Abstract:
A modular boiler having a cylindrical combustion chamber made of three modules and a module comprising a cover and a module comprising an expansion vessel mounted coaxially on the boiler adjacent the combustion chamber at opposite ends thereof. The cover has a frustro-conical configuration with the inner walls thereof diverging from an opening in the cover toward the interior of the combustion chamber at an angle of between 30° and 110°. The combustion chamber is formed of three castings that define the cylindrical combustion chamber and six axial hot gas flow paths spaced circumferentially from each other and disposed axially of and radially of the combustion chamber. Hot gases from the downstream end of the combustion chamber are recirculated to the upstream end of the combustion chamber to improve the combustion. Hot gas is diverted from these hot gas flow paths and flowed spirally of these flow paths along axially spaced flow paths immersed in the water circuit of the boiler to improve heat transfer. Liquid fuel is fed into the combustion chamber from a burner at the opening of the cover. The fuel is mixed with air to which a rotation has been imparted about the axis of the arrangement and entering the opening of the cover. The cover is jacketed and the jacket defines part of the inlet cold water circuit and the heated hot water circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to boilers and more particularly to a new and improved modular boiler. 
     It has already been proposed to recirculate a part of the burned gases in solid, liquid or gaseous fuel boilers by returning them into the burner. On the other hand, it has not been proposed that a recirculation of the gases within the inside of the combustion chamber itself be effected. This recirculation could nevertheless have numerous advantages. It would increase the time of stay of the burned gases in the boiler and therefore would decrease the rate of formation of NO x . This reinjection could be used also to protect the walls of the combustion chamber, at least in part, from the flame, reducing the rate of formation of NO x  by heat. 
     SUMMARY OF THE INVENTION 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to utilize such a type of internal recirculation of the burned gases. 
     For this purpose, the present invention relates to a liquid or gaseous fuel boiler comprising a combustion chamber formed of sidewalls, a bottom, and a cover which has an opening for a burner, a water circulation circuit surrounding said chamber and connecting a source of cold water to a hot water collector, and a burned-gas circulation circuit which is in contact with the water circulation circuit and connects the combustion chamber to at least one exhaust collector. This boiler is characterized by the fact that the burned-gas circulation circuit comprises ducts extending along the combustion chamber which are open on the bottom side of the combustion chamber and are connected to the burned-gas collector, and by the fact that the other ends of the the ducts discharge into the front portion of the the combustion chamber through respective injection passages which are directed so as to form jets of burned gases directed towards the bottom of said chamber. 
     The following description refers to a special boiler forming the object of several inventions. It is pointed out immediately that the features forming the object of the present invention are applicable, with advantage, to other types of boilers, and particularly to boilers provided with a flat cover as well as to boilers having no injection nozzles, as in the case of the boiler described below. It is also obvious that the expansion vessel may be dissociated from the boiler. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawing shows, by way of example, one embodiment of the boiler forming the object of the present invention. 
     FIG. 1 is a sectional view taken along the section line I--I of FIG. 2. 
     FIG. 2 is a sectional view taken along the section line II--II of FIG. 1. 
     FIG. 3 is a sectional view taken along the section line III--III of FIG. 1. 
     FIG. 4 is a sectional view taken along the section line IV--IV of FIG. 1. 
     FIG. 5 is a developed view taken along the section line V--V of FIG. 2. 
     FIG. 6 is a sectional view through a convection duct shown on a larger scale, in which the secondary movements of the gaseous mixture have been shown. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The boiler shown in FIG. 1 is a modular boiler which comprises a hollow cover 1, a bottom 2, three intermediate elements 3, and an expansion vessel 4 fastened to the bottom 2. The cover 1 has an opening 5 adapted to receive a burner 6. This opening 5 communicates with a combustion chamber 7 formed by the inner walls of the cover 1 and of the bottom 2 as well as by the central openings 8 provided through each of the intermediate elements 3. The inner wall of the cover 1 has a spape whose aerodynamic properties have been designed for a purpose which will be explained further below. 
     The bottom of the boiler, which closes off the combustion chamber 7, gives access to six ducts 9 having the shape of annular segments, which are concentric to the longitudinal axis of said chamber 7. 
     Before describing the boiler in further detail, we shall describe an intermediate element 3, with reference to FIG. 2. This element, shown in plan view, is of generally annular shape. On this element there can be noted the central opening 8 as well as the six ducts 9. The opening 8 and the ducts 9 pass through the element 3 which extends between two parallel planes perpendicular to the axis of the opening 8. This element is a hollow cast iron body produced by casting. The inner space 3a (FIGS. 1 and 5) of this hollow element communicates with two openings 10 and 11 which are diametrically opposite each other with respect to the opening 8 and pass through the element 3 parallel to the axis of the central opening 8. The opening 10 is connected to the cold water feed circuit while the opening 11 is connected to the hot water distribution circuit. Six radial segments 12 provided between the ducts 9 connect the body of the element 3, that is to say the portion located outside the ducts 9, to an inner ring 13 which surrounds the central opening 8. These radial segments 12 and the ring 13 are hollow on the inside so that they communicate with the inner space 3a located at the periphery of the ducts 9. 
     As shown in FIG. 1, the ring 13 extends over the entire width or length of the intermediate element 3 so that these rings 13 are assembled alongside of each other. This is not true of the portion of these elements 3 which extends along the periphery of the ducts 9. In this portion, the hollow space does not extend over the entire width or length of the element, the rest of this width or length being occupied by the three ribs 16, 19 and 20 provided on each of the two faces of the element and intended to form convection conduits 17 and 18 between the ducts 9 and the exhaust gas collectors 14 and 15 respectively which are diametrically opposite each other with respect to the axis of the combustion chamber 7. These collectors are closed by covers only some of which, 15&#39;, are visible in FIG. 1. As can be seen from this figure, the convection conduits 17, 18 alternate with the inner spaces 3a of the elements 3. 
     If one refers again to FIG. 2, it will be noted that the provision of the conduits 17 and 18 is obtained by means of two spiral ribs 19 and 20 which are 180° apart from each other and extend around a circular rib 16 forming the periphery wall of the ducts 9. Each of these ducts is connected to the conduits 17 and 18 or even to both of these conduits by two injection nozzles 21 extending over a portion of the length of the conduit, for the purposes which will be explained subsequently. 
     From FIGS. 1 and 3 it can be noted that a series of spaces 22, distributed over the same circumference, is formed between the cover 1 and the inner ring 13 of the modular element 3 adjacent the cover. These spaces 22 cause the downstream ends of the ducts 9 to communicate with the combustion chamber 7 so as to permit the reinjection of a certain amount of hot gases upstream in the combustion chamber and better balance the pressure in the ducts 9. The temperature of the burned gases thus becomes more uniform in these ducts, so that the heat transfer is better distributed. This reinjection favors blue-flame combustion which gives better efficiency and is less noisy than yellow-flame combustion. 
     This film of gas is thus reinjected along the wall of the chamber 7 in a zone which is particularly exposed by virtue of the temperature of the flame. As the reinjected gases are not as hot as the flame, they form a protective film locally. This is of particular importance when the boiler is provided with a cover such as that shown, which, as will be seen subsequently, causes the flame to hug the wall of the chamber. In this case, particularly if the boiler is powerful and has numerous intermediate elements 3, it is advisable that the film of reinjected gas at least partially prevent the flame from coming into contact with this wall and make it possible to avoid reactions between the flame and the carbon of the cast iron of the walls of the combustion chamber. 
     Finally, the internal recirculation of the burned gases causes a diluting of the gases in the boiler and leads to a reduction in the rate of formation of NO x . 
     The bottom 2 of the boiler also has an inner ring 23. The six ducts 9 having the shape of annular segments, commence between said ring 23 and the wall 24 which closes off the chamber 7. Like the other rings 13, the ring 23 communicates on the one hand with an opening 10&#39; and on the other hand with an opening 11&#39;. These openings are located in the extension of the openings 10 and 11 respectively, thus forming a conduit for the distribution of cold water to the boiler and a hot water collector respectively. 
     The bottom 2 also has an annular wall 25 which extends around the wall 24 and creates a communication with the openings 10&#39; and 11&#39;. 
     This annular wall 25 is intended for the attachment of the expansion vessel 4. This expansion vessel 4 has a wall 26 provided with a small opening 27 and is fastened in airtight manner to the end of the annular wall 25 thus forming, except for the opening 27, a closed space between the walls 24 and 26. The expansion vessel also has a diaphragm 28 whose edges are clamped between the edge of the wall 26 and the edge of a receptacle 29. These three elements are assembled on the annular wall 25 by a fastening collar 30. A guide ring 31 is fastened to the back of the wall 26, concentrically to the sidewall of the receptacle 29, and constitutes a guide support when the diaphragm 28 is folded towards the wall 26. This expansion vessel 4 also has an opening 32 through the wall of the receptacle 29, which serves to introduce a fluid between the diaphragm 28 and the receptacle in order to exert a certain pressure on the diaphragm 28. 
     The burner 6 is mounted coaxially to the chamber 7. It has a spiral supply well 36 fastened in the opening 5 of the cover 1. This well 36 is provided with vanes 37 intended to impart a pre-rotation to the jet of recirculated gases and air entering the chamber 7, the well being connected to the recirculation device for the burnt gases (not shown), which is connected to one of the exhaust collectors 14 and 15. 
     In operation, the combustion gases produced in the chamber 7 penetrate into the six ducts 9 having a shape of annular segments and flow in the direction towards the cover 1. As they advance in the ducts 9, the combustion gases enter the spiral conduits 17 and 18 via the injection nozzles 21 provided through the circular ribs 16. These spiral conduits 17 and 18 guide the combustion gases towards the exhaust collectors 14 snd 15 respectively. One of the collectors is connected to the stack while the other is connected to the burner by a recirculation circuit (not shown). As has already been stated, the downstream ends of the channels of the ducts 9 communicate with the combustion chamber 7 via spaces 22. Thus a part of the combustion gases is reinjected into the combustion chamber through the spaces 22. This reinjection, as well as the recirculation of the gases in the burner, assures blue-flame combustion. 
     Various works have shown the curvature effect of a conduit of a given length on the flow of a fluid in said conduit. This curvature effect causes secondary movements within the flow in a plane perpendicular to the direction of advance of the fluid. The arrows included in the sectional view of such a conduit, shown on a larger scale in FIG. 6, indicate the path of these secondary movements. Now, these secondary movements greatly increase the heat transfer between the fluid and the walls of the conduit. They come from the centrifugal effect caused by the curvature, which effect is substantial only if the Dean&#39;s number of the flow is greater than a certain maximum. This maximum is a function of the Prandtl (Pr) number of the fluid, given by the ratio of the kinematic viscosity of the fluid to the thermal diffusivity of this fluid. The Dean&#39;s number is defined by the formula: ##EQU1## in which Re is the Reynolds number of the flow D H  is the hydraulic diameter of the duct Rc is the radius of curvature of the duct. 
     By way of example, it may be stated that for a gas or a gaseous mixture in which Pr is of the order of 0.7, the minimun Dean&#39;s number which must be present in order for the secondary movements to be substantial is about 10. If Pr is about 5 (as in the case of water) De min is about 5 and if Pr is about 30 (as in the case of a light oil), De min is about 1. 
     The presence of injection nozzles 21, located along the inner face of the spiral convection conduits, has the effect of locally reinforcing these secondary movements by a factor which is a function of the difference between the velocities produced by the curvature, along the direction of the radius of curvature, and the velocity of injection. It can be said that if a flow of gas is injected through the nozzles extending through the inner face of the curvature (see FIG. 6) at a velocity 20 times greater than the secondary velocities produced by the curvature, the reinforcement factor of the curvature effects is of the order of 2, which is considerable. 
     The secondary movements effectively distribute the injected gases and make the temperature field at the periphery of the spiral duct more uniform. This results in a greater transfer of heat and a decrease in the thermal stresses in the metal. 
     It has been stated that the cross section of the different injection nozzles 21 decreases from nozzle to nozzle, in the downstream direction of the spiral convection conduits 17 and 18. This feature takes into account the losses in head present upon going from the upstream end towards the downstream end of these conduits and makes it possible to obtain uniform rates of flow for all of the injection nozzles. 
     Aside from the curvature of the convection ducts, the existence of the nozzles has several advantages, particularly the advantage of making the weight rate of flow uniform between the different elements 3 so that the last element will have substantially the same rate of flow as the first element, and moreover of maintaining an intense turbulence in the convection conduits thus increasing the heat transfer coefficient, and finally of reinjecting hot gases into the gases which have already cooled down, which increases the average temperature of the gases and therefore the flow of heat transferred from the gases to the water. 
     One will also note the equiangular arrangement of the nozzles with respect to the longitudinal axis of the combustion chamber 7, which distributes the hot points in the metal uniformly, better distributing the thermal stresses. 
     It will furthermore be noted from FIG. 5 that the cross section of the convection ducts decreases from one nozzle 21 to the next, then increases suddenly again at each nozzle. This cross-section is selected so as to take into account the decrease in volume of the gases as a result of the cooling down thereof and the new conditions resulting from each reinjection. This cross-section is therefore calculated so as to maintain a substantially constant velocity of flow of the gases in the convection ducts. 
     While the combustion gases flow spirally in two separate streams between each element 3, the flow of the water takes place within these elements from the opening 10 to the opening 11. A part of the cold water entering into the inner space 3a of the intermediate elements 3 passes into the ring 13 via the radial segments 12 connecting the body of the element 3 to said ring. 
     Upon the placing in operation of the boiler, a certain pressure is created in the expansion vessel 4 between the receptacle 29 and the diaphragm 28 by introducing a gas under pressure through the opening 32, which is then hermetically closed. When the water is introduced, the pressure within the expansion vessel 4 is equalized via the opening 27. This arrangement of the expansion vessel is advantageous due to the fact that it makes it possible to integrate it in the boiler, thus forming a more compact installation. 
     During the course of the description mention has already been made of certain advantages of the boiler which is the object of the present invention. Still others may be mentioned which make is possible to solve many problems posed by the boilers today on the market. 
     Among such advantages, we may first of all mention the fact that the flow of the combustion gases between the ducts 9 and the collectors 14 and 15 takes place via convection conduits 17, 18, connected in parallel to the ducts 9. This arrangement of the convection conduits in parallel is extremely important due to the fact that it makes it possible to adapt the area of the cross-sections of passage of the combustion gases to the power of the boiler. 
     Each modular element is provided with two convection conduits 17, 18 which lead to two exhaust collectors 14 and 15, which makes it possible to effect the recirculation of the exhaust gases coming from one of the two collectors. 
     As can be noted particularly well from the cross-sectional views of the boiler, its geometry is symmetrical both with respect to the water, feed, and discharge conduits and with respect to the convection conduits and the exhaust collectors. This symmetry makes it possible to have uniformly distributed specific heat loads, thus avoiding strong internal stresses in the cast iron. 
     From these same cross-sectional views of the boiler it can also be seen that the second half of each convection conduit, located downstream of the nozzles 21 which discharge into said conduits, decreases in cross section as one approaches the exhaust collectors 14 and 15. As the cooling of the gases leads to a decrease in their specific volume, their absolute pressure remaining substantially constant, this decrease in cross-section makes it possible to make the velocity of these gases uniform and contributes to a godd heat transfer. Turbulence generators (not shown) can also be placed in these conduits. This measure is however optional. 
     FIG. 1 shows that the ribs 16, 19 and 20 forming the convection conduits 17 and 18 constitute heat transfer vanes for the water circulation ducts. 
     It has been mentioned that the inner wall of the hollow cover 1 is of a special shape which, starting at the opening 5, provides a space of progressively increasing cross section of generally frusto-conical shape with an angle of between 30° and 110°. This cover 1 closes the combustion chamber 7 which is cylindrical. The conical portion connecting the opening 5 to the cylindrical chamber 7 is cooled by the circulation of water within the hollow cover. Moreover, the pre-rotation imparted to the feed gases by the vanes of the spiral well 36 imparts to these gases or to the gas-liquid mixture a turbulent movement which follows the conical portion of the cover. The value of the angle θ is selected as a function of the angular speed imparted to these gears or to the gas-liquid mixture. The inner shape of the cover 1 has the advantage of eliminating the dead eddyings which occur in the corners of boilers with flat covers. This conicity makes it possible to stabilize the flow and to elongate the flame, which spreads out on the periphery of the combustion chamber, located in the extension of the conical portion of the cover. The temperature of the flame is made more uniform and the volume of radiating burned gases is greater, which increases the heat transfer to the wall of the combustion chamber 7. 
     The elimination of the dead eddy which takes place in boilers with a flat cover at the corner between said cover and the combustion chamber, decreases the total loss in head of the boiler and increases the transfer or heat by radiation. This is due to the fact that the dead eddy is relatively cold and constitutes a screen against the radiation of the flame. 
     The suppression of this dead eddy therefore makes it possible to utilize the volume provided within the hollow cover in order to increase the total exchange surface of the boiler. Another reason for this circulation of water in the cover is that the water lowers the temperature of the surface of the cover. This cooling of the wall of the cover reduces the formation of nitrogen oxides NO x  by the action of heat and reactions between the flame and the carbon of the cast iron of the cover.