferential direction. To ensure that the insert 6 is also fixed longitudinally on the support frame 46, each of the ends of the members 53 has an upwardly extending lug 55 formed thereon and the internal distance between said lugs corresponds approximately to the length of the insert 6. In the inserted state, the lugs 55 therefore grip around the insert 6 on the outsides of the reversing discs 1 0, 11.

To facilitate compensating for longitudinal differences between the insert 6, one of the lugs 55 of each member 53 is moved outwardly and is provided with an inwardly projecting tunnel 56.

The clear distance between the lugs 55 of each of the members 53 is then greater than the distance between the ends of the tongue 56 and the oppositely disposed lug 55 which is smaller than the length of the insert 6. When the insert 6 is placed in position, the tongues 56 are laterally bent to the extent that the insert 6 can be mounted on the support frame 46 with slight pressure or without clearance or with slight clearance.

Downwardly extending feet 71 can be provided on the members 53 to improve stability. These permit recirculation in the radial direction if such recirculation has a radial component due to the twist of the flame.

Advantageously, the support frame 46 is constructed of high temperature resistant steel.

Stress cracks of the kind which occur as a result of a rigid system of connection and more particularly as a result of the tolerances due to dimensional differences which always occur in the manufacture of ceramic magerials, are compensated in simple manner by this retaining system.

Claims (54)

1. An insert for an oven or boiler provided with an atomizing burner for liquid fuel, comprising a tube having an internal diameter which is greater than the external diameter of a mixing tube of the burner, an annular reversing disc mounted at each tube end with a central aperture whose diameter is smaller than the internal diameter of the tube and whose external diameter is greater than the external diameter of the tube, open apertures being provided between the internal surface of each reversing disc nearest to the tube and end face of each tube end, the tube and the reversing discs comprising ceramic material with a high thermal shock stability.

2. An insert as claimed in claim 1, in which the reversing discs are provided with an extension which is constructed as a base.

3. An insert as claimed in claim 1 or 2, in which support strips are formed on the tube.

4. An insert as claimed in claim 1,2 or 3, in which the reversing discs are mounted on the tube by mechanical means.

5. An insert as claimed in claim 4, in which the mechanical means are constructed in the manner of a bayonet closure which is mounted on the tube or on the reversing discs or is integrally formed thereon during the production thereof.

6. An insert as claimed in claim 4, in which the mechanical means comprise a plurality of tie rods which extend outside the tube and act on the reversing discs.

7. An insert as claimed in any one of claims 1 to 6, in which the reversing discs are connected to the tube by ceramic bonding means.

8. An insert as claimed in any one of claims 1 to 7, in which the tube has the shape of the external surface of a truncated cone.

9. An insert as claimed in any one of claims 1 to 8, in which each of the external edge and internal edge and internal edge of the reversing discs is provided with an edge bead which is oriented in the direction of the tube.

10. An insert as claimed in any one of claims 1 to 9, in which the end faces of the tube ends are constructed in the manner of a dog clutch and the recesses thereof in conjunction with the internal surfaces of the reversing discs form the open apertures.

11. An insert as claimed in claim 10, in which the reversing discs are mounted on lugs of the end faces by ceramic bonding means.

12. An insert as claimed in any one of claims 1 to 11, in which the reversing discs and the tube have matching centring means in the form of indentations and centring pins.

13. An insert as claimed in claim 12, in which each of the reversing discs or the tube is provided with a plurality of indentations at different planes.

14. An insert as claimed in any one of claims 1 to 13 in which the open apertures at the flame entry side are of larger cross-section than those at the exit side.

1 5. An insert as claimed in any one of claims 1 to 14, in which the ceramic material of the insert wholly or substantially comprises at least one of the following material; fire clay containing 30 to 50% by weight of clay; silicon carbide containing 3050% by weight of clay; alumina containing 30--60% by weight of clay and, where appropriate, 20-20% by weight of zirconium silicate and/or zirconium dioxide; zirconium dioxide containing 3060% by weight of clay.

1 6. An insert as claimed in any one of claims 1 to 15, in which the ceramic material comprises silicon carbide or a mixture of silicon carbide with up to 70% by weight of a ceramic blend comprising 3570% by weight of Six2,20 to 40% of alumina, 7 to 14% by weight of magnesia, 0.5 to 2% by weight of Foe203 and where appropriate up to 1% by weight of CaO and/or TiO2 and/or Na20.

1 7. An insert as claimed in any one of claims 1 to 1 6, in which the ceramic material for the insert contains 0.5 to 7% by weight of lithium oxide.

18. An insert as claimed in any one of claims 1 to 17, in which the external surface of the reversing discs is provided with a plurality of indentations which are arranged in mutually offset configuration, and a hook of a tie rod engages in one indentation of one reversing disc and a stressing screw, screwmounted into a bent lug of the tie rod, engages into the oppositely disposed indentation of the second reversing disc.

19. An insert as claimed in claim 18, in which the indentations have a support surface, which slopes in relation to the external edge of the reversing discs so that when the stressing screw is tightened, the discs are abie to pull the tie rod in the direction of the tube axis.

20. An insert as claimed in claim 19, in which, in the region of the indentations, the edge of each of the reversing discs is provided with agroove for the extended part of the tie rod.

21. An insert as claimed in claim 20, in which the depth of the groove corresponds at least to the thickness of the tie rod.

22. An insert as claimed in any one of claims 19 to 21, in which the hook is bent at an angle which is such that it corresponds at least approximately to the angle of the sloping support surface.

23. An insert as claimed in any one of claims 1 8 to 22, having at least three indentations arrange in symmetrical distribution.

24. An insert as claimed in any one of claims 18 to 23, in which, in accordance with the distribution of the indentation, the tube is provided with integrally formed support ribs for the extended part of the tie rod.

25. An insert as claimed in any one of claims 1 8, to 24, in which the tie rod is constructed of flat strip material and the grooves of the reversing discs have an elongate, matched rectangular shape.

26. An insert as claimed in claim 24 or claim 25 when dependent on claim 24, in which the support ribs have a piane external surface.

27. An insert as claimed in any one of claims 1 to 26, in which the tube comprises two identical and lightly conical tube members whose smaller cross-sectional apertures abut against each other so that the tube cross-section diminishes from both ends towards the middle.

28. An insert as claimed in any one of claims 1 to 27, in which two identical reversing discs are provided and the open apertures on both sides of the tube have the same cross-section.

29. An insert as claimed in claim 27 or claim 28 when dependent on claim 27, in which the two tube members are stressed to each other by means of tie rods.

30. An insert as claimed in any one of claims 1 to 29, in which grooves of the reversing discs and external surfaces of support ribs are provided to fix and centre the individual components during and after assembly.

31. An insert as claimed in any one of claims 1 to 26, in which the tube and the reversing discs consist of refractory ceramic material and the tube tapers outwardly in barrel configuration from the tube ends towards the middle thereof.

32. An insert as claimed in any one of claims 1 to 26, in which the tube comprises two truncated cones.

33. An insert as claimed in any one of claims 1 to 32, in which a middle section of the tube is of cylindrical construction.

34. An insert as claimed in any one of claims to 33, in which the reversing discs and the tube are provided with a continuous longitudinal slit or with a groove which forms a place of preferred frangibility when stressed in thermal shock.

35. An insert as claimed in any one of claims 1 to 33, in which each of the reversing discs is provided with a place of preferred frangibility or a separating slit arranged to respond to temperature changes to form a crack.

36. An insert as claimed in any one of claims 1 to 33, in which a place or places of preferred frangibility are defined by a notch.

37. An insert as claimed in claim 36, in which a longitudinal notch situated on the tube extends over the entire external length thereof.

38. An insert as claimed in claim 36 or 37, in which notch situated on each reversing disc, extends in the radial direction thereof.

39. An insert as claimed in any one of claims 1 to 38, in which the reversing discs and the tube bear on the ends of members or top edges thereof associated with a U-shaped support frame.

40. An insert as claimed in claim 39, in which the top edges are adapted to the contour of the insert.

41. An insert for an oven or boiler, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

42. A retaining device for an insert as claimed in any one of claims 1 to 41, comprising a support frame and a fixing component, the support frame being provided with two upwardly extending members whose distance from each other is less than the diameter of the insert, the top edge of the members being adapted to the contour of the' external surface of the insert so that a multipoint support is provided for the or each contacting insert member, the fixing component comprising-a metal strip with ends which are slightly bent back and being arranged to be placed upon or inverted over the insert loosely or with a slight prestress to retain the individual components of the insert in the longitudinal direction.

43. A retaining device as claimed in claim 42, in which the members are cut free in the region in which two individual insert members are in contact with each other.

44. A retaining device as claimed in claim 42 or 43, in which the ends of the members project beyond the insert and are provided on the projecting part with upwardly extending lugs the internal distance between which corresponds to the length of the insert.

45. A retaining device as claimed in claim 44, in which at least one of the lugs is resiliently and/or plastically bendable in the longitudinal direction.

46. A retaining device as claimed in claim 45, in which one lug is provided with a laterally bendable tongue which projects inwardly along the longitudinal extent of the insert and can be brought into active connection with the outside of the reversing disc.

47. A retaining device as claimed in any one of claims 42 to 46, in which the members comprise a sheet metal component and are rigidly joined to each other by means of at least one U-bar.

48. A retaining device as claimed in claim 47, in which the members and the U-bar are joined to each other by spot welding.

49. A retaining device as claimed in any one of claims 42 to 48, in which the insert bears upon the support frame so as to be prevented from rotation.

50. A retaining device as claimed in claim 49, in which part of the member engages with a longitudinal groove on the external surface of the reversing discs.

51. A retaining device as claimed in any one of claims 42 to 50, in which the support frame and a retaining bar consist of high temperature resistant steel.

52. A retaining device as claimed in any one of claims 42 to 51, in which two support feet are formed on each member.

53. A boiler or oven including an insert as claimed in any one of claims 1 to 41.

54. A boiler or oven as claimed in claim 53, further including a retaining device as claimed in any one of claims 42 to 52.