GB2070211A

GB2070211A - Heat exchanger for a domestic central heating boiler

Info

Publication number: GB2070211A
Application number: GB8103379A
Authority: GB
Original assignee: Saunier Duval Eau Chaude Chauffage SDECC SA
Current assignee: Saunier Duval Eau Chaude Chauffage SDECC SA
Priority date: 1980-02-22
Filing date: 1981-02-04
Publication date: 1981-09-03
Also published as: FR2476808B1; DE3105598A1; FR2476808A1; DE3105598C2; GB2070211B

Abstract

A central heating boiler heat exchanger comprises an array of longitudinal tubes 4 with an elongated cross-section arranged along the generatrix of a cylinder to form a cylindrical inner combustion chamber 9 in such a way that the side surfaces 11 of the said tubes define between them a substantially constant gap 12 of a small width a, the length b of the side surface 11 in relation to the inter-spacing width a is greater than or equal to the flame-arresting distance. <IMAGE>

Description

SPECIFICATION Heat exchanger for a domestic central heating boiler The present invention relates to a heat exchanger for a domestic hot water central heating boiler of the room-sealed forced draught type.

In a known embodiment of a forced draught boiler, where the combustion chamber is constituted by an array of longitudinal water tubes arranged around a central burner whose combustion gases are emitted radially and pass through this array of tubes, the best heat exchange conditions between the water and gas are obviously sought, for example by means of the shape and positioning of the tubes.

Those longitudinal water tubes may have a circular or elliptical cross-section or have different contours on the upside and downside as, for instance, tubes with a so-called orange segment cross-section or tear-drop cross-section.

Moreover, in this kind of device, the combustible mixture is supplied by a fan upstream of the combustion chamber which is therefore situated in an enclosure sealed off from the atmosphere of the premises wherein the apparatus is installed. It will therefore be seen that it is necessary to make provision for a preliminary clearance of the combustion circuit before any lighting operation within this circuit.

This preliminary clearance consists in causing the fan to operate to evacuate possible accumulations of the combustible mixture and thus to avoid the risks of explosion.

But in the case where this preliminary clearance has not taken place, for example because of mis-handling or defective working or if the strength of the gaseous mixture is outside the flammability limits, an accumulation of combustible gas in the combustion chamber will occur and, the moment the igniter is caused to operate, an explosion may take place with the attendant risks entailed thereby.

The present invention aims to provide a new type of heat exchanger with an array of water tubes having an elongated cross-section arranged concentrically with respect to a radial burner which avoids the risk of explosions by affording additional safety in the event that an accumulation of the combustible mixture occurs in the apparatus.

According to the present invention we provide a heat exchanger for a water heating domestic boiler of the room-sealed forced draught type comprising an array of longitudinal tubes with an elongate cross-section, with the side surfaces of adjacent tubes having rectilinear profile portions bounding between themselves a substantially constant gap of small thickness, the said tubes being disposed along the generatrix of a,cylinder to define a cylindrical inner combustion chamber, wherein the ratio between the dimension corresponding to the length of the substantially rectilinear portion of the tube profile and the separation defining the gap between the said tubes, is greater than or equal to the flamearresting distance so as to establish an explosion-resisting barrier.

Other special characteristics and advantages of the invention will become apparent from the following description of one embodiment with reference to the accompanying drawings, in which: Figure 1 is a partly cut away side elevational view of the heat exchanger; Figure 2 is a half cross-section of the heat exchanger wherein only a part of the array of water tubes has been shown, in order to clarify the Figure; and Figure 3 is a view of the water tubes on an enlarged scale.

As shown in the Figures, the exchanger comprises within an enclosure 1 which collects the burned gases and evacuates them via flue 2, a radial burner 3 arranged at the centre and at least one array of vertically extending water tubes 4 concentrically around the burner 3. The water admitted by an inlet tube 5 into a lower collector 6 flows through a cluster of the tubes 4 to reach the top collector 7 then it returns again via another tube cluster to the bottom collector and so on before ending at the outlet pipe 8. Between the inlet pipe 5 and the outlet pipe 8, the water has therefore traversed the exchanger circuit via successive tube clusters.

The array of tubes 4 forms, in enclosure 1, two distinct concentric compartments; the first, the so-called combustion chamber 9 proper, is situated on the inside around burner 3 and the second, the exhaust chamber 10 for the combustion products, is situated between the downstream side (with reference to gas flow) of tubes 4 and the external walls of enclosure 1.

Tubes 4 have a cross-section elongated in such a way that the side surfaces 11 of each tube 4 should be substantially parallel to the side surfaces of the two adjacent tubes 4 of the same array so as to obtain between each one of them a substantially constant gas flow gap 1 2 of small width and yet bounded by two large substantially planar surfaces, facilitating the laminar flow of the hot gases from combustion chamber 9 towards the evacuation chamber 1 0.

Gap 1 2 (Figure 3) between each tube 4 is defined by an interspacing "a" which is less than or equal to 1.5 mm extending over the whole length "b" (which is greater than or equal to 3 mm) of the substantially rectilinear part of the tube profile. One thus has a gap whose cross-section is greater than or equal to the arresting distance of the flames.

If, for any reason, the combustible mixture arrives via burner 3 in the combustion chamber 9 without ignition, it passes through the array of tubes 4 along gaps 12. This is easily effected since the pressure losses between the combustion chamber 9 and the evacuation chamber 1Q, are very low. The combustible mixture therefore has a tendency to accumulate in the evacuation chamber 10.

When the burner becomes operative, the combustible mixture in the combustion chamber 9 is ignited. But the flames cannot pass through the array of tubes 4 because the length "b" of the gaps 1 2 in relation to interspacing "a" is greater than or equal to the arresting distance, so much so that the combustible mixture accumulated in the evacuation chamber 10 does not ignite and the chemical reaction produced by the combustion within the combustion chamber 9 is arrested at the tubes 4.

This heat exchanger thus forms an explosion-resisting barrier and prevents the risk of explosion if an accumulation of explosive mixture occurs in the apparatus.

Claims

1. A heat exchanger for a water heating domestic boiler of the room-sealed forced draught type comprising an array of longitudinal tubes with an elongate cross-section, with the side surfaces of adjacent tubes having rectilinear profile portions bounding between themselves a substantially constant gap of small thickness, the said tubes being disposed along the generatrix of a cylinder to define a cylindrical inner combustion chamber, wherein the ratio between the dimension corresponding to the length of the substantially rectilinear portion of the tube profile and the separation defining the gap between the said tubes, is greater than or equal to the flamearresting distance so as to establish an explosion-resisting barrier.

2. A heat exchanger according to claim 1, wherein said length of the rectilinear profile portion of the tubes defining said gaps is greater than or equal to 3 mm.

3. A heat exchanger according to claim 1 or 2, wherein said separation between the tubes defining the gaps is smaller than or equal to 1.5 mm.

4. A heat exchanger according to claim 1, 2 or 3, wherein each said gap is of constant width in that said separation is uniform along a directi,on radially of the heat exchanger cyclinder and each tube has a cross-section which is divergent when considered along a direction radially outwardly of the heat exchanger cylinder.

5. A heat exchanger for a water heating domestic boiler substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.

6. A domestic water heating boiler of the room-sealed forced draught type, comprising a heat exchanger according to any one of the preceding claims.