EP0691516A1

EP0691516A1 - Heat generator with triple-pass flue gas circulation

Info

Publication number: EP0691516A1
Application number: EP95108261A
Authority: EP
Inventors: Riccardo Belleli
Original assignee: Belleli SpA
Current assignee: Belleli SpA
Priority date: 1994-07-04
Filing date: 1995-05-30
Publication date: 1996-01-10
Also published as: IT1271205B; ITMI941395A1; ITMI941395A0

Abstract

A heat generator with triple-pass flue gas circulation comprising a flue (2), which constitutes the first flue gas pass and is connected, at its ends, respectively to a front tube plate (3), with which a burner is associable, and to a rear tube plate (4). There is also a tube nest (5), which forms the second pass, and there are tubes (6) that form the third flue gas pass. The flue (2) has, at the openings (10, 11) for connection to the tube plates (3, 4), a cross-section that is smaller than the median transverse cross-section of the flue (2) itself.

Description

The present invention relates to a heat generator with triple-pass flue gas circulation.
Conventional heat generators with triple-pass flue gas circulation are typically constituted by a flue connected, at its ends, to a front tube plate and to a rear tube plate. A first tube nest is provided around the flue and in practice forms the second flue gas circulation, which is in turn connected to variously arranged tubes that form the third flue gas pass.
In conventional solutions, the flue has a substantially cylindrical shape and is connected to the burner at one end; accordingly, it is necessary to provide a flange, made of refractory material, capable of withstanding the high temperatures caused by combustion.
Another problem resides in the fact that due to the differential thermal expansions of the flue and of the flue gas tubes, a certain minimum distance must be kept on the tube plates between the region where the flue is connected and the first series of tubular elements that constitute the tube nest, with a consequent considerable increase in the size of the heat generator.
Accordingly, the solutions of the known art are generally quite bulky, also in view of the fact that the main dimensions of the flue are due to the need to have a sufficient volume for combustion and a flue surface that absorbs an amount of heat sufficient to limit the temperature and accordingly the formation of nitrogen oxides.
A principal aim of the invention is indeed to solve the problem described above by providing a heat generator with triple-pass flue gas circulation in which it is possible to significantly reduce the overall dimensions while maintaining an adequate distance between the region where the flue couples to the tube plate and the first row of tubes that constitute the first tube nest.
Within the scope of the above aim, a particular object of the invention is to provide a heat generator in which the heat exchanges with the heat-exchange fluid present in the vessel or casing of the heat generator are optimized, and which can also eliminate or in any case significantly limit any formation of condensation in the third flue gas pass.
Another object of the present invention is to provide a heat generator which by virtue of its particular constructive characteristics is capable of giving the greatest assurances of reliability and safety in use.
Another object of the present invention is to provide a heat generator with triple-pass flue gas circulation, according to the invention, comprising: a flue, which constitutes the first pass and is connected, at its ends, respectively to a front tube plate, with which a burner is associable, and to a rear tube plate; a tube nest, which forms the second pass; and tubes that form the third flue gas pass; characterized in that said flue has, at the openings for connection to said tube plates, a cross-section that is smaller than the median transverse cross-section of said flue.
Further characteristics and advantages will become apparent from the description of a preferred but not exclusive embodiment of a heat generator with triple-pass flue gas circulation, illustrated only by way of non-limitative example in the accompanying drawings, wherein:

figure 1 is a schematic longitudinal sectional view of a heat generator according to the invention;
figure 2 is a main sectional view of the heat generator;
figure 3 is a view of a heat generator in which the tubes that form the third pass are located outside the casing;
figure 4 is a schematic view of a heat generator in which the first tube nest is arranged to the side of the flue;
figure 5 is a view of a heat generator in which the tubes that form the third pass are placed in contact with the casing;
figure 6 is a view of a particular embodiment of the tubes that form the third pass;
figure 7 is a longitudinal sectional view of a heat generator with only one narrow opening.

With reference to the above figures, and particularly to figure 1, the heat generator with triple-pass flue gas circulation according to the invention, generally designated by the reference numeral 1, comprises a flue 2 that constitutes the first flue gas pass and is connected, at its ends, respectively to a front tube plate 3 and to a rear tube plate 4.
There is also a tube nest, generally designated by the reference numeral 5, that forms the second flue gas pass and is operatively associated with tubes 6 that form the third flue gas pass.
The important particularity of the invention resides in the fact that the flue 2 has, at its front and rear openings 10 and 11 for connection to the front plate 3 and to the rear plate 4 respectively, a cross-section that is smaller than the transverse cross-section of the flue.
In particular, with a flue having a substantially circular cylindrical shape, the openings 10 and 11 have a narrower diameter than the median portion, and a shoulder 12 is thus formed at the axial ends of the flue 2 which performs a dual function; furthermore, as shown in figure 7, it is possible to provide only the narrow opening 10, while the opening 11a has the same diameter as the flue.
First of all, it allows to provide, on the tube plates 3 and 4, an adequate minimum distance between the region where the openings 10 and 11 are connected and the region where the tube nest 5 is connected; however, said tube nest is arranged in practice very close to the outer surface of the flue.
This solution furthermore allows to have a large-volume flue, with consequent advantages for combustion and with an increased overall surface (since the end faces must be considered); the larger surface allows greater heat exchange, mainly due to radiation, with a consequent lower temperature and reduced formation of nitrogen oxides.
Another important aspect is furthermore constituted by the fact that the particular shape of the axial end of the flue, especially at the burner coupling region, allows to use a flange 20 which, since it is not directly exposed to combustion, needs not be made of refractory material or is in any case not subjected to wear.
Another aspect that must be stressed is constituted by the fact that the shoulders 12 in practice form a recessed region at the ends of the flue, so that the fluid contained inside the container or casing 21 of the generator optimizes heat exchange and furthermore acts as a cooling element for the flange 20.
As previously mentioned, the tube nest 5 is arranged close to the outside wall of the flue, thus significantly reducing the overall dimensions of the heat generator and therefore its cost.
The third flue gas pass can be optionally provided outside the casing, as shown in figures 1 and 2, thus allowing to further limit the overall dimensions.
It is also possible to provide a solution, as shown in figures 5 and 6, in which the tubes that form the third flue gas pass, generally designated by the reference numeral 30, are constituted by contoured tubes that are in contact with the casing and are for example welded on said casing.
In the case of tubes that are separate from the casing, there is no particular need to limit heat exchange in order to avoid condensation, whereas in the case of tubes that are arranged in contact with the casing, the ducts can be partially or fully insulated, or optionally it is possible to provide, inside the tube or inside part of the extension of the tube, a C-shaped sheet 31 for protecting the wall of the casing from corrosion caused by any condensation in the transients and to limit heat exchange, preventing the formation of dew.
The C-shaped sheet also has the purpose of protecting the weld between the tube and the casing which, by expanding, would tend to reduce the part of its surface that is in contact with the exchange wall, accordingly limiting heat exchange.
With the basic concept that constitutes the subject of the invention, namely the use of a flue having a narrower cross-section at the openings for connection to the tube plates, it is possible to use any distribution of the tube nest which, besides being concentric as shown in the above proposed solutions, can be arranged to the side of the flue, as shown in figure 3.
It should be added to the above that with the described solution all flue gas passes are easily accessible for cleaning by simply opening the door on which the burner is mounted.
The preferred constructive solution for the heat generator, which is particularly advantageous for high heat power levels, is to provide a flue with a circular cross-section, arrange the tubes of the second pass in an annular region that is very close to the wall of said flue, and provide ducts of the third pass that are fully separated from the casing, but without interposing any material between the ducts and the casing.
Heat can thus be transmitted between the ducts of the third flue gas pass and the water contained in the casing, recovering heat and thus contributing significantly to an increase in the efficiency of the generator.
The insulation surrounds the casing and the duct together, not separately, so as to facilitate this heat recovery.
From the above description it is thus evident that the invention achieves the intended aim and objects, and in particular the fact is stressed that a heat generator with triple-pass flue gas circulation is provided which allows to considerably reduce the overall dimensions and accordingly the costs, without however having drawbacks of any kind, since the tube nest, despite being close to the flue, is slightly spaced in the regions where it is coupled to the tube plates.
The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept.
All the details may furthermore be replaced with other technically equivalent elements.
In practice, the materials employed, as well as the contingent shapes and dimensions, may be any according to the requirements.
Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

Heat generator with triple-pass flue gas circulation comprising: a flue (2), which constitutes the first pass and is connected, at its ends, respectively to a front tube plate (3), with which a burner is associable, and to a rear tube plate (4); a tube nest (5), which forms the second pass; and tubes (6) that form the third flue gas pass; characterized in that said flue (2) has, at least at one of the openings (10,11) for connection to said tube plates (3,4), a cross-section that is smaller than the median transverse cross-section of said flue (2).
Heat generator according to claim 1, characterized in that said flue (2) has, at both said openings (10,11) for connection to said tube plates (3,4), a cross-section that is smaller than the median transverse cross-section of said flue.
Heat generator according to the preceding claims, characterized in that said flue (2) has a substantially circular shape in transverse cross-section and in that said openings (10,11) have a diameter that is smaller than the diameter of the median portion of said flue (2).
Heat generator according to one or more of the preceding claims, characterized in that it comprises, at at least one of the axial ends of said flue (2), a shoulder (12) for mutually joining the openings (10,11) for connection to the tube plates (3,4) and the outer surface of the flue.
Heat generator according to one or more of the preceding claims, characterized in that it comprises, at both of said axial ends of said flue (2), a shoulder (12) for mutually joining the openings (10,11) for connection to the tube plates (3,4) and the outer surface of the flue.
Heat generator according to one or more of the preceding claims, characterized in that the open ends of the flue (2), said front tube plate (3), and said rear tube plate (4) are substantially identical to each other.
Heat generator according to one or more of the preceding claims, characterized in that said tubes (30) that form the third pass are arranged outside the casing (21) that contains said flue (2) and said tube nest (5).
Heat generator according to one or more of the preceding claims, characterized in that said tubes (30) that form the third pass are constituted by ducts associated with the extrados of said casing (21).
Heat generator according to one or more of the preceding claims, characterized in that it comprises an insulation wrapping around said casing (21) and said tubes (6) that provide said third passage.
Heat generator according to one or more of the preceding claims, characterized in that it comprises, in at least one portion of the extension of said tubes (30) that form the third pass, a C-shaped element (31) placed in contact with said casing (21).
Heat generator according to one or more of the preceding claims, characterized in that said C-shaped element (31) is adapted to protect the wall of the generator from corrosion caused by possible condensation, to limit heat exchange, and to modify the surface of contact by thermal expansion.
Heat generator according to one or more of the preceding claims, characterized in that it comprises an insulating layer interposed between said tubes (6,30) that form the third flue gas pass and said casing (21).