FR3037389B1 - TUBULAR HEAT EXCHANGER COMPRISING A CRACKING CONTROL MEMBER, ITS IMPLEMENTATION METHOD AND ITS MOUNTING METHOD - Google Patents

Abstract

Heat exchanger comprising - an enclosure (3, 4, 16), - a plurality of tubes (2) arranged in the enclosure, each tube comprising a peripheral wall, - input means of a first fluid in the tubes - means for outputting the first fluid out of the tubes - input means for a second fluid in the chamber, towards the outer periphery of the tubes - means for outputting the second fluid out of the chamber, said exchanger being characterized in at least one tube (2) is equipped with at least one member (5) for controlling the fouling of its peripheral wall, this control member comprising - a body (51) disposed on the outer face (2 ') of this wall, - an electrical resistance (52) extending in the body (51) - measuring means (53), for measuring the temperature of the electrical resistance (52).

Description

Tubular heat exchanger comprising a fouling control member, its method of implementation and its method of assembly Technical field of the invention

The invention relates to the field of industrial heat exchangers. It relates more specifically to the monitoring of fouling (also called fouling) inside these exchangers. The invention also relates to the probes for carrying out this monitoring, and the methods for detecting fouling using appropriate probes. More particularly, the invention relates to the monitoring of fouling of a shell-and-tube heat exchanger type heat exchanger comprising tubes made of graphite and or graphite-based. The invention more specifically targets such an exchanger, in which the fluid flowing outside the aforementioned tubes undergoes a phase change, namely that it is admitted in the vapor state and is evacuated in the predominantly liquid state. Such heat exchangers are used in particular in installations for concentrating phosphoric acid.

State of the art

The fouling (in English "fouling") of pipes in which a fluid circulates is a well-known phenomenon. It is related to the undesirable deposition of material from said fluid; this material is deposited on the internal walls of the pipes. This deposit often shows a structure in successive layers. It modifies the conditions of heat exchange between the fluid and the tube. It ends up narrowing the inner diameter of the tube; thus it modifies the hydrodynamic flow conditions of the tube and possibly of the whole piping system of which the tube is a part: to guarantee a constant flow the flow velocity must be increased, which can lead to certain parts of the piping system to phenomena of turbulence or cavitation. Alternatively, at constant flow rate, the effectiveness of the tube as a conveying means and / or heat exchanger decreases. Similarly, the degradation of the heat exchange due to the undesirable deposition of material requires a change in the operating regime of the piping system. Thus, the piping system may be forced to operate under conditions for which it has not been sized, or it must operate under operating conditions that no longer fulfill the function for which it was sized.

Moreover, the deposited layer, due to the modification of the flow conditions in the tube, can spontaneously release particles of deposited material which is driven by the fluid, and which can cause problems in the fluid system: it is susceptible obstruct filters, orifices, nozzles, or contaminate the fluid. This release may be related to the erosion of the layer, the flaking, the internal stresses in the deposited layer and the interface between this layer and the tube, or to a spontaneous rupture related to an external effect (blow , vibration, thermal expansion).

The deposited layer may change over time, not only in thickness but also by transformation of its chemical structure, for example under the effect of the temperature of the transported fluid.

In general, the fouling of the tubes of an industrial exchanger represents a cost factor: the efficiency of the tube, in terms of flow and heat exchange, decreases; the maintenance intervals of the exchanger must take into account the fouling; and finally an undetected clogging can cause irreversible damage to the equipment. In certain types of industrial installations, for example certain heat exchangers (for example for the concentration of phosphoric acid), clogging of the tubes requires the complete shutdown of the exchanger for maintenance or repair.

For example, the fouling of the heat exchangers used in the phosphoric acid concentration units is a well-known phenomenon. These heat exchangers are generally of the shell and tube heat exchanger type and include several hundred (or more than a thousand) straight and parallel tubes made of graphite and / or graphite within which phosphoric acid circulates; between said tubes circulates a coolant, confined in the interior volume formed by the aforementioned calender. In these tubes, a deposit is formed over time that progressively turns into gypsum, an insoluble compound that is very difficult to remove. A typical maintenance procedure to remove this deposit involves shutting down the installation and rinsing the tubes with a hot solution of 5% sulfuric acid: this process works well for fairly thin layers, but becomes more difficult for thick layers. Indeed, the layers of gypsum tend to swell under the effect of sulfuric acid, and this can cause significant internal stresses in the pipes, with a risk of cracking or even rupture; this risk is particularly strong in the case where the material used for the tube is non-metallic (ceramic or graphite in particular).

Each stop of the exchanger causes a loss of production. According to a general trend of preventive maintenance, it would be desirable to be able to carry out the maintenance operations when they are necessary instead of at regular intervals, as maintenance at regular intervals can lead to a maintenance operation that is too frequent. late, but also to an early maintenance operation. In order to be able to carry out predictive maintenance, it would be necessary to be able to detect the fouling condition of a tube: this makes it possible to take a decision as to the desirability of a maintenance operation.

An extremely undesirable phenomenon, the fouling of tubular heat exchangers can not be easily detected. There are indirect methods: a pressure drop in the system can be detected by local pressure measurements, but this does not make it easy to locate the narrowed area of the tube. Local temperature measurements can also detect a change in the temperature difference between two points of the tube, indicating a decrease in the heat exchange in the tube; to locate the location of the narrowed tube requires a large number of thermal probes.

These methods work pretty well. Their major disadvantage is that the probes are external to the system: as such they are usually added to the installation by the operator of the installation, which is also responsible for instrumentation and monitoring. As such, the probes can be omitted, or remain disconnected by the operator after maintenance operations, or be stolen. In addition, the detection thresholds and the generation of alerts can be set (or disordered) by the operator.

It would therefore be desirable to be able to dispose in an industrial installation having tubular elements, and in particular in an industrial installation having tubular elements inside which continuously circulate corrosive, flammable, hot or toxic fluids, and therefore in an installation industrial, whose shutdown and maintenance require specific precautions, a system for detecting the fouling of said tubular elements, said system showing good reproducibility and reliability, without drifting the signal over time, and which system uses probes fouling integrated with said tubular elements to be monitored.

More particularly it would be desirable to have a tubular heat exchanger provided with a reliable and simple device for detecting the fouling condition of the tubes, in particular for graphite and graphite-based tubes in which the tube circulates. 'Phosphoric acid.

WO-A-2007/099240 discloses a heat exchanger, equipped with a device for evaluating its fouling state. This device can be placed at different points of the exchanger. Typically it is interposed between, on the one hand, the plate located at the end of the stack and, on the other hand, the frame of the exchanger or an additional plate of dummy type. The solutions described in this document are not applicable to a tubular heat exchanger such as that targeted by the invention.

WO-A-2009/153323 discloses a method for the detection and / or measurement of fouling in exchangers. A resistor is placed at a wall of this exchanger and this resistor is subjected to two successive power levels. This document deals more specifically with the electrical aspect of this detection and does not provide detailed structural information as to which exchangers can be controlled by this method. Again, the teaching of this state of the art can not be transposed in a simple way to an exchanger covered by the present invention.

Object of the invention

Thus a first object of the invention is a heat exchanger comprising:

an enclosure having a peripheral grille delimiting an interior volume,

a plurality of tubes disposed in said interior volume, each tube comprising a peripheral wall,

input means of a first fluid in said tubes,

means for outputting the first fluid out of said tubes,

input means of a second fluid in said interior volume, towards the outer periphery of the tubes,

means for outputting the second fluid from said interior volume, said exchanger being characterized in that at least one tube is equipped with at least one member for controlling the fouling of its peripheral wall, this control member comprising

a body disposed on the external face of this wall,

- an electrical resistance extending into the body

measuring means for measuring the temperature of the electrical resistance.

According to other features of the invention:

a) the body of the control member is a collar surrounding at least partially the outer face of the wall of the tube.

b) the collar is surrounded by an insulating sleeve to prevent a substantial loss of heat opposite the wall of the tube.

c) the enclosure comprises at least one end plate, which is pierced with axial holes for the passage of the tubes, and at least one auxiliary channel connecting the outside and an axial orifice occupied by a tube equipped with a control member for fouling, this control member being equipped with a sheath containing at least one functional wire of said member, this sheath extending in the auxiliary channel.

d) several tubes are equipped with a fouling control member, in particular between two and eight tubes, in particular between four and six tubes.

e) the length of each tube is between 1 and 9 meters, in particular between 3 and 9 meters.

f) the outer diameter of each tube is between 45 and 55 millimeters, especially between 50 and 52 millimeters.

g) the inside diameter of each tube is between 35 and 45 millimeters, in particular between 37 and 39 millimeters.

h) the so-called safety length separating the top of the tube, adjacent to the fouling control member, and the end of the fouling control member opposite said top, is between 0.1 and 3 meters, especially between 0.5 and 2 meters.

These additional features (a) to (h) can be implemented individually or in any technically compatible combination.

Another subject of the invention is a method for controlling the fouling of a heat exchanger comprising

an enclosure having a peripheral grille delimiting an interior volume and

a plurality of tubes extending in said interior volume, each tube comprising a peripheral wall, a process in which

a first fluid, in particular phosphoric acid, is admitted into the tubes and this first fluid is discharged from the tubes;

a second fluid is circulated in said interior volume, at the outer periphery of the tubes, countercurrently to said first fluid,

said method being characterized in that

the body of a fouling control member is provided on the outer face of the wall of at least one tube, this control member further comprising an electrical resistance extending in the body of the measuring means for measuring the temperature of the electrical resistance

the electrical resistance is electrically powered,

at least one so-called instantaneous curve is produced, each instantaneous curve showing the evolution as a function of time of the temperature of the resistance,

the instantaneous or each instantaneous curve is compared with at least one reference curve.

According to other features of the invention:

i) the reference curve shows the evolution as a function of time of the temperature of the resistance, for a tube whose fouling has a known threshold thickness.

j) flowing the first fluid into the tubes at a substantially constant temperature as a function of time, at least in the vicinity of the control member.

k) flowing the first fluid in a substantially vertical direction, from bottom to top.

l) the fouling control member is placed so that the so-called safety length, separating the top of the tube, adjacent to the fouling control member, and the end of the control member fouling opposite said vertex, is between 0.1 and 3 meters, especially between 0.5 and 2 meters.

m) causing at least a partial condensation of the second fluid during its journey into the interior volume.

n) phosphoric acid is flowed inside the tubes.

These additional features (i) to (n) can be implemented individually or in any technically compatible combination.

Yet another object of the invention is a method of mounting a heat exchanger as above, wherein:

one end of each tube equipped with its fouling control member is fixed in the axial orifice formed in the end plate of the enclosure,

the auxiliary channel is arranged in the plate and the sheath of said control member is passed through said auxiliary channel,

- Rebunching said auxiliary channel.

Filling of said auxiliary channel can be done using a suitable cement.

Yet another subject of the invention is a heat exchange tube belonging to the exchanger as above, this tube being equipped with at least one device for controlling the fouling of its peripheral wall, this control comprising a body disposed on the outer face of this wall, an electrical resistance extending in the body, measuring means for measuring the temperature of the electrical resistance.

Yet another object of the invention is a phosphoric acid concentration plant comprising at least one heat exchanger as above.

The Applicant has realized that the problem presented above can be solved by integrating at least one fouling control element, of the active probe type, in at least one of the tubular elements of the heat exchanger to be monitored. . A probe is said to be active because it measures the response of the perturbation it has itself generated.

More precisely, the so-called active probe can locally cause a controlled thermal disturbance (advantageously a controlled heating and / or the emission of a predetermined quantity of energy). In addition, a means is provided for measuring the local consequence of this controlled disturbance or the response to this controlled disturbance.

Thus, the active probe comprises at least one means for emitting a certain amount of thermal energy. In one embodiment, said active probe comprises means for rapidly heating a surface area of said tubular element, for creating in said surface zone a determined temperature difference, and / or for dissipating in said surface zone a predetermined quantity of energy.

The system according to the invention also comprises at least one heating measurement means caused by said energy emission. In one embodiment, this measuring means is a thermocouple, positioned in a manner adapted to collect a usable signal. It can be inserted into a hole provided for this purpose in a surface area of said tubular element.

Advantageously, the active probe has a collar which is fixed around the outer surface of the tubular element, so as to cover at least one segment of said outer surface. Said active probe can be connected to at least one means for measuring the temperature.

The heat exchanger of the invention comprises a plurality (and typically a large number, for example several hundred or more than a thousand) of tubes, also called tubular elements, which are typically identical to each other, as well as at least a tubular element (typically identical to the others, except for the following) which comprises on its outer wall or in its outer wall at least one active sensor capable of locally causing a thermal disturbance, and at least one means for measuring said thermal disturbance.

In one embodiment of this exchanger, said tubular element extends between two parallel end plates, also called tubular plates, and each end of said tubular element is inserted into an orifice formed in one of the two tubular plates. One of these tubular plates is fixed, the other can be mobile.

The installation of the active probe on the outer face of the wall of the tubular element is advantageous. Indeed, it avoids a direct contact between the probe and the fluid flowing inside the tubular element. However, such contact is undesirable for reasons related to corrosion, the difficulty of ensuring a tight insertion of the probe in the channel of the tubular element, and the possible disruption of hydrodynamic flow conditions. of said fluid by the probe itself.

The term "on" the outer face means first of all that the body of the control member, for example made in the form of a collar, can be fixed against this external face, which is then smooth, or in a recess formed in this outer face. This body can also be placed in the immediate vicinity of this external face, without however being in direct contact with it.

Said heat exchanger can be integrated into an industrial installation. More particularly, said installation may be a phosphoric acid concentration plant.

It is the merit of the Applicant to have developed a fouling control member, which is adapted to a shell and tube heat exchanger type. In addition, it is advantageous to position this exchanger in an area where the first fluid circulating in the tubes has the highest temperature. Indeed, the fouling layer has a maximum thickness in this area, particularly in the case where circulates phosphoric acid, whose temperature rise leads to a significant deposition of gypsum.

Said tubular elements may be made of any material usually used for heat exchanger tubes, and more particularly of metal (such as: steel, stainless steel, plated steel, zirconium, titanium, aluminum, copper and their alloys) or non-metallic material (such as: PTFE, ceramic, graphite). Advantageously, said tubular elements are made of graphite or graphite-based.

figures

Figures 1 to 6 illustrate embodiments of the invention.

Figures 1a to 1f show a vertical section through a heat exchanger according to the invention; to simplify the figure it shows only a single tube 2 taken between two tubular plates 3,4; these figures illustrate an embodiment of the method according to the invention.

Figure 2 shows schematically a perspective view of a tube 2 according to the invention with the fouling control member 5 in the form of a collar which at least partially surrounds the circumference of the tube.

FIG. 3 is a view in longitudinal section, illustrating in more detail the constituent elements of the control member 5.

Figure 4 is a longitudinal sectional view of a tube 2 according to the invention with a curve which schematically shows the heat transfer along the different media.

FIG. 5 is an electrical diagram of the collar, belonging to the fouling control member,

FIG. 6 represents several curves, illustrating the variation of the temperature of the integrated resistance in this collar, for several states of fouling of the exchanger.

The following reference signs are used in the figures:

1 Heat exchanger 51 Necklace 2 Graphite or graphite tube L51 Length of 51 2 ' External side of 2 51 ' Active surface 51 2 " Internal side of 2 51B End of 51 3 First tube plate (fixed) 52 Resistance 4 Second tube plate (Sliding) 53 Thermocouple 5 Fouling control body 54 52 Power Terminals 6 Sheath 55 Feeding wires 52 7 Gluing area 1 ^st end tube 58 Muff 8 Sealing area hole 2 ^nd plate E Fouling layer 9 Seaming area 2 ^nd end tube F1 Flow of the first fluid 10 Sealing area hole 1 ^ere plate F2 Flow of the second fluid 11 Auxiliary channel S2 Top of the tube 2 12 Orifice LS Security length 13 Shirt for hole 2 ^nd plate CO Curve 14 Shirt for hole 1 ^ere plate C1 Curve 15 Sealing plug C2 Curve 16 Outer wall C3 Curve 17 Flange Dint Inside diameter of 2 Dext Outside diameter of 2

detailed description

Here we mean by "tube" or "tubular element", a construction element 5 having at least one channel within which a liquid can flow when said tubular element is used in accordance with its intended purpose.

FIG. 1 illustrates a heat exchanger according to the invention, designated as a whole by reference numeral 1. This exchanger firstly comprises, in a manner known per se, an enclosure formed by end tube plates 3, 4 as well as by a peripheral shell forming an outer wall, or a shell 16.

This exchanger, used in installations of the chemical industry, typically comprises a plurality of parallel straight tubes 2 graphite or graphite-based, which are received in the enclosure. Typically it involves several hundred or more than a thousand graphite or graphite tubes. These "tubes and shell" type exchangers are known as such. They include the aforementioned shell 16 and said plurality of straight and parallel tubes 2.

Preferably, these tubes 2 are graphite and or graphite-based. Their length L2 is typically between 1 and 9 meters, especially between 3 and 9 meters. In a preferred embodiment of the invention, this exchanger is arranged so that the tubes 2 are oriented vertically. This arrangement has advantages which will be explained below.

Note also Dlnt and Dext diameters respectively inside and outside of each tube 2, which are measured in the uncured state of the tube. The internal diameter Dlnt is typically between 35 and 45 millimeters, especially between 37 and 39 millimeters; in one embodiment it is 38.1 millimeters. The outside diameter Dext is typically between 45 and 55 millimeters, in particular between 50 and 52 millimeters; in one embodiment it is 50.8 millimeters.

These tubes 2 are held by the plates called tubular plates 3, 4 into which they are inserted. Each tube 2 is inserted into an orifice 12b arranged in the first tube plate 3 and fixed, for example by gluing, and each tube 2 is also inserted into an orifice 12a formed in the second tube plate 4 and fixed, for example by gluing. Said first and second tubular plates 3, 4 may be made of graphite.

The exchanger further comprises means for inputting a first fluid into the tubes, means for outputting the first fluid out of the tubes, means for introducing a second fluid into the chamber, towards the outer periphery. tubes, and means for outputting the second fluid out of the envelope. These different input and output means of known type are not illustrated in the figures. Preferably, the first fluid is phosphoric acid while the second fluid is water vapor.

In an advantageous embodiment the first tubular plate is a fixed plate 3, that is to say that it is fixed on the not shown head of the exchanger 1, typically using a flange 17 of Tightening. The second tube plate 4 is advantageously a sliding plate, that is to say movable relative to the shell 16. Each tube 2 is inserted into the orifice 12a formed in this sliding tubular plate 4 and fixed by gluing.

According to the invention at least one tube 2 of said heat exchanger 1 is equipped with a fouling control member, generally designated by the reference 5. Preferably a tube is selected near the outer wall 16 of the exchanger 1, as shown in Figure 1. Advantageously thus equips several tubes 2, for example two, three, four or five tubes 2.

Said control member 5, which is of the active probe type, is for example in accordance with that marketed by NeoTim in Albi (France), which is the subject of FR-A-2 932 886. As shown in particular in FIGS. and 3, this control member comprises first of all a body made in the form of a collar 51, which at least partially surrounds the tube 2, namely that it is arranged against the outer face 2 'of the wall of this tube. Note also 2 "the inner face of the wall of this tube.

This collar 51, which is in the form of a planar and flexible element, of integrated circuit type, comprises an electrical resistance 52, shown schematically in FIG. 3. In practice, the electrical resistance can be realized, as illustrated in FIG. Figure 5, in the form of a metal track deposited on the collar. This resistor 52 is associated with one or more thermocouples 53, making it possible to measure the temperature in the direct vicinity of this resistance. Two power supply terminals 54 are provided, with all the necessary connectors, to be able to supply this resistance from outside the exchanger. In addition the thermocouples are associated with at least one output terminal not shown, for transmitting the electrical signal delivered in use by this thermocouple.

The control member 5 is further equipped with a sheath 6, the implementation of which will be detailed in the following. This sheath 6 encloses at least one functional wire of the active probe, in particular wires 55 for supplying the resistor, electrically connected to the terminals 54, and at least one signal wire connected to the output terminal of the thermocouples. . These different functional threads can be distinct or confused. This sheath is placed in electrical communication with an alarm, not shown, of any appropriate type, which makes it possible to alert the operator in the event of a malfunction, as will be explained in what follows.

L51 is the length of the contact surface between the collar and the wall of the tube 2, according to the main direction of the latter. This length is for example between 0.02 and 0.1 meters. Preferably, the collar 51 is provided in the upper part of the tube, namely that in which the fluid flowing inside this tube, typically phosphoric acid, is the hottest. Indeed, the fouling is maximum in this zone. LS is the so-called safety length, separating the top S2 of the tube and the lower end 51B of the collar 51, which is opposite said vertex. This length LS is for example between 0.1 and 3 meters, especially between 0.5 and 2 meters.

The control member 5 is surrounded, on its opposite end faces as well as on its outer face, by a sleeve 58 made of a thermally insulating material. This sleeve thus allows anisotropic heat transfer from the collar to the tube. In other words, when the collar is heated, this heat dissipates substantially completely in the wall of the tube 2.

The method of placing a tubular element provided with an active probe according to the invention is illustrated in FIGS. 1a to 1f. They show schematically a graphite tube 2 intended to be inserted in the orifice 12b of the first tubular plate 3 (fixed plate) and in the orifice 12a of the second tubular plate 4 (sliding plate); this tube 2 is situated at the periphery of the tube bundle, close to the external wall 16 of the heat exchanger 1.

In a first step illustrated in Figure 1a is carried out a tracking, or prior control. For this purpose the tube 2 is positioned within the fixed tubular plate 3, in the precise position which it must adopt in use. The walls facing the tube and the openings of the plagues 3 and 4 are not glued. This identification makes it possible to check whether the sheath 6 protrudes out of the orifice 12a of the tubular flap 4 sliding, at a distance appropriate for the implementation of the subsequent steps. If this distance is not suitable, the position of this sheath is modified in a detailed manner.

In a second step illustrated in FIG. 1b, said tube 2 is moved through the orifice 12a of the sliding tubular plate 4, and a suitable glue is applied to a gluing zone 7 which represents at least a part of the external surface. tube mistletoe protruding from said sliding tubular plague, and applying a suitable glue on a gluing area 8 which represents at least a portion of the inner surface of the orifice 12b of the fixed tubular plate 3.

In a third step illustrated in FIG. 1c, said tube is moved through the orifice 12b of the fixed tubular plate 3 and through the orifice 12a of the tubular sliding plate 4, in order to spread the glue on the surfaces to be glued. .

In a fourth step illustrated in FIG. 1d, the tube 2 is placed in the orifices 12a, 12b and the sheath 6 is recovered in an auxiliary channel formed by a lateral orifice 11 formed in the tubular sliding plate 4. and which communicates with the orifice 12a. As a variant this orifice 11 can be arranged in an axial direction. Its diameter may be of the order of 10 mm.

In a fifth step illustrated in FIG. 1e, a jacket 13, 14, that is to say a tubular insert, is inserted respectively into the orifice 12b of the fixed tubular plate 3 and into the orifice 12a of the sliding tubular plate. 4. At least a portion of the outer surface of said shirts 13,14 is coated with a suitable glue.

In a sixth step illustrated in FIG. 1f, the lateral orifice 11 is closed by a suitable cement forming a sealing plug 15.

In operation, the two fluids are flowed according to the arrows F1 and F2, respectively in the tubes and around these tubes, in order to put the aforementioned fluids in exchange for mutual heat. In an advantageous embodiment, the exchanger according to the invention is used in position "vertical tubes" with a first fluid, called "process fluid", which is heated by the second fluid, called "fluid service". This exchanger is used in such a way that at least the process fluid, which is advantageously phosphoric acid in the liquid phase, undergoes a temperature change such that the temperature at the top of the exchanger is higher than the temperature. in foot of exchanger.

Advantageously, the service fluid is water vapor, which undergoes partial condensation as it transfers its heat to the process fluid. Due to the heating of the process fluid, fouling of the tube can be rapid and considerable, especially in the upper zone of this tube. The inventors have surprisingly found that the fouling member, disposed in the vicinity of the outer wall of the tube, is capable of detecting this fouling present against the inner wall of this tube. This is even more surprising in the case of using graphite or graphite tubes which have a significant thickness, of the order of 5 to 10 millimeters.

Advantageously, the first fluid flows in the tubes 2, at a substantially constant temperature as a function of time. This means that the temperature of this first fluid can vary in space, ie from one end to the other of the tubes. However, this temperature is invariant in time, at least in the vicinity of the control member, or at any point of the tube.

The resistor 52 is then energized electrically. This supply may be effected by pulses or, preferably, by a current step. The resistance heats up and the calories thus generated dissipate substantially completely in the wall of the tube, due to the presence of the insulation. Thermocouples measure the evolution of the temperature of this resistance, as a function of time. Since the temperature of the first fluid is constant, as explained above, it will be appreciated that the measured temperature of the resistor is representative of the temperature difference between the resistor and the first fluid.

Typically, the power supply phase of the resistor is of the order of seconds to minutes. It is conceivable that, as a function of time t, the temperature T52 of the resistance increases and then reaches a plateau. Characteristic curves of this evolution are shown in FIG. 6, which will be explained in what follows.

The appended FIG. 4 further shows the heat transfer between the collar and the first fluid, along the wall of the tube and then with a fouling layer. The temperature decreases firstly by conduction (k conductivity) through the wall of the tube, according to a DT2 value. Then there is a layer E said fouling, which tends to form as the operating time of the exchanger. This layer induces an additional thermal resistance rE per unit area, which is associated with a decrease in temperature DTE. The thicker the fouling layer, the higher the DTE value and the higher the temperature difference between the active surface and the first fluid. In other words, the assembly formed by the wall and the fouling layer has a thermal conductivity lower than the thermal conductivity of the wall alone.

Therefore, the thicker the fouling layer E is, the higher the temperature of the resistance. Referring again to FIG. 6, the CO curve illustrates the change in temperature for a non-fouled heat exchanger. The curves C1 to 03, which illustrate increasingly clogged exchangers, are therefore located above this CO curve.

Advantageously, a reference measurement is first made, corresponding to the change in temperature for an exchanger having a fouling layer, the known thickness of which has a threshold value. In other words, for a thickness greater than this threshold value, the exchanger is typically stopped and the interior of the tubes is cleaned. The evolution of the temperature of the resistance as a function of time, for this exchanger having this threshold fouling layer, is materialized by the curve C2.

As the heat exchanger is put into operation, successive, so-called instantaneous measurements of the temperature variation T52 are made as a function of time t. When a curve resulting from an instantaneous measurement is located above the reference curve C2, this means that the fouling layer, of unknown thickness, is thicker than the threshold layer and that it is therefore necessary to stop the exchanger. In Figure 5, the curve C1 below the curve C2 materializes an acceptable fouling situation. In contrast, the curve C3 located above this curve C2 illustrates a situation where it is necessary to stop and clean the exchanger.

The process according to the invention can be carried out in a phosphoric acid concentration plant. In particular, such an installation comprises a heat exchanger in which the phosphoric acid is heated using a heat-transfer medium (usually water vapor) to be concentrated in an evaporator. It is preferred that the temperature difference between the coolant (in this case: water vapor) and the process fluid (in this case phosphoric acid) is not too high in order to limit the fouling of the elements. tubular and thermal stresses in the installation. Advantageously, the temperature of the steam used will be between 110 ° C and 160 ° C while the temperature of the phosphoric acid will be between 70 ° C and 90 ° C.

By way of example, a heat exchanger is considered in which the water vapor, forming a coolant, heats the phosphoric acid, which forms the process fluid, with a view to its concentration in an evaporator. In this case, it is preferred to conduct the process so that the phosphoric acid enters the tubular element of the heat exchanger at about 82 ° C, is heated with water vapor at a temperature of of 133 ° C, and leaves said tubular element at a temperature of 85 ° C.

Claims (18)

1. Heat exchanger comprising; an enclosure (3, 4, 16) having a peripheral grille delimiting an interior volume (V), a plurality of tubes (2) made of graphite or graphite, arranged in said interior volume (V), each tube comprising a peripheral wall, phosphoric acid inlet means in said tubes (2), phosphoric acid outlet means out of said tubes (2), input means of a second fluid in said internal volume (V) towards the outer periphery of the tubes means for outputting the second fluid from said interior volume (V), said exchanger being characterized in that at least one tube (2) is equipped with at least one member (5) for controlling the fouling of its peripheral wall, this control member comprising »a body (51) disposed on the outer face (2 ') of this wall,» an electrical resistance (52) extending in the body (51) measuring means (53) for measuring the temperature of the electrical resistance (52). 2. Heat exchanger according to the preceding claim, characterized in that the body of the control member (5) is a collar (51) at least partially surrounding the outer face of the wall of the tube. 3. Heat exchanger according to the preceding claim, characterized in that the collar (51) is surrounded by an insulating sleeve (58) adapted to prevent a substantial loss of heat opposite the wall of the tube (2). 4. Heat exchanger according to one of the preceding claims, characterized in that the enclosure comprises at least one end plate (4), which is pierced with axial orifices (12a) for the passage of the tubes, and at least one auxiliary channel (11), connecting the outside and an axial orifice occupied by a tube equipped with a fouling control member (5), this control member being equipped with a sheath (6) enclosing at least one functional wire of said member, which sheath extends into the auxiliary channel (11). 5. Heat exchanger according to one of the preceding claims, characterized in that several tubes are equipped with a fouling control member, in particular between two and eight tubes, in particular between four and six tubes. 6. Heat exchanger according to one of the preceding claims, characterized in that the length of each tube is between 1 and 9 meters, especially between 3 and 9 meters. 7. Heat exchanger according to one of the preceding claims, characterized in that the outer diameter (Dext) of each tube is between 45 and 55 millimeters, in particular between 50 and 52 millimeters. 8. Heat exchanger according to one of the preceding claims, characterized in that the inner diameter (Dint) of each tube is between 35 and 45 millimeters, in particular between 37 and 39 millimeters. 9. Heat exchanger according to one of the preceding claims, characterized in that the so-called safety length (LS), separating the apex (S2) of the tube, adjacent to the fouling control member, and the end (51 B) of the fouling control member opposite said vertex, is between 0.1 and 3 meters, especially between 0.5 and 2 meters. 10. Method for controlling the fouling of a heat exchanger comprising an enclosure (3, 4, 16) having a peripheral grille delimiting an interior volume (V) and a plurality of graphite or graphite-based tubes (2) extending into said interior volume (V), each tube comprising a peripheral wall, a process in which Phosphoric acid is admitted into the tubes and this phosphoric acid is discharged from the tubes; a second fluid is circulated in said inner volume (V) at the outer periphery of the tubes, counter-current to said phosphoric acid, said method being characterized in that the body (51) of a fouling control member (5) is arranged on the outer face (2 ') of the wall of at least one tube (2), this control member comprising in addition> an electric resistor (52) extending in the body (51)> measuring means (53) for measuring the temperature of the electrical resistance (52) the electrical resistance is electrically powered, at least one so-called instantaneous curve (C1, C3) is produced, each instantaneous curve showing the evolution as a function of time of the temperature of the resistance, the instantaneous or each instantaneous curve is compared with at least one reference curve (C2). 11. Method according to the preceding claim, characterized in that the reference curve (C2) shows the evolution as a function of time of the temperature of the resistance, for a tube whose fouling has a known threshold thickness. 12. Method according to one of claims 10 or 11, characterized in that flows phosphoric acid in the tubes at a substantially constant temperature as a function of time, at least in the vicinity of the control organ (5). 13. Method according to one of claims 10 to 12, characterized in that flows phosphoric acid in a substantially vertical direction, from bottom to top. 14. Method according to one of claims 10 to 13, characterized in that places the fouling control member so that the so-called safety length, separating the top of the tube, adjacent to the body of the control of fouling, and the end of the fouling control member opposite said vertex, is between 0.1 and 3 meters, in particular between 0.5 and 2 meters. 15. Method according to one of claims 10 to 14, characterized in that causes at least partial condensation of the second fluid during its journey into the interior volume. 16. A method of mounting a heat exchanger according to claim 4, wherein; * one end of each tube (2) equipped with its control member (5) of fouling in the axial orifice (12a) arranged in the end plate (4) of the enclosure, - the auxiliary channel (11) is arranged in the plate and the sheath (6) of said control member (5) is passed through said auxiliary channel, - Rebunching said auxiliary channel (11). 17. Heat exchange tube graphite or graphite-based exchanger according to any one of claims 1 to 9, this tube being equipped with at least one member (5) for controlling fouling of its peripheral wall, this control organ comprising A body (51) disposed on the external face (2 ') of this wall, an electrical resistor (52) extending into the body (51) measuring means (53) for measuring the temperature of the electrical resistance (52). 10 18. Phosphoric acid concentration plant comprising at least one heat exchanger according to any one of claims 1 to 9.