EP0395457B1 - Process and apparatus for heating a flow of a gaseous fluid by successive heat exchanges - Google Patents

Abstract

The invention relates to a process and a device for indirectly heating a flow of gaseous fluid. According to the invention, the flow of fluid to be heated is first subjected to a first heat exchange with combustion products, in a heat exchanger (3), and is subsequently subjected to a second heat exchange with a heat-exchanging fluid circulating in a second heat-exchanger (5). The flow of fluid to be heated is then circulated in a third exchanger (7) where it is subjected to a heat exchange with the combustion products before the latter are used at the time of the first heat exchange. The third heat exchanger (7) can comprise radiating tubes (51) and convection surfaces for heating the flow of fluid in circulation, essentially by convection.

Description

The invention relates to a method and an apparatus for heating a flow of gaseous fluid, in particular that of air.

• par chauffage direct du flux d'air via des produits de combustion gazeux produits par un brûleur à gaz, le flux d'air et les produits de combustion entrant en contact et se mélangeant,
• ou par réchauffage indirect de l'air au moyen de résistance électriques,
• ou encore par réchauffage indirect d'air via un échange thermique sans contact direct entre cet air et un ou plusieurs fluide(s) caloporteur(s) chauffé(s) au gaz ou au fuel.

The production of gas and in particular hot air at high temperature (which can be estimated a priori between 350 and 450 ° C) with a significant flow rate (for example of the order of 10,000 to 30,000 Nm³.h) is currently carried out:

• by direct heating of the air flow via gaseous combustion products produced by a gas burner, the air flow and the combustion products coming into contact and mixing,
• or by indirect heating of the air by means of electrical resistance,
• or by indirect heating of air via a heat exchange without direct contact between this air and one or more heat transfer fluid (s) heated with gas or fuel oil.

The first two systems appeared to be unsuitable for the present case, due in particular to the mixture between the products of combustion and the air to be heated and the cost of electrical energy, in particular in winter.

• l'utilisation de parois en matériau réfractaire qui se détériorent assez rapidement par suite des écarts de régime et des remises en route successives imposés à l'appareil ;
• l'utilisation également d'aciers spéciaux au niveau de l'étage d'échange entre les produits de combustion sortant du brûleur et l'air à réchauffer, ceci n'empêchant d'ailleurs pas de fréquentes fissurations des parois.

As for indirect heaters, if in practice they are the only devices (with electric heaters) that allow indirect heating of air to a temperature above about 300 ° C, they still have some drawbacks among which we can note :

• the use of walls of refractory material which deteriorate fairly quickly as a result deviations in speed and successive restart operations imposed on the aircraft;
• the use also of special steels at the level of the exchange stage between the combustion products leaving the burner and the air to be heated, this moreover not preventing frequent cracking of the walls.

Finally, we can note the existence of significant pressure drops resulting from the desired compactness of these exchangers.

To remedy these imperfections in known devices, the invention proposes a new type of heating system making it possible to increase the efficiency of the device, in particular by providing several exchange stages and by reducing, on each stage, the temperature differences. between the heating products and the heated products.

A well-studied system for recovering calories also makes it possible to produce a high-performance heating device, flexible to use, reliable and less expensive than existing devices.

In the past, heat generators with multiple exchangers have already been proposed.

Publication CH-A-428 140 thus proposes a heat generator comprising three exchangers in series where the first and third exchangers heat a gas stream by indirect heat exchanges with combustion products.

However, this device does not allow the same gas flow to be heated in several exchange steps, since the second exchanger, which receives the fumes from the first, is used to heat a liquid (water), while the third exchanger, which receives the second flue gas, used to heat a different volume of air than that heated by the first exchanger.

• a) on fait tout d'abord subir au fluide en question un premier échange thermique avec des produits de combustion ayant une température supérieure à celle dudit flux de fluide,
• b) on fait ensuite subir à ce même flux un second échange thermique avec un fluide caloporteur,
• c) puis, on fait subir toujours au même flux un troisième échange thermique avec lesdits produits de combustion avant que ces derniers soient utilisés lors du premier échange thermique.

With the aim therefore in particular of improving the thermal efficiency of a heating installation of a the only gaseous fluid, the invention proposes an improved process which is characterized in that:

• a) the fluid in question is first subjected to a first heat exchange with combustion products having a temperature higher than that of said fluid flow,
• b) this same flow is then subjected to a second heat exchange with a heat transfer fluid,
• c) then, the same flow is always subjected to a third heat exchange with said combustion products before the latter are used during the first heat exchange.

• transférer par rayonnement l'énergie calorifique contenue dans les produits de combustion,
• faire absorber l'énergie ainsi rayonnée par des surfaces de convection,
• et chauffer alors essentiellement par convection ledit flux de fluide que l'on fera alors avantageusement circuler au contact desdites surfaces de convection.

According to an additional characteristic of the invention, it will be preferred in practice, to carry out the third heat exchange,

• transfer the heat energy contained in the combustion products by radiation,
• absorb the energy thus radiated by convection surfaces,
• and then essentially heating by convection said fluid flow which will then advantageously be circulated in contact with said convection surfaces.

In accordance with an additional characteristic of the invention, it even appeared a priori preferable to carry out a fourth heat exchange between the flow of fluid leaving the heat exchange carried out according to step b) above and the flow of fluid undergoing the heat exchange according to step c), then essentially circulating the flow of fluid leaving the exchange carried out according to said step b) around and in contact with thermally conductive walls externally limiting an interior volume in which the heat exchange is carried out following step c).

As mentioned at the beginning of this description, the invention also relates to a heating device for heating the same gaseous fluid flow in several stages of heat exchange, this device comprising a first, a second and a third exchanger each with an internal volume, these volumes communicating with each other in pairs to successively receive said flow of fluid which heats up via heat exchange pipes which pass or wind through each of the exchangers and in which a thermal fluid circulates, the thermal fluid circulating in said pipes, which are interconnected, first and third exchangers consisting of combustion products, said gaseous fluid being thus heated in each of the three exchangers before being recovered by a recovery pipe connected at the outlet of the third exchanger.

• La figure 1 est une vue schématique d'ensemble d'un mode possible de réalisation de l'appareil de chauffage de l'invention,
• et la figure 2 illustre schématiquement un détail de réalisation d'une partie intérieure du troisième échangeur.

The characteristics of the invention which have just been presented, as well as other additional characteristics, will appear in more detail from the description which follows, given in relation to the appended drawings given by way of nonlimiting examples and in which :

• FIG. 1 is a schematic overview of a possible embodiment of the heating apparatus of the invention,
• and FIG. 2 schematically illustrates a detail of an interior part of the third exchanger.

If we refer first to FIG. 1, we therefore see illustrated a heating device for gaseous fluid, marked 1.

The apparatus 1 comprises three successive chambers forming heat exchangers 3, 5, 7 and arranged in series, one after the other.

As illustrated, the first exchanger 3 comprises an enclosure with walls 6 which can be metallic defining a chamber with an interior volume 8 into which opens, at one end, an inlet pipe 9 for admitting the flow of fluid to be heated (such as air).

Inside the chamber 8 winds at least one recycling pipe 11 which may have exchange fins 12 and in which it is planned to circulate gaseous combustion products for indirect heat exchange with the flow of fluid passing through the volume 8, and before these same combustion products are evacuated from the exchanger 3, via the recovery pipe 13 to which the pipe 11 is connected.

Significantly opposite the fluid inlet pipe 9, the interior chamber of the first exchanger 3 is connected, via a connection pipe 17, to one end of the interior volume 15 of the second exchanger 5, of so as to supply this exchanger with preheated gaseous fluid.

This exchanger 5 can in particular be produced so that its internal chamber 15 is limited by metal walls 19 covered externally with a thermally insulating sheath 21.

Inside the chamber 15 is disposed a heat exchange battery 23 comprising several tubes (possibly with fins) 25 extending substantially perpendicular to the direction of circulation of the gaseous fluid in the chamber 15 (direction materialized by the arrow 27 ). These different tubes 25 are connected towards their two opposite ends to two collectors 29, 31. To the inlet manifold 29 is connected a pipe 33 for supplying heat transfer fluid. As heat transfer fluid or thermal fluid, provision may in particular be made for the use of steam or synthetic oil. In this case, it will be preferable to connect the manifold 31 to a pipe 35 for evacuating the steam condensates, a purge valve 36 advantageously making it possible to regulate the evacuation of these condensates.

So as to also obtain a regulation of the flow rate of thermal fluid admitted into the exchange battery 23, it appeared preferable to have a regulating valve 38 on the intake pipe 33.

Towards its end opposite to the pipe 17, the chamber 15 communicates with the interior volume of the third heat exchanger 7, by means of a connection channel 37 opening, on one side, into the lower part of the chamber 15 and, on the other, in the upper part of the interior volume of the third exchanger 7.

As is still clearly illustrated in FIG. 1, the internal volume of this exchanger 7 is divided into a large internal chamber 39 externally limited by thermally conductive walls (in particular metallic) 41, themselves arranged at a certain distance from a thermally insulated outer enclosure 43.

In practice, the connection channel 37 between the exchangers 5 and 7 will pass locally and in the upper part of the enclosure 43 to open at one end of the space 45 whose width l will be sufficient to ensure correct circulation of the flow of fluid around and in contact with the conductive outer walls 41 of the inner chamber 39.

Noticeably opposite the connection tube 37, and preferably at the bottom, the chamber 39 communicates with the space 45 through a communication orifice 47, so that the gaseous fluid which has circulated in this space 45 can penetrate inside the chamber 39 to undergo a new heat exchange with combustion products circulating inside the exchange tubes 51 connected, upstream to the burners 53 which can in particular be supplied with combustible gas and combustion air .

In order to limit the problems linked to a faulty operation of a burner, it has been provided in the invention to preferably use several exchange tubes 51 each connected, outside but in the immediate vicinity of the enclosure 43 , to a burner 53.

As exchange tubes 51, it is preferable to use radiant tubes, for example U tubes. extending widely into the interior chamber 39 before being connected, passing through the enclosure 43, each to a recovery pipe 55 provided for recycling the combustion products from the third exchanger 7 to the pipe 11 of the first exchanger 3.

In FIG. 1, it will also be noted that, to ensure the evacuation and recovery of the heated gas flow in the third exchanger 7, the chamber 39 is connected locally and preferably in the upper part, to a recovery pipe 57. Advantageously, this pipe 57 will be connected to the chamber 39 in a place capable of promoting a circulation of the gaseous fluid to be heated therein which is generally oriented in a direction transverse to that in which the tubes 51 extend which will then preferably be arranged substantially parallel each other.

To promote this circulation of the fluid to be heated, the chamber 39 may also be equipped, at the connection of the pipe 57, with a deflector 59.

To this set, it also appeared useful to add a regulation system constituted by a thermal probe 61 engaged on the "air" outlet pipe 57 and connected to a regulator 63 capable of acting on the one hand the automatic valve 38 for adjusting the flow of thermal fluid through the pipe 33 and, on the other hand, on another automatic valve 65 for adjusting the supply flow, for example of combustible gas, to the burners 53.

An important characteristic of the invention residing in the design of the third heat exchanger 7, particular attention has also been paid to the production thereof and in particular to the configuration of the radiant tubes 51.

Of course, these can be made with heat exchange fins, as shown at 67 in Figure 1, these fins then preferably extending transversely to the general direction (shown in 69) of the flow of gaseous fluid inside the chamber 39.

However, in Figure 2, there is schematically illustrated in perspective view with cutaway the chamber 39 in which are disposed radiating tubes 51 'bent extending substantially parallel over the entire length of the chamber. These radiant tubes 51 ′, which may have a metallic radiation surface, are in this case devoid of fins. On the other hand, between two consecutive tubes and at a distance from each of them, there are provided convection means, for example in the form of plates 71 with metallic convection surfaces, capable of absorbing the energy radiated by the tubes so as to heat in particular by convection in contact with these plates the fluid (shown diagrammatically by the double arrow 73) which is always admitted into the chamber 39 through the orifice 47.

Preferably, the convection plates 71 will be equipped on their two opposite faces with heat exchange fins 75 advantageously extending transversely relative to the direction in which a circulation of the fluid is favored inside the chamber 39.

It should be noted that, as illustrated in FIG. 2, the plates 71 will preferably be arranged so as to constitute, with respect to each other, baffles extending the path of the fluid inside the chamber 39 and promoting its stirring, the fluid thus recovering the calories concentrated around the plates, between the fins 75, which may in particular be metallic.

Of course, other types of structurally different convection surfaces of the plates 71 could be envisaged, without departing from the scope of the invention.

We will now briefly describe the operating principle of the device which has been described above.

This is how it works:

The gaseous fluid to be heated, for example air, is firstly introduced into the first exchanger 3 through the intake manifold 9. This fluid which can, for example, be admitted at an ambient temperature of 25 ° C. heats up in contact with the coil formed by the transverse pipe 11 inside which therefore circulate the combustion products emanating from the burners 53, after these products have lost part of their calories by heat exchange in the third exchanger 7.

While these same combustion products are evacuated via the recovery line 13, the fluid current passes from the chamber 8 of the first exchanger to the chamber 15 of the second exchanger 5 where it is again heated by indirect heat exchange essentially through the exchange walls of the battery of tubes 25 inside which therefore circulates a vaporized thermal fluid, such as for example water vapor which can be admitted under a pressure of the order of 10 to 15 bars and with a temperature of the order of 230 to 260 ° C.

In this way, the gaseous fluid which enters the second exchanger 5 for example at a temperature of 60 to 80 ° C can exit therefrom at 180 or even 200 ° C, or even possibly more, the thermal gradient of heating can be adapted thanks to the regulator 63 which will preferably be programmed so that variations in heat flow rates are primarily absorbed by the vaporized thermal fluid, thus making it possible to minimize sudden thermal variations at the level of the burners 53 and the radiant tubes 51 of the third exchanger 7.

Leaving the second exchanger 5, the fluid flow, already heated by two successive heat exchanges, is then admitted into the peripheral space 45 of the third exchanger 7.

Given the location of the orifice 47 for access to the inner chamber 39 of this third exchanger 7, the fluid to be heated will therefore first of all circulate essentially in contact with the outer walls thermally conductors of this chamber 39, thus recovering in particular by convection part of the calories contained in the chamber 39 and released via the wall 41 of the latter, either by the current of circulating fluid, or by the radiant tubes 51 or 51 'and / or by the convection plates 71 and their fins 75 (see FIG. 2).

However, most of the heat exchange conducted inside the third exchanger 7 will take place inside the chamber 39, when the fluid flow comes to circulate in the immediate environment of the tubes 51 (or 51 ' ) through which can be transferred, by radiation, the heat energy contained in the combustion products just out of the burners 53 (usually at a temperature of about 800 to 1200 ° C).

By providing convection plates 71 (preferably with fins) the energy thus radiated can be absorbed and then returned to the fluid which will thus be heated in chamber 39 by convection by circulating in contact with the convection surfaces provided for this purpose, before to be evacuated at a temperature which can generally be estimated between 350 and 450 ° C, via the recovery pipe 57 where the thermal sensor 61 will allow the regulator 63 to dose the supply, on the one hand in thermal fluid vaporized from the second exchanger 5 and, secondly, fuel from the burners 53, via the valves 38 and 65 respectively, preferably with priority to the "steam" circuit of the exchanger 5.

It will be noted that with such a heating process, without direct contact between the gaseous fluid to be heated and the heat transfer fluids for heating, it will be possible to deliver at the outlet of the apparatus a hot fluid at high temperature free of pollution which can for example be used. in the agro-food or pharmaceutical industry, especially for drying products or for various heat treatments.

Claims (10)

1. Process for heating a stream of gaseous fluid, characterised in that:

   a) the stream of fluid to be heated is firstly subjected to a first heat exchange with products of combustion having a temperature which is greater than that of the said stream of fluid;

   b) this same stream of fluid to be heated is then subjected to a second heat exchange with a heat-transfer fluid;

   c) the said stream of fluid to be heated is then subjected to a third heat exchange with the said products of combustion before the latter are used during the first heat exchange.
2. Process according to Claim 1, characterised in that, before the second heat exchange, the temperature of the heat-transfer fluid used in this step is intermediate between the temperatures, before the heat exchange, of the products of,combustion used during the said first and third heat exchanges.
3. Process according to Claim 1 or Claim 2, characterised in that, in order to perform the third heat exchange:

   - the thermal energy present in the products of combustion is transferred by radiation;

   - the energy thus radiated is absorbed by convection surfaces (41, 71, 75); and

   - the said stream of fluid to be heated which is circulated such that it is in contact with the said convection surfaces is heated substantially by convection.
4. Process according to any one of the preceding claims, characterised in that a fourth heat exchange is performed between the stream of fluid to be heated and emerging from the heat exchange performed according to stage b) and the stream of fluid to be heated being subjected to the heat exchange according to stage c), substantially as a result of circulation of the said stream of fluid emerging from the heat exchange performed according to stage b) around and in contact with heat-conducting walls (41) which delimit externally an inner space (39) inside which the heat exchange according to stage c) is performed.
5. Heating apparatus for heating in several heat exchange stages a given stream of gaseous fluid comprising a first (3), a second (5) and a third (7) heat exchanger each with an inner space (8; 15; 39, 45), which spaces communicate with one another in pairs in order to receive successively the said stream of fluid which is heated via heat exchange conduits (11; 25, 51, 51') which pass or meander through each of the heat exchangers and in which a thermal fluid circulates, the said thermal fluid circulating in the said conduits (11; 51, 51'), which are connected to one another, the first and third heat exchangers (3, 7) consisting of products of combustion, the said stream of gas being heated in this way in each of the three heat exchangers before being recovered by means of recovery conduit (57) connected at the outlet of the third heat exchanger (7).
6. Apparatus according to Claim 5 in which the heat exchange conduits (11; 51, 51') of the first and third heat exchangers (3, 7) are connected to one another without passing through the second heat exchanger (5) of which the exchange conduit (25) is supplied with a heat-transfer fluid other than the said products of combustion, such as a synthesis oil or steam, the said fluid to be heated circulating successively in the first (3), second (5) and then third heat exchanger (7).
7. Apparatus according to Claim 5 or Claim 6, characterised in that the said exchange conduits of the third heat exchanger (7) comprise a plurality of conduits (51, 51') which are made of heat-conductive material and are each connected on the exterior of this heat exchanger, upstream, to a burner (53) supplying these conduits with the said combustion products of combustion.
8. Apparatus according to Claim 7, characterised in that, inside the third heat exchanger (7), the said conduits (51, 51') are curved and extend in an internal chamber (39) which is delimited by heat-conductive walls (41) and which is disposed inside an enclosure (43) with external heat-insulating walls, the said enclosure communicating, for the circulation of the fluid to be heated, at one end with the inner space (15) of the said second heat exchanger (5) and at the other end with the said chamber (39) which it contains, which is connected to the said conduit (57) for recovering the heated gaseous fluid after it has circulated in the said chamber.
9. Apparatus according to Claim 7 or Claim 8, characterised in that the said conduits (51') comprise radiating tubes at a distance from which there are disposed convection means (71, 75) which can absorb the energy radiated by the said conduits (51') and increase the temperature of the said gaseous fluid to be heated introduced into the third heat exchanger (7).
10. Apparatus according to Claim 9, characterised in that the convection means (71, 75) consist of metal plates (71) equipped with heat exchange ribs (75) extending transversely relative to the preferred direction of circulation (69, 73) of the stream of fluid to be heated, inside the said third heat exchanger (7).

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