FR2669414A1 - Heat-exchanger method and exchanger for implementing this method - Google Patents

Abstract

The exchanger comprises a casing passed through, from its face 2 to its face 3, by a flow of fluid to be heated. Radiator tubes with incorporated gas burner (41 to 45) are distributed transversely in the casing, in a zigzag. Corrugated partitions (51 to 56) divide the stream of fluid into adjacent elementary streams, between which no flow of fluid is possible. Constrictions (7i i) regulate the flow in each elementary stream, upstream of each radiator tube. Application to improving the efficiency and the uniformity of the heating of a flow of fluid.

Description

 The present invention relates to a heat exchange process and to a heat exchanger designed for the implementation of this process. More particularly, the invention relates to such an exchanger designed to uniformly heat a stream of a gaseous fluid by passing over heating elements, using raw, manufactured or recovered natural gas (for example biogas from biomass). In the following, the expression "heating element" or "radiant tube" will mean organs of the type defined above, elongated transversely to the direction of circulation of the fluid.

 We know from French patent application No.

89 04214 of March 24, 1989 and filed on behalf of GAZ DE
FRANCE, a heating device comprising in particular an exchanger 7 delimiting a chamber 39 in which are arranged U-shaped radiant tubes arranged transversely to the general direction of circulation of a fluid to be heated. In the embodiment described in FIG. 2 of this patent application, plates 71 fitted with fins 75 constitute baffles extending the path of the fluid inside the chamber 39 by promoting its mixing, the fluid thus recovering calories concentrated around the plates.

 There are also known exchangers consisting of several parallel layers of elongated heating elements, superimposed in the thickness of a stream of a fluid to be heated. The heating elements are arranged transversely to the direction of flow of the fluid.

They thus constitute obstacles to the progression of the fluid, obstacles which are moreover necessary for bringing the fluid into contact with the heating elements. However, the current lines of the fluid, deflected from one sheet to the other by their meeting with these heating elements, create erratic turbulences in the flow, which affect the uniformity of the heating of the vein of fluid.

The efficiency of the heat transfer operation is adversely affected.

 The present invention therefore aims to provide a heat exchange process, and to provide an exchanger for the implementation of this process, which ensure good uniformity of heat transfer, throughout the thickness of the stream to be heated, so as to improve the overall efficiency of the heat exchange between the fluid and the heating elements.

 This object of the invention is achieved, as well as others which will appear on reading this description, with a process of heat exchange between a fluid and a bundle of parallel cylindrical radiant tubes arranged in superimposed layers, in a volume crossed by a stream of fluid substantially perpendicular to the axes of the radiant tubes and parallel to the planes of the layers of tubes. According to the invention, the fluid stream is divided into several independent elementary veins shaped to each envelop only the radiant tubes with a single layer to create substantially identical fluid flow conditions around each tube, throughout the volume. .

 According to another characteristic of the method according to the invention, the flow conditions are reset in each elementary stream, upstream of each of the successive radiant tubes of the same layer, by passing the fluid through an elongated constriction, of substantially section constant throughout the volume.

For the implementation of this method, the invention provides a heat exchanger of the type which comprises a plurality of radiant tubes arranged in parallel layers in a box traversed by a stream of fluid substantially perpendicular to the axes of the tubes and parallel to the planes of the layers of tubes. According to the invention, this exchanger further comprises partitions each interposed between two adjacent plies, two partitions each framing the plies to delimit a corresponding elementary stream of the fluid
Advantageously, the partitions are corrugated, the pitch of the waves of the partitions being substantially equal to that of the tubes in each ply so that each partition passes between two plies at a substantially constant distance from the elements of the two plies, dividing the current of fluid in veins. adjacent elementaries.

 The corrugated partitions are arranged relative to each other so as to delimit, upstream of each radiant tube, an elongated throttle of substantially constant section throughout the box, of the elementary stream of fluid which passes over the tube.

 According to a preferred embodiment of the invention, the radiant tubes are regularly distributed in staggered rows in the box. The partitions are made of corrugated sheet.

Other characteristics and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which
- Figure 1 is a schematic perspective view of
the exchanger according to the present invention,
- Figure 2 is a longitudinal sectional view of
the exchanger of FIG. 1, and
- Figure 3 is a detail view, in axial section
of one end of a radiant tube fitted
the exchanger according to the invention.

 The exchanger shown in Figure 1 of the accompanying drawing, comprises a parallelepipedic box 1 open on two opposite faces 2 and 3. A stream of gaseous fluid to be heated enters through face 2 and exits through face 3. Radiant tubes, generally cylindrical, are mounted transversely in the box 1 along n superimposed beds of m elements 41 '42' 43 '44' 45 respectively. In the exchanger shown, n = 5 and m = 3.

The radiant tubes may for example be of the type described in GAZ DE FRANCE publication 84.04 entitled "High temperature gas radiant tubes". Such radiant tubes include a gas burner and are capable of reaching a surface temperature of 1000 "C. They have the advantage of allowing the heating of an enclosure at high temperature without the combustion gases coming to mix. to the product heated in this enclosure.

 The fluid which circulates in the box 1 therefore heats up in contact with the radiant tubes 41 to 45. If the box contains only the radiant tubes, it is understandable that the obstacles which these oppose to the circulation of the fluid cause in the box a turbulent and uncontrolled macrocirculation of the fluid, generating local non-uniformities of the temperature thereof. A good thermal transfer requiring uniformity of transfer throughout the volume delimited by the enclosure, it appears that this turbulent macrocirculation degrades the efficiency of thermal transfer.

 According to the present invention, the heat transfer is homogenized in the fluid stream, inside the box 1, by dividing this stream into several independent and adjacent elementary veins shaped to envelop only each of the radiant tubes in one of the layers, in substantially uniform heat exchange conditions throughout the volume delimited by the box 1. This division is obtained by interposition between the adjacent beds of radiant tubes, partitions 51 to 5 6 According to a preferred embodiment, these partitions are corrugated. In Figure 2, the relative positions of these partitions and tube beds are shown more precisely. The radiant tubes at 45 being regularly arranged in staggered rows as shown in this figure, the corrugated partitions 51 to be used have the same pitch as the tubes and are arranged so as to partially envelop them, at a substantially constant distance from their external surface. .

 Thus the partitions 51 and 52 for example, define a longitudinal passage in the box 1, for an elementary stream of fluid entering through a slot 21 delimited by these two sheets at the level of the face 2 of the box 1. The elementary stream of fluid passes first in a transverse neck or constriction 7 delimited by two longitudinally adjacent extrema of the undulations of the partitions 51 and 52. After having licked the radiant tube 41 'by dividing into two branches, the elementary vein is reconstituted to cross a second constriction 712 geometrically identical to throttle 711.

 The fluid flow conditions in the constriction 711 then reproduce in the constriction 712 'before the vein divides again to pass over the next heating element of the same bed. We thus "reinitialize", in a way, the flow upstream of each radiant tube, over the entire length of the box, and in each of the adjacent elementary veins delimited by pairs of neighboring corrugated partitions. copied into a plurality of throttles 711 to 754 regularly distributed in the volume of the box, each volume element of the fluid to be heated encounters in its passage through the box, a sequence of identical flow and heating conditions which ensure the uniformity of the heating of the fluid exiting from the face 3 of the box, throughout the thickness of the flow of outgoing fluid constituted by the parallel gathering of the various elementary veins. It will also be noted that, according to the invention, the corrugated partitions oppose an uncontrolled turbulent macrocirculation of the fluid in the box, generating heating non-uniformities.

 The partitions also absorb radiation that has not been absorbed by the fluid. They thus create new heat exchange surfaces to heat this fluid. The total exchange surface is substantially doubled with the consequence of an improvement in the efficiency of the exchange.

 It is further observed that the partitions tightly envelop the surface of each radiant tube which ensures good heat transfer, uniform and high efficiency, the tube being almost enclosed between two partitions which follow its surface at a constant distance.

 The staggered arrangement of the tubes makes it possible to heat the partitions over practically the entire surface of their two opposite faces. Thus these partitions are completely used both for guiding and heating the fluid.

 Various technological characteristics of the exchanger according to the invention appear in FIGS. 1 and 2. In FIG. 1, it can be seen that the inlet 2 and outlet 3 faces of the box are lined with peripheral flanges 8 and 9 respectively. In FIG. 2, plates 10, 11 appear which surround the lateral surface of the box 1, these plates being made of material suitable for thermally insulating this box. Handling elements 12 and support 13 can be incorporated into the box.

 In FIG. 1 also appear, across the inlet face 2 of the box, notched spacers 14 suitable for supporting the edges of the corrugated partitions, in sheet metal for example, which are installed in the box. The box itself can be made in sheet metal. Figure 3 shows in axial section the means then used to support the sheets and radiant tubes on the side face 22 'of the box which is hidden in Figure 1 and which is opposite to the face 22 visible in this figure.

This face 22 is moreover pierced with openings arranged in staggered rows for the introduction of the radiant tubes into the box, these tubes being supported in these openings, at the level of the face 22. The face 22 is also constituted by a sheet flat, the adjacent edges of two corrugated sheets, 51 and 52 for example, are supported by U-shaped profiles (IPN) 15 and 16 respectively, themselves welded to the sheet 14.

 When the radiant tubes are of the recovery thermowell type, as shown in FIG. 8 of the aforementioned GAZ DE FRANCE document, these tubes have a stud at the end opposite to that which is carried by the face 22 of the box. of support 17. To avoid an overhang of the radiant tube in the box 1, a ring 18 is welded on the face 22 of the box and the pin 17 of the radiant tube 41 is engaged in this ring.

 Note in the diagram of Figure 3, the wide adjustments of the support elements relative to the supported members, radiant tubes or corrugated sheets. Large thermal expansions are thus acceptable whereas, all these means being internal to the casing 1, the clearances thus established are without disadvantage due to the absence of pressure gradient at the level of these clearances.

 Spacing means 19, 20 are provided between the sheets to prevent them from moving substantially relative to one another, in particular under the effect of thermal expansion. These spacing means can be constituted by a rivet 19 passing through a spacer ring 20, for example. Any other suitable means, for example, can be substituted for these means, at the choice of a person skilled in the art. Fins 21 can be used to separate the corrugated sheets from the heating elements.

 It will be noted that the casing of the exchanger according to the invention is capable of absorbing the inevitable thermal expansions undergone without difficulty. In addition to the games of the support means for the radiant tubes and the sheets, it is clear that the corrugated shape of these sheets which separate the elementary fluid streams ensures excellent absorption of these expansions.

It was thus possible to produce an exchanger having the following characteristics and performances, given only by way of example
- interior volume of the box: 3.2 m3
- number of radiant tubes: 15
- yield on PCI: 72%
- thermal power: 550 kw
- treated air flow: 16,000 kg / hour
- incoming air temperature: 1800C
- exhaust air temperature: 300 "C
Of course, the invention is not limited to the embodiment described and shown which has been given only by way of example. Indeed, it is clear that the exchanger according to the invention could be adapted to the cooling of a stream of fluid rather than to its heating. The shape of the sheets used to constitute the separating partitions isolating the various elementary veins, could differ from that shown in Figure 2. Thus one could use partitions made of sheet folded according to trapezoidal slots, if economic considerations, for example, militate in favor of such an achievement. Similarly, the supply of the radiant tubes could be modulated, for example as a function of the position of the tube in the box, so as to further improve the regularity of the heat exchange.

Claims (9)

 1. A method of heat exchange between a fluid and a bundle of parallel cylindrical radiant tubes arranged in superimposed layers, in a volume traversed by a stream of fluid substantially perpendicular to the axes of the radiant tubes and parallel to the planes of the layers of tubes, characterized in what we divide the fluid stream into several independent and identically shaped elementary veins each enveloping only the radiant tubes with a single layer to create substantially identical fluid flow conditions around each tube, throughout the volume.  2. Method according to claim 1, characterized in that the flow conditions are reset in each elementary stream, upstream of each of the successive radiant tubes of the same layer, by passing the fluid through an elongated constriction of substantially constant section throughout the volume.  3. Heat exchanger for implementing the method according to any one of claims 1 and 2, of the type which comprises a plurality of radiant tubes to 4), arranged in parallel sheets in a box (1) crossed by a flow of fluid substantially perpendicular to the axes of the tubes and parallel to the planes of the layers of tubes, characterized in that it comprises partitions (51 to 5 each interposed between two adjacent layers, two partitions each framing the layers to delimit a corresponding elementary vein of fluid.  4. Exchanger according to claim 3, characterized in that the partitions are corrugated, the pitch of the waves of the partitions being substantially equal to that of the elements in each ply so that each partition passes between two plies at a substantially constant distance from the tubes of the two tablecloths.  5. Exchanger according to claim 4, characterized in that the corrugated partitions (51 to 5 are arranged relative to each other so as to delimit, upstream of each radiant tube, an elongated constriction (711 to 754) of section substantially constant throughout the box, of the elementary stream of fluid which passes over the tube.  6. Exchanger according to any one of claims 4 and 5, characterized in that the radiant tubes (41 to 45) are regularly distributed in staggered rows in the box.  7. Exchanger according to any one of claims 4 to 6, characterized in that the partitions at to 56) are made of corrugated sheet.  8. Exchanger according to any one of claims 4 to 6, characterized in that the partitions are made of sheet metal folded in trapezoidal slots.  9. Exchanger according to any one of claims 4 to 8, characterized in that spacing means (19, 20; 21) are provided between the partitions between them and between the partitions and the radiant tubes to maintain the dimensions transverse elementary veins.