EP3794295B1 - Rotary tube apparatus - Google Patents

Description

The invention relates to a rotary tube apparatus, in particular a sectional cooler for cooling a free-flowing solid, with structures attached to its walls to increase heat conduction. A rotary tube apparatus is used to cool or heat a free-flowing material, in particular bulk material. A rotary tube apparatus, particularly in its configuration as a sectional cooler, is used for continuous processes in chemical engineering.

Various devices and methods for cooling very hot products are known from the prior art. In various industrial sectors, such as metallurgy, the chemical industry, the building materials and cement industry as well as the recycling industry, coolers are required to cool very hot products such as burnt pigments, slag, metal oxides and hydroxides, cement clinker, sponge iron, scale, activated carbon, catalysts, coke, smelting works residues, etc. Further processing is often not possible without cooling the very hot products. In many cases, as part of the technologically necessary cooling, the thermal energy contained in the solid should be at least partially recovered.

There are therefore various technologies, i.e. devices and processes for cooling bulk materials that have to be cooled from an initial temperature of e.g. 700°C to 1,400°C to final temperatures of e.g. 80°C to 200°C.

In addition to the use of coolers that use direct contact between ambient air and the goods to be cooled, rotary tube coolers operated indirectly with air or water are used for this task. "Indirectly" means that the cooling medium, for example water or air, does not come into direct contact with the hot product to be cooled, but heat is exchanged from the hot product to the cooling medium via an apparatus wall separating the media.

Out of U.S. 1,218,873 A , U.S. 2,283,129 A and U.S. 2,348,446 A indirectly operated with air solids coolers are known that work both with a single, closed drum housing, as well as those that lead the solids in several tubes within a drum.

Furthermore it is over DE 44 06 382 C2 , DE 33 31 744 C2 , U.S. 3,829,282 A , U.S. 3,920,381A ; U.S. Patent No. 4,021,195 ; U.S. 4,089,634A and U.S. 4,131,418A known to introduce hot bulk material, such as in the cement industry accumulating, hot and to be cooled clinker in several tubes arranged around a discharge end of a rotary kiln and conveyed by the rotation of the kiln and thus the cooling tubes. In such coolers, the cooling tubes carrying the hot product are cooled by free convection of the ambient air.

In the simplest designs of rotary tube coolers indirectly cooled with water, a rotary tube is sprayed with water from the outside; or the drum is run through a water bath, as in U.S. 4,557,804 A described, whereby the surface of the rotating drum is wetted with water and cools the apparatus wall, while in turn the hot product located in the drum is cooled by heat dissipation to the cooled apparatus wall.

Out of EP 0 567 467 B1 a rotary tube cooler with a rotary tube is known, which rotates within a stationary, brick-lined enclosure and in which the cooling medium, for example air or water, flows in the cavity formed between the rotary tube and the lining.

A similar solution, in which the drum shell is formed by a pipe system through which cooling water flows, is out U.S. 1,711,297 A ; U.S. 4,711,297 A , EP 0 217 113 A2 and DE 35 34 991 AI known. The design of such a simple drum requires a small surface area for heat exchange and, as a result, a low cooling capacity of the apparatus. In the patent U.S. 2,362,539 A a cooler is described which works with a plurality of product-carrying pipes arranged on a circular circumference, the pipes being sprayed with water from above for cooling and the water running off into a trough underneath.

U.S. 2,283,129 A and EP 2 889 569 A1 disclose rotary tube apparatus. Out of DE 160 351 C and U.S. 4,637,034 A stationary arrangements for improving the heat transfer are known.

In the case of sectional coolers, as they have become known from the Grenzebach BSH GmbH, a plurality of chambers, for example six or eight chambers, the so-called sections, are created to increase the heat exchanger surface, which are located in a rotary drum housing, creating a cavity between the chambers. In relation to the cross-section of a cylindrical housing, each chamber thus fills out a sector of a circle or cross-section of a circle.

To cool the hot product in the chambers (sections) or conveyed through the chambers, cooling water is passed through the cavities formed in the drum housing between the sections. The cooling water is supplied and removed via a sealed swivel joint on the product discharge side of the drum and pipe connections to and from the individual double pipes.

Such sectional coolers have a special design that leads to high material and labor costs in production, especially due to the extensive welding work required. In addition, the drum housing itself necessarily has a high weight, because the drum and the walls of the chambers must be made thick-walled for reasons of strength. Although both lead to a high overall weight of the apparatus, they permit particularly effective heat dissipation.

Sectional coolers essentially consist of a rotating rotor, which is usually driven by a chain. At the ends of the rotor are rigid housings for product infeed and outfeed. Depending on the size of the cooler, the rotor is either mounted on the ends of its own axis (axle cooler) or has a rotary tube typical race bearing. Inside, the rotor consists of several sectional chambers, which are arranged in the shape of a pie around a central hollow shaft. This arrangement is completely surrounded by the outer jacket. Conveying elements are located in the sectional chambers. Depending on the requirements, these can be shovels, chains or the like.

Depending on the requirements, sectional coolers are built with diameters between 0.8 and 4 m and lengths of 3 to 30 m.

Sectional coolers work with indirect water cooling. The cooling water passes through an inner central hollow shaft between the individual sections, flows around them and exits again through an outer central hollow shaft. The product to be cooled usually falls directly into the product drop housing and is transported to the other end of the cooler by the rotary movement and the conveying elements. The rotation ensures that the product is constantly mixed in the sections and thus good heat transfer is achieved. The product can be conveyed in parallel or countercurrent to the cooling medium.

Sectional coolers can be used to cool almost all free-flowing bulk materials. They are often found behind rotary kilns in calcination processes or the like. Their main goal is usually to cool the products down enough so that they can be handled with other equipment (conveyors, mills, etc.). However, the cooling itself is often an important part of the manufacturing process. Typical products are e.g. B. petroleum coke, zinc blende roasted goods, soda, pigments and many more. The inlet temperatures of the products can be up to 1400 °C.

In contrast to directly air-cooled devices, there are no problems with product discharge in the air flow when cooling powders in the sectional cooler. Thanks to the robust construction, even larger particles do not cause any problems. By using appropriate seals, it is possible to create an inert space in the sections, which means that reactive products can also be treated.

It is the object of the invention to improve a rotary tube apparatus, in particular a sectional cooler, of the type mentioned at the outset in such a way that it achieves optimized heat transfer from the material to be cooled to the cooling medium.

According to the invention, this object is achieved as specified in patent claim 1.

Advantageous developments result from the dependent claims and the description, in particular in connection with the figures.

The invention relates to any rotary tube apparatus that is used to cool or heat a free-flowing material. In the following, reference is always made to a rotary tube cooler and its cooling function as an example of such a rotary tube apparatus; nevertheless, the invention is intended to be used for any pourable material introduced into such a rotary tube cooler. The hollow tubes are preferably arranged in rows, which extend in the longitudinal direction of the rotary tube apparatus.

Advantageously, two adjacent rows of hollow tubes each have a staggered arrangement of the hollow tubes.

The hollow tubes can be attached to the walls of sections by screwing, gluing or riveting, for example.

Welding methods are also suitable, for example, in particular submerged arc welding, gas metal arc welding, friction welding or stud welding. A method that is particularly adapted to hollow pipes and is therefore particularly suitable is sleeve welding.

The hollow tubes have a length of less than 10 cm, in particular less than 5 cm. Most preferably they have a length of 3.6 cm.

The hollow tubes advantageously have a diameter of less than 5 cm, in particular 3.0 cm.

It also proves to be advantageous if the hollow tubes have a wall thickness of 1 cm or less, in particular 0.5 cm.

Preferably, the rotary tube cooler has a plurality of sections having a higher density of hollow tubes on the radial walls and on the circular arc wall exhibit than in the corner areas between the radial walls and between the radial walls on the one hand and the arc wall on the other hand.

It is advantageously provided that the sections each have about 500 ribs or 500 hollow tubes per meter of length of the rotary tube cooler.

The invention also relates to a method for operating a rotary tube apparatus, in particular a rotary tube cooler, as described above. The method is characterized in that the solid moves in a turbulent flow around the hollow tubes.

Fig. 1

eine Darstellung des Verschleißes eines Bauteils (senkrechte Achse), beispielsweise des Drehrohrapparats, als Funktion des Verhältnisses der Härte des Werkstoffs des Bauteils zur Härte eines Verschleißkörpers (waagrechte Achse), beispielsweise von Zinkoxid,

Fig. 2

eine Darstellung des Verschleißes eines Bauteils (senkrechte Achse), beispielsweise des Drehrohrapparats, als Funktion des Verhältnisses der Härte des Werkstoffs des Bauteils zur Härte von Zinkoxid (waagrechte Achse) für verschiedene zum Einsatz in einem Drehrohrapparat verwendbare Materialien,

Fig. 3

eine Darstellung der Brinellhärte [HBW] (senkrechte Achse) als Funktion der Bruchdehnung, gemessen in [%], für verschiedene zum Einsatz in einem Drehrohrapparat verwendbare Materialien (waagrechte Achse), insbesondere für dessen eine Kühlfunktion ausübende Bauteile wie die Kühlrippen,

Fig. 4

eine Darstellung der Wärmeleitfähigkeit λ, gemessen in [W/(m K)], (senkrechte Achse) verschiedener Materialien in Abhängigkeit zur Differenz des Wärmeausdehnungskoeffizienten dieser Materialien zu dem Wärmeausdehnungskoeffizienten α [10^-6 K^-1] des für die Wände der Sektionen des Drehrohrapparats eingesetzten Baustahls IS235JR (waagrechte Achse),

Fig. 5

den von verschiedenen Materialien übertragenen Wärmestrom Q [W] (senkrechte Achse) in Abhängigkeit von deren Wärmeleitfähigkeit A [W / (m K)] (waagrechte Achse),

Fig. 6

die Wärmeleitfähigkeit verschiedener Materialien (senkrechte Achse) als Funktion ihrer Temperaturleitfähigkeit (waagrechte Achse),

Fig. 7

eine Schnittansicht eines Ausschnitts einer Sektion eines Sektionalkühlers mit einer Rechteckrippe, die über eine Schraube und eine Mutter mit einer Wandung der Sektion verbunden ist,

Fig. 8

eine Schnittansicht eines Ausschnitts einer Sektion eines Sektionalkühlers mit einer im Querschnitt wellenförmigen Rippe,

Fig. 9

eine Schnittansicht einer auf der Wandung einer Sektion eines Sektionalkühlers aufgebrachten Hohlrippe oder Rohrrippe,

Fig. 10

einen Querschnitt durch einen schematisch dargestellten Sektionalkühler mit acht Sektionen, die jeweils teilweise mit einem rieselfähigen Gut (in Schwarz dargestellt) gefüllt sind,

Fig. 11

eine isometrische Querschnittsdarstellung eines Sektors eines Sektionalkühlers gemäß Fig. 10, der mit reihenförmig angeordneten Hohlrippen gemäß Fig. 9 ausgestattet ist,

Fig. 12

eine Draufsicht auf reihenförmig in auf einer inneren Wandung eines Sektors des Sektionalkühlers angeordnete Rohrrippen im Bereich einer der Zonen, in denen das zu kühlende Gut eine höhere Teilchengeschwindigkeit aufweist, und

Fig. 13

eine Darstellung einer von Teilchen eines zu kühlenden Guts umströmten Rohrrippe.

The invention is explained in more detail below in exemplary embodiments with reference to the drawings. Show it:

1

a representation of the wear of a component (vertical axis), for example the rotary kiln, as a function of the ratio of the hardness of the material of the component to the hardness of a wear body (horizontal axis), for example zinc oxide,

2

a representation of the wear of a component (vertical axis), for example the rotary kiln, as a function of the ratio of the hardness of the material of the component to the hardness of zinc oxide (horizontal axis) for various materials that can be used in a rotary kiln,

3

a representation of the Brinell hardness [HBW] (vertical axis) as a function of the elongation at break, measured in [%], for various materials that can be used in a rotary tube apparatus (horizontal axis), in particular for components that perform a cooling function, such as the cooling fins,

4

a representation of the thermal conductivity λ, measured in [W/(m K)], (vertical axis) of various materials as a function of the difference in the coefficient of thermal expansion of these materials to the coefficient of thermal expansion α [10 ^-6 K ^-1 ] of the for the walls of the IS235JR structural steel (horizontal axis) used in rotary kiln sections

figure 5

the heat flow Q [W] (vertical axis) transferred by different materials as a function of their thermal conductivity A [W / (m K)] (horizontal axis),

6

the thermal conductivity of different materials (vertical axis) as a function of their thermal diffusivity (horizontal axis),

7

a sectional view of a detail of a section of a sectional cooler with a rectangular rib which is connected to a wall of the section via a screw and a nut,

8

a sectional view of a detail of a section of a sectional cooler with a rib that is wavy in cross section,

9

a sectional view of a hollow rib or tubular rib applied to the wall of a section of a sectional cooler,

10

a cross section through a diagrammatically shown sectional cooler with eight sections, each of which is partially filled with a free-flowing material (shown in black),

11

an isometric cross-sectional view of a sector of a sectional cooler according to FIG 10 , according to the hollow ribs arranged in rows 9 Is provided,

12

a top view of tube ribs arranged in rows on an inner wall of a sector of the sectional cooler in the area of one of the zones in which the material to be cooled has a higher particle velocity, and

13

a representation of a pipe fin around which particles of a material to be cooled flow.

According to the invention, a large number of criteria are taken into account when optimizing a rotary tube cooler. The best possible combination of material, joining process and geometry is determined. However, the optimization of the heat transfer of the rotary tube cooler, in particular of the sectional cooler, is mainly improved by the introduction and optimization of the cooling fins.

The substrate to be cooled is introduced into a rotary tube cooler, for example a sectional cooler, at a high temperature, for example at up to 950° C. The temperatures of the sections are lowered by the constant cooling of the sections by a cooling fluid, for example water. Depending on the geometry, cooling fins in the sections can reach a temperature of 550 °C, for example, at the point at which the product enters. However, the mechanical stresses on the ribs are low. They are limited to stress caused by contact with the product. The ribs do not have a supporting or strength-enhancing role within the sectional cooler. Therefore, materials can also be considered whose application limit is below the mentioned 550 °C. The main stress lies in the area of wear resistance due to the substrate to be cooled or heated, for example powdered zinc oxide. Depending on the composition of the atmosphere inside the sectional cooler, high-temperature corrosion processes can also take place.

Due to the temperatures that occur, the choice of materials is limited to metals and their alloys as well as ceramic materials. Despite their good properties in terms of corrosion resistance, ceramic materials have poor thermal conductivity. In addition, their brittle behavior must be assessed critically. Thus, preference is given to metallic alloys in the selection of materials. The materials that can be selected are shown in Table 1 with some of their properties. As can be seen in the selection, materials from different categories are included in the selection process. For example, the sectional cooler with all its Built-ins mostly made of structural steel S235JR with the material number 1.0038. However, other alloys, for example aluminum or magnesium, and various types of steel are also suitable.

Table 1 shows the materials.

The selection of the material to be used is made based on a number of criteria. Since the main stress on the cooling fins is the wear caused by the zinc oxide, it is important to keep this as low as possible. The types of wear that occur are sliding wear and impact wear. High resistance to both types, composed of the mechanisms of abrasion and surface distress, can be achieved through a combination of high hardness and ductility. The abrasion mechanism can be counteracted by making the material very hard.

As in 1 represented schematically by the ratio of the hardness of the component to the hardness of the wear body, abrasion wear is divided into three zones. In the zone with a ratio of less than 0.6, the greatest wear occurs due to the low hardness of the component. In an area with a hardness ratio of the two components between 0.6 and 1.2, there is a transition from the high wear level to the low wear level. From a value of 1.2, wear from abrasion is minimized, since the wear body cannot penetrate the component due to its lower hardness. Table 1 category short name material number Thermal conductivity [W/(mK)] Expansion coefficient [*10^-6K^-1] Specific heat capacity [J/(kgK)] Modulus of elasticity [N/mm ² ] Density [kg/m ³ ] Hardness Brinell [HBW] aluminum alloy AlMg1SiCu 3.3211 170 23.0 895 70,000 2,700 88 magnesium alloy AM50A (EN MCMgAl5Mn) EN-MC21220 65 14.0 1020 45,000 1,770 58 pure nickel nickel 201 2.4068 79 1, 8 456 205,000 8,900 95 Structural steel S355JR 1.0045 54 0.0 461 210,000 7,850 170 tempered steel 25CrMo4 1.7218 49 0.4 435 210,000 7,750 216 Structural steel S235 1.0038 54 0.0 461 210,000 7,850 123 carbon steel SAE AISI 1008 1008 65 13.1 470 190,000 7,900 97 High-temperature steel P235GH 1.0345 57 13.0 461 210,000 7,850 143 Heat-resistant pressure vessel steel P265Gh 1.0425 51 13.0 461 210,000 7,850 155 Stainless ferritic steel X6CrMoS17 1.4105 25 13.0 460 220,000 7,700 200

Zinc oxide is a mineral. Accordingly, the hardness of zinc oxide is classified according to the Mohs hardness scale, which is based on the scratch hardness of minerals. The value is approx. 4. Although an exact conversion into a Brinell hardness value that is typical in mechanical engineering is not possible, the guide value for the hardness of zinc oxide according to Brinell is approx. 180 HBW (HBW = Brinell tungsten carbide hardness). If you form the ratio of the hardness of the materials under consideration to that of the zinc oxide and enter this in the graph 1 the following diagram results: The heat-treated steel 25CrMo4 is the only material that is in the low wear zone. The magnesium alloy, the pure nickel and the carbon steel are in the area of maximum wear due to abrasion. All other materials are in the area of the transition ( 2 ).

Since surface distress is also important in addition to the mechanism of abrasion, the materials are also assessed in relation to their resistance to wear. The elongation at break can be used as a measurable variable for the resistance. This reflects the ductility of the materials, which counteracts the increasing degree of surface distress. figure 3 compares the material properties of hardness versus elongation at break, since wear depends on the combination of both properties.

zu $\frac{1}{3}$

festgelegt.Correspondingly, materials that are in the upper right area of the diagram are preferable for use in a rotary tube cooler due to their combination of hardness and elongation at break. Materials found in the lower right, such as nickel, have good wear resistance to surface distress, but their low hardness makes them susceptible to abrasion. The two alloys of aluminum and magnesium do not show particularly good resistance for either mechanism. However, it must be taken into account that the proportion of abrasion outweighs that of surface distress. This is due to the small particle diameters of zinc oxide between 0 mm and 6 mm. Accordingly, a weighting factor must be included, which is figure 3 is not taken into account. For example, the ratio of abrasion to surface distress is given by $\frac{2}{3}$

to $\frac{1}{3}$

fixed.

Since according to the invention mainly the heat transfer of the sectional cooler is improved, the thermal conductivity of the individual materials is primarily taken into account. Regardless of the geometry, increased heat flows can be achieved by using particularly suitable materials with higher thermal conductivity. However, it should be noted that the number of materials that can be used may be limited depending on the joining process. In addition, the coefficient of thermal expansion must be taken into account. If the sections are made of mild steel, which has a coefficient of about 12 × 10 ^-6 K ^-1 , if the cooling fins are made of other materials, stresses can arise. The sections and cooling fins are at room temperature during the joining process. If the cooler is now put into operation, the temperature rises and the components expand. Materials with different coefficients of thermal expansion therefore expand to different extents. This difference in expansion creates stresses in the area of the joining zone. Depending on the temperature and the difference in thermal expansion coefficients, these are larger or smaller. Depending on the joining method, critical stresses can be exceeded. In 4 Therefore, the thermal conductivity is plotted against the difference in the coefficient of thermal expansion between the considered material of the cooling fins and the structural steel IS235JR used in the sections.

It turns out that the aluminum alloy has the greatest thermal conductivity, but also a large difference in the thermal expansion coefficient of structural steel. Together with the magnesium alloy, which has a significantly lower thermal conductivity compared to the aluminum alloy, the greatest stresses are to be expected in the area of the joining zone. All other materials are in a similar range of thermal expansion coefficients and thermal conductivity, with the stainless ferritic steel X6CrMoS17 having the lowest thermal conductivity.

If one compares the transferred heat flow under the same conditions only with the different materials, the result is in figure 5 heat flow shown as a function of thermal conductivity. A course is shown that is similar to that of a square root function. At low values of thermal conductivity increases the heat flow increases sharply. With increasing thermal conductivities, the heat flow continues to increase, but the slope of the curve decreases sharply. For this reason, the heat flow of X6CrMoS17 is about 20% lower than that of S235JR, although the thermal conductivity is over 50% lower. The thermal conductivity of the aluminum alloy exceeds that of mild steel by more than 200%. The gain in heat flow, on the other hand, is only 20%. The course therefore approaches a maximum heat flow.

figure 5 shows the transferred heat flow as a function of the thermal conductivity. Another evaluation criterion is the thermal conductivity in relation to the explained thermal fatigue. Although the number of operating cycles of a sectional cooler is low, since they are almost exclusively taken out of service for maintenance and repair, thermal fatigue of the cooling fins can still occur if the temperature conductivity is too low. Therefore, higher thermal conductivity of the materials, as well as the geometries, are to be preferred in order to avoid cracks in the components and signs of fatigue.

6 shows the thermal conductivity of the materials in relation to the thermal conductivity. With regard to the thermal properties, the aluminum alloy again achieves the best result due to the high thermal conductivity as well as the thermal conductivity. Since the thermal conductivity is composed of the thermal conductivity, the density and the specific heat capacity, it becomes clear why the aluminum alloy with the low density and the high thermal conductivity has a high thermal conductivity. The magnesium alloy also has a high thermal conductivity. In terms of thermal conductivity, the alloy X6CrMoS17 has the worst properties. The other materials have approximately the same thermal conductivities, with the known differences in thermal conductivities.

In order to determine the most suitable material, the factors and evaluation criteria presented above, such as hardness, elongation at break, thermal conductivity, coefficient of expansion, thermal conductivity, heat flow and costs are used evaluated. Depending on the importance of the individual evaluation criteria, these are given weighting factors, for example (see Table 2). Table 2: Weighting factors of the evaluation criteria evaluation criterion weight factor hardness 0.30 elongation at break 0.20 thermal conductivity 0.20 Difference in thermal expansion coefficient 0.15 thermal conductivity 0.05 heat flow 0.20 Total: 1

In addition to the thermal conductivity, the transferred heat flow is also included in the evaluation with the same weighting factor, since it has been shown that the thermal conductivity is crucial for the heat flow, but does not have a linear course. Accordingly, the determined heat flow serves as an additional factor to compensate for this non-linearity. The criteria associated with wear and tear or fatigue of the materials also have a major influence.

The evaluation is carried out by assigning the value one to the highest value of an evaluation criterion. The value zero forms the lower limit in each case. A linear progression is formed between the upper and lower value, so that the remaining values lie between the two limits. The determined values are then multiplied by the respective weighting factor. This is carried out for the various evaluation criteria and finally the individual results are summed up. The best possible evaluation of the total is therefore one.

Example: The alloy 25CrMo4 has the highest hardness with 216 HBW. This corresponds to the value 1. It follows that the other materials each receive 0.01 evaluation points for every 2.16 HBW. This results in a value of 0.57 for the structural steel S235JR with a hardness of 123 HBW. Multiplied by the weighting factor, the values are 0.3 and 0.171.

The full evaluation is presented in Table 3. The heat-treated steel 25CrMo4 achieved the best result with an overall rating of 0.8032. S355JR mild steel follows with 0.7972 points. Since the two materials have achieved similarly good results, a final decision is made on the choice of material depending on the joining process used.

The heat-treatable steel has the considerable disadvantage that in the case of a weld, it has to be stress-relieved for several hours at temperatures between 680 °C and 720 °C and then slowly cooled down in order to reduce stresses within the heat-affected zone by welding. In the case of the large components of a sectional cooler, this also means a great deal of technical effort in addition to the time. The easy-to-weld structural steel S355JR does not require such a time-consuming, but also costly post-treatment. The heat-treated steel 25CrMo4 is therefore to be preferred for all joining processes, except for welding, where the advantages of mild steel in terms of easier handling predominate.

einzelnen Fügeverfahren eingegangen und jeweils mit den übrigen Verfahren verglichen.The manner in which the fins are attached to the sections of the sectional cooler has a significant impact on the service life and the transferred heat flow. Below is the pros and cons of the

individual joining processes and compared with the other processes.

A major advantage of bonding is that an equivalent result can be achieved with all metals with good pre-treatment. Accordingly, different material combinations are possible. However, depending on the type of adhesive used, there are other factors to consider.

Structural adhesives can absorb stresses of up to 30 MPa. Compared to the other joining methods, this is many times lower. However, in order to be able to withstand these stresses, very complex pre-treatment of the workpieces is necessary, since this is the only way to ensure good wetting of the surfaces, which is decisive for the quality of the bond. Since an even and thin layer thickness of the adhesive is also crucial, both sections and cooling fins must meet high tolerance requirements. Despite the low thermal conductivity of the adhesive, the heat flow is only imperceptibly changed due to the small layer thickness.

It must also be taken into account that the adhesives must be subjected to even pressure during the time-consuming drying process. In addition, the sections must be fully heated during drying. This requires a high energy requirement and high technical complexity. Although there are adhesives that can be used at temperatures above 1000 °C, all of these are affected by aging processes. In addition, there is a risk of creeping at high temperatures, which can drastically reduce the service life of the sectional cooler.

In the case of elastic adhesives, the requirements for the tolerances of the components are lower due to the larger layer thicknesses. However, the transferred heat flow decreases drastically. In addition, lower stresses can be tolerated than with structural adhesives. In order to be able to absorb an identical force, a larger contact area is therefore required.

Screw connections, which can also be used to connect different materials to one another, are even more advantageous than adhesive connections. Since the connections are not material-to-material but friction-locked, high dimensional accuracy must also be maintained in order to produce full-surface contact between the rib and the section so that the heat is transferred via thermal conduction. Voids between section and rib result in free convection between the two components. This would significantly reduce the transferred heat flow.

In contrast to adhesive connections, screw connections can withstand significantly higher stresses by adapting the components used, such as screws and nuts. However, a large number of holes must be drilled into the sections through which the screws are guided. This reduces the rigidity of the sections. In addition, this area must be sealed. This requires the use of additional components.

In addition to the weakening of the sections by the bores, the clamping force between the screw head and the nut generates stresses in the sections which are superimposed on the stresses occurring during operation.

In a section 1 ( 7 ) of a sectional cooler has a rib 2 with a rectangular shape (rectangular rib) and is connected to a wall 5 of the section 1 via a screw 3 and a nut 4 . In this way, the rib 2 forms a bearing surface for the screw head of the screw 3. By using screws 3, the ribs 2 can be exchanged without being destroyed.

Rivet connections can also be used as an alternative to using screw connections

The press connection method requires the use of ribs which are pushed through the wall of the section at least in some areas.

After pressing in, the wall of the section and the respective rib can also be glued or welded.

Another method of making a connection between the ribs and the wall is joining by welding, divided into two categories. Both submerged arc welding and metal inert gas welding are used, as well as friction welding and stud welding.

Submerged arc welding is not suitable for all welding positions because the powder lies loosely on the welding zone. This means that only welding positions with a low inclination can be realized. Each section of a sectional cooler consists of two parts joined together. These are welded together after the turning bars and conveyor blades have been installed.

In comparison to submerged arc welding, the welding torches for both automated and manual MSG welding (MAG gas-shielded metal arc welding) have significantly smaller dimensions. The preparations to weld the ribs to the sections are less than the preparations required when gluing, screwing or riveting. Inaccuracies can be compensated for by introducing additional filler metal. With regard to the heat flow, the ribs only have to be provided with chamfers in order to be able to guarantee full-surface contact. Within the weld seam, the material's thermal conductivity is almost identical to that of the starting material. Thus, very good results can be achieved in terms of the heat flow transferred between the two components by welding with full-surface contact between the rib and the section.

Despite the influence of the high thermal load on the structure during welding, the tolerable stresses are significantly higher in comparison to bonding with structural adhesive or those of a press connection, despite the welding residual stresses. Also, no additional contact surfaces are necessary as with screws and rivets. Since the ribs are completely surrounded by welds, it is only necessary to reduce the length of the ribs. Accordingly, instead of one long rib, three to four shorter ribs are introduced along the sections; this can also be referred to as a broken rib. This reduces distortion and stress. Subsequent treatment of the welds is not necessary, since the mild steel S355JR is very easy to weld and repairs can also be carried out on the job sites in the same way. Additional components are limited to the welding wire, which means that production is not unnecessarily more complex or error-prone than with screw connections.

For rotationally symmetrical cooling fins, on the other hand, friction welding is opposed to stud welding. Friction welding is characterized by very good quality in the area of the welding zone. The strength is higher than that of the base material. The thermal load and the associated distortion and residual stresses are also lower than with a fusion welding process.

This shows that gas metal arc welding is the preferred option for joining cooling fins.

Stud welding is characterized by very short welding times. These are significantly below those of friction welding. Due to the short welding times, the thermal load is lower than, for example, with MSG welding. The strength of the material connection is higher than that of the base material. The connection is also not affected by aging processes, as is the case with adhesive bonds.

The preparation of the welding zone is identical to that of MSG or submerged arc welding (submerged arc welding) and therefore also significantly less compared to the other processes considered. With a round cross-section of the cooling fins, cutting the long rod to the desired length is sufficient as preparation in the area of the fins. The sections do not have to be provided with complex bores with minimum tolerances. Welding fillers are not required, only shielding from the atmosphere with a protective gas is necessary.

The small dimensions of the welding gun of a stud welder allow easy attachment of the ribs in all areas of the section. In addition, the manual skill required is very low due to the easy handling of the welding gun.

However, it should be noted that the maximum weldable diameter of the cooling fins is limited to 30 mm. The blowing effect must also be taken into account in order to achieve full-surface contact and thus the best possible heat transfer. Despite the limitation of the outer diameter to 30 mm, stud welding offers the best compromise due to the good mechanical properties of the joining zone in combination with the easy handling of the welding gun and the very short welding times. Stud welding should therefore be used for round geometries of the cooling fins.

The cooling fins are therefore welded to the sections regardless of their geometry. Therefore, the structural steel S355JR is to be preferred over the heat-treated steel 25CrMo4, as it is very easy to weld and does not require any post-treatment. Since S355JR structural steel is a low-alloy structural steel, an active gas is recommended as the shielding gas because it is cheaper than an inert gas.

According to the invention, a geometry of the cooling fins is also created which satisfies a number of criteria, in particular with regard to the heat flow.

The heat flow, based on the contact surface between the cooling fin and the section, is used to determine the heat flow, based on 1 mm ² . This allows the efficiency of the different geometries to be estimated independently of the size of the rib or its contact area with the section. Since some ribs, such as the bladed ribs, occupy a significantly larger area of the section than their contact area, this is accounted for by a projected area, ie the area covered by the contour of the rib

This must be taken into account with regard to the number of ribs to be installed, since the possible number depends heavily on the projected area. For this reason, the heat flow is also related to the projected area. In addition to the areas, the weight of the ribs is also included in the evaluation. The heat flow, related to the weight of the cooling fin, serves as a further criterion for the efficiency of the geometry under consideration. With a high quotient from heat flow and weight, better use of resources is achieved, material consumption and the associated material costs are reduced. As a further criterion, the ratio of the heat flow at a point in time t, for example t = 28s, is compared with the stationary heat flow towards the end of the simulation. This ratio can be used to determine the thermal conductivity of the geometry. A high thermal conductivity of the geometry also prevents or reduces the risk of thermal fatigue.

Table 4 shows the weighting of the various criteria. The two heat flows related to the surfaces are the decisive criteria of the geometries. For this reason, the weighting factors together are 0.65. Relating heat flow to fin weight gives an indication of the efficiency of the fin, but is not a definitive indication of the overall improvement in heat flow to the currently used cooling fin. For this reason, the criterion should not be neglected, but with a weighting factor of 0.2 it is factored less than the heat flows, based on the areas. With a weighting factor of 0.15, thermal conductivity is below the other factors. This is justified because the ratio of the heat flows at different times is primarily decisive for thermal fatigue. Table 4: Weighting factors of the geometry evaluation criteria evaluation criterion weight factor heat flow per area 0.4 Heat flow per projected area 0.25 heat flow per kilogram 0.2 thermal conductivity 0.15 total 1

The different geometries are evaluated in a similar way to the material pre-selection. The highest value of an evaluation criterion is given the value 1. A linear gradation down to the value 0 is then formed and the remaining geometries are provided with the corresponding value.

The values are multiplied by the weighting factors and then summed up. The maximum achievable sum is therefore 1.

The evaluation is shown in Table 5. The best total score of 0.859 points is achieved by a wavy rib 6 ( 8 ) (shown only as "Wavy" in Table 5). This is due to the large area achieved by the geometry. However, it must be taken into account that elongated ribs are to be attached to the sections by MIG welding. However, due to the contour, it is possible to apply the required chamfer to ensure full-surface contact between the rib and the section, but due to the curvature on the left side of the rib ( 8 ) it is not possible to feed the welding torch. By adapting the geometry to ensure weldability, the score is reduced by almost 0.2 points to 0.672. Although in 8 only a "half wave" of a cross section of such a rib 6 is shown in cross section, it is understood that according to the invention each rib 6 can have a large number of wave crests and wave troughs.

Table 5 shows the evaluation of the geometry.

The unadapted wavy rib is followed by the optimized rectangular rib with a difference of 0.084 points. This has due to the optimally calculated height <b>Table 5</b> Heat flow per area Calculation with weighting factor heat flow per proj area Calculation with weighting factor heat flow per kg Calculation with weighting factor temperature conductivity Calculation with weighting factor TOTAL Convex reversed around 1,000 0, 400 0.343 0.086 0, 330 0.066 0.879 0.132 0.684 Wavy 0, 0, 322 0.769 0.192 0.973 0.195 1,000 0.150 0.859 Tree 0.796 0.319 0.327 0.082 0.736 0.147 0.958 0.144 0.691 reverse trapezoid 0.767 0, 307 0, 404 0.101 0.512 0.102 0.827 0.124 0.634 tree reversed 0.761 0, 305 0.313 0.078 0.704 0.141 0.959 0.144 0.667 Shovel ratio 0.75 0.727 0.291 0.319 0.080 0.542 0.108 0.925 0.139 0.618 Convex reversed 0.699 0.279 0.383 0.096 0.361 0.072 0.711 0.107 0.554 Fork 0.664 0.266 0.520 0.130 0.843 0.169 0.942 0.141 0.705 Shovel connected 1 0.643 0.257 0.211 0.053 0.412 0.082 0.931 0.140 0.532 Shovel ratio 0.5 0.630 0.252 0.414 0.103 0.560 0, 112 0.912 0.137 0.604 rectangle 0.608 0.243 1,000 0.250 0.754 0.151 0.867 0.130 0.774 jagged rectangle 0.588 0.235 0.9661 0.2421 0.752 0.150 0.876 0, 131 0.758 Round hollow 1 to 3 around 0.551 0.220 0.906 0.226 0.698 0.140 0.872 0.131 0.717 Round hollow 1.5 to 3 around 0.549 0, 220 0.903 0.226 0.792 0.158 0.926 0.139 0.743 Square around 0.542 0.217 0.892 0.223 0.579 0.116 0.774 0, 116 0.672 Round hollow 2 to 3 around 0.527 0.211 0.867 0.217 0.944 0.189 0.990 0.149 0.765 parabola 0.524 0.210 0.861 0.215 0.776 0.115 0.827 0.124 0.704 rectangle default 0.523 0, 209 0.859 0.215 1,000 0, 200 0.797 0.120 0.743 trapeze 0.508 0.203 0.835 0.209 0.944 0.189 0.862 0.129 0.730 Around around 0.463 0.185 0.761 0.190 0.534 0.107 0.729 0.109 0.592 triangle pointed 0.4411 0.177 0.725 0.181 0.923 0.185 0.794 0.119 0.662 Convex 0.430 0.172 0.707 0.177 0.606 0.121 0.721 0.108 0.578 claw 0.409 0.164 0.270 0.067 0.539 0.108 0.709 0.106 0.445 Convex around 0.396 0.158 0.651 0.1631 0.679 0.136 0.671 0, 101 0.557 already the best possible result, whereas the other geometries have the potential to achieve even better results through further adjustments. Another reason for the good result of the optimized rectangular rib is that the high efficiency of the geometry is due to the low factor m × h .

The next best result is achieved by the round geometry with an indentation with the ratio between the inner radius R _i of a circular hollow rib 7 and the outer radius R _a (cf. 9 ) from 2 to 3. With a score of 0.765, this is 0.009 rating points below the value of the optimized rectangular rib. Each of the ribs 7 is provided with a hole in the middle. In addition to the considerable additional effort, this is associated with increased tool costs.

However, the simulation of a tube, the production effort for which is significantly lower, with the identical diameter of the drilled rib shows the potential of this geometry. With a heat flow of Q̇ = 62.2 W , this geometry achieves a value of 0.787. This exceeds the value of the optimized rectangular rib without having exhausted the full potential of the geometry. In relation to attaching tube fins to the sections, a relatively newly developed modification of stud welding, Magnetic Rotating Arc (MARC) also called sleeve welding, can be used.

This has almost identical properties to stud welding and differs primarily in the shape of the arc. A magnetically moved circular arc is generated between the rib and the section. This ensures a ring-shaped weld pool of the two components. The advantage of the extremely short welding times is also not lost with this process. The quality of the weld is very good with strengths exceeding those of the base materials. In addition, sleeve welding is not as susceptible to the blowing effect.

Since the geometry of the pipe delivers almost the best results in terms of heat flow, this is explained in detail below in combination with MARC welding using an exemplary embodiment.

The tubular geometry of the cooling fin is explained below using a standardized tube as an example. The dimensions are taken from DIN EN 10220, for example. The diameter for which the MARC welding process is possible is approx. d = 30 mm, for example, as with stud welding. The smallest diameter chosen, for example, is d=25 mm. The wall thickness is varied between T=6.3 mm and T=5 mm.

The evaluation is carried out identically to the evaluation given above. The same evaluation criteria with the same weighting factors are used. However, another evaluation criterion, the heat flow, is added. Since each is a tube rib, this addition is possible without further adjustments. The heat flow is weighted with a factor of 0.3. The maximum achievable sum thus increases to a value of 1.3. The length of the ribs is fixed at L = 50 mm, regardless of the diameters and wall thicknesses.

Table 6 shows the evaluation of the optimization of diameter and wall thickness.

The evaluation listed in Table 6 shows that the geometries with a wall thickness of T = 5 mm generally achieve better results. This can be attributed to the larger heat exchange surface. Despite the lower wall thickness, the tubular ribs achieve similar strengths to a comparable rectangular rib with a thickness of T = 10 mm due to their round geometry.

The best result is achieved with a diameter of d = 30 mm and a wall thickness of T = 5 mm. Based on these specified characteristics of the <b>Table 6</b> diameter [mm] wall thickness [mm] heat flow per area heat flow per proj area heat flow per kg thermal conductivity heat flow TOTAL 30, 0 6, 3 0, 129 0.086 329, 249 0.816 60, 620 1, 109 30, 0 5, 0 0, 145 0.081 369, 210 0.864 56, 910 1, 146 26, 9 6, 3 0, 129 0.093 329, 317 0.816 52, 700 1, 089 26, 9 5, 0 0, 147 0.089 373, 267 0.864 50, 400 1, 142 25, 4 6, 3 0, 129 0.096 329, 096 0.816 48, 830 1, 080 25, 4 5, 0 0, 146 0.093 372, 885 0.864 46, 900 1, 135 25, 0 6, 3 0, 129 0.097 328, 978 0.816 47, 790 1, 078 25, 0 5, 0 0, 146 0.094 373, 037 0.865 46,000 1, 134

Geometry determines the most preferred length of the rib. The length of the rib is varied with a pitch of 2 mm in a range between L = 30 mm and L = 60 mm. Since both the area and the projected area are identical, the evaluation criteria are limited to the heat flow (weighting factor 0.65), the heat flow related to the weight (weighting factor 0.2) and the thermal diffusivity (weighting factor 0.15).

In Table 7, the results of the evaluation are plotted against the length of the rib. It can be seen that there is a maximum at a length of L = 36 mm. With increasing length, the heat flow increases significantly less from the maximum in relation to the increasing mass. For this reason, the course of the graph falls from the maximum. Consequently, the rib with the length of L = 36 mm is chosen. This offers the best compromise of the considered criteria. Since the rib is compressed by about a length of L = 1.5 mm during the welding process, this amount must be added to the optimal length of the rib. This results in a length of L = 37.5 mm.

The dimensions of the optimization result in an outer diameter of the tube of d = 30 mm with a wall thickness of T = 5 mm and a length of L = 36 mm or L = 37.5 mm, taking into account the joining method used and the associated reduction in length.

In addition to the already determined and optimized geometry of the fins, the arrangement in combination with the number is also decisive for the transferred heat flow.

In order to determine the distribution of the material to be cooled, for example zinc oxide, within the sections and thus to be able to define the distribution of the ribs in these, the degree of filling φ is determined. This is made up of the residence time, the volume flow of the zinc oxide and the volume of the sections. The degree of surface coverage can be determined based on the degree of filling. The degree of area coverage indicates the area of the sections that are covered with the product. This results in the degree of filling: ϕ = 4.17% and for the degree of area coverage: λ _A = 17.61%. With a cross-sectional area of the chamber (section) of Q _K = 0.342 m ² , this corresponds to a surface coverage of A = 0.060 m ² . In combination with the dynamic angle of repose of the zinc oxide of θ _dyn = 40°, the distribution of the zinc oxide within the sections in the different layers can be determined.

The graphical determination of the area coverage of a preferably inclined or alternatively horizontally mounted sectional cooler 8 is in 10 shown in cross section. It shows that each area of the section is covered for a similar period of time. There is therefore no area in which the attachment of cooling fins would not have a positive effect. If you take a closer look at the distribution of the zinc oxide, you will notice that the product has different speeds in the different areas. In the 10 Areas labeled A, A' and A" are the zones where the zinc oxide is flowing at lower velocities, while in areas B, B' and B" it is moving at higher velocities.

Higher speeds also result in more turbulent flows, which in turn result in better convective heat transfer. The main purpose of the turning bars is to reduce the speed of the product in order to reduce the wear of the sections. Thus, according to the invention, an increased number of cooling fins in the areas is preferred B, B' and B" of the sections, on the one hand to take advantage of the flow in terms of heat transfer, but also to reduce the speed of the product to the extent that wear is kept low. Nevertheless, according to the invention, cooling ribs are also attached in areas A, A' and A", since the heat transfer through the ribs is also significantly improved at the lower speeds of the product.

The calculated temperature profile can be used to determine the positions within the cooler for the heat transfer coefficients.

Simulations whose boundary conditions are identical except for the heat transfer coefficients are carried out once with and once without cooling fins. The efficiency in the different areas of the cooler can be determined by forming the quotients between the heat flow with cooling fins and the heat flow without cooling fins. The results of the simulation are shown in Table 8. Table 8: Ratio of heat flows with different heat transfer coefficients with and without cooling fins Temperature T [°C] Heat transfer coefficient α [W/(mK)] Heat flow with cooling fin Q̇ [W] Heat flow without cooling fins Q̇ [W] 817 225.67 1268.3 245.8 600 200.46 872.1 160.1 280 156.95 337.9 55.9

$$X_2=\frac{{\dot{Q}}_{\mathit{Rippe}2}}{{\dot{Q}}_2}=\frac{\mathrm{872,1}}{\mathrm{160,1}}=\mathrm{5,4}$$

$$X_3=\frac{{\dot{Q}}_{\mathit{Rippe}3}}{{\dot{Q}}_3}=\frac{\mathrm{337,9}}{\mathrm{55,9}}=\mathrm{6,0}$$

The ratios between the heat flows with and without cooling fins result in: $${\mathrm{<figure-callout\; id="1"\; label="X"\; filenames="imgf0001.png"\; state="\{\{state\}\}">X</figure-callout>}}_1=\frac{{\dot{Q}}_{\mathit{rib}1}}{{\dot{Q}}_1}=\frac{1268.3}{245.8}=5.2$$

$$X_2=\frac{{\dot{Q}}_{\mathit{rib}2}}{{\dot{Q}}_2}=\frac{872.1}{160.1}=5.4$$

$$X_3=\frac{{\dot{Q}}_{\mathit{rib}3}}{{\dot{Q}}_3}=\frac{337.9}{55.9}=6.0$$

As the conditions show, the additional gain in transferred heat flow can be clearly seen in all areas of the cooler. With decreasing temperature and thus decreasing heat transfer coefficient, the ratio of the heat flow between finned and non-finned surface increases by a further 15%. However, since the conditions are all in a similar area, the distribution of the ribs over the length of the cooler should be even. An even distribution of the cooling fins means that assembly can be kept simple. This benefit outweighs the small benefit of the increased heat flow ratio in the lower temperature region.

According to the invention, the preferred number of cooling fins to be introduced is also determined. Here, both the heat flows from the contact surface to the cooling rib, but also the heat flows from the base plate surrounding the rib, are included. The geometry of rectangular strips, for example with the dimensions 9.9 m × 0.01 m × 0.03 m, and the tube ribs used are considered. In order to comply with the minimum distance between two tube fins of a = 18 mm, the maximum number of fins per section is limited to 917 per meter of cooler. With this number of fins, a heat flux twice that of the prior art is achieved.

The heat flow at 971 tube ribs with a length of the cooler of L = 1 m is Q̇ = 126,182 W per section. With 16 turning strips that are not continuously welded, a heat flow of Q̇ = 63,146 W is achieved under identical conditions.

An equation can be set up that determines the heat flow as a function of the number of tube fins: $$\dot{Q}=\mathrm{46,287}W+x_{\mathit{ribs}}\times 82.23W$$

The heat flow of the rectangular fins is already achieved from a number of 205 tube fins. Table 9: Radiator data at 500 tube fins compared to a conventional rectangular fin radiator number of ribs 500 increase in heat flow 38.41% Length ratio L _new to L 0.722 Net length of cooling chamber new [m] 7,080 Change in weight without internals due to the new net length of the cooler [kg] -8,484 Total weight of ribs [kg] 3,274 Weight difference to built-in turning bars [kg] 24.5 Total weight difference [kg] -8,460

With the number of 500 ribs, these achieve almost the same weight as 16 of the built-in rectangular turning strips, based on one meter. By increasing the heat flow by approx. 38%, the sectional cooler can be significantly shortened. With a net length of the cooling chamber of L = 9.8 m, 2.7 m can already be saved, resulting in a new net length of the cooling chamber of L = 7.1 m. Considering the weight of the cooling fins, around 8.5 tons of material can be saved.

According to one embodiment, the geometry of the ribs in a section 9 of a sectional cooler 8 according to the invention is as shown in 9 is shown.

Taking into account the knowledge gained regarding the geometry of ribs 10, the different zones A, A', A"; B, B', B" and the number of ribs 10, the following rough draft has been created. As in 11 As can be seen, there are significantly more ribs 10 in the elongated zones B, B', B" of the sections 9 than in the three corners A, A', A". This is due to the different speeds of the bulk material. In the elongated zones B, B', B" the speed is higher, which is why there is an increased heat transfer in these areas takes place, which can be further improved by an increased number of cooling fins 10. In addition, the speed of the particles in the vicinity of the wall of the section must be reduced in order to keep the wear on section 9 low. The section 9 shown contains about 500 ribs 10 over a length of one meter.

figure 12 shows the top view of the tube ribs 10 in one of the zones of higher particle velocity. Due to the offset of the ribs 10 between the rows of ribs 11, 12, the fine-grained zinc oxide constantly flows against them. On the one hand, this reduces the speed of the zinc oxide, but on the other hand, the deflection of the grains creates a turbulent flow, which improves the convective heat transfer. the inside 12 The arrow shown indicates the direction of flow. An example of what the flow around one of the ribs 10 might look like is in 13 shown. Immediately in front of the rib, the particles are deflected outwards. Several turbulences arise behind the rib, which are characteristic of turbulent flows. It also shows that particles with lower speed are directly behind the rib. With this distribution of the ribs 10, there is no cast shadow behind the ribs 10. The zinc oxide is in contact with the rib 10 all around. According to the invention, conveyor blades are also provided within the sections. In order to achieve a dwell time of the particles of, for example, t = 5.32 minutes in the respective section of the cooler, the conveying blades must also be adapted. This can be achieved by reducing the number of blades, one less bladed wall and adjusting the axial offset of the blades. Table 10: Adjustment and comparison of the residence time of the particles according to the invention (New) versus the prior art (Old) Surname Old New Length of cooling chamber _LK 9.9 7:18 m number of bladed walls n _Ws 3 2 - pumping efficiency n 0.3 0.3 - Number of blades per wall ns 15 11 - Offset blades axial ss 0.22 0.18 m number of revolutions n 4.7 4.7 min ^-1

These adjustments result in a feed of s = 0.47 m and consequently a dwell time of t = 5.49 min. This differs only slightly from the previous dwell time. The installed ribs 10 can serve as attachment points for welding the conveyor blades. Since there is no blading on one wall of the cooler, the assembly work is reduced in this area.

The selected and optimized geometry in combination with the selected material, the structural steel S355JR and the joining through the special modification of stud welding improve the heat transfer in a sectional cooler significantly compared to designs known from the prior art.

The selected joining process, MARC welding, is characterized by very short welding times, so that the many ribs can be welded in the shortest possible time. These short welding times are associated with lower thermal loads than with other fusion welding processes. This is also reflected in the low distortion of the sections and low residual welding stresses in the heat-affected zone. Another advantage is the easy handling of the welding gun, so that even less trained personnel can carry out the welding; however, the welding can also be automated by a welding robot. through the The small dimensions of the welding gun also ensure accessibility to the sections.

For example, the diameter of the ribs is 10 d = 30 mm. However, looking at the results in Table 6, it can be seen that the larger the diameter, the better the results.

The mechanical properties of the material exceed those of the base material in the area of the joining zone. In combination with the material selected for the ribs 10, this results in a high resistance to the predominant proportionate abrasion in the area in which the product hits the ribs 10. The hardness of S355JR mild steel exceeds that of the section by almost 40%. Due to the low weight of the selected geometry, the additional costs due to the higher-quality structural steel are negligible. With regard to the thermal conductivities, the walls of the section 8 and the ribs have at least substantially equal values. Due to the same thermal expansion coefficients, there are no stresses caused by different degrees of expansion of the components in the case of temperature differences. The problem of thermal fatigue also does not apply due to the same thermal conductivity of the two materials, since no signs of fatigue of this type have occurred in previous coolers with turning strips made of S235JR.

Since both materials are construction steel or low-alloy steels, they can be welded very well. In addition, no post-treatment of the joining zone is necessary. The fins 10 are easily made by cutting through tubes. Another advantage is that the selected steel is a very common steel.

The geometry of the rib convinces with a very good result even without optimization. The values exceed those of the optimized rectangular rib. Optimization achieves even better results. The geometry is characterized by a large heat exchange surface with a low weight. The optimum length of the rib 10 for the cooler under consideration is I=36 mm. This value is approx. 10 mm below the value of the optimal one rectangular corner rib. Material and weight can thus also be saved as a result of these properties.

Regardless of the number of ribs 10 to be used, they should preferably be arranged in a staggered manner. This means that the original task of the turning bars, which was to reduce wear on the sections, is fulfilled despite the new geometry. The round geometry, paired with the offset arrangement of the fins, creates a more turbulent flow, which improves heat transfer. In addition, there is no cast shadow behind the ribs. Thus, the outside of the rib is constantly in contact with the product to be cooled, which also ensures high heat transfer.

However, the number of cooling fins to be installed has yet to be determined. The considered value of 500 cooling fins 10 per section 8, based on a meter length, is just an example.

A reduction in the weight of the cooler is accompanied by further advantages. On the one hand, the torque required to set the cooler in rotation is lower. Depending on the degree of reduction in the required power of the motor, its load drops, or a cheaper motor with less power can be installed. This reduces the energy requirement of the system. In addition, the mechanical loads in the area of the pinion and the ring gear for the transmission of the motor drive to the outer wall of the rotary tube cooler are reduced. Furthermore, the loads acting on the bearings are reduced. Depending on the number of ribs, the load or dimensioning of the foundations can also be lower or designed to be smaller. The sectional coolers are used all over the world. However, the coolers are always manufactured at the same location. Thanks to their lower weight and smaller dimensions, the sectional coolers require less effort to handle during transport and installation of the cooler. The space costs of the sectional cooler, which are incurred when calculating a system, are also lower.

The knowledge gained about the selected combination of joining process, material and geometry of the cooling fin offers a clear advantage over the prior art thanks to the consequences mentioned.

Another decisive factor for improved heat transfer is that the ribs 10 must be connected to the section 9 over their entire contact surface. This ensures that the energy transferred from the product to the fins is transported to the water-cooled surface as efficiently as possible. For example, a cooler has a length of I = 10.5 m. With an outer diameter of d = 2.3 m and a weight of m = 35,000 kg, a granular substrate is cooled in 8 sections from temperatures above T = 700 °C to T = 150 °C. Based on the known data of the cooler, the temperature curve and the heat transfer coefficients can be determined at different points of the cooler.

Each of the eight sections of this cooler is provided with 16 turning strips, for example. Their task is to reduce the speed of the particles in order to minimize wear on the sections. Since it has been shown that more thermal energy is also transferred through the turning strips, they also serve as cooling fins. With regard to the optimization of this property, the turning bars are examined.

In order to ensure full-surface contact between the fin and section, but also to achieve a high heat flow, taking into account the conditions prevailing in the section cooler, it is important to determine the most suitable material in addition to the joining process.

The material is determined by considering seven different relevant properties. The wear mechanisms that stress the ribs are abrasion, which can be reduced by high hardness of the material, and surface distress, which is reduced by ductility. In addition to the costs and the thermal conductivity, the differences in the thermal expansion coefficients are also included in the evaluation. To the goal to improve the heat transfer, the thermal conductivity and the heat flow are also included in the evaluation.

The evaluation of the ten materials provides the result that the structural steel S355JR is best suited for use as the material for the cooling fins, taking into account the joining process selected afterwards. Due to a higher hardness compared to the S235JR alloy, abrasion wear is reduced. Due to the identical values of the heat conduction and the heat flow of the structural steel S355JR to the structural steel S235JR, there are no losses in the area of heat transfer. Since both materials also have the same coefficient of thermal expansion, there are no stresses in the contact area between the rib and the section due to temperature differences between the condition when the cooler is in operation and when it is not in operation.

In order to attach the ribs to the sections over their entire surface, two joining processes are particularly suitable, which are to be used depending on the geometry of the rib. MAG welding is used for elongated cooling fins. The cooling fins are to be provided with two chamfers and to be connected to the sections by a double HV seam over the entire surface. Stud welding is suitable for round geometries due to its very short welding times and very good mechanical properties of the joining zone. In addition, no additives are necessary. Preparation is limited to cutting the ribs to the required length and the manual skill required to operate a stud welder is minimal.

The other decisive factor of the cooling fin, the geometry, is also achieved by evaluating different criteria. The heat flow related to the contact area, the heat flow related to the projected area, the heat flow related to the weight of the cooling fin, and the thermal conductivity of the geometry are considered. After evaluating the various geometries, a bar rib with a hole is chosen.

However, since this geometry is associated with high manufacturing costs, a tubular rib is simulated, which achieves an even better result. Since open geometries cannot be joined with stud welding, a modification, MARC welding, must be used. The chosen geometry of the tube is optimized with regard to the outside and inside diameter. For reasons of cost, only standardized diameters are considered. The optimum results with an outside diameter of d = 30 mm and a wall thickness of T = 5 mm. Another series of simulations and their evaluation gives the result that the cooling fin with a length of I = 36 mm delivers the best possible result.

The consideration of the material flow shows that there are areas with higher and lower particle velocities. Due to the more turbulent flow and the additional task of reducing the particle speed, more ribs are to be installed in the higher speed areas than in the lower particle speed area. In addition, the ribs are to be arranged in a staggered manner. This ensures that material flows directly against each rib. Another positive effect of the selected geometry is that turbulence of the product occurs behind the rib, which further improves the heat transfer through a more turbulent flow. With the help of the heat transfer coefficients determined for the different positions under temperatures within the cooler, it can be determined that the ribs have an almost identical positive influence on the transferred heat flow along the cooler.

A list of the weight difference depending on the number of cooling fins used shows the possible potential of the optimized tube fins. The economic optimum can be determined from the costs due to the increasing assembly effort in relation to the material saved, the weight and the resulting further possible savings with an increasing number of cooling fins. As a result, the corresponding economic and technical design of the cooler must be carried out.

Claims (12)

1. A rotary tube apparatus for cooling or heating flowable granular materials, with structures mounted on its walls for increasing the heat-transferring surface area as well as the thermal conduction, characterized in that the rotary tube apparatus has sections and that hollow tubes (10) are arranged in the sections.
2. The rotary tube apparatus according to claim 1, characterized in that the hollow tubes (10) are arranged in rows extending in the longitudinal direction of the rotary tube apparatus.
3. The rotary tube apparatus according to claim 2, characterized in that two adjacent rows of hollow tubes (10) respectively have an offset arrangement of the hollow tubes (10).
4. The rotary tube apparatus according to one of claims 1 to 3, characterized in that the hollow tubes (10) are mounted by screws, adhesive bonding or rivets on the walls of sections (9).
5. The rotary tube apparatus according to one of claims 1 to 3, characterized in that the hollow tubes (10) are mounted by a welding process, in particular submerged arc welding, metal inert gas welding, friction welding, stud welding or MARC welding.
6. The rotary tube apparatus according to one of claims 1 to 5, characterized in that the hollow tubes (10) have a length of less than 10 cm.
7. The rotary tube apparatus according to claim 6, characterized in that the hollow tubes (10) have a length of 3.6 cm.
8. The rotary tube apparatus according to one of claims 1 to 7, characterized in that the hollow tubes (10) have a diameter of less than 5 cm.
9. The rotary tube apparatus according to one of claims 1 to 8, characterized in that the hollow tubes (10) have a wall thickness of 1 cm or less.
10. The rotary tube apparatus according to one of claims 1 to 9, characterized in that it is subdivided into at least three sections (9) and in that the sections have a higher density of hollow tubes (10) on the radial walls and on the peripheral wall (B, B', B") than in the corner areas (A, A', A") between the radial walls and between the radial walls on the one hand and the peripheral wall on the other.
11. The rotary tube apparatus according to claim 10, characterized in that the sections (9) respectively have approximately 500 ribs or 500 hollow tubes (10) per metre of length of the rotary cooler.
12. A method for operating a rotary tube apparatus according to one of claims 1 to 11, characterized in that the free-flowing solid material is fluidized on the hollow tubes (10) during rotation of the rotary tube apparatus.

Images