GB2064749A

GB2064749A - Recuperators for heating air by furnace gases

Info

Publication number: GB2064749A
Application number: GB8037405A
Authority: GB
Original assignee: Saint Gobain Emballage SA
Current assignee: Verallia France SA
Priority date: 1979-11-30
Filing date: 1980-11-21
Publication date: 1981-06-17
Also published as: ES497273A0; BE886414A; IT1130542B; IT8068833A0; NL8006414A; DK506780A; ES8107384A1; LU82963A1; GB2064749B; FR2470942B1; FR2470942A1; DE3043804A1

Abstract

Heat from hot furnace gases passing through a flue 21 is recuperated to heat air by passing the gases through a high temperature exchanger 1a from top to bottom and then through a lower temperature exchanger 2a from bottom to top, the exchangers being positioned with their longitudinal axes extending vertically and being laterally spaced from each other (e.g. side-by-side) the air (e.g. combustion air) passing first through the heat exchanger 2a in the opposite general direction to that of the gases, and then through the heat exchanger 1a in the same general direction as the gases. Deposits from the hot gas which form on the heat exchange surfaces and subsequently become detached therefrom, collect in a chamber 20 of the flue and are removed at 22. <IMAGE>

Description

SPECIFICATION Heat exchange The invention relates to a method and device for recuperating heat from furnace gases, especially but not exclusively gases from glass furnaces.

In order to heat the air for combustion fed to such furnaces it is known to use regenerators formed of stacks of bricks, traversed alternatively by the furnace gases and then the air for combustion.

In furnaces of smaller size the installation of such regenerators is frequently inconvenient and excessively expensive. Preferably there are then used recuperators which are in fact simple metal vertical heat exchangers, in which the gases travel in principle in ascending movement, taking advantage of natural draw before being discharged into a chimney.

There are used in practice two principal types of exchanger. The exchangers of one type are double envelope devices in which the air passes through a peripheral space and the furnaces gases through a central part. Others have a single insulated envelope in which is arranged an annular group of suspended tubes inside which passes the air to be heated. The latter are of more complicated construction, but allow achievement of a better yield having a temperature of heating of the air which is greater at the output.

In fact these two types of exchangers are generally combined in recuperators, arranging them one above the other and feeding them by a lower flue. The lower exchanger which operates at high temperature is a tubular exchanger of chrome stainless steel in which there is carried out an antimethodic exchange, that is to say the air, already heated, circulates from bottom to top in the same direction as the hot furnace gases which allows the tubes to be maintained at substantially constant temperature, of the order of 750"C, over the whole length of the exchanger, thus reducing thermal stresses.

The upper exchanger, functioning at a relatively low temperature, is a double envelope exchanger formed of austenitic chrome nickel stainless steel sheet, in which there is carried out methodic exchange, the cold air passing from top to bottom, in the reverse direction with respect to the already cooled furnace gases. This type of flow allows obtaining of a better extraction of heat from the gases, but it involves higher stresses in the sheets which however remain acceptable in this relatively cold section.

In the various types of recuperators which have been known up to now, and in particular in those which have just been described, the principal difficulty encountered arses from a fouling of the walls exposed to the gases, fouling which causes reduced heat exchange and thus a drop in temprature to which the combustion air is heated and which is likely to cause a premature deterioration of the installation by the formation of hot points which generate disymmetrical forces, bending of the tubes and accelerated corrosion.

This fouling is caused essentially by a pasty deposit of mixtures of acid salts, such as sulphates and in some cases borates, which are easily observed in the feed flue situated at the lower part of the recuperator. This deposit is produced in a zone of temperature of the order of 800"C, that is to say on the tubes and in particular in the upper part of the exchanger containing bundles of tubes.

In this region, the deposit prevents transmission of heat and becomes solidified on contact wiht the walls, so that not all the deposit flows downwardly. Thus the yield of the recuperator is lowered progressively and bending may commence. At a higher temperature the deposition is fluid or even no condensation may be produced. At a lower temperature, on the other hand, and especially in the upper exchanger, the deposition takes place in solid form and generates dust which a cleaner placed at the input of the chimney has the function of collecting, at least partially.

The invention is intended to remove the difficulties caused by fouling of the usual heat recuperators in providing a recuperator in which a tubular heat exchanger and a double envelope exchanger are combined in such a way that they produce practically no fouling.

According to one aspect of the invention there is provided a method of recuperating heat from furnace gases and heating air by exchange of heat between these gases and air in heat exchangers, in which the furnace gases successively pass through a high temperature exchanger and a lower temperature exchanger and the air to be heated first passes through the lower temperature exchanger countercurrent to the gases and then passes through the high temperature exchanger in the same direction as the gases, the two exchangers being arranged vertically and parallel and the gases passing from top to bottom in the high temperature exchanger, then from bottom to top in the lower temperature exchanger.

According to another aspect of the invention there is provided a device for recuperating heat from furnace gases and heating air by exchange of heat between these gases and air, comprising a high temperature heat exchanger and a lower temperature heat exchanger through which the gases leaving the furnaces may pass successively, and means for passing air firstly through the lower temperature exchanger countercurrent to these gases and then through the high temperature in the same direction as the gases, the exchangers being parallel and arranged such that the gases pass through them vertically, a feed duct for feeding the gases into the high temperature exchanger opening into the upper part of the latter, and a feed duct for feeding gases of the lower temperature exchanger opening into the lower part of the latter.

Using the method of the invention in the high temperature exchanger the gases and air pass from top to bottom, whereas in the lower temperature exchanger the gases pass from bottom to top and the air from top to bottom.

The bottoms of these two exchangers can be connected by a flue of refractory material and the exchanger at the higher temperature can be fed with gases at its upper part by a flue also made of refractory material.

As in known heat recuperators, the high temperature exchanger may be a tubular ex changer, whereas the medium temperature exchanger may be a double envelope ex changer.

Because of the flow of gases from top to bottom in the high temperature exchanger, this device may disturb the natural draught of the furnace, but it is a simple task to re balance the pressures by means known in the art if necessary.

It is found, surprisingly, that if the quantity of solid material recovered at the foot of the exchanger, that is to say with the above mentioned arrangement on the bottom of the connecting flue between the exchangers, and essentially at the foot of the cold exchanger, is relatively large, any deformation of the hot exchanger having a bundle of tubes disap pears. There is simply created on this bundle a relatively thin deposit of which the heat resistance lowers slightly the temperature of the tubes of the bundle.

In the case of application of the device according to the invention to a glass furnace, there is deposited on the bundle of tubes a layer of small thickness of salts, essentially borates and sulphates, of which the propor tion varies according to the composition of the glass being manufactured, and the tempera ture of the surface exposed to the gases may stabilise itself between 800 and 900"C.

A device according to one embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagram of a heat recuperator of known type; Figure 2 is a diagram of a heat recuperator according to the invention.

The recuperator which is known in the art, which is shown in Fig. 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 toroidal collector 3 and a lower toroidal collector 4 with which communicates a bundle of parallel tubes 5. The upper exchanger 2 is a double envelope exchanger between the walls 6 and 7 of which are arranged fins 8 forming zig-zags. An annular input box 9 arranged at the upper part of the exchanger 2 communicates with the space formed between the walls 6 and 7. Likewise an annular output box 10 is arranged at the base of the exchanger 2.

The lower exchanger 1 is fed at its base by a flue 11 of refractory material with furnace fumes at a high temperature leaving the furnace. After partial cooling in the exchanger 1 which they traverse from bottom to top, the gases pass in the central part of the upper exchanger 2 in which they travel also from bottom to top and then are evacuated by the chimney 12.

The air to be heated flows in the inverse direction to the gases in the upper exchanger 2. It enters in fact by the upper box 9 in the annular space separating the walls 6 and 7 and is removed at the lower part by the box 10. It is then brought by a duct 1 3 to the lower collector of the exchanger 1 from which it passes in the tubes 5, then in the upper collector 3 to be evacuated at 14 towards the feed of the furnace. In the lower exchanger 1 (high temperature exchanger) the air and the gases thus pass in the same direction.

In the case of application of such a device to a glass furnace, the gases arriving at the bottom of the tubular exchanger 1 have a temperature of about 1 250 C. They enter at about 900"C the double envelope exchanger 2 and are removed to a chimney at about 700"C.

The air entering the housing 9 at 20"C in the exchanger 2 leaves at its base by the housing 10 at about 250"C, to be heated to about 750"C in exchanger 1.

As has been explained above such a heat recuperator has the disadvantage that the functioning of each heat exchanger produces effects on the other exchanger. More precisely the powder particles resulting from condensation of the gases in exchanger 2 may possibly be eliminated with the gases which are deposited on cleaning devices arranged at the base of the exchanger, but the particles which are deposited on the lateral walls and then become detached have a tendency to fall onto the bundle of tubes 5 of the lower exchanger 1. However, at the upper part of these tubes of which the temperature is of the order of 750 to 800"C, this temperature is not low enough for the gases to condense in the form of a solid, and it is a pasty deposit of acid salts which is produced. This deposit is not sufficiently fluid to flow towards the bottom in the direction of the flue 11, but, on the contrary, because of the addition of dust proceeding from the upper exchanger forms a mass on the tubes 5 rendering the heat recuperator progressively unusable.

The device according to the invention shown in Fig. 2, allows this difficulty to be solved. In this figure the members already described in relation to Fig. 1 have the same reference numerals with the addition of the index a.

In Fig. 2 a high temperature exchanger is formed of a tubular exchanger 1 a and a medium temperature exchanger is formed by a double wall exchanger 2a, but instead of being arranged one above the other these exchangers are arranged side by side and their bases are connected by a lower flue 20 of refractory material.

The hot gases which leave the furnace feed, through a flue 21, the upper part of the exchanger 1 a in which they pass from top to bottom, then they pass through the flue 20 to arrive at the base of the exchanger 2a which they traverse from bottom to top finally to be evacuated by the chimney 1 2 a.

As before the air to be heated enters the exchanger 2a by an annular box 9a arranged at its upper part and it is evacuated at its base by the box 1 0a to be conducted by a duct 1 3a towards the upper collector 3a of the tubular exchanger 1 a from which it passes in the tubes 5a then into the lower collector 4a and finally into a duct 1 4a which opens into the feed for the furnace.

In the case of application of the device according to the invention to glass furnaces the gases arriving at 21 at the upper part of the exchanger 1 a are at about 1 250 C. At the base of this exchanger 1 a and in the flue 20 their temperature is about 800"C and they are finally evacuated by the chimney 1 2a at about 600"C.

The air enters the upper part of the exchanger 2a at about 20"C and leaves at the base of the latter by the duct 1 3a at about 200"C and, after passage in the exchanger 1 a, it reaches about 750"C.

If there are compared this device for recuperating heat with that described with reference to Fig. 1 it will be understood that the dust resulting from the condensation of gases in the medium temperature exchanger 2a cannot pass into the high temperature exchanger 1 a. It simply falls into the flue 20 where it melts and from which it may be removed by ports such as 22 without damaging the exchanger 1 a. Further, the pasty condensate which forms on the tubes 5a of the bundle in exchanger 1 a when the gases are already cooled only appears at the lower part of the tubes, from which it flows spontaneously in the same direction as the gases towards the intermedite flue from which it may also be removed.

The pasty deposit produced on the base of tubes 5a thus rapidly reaches a state of equilibrium and it is easy to take account of this constant degree of fouling to calculate the optimum arrangement of heat exchange surfaces.

Claims

1. A method of recuperating heat from furnace gases and heating air by exchange of heat between these gases and air in heat exchangers, in which the furnace gases successively pass through a high temperature exchanger and a lower temperature exchanger and the air to be heated first passes through the lower temperature exchanger countercurrent to the gases and then passes through the high temperature exchanger in the same direction as the gases, the two exchangers being arranged vertically and parallel and the gases passing from top to bottom in the high temperature exchanger, then from bottom to top in the lower temperature exchanger.

2. A device for recuperating heat from furnace gases and heating air by exchange of heat between these gases and air, comprising a high temperature heat exchanger and a lower temperature heat exchanger through which the gases leaving the furnace may pass successively, and means for passing air firstly through the lower temperature exchanger countercurrent to these gases and then through the high temperature exchanger in the same direction as the gases, the exchangers being parallel and arranged such that the gases pass through them vertically, a feed duct for feeding the gases into the high temperature exchanger opening into the upper part of the latter, and a feed duct for feeding gases of the lower temperature exchanger opening into the lower part of the latter.

3. A device according to Claim 2, in which the lower part of the high temperature exchanger and the lower part of the lower temperature exchanger are connected by a flue of refractory material.

4. A device according to Claim 3, in which said flue is provided with at least one means of access from the exterior, allowing removal of material deposited therein.

5. A device according to any one of Claims 2 to 4, in which the high temperature exchanger is a tubular exchanger whereas the lower temperature exchanger is a double envelope exchanger.

6. A method of recuperating heat from furnace gases, substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

7. A device for recuperating heat from furnace gases, substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.