WO2010037719A2

WO2010037719A2 - High efficiency heat exchanger element

Info

Publication number: WO2010037719A2
Application number: PCT/EP2009/062552
Authority: WO
Inventors: Peter De Jaeger; Jan Van Peteghem
Original assignee: Bekaert Combust. Technol. B.V.
Priority date: 2008-10-03
Filing date: 2009-09-28
Publication date: 2010-04-08
Also published as: WO2010037719A3

Abstract

The present invention relates to a co-cast heat exchanger element intended for a central heating boiler, which heat exchanger element is made from substantially aluminum, the heat exchanger element being provided with walls which enclose a water carrying channel, and with at least one wall which encloses at least one flue gas draft to which a burner can be connected, at least one wall which encloses the at least one flue gas draft being water-cooled in that it also forms a boundary of the water-carrying channel, while one of the water-cooled walls is provided with heat exchanging surface enlarging wires and/or strips which extend in the respective flue gas draft.

Description

High efficiency heat exchanger element

Technical Field

[0001] The present invention relates to a co-cast heat exchanger element intended for a central heating boiler, which heat exchanger element is made from substantially aluminum, the heat exchanger element being provided with walls which enclose a water carrying channel, and with at least one wall which encloses at least one flue gas draft to which a burner can be connected, at least one wall which encloses the at least one flue gas draft being water-cooled in that it also forms a boundary of the water- carrying channel, while at least one of the water-cooled walls is provided with heat exchanging surface enlarging pins and/or fins which extend in the respective flue gas draft and is also provided with other heat exchange surface enlarging metallic structures.

[0002] The present invention also relates to a method for obtaining such a co-cast heat exchanger element and its use in a central heating boiler.

Background Art

[0003] For a good heat transfer in a heat exchanger, two main characteristics have to be taken into account: the actual heat transfer, being the merit and the pressure drop, expressing the required effort.

[0004] Nowadays, the most commonly used systems for domestic and industrial heating of water for central heating and tap water all comprise finned/pin finned or blank tubes which are circulating around a flue gas draft, wherein the flue gases are produced by a burner.

[0005] The heat transfer mechanism in these systems can be described as follows: the heat in the flue gases is transferred to the pins or fins on the flue gas draft walls, by the rules of convection. From there, heat is transferred to the walls of the flue gas draft, which also are the walls of the water channels of the water to be heated by the boiler, via conduction rules. The water passing through the water channels of the boiler is then heated via convection rules. [0006] The heat exchange in such systems is thus dependent on conduction and convection rules and this conduction and convection can be tailored by adapting the form and structure of the heat exchange enlarging fins and/or pins.

[0007] The pressure drop, which is inevitably related to the heat exchanging capacity of the system, should be kept as low as possible to reduce the required pumping power. Thereby minimising size, weight, cost and power consumption of pumping devices.

[0008] Prior art already describes some heat exchangers for boilers adapting the pin and or fin configuration accordingly.

[0009] EP 1722172 describing a heat exchanger for a boiler wherein the cross- sectional surface of the pins and/or fins is smaller than 25 mm²; the heat exchanger is a mono-casting. Such heat exchanger, with pins with a length of e.g. 15 mm and having a greater surface-content ratio, has a low weight. This results optimally in a thermal inertia of 0,16 kg/kW, which makes the heat exchanger element heating up much more rapidly, thereby reducing the time required for obtaining hot water for domestic use. Such heat exchanger, due to the smaller length of the pins and/or fins, has a smaller cross-section of the flue gas draft. This leads to a higher flow velocity of the flue gases and results in a higher heat transfer coefficient and thus a better efficiency. However, the lengths of the pins and or fins in this heat exchanger are limited, due to casting limitations, to lengths of 25 mm. And also casting limitations, e.g. cold flow, have a limiting effect on the length/diameter ratio. The limited length/diameter ratio forces the width of the flue gas draft to smaller dimensions in order to keep more optimal heat extraction from the flue gases. The smaller the equivalent pin diameter the shorter the pin will be. Typical: for an equivalent diameter of 4mm, the pin can be 15mm long. So using pins with a diameter smaller than 4mm is only possible with a narrow flue gas draft; thus reducing the total amount of heat exchanging surface and increasing the pressure drop considerably.

[0010] The use of such rather small flue gas drafts also reduces the serviceability of the flue gas draft of the heat exchanger element. [0011] The known heat exchanger element is already relatively small for a boiler with such an output. When this boiler is used for heating not only central heating water but also domestic hot water, there is still a need for further improving the compactness, and for a still more rapid heating of the domestic hot water. Due to decreasing flue gas speeds and available temperature difference between the water and the flue gasses, the biggest effort to extract heat out of the flue gasses has to be made in the last part of the heat exchanger. Therefore, increasing the heat exchanging surface area towards the end of the heat exchanger is favourable.

[0012] Traditional casting technology has limitations in increasing this surface because of limited pin diameters, pin lengths and pin configurations, e.g. sand core becomes weak when pins are positioned to close to each other. The use of smaller pins also has the advantage to increase the heat transfer coefficient of the flue gas flow around the pin.

Disclosure of Invention

[0013] An aspect of the claimed invention provides a heat exchanger element for a condensing boiler with improved efficiency.

[0014] In another aspect of the claimed invention provides a heat exchanger element intended for condensing boiler having a higher output than the known boilers with comparable dimensions, the intended heat exchanger element being more compact and having low weight.

[0015] To this end, the heat exchanger element according to the invention is manufactured as a co-casting product from substantially aluminium, the heat exchanger comprising the features of claim 1.

[0016] The heat exchanger element for condensing boiler has a very high design freedom. The heat or energy available in the flue gases can be efficiently extracted without increasing the pressure drop in the flue gas draft due to the heat exchange enlarging structures used. By using co-casted pins with a smaller diameter and a denser configuration (smaller horizontal and vertical pitch) one can create a heat exchanging structure with up to 25% more heat exchanging surface area and the same density than the state of the art mono-cast heat exchangers with reduced pin structure. Thus giving a higher heat exchange for the same pressure drop. [0017] The use of the wires and/or strips, with their great surface-content ratio and heat exchanging action, makes it possible to cool down the flue gas and to transfer the heat efficiently to the water-cooled walls.

[0018] In a preferred embodiment the cross sectional surface of the wires and/or strips is smaller than 12 mm². Thickness of the strips smaller than 2mm. Preferred pitches are smaller than 6mm.

[0019] Preferably, the metal wires and/or strips are straight. In another preferred embodiment, the metal wires and/or strips are profiled and/or preformed. In an alternative preferred embodiment, the metal wires and/or strips are preformed in a 3D structure, e.g. one wire which is pleated and cast into both opposing walls of the heat exchanger element.

[0020] In another preferred embodiment the cross sectional surface of the wires and/or strips are different over the entire heat exchanger. The wires and/or strips can be randomly distributed, or they can gradually have a smaller diameter in the direction of flow of the flue gases in the flue gas draft. This enables the tuning of the optimal convection and conduction resistances to the energy which is still available for extraction in the flue gases.

[0021] The distance between the wires and/or strips can be chosen in relation to the desired pressure drop over the complete flue gas draft. More preferably, the distance between the wires and/or strips is such that an increasing density of the pin structure towards the bottom of the heat exchanger is obtained.

[0022] The incorporation of the metal wires and/or strips into the heat exchanger element is a relatively simple method: this metal wires and/or strips are incorporated in the internal sand core of the heat exchanger during the casting process. Alternatively, the metal wires and/or strips are built in into the, e.g. polystyrene, positive model in a lost foam casting process.

[0023] Surprisingly, it was found that the metal wires and/or strips were not affected by the hot molten metal, cast onto the wires and/or strips and that a good metallic bond was obtained between the wires and/or strips and the cast heat exchanger element. And also that the aluminium-oxides present at the surface of the aluminium material did not inhibit a good connection between the metal wires and/or strips and the heat exchanger element. The metal wires and/or strips stay intact into the complete co- cast structure and are properly surrounded by the melt, creating a big contact surface, which results in a very good heat transfer.

[0024] Hence, with the heat exchanger element according to the invention, a central heating boiler can be made having a greater output than the known central heating boilers with comparable dimensions, while the same or even a better degree of compactness and thermal inertia is achieved.

[0025] The heat exchanger element is manufactured as a co-casting, comprising the steps of claim 6, 7, 8, 9 or 10, and can be manufactured in a relatively quick and efficient manner.

[0026] According to a further aspect of the invention the wall forming the boundary between the water carrying channel and the flue gas draft in the heat exchanger element may further be provided with heat exchange surface enlarging pins and/or fins which are cast integrally with the heat exchanger element around the metal wires and/or strips.

[0027] According to another aspect of the invention the flue gas draft has a minimal width of 40 mm. Such flue gas draft provides the further advantage of a decreased pressure drop, requiring smaller ventilator capacity, and efficient heat or energy extraction from the flue gases. Another advantage of such large conduits is the good serviceability of the heat exchanger element.

[0028] Another aspect of the invention relates to a central heating boiler comprising at least one heat exchanger element according to the invention.

DEFINITIONS

[0029] The heat exchanger element is made from substantially aluminium meaning that the heat exchanger element can be made out of pure aluminium or an aluminium alloy. Wherever in this description is referred to metal, aluminium or one of its alloys is referred to. It should be noted that the terms metal, aluminium and aluminium-alloy will be used throughout this text without meaning anything else than aluminium or one of its alloys. [0030] The term co-casting is explained in claim 5, and can be described in short as casting around an already existing product which stays intact during that casting. [0031] The term "pitch" should be understood as meaning the distance between two adjacent pins or fins or wires or strips either in horizontal or vertical direction.

Brief Description of Figures in the Drawings

[0032] Fig. 1 is a perspective view of an exemplary embodiment of a heat exchanger according to the invention.

Fig. 2 is an exemplary embodiment of a sectional view taken on the plane M-Il' of Fig. 1.

Fig. 3 is another exemplary embodiment of a sectional view taken on the plane III-IH' of Fig. 1.

Fig. 4 is a perspective view of an alternative exemplary embodiment of a heat exchanger according to the invention.

Fig. 5 is an exemplary embodiment of a sectional view taken on the plane V-V of Fig. 4.

Fig. 6 is another exemplary embodiment of a sectional view taken on the plane Vl-Vl' of Fig. 4. Reference numbers list

1 heat exchanger element

2 walls

3 water-carrying channel

5 burner

6 combustion chamber

7 flue gas draft

8 fins

9 pins

10 metallic wires and/or strips

Mode(s) for Carrying Out the Invention [0033] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual reductions to practice of the invention.

[0034] Figures 1 , 2 and 3 shows exemplary embodiments of the heat exchanger 1 according to the invention. Heat exchanger 1 is manufactured as a co- casting substantially from aluminium. The heat exchanger comprises a number of walls 2, which walls enclose on one side a water carrying channel 3 and on the other side a flue gas draft 7. The flue gas draft 7 extends from the burner space 6. The burner space 6 is intended for accommodating a burner, as shown for example in fig. 2, as reference number 5. Preferably, the burner is a metal fiber burner membrane, as described in WO 2004/092647. The flue gas draft comprises metal wires made from substantially aluminium which connect two opposite water cooled walls 2 of the flue gas draft 7. Alternatively, the heat exchanger comprises metal strips of substantially aluminium. In another alternative embodiment the heat exchanger comprises both wires and strips for heat exchange surface enlargement. Preferably, the pins and/or wires have a cross-sectional surface which is smaller than 12 mm². Figure 2 shows one example embodiment of the present invention wherein the flue gas draft is filled with equal-spaced wires. Figure 3 shows another example embodiment wherein the flue gas draft is filled with wires, wherein the wires closest to the burner chamber 6 have a bigger cross section and are at bigger distance, also called pitch, from each other, whereas further downstream the wires are gradually getting smaller in cross section and are closer to each other.

[0035] In a preferred embodiment, the cross sectional surface of the metal wires and/or strips are varied over the flue gas draft. In an even more preferred embodiment the cross sectional surface of the metal wires and/or strips is gradually decreasing in the direction of the flue gases, this together with a decreasing pitch of the different pins, tuning the conductive and convective resistances to the energy still available in the flue gases. This more efficient heat transfer structure can be translated in more compact and lower weight heat exchanger elements for exchanging the same amount of power (kW's).

[0036] The heat exchanger element 1 is preferably manufactured by means of a casting process, such as, for instance, sand casting or die-casting. Preferably, use is then made of at least one core to form the water channel and at least one second core for forming the flue gas channel(s). These flue gas draft cores comprise the metal wires and/or strips. Alternatively, also a lost foam casting process can be used. The metal wires and/or strips sand core is then build in into the (polystyrene) foam positive model. Alternatively, in lost foam casting, the metal strips and/or wires can be build in into the (polystyrene) foam positive model. The metal wires and/or strips will than be filled with the sand used for the lost foam casting, and no separate step for making a sand core is necessary.

[0037] The heat exchanger 1 of figures 1 and 4 are produced by the sand co- casting process. First a core box for sand casting of the heat exchanger element is provided. A mixture of sand and binder is then blown into the void space in the core box thereby obtaining a sand core, which is subsequently left to harden. Thereafter the core box is removed. A plurality of metal wires and/or strips is then integrated into this sand core by insertion of said metal wires and/or strips through the sand core. This "metal wire and/or strip - sand core" is then integrated in a flue gas draft sand core. The wire or strips van also be integrated into the mould before the sand is blown in. In an alternative embodiment, the sand core is already the complete flue gas draft sand core. This flue gas draft sand core is placed in a moulding box together with a water side core, which is sufficiently known in the art and of which no further details will be given. Molten metal is then poured into the moulding. The cast workpiece is left to cool and thereafter the sand cores are removed. This results in exemplary heat exchanger element 1 as depicted in figures 1 to 6. [0038] In an alternative embodiment, the heat exchanger element 1 is made via a lost foam co-casting method. Here, the production of a metal wires and/or strips containing heat exchanger element comprises following steps. First, a metal wires and/or strips-sand core, obtained as in paragraph 37, is build in into a polystyrene pattern (or positive) of the heat exchanger element and further prepared as known in the art. The "polystyrene pattern - metal wires and/or strips sand core" hybrid cluster is placed into the casting flask and backed-up with un-bonded sand. After the mold compaction, the polystyrene pattern is poured with the molten metal. Then only a relative simple filter action is needed to remove the un-bonded sand from around, and out of, the co-cast heat exchanger element. And also the sand of the metal wires and/or strips-sand core needs to be removed. Alternatively, a plurality of metal wires and/or strips is built into the polystyrene pattern of the heat exchanger element. Then also the metal wires and/or strips will be backed up with unbonded sand, which will be easily removed after co-casting of the heat exchanger element

[0039] Figure 4 shows an alternative embodiment of the invention. Same reference numbers describe same structures as in figure 1. The embodiment of figure 4 is similar to the embodiment in figure 1 , so only the differences will be explained. As can be seen in figures 5 and 6, in a first part of the flue gas draft, pins and fins are cast together with the heat exchanger element, providing less risk of a bad connection in the highly loaded first part of the heat exchanger element, where the pins are in contact with the flame. In the exemplary embodiment of figure 5, the remainder of the flue gas draft is filled with equal-spaced wires. Figure 6 shows another example embodiment wherein the remainder of the flue gas draft is filled with wires, wherein the wires closest to the burner chamber 6 have a bigger cross section and are at bigger distance, also called pitch, from each other, whereas further downstream the wires are gradually getting smaller in cross section and are closer to each other.

[0040] A first worked example embodiment as in figure 1 , gives a heat exchanger element with an output of approximately 35 kW. The weight of the heat exchanger element per kW to provide, is less than 0,25 kg/kW. In the present exemplary embodiment, the thermal inertia is only 0,2 kg/kW with a compactness of 5,5 kW/l, resulting in a heat exchanger element of 7,0 kg and a volume of 6,41. The water carrying channel has a volume of 1 ,3 litre.

[0041] An alternative worked example embodiment as in figure 4, gives a heat exchanger element with an output of approximately 25 kW. For this exemplary embodiment, the thermal inertia is also only 0,2 kg/kW with a compactness of 5,5 kW/l, resulting in a heat exchanger element of 5,0 kg and a volume of 4,61.

[0042] Figures 7A, 7B and 7C show some examples of different embodiments of the present invention. Figure 7A shows an exemplary corss section of the flue gas draft wherein the wires and/or strips are corrugated or pleated. The wires and/or strips connect the two opposing water-cooled walls. Figure 7B shows single wires or strips which are in zig zag shape such that one wire or strip has multiple contacts with both water cooled walls. Figure 7C shows strips in the flue gas draft which are positioned in an angle with respect to one another, as an example of possible strip configurations.

[0043] There is thus described a new heat exchanger element intended for central heating boilers, which is very compact and at the same time is able to extract more energy out of the flue gases. This new heat exchanger element is a co-cast heat exchanger element made from substantially aluminum, the heat exchanger element being provided with walls which enclose a water carrying channel, and with at least one wall which encloses at least one flue gas draft to which a burner can be connected, at least one wall which encloses the at least one flue gas draft being water- cooled in that it also forms a boundary of the water-carrying channel, while one of the water-cooled walls is provided with heat exchanging surface enlarging wires and/or strips which extend in the respective flue gas draft.

Claims

1. A heat exchanger element (1) comprising walls (2) from substantially aluminium, said walls (2) enclosing at least one water carrying channel (3) and having at least one flue gas draft (7), at least one wall forming a boundary between said water carrying channel (3) and said flue gas draft (7), said at least one wall at least being provided with a plurality of heat exchange surface enlarging wires and/or strips (10) from substantially aluminium, which extend in the flue gas draft (7), characterised in that said walls are cast around said wires and/or strips to form a co-cast heat-exchanger element.

2. A heat exchanger element (1) according to claim 1 , wherein said wires and/or strips (10) have a cross sectional surface which is smaller than 12 mm².

3. A heat exchanger element (1) according to claims 1 or 2, wherein said wires and/or strips (10) are spaced apart on a pitch which is equal to or smaller than 6 mm.

4. A heat exchanger element (1) according to claims 1 to 3, wherein said at least one wall is further provided with heat exchange surface enlarging pins and or fins (8,9) which are cast together with the heat exchanger walls.

5. A heat exchanger element (1) according to claims 1 to 4, wherein said flue gas draft has a minimum width of 40 mm.

6. Process for the production of a metal wires and/or strips containing heat exchanger element, said process comprising the steps of: a) providing a core box; b) blowing a mixture of sand and binder into the void space of said core box, thereby obtaining a sand core; c) hardening said sand core; d) removing said core box; e) integrating a plurality of metal wires and/or strips into said sand core by insertion of said metal wires and/or strips through said sand core; f) integrating said "metal wire and/or strip - sand core" in a flue gas draft sand core; g) placing said flue gas draft sand core in a moulding box together with a water side core; h) pouring molten metal into said moulding; i) cooling of the cast workpiece; j) removing the sand cores.

7. Process for the production of a metal wires and/or strips containing heat exchanger element, said process comprising the steps of: a) providing a flue gas draft-core box; b) blowing a mixture of sand and binder into the void space of a flue gas draft- core box, thereby obtaining a flue gas draft sand core; c) hardening said flue gas draft-core; d) removing said core box; e) integrating a plurality of metal wires and/or strips into said sand core by insertion of said metal wires and/or strips through said sand core; f) placing said flue gas draft sand core in a moulding box together with a water side core; g) pouring molten metal into said moulding; h) cooling of the cast workpiece; i) removing the sand core.

8. Process for the production of a metal wires and/or strips containing heat exchanger element, said process comprising the steps of: a) providing a flue gas draft-core box; b) placing the wire inside of the core mould c) blowing a mixture of sand and binder into the void space of a flue gas draft- core box, thereby obtaining a flue gas draft sand core; d) hardening said flue gas draft-core; e) removing said core box; f) integrating a plurality of metal wires and/or strips into said sand core by insertion of said metal wires and/or strips through said sand core; g) placing said flue gas draft sand core in a moulding box together with a water side core; h) pouring molten metal into said moulding; i) cooling of the cast workpiece; j) removing the sand core.

9. Process for the production of a metal wires and/or strips containing heat exchanger element via lost foam investment casting, said process comprising the steps of: a) providing a flue gas draft-core box; b) blowing a mixture of sand and binder into the void space of a flue gas draft- core box, thereby obtaining a flue gas draft sand core; c) hardening said flue gas draft-core; d) removing said core box; e) integrating a plurality of metal wires and/or strips into said sand core by insertion of said metal wires and/or strips through said sand core; f) building in said "metal wire and/or strip - sand core" into the polystyrene pattern of the heat exchanger element; g) coating of the polystyrene pattern - "metal wire and/or strip - sand core" hybrid cluster with ceramic; h) drying the ceramic coating; i) placing said polystyrene pattern - metal wire and/or strip hybrid cluster into a casting flask and backing up said cluster with un-bonded sand; j) performing mold compaction; k) pouring the polystyrene pattern with the molten metal;

I) cooling of the cast workpiece; m) removing the sand cores.

10. Process for the production of a metallic porous body containing heat exchanger element via lost foam investment casting, said process comprising the steps of: a) providing a polystyrene heat exchanger pattern; b) integrating a plurality of metal wires and/or strips into said polystyrene pattern by insertion of said metal wires and/or strips in said polystyrene pattern; c) coating of the polystyrene pattern - "metal wire and/or strip - sand core" hybrid cluster with ceramic; d) drying the ceramic coating; e) placing said polystyrene pattern - metal wire and/or strip hybrid cluster into a casting flask and backing up said cluster with un-bonded sand; f) performing mold compaction; g) pouring the polystyrene pattern with the molten metal; h) cooling of the cast workpiece.

11. Process according to claim 6, 7, 8 or 9, wherein said metal of said metal wires and/or strips is aluminium or an aluminium alloy.

12. A heat exchanger element obtained by the methods as in any of the claims 6 to 10.

13. A heating boiler provided with a heat exchanger element according to any one of the claims 1 , 2, 3, 4, 5 or 12.