EP4736575A1

EP4736575A1 - Heating panel, process gas heater comprising said panel and method for monitoring the conditions of said panel

Info

Publication number: EP4736575A1
Application number: EP24746053.8A
Authority: EP
Inventors: Matteo Giacomo RICCI; Giacomo COZZAROLO; Andrea Codutti
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2023-06-30
Filing date: 2024-06-28
Publication date: 2026-05-06
Also published as: KR20260048552A; MX2026000107A; CN121773705A; IT202300013626A1; WO2025003980A1

Abstract

The present invention relates to a heating panel which allows to easily monitor the status of each heating element so as to quickly identify a malfunction and act by replacing the panel inside a heating module. The heating panel is highly reliable and easy to maintain. The heating panel (P) comprises at least one heating element (T) formed by a triplet of resistors (R1, R2, R3) mutually connected in a star pattern, in which each free end of each resistor (R1, R2, R3) of said triplet receives power from a different phase of a three-phase system and is connected, by at least one controlled switch (SCR, 4), to a first busbar (3). A first control switch (2A) is interposed between the controlled switches (SCR, 4) and said first busbar (3). Furthermore, the first busbar (3) is connected to at least one transformer (1 ) by at least one second control switch (2B). The star center (CS) of the triplet (R1, R2, R3) is in turn connected to a second busbar (8). Furthermore, a current measuring device (6) is provided, positioned between the star center (CS) of the triplet (R1, R2, R3) and the second busbar (8) or along said second busbar (8), so as to detect the extent of the possible current passing through the star center (CS) of the triplet, and in which a control unit (PLC) is provided which is adapted to receive measurement data coming from said current measuring device (6) in order to monitor the conditions of the heating panel (P).

Description

HEATING PANEL, PROCESS GAS HEATER COMPRISING SAID PANEL AND METHOD FOR MONITORING THE CONDITIONS OF SAID PANEL

* * * * *

FIELD OF THE INVENTION

The present invention relates to the field of electric heaters. In particular, the present invention relates to a heating panel, to a process gas heater comprising at least one heating panel and to a method for monitoring the conditions of said heating panel, preferably all applied to steel plants. Nonetheless, it may be applied to any panel that electrically heats any industrial process.

BACKGROUND ART

In the prior art, for example in plants for heating steel products, such as, for example, tunnel furnaces, or in ladle and tundish preheating units, or in Process Gas Heaters (PGH) used in direct reduction, the use of gas heating systems to reach the thermal targets useful for the process and bring the elements which need to be heated to the desired temperature is known.

Many of these gas heating systems use burners, resulting in emissions of CO2 and other pollutants due to gas combustion. Furthermore, such known solutions also have a low performance.

Solutions have already been proposed in the prior art in which the concept of electric heating of steel products, components of a steel plant, or a process gas is introduced. However, such known solutions have a low reliability.

In fact, the resistors used undergo deterioration over time, which depends both on the maximum temperatures reached as well as on the number, amplitude and gradients of the cycles of temperature itself, to which they are subjected.

The most commonly used configurations for the electrical connection of the resistors are those that provide electric power supply with a three-phase alternating current, in which each resistor (where the term "resistor" may also mean a group of electrical resistors connected to one another in series or in parallel), is connected and is a phase of the three-phase system. If the three phases, and therefore the resistors which compose them, are identical, then the electrical load is defined as "balanced" and the currents passing through the three phases are identical, producing, due to the Joule effect, the same thermal power. Such three-phase resistor systems may be connected in parallel to a single three-phase power line, in which the electric current is controlled by switches, for example solid state switches, called thyristors.

Line electric currents, in relation to the thermal powers required by the process, may easily reach a few thousand amperes.

When more than one of such systems are connected in this manner it becomes difficult, if not impossible, to monitor the status of one of the phases of the system in the event of malfunction of one of said resistors, since the extent of the resulting unbalance of the three-phase currents may be insufficient to be detected by the instrumental systems. This is even more true as the line currents and the number of three-phase groups connected in parallel increase.

In other words, the status of the resistors is not adequately monitored, therefore it is difficult, if not impossible, to predict and/or identify a possible malfunction of one of the resistors in a time useful to reduce the risks of low quality production or of a downtime of the production itself.

Therefore, the need arose for a solution which allows to have a plant which is highly reliable and with low emissions.

SUMMARY OF THE INVENTION

It is the object of the present invention to create a heating panel which allows to easily monitor the status of each heating element so as to quickly identify a malfunction and act by replacing the panel inside a heating module.

It is a further object of the present invention to create a heating panel which is highly reliable and easy to maintain.

It is another object of the present invention to provide an efficient method for monitoring the conditions of said heating panel.

The objects of the invention have been achieved by a heating panel as defined in claim 1 .

One of the two schemes most frequently used for this purpose is the one with the resistors connected in a “star pattern", since, in this manner, the line currents are lower than the corresponding currents with a "delta" configuration, given the same power delivered. A second advantage consists in the fact that the star center, to which the three phases, and therefore the three resistors, converge, is accessible and, therefore, the status of the electrical potential thereof may be monitored. In the event that the three phases, and therefore the three resistors which form them, are not identical, or undergo variations in resistivity and/or size over time, or, even, a phase is interrupted, the load becomes "unbalanced" and, consequently, the potential of the star center increases and a current flow is generated through the neutral line, connected from the star center of the three-phase system to the earth potential.

The heating panel according to the present invention comprises at least one heating element formed by a triplet of resistors mutually connected in a star pattern, in which each free end of each resistor of said triplet receives power from a different phase of a three-phase system and is connected, by at least one controlled switch, to a first busbar. The heating element further comprises a first control switch, placed between the outputs of the controlled switches and said first busbar, and the first busbar is connected to at least one transformer by at least one second control switch. Furthermore, the star center of the triplet is in turn connected to a second busbar and a current measuring device is provided, positioned between the star center of the triplet and the second busbar or along the second busbar so as to detect the extent of any current passing through the star center of the triplet. Finally, a control unit is provided as adapted to receive measurement data from the current measuring device in order to monitor the conditions of the heating panel.

It is another aspect of the invention a heating module comprising at least one heating wall provided with at least one aforesaid heating panel.

It is a further aspect of the invention a process gas heater, adapted to heat the process gas before using it in an industrial process, in which said heater is crossed by at least one pipe to convey the process gas and comprises at least one aforesaid heating module.

It is another aspect of the invention a method for monitoring the conditions of a heating panel, as described above, said method comprises the following steps:

- detecting a current variation in the star center of the triplet of resistors of said at least one heating element by means of the current measuring device, and

- monitoring the conditions of the heating panel by means of the control unit which receives the measurement data coming from the current measuring device and processes them to detect possible malfunctions of said at least one heating element.

More specifically, the invention may be used in a direct reduction plant, in particular in process gas heaters.

Advantageously, the panel of the present solution may also be used in other plants, for example in furnaces for heating steel products, in ladle and tundish preheating units, and in annealing furnaces for strips to be galvanized.

BRIEF DESCRIPTION OF THE FIGURES

Reference is made later in this description to the drawings in the accompanying Figures, in which:

Figure 1 shows an example of a heating panel with details of at least one heating element,

Figure 2 shows an example of a heating panel with a plurality of heating elements,

Figure 3 shows an example of a heating wall comprising two heating panels,

Figure 4 diagrammatically shows an example of a heating module comprising a plurality of heating walls, and

Figure 5 diagrammatically shows an example of a heating module.

The parts according to the present description are shown in the drawings, where suitable, employing conventional symbols, showing only the specific details which are pertinent to the understanding of the embodiments of the present invention, so as not to highlight details which will be immediately apparent to the person skilled in the art, with reference to the description provided below.

DETAILED DESCRIPTION OF THE INVENTION

The solution of the present invention is now described with the aid of the drawings.

It is the object of the present invention a heating panel, a process gas heater comprising said panel and a method for monitoring the conditions of said panel. In order to further reduce emissions, a concept of electrical heating for process gas (e.g., CH4, H2, combinations thereof) in a Process Gas Heater is proposed which allows great reliability by monitoring the heating elements.

To preventively diagnose the breaking of a resistor, the solution involves connecting triplets of resistors in a star pattern and monitoring the star center in order to recognize, with a continuous monitoring, errors or malfunctions in one or more resistors which lead to having a non-zero (or rather, not negligible) current in the star center.

When connecting three resistors in a star pattern, each phase is (individually) connected to the three ends of three resistors and the other three ends are connected to each other to form the star center. The sum of the three currents circulates on the neutral, but three equal currents out of phase with each other by 120° give as sum a zero value.

If the star is balanced, i.e. , the three resistors have the same value, the potential of the star center is zero, and therefore there would be no neutral current on the star center.

With reference to Figure 1 , a heating panel P comprises at least one heating element T formed by a triplet of resistors R1 , R2, R3.

The three resistors R1 , R2, R3 are mutually connected in a star pattern, and each free end of each resistor R1 , R2, R3 of the triplet receives power from a different phase of a three-phase system.

Each free end of each resistor R1 , R2, R3 is also connected, by means of a respective controlled switch 4, for example a Silicon Controlled Rectifier SCR, to a first busbar 3.

The opposite ends of the three resistors R1 , R2, R3 are connected to each other to form the so-called star center CS.

A first control switch 2A is interposed between the outputs of the controlled switches SCR 4 and the first busbar 3.

The first busbar 3 is in turn connected to a transformer 1 by a second control switch 2B. For example, conventionally, transformer MV/LV is a 33/0,69 kV ONAN (Oil Natural Air Natural) transformer.

In heating element T, the star center CS of the triplet R1 , R2, R3 is in turn connected to a second busbar 8. Advantageously, a current measuring device 6 is provided, positioned between the star center CS of the triplet R1 , R2, R3 and the second busbar 8.

Otherwise, the current measuring device 6 may be positioned along the second busbar 8.

It is the object of the current measuring device 6 to detect the extent of the possible current passing through the star center CS of the triplet of resistors.

Furthermore, a control unit PLC is provided adapted to receive measurement data from the current measuring device 6 to monitor the conditions of the heating panel P.

The element 7 is a remote input/output unit for collecting analog or digital inputs and transmitting them over the network.

The system provides access to the neutral of the triplets of resistors by means of measuring instruments so as to read any currents passing through the star center CS. If the star is perfectly balanced, the potential of the star center CS is zero, therefore there is no current on the neutral.

Otherwise, given that the phase currents may be significant and of the order of 300 A, even a small imbalance in one of the three resistors of the star produces effective current values which are easily measurable, since they are of the order of a few A.

If the current of the star center CS is continuously monitored by means of measurement systems, it is possible to promptly identify variations in the current of the star center CS, predict the evolution of the resistivity of the heating element T and provide maintenance procedures which include the replacement of the heating element T or of the heating panel P containing it.

Since the resistors used in the heating elements T are made of metal alloys, over time the resistivity value thereof will be subject to changes due to the expansions to which they are subjected.

Therefore, by monitoring the current of the star center CS, i.e. , the current circulating on the neutral wire which connects the star center of the three-phase system of resistors to the neutral line, it is possible to analyze the change in the resistivity of the heating elements and, in the case where it exceeds an alarm threshold, provide for a replacement at the first production pause, or if necessary, a production downtime, to carry out the replacement. Preferably, the controlled switch SCR 4 is a thyristor, preferably, a 3P zero crossing Thyristor.

In some embodiments the heating panel P comprises a plurality of heating elements T, each formed by a triplet of resistors R1 , R2, R3 mutually connected in a star pattern, and the controlled switch SCR 4 simultaneously controls the plurality of heating elements T1 -Tn.

By way of explanation, Figure 1 shows two heating elements T1 and T2.

In the preferred embodiments, the first busbar 3 is a three-phase (busway or cable) line and the second busbar 8 is the neutral (busway or cable) line.

Preferably, the current measuring device 6 is a current transducer 6.

By installing current transducers 6, it is possible to obtain an early diagnosis of the remaining life of the heating elements.

At the beginning of their life, the resistors are all "approximately" equal, but, as time passes, resistivity changes and some may be damaged by the different heating - cooling cycles and may therefore alter the nominal resistance value thereof, thus unbalancing the star.

The fact that the resistors, at the beginning of their life, are not strictly "identical" is not relevant for the purposes of the validity of the invention. In fact, the absolute value of the current circulating from the star center to the neutral line at the beginning of the resistors' life is not relevant, but rather the variation thereof over time, which indicates the degradation of one or more resistors and, by means of condition monitoring techniques it defines predictive maintenance intervention logics.

The resistors R1 , R2, R3 which make up the triplets forming the heating elements T are selected from among different types:

• metal resistors, made as wires or plates or other geometric types with Iron-Chrome-Aluminum or Nickel-Chrome alloys, made with sintered powders and drawn or rolled;

• resistors made of ceramic materials, for example, Silicon Carbide, SiC;

• resistors made of ceramic metal (cermet), for example, Molybdenum Disilicide, MoSi2.

Each resistor, as well as each triplet of resistors, has a power ranging from a few tens of kW to a few hundreds of kW, the power limitation being exclusively due to the maximum capacity of the switches and thyristors, in terms of maximum interruptible and/or manageable currents, available on the market.

The power and number of triplets, placed in parallel, is given by:

• required thermal power,

• geometric configuration of the furnace/heater,

• volumetric uniformity of the required temperature range,

• precision and accuracy of temperature control, and

• heating gradient.

The resistors used are selected on the basis of the temperature of use and optimization of current management.

Metallic resistors are usually used as the basis for producing the heating elements T.

Preferably, the resistors are selected from among those made with iron, chromium and aluminum alloys (Fe-Cr-AI).

In various embodiments, the resistors are wave shaped.

The selected diameter of the wires which make up the resistors is conventionally from 3 to 10 mm.

Usually, for example, a line voltage of 690V is selected.

Each triplet of resistors is anchored to a refractory part so as to allow the free thermal expansion of the resistors.

Preferably, the heating panel P further comprises a remote input/output unit RIO receiving the measurement data detected by the current measuring device 6. The remote input/output unit RIO sends the data towards a network NET for the remote monitoring of the conditions of the heating panel P. In particular, the remote input/output unit RIO receives the measured data in analog or digital format and sends it to the network for processing.

The control unit PLC is adapted to continuously control the measurement data detected by the current measuring device 6 and, in the case where a measurement data exceeds an alarm threshold, said control unit PLC is adapted to generate a request for replacing the panel P.

Figure 2 shows an example of a panel P which comprises a plurality of heating elements T1 , T2, T3, ... Tn. In a preferred application of the invention, it is possible to panel the Process Gas Heater with a plurality of panels P1 ... Pn which in turn comprise a plurality of heating elements T1 ...Tn each.

The present invention is also aimed at a heating wall, indicated with reference W, comprising at least one heating panel P as those described above.

Figure 3 shows a wall W which comprises two heating panels P1 and P2.

Finally, as shown in Figure 4, a heating module M comprises a plurality of heating walls W1 ... Wn.

The modularity provided by the panel P, which is the smallest element or basic block, allows to create different walls W and heating modules M with high flexibility and availability.

The present invention also relates to a process gas heater, such as a CH4 and/or H2 heater, adapted to heat said process gas before using it in an industrial process, in which the heater is crossed by at least one pipe to convey the process gas and comprises at least one heating module (M) as the one just described above.

Finally, the present invention also relates to a method for monitoring the conditions of a heating panel P comprising the following steps:

- detecting a current variation in the star center CS of the triplet of resistors R1 , R2, R3 of the heating element T by means of the current measuring device 6, and

- monitoring the conditions of the heating panel P by means of the control unit PLC which receives the measurement data coming from the current measuring device 6 and processes them to detect possible malfunctions of the heating element T.

In particular, the control unit PLC identifies a current flow variation trend between the star center CS of the triplet R1 , R2, R3 and the second busbar 8 or along said second busbar 8, and in the event that the current flow exceeds an alarm threshold, the control unit PLC generates a request for replacing the heating panel P.

An embodiment is now described purely by way of explanation.

Preferably, a process gas heater comprises at least two heating modules, for example four modules M1 , M2, M3 and M4. With reference to Figure 5, each module M preferably comprises two thermal zones A and B.

In each thermal zone A, B there are two walls, respectively. As shown in Figure 5, two walls W1 , W2 for the zone A and two walls W3, W4 for the zone B will be provided.

Each wall W is in turn composed of two heating panels P1 and P2. Each heating panel P comprises several heating elements T1 , T2 and Tn.

By way of explanation only, numerically, each module M comprises 216 resistors, and each thermal zone A and B comprises 108 resistors.

In particular, the thermal zone A comprises two walls W1 and W2 and each wall W comprises fifty-four resistors divided into two panels P1 and P2. Each panel P is therefore composed of twenty-seven resistors. Each triplet comprises three resistors, therefore for each panel P there will be nine triplets, i.e., nine heating elements T.

A further embodiment will now be described with reference to Figures 6-10.

Phasors are mathematical tools used to represent variable quantities in the frequency or harmonic time domain. They are widely used in the analysis of electrical circuits and sinusoidal frequency systems.

A phasor represents a complex sinusoidal quantity as a vector in the complex plane. A phasor is described by a length (amplitude) and by an angle with respect to a reference axis. The amplitude of the phasor is the peak or effective value of the sinusoidal quantity, while the angle indicates the phase of that quantity with respect to a reference point in time.

In summary, phasors are a vector representation of sinusoidal quantities which simplify the analysis of sinusoidal frequency systems, allowing mathematical calculations to be performed using complex algebra.

In the case of a three-phase alternating voltage system, the choice of phase reference depends on the convention used in the specific system or on the applicable norms and standards. There are two common conventions for connecting the load: the star convention and the delta convention.

In the star convention, one of the phases is selected as the zero phase voltage reference or reference phase. The other two phases are, conventionally, out of phase with each other by 120 degrees (2TT/3 radians) each. The star center is the point to which all loads are connected. This point may or may not be connected (i.e. , connected to the relevant point in the power supply).

The difference between connecting or not the neutral or star center in the star convection is useful in the case of unbalances.

In detail, in the case of balanced operation, i.e., with all the loads equal to each other, there are no differences whether the star center is connected or not, this is because, if the three loads are identical, the voltage in the star center is zero, and no current flows in the neutral wire (since the vector sum of the three phase currents is equal to zero).

Otherwise, if the three loads are unbalanced, the voltage at the star center is zero, therefore without the neutral wire, the voltages on the individual loads would change. The neutral wire therefore serves to "force" the central voltage to zero, maintaining voltages at the nominal value thereof even for unbalanced loads. Finally, if connected, the current on the neutral wire is equal to the vector sum of the phase currents.

Taking the star convention as reference, thus four wires of which one is neutral, as in the configuration of Figure 1 , and the fact of taking one of these phases as a reference to calculate the phase shift, knowing that the other two phases are out of phase by 120° and 240° with respect to the reference one, and considering the definition of phasors reported above, it is possible to understand which phases (i.e., the resistive elements of the triplet) are going to deteriorate over time, generating malfunctions.

Considering the loads as resistive, the phase current has no phase shift with respect to the power supply voltage which generates it. This allows to measure the phase of the power supply voltage and consider it equal in extent to the phase of the corresponding current. Furthermore, it should be kept in mind that the neutral current is equal to the vector sum of the three phase currents.

In perfect operation, the phasors of the currents of the three phases should essentially be equal, therefore the fourth neutral phasor should be almost zero (taking into account the tolerances) or zero.

By measuring the voltage of the reference phase, by means of a suitable voltage meter, and using it as a phase reference, it is possible to verify the phase shift, with respect thereto, of the current which is created in the neutral wire under conditions of unbalanced loads (it is advisable to operate voltage measurement, since it is easier and less invasive, in fact the direct measurement of the current would require the use of in-line probes, with potential issues of accessibility and space).

The power supply voltages are imposed by the network and, by connecting the loads in a star pattern with a four-wire method, they remain constant even in the case of unbalanced loads.

In the case of degradation of a resistor with a consequent increase in value, as the voltage is kept constant by the four-wire configuration, the relative current flowing therethrough will decrease. This means that the sum between the three phase currents (i.e., the current flowing on the neutral) will be non-zero and the phase shift of this neutral current will provide information on which current or currents are reduced (due to the increase in the relative resistance).

In the following three examples, some possible combinations are shown of decreases in the performance of the heating elements (with reduction of current flow) by virtue of the use of the method described above. a) In the event that the load associated with the 0° phase has a deterioration (see Figure 6): the three arrows indicated with cFO, cF120 and cF240 are the phase currents (therefore, which flow on the loads, i.e., on the resistors) while the one indicated with cN is the neutral current. It may be noted that a phase current cFO has a reduced extent, a symptom of deterioration of the load associated with the 0° phase. The resulting neutral current cN will have a phase equal to the reduced current, and with a negative sign (which, at phase level, means a further phase shift of +180°). Therefore, the resulting phase cN is 0° + 180° = 180°. b) In the event that the load associated with the 0° phase and the load associated with the 120° phase have an equal deterioration, i.e., a decrease in the current caused by an increase in the resistance of the loads (see Figure 7): the three arrows with cFO, cF120 and cF240 are the phase currents (therefore flowing on the loads or on the resistors) while the one indicated with cN is the neutral current. It may be noted how the two phase currents cFO and cF120 have a reduced extent, which is symptom of deterioration of the loads with phase at 0° and 120°. In this example, it should be noted that the two phases have the same reduction. The resulting neutral current cN will have an intermediate phase between the two reduced currents with a negative sign (which, at phase level, means a further phase shift of +180°). Therefore, the resulting phase is [(0° + 120°) / 2] + 180°=240°. Thus, the resulting neutral current cN is superimposed on cF240. c) In the event that the load associated with the 0° phase and the load associated with the 120° phase have a different deterioration, i.e. , a decrease in the current caused by an increase in the resistance of the loads (see Figure 8): the three arrows with cFO, cF120 and cF240 are the phase currents (therefore flowing on the loads on the resistors) while the one indicated with cN is the neutral current. It may be noted how the two phase currents cFO and cF120 have a reduced extent, which is symptom of deterioration of the loads with phase at 0° and 120°. In this example, it should be noted that the two phases have different reductions. The resulting neutral current cN will therefore have an intermediate phase, although weighted with respect to the reduction to the two reduced currents, and with a negative sign (which, at phase level, means a further phase shift of +180°). It may be noted how the resulting phase is close to the previous case (thus 240°) but shifted as to get closer to the case in which only the 0° phase is reduced (in fact, the 0° phase is the one with the greatest reduction). Therefore, from the diagram it is possible to understand that, also in this case, two phases have issues, although different in extent.

With reference to Figure 9, the component or current measuring device 6 is an amperometric transformer which already integrates the phase measuring function. Therefore, the circuit of Figure 9, with respect to the one of Figure 1 , provides a connection 9 of the current measuring device 6 with one of the controlled switches 4, since it is known that the phase meter must have a reference phase to understand the unbalance with respect to the other two.

Alternatively, in the case where the current measuring device 6 does not have the function of measuring the phases, see Figure 10, it is necessary to add a further phase measuring device 10, which shall be connected, by means of a connection 9, with at least one of the controlled switches 4 which will be, in fact, used as a reference, passing through a device 11 adapted to calculate the phase difference between the controlled switch 4 and the star center CS (current on the branch which comprises the current measuring device 6) so as to understand which of the three is the unbalanced phase, according to the concept described. The above description of specific embodiments is capable of showing the invention from a conceptual point of view so that others, using the prior art, will be able to modify and/or adapt such specific embodiments in various applications without further research and without departing from the inventive concept, and thus it is understood that such adaptations and modifications will be considerable as equivalents of the specific embodiments.

Means and materials for performing the various functions described may be of various nature without departing from the scope of the invention.

It is worth noting that the terminology or expressions used are only descriptive and therefore non-limiting.

Obviously, without prejudice to the principle of the invention, the construction details and the embodiments may widely vary with respect to what is described and illustrated above by way of example, without however departing from the scope of the present invention.

Where the constructive features and techniques mentioned in any successive claims are followed by references signs or numerals, such reference signs were introduced for the sole purpose of increasing the intelligibility of the same claims and consequently, such reference signs have no limiting effect on the interpretation of each element identified, by way of example only, by such reference signs.

Claims

1 ) A heating panel (P) comprising at least one heating element (T) formed by a triplet of resistors (R1 , R2, R3) mutually connected in a star pattern, wherein each free end of each resistor (R1 , R2, R3) of said triplet receives power from a different phase of a three-phase system and is connected, by at least one controlled switch (SCR, 4), to a first busbar (3), and wherein at least one first control switch (2A) is interposed between the outputs of the controlled switches (SCR, 4) and said first busbar (3), wherein said first busbar (3) is connected to at least one transformer (1 ) by at least one second control switch (2B), wherein the star center (CS) of the triplet (R1 , R2, R3) is in turn connected to a second busbar (8), wherein a current measuring device (6) is provided, positioned between the star center (CS) of the triplet (R1 , R2, R3) and the second busbar (8) or along said second busbar (8), so as to detect the extent of the possible current passing through the star center (CS) of the triplet, and wherein a control unit (PLC) is provided which is adapted to receive measurement data coming from said current measuring device (6) to monitor the conditions of the heating panel (P).

2) The heating panel according to claim 1 , wherein said at least one controlled switch (SCR, 4) is a thyristor, in particular a 3P zero-crossing thyristor.

3) The heating panel according to claim 1 or claim 2, wherein said heating panel comprises a plurality of heating elements (T), each formed by a triplet of resistors (R1 , R2, R3) mutually connected in a star pattern, and wherein said at least one controlled switch (SCR, 4) simultaneously controls said plurality of heating elements (T).

4) The heating panel according to one of the preceding claims, wherein said first busbar (3) is a three-phase busway or cable line and the second busbar (8) is the neutral busway or cable line.

5) The heating panel according to one of the preceding claims, wherein said current measuring device (6) is a current transducer (6).

6) The heating panel according to one of the preceding claims, wherein said resistors (R1 , R2, R3) are selected from the following types: metal resistors, made as wires or plates or other geometric types with Iron-Chrome-Aluminum or Nickel-Chrome alloys, made with sintered powders and drawn or rolled; resistors made of ceramic materials, e.g., Silicon Carbide, SiC; or resistors made of ceramic metal (cermet), e.g., Molybdenum Disilicide, MoSi2.

7) The heating panel according to one of the preceding claims, wherein said heating panel (P) comprises a remote input/output unit (RIO) for receiving said measurement data detected by the current measuring device (6) and sending them towards a network (NET) for the remote monitoring of the conditions of the heating panel (P).

8) The heating panel according to one of the preceding claims, wherein said control unit (PLC) is adapted to control the measurement data detected by said current measuring device (6) and, in the case where a measurement data exceeds an alarm threshold, said control unit (PLC) is adapted to generate a request for replacing the panel (P).

9) The heating panel according to one of the preceding claims, wherein said current measuring device (6) connected to the star center (CS) is an amperometric transformer which integrates the phase measurement function, and wherein a connection (9) of said current measuring device (6) with one of the controlled switches (4) serving as the reference phase is provided.

10) The heating panel according to one of claims 1 to 8, wherein for a current measuring device (6) connected to the star center (CS) without phase measurement, a phase measuring device (10) is added, connected to the star center (CS), and a device (11 ) adapted to calculate the phase difference between a controlled switch (4) and the star center (CS), connected to said phase measuring device (10) by means of a connection (9).

11 ) A heating wall (W) comprising at least one heating panel (P) according to any one of the preceding claims 1 to 7.

12) A heating module (M) comprising at least one heating wall (W) according to claim 11 .

13) A process gas heater, adapted to heat the process gas before using it in an industrial process, wherein said heater is crossed by at least one pipe to convey the process gas and comprises at least one heating module (M) according to claim 12. 14) A method for monitoring the conditions of a heating panel according to one of claims 1 to 10, said method comprising the following steps:

- detecting a current variation in the star center (CS) of the triplet of resistors (R1 , R2, R3) of said at least one heating element (T) by means of the current measuring device (6), and

- monitoring the conditions of the heating panel (P) by means of the control unit (PLC) which receives the measurement data coming from said current measuring device (6) and processes them to detect possible malfunctions of said at least one heating element (T). 15) The method according to claim 14, wherein said control unit (PLC) identifies a current flow variation trend between the star center (CS) of the triplet (R1 , R2, R3) and the second busbar (8) or along said second busbar (8), and in the event that said current flow exceeds an alarm threshold, said control unit (PLC) generates a request for replacing the heating panel (P).