EP0219882B1 - Process and automatic cleaning device for gaseous fluids - Google Patents

Abstract

Automatic device for regular cleaning of the surfaces of a heat exchanger for gaseous fluids flowing in vertical channels (1) defined between the said surfaces, comprising resilient members (5) arranged permanently in the said channels (1) and capable of being caused to vibrate in order to perform the cleaning of the said surfaces, characterized in that it comprises conduits (10, 11, 12) for the injection of additional compressed gas, opening out in front of the openings of groups of channels (1) and an injection control device designed to produce, successively and at regular intervals for each group of channels, an injection of additional compressed gas inducing in the said group of channels (1) a flow of gaseous fluid originating from the exchanger, causing the resilient members (5) present in each group of channels (1) to vibrate.

Description

The present invention relates to a method and an automatic device for periodically cleaning the surfaces of a heat exchanger intended to treat gaseous fluids flowing in vertical channels defined between said surfaces.

The problem posed by fouling of the heat exchanger exchange surfaces is an important obstacle for their operation. For heat exchangers intended to treat gaseous fluids, there is first of all a reduction in the heat flow exchanged due to the fouling of the walls between which the gaseous fluid flows. In addition, there are deposits of dust which can quickly reach significant thicknesses, which leads to a considerable increase in pressure losses.

As heat exchangers are generally made up of a number of parallel channels, fouling leads to a risk of blockage of part of the gas passage section, which results in a loss of exchange surface. resulting in a decrease in the efficiency of the exchanger. In fact, it is very difficult to ensure a strictly uniform distribution of the flow rates in all of the channels in parallel with such a heat exchanger. Thus, it is conceivable that a certain number of channels receive less bit rate than others. All the channels being generally geometrically identical, the flow velocities in certain channels can therefore be lower than in others. The deposition rate of the dust particles varying in opposite direction to the gas flow rate, this results in a preferential deposition of the solid particles along the walls of the less well-supplied zones of the exchanger. The pressure drop characteristics of clogged channels increase rapidly with the decrease in their hydraulic diameter, which again leads to a reduction in flow velocities. Thus the phenomenon is self-sustaining and accelerates even until the complete closure of certain channels.

To avoid these drawbacks, regular maintenance of the exchange surfaces is generally carried out. This maintenance can be done discontinuously with periodic human intervention. However, such a process has the disadvantage of causing significant labor costs as well as loss of productivity due to the necessary shutdowns of the installation.

It has therefore been envisaged to combat fouling of the heat exchangers by acting at the very source of the phenomenon. We first imagined to fight against clogging of heat exchangers by organizing flows in a strictly uniform manner so that solid deposits occur simultaneously on all surfaces. The thickness of the deposits then tends asymptotically towards a limit for which the speed of erosion counterbalances the speed of deposit. Such a stable limit for the thickness of the deposits prevents partial clogging of the gas passage section in the exchanger. FR-A-2 524132 describes such an embodiment in which the passage of gases is organized in a perfectly uniform manner. However, an accidental imbalance in the diet cannot be ruled out, which would then lead to clogging.

Another means of combating fouling and clogging is to provide cleaning of the inside of the tubes. Devices for this purpose have been provided, in particular for tubular heat exchangers intended for the treatment of liquids.

We know for example the use of flexible bodies of size slightly greater than the diameter of the tubes, which are forced by hydraulic thrust over their entire length (see EP-A 41 698). Such a method is however not applicable in the case of a compressible gas flow.

In some cases, it has been recommended to use the friction friction of the fluids on the exchange walls and their sudden variation to detach the particulate deposits. For example, a device for sweeping with steam or compressed air from the tubes is known. smoke from boilers (see EP-A-29 933). However, not all solid deposits can be detached under the influence of friction constraints alone, and only mechanical scraping could guarantee regular maintenance of the exchange surfaces.

For heat exchangers intended to treat liquids, it has already been imagined to use an elastic metal spiral stirred by the circulation of the liquid FR-A-2 479 964). In such a process, the amount of movement transmitted by the liquid is sufficient to cause the metal wire to stir, which comes into repetitive contact with the internal walls of the tubes, thus effecting cleaning. However, in the case of a gaseous fluid, such a method could not be used, the amounts of movement transmitted by the gases being insufficient under normal conditions of use. An increase in the flow rate of the gaseous fluid to obtain the desired effect would cause the appearance of far too great pressure drops.

FR-A-2 435 292 also suitable in the case of a heat exchanger for the treatment of liquids, uses a mechanical device to periodically stretch a helical spring whose function is to scrape the substances deposited along the walls, thus preventing their deterioration by local overheating. It is recommended to use a tight fit along the wall of the tube.

The use of such a mechanical device for a heat exchanger intended to treat gaseous fluids, in particular at high temperature may exceed 800 ° C, dusty and possibly aggressive, would present considerable difficulties both in terms of design and reliability.

The present invention relates to an automatic method and device for periodic cleaning of the internal surfaces of a heat exchanger for gaseous fluids which makes it possible to obtain a vibration of elastic scraping elements placed inside the channels of the heat exchanger, and this by simple pneumatically actuated means, in order to solve the problems posed by the adaptation of known cleaning devices to heat exchangers intended to treat gaseous fluids.

According to the invention, the automatic method of periodic cleaning of the surfaces of a heat exchanger for gaseous fluids flowing in vertical channels between said surfaces, makes use of elastic elements permanently arranged in said channels and capable of being vibrated to perform cleaning of said surfaces. According to the invention, the elastic elements are vibrated successively for at least one group of channels of the heat exchanger, by means of an injection of an additional compressed gas in a position such that it induces in said group of channels a flow of gaseous fluid coming from the exchanger.

The additional gas injection control can be done manually intermittently, or according to a sequence determined for each group of channels of the exchanger under the control of an automatic control system.

The injection of the additional gas under pressure can be done in the axis or in the plane of symmetry of the channels, or even in an inclined manner depending on the applications.

The injection of the additional compressed gas is preferably carried out by means of nozzles placed in a position upstream of the mouth of each channel of the heat exchanger.

The invention also relates to an automatic device for periodically cleaning the surfaces of a heat exchanger for gaseous fluids which allows the implementation of the method of the invention. The device of the invention comprises injection pipes for additional compressed gas opening upstream, in front of the openings of the channel groups and an injection control device suitable for successively and periodically controlling for each group of channels, an injection of additional compressed gas inducing in the group of channels a flow of gaseous fluid coming from the exchanger, thus causing the vibration of the elastic elements being in the group of channels. These vibrations, which occur both longitudinally, transversely and in rotation, cause a multitude of contacts between the elastic elements and the internal walls of the channels of the heat exchanger, thus causing scraping of these walls and detachment of the solid particles which can then fall under the action of gravity, in the vertical channels and / or be entrained by the gas flow.

The elastic elements are preferably fixed at their two ends in the vicinity of the two ends of the channels.

In a variant, the elastic elements may only be fixed at their upper end, close to the opening, the lower end of the elastic elements then being free.

The injection pipes preferably have injection nozzles which direct the flow of additional compressed gas towards the upper opening of the channels. These nozzles can also serve as a fixing for the upper part of the elastic elements, either directly or by means of additional elements integral with the nozzles or fixed to the nozzles.

The elastic elements are arranged in the vertical channels of the heat exchanger in the immediate vicinity of their internal walls, without however coming into contact with said walls during normal operation of the exchanger, that is to say outside cleaning periods.

During normal operation of the exchanger, the elastic elements therefore act as turbulators disturbing the boundary layer in the vicinity of the internal walls of the channels, which makes it possible to circulate the gas flow at a low speed which is preferably between approximately 8 and 12 m / second, and more particularly between approximately 8 and 10 m / second.

In a preferred embodiment, the elastic elements consist of metal wires wound in a helix. Alternatively, it is possible to use metal wires provided with a plurality of blades extending radially, advantageously aerodynamically profiled, so as to vibrate the whole of the elastic element by the action of the gas flow induced by the additional compressed gas from the injection lines.

• la figure 1 est une vue partielle en élévation en coupe d'un échangeur de chaleur tubulaire comportant un dispositif automatique de nettoyage périodique selon l'invention;
• la figure 2 est une vue partielle en coupe de côté de l'échangeur de la figure 1;
• les figures 3 et 4 illustrent deux variantes de fixation de la partie haute des éléments élastiques;
• la figure 5 illustre en vue agrandie schématiquement et en coupe, une variante de réalisation de l'extrémité haute d'un tube d'échangeur de chaleur; et
• la figure 6 illustre schématiquement en coupe une variante d'élément élastique pouvant être utilisé pour la mise en oeuvre de la présente invention.

The invention will be better understood on studying the detailed description of some embodiments taken by way of non-limiting examples, and illustrated by the appended drawings, in which:

• Figure 1 is a partial sectional elevation view of a tubular heat exchanger comprising an automatic periodic cleaning device according to the invention;
• Figure 2 is a partial side sectional view of the exchanger of Figure 1;
• Figures 3 and 4 illustrate two variants of fixing the upper part of the elastic elements;
• Figure 5 illustrates in enlarged view schematically and in section, an alternative embodiment of the upper end of a tube heat exchanger; and
• FIG. 6 schematically illustrates in section an alternative elastic element which can be used for the implementation of the present invention.

As illustrated in FIGS. 1 and 2, the heat exchanger is of the cross-flow tubular type in which hot and dusty gases flow inside vertical tubes 1, preferably from top to bottom. low. The cooling air flows transversely to the direction of the hot and dusty gases, outside of the tubes 1 and between them. It will of course be understood that the invention could also be applied, without notable modifications, to an exchanger of the tube-shell type with flow parallel to the tubes of the cooling gases, or to another type, and in particular to a heat exchanger. with plates.

The tubes 1 are fixed to the upper 2a and lower 2b head plates by welding according to a conventional method in the construction of exchangers of this type. The tubes 1 thus communicate with an upper plenum 3 which is used for the admission or extraction of hot and dusty gases through an intake or extraction orifice which is not shown in the figures, and with a lower plenum 4 comprising an extraction or admission orifice also not shown. The lower plenum 4 preferably has, as illustrated in FIG. 1, the shape of a hopper making it possible to facilitate the recovery of the solid particles which will settle there during the cleaning operations.

The dimensioning of the gas passage sections is chosen so that a flow speed of between approximately 8 and 12 m / second, and preferably approximately 8 and 10 m / second, is obtained. It is indeed advisable not to adopt a too high flow speed, in order not to create exaggerated pressure drops. Furthermore, a flow rate that is too low would cause prohibitive bulk for the entire apparatus. The choice of the diameter of the tubes is made so as to allow the passage of gases with the suitable flow speed which has just been mentioned, while allowing the insertion of the elastic cleaning elements.

The elastic elements are formed in the example illustrated in Figures 1 and 2 by a metal wire 5 wound in a helix and forming a spring. The springs 5 are rigidly fixed at their upper 6 and lower 7 ends, which both protrude from the upper and lower ends of the tubes 1. The lower ends 7 of the springs 5 are fixed to a grid 8 itself rigidly mounted by means not illustrated in the figures, in the lower plenum 4. In the example illustrated, the grid 8 has a mesh identical to that of the axes of the tubes 1 of the exchanger. It will however be understood that a different attachment could perfectly well be envisaged.

The lower part 7 of the spring 5 is fixed by means of hooks 9 allowing easy disassembly. Here again, other means could be used, and in particular a bolting or pinning fastening, insofar as easy disassembly remains possible.

In the upper part of the heat exchanger and inside the upper plenum 3, are arranged a plurality of injection nozzles 10 for an additional compressed gas which can for example be compressed air or steam pressurized water. The nozzles 10 have ends of small diameter which can be for example between 4 and 10 mm approximately, it being understood that the choice of the diameter of the injection nozzle depends on the diameter of the tubes 1 of the exchanger.

The nozzles 10 are centered on the axes of the tubes 1 and placed at a certain distance above the opening of the tubes 1. It would be possible, in a variant, for the axis of the nozzles 10 to have a certain inclination relative to the 'axis of the tubes 1, which would then direct the compressed additional gas jet towards the periphery of the elastic elements 5, causing a different excitation.

The injection nozzles 10 are connected by small vertical tubes 11 to an injection pipe 12 itself connected to a reservoir of compressed gas 13. It will be noted that in the example illustrated, each injection pipe 12 equipped with its plurality of vertical tubes 11 and injection nozzles 10, allows the injection of gas into a row of tubes 1 (Figure 2).

A control valve 14 which can be actuated manually or by means of a solenoid valve controlled by an automatic mechanism, allows the periodically controlled injection of the additional compressed gas contained in the reservoir 13, for this row of tubes 1.

The attachment of the upper end 6 of the springs 5 can be made directly on the vertical tubes 11. Referring to FIG. 3, there is seen a first embodiment of such an attachment. According to this embodiment, the injection tube 11 is provided with longitudinal fins 15 in which are made perforations 16 allowing the passage and the winding of the upper end of the spring 5. FIG. 4 shows an alternative embodiment in which the spring 5 is terminated by a winding 17 of smaller diameter than the spring 5, the winding 17 being threaded on the end of the injection tube 11 and blocked by a clamping element 18. It will of course be noted that 'it would be perfectly possible to fix the upper ends of the springs 5 by other means, for example directly on the injection pipe 12, or else on a separate support rigidly mounted in the upper plenum 3.

The device of the invention operates in the following manner. To carry out periodic cleaning of the internal walls of the tubes 1, a gas is injected into a row of tubes 1 additional tablet at a pressure of the order of 2 to 6 bars via the nozzles 10. This injection, which is carried out for a relatively short duration, for example between 1 / 10th of a second and a few seconds, instantly induces a flow of fluid gas from the upper plenum 3 and the tubes 1 of the adjacent rows. This induced gaseous fluid flow rate is of the order of four to six times the flow rate of the additional compressed gas injected through the nozzles 10. The flow speed thus created inside the tubes 1 is therefore very high. The amount of movement thus provided is communicated to the springs 5 and the resulting agitation is damped in the flow and along the walls of the tubes 1 by impact and scraping, which results in cleaning and maintenance of the state. internal surfaces of the tubes 1 of the heat exchanger.

It is thus possible to prevent fouling of the exchanger tubes without generating excessive pressure losses since the flow rate, in normal operation outside of the cleaning periods, can be chosen at a relatively low value as mentioned earlier. The heat exchange performance is also improved thanks to the insertion inside the tubes of the springs 5 which play the role of turbulators whose effect of removing the boundary layers compensates for the reduction in the flow speed. The use of a manual or automatic control system makes it possible to perfectly control the frequency of injection of the additional compressed gas and thus to optimize the wear and the frequency of replacement of the elastic elements constituted by the springs 5.

FIG. 5 illustrates a variant of the device of the invention in which the upper end of each tube 1 of the exchanger is fitted with a mouth piece 19 partially penetrating inside the tube 1. The piece of mouth 19 can be fixed to tube 1 by threading as illustrated in FIG. 5, or by any other means such as snap-fastening, welding, gluing, etc. The mouthpiece 19 is profiled in the manner of a nozzle convergent, so as to induce a greater flow of gaseous fluid under the effect of the injection of the additional compressed gas by the nozzles 10 placed as before at a certain distance from the mouth of the tubes 1. It is thus possible to further reduce the flow rate of the additional compressed gas necessary for the periodic cleaning operation.

FIG. 6 schematically shows an elastic element of different structure which can be used in the context of the invention. In this figure, there is shown a tube 1 inside which the elastic element is constituted by a cable 20 having slight undulations and provided with a plurality of blades 21 extending radially and having an aerodynamic profile so as to be able to be driven in a vortex manner in the gas flow parallel to the axis of the tube 1. The blades 21 then cause the cleaning by impacts and scraping as previously.

In all cases, it will be noted that it is important that the elastic element constituted by the spring 5 or by the cable 20 provided with the fins 21, or even by any other equivalent means, be placed inside the tube 1 or of the vertical channel of the exchanger, so as to be in close proximity to its internal walls, without however coming into contact with said walls during normal operation of the heat exchanger outside of the cleaning periods. As a result, the boundary layer is effectively disturbed by portions of the elastic element located in the vicinity of the internal walls of the tubes 1 and cleaning is better ensured during the injection of compressed gas.

In the examples illustrated, the elastic elements have been rigidly fixed at their upper and lower ends. It will however be understood that it would be possible in a variant not to fix the lower ends of the elastic elements. These then remain free of any hindrance in the vicinity of their lower end 7 and can somehow float in the gas flow. The vibration characteristics caused by the injection of additional compressed gas and the induced gas flow are then different and can be adapted to certain particular problems of clogging.

Claims (16)

1. Automatic process for periodically cleaning the surfaces of a heat exchanger for gaseous fluids flowing inside vertical channels between the said surfaces, by means of elastic members (5, 20, 21) arranged permanently inside the said channels (1) and capable of being made to vibrate so as to perform cleaning of the said surfaces, characterized in that vibration of the elastic members (5, 20, 21) is performed successively for at least one group of channels (1) of the heat exchanger by injecting an additional supply of compressed gas in a position such it induces a flow of gaseous fluid from the exchanger, inside the said group of channels.

2. Automatic process according to Claim 1, characterized in that the injection of additional gas is controlled manually at intervals.

3. Process according to Claim 1, characterized in that the injection of additional gas is performed in a given sequence for each group of channels of the exchanger under the control of an automatic control device.

4. Process according to any one of Claims 1 to 3, characterized in that the additional gas is injected along the axis or plane of symmetry of the channels.

5. Process according to any one of Claims 1 to 3, characterized in that the additional gas is injected at an angle relative to the axis or plane of symmetry of the channels.

6. Process according to Claim 4 or Claim 5, characterized in that the additional gas is injected by means of nozzles (10) located in a position upstream of the opening of each channel (1).

7. Automatic device for periodically cleaning the surfaces of a heat exchanger for gaseous fluids flowing inside vertical channels (1) defined between the said surfaces, comprising elastic members arranged permanently inside the said channels (1) and capable of being made to vibrate so as to perform cleaning of the said surfaces, characterized in that it comprises pipes (10, 11, 12) for injecting additional compressed gas, the ends of which pipes are located in front of the openings of groups of channels (1), and an injection control device (14) designed to control, successively and periodically, for each group of channels, injection of an additional supply of compressed gas inducing inside the said group of channels (1) a flow of gaseous fluid from the exchanger, causing vibration of the elastic members (5, 20, 21) inside the said group of channels.

8. Device according to Claim 7, characterized in that the elastic members are fixed at least at their top ends close to the upstream opening of the channels.

9. Device according to Claim 7 or Claim 8, characterized in that the elastic members are fixed at their two ends in the vicinity of the ends of the channels.

10. Device according to any one of Claims 7 to 9, characterized in that the injection pipes comprise the injection nozzles (10) directing the flow of additional compressed gas towards the upper opening of the channels (1).

11. Device according to Claim 10, characterized in that the elastic members (5) are fixed to the said nozzles (10, 11).

12. Device according to any one of Claims 7 to 11, characterized in that the elastic members (5, 20,21) are arranged inside the channels (1) in the immediate vicinity of their internal walls without, however, coming into contact with the said walls during normal operation of the exchanger, when cleaning is not being performed.

13. Device according to Claim 12, characterized in that the elastic members consist of helically wound metal wires (5).

14. Device according to Claim 12, characterized in that the elastic members consist of metal wires (20) provided with a plurality of radially extending vanes (21).

15. Device according to any one of Claims 7 to 14, designed for a tubular heat exchanger, characterized in that each tube is equipped internally with an elastic member cooperating with an injection nozzle (10) communicating with a pipe (12) for injecting additional pressurized gas.

16. Device according to Claim 15, characterized in that the top end of each tube (1) has a mouthpiece (19) with a converging profile cooperating with the injection nozzle (10) in order to increase the flow of gaseous fluid induced by the injection of additional compressed gas.

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