TECHNICAL FIELD

The present invention concerns a heat- recovery combustion apparatus.

In particular, the present invention finds an advantageous, but not exclusive, application for ceramic kilns, which will be explicitly referred to in the following description, without thereby losing its' general character.

BACKGROUND ART

In the field of ceramic kilns it is known the use of heat-recovery combustion apparatuses, also " called "self-recovery combustion apparatuses". ^■

For example, the patent application EP-A1-2 278 244 (SACMI) describes the use of a burner of the aforesaid kind in a ceramic kiln.

As already known, self-recovery burners are widely employed, since they allow to increase the thermal efficiency while reducing fuel consumption.

The self -recovery burner illustrated and described in EP-A1-2 278 244 (SACMI) is a . free flame .burner comprising fuel gas feeding means, oxidant gas feeding means and exhaust fumes scavenging means . The burner can suck the exhaust fumes generated by means of combustion, using them for pre-heating an oxidant gas through a ceramic heat recovering pipe . In particular, the exhaust fumes flow flows counter current to the secondary air flow, thus heating it before it is mixed with the products of primary combustion.

In other words, at least some of the latent heat contained in the exhaust fumes is used for preheating the secondary air. The heat exchange occurs through the aforesaid ceramic heat recovering pipe .

In the past, attempts have been made to use self-recovery burners employed in fields close to the firing of ceramic articles.

In the present context, "ceramic articles" means items such as tiles, sanitary ware, roof tiles, bricks, etc.

Such ceramic items have therefore been obtained by compression of ceramic powders, or have been produced by casting of slurry or by extrusion; both kinds of items can be enamelled or raw.

A first stage of testing tried to employ in ceramic kilns some burners which had been successfully used in furnaces for the thermal treatment of metal articles.

A self-recovery burner, for example advantageously used in furnaces for thermal treatments, has been described in the patent EP-Bl-0

773 407 (WS Warmeprocesstechnik GmbH) . However, these burners, expressly designed for working for other applications, have shown several problems when used with kilns for firing ceramic articles.

In fact, the burner described in the aforesaid patent EP-Bl-0 773 407 (WS Warmeprocesstechnik GmbH) works very well in the applications for which it had been designed, namely in furnaces for the thermal treatment of metals (e.g., in draining or stretching furnaces) , characterized by very controlled reducing environments with a low inlet of combustion products in the furnace chamber.

However, as previously stated, said burners are not suitable to ceramic kilns because, when thermal power is increased in said kilns, they develop a huge amount of fumes entering the furnace chamber and putting in overpressure the chamber itself. Therefore, the furnace chamber must be designed so that it is capable to sustain huge stresses due to overpressure and obviously, as a consequence, this remarkably increases the building costs of the furnace .

Furthermore, as for instance described in EP-A1- 2 278 244 (SACMI) , the different chambers for firing transiting ceramic materials, with usually varying thermal conditions, follow one another, and therefore a possible section overpressure should be avoided because it could cause the migration of fumes to adjacent sections, with negative consequences on the quality of the finished ceramic product which is not therefore subjected to a suitable thermal cycle.

Moreover, if a high power is applied to the heat-recovery burners of the aforesaid type, as usually required by ceramic kilns, they will give very hot fumes, with environmental negative consequences and low returns of the heat recovering pipe.

DISCLOSURE OF INVENTION

Therefore, it is the main object of the present invention to provide a heat-recovery combustion apparatus lacking the aforesaid drawbacks and, at the same time, easy and economical to produce.

Another object of the present invention is to provide an innovative gas burner which can be advantageously used in the aforesaid heat-recovery combustion apparatus .

According to the present invention, therefore, a heat-recovery combustion apparatus and a recovery gas burner are produced according to what claimed in the independent claims, or in any one of the claims directly or indirectly dependent on said independent claims . BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention it will be now described a preferred embodiment, purely as a non limitative example and with a reference to the alleged drawings, wherein:

- figure 1 shows a longitudinal section of the heat-recovery combustion apparatus which is the main object of the present invention;

figure 2 shows an enlarged view of some details of the apparatus of figure 1;

- figure 3 shows a three-dimensional view of swirling means for the secondary oxidant gas used in the apparatus according to figures 1, 2;

- figure 4 shows a three-dimensional view of a heat recovering pipe used in the apparatus according to figure 1, 2;

- figure 5 shows an enlarged view of the A-A section of the apparatus shown in figure 1;

- figure 6 shows a first three-dimensional view of a combustion head, mounted at the distal end of a burner used in the apparatus according to figure 1;

- figure 7 shows a second three-dimensional view of the combustion head according to figure 6.

BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1, reference number 100 indicates as a whole a heat -recovery combustion apparatus, in particular for ceramic kilns, which is the main object of the present invention.

As shown again in figure 1, the apparatus 100 comprises a burner 10, of the type which will be better described hereinafter, a heat recovering pipe 20, recovering the heat contained in the exhaust fumes, and scavenging means 30 of the exhaust fumes.

The apparatus 100 is mounted on a wall (WL) of a kiln (KL) for firing ceramic articles. The wall (WL) is made of a refractory material.

More precisely, at least a part of the apparatus 100 is housed inside a through hole (HL) pierced in the aforesaid wall (WL) .

Actually, in the embodiment shown in figure 1, the scavenging means 30 are directly fastened to the wall (WL) with known and not shown means.

Said scavenging means 30 comprise, in turn, a fumes box 31 linked to an evacuation duct 32 directing the combustion products to an exhaust ^{^} chimney ^' (no ^~ shown) .

As shown in figure 1, the fumes box 31 is partially padded with an insulating material (IS) .

The fumes box 31 has a bottom 31A provided with a through hole 33 symmetric to a longitudinal axis (X) (figure 1) .

With a particular reference to figure 4, 5 the heat recovering pipe 20 comprises, in turn, a main tubular body 21 from which straight radial projections 22 protrude outwards, which are arranged in a regular radial pattern with regard to the axis (X) . It must be specified that the radial projections 22 are "straight" in that each longitudinal axis of each radial projection 22 is substantially parallel to the longitudinal axis (X) .

The straight radial projections 22 remarkably develop in height if compared to the diameter of the main tubular body 21.

In this specific case, the projections 22 are twelve, but their effective number essentially depends from time to time by the amount of heat which must be exchanged between the outgoing . fumes and the incoming secondary air.

The main tubular body 21 ends on the one side with a flaring 23 whereas, on the opposite side, it ends with a "bottleneck" tapering 24 (figure 4).

By the way, with the same burner 10 the heat recovering pipe 20 (whose aim is composing different combustion apparatuses) can be selected according to its geometrical characteristics in order to guarantee the passage and the transmission mode of the heat amount required for that particular treatment.

The whole heat recovering pipe 20 can be advantageously, but not necessarily, made of a refractory ceramic material.

As shown again in figure 1, the through hole 33 is partially closed by a flange 40, also provided with its through hole 41.

In actual use, the flange 40 is fastened on the fumes box 31 by means of screws 60, only one of which is visible in figure 1. Furthermore, the flange 40 holds a shaped sealing ring 50 made of a special refractory material. The shaped sealing ring 50 is housed in the aforesaid through hole 33 and avoids the leaking of fumes in the environment .

The burner 10 comprises a fastening flange 11 provided with a plurality of through holes 12. In actual use, each through hole 12 is equipped with a traditional fixing device 13.

Therefore, in the assembled configuration, the fastening flange 11 is locked to the flange 40, tightening in the meanwhile the flaring 23 of the heat recovering pipe 20 arranged between the flange 40 and the fastening flange 11.

Obviously, interposed gaskets are provided among the various mutually packed elements, although said gaskets will not be described in detail.

Moreover, the burner 10 comprises a fuel gas feeding duct 14 whose longitudinal symmetry axis is the aforesaid axis (X) . The fuel gas flows in the feeding duct 14 according to a direction and an orientation defined by an arrow (Fl) .

In particular, the fuel gas is methane CH4 or GPL; but the present apparatus 100 has been designed to work with any kind of fuel , both in the form of a gas and in the form of a nebulised liquid fuel.

Furthermore, the burner 10 comprises an oxidant feeding box 15 provided, in turn, with a feeding junction 15A and with an oxidant distribution chamber 15B ending with the aforesaid fastening flange 11. The oxidant distribution chamber 15B is crossed by a portion of the feeding duct 14.

In particular, the oxidant gas is air, but the present apparatus 100 can also work with other types of oxidant gas, such as, for instance, pure oxygen.

A primary air feeding duct 16 is arranged coaxially with respect to the fuel gas feeding duct 14 and to the heat recovering pipe 20.

In the embodiment illustrated in figure 1, the duct 16 starts at the fastening flange 11 and is hold in place with regard to the duct 14 and to the heat recovering pipe 20 by special spacers (not shown) .

It is thus created a first passage 56, inside which the primary air flows according to a direction and an orientation defined by an arrow (F2) . Said first passage 56 is therefore defined by the annular space limited on the one side by the fuel gas feeding duct 14 and, on the other side, by the duct 16.

By the way, the first passage 56 also houses a pair of electrodes (not shown) for the ignition of the primary mixture and the control of the relating flame .

Analogously, between the duct 16 and the inner cylindrical surface of the heat recovering pipe 20 a second passage 57 is defined, in which the secondary air flows according to a direction and an orientation represented by an arrow (F3) .

The inner surface of the through hole (HL) , together with the outer surface of the heat recovering pipe 20, defines a third passage 58 for discharging the combustion fumes flowing to the scavenging means 30 according to a direction and an orientation represented by an arrow (F4) .

Swirling means 65 of the kind shown in more detail in figure 3 are arranged again at the fastening flange 11, and, thus, also at the beginning of the duct 16.

Said swirling means 65 comprise a main cylindrical body 65A provided, on its outer surface, of a plurality of helical projections 65B helically wound around the main cylindrical body 65A (figure 3) .

As shown in figure 1, the main cylindrical body 65A is fitted on the proximal end of the duct 16, whereas the ends of the helical projections 65B rest on the inner walls of the oxidant distribution chamber 15B.

In another embodiment, not shown, the swirling means 65 are moved forward, namely in the second passage 57, substantially at the height of the flaring 23 of the heat recovering pipe 20.

In a further embodiment, not shown, the swirling means 65 are arranged in a region in between the oxidant distribution chamber 15B and the flaring 23 of the heat recovering pipe 20.

In other words, the swirling means 65 are in a region 85 in between the oxidant distribution chamber 15B and the heat recovering pipe 20; said region 85 comprises the end portion of the oxidant distribution chamber 15B, the flaring 23 of the heat recovering pipe 20 and the proximal end of the duct 16, which divides the secondary air and the primary air.

In other words, the swirling means 65 are arranged at the beginning of the secondary air path, in order to create swirls in the secondary air of the second passage 57.

Of course, the main purpose is to maximize the heat exchange between fumes and secondary air through the heat recovering pipe 20.

In a first group of embodiments, not shown, the swirling means 65 can be formed in one piece with at least one of the following elements comprised in the region 85:

- the end portion of the oxidant distribution chamber 15B; and/or

- the flaring 23 of the heat recovering pipe 20; and/or

- the proximal end of the duct 16.

Moreover, in a second group of embodiments, not shown, the swirling means 65 can be fastened in any way (welding, mechanical interference, etc.) to at least one of the following elements comprised in the region 85:

- the end portion of the oxidant distribution chamber 15B; and/or

- the flaring 23 of the heat recovering pipe 20; and/or

_, - the proximal end of the duct 16.

The arrangement of the swirling means 65 just at the very beginning of the second passage 57 remarkably influences the thermal flow in the heat recovering pipe 20, said thermal flow occurring between the hot fumes discharged through the third passage 58 and the secondary air typically entering at room temperature the second passage 57.

Also the presence of the aforesaid radial projections 22 of the heat recovering pipe 20 plays a relevant role on the increase of the heat exchange efficiency between hot fumes and secondary air.

In other words, the coexistence of radial projections 22 and swirling means 65 (these latter being arranged at the beginning of the heat recovering pipe 20) leads to exceptional and unexpected thermal efficiency, thus causing a remarkable cooling of the fumes which are therefore released in the environment at temperatures lower than those of prior art solutions.

As shown in figure 1, the fuel gas, the primary air and the secondary air are directed to a combustion head 80, shown in more detail in figure 2 and forming the end part of burner 10.

The combustion head 80 is substantially shaped as a circular plaque fitted on the distal ends of ducts 14 and 16.

As shown in more detail in figures 6, 7, the combustion head comprises in turn a first disc 82, whose diameter is substantially equal to the one of duct 16, on which a second disc 83 is overlying, whose diameter is substantially equal to the one of duct 14.

Between the two discs 82, 83 there is a cylinder 84 connecting said two discs 82, 83. The two discs 82, 83 and the cylinder 84 are advantageously made en bloc.

Two through holes 82A crossed in use by the aforesaid electrodes (not shown) are provided on disc 82. Helical projections 82B are provided on the periphery of disc 82 for performing a swirl distribution of primary air.

A plurality of through holes 82C, again for primary air, is circumferentially arranged. Said through holes 82C have inclined axes converging on a point lying on the aforesaid axis (X) .

A plurality of holes 82D, through which the cooling air of disc 82 flows, is then arranged between said through holes 82C and said helical projections 82B. Obviously, said cooling air will then take part in the combustion and in the formation of the flame.

In order to achieve optimal results the helical projections 82B for distributing primary air must have the same direction of inclination of helical projections 65B (figure 3) comprised in swirling means 65 of the secondary air, so that both swirls (of primary air and, respectively, of secondary air) have the same direction of rotation, to the right (or clockwise) in this case. It has been experimentally observed that this . fact, that is to have the same direction of rotation of the two series of helical projections 65B, 82B, leads to a better combustion resulting in a considerable reduction of NOx.

On the other hand, with regard to the fuel gas, observing again figures 6, 7 it can be noted that it flows radially through a series of through holes 84A provided on the cylinder 84, and axially through a series of through holes 83A (having an axis parallel to axis (X)) provided on the second disc 83.

As well known in the field of combustion the fuel gas and the primary air exiting the aforesaid openings provided on the circular plaque 81 are mixed downstream of the combustion head 80, thus forming a primary mixture which is ignited by the electrodes (not shown) .

Therefore, the primary combustion occurs in a combustion chamber 90 ideally axially enclosed by the combustion head 80 and by the tapering 24. The combustion chamber 90 is also laterally limited by the inner surface of the heat recovering pipe 20.

From the primary combustion takes form the so- called "dart" (not shown) which is hottest part of the flame. The periphery of the dart is then invested by the swirls of secondary air coming up through the second passage 57. As known, the primary combustion is thus completed.

The secondary combustion air does not reach the dart in a particularly swirling state, because the swirls have been formed only at . the beginning of the second passage 57 thanks to the swirling means 65. In the remaining part of the second passage 57 the secondary air flow regularizes, thus becoming almost laminar. In fact, it has been experimentally observed that, in order to obtain an optimal combustion in this kind of burners, it is preferable that the secondary air flow reaches the dart (given by the combustion of fuel gas with primary air) in the most delicate way possible, namely as free of swirls as possible .

In other words, in this kind of burners the secondary air swirls, as already stated, are quite useful only in the starting phase of the second passage 57 whose aim is improving the heat exchange with the exhaust fumes, but the secondary air swirls must be avoided, as far as possible, at the combustion head 80.

The (primary and secondary) combustion products are then expelled to the chamber of the kiln (KL) for ceramic articles through the tapering 24. The possible presence of the tapering 24 and the extent of reduction of cross section possibly occurring in the tapering 24 itself are given by the design parameters and are choices dictated by the desired inlet speed of the fumes in the chamber of the kiln (KL) .

The main advantages of the previously described heat-recovery combustion apparatus can be summarized as follows:

- remarkable improvement of the heat exchange between the combustion products and the secondary air;

- remarkable cooling of the fumes, with resulting environmental benefits;

- fast evacuation of the fumes from the kiln chamber, even in case of huge amounts;

- drastic reduction of the danger of possible overpressure in the kiln chamber; and

- outflow of the fumes almost by natural draft, resulting in a saving due to the reduced power of the aspirators and to the absence of ejectors for expelling the fumes through the chimney .