LU82963A1 - METHOD AND DEVICE FOR RECOVERING HEAT FROM OVEN FUMES, ESPECIALLY GLASS OVENS - Google Patents

Description

DESCRIPTIVE MEMORY

FILED IN SUPPORT OF A REQUEST FOR

PATENT

FORMED BY

SAINT-GOBAIN EMBALLAGE and S. E. T. R. E. M. for

Method and device for recovering heat from smoke from ovens, in particular from glass ovens.

The invention relates to a method and a device for recovering heat from oven fumes, in particular from glass oven fumes.

In order to heat the combustion air of such ovens, it is known to use regenerators formed from stacks of bricks, crossed alternately by the fumes, then by this combustion air.

In smaller ovens, the installation of such regenerators is often less convenient and also too expensive. PVD recuperators are then preferably used. 11 (5F) JL2 79570 which are in fact simple metallic vertical heat exchangers, in which the fumes circulate in principle in an upward movement, taking advantage of the natural draft, before being discharged to the chimney.

There are two main types of exchangers in practice. Some are double-envelope devices, where the air circulates in the peripheral space and the fumes in the central part. The others have a single insulated envelope, in which is placed an annular bundle of hanging tubes, inside which the air to be heated circulates. They are more complex in structure, but allow better performance, due to the higher air heating temperature at the outlet.

In fact, these two types of exchangers are generally combined in recuperators by placing them one above the other and supplying them with a lower flue. The lower exchanger, which operates at high temperature, is a tubular exchanger made of refractory chromium steel, in which an anti-methodic exchange is carried out, that is to say that the air, already heated, circulates there from bottom to top. in the same direction as the hot fumes, which makes it possible to maintain the tubes at a substantially constant temperature, of the order of 750 ° C., over the entire length of the exchanger, and thus to reduce the thermal stresses.

The upper exchanger, operating at relatively low temperature, is a double-shell exchanger made of sheets of austenitic stainless steel with chromium nickel, in which a methodical exchange is carried out, the cold air circulating there from top to bottom, in opposite direction smoke already cooled.

This type of circulation makes it possible to obtain better exhaustion of the heat of the fumes, but it generates higher stresses on the sheets, which however remain acceptable in this relatively cold section.

In the various types of recuperators known up to now, and in particular in the one which has just been described, the main difficulty encountered comes from fouling of the walls exposed to the fumes, fouling which causes partial clogging, which lowers the exchange efficiency of the appliance by causing a drop in the temperature for reheating the combustion air and which is even capable of causing premature deterioration of the installation by the formation of hot spots which cause asymmetrical forces, tube bending and accelerated corrosion.

This fouling comes essentially from a pasty deposit of mixtures of acid salts such as sulphates and, if necessary, borates, which can easily be seen in the flue pipe located at the bottom of the recuperator. This deposition occurs in a temperature zone of the order of 800 ° C, that is to say also on the tubes and in particular in the upper part of the tube bundle exchanger.

In this region, the deposit prevents the transmission of heat and solidifies on contact with the walls, without managing to flow completely downwards. This is how the recovery of the recuperator gradually decreases and clogging can occur. At higher temperatures, the deposit is fluid or even there is no condensation. At a lower temperature, on the contrary, and in particular in the upper exchanger, the deposition takes place in solid form and generates dust, which the purifier placed at the entrance of the chimney has in particular the function of capturing, at least in part.

The invention aims to eliminate the drawbacks associated with '1' t 'fouling of conventional heat recovery units by providing a recovery unit in which a tubular heat exchanger and a double jacket exchanger are combined in such a way that they cannot practically not clog.

To this end, the subject of the invention is a method for recovering heat from the fumes of an oven and preheating the combustion air of these ovens by heat exchange between these fumes and this air in two exchangers , one at high temperature, the other at medium temperature, which successively pass through the fumes from the oven, while the air to be preheated first passes through the medium temperature exchanger, where it circulates against the current flue gases, then in the high temperature exchanger, where it circulates in the same direction as the fumes, this process being characterized in that, the two exchangers being arranged in parallel in a vertical position, the fumes circulate from top to bottom in the high temperature exchanger, then from bottom to top in the medium temperature exchanger.

The invention also relates to a device for recovering heat from the fumes of an oven and for preheating the combustion air of this oven, by heat exchange between these fumes and this air, this device comprising a high temperature heat exchanger and a medium temperature heat exchanger which successively pass through the fumes leaving the furnace, while the air first enters the medium temperature exchanger, counter-current to the fumes, then into the high temperature exchanger, where it circulates in the same direction as the fumes, this device being characterized in that the exchangers are arranged in parallel in positions such that the fumes circulate there vertically, the smoke supply duct of the high temperature exchanger opening at the top of the latter, while the smoke supply duct of the medium temperature exchanger opens at the bottom of the latter.

According to the invention, therefore, in the high temperature exchanger, the fumes and air circulate from top to bottom, while, in the medium temperature exchanger, the fumes circulate from bottom to top and the air from high to low. the bases of these two exchangers will therefore be joined by a flue in refractory products and the high temperature exchanger will be supplied with smoke at its upper part by a flue also in refractory products.

As in the usual heat recuperators, the high temperature exchanger is a tubular exchanger, while the medium temperature exchanger is a jacketed exchanger.

Due to the circulation of the fumes from top to bottom, in the high temperature exchanger, this device can disturb the natural draft of the ovens, but, if necessary, it is easy for those skilled in the art to rebalance the pressures by measurements known in the art.

It is surprisingly found that, if the quantity of solid matter collected at the foot of the exchanger, that is to say in the case under consideration, on the bottom of the flue connecting the exchangers and essentially at the foot of the cold exchanger, is relatively large, all clogging of the hot exchanger with a tube bundle has disappeared. There is simply created on this beam, a relatively thin deposit, the thermal resistance of which somewhat lowers the temperature of the sheets of the beam.

In the case of the application of the device according to the invention to a glass furnace, a thin layer of salts, essentially borates and sulfates, is deposited on the bundle of tubes, the proportion of which varies according to the composition of the elaborate glass, and the temperature of the surface exposed to the smoke stabilizes between 800 and 900 ° C.

The appended schematic drawings illustrate the prior art and the device according to the invention. In these drawings:

Figure 1 is a diagram of a heat recovery unit of a known type;

Figure 2 is a diagram of a heat recovery unit according to the invention

The recuperator known in the art, which is shown in Figure 1, comprises two heat exchangers 1 and 2, arranged one above the other. The lower exchanger 1 is a tubular exchanger of the cage type, comprising an upper toric collector 3 and a lower toric collector * +, with which a bundle of parallel tubes communicates 5 · The upper exchanger 2 is a double-shell exchanger, between the walls 6 and 7 of which are arranged fins 8 forming spacers. An annular inlet box 93 disposed at the upper part of the exchanger 2 communicates with the space between the two walls 6 and 7 · Similarly, an annular outlet box 10 is arranged at the base of the exchanger 2 .

The lower exchanger 1 is supplied at its base, by a flue 11 in refractory products, in high temperature fumes leaving the oven. After partial cooling in the exchanger 1, which they pass from bottom to top, the flue gases pass into the central part of the upper exchanger 2, in which they also circulate from bottom to top, then are evacuated through the chimney 12.

The air to be preheated circulates in the opposite direction to the fumes in the upper exchanger 2 · It enters, in fact, through the upper box 9 into the annular space separating the walls 6 and 7} and is evacuated therefrom by the the box 10. It is then led by a pipe 13 to the lower collector of the exchanger P, from where it passes through the tubes 5, then into the upper collector 3 "to be discharged in 1 * f to the supply of ovens. In the lower exchanger 1 (high temperature exchanger), the smoke air therefore flows in the same direction.

In the case of the non-application of such a device to a glass furnace, the fumes arriving at the base of the tubular exchanger 1 have a temperature of approximately 1250 ° C. They enter about 900 ° C in the double jacket exchanger 2 and are evacuated to the chimney at about 700 ° C.

The air entering through the cage 9, at 20 ° C, in the exchanger 2 leaves at its base, through the cage 10, at around 250 ° C, to be reheated to around 750 ° C in the exchanger 1 .

As explained above, such a heat recovery unit has the disadvantage that the operation of each heat exchanger produces effects on the other exchanger. More specifically, the pulverulent particles resulting from the condensation of the fumes in the exchanger 2 can optionally be eliminated with the fumes which are deposited on the purification devices placed at the base of the exchanger, but those which are deposited on the walls. lateral, then detach from it, tend to fall back on the bundle of tubes 5 of the lower exchanger 1. Now, at the upper part of these tubes, the temperature of which is of the order of 750-800 ° C., this temperature is not low enough for the fumes to condense therein in solid form and it is a pasty deposit of acid salts which occurs. This deposit is not fluid enough to flow down towards the flue 11, but, on the contrary, due to the addition of the dust coming from the upper exchanger, it ends up taking * mass on the tubes 5 by gradually rendering the heat recovery unit unusable.

The device according to 1; invention, which is shown in Figure 2, overcomes this drawback.

In this figure, the bodies already described in relation to Figure 1 keep the same reference numbers, but assigned the index a.

There is therefore a high temperature exchanger constituted by a tubular exchanger 1a and a medium temperature exchanger constituted by a double wall exchanger 2a, but, instead of being arranged one above the other, these exchangers are arranged one next to the other and their bases are joined by a lower flue 20 made of refractory material.

The hot fumes coming out of the furnace feed the upper part of the exchanger 1a through a flue 21, in which they circulate from top to bottom, then they pass through the carneai 20 to arrive at the base of the exchanger 2a, which they cross from bottom to top, to be finally evacuated by the chimney 12a.

As before, the air to be preheated enters the exchanger 2a through the annular box 9a disposed at its upper part and it is evacuated at its base by the box 10a, to be entrained by the pipe 13a towards the upper manifold 3a of the tubular exchanger 1a, from where it passes through the tubes 5a, then into the lower collector at-a and finally in the conduit there-a which leads to the supply of the ovens.

In the case of the application of the device according to the in. For glass ovens, the fumes arriving at 21 at the top of the heat exchanger 1a are around 1250 ° C. At the base of this exchanger 1a, and in the flue 20, their temperature is around 800 ° C, and they are finally evacuated through the chimney 12a at around 600 ° C.

The air penetrates at about 20 ° C at the top of the exchanger 2a and exits at the base of it by the pipe 13a at about 200 ° C and, after passage in the exchanger 1a, it reaches about 750 ° C

If this heat recovery device is compared with that which has been described with reference to FIG. 1, it can be seen that the dust resulting from the condensation of the fumes in the medium-temperature exchanger 2a cannot pass through the exchanger at high temperature 1a. They simply fall into the flue 20, where they melt and from where they can be eliminated by doors such as 22, without damage to the exchanger 1a · Furthermore, the pasty condensates which form on the tubes of the bundle of l exchanger 1a, when the fumes are already cooled, appear only at the bottom of the tubes, from which they flow spontaneously in the same direction as these fumes towards the intermediate flue, where they will also be collected.

The pasty deposit occurring on the base of the tubes 5a. therefore quickly reaches a state of equilibrium and it is easy to take account of this constant fouling in order to optimally calculate the heat exchange surfaces.

Claims (4)

1.- Process for the recovery of heat from the fumes of an oven and the preheating of the combustion air of these ovens, by heat exchange between these fumes and this air in two exchangers, one at high temperature , the other at medium temperature, which successively pass through the fumes from the oven, while the air to be preheated first passes through the medium temperature exchanger where it circulates against the fumes, then through the heat exchanger high temperature, where it circulates in the same direction as the fumes, this process being characterized in that, the two exchangers being arranged in parallel in a vertical position, the fumes circulate from top to bottom in the high temperature exchanger, then bottom up in the medium heat exchanger. 2 · - Device for recovering heat from the fumes from an oven and for preheating the combustion air of this oven, by heat exchange between these fumes and this air, this device comprising a heat exchanger with high temperature and a medium temperature heat exchanger which successively pass through the fumes leaving the oven, while the air first enters the medium temperature exchanger, counter-current to the fumes, then into the high heat exchanger temperature, where it circulates in the same direction as the fumes, this device being characterized in that the exchangers are arranged in parallel in positions such that the fumes circulate there vertically, the smoke supply duct of the exchanger i high temperature opening at the upper part thereof, while the smoke supply duct of the medium temperature exchanger opens at the base thereof. 3 · - Device according to claim 2, characterized in that the base of the high temperature exchanger and the base of the medium temperature exchanger are joined by a flue made of refractory products. if. - Device according to claim 3? characterized in that said flue is equipped with at least one access means such as a door, making it possible to collect and eliminate the materials which are deposited there. 5 · - Device according to one of claims 1 to 3, characterized in that, the high temperature exchanger is a tubular exchanger, while the medium temperature exchanger> „is a double jacket exchanger. % Λ *