System for temperature control, power management and distribution in an electric heating system

Abstract

A system designed for temperature control and power management and distribution in an electric heating system which reduces / avoids producing disturbances in the grid as the electric power delivered to the heating process varies. The control system manages the distribution of power over time, as a function of the thermal demands of an electric heating system, and comprises a plurality "m" of heaters, in which each heater in turn comprises a plurality "n" of SCRs. The control system comprises a load switching device for distributing power to the load so as to achieve the desired process temperature.

Description

[0001] SYSTEM FOR TEMPERATURE CONTROL, POWER MANAGEMENT AND DISTRIBUTION IN AN ELECTRIC HEATING SYSTEM

[0002] * * * * *

[0003] FIELD OF THE INVENTION

[0004] The present invention relates to the field of electric heating systems, for example, a heater of a process gas or of fluids, liquids or materials in general, preferably applied in metallurgical or steelmaking plants, or in steelworks.

[0005] In particular, the present invention may be applied to a process gas heater, such as reheating furnaces for semi-finished products, annealing furnaces, gas heating for direct reduction and / or blast furnaces.

[0006] In general, it may be applied to any system that electrically heats a gas, or a fluid in general, for any industrial process.

[0007] The present invention may further be applied to heat treatments that exploit electric heating in fields of application that involve glass processing, in the fields of metallurgy, steelmaking, and in all fields in which it is necessary to heat a process gas for use in subsequent processing.

[0008] PRIOR ART

[0009] In the present context, the growing demand for efficient and sustainable energy solutions has driven the development of advanced electric heating systems. Optimal temperature management, together with precise power distribution, represents a crucial aspect to ensure not only to obtain the temperatures required by the process with the necessary uniformity, but also the energy efficiency and the environmental sustainability.

[0010] Conventional electric heating systems often suffer from inefficiencies due to inadequate management of power distribution. These problems may generate large fluctuations of the electrical parameters on the grid and considerable disturbances, such as flickering (sudden and repetitive variations of the voltage) and harmonics (waveforms that disturb the grid and induce erratic behavior of the equipment connected thereto), which may compromise the proper operation of the apparatuses connected thereto. Furthermore, such fluctuations induce the need to oversize the networks and the related components (busbar ducts, cables, switches, etc.), since they must be sized for the maximum values (peaks) of electrical power draw, generating unnecessary expenditures and greater wear of the components of the system, in particular of the heating elements. And, finally, they may induce excessive energy consumption. There exist, in the background art, solutions that provide logic for the distribution of electrical energy to a load. These systems provide for the use of semiconductor devices, specifically referred to as “Thyristors” or Silicon Controlled Rectifiers - SCRs to feed electrical energy to the load. They are substantially equivalent to diodes, with the only difference that forward conduction only occurs after the application of an appropriate trigger signal to a third terminal called a “gate”. They can operate in two modes: “phase-angle” activating control or full cycle activating control (also “called burst firing mode” or “zero crossing mode”). The system also comprises a memory that stores an electrical energy distribution strategy and a central unit that controls the load switch to switch from the phaseangle operating mode to the full-cycle operating mode based on the electrical energy distribution strategy.

[0011] The phase-angle control mode regulates the power by varying the point in the alternating current (AC) cycle in which the switching device is activated. The current is thus “cut” at a certain phase angle and has the advantage of allowing fine and continuous power adjustment. Conversely, it produces high-frequency electromagnetic waves, radio interferences and, in general, electromagnetic interference (EMI). The electromagnetic interference may comprise harmonics that may disturb the operation of other apparatuses connected to the same network, the waveform distortion that causes operating problems in electronic devices that require a clean sinusoidal current, the acoustic hum in components such as the transformers that may emit an audible hum due to vibrations induced by switching currents, as well as more complex and costly control circuits. It should be noted that the generation of electromagnetic interference (EMI) on the network is punishable under the law with fines, cessation orders, civil liability with claims for damages, and further penalties, depending on the severity of the harm caused to other users connected to the network.

[0012] Vice versa, the zero-crossing mode control only activates the switching device when the AC alternating voltage crosses zero. Thereby, the current is only applied to the load at the start of each half-wave. This minimizes the generation of electrical noise, electromagnetic interference and production of harmonics, and features simpler and less expensive control circuits. Its adjustment is less accurate, but entirely compatible with the accuracy required in temperature control of industrial plants.

[0013] It is therefore understood that phase control is only usable in the event of medium - low powers, in the order of hundreds of kW up to a few MW (such value depends, obviously, strongly on the type and configuration of the grid).

[0014] SUMMARY OF THE INVENTION

[0015] The present patent relates to an innovative system designed for temperature control, power management and distribution in an electric heating system, which reduces / avoids producing disturbances on the grid upon variation of the electric power delivered to the heating process.

[0016] This system is designed to overcome the limitations of existing systems, providing a significant improvement in terms of control precision, energy efficiency, and adaptability to variable environmental conditions.

[0017] The object of the present invention is to manage power in electric heating systems, as a function of the time variation of thermal demands, reducing or limiting the generation of grid disturbances.

[0018] It is a further object of the present invention to achieve a system for the temperature control, power management and distribution in an electric heating system to reduce power fluctuations.

[0019] It is another object of the present invention to achieve an efficient method for monitoring the power in electric heating installations for heat treatments.

[0020] It is necessary to emphasize that the optimization of electrical loads, and the consequent functional parameters of the grids, will become increasingly stringent in the future, with increase of electric power required by industrial decarbonization and electrification processes, replacing thermal processes obtained by combustion of fossil fuels. Indeed, electric powers in the order of tens and hundreds of MW are expected, thus one to two orders of magnitude greater than what is typically currently used.

[0021] The proposed solution provides a system that integrates sensing and control technologies to optimize temperature regulation and power distribution in real time. The system is provided with high precision sensors and control algorithms that continuously monitor and regulate the operating conditions of the electric heating. Further, the system comprises a modular architecture allowing easy integration and adaptation to different configurations of heating systems.

[0022] Through optimized power distribution, the system minimizes power fluctuations, reduces waste, and improves overall energy efficiency.

[0023] The modular architecture enables easy adaptability to various settings and heating configurations, offering flexibility for future upgrades and expansions.

[0024] In summary, the present patent proposes a highly innovative system for temperature control, power management and distribution in electric heating systems, intended to significantly improve the performance and energy efficiency of modern heating systems.

[0025] The object of the invention has been achieved by a control system as defined in claim 1.

[0026] The control system, for temperature control, power management and distribution over time, as a function of the thermal demands of an electric heating system, comprises a plurality “m” of heaters, in which each heater in turn comprises a plurality “n” of SCRs. The control system comprises a load switching device for distributing power to the load so as to achieve the desired process temperature,

[0027] - in which the SCRs are subdivided into “j” groups, with j = 1 , ... m, and for groups “j” from 1 to “m-1” the control system is configured to sequentially switch the different groups on and off with a delay between each group,

[0028] - in which for each group “j” of SCRs, with “j” from 1 to “m-1”, the control system is configured to command switch-on with a delay with respect to the previous group “j- 1”,

[0029] - in which the control system is configured to select, for each group “j”, a required power value P’ %,j from those calculated according to the equation:

[0030] 100 * k P'%‘i =^CRj where k is an integer number selected from the range from 1 to nSCRj and where nSCRj is the number of active SCRs present in the group “j”,

[0031] - and in which the last group “m” of SCRs is controlled with a power P%,m selected from those calculated according to the equation: where Ptot = YJ^i Pj, where Pj is the rated power of the “j-th” heater (HEXj), where Pm is the rated power of the “m-th” heater (HEXm), where P% =100*Atowwhere

[0032] Aton is the time interval in which the SCRms of said last group “m” switch on at the rated power, and where Atoff is the time interval in which the SCRm switch off.

[0033] The control system is configured to calculate, for each group “j” of SCRs the switch- p% on instant Aton

[0034] Ji calculated according a to the equation Aton = - Atoff, the 100-P% corresponding switch-off instant AtoffJ calculated according to the equation Atoff =10p~P%Aton, and the switch-on delay with respect to the previous group “j-1 ” Atdj calculated according to the equation Atdj =Atcycle, where Atcycle is calculated as nSCRj

[0035] Atcycle = (Aton + Atoff).

[0036] The load switching device is adapted to distribute power to the load according to predetermined selection criteria, in which said selection criteria depend on the constructional and functional characteristics of the Heaters (HEXj) and the specific heating process.

[0037] The control system uses selection criteria selected from:

[0038] A) Defining a minimum P% value for automatic control, depending on the capacity of the system and / or on the installed power of the heater HEX; and / or

[0039] B) Maximizing the power of the Heaters HEXj powered with Medium

[0040] Voltage, which have higher energy efficiency; and / or

[0041] C) Maximizing the percentage power without fluctuations P’%,j of heaters HEXj working at lower temperatures, and / or

[0042] D) Equalizing the percentage powers without fluctuations P’%,j of the first two heaters HEX1 and HEX2, and / or

[0043] E) Minimizing the value P’%,m of the last Heater HEXm, which works at a higher temperature, or

[0044] F) Selecting the value P’%,m of the last Heater HEXm within a predetermined range.

[0045] In various embodiments the rated power of each Heater varies according to the requirements of the thermal process and such rated power is to be equally divided between the “n” SCRs of the “j-th” Heater, defining a unit power P1 ,1 = P1 ,2 = ... P1 ,n equal for the number nSCRj of active SCRs present in the group “j”. The control system comprises an external control loop, based on Proportional- Integrative-Derivative controllers PID, adapted to process the temperature measurements TTg of gas input into the heater and gas output from the heater and the gas flow rate measurement, FTg, from the field instrumentation, and generate an output Yg, digitized and filtered with integers from 0 to 100, in which the value Yg is exactly the parameter P%.

[0046] The control system comprises an internal control loop, adapted to control the internal temperature of each “j-th” Heater, and also based on a respective Proportional-lntegrative-Derivative PID controller, with the main purpose of preventing the “j-th” Heater from incurring overtemperatures, ensuring that, under any condition, it works at temperatures below those set, Tjset, which are the maximum permissible temperatures defined at the design stage for each Heater HEXj, and in which the output Yj of the internal control loop is digitized and filtered with integers from 0 to 100.

[0047] The control system comprises functional blocks adapted to perform the following respective operations:

[0048] - selecting on the basis of P% a set of (m-1 ) percentage power values without fluctuations P’%,j, applicable to the first (m-1 ) Heaters HEXj;

[0049] - comparing the values of the percentage powers without fluctuations P’%,j selected in the previous step with the corresponding value Yj calculated from the internal control loop; if the condition P’% ,j < Yj is met, then P’% ,j is used as the percentage power value without fluctuations for the “j-th” Heater, otherwise the value P’%,j* is used, calculated with a lower number k, such as to satisfy this condition,

[0050] - calculating the percentage power value without fluctuations P%,m according to equation P%,m = , based on the (m-1 ) percentage power values without fluctuations P’%,j selected in the previous step, with the limitation P%,m < 100%,

[0051] - checking that the value P%,m calculated in the previous step is less than or equal to the value of the output Ym, generated by the internal control loop, otherwise the lower value, between P%,m and Ym, is selected to drive the SCRs of the last “m-th” Heater; - calculating for each group “j” of SCRs the switch-on instant Atonj calculated p% according to the equation Aton = - Atoff, the relative switch-off instant Atoff i calculated according to the equation Atoff =10°~P%Atonand the switch-on delay with respect to the previous group “j-1” AtdJ calculated according to the equation Atd = for each HEXj, and the on-off sequence for each “j-th” Heater is transmitted to a corresponding board for performing the on / off cycles.

[0052] The electric heating system of a process gas, suitable for heating the process gas prior to use thereof in an industrial process, in which the electric heating system is traversed by at least one pipe for conveying the process gas and comprises a plurality “m” of heaters, in which each heater in turn comprises a plurality “n” of SCRs, and in which the heating system provides a control system as described above.

[0053] The invention also relates to a control method for controlling temperature, managing and distributing power over time as a function of the thermal demands of an electric heating system.

[0054] The control method comprising the steps of: defining an external control loop, based on Proportional-Integrative-Derivative PID controllers, which process the temperature measurements TTgdi of gas input into the heater and gas output from the heater and the gas flow measurement, FTg, from the field instrumentation, and generate a digitized and filtered output Yg with integers from 0 to 100, defining an internal control loop of the internal temperature of each “j-th” Heater, based on a respective Proportional-Integrative-Derivative PID controller, with the object of avoiding overtemperatures, ensuring that each “j-th” Heater operates at temperatures lower than those set, Tjset, and calculating a digitized and filtered output Yj with integers from 0 to 100,

[0055] - selecting on the basis of P% a set of (m-1 ) percentage power values without fluctuations P’%,j, applicable to the first (m-1 ) Heaters HEXj;

[0056] - comparing the values of the percentage powers without fluctuations P’%,j selected in the previous step with the corresponding value Yj calculated from the internal control loop; if the condition P’% ,j < Yj is met, then P’% ,j is used as the percentage power value without fluctuations for the “j-th” Heater, otherwise the value P’%,j* is used, calculated with a lower number k, such as to satisfy this condition,

[0057] - calculating the percentage power value without fluctuations P%,m according to ptot*p%- ;™-1)p'%,j*pj equation P%,m = - - , based on the (m-1 ) percentage power values without fluctuations P’%,j selected in the previous step, with the limitation P%,m < 100%,

[0058] - checking that the value P%,m calculated in the previous step is less than or equal to the value of the output Ym, generated by the internal control loop, otherwise the lower value, between P%,m and Ym, is selected to drive the SCRs of the last “m-th” Heater;

[0059] - calculating for each group “j” of SCRs the switch-on instant Atonj calculated p% according to the equation Aton = - Atoff, the relative switch-off instant Atoff i calculated according to the equation Atoff =10°~P%Atonand the switch-on delay with respect to the previous group “j-1” AtdJ calculated according to the equation Atd =Atcycleand the on / off sequence for each “j-th” heater is transmitted to the control module for performing the on / off cycles.

[0060] BRIEF DESCRIPTION OF THE DRAWINGS

[0061] In the following description, reference will be made to the Figures shown in the accompanying drawings, in which:

[0062] Figure 1 shows two possibilities of providing the same power in a same cycle time with duration Dtcycle,

[0063] Figure 2 shows the SCR on / off sequence, in which the turning-off of one SCR corresponds to the turning-on of the subsequent SCR placed in time sequence,

[0064] Figures 3 and 4 show the range of fluctuation of the power in the case without load control strategy and in the case with load control strategy, respectively, and

[0065] Figure 5 shows a block diagram in which the temperature control system and the load control strategy are integrated for a system composed of three heaters.

[0066] The parts according to the present description are depicted in the drawings, where appropriate, using conventional symbols, showing only the specific details relevant to understanding the embodiments of the present invention, so as not to highlight details, which will be immediately apparent to those skilled in the art, with reference to the description provided below.

[0067] DETAILED DESCRIPTION OF THE INVENTION

[0068] The solution according to the present invention will now be described with the aid of the drawings.

[0069] The following description relates to the temperature control of a heating process carried out by a plurality of SCRs and their load management, so as to avoid large fluctuations of the electrical parameters on the power grid.

[0070] It is assumed that the operation of the SCRs is known, as well as their operating mode, being standard electronic components applied as solid state switches for the power supply of electrical loads since 1957, the year of their invention, capable of being turned on and off more than one billion times, with high electric currents of thousands of Amperes, both in Medium Voltage MV and in Low Voltage LV.

[0071] For the purposes of the present description, the SCRs operate in burst mode (also referred to as zero-crossing), driven by a manipulated output variable, computed by means of a Proportional-Integral-Derivative (PID) algorithm or by any other control algorithm or control system, in which the manipulated output variable represents the quantity of power provided to be introduced into the process, so as to obtain the required process target temperature Tg.

[0072] The SCRs control the electric power supply to a series of electric resistors, also called Heating Elements, when assembled with other materials to form a complex element, which heat the volume into which they are inserted, by the Joule effect.

[0073] In general, the solution described herein is applicable to a group of “m” Heaters (“HEX”) or Thermal Zones (“TZ”), supplied at Medium Voltage MV or at Low Voltage LV, each of which has a plurality of SCR-controlled switches, ranging from 1 to “n”, onto which the required power is divided.

[0074] In the following, the acronym “HEX” (Heat Exchanger) will be used to indicate, indifferently, both types of groups:

[0075] - so-called Electric Heaters, for example so-called Circulation Heaters or Immersion Heaters, in which a fluid (gas or liquid) is heated in a tank or in a vessel by electric heating elements, so as to form a heat exchanger such as shell and tube heat exchangers, or

[0076] - the thermal zones of electric furnaces, in which an inner volume of a furnace is heated by groups of electric resistors, which irradiate a solid body.

[0077] They are also applicable to any other apparatus in which groups of resistors or heating elements are supplied at medium or low voltage and controlled by semiconductor devices, such as SCR-controlled switches.

[0078] In particular, the proposed solution may be applied to the heating of a Process Gas in heating installations, such as heat treatment or annealing furnaces, but also to process gas heaters used in the Direct reduction sector (Process Gas in Direct Reduction Plants), where a Process Gas (PG) enters into a set of Heaters HEX and into a set of Thermal Zones TZ to be heated.

[0079] The Thermal Zones TZ, in which the process gas that has to be heated flows, generally consist of pressurized parallel tubes, specifically known as “coils”, in which a Process Gas flows, at fixed or variable flow rate, arranged in series and / or in parallel, to bring the temperature of the Process Gas PG to the required value Tg. Considering the subdivision of the flow rate of the Process Gas PG into parallel trains, each of them will be composed of a certain number “m” of Heaters HEX (also defined as a “group” or a “set”), arranged in sequence from the first “1” to the last “m-th”.

[0080] Similarly, the SCRs belonging to each heater HEX will be arranged in sequence from the first to the “n-th”.

[0081] The rated power of each Heater HEX may vary based on the requirements of the thermal process, and must be equally divided between multiple SCR-controlled switches, i.e., the rated power of each SCR, belonging to a Heater HEX, must be the same.

[0082] As a general example (Example 1 ), having five Heaters HEX in series (m=5), i.e. HEX1 , HEX2, HEX3, HEX4, HEX5, HEX1 and HEX2 each being composed of six SCRs (nSCR=6), and the remaining HEX3, HEX 4 and HEX 5 composed of four SCRs (nSCR=4), the following table may be defined for the arrangement of the SCRj.i for each Heater HEXj:

[0083] TABLE 1 where there will be:

[0084] 6 SCRs in the first Heater HEX1 , all having the same power P1 ,i;

[0085] 6 SCRs in the second Heater HEX2, all having the same power P2,i;

[0086] 4 SCRs in the third Heater HEX3, all having the same power P3,i;

[0087] 4 SCRs in the fourth Heater HEX4, all having the same power P4,i;

[0088] 4 SCRs in the fifth Heater HEX5, all having the same power P5,i.

[0089] For the described arrangement:

[0090] - the Heaters HEXj, with j=1 ...m, will have the unit rated power Pj , with j=1 ...m, which may be equal or may differ between them (for example: P1 + P2 + P3 + P4 + P5).

[0091] - the SCRI .i, with i=1 ... n, instead, belonging to the same Heater HEX1 , will have the same unit rated power P1 ,1 = P1 ,2 = ... P1 ,n and so on.

[0092] When the switches are grouped into large groups, operating at high power, it is important to avoid the SCRs turning on and off simultaneously, and, in general, in an uncontrolled sequence, so as to generate power peaks and large variations of the electrical grid parameters over a wide range, in particular voltage variations (also called “flickering”), which may negatively affect other electrical equipment connected to the same grid.

[0093] On the other hand, the group of SCRs must, as a priority, satisfy the power demand originating from the heating process, therefore it must be driven and managed so as to distribute and equalize such power demand between the SCRs, avoiding or reducing as much as possible the aforementioned fluctuations.

[0094] Therefore, each group of controlled Heaters HEXs must achieve the following objectives:

[0095] - performing stable and precise temperature control, and

[0096] - limiting as much as possible the maximum amplitude of the power fluctuation on the electrical grid of the group, under any operating condition, and optimizing the absorbed electric power. A temperature control and load management strategy is described below that satisfies both objectives, limiting the amplitude of the load fluctuation to the power of a single SCR, belonging to the last HEXm of the set.

[0097] Therefore, with reference to Example 1 described above, in any condition, the maximum amplitude of the power fluctuation must be equal to the power of one SCR5.L

[0098] Let us examine in detail the sequential switch-on mode, taking into consideration a Heater HEX, having nSCR.

[0099] The energy released by a single SCR is defined by equation (1 ) set forth below:

[0100] AE = P * Atcycle = P1 ,i * Aton (1 ) where:

[0101] - P is the power required by the thermal process,

[0102] - P1 ,i is the rated power of the i-th SCR, denoted as SCRi, of the first Heater HEX1 ,

[0103] - Atcycle is the time interval considered,

[0104] - Aton is the time interval wherein the i-th SCR, denoted as SCRi, is on at the rated power, and

[0105] - Atoff is the time interval wherein the i-th SCR, indicated as SCRi, is off.

[0106] If in the same cycle At it is desired to provide a lower quantity of energy P < P1 , i, it is possible to proceed in two ways:

[0107] - by “modulating” - i.e. reducing - the power from P1 ,i to P, or

[0108] - by injecting P1 ,i for a shorter time, reducing the switch-on time, as shown in Figure 1 .

[0109] The equation (1 ) may be written:

[0110] P * (Aton + Atoff) = P1 ,i * Aton

[0111] Setting P1 ,i = 100 and expressing the required power P in terms of percentage of P1 , i, it may be written as:

[0112] P% * (Aton + Atoff) = 100 * Aton

[0113] From this equivalence, it is easy to derive the switch-on / switch-off time of a single SCR-controlled switch:

[0114] P%

[0115] Aton = toff 100-P% where IOO-P%

[0116] Atoff = - Aton (3)

[0117] P%X'

[0118] As already mentioned, if in a Heater HEXj “n” SCRi have been installed, with i=1 ... n, the total required power must be equally divided among all the SCRi present in the Heater HEXj and active or operating.

[0119] An “active” SCR means a functioning SCR, thereby the exact number of devices to be considered is obtained.

[0120] Obviously, operating all the SCRi with the same on / off sequence is a trivial solution, which:

[0121] - from a thermal point of view, may entail considerable thermal non-uniform ities in the HEXj and,

[0122] - from an electrical point of view, entails an enormous variation in electrical parameters, as already mentioned previously.

[0123] Therefore, the SCRi must be turned on / off “in sequence”, each with a certain delay

[0124] - Atd - with respect to the preceding SCRi-1 .

[0125] In general, for a multiple-SCR system, the selection of the position of the specific SCR in the “switch-on sequence” depends on the number “n” of SCRi installed and active in the HEXj, and on the geometric configuration and arrangement of the Heating elements.

[0126] Further, they may operate simultaneously in pairs or grouped into subgroups, turned on and off as individual SCRs.

[0127] In any case, the cycle time being equal to Atcycle = (Aton + Atoff), the delay time for the switch-on of each controlled switch SCRi, with respect to the preceding SCRi-1 , will be provided by:

[0128] By combining the above-described equations (1 ) and (4), a “fixed time cycle” is obtained, also called “time proportional control”.

[0129] It is therefore called a “fixed time strategy” because the cycle time Atcycle, derived from equation (4), is assigned and fixed, and Atd depends only on the number “n” of SCRs, defined as nSCR.

[0130] It should be noted that equation (4) allows the possibility to be considered that one or more SCRi installed in a Heater HEXj may be out of service. Consequently, by detecting the status of the SCRs, it is immediately possible to set the number nSCR of “active” SCRs and assign to them the missing contribution of the SCRs out of service.

[0131] Therefore, rather than the number of SCRs installed in a HEXj, nSCRj represents the number of installed and “active” SCRs in the j-th HEX.

[0132] Given a certain number of nSCRj, grouped in a Heater HEXj, having a total power:

[0133] Pj= nSCRj*Pj,i (5) where Pj, i is the rated power of each SCRi-controlled switch of the Heater HEXj, theoretically there are specific percentage power values P% ,j , known as percentage powers without fluctuations P'%,j: with “k” an integer number selected in the range 1... nSCRj, so that there are no power fluctuations.

[0134] This situation is shown in Figure 2 for a number “n” of nSCRj equal to n=4 with k=3 and P%,j = 75%.

[0135] In this situation, the switch-off of an SCRi always corresponds to the switch-on of the subsequent SCRi+1 .

[0136] In Figure 2 an example is shown in which the total power released is constant and equal to 3 times Pj, i throughout the cycle At. In this situation, the switch-off of an SCRi always corresponds to the switch-on of the subsequent SCRi+1 .

[0137] In the Table 2, the percentage power values without fluctuations P'%,j are reported which, for any nSCRj, avoid the power fluctuation of the Heater HEXj.

[0138] TABLE 2

[0139] Obviously, the situation shown in Figure 2 is an ideal situation: in reality, the value of P%,j may vary continuously from 0 to 100, based on the process thermal energy required of the Heater HEXj. Further, it is necessary to consider the discretization errors deriving from the calculation of Atd and Aton.

[0140] For example, by selecting a cycle time Atcycle = 1000 ms, if the number of active controlled switches nSCRj is equal to n=6 then the switch-on delay is Atd = 1000 / 6 = 166.6666 and the power is P%,j = 100*2 / 6 = 33.3333%.

[0141] As a result, in the real situation, power peaks are generated with a duration less than or equal to the minimum time interval Aton and with an amplitude equal to Pj, i. Considering now multiple Heaters HEXj, arranged in a group, each of which has a number of active SCRj equal to nSCRj, with j=1 ...m, with a value P% that represents the power demand for all the Heaters HEXj belonging to the group.

[0142] If there is no load management policy, i.e. if the multiple SCRs are free to provide the power required by the heating process, i.e. by the temperature control system TCS, large power fluctuations may occur, due to the variation of P% and the combinations of switching on and switching off the SCRs, belonging to different Heaters HEXj.

[0143] This situation becomes much more critical as the power involved increases. Therefore, considering that the temperature control system TCS must, in any case, be prioritized, temperature control being the first objective, it is necessary to find a load management policy, to avoid or reduce these peaks.

[0144] The main assumptions, preparatory to the definition of a load management strategy, are as follows:

[0145] A) cycle time, Atcycle: to avoid discretization errors, the cycle time, Atcycle, must be a whole multiple of the product of the parameters Aton and nSCR. The typical Atcycle for the heating of the Process Gas PG varies from 1 to 2 s. For example, with Aton = 20 ms and nSCR = 6, it could be 10 times their product, i.e. Atcycle = 10 * 20 * 6 = 1200 ms. Higher values should be avoided, because they could lead to undesired fluctuations of the temperature of the resistors at low loads (when the Atoff value is comparable to Aton or higher).

[0146] When a plurality of Heaters HEX (for example m=5 as in Table 1 HEX1 ... HEX5) is composed of a plurality of SCRs (i.e., in each HEX there are nSCRj), then the cycle time Atcycle must be a whole multiple of all the products calculated as the multiplication of the number of nSCRj, included in the group of HEX, and Aton. In other words, in a plant with “m” Heaters HEXj (j = 1 ... m), the Atcycle must be selected as a whole multiple of the “m” values calculated as the product of the number “nSCRj” and Aton.

[0147] Therefore, for all the Heaters HEXj (j=1... m), the value Vj=nSCRj*Aton must be calculated, and Atcycle shall be a multiple of all the Vj calculated.

[0148] With reference to Example 1 above, with Aton = 20 ms, Atcycle must be a simultaneous multiple of 120 = (6*20), for the HEXIand HEX2 that each have six SCR, and of 80 = (4*20), for the HEX3, HEX4 and HEX5 that each have four SCR. Therefore, Atcycle = 1200 ms or Atcycle = 1440 ms are both acceptable, being at the same time multiples of 80 and 120, but not Atcycle = 1320 ms, this latter being a whole multiple of 120 only, but not a whole multiple of 80.

[0149] B) switch-on time, Aton: the frequency of the alternating current being 50 Hz, the minimum switch-on period corresponds to the first half-wave, i.e. 10 ms, while the other periods must be selected as whole multiples of 10 ms. In actual fact, this value also depends on the available hardware features, in particular on the available digital processors and on the speed of the network, whereby, in the general cases, it may be assumed that it is equal to 20 ms, i.e. a period. This assumption does not affect the capability of the control system, because the dynamics of the thermal process are typically slow with respect to this value.

[0150] Further, being:

[0151] Aton Atcycle Atd (7) the value of Aton must be rounded so as to be a whole multiple of 20 ms. The maximum period, when the required power percentage P% is equal to 100, is obviously equal to the selected Atcycle.

[0152] C) delay time, Atd: to avoid discretization errors, the delay time, Atd, must be a whole multiple of Aton. Selecting Atcycle as above, this condition is automatically satisfied.

[0153] D) required power, P%: the required power depends on the temperature difference between the temperature Tg set and desired for the Process Gas PG and the actual temperature Te measured for the Process Gas PG. It is a manipulated variable, generated by a control system, for example a Proportional-Integral- Derivative PID control system or other control systems such as Model Predictive Control (MPC), fuzzy logic algorithms, and so on. This parameter must be digitized from 0% to 100%, where 0% corresponds to no Power (i.e. Aton = 0) and 100% corresponds to full Power (i.e. Aton = Atcycle). It may be rounded to the unit (this means having 100 digital steps), without losing significant control sensitivity, to avoid discretization errors. From the equation (6), it is known that there are specific values of percentage powers without fluctuations P'% ,j that avoid power fluctuations. But, in general, P% = P%,j + P'%,j.

[0154] The solution proposed here provides for using a number “m” of Heaters HEXj, each of which has a number “n” of SCRi, to electrically heat a body up to a required temperature Tg (the process temperature).

[0155] In particular, it applies to the heating of the process gas in a direct reduction DRI (Direct Reduced Iron) plant, where the heating process is obtained by dividing the process gas flow PG into a series of parallel trains, each composed of a plurality of SCRi-controlled switches placed in series, to bring the temperature of the entire flow rate to the value required by the direct reduction DRI process.

[0156] All the SCRi-controlled switches belonging to a Heater HEXj group must be switched on and off in an operating sequence, independently of the supply voltage or other functional parameters.

[0157] This sequence must be constructed from the coldest Heater to the hottest one, although other combinations are possible, based on the specific thermal process. With reference to Example 1 above and making use of Table 1 , the sequence may be established as follows: for each Heater HEXj with j=1 ... m, there are the controlled switches SCRj.i, with i = 1 ... n, where the coldest Heater will be for j=1 , namely HEX1 , and the hottest one will be for j =5, namely HEX5.

[0158] As already mentioned, the same required power P%, should be sent to each Heater HEXj, with j=1 ...m, to drive the controlled switches SCRj.i which make up the same Heater HEXj.

[0159] However, this strategy may lead to large power fluctuations on the electrical grid. As previously anticipated, the objective of the proposed strategy is to minimize power fluctuations on the electrical grid.

[0160] This may be carried out according to the following strategy:

[0161] 1 ) not having any power fluctuations for the (m-1 ) Heaters HEX1 ... HEXm-1 , i.e. from the first to the penultimate of the series, and 2) allowing precise temperature regulation by the last Heater HEXm of the series, allowing a power fluctuation that must be limited to the power of an SCRj.i-controlled switch belonging to the last Heater HEXm.

[0162] It is possible to generate a numerical table, for the (m-1 ) Heaters HEX1 ... HEXm-1 , containing the values of percentage power without fluctuations P'% ,j (see Table 2). With reference to Example 1 , Table 3 instead reports the values of the percentage powers P%,j, in particular the percentage powers without fluctuations P'%,j, which, for any nSCRj = 4 and 6, avoid power fluctuation in the Heaters HEX.

[0163] Considering:

[0164] Pj the rated power of the “j-th” Heater HEXj;

[0165] Pm the rated power of the “m-th” Heater HEXm; and

[0166] Ptot the total rated power of all the Heaters HEXj installed, the total power is calculated as:

[0167] Ptot

[0168] Calculating the values:

[0169] P% the required power, where P% * (Aton + Atoff) = 100 * Aton,

[0170] P'%,j the percentage power without fluctuations calculated according 100*k to equation (6) P'%, j = - with k an integer number between 1 and nSCRj, and nSCRj where nSCRj is the number of active SCR-controlled switches present in group “j”, it follows that the required power for the last Heater HEXm, indicated as P%,m, may be calculated as follows: with the mandatory criterion that P%,m < 100%.

[0171] Obviously, it is possible to generate different values of P%,m based on the value P'% ,j of percentage power without fluctuations used in the equation (8). With reference to Example 1 , selecting the numeral values reported below:

[0172] P1 = P2 = 5000 kW,

[0173] P3 = P4 = P5= 3000 kW,

[0174] Ptot = 18000 kW, - P'%,j according to Table 3, assuming P% = 81 %, the following Table 4 is generated for the fifth and last Heater HEX5 in which P%,5:

[0175] TABLE 4 The selection of the row of Table 4, containing the most suitable values, may depend on the functional and constructional features of the Heater HEX and on the specific heating process.

[0176] Below, by way of non-limiting example, some examples of selection criteria are reported:

[0177] A) Defining a minimum P% value for automatic control, as a function of the system flow rate and / or the installed power of the Heater HEX;

[0178] B) Maximizing the power of the Heaters HEX supplied at Medium Voltage, which have greater energy efficiency;

[0179] C) Maximizing the percentage power without fluctuations P’%,j of heaters HEXj working at lower temperatures, and / or

[0180] D) Making equal the percentage powers without fluctuations P'%,j of the first two Heaters HEX 1 and 2, and / or

[0181] E) Minimizing the value P'%,5 of the fifth HEX5 Heater, which operates at higher temperature, or

[0182] F) Selecting the value P'%,5 of the fifth Heater HEX5 within a certain range, and so on.

[0183] Similarly, other criteria may be selected to optimize electrical energy consumption, for example, if the SCR switches are connected to different networks with different capacities and features, selecting the parameters so as to exploit the features of the various networks, etc.

[0184] These criteria may be applied automatically, adopting an appropriate algorithm that leads to the selection of a row of P'% ,j values.

[0185] This means that, for each P%, the algorithm may automatically select the reference P'%,j to be applied to (m-1 ) HEX and, consequently, calculate the P%,m of the last Heater HEXm.

[0186] Further, on the basis of these values, for each Heater HEX, the algorithm calculates the data listed below, using the equations provided above:

[0187] - Aton, according to equation (1 )

[0188] - Atd, according to equation (4)

[0189] - Atoff = Atcycle - Aton These values are applied to sequentially switch on / off each SCR inserted in the group.

[0190] With reference to Table 4, by applying conditions c), d), e) the set of values that satisfy them all is contained in the last row below:

[0191] TABLE 5

[0192] A simulation of the load diagram on the electrical network for two cases is shown below:

[0193] 1 ) Application of the P% = 81 % to all the HEX, without any load management strategy.

[0194] 2) By applying the load management strategy using, for the same required power P% = 81 %, the values reported in Table 5.

[0195] The charts shown in Figure 3 and Figure 4 show on the vertical axis the number nSCR of active controlled switches and on the horizontal axis the time intervals, in which each line has a width of 20 ms.

[0196] In Figure 3, the power fluctuation varies from zero to 5 SCR, namely from the 17th to the 22nd SCR, to follow the required power P%=81 %, for a total power fluctuation of 5 MW.

[0197] Conversely, in Figure 4 the power fluctuation varies from zero to 1 SCR, in particular from the 19th to the 20th, to follow the required power P%=81 %, for a total power fluctuation of 1 MW.

[0198] Figure 3 shows that the maximum fluctuation, within one interval, of the power is 5 SCRs, whereas this fluctuation in Figure 4 is limited to the power of a single SCR, which, by adopting a zero crossing switch-on mode, is the best achievable result.

[0199] Obviously, for the same rated power, the higher the number of controlled switches nSCR, the better, approaching a continuous modulation.

[0200] The drawbacks of an increase in the number of nSCR switches are equally obvious (costs and space). Therefore, as usual in engineering, it is necessary to find the best compromise, considering the specific case. In addition to the final temperature of the body to be heated, which must be continuously controlled by the temperature control system TCS that generates the output Y = P%, the internal temperature of the Heater HEX must also be monitored and controlled. For example, in the case of an electric Process Gas Heater e-PGH, the temperature of the process gas PG must be measured at the outlet of the last Heater HEX of the series, and continuously controlled, so as to ensure that it is maintained at the value required by the process.

[0201] Further, also the temperature of the process gas PG inside each Heater HEXj must be measured and controlled, mainly for the following reasons:

[0202] - to anticipate and avoid each possible local overheating, which may lead to the shutdown of the Heater HEX for safety reasons, compromising the continuity of operation of the plant or, in the worst case scenario, damage to the heating elements; and

[0203] - to accelerate and stabilize the control action.

[0204] The actual measured temperature Te, indicated as TTj, must be representative of the internal “thermal state” of the entire unit (Heater HEX or Thermal Zone TZ).

[0205] Obviously, it is possible to install some of these sensors, so as to select one or a combination thereof.

[0206] It is further necessary to provide a certain number of safety sensors TZTj,i to measure the temperature of the heating elements, so as to prevent it from exceeding the maximum allowed value.

[0207] Therefore, the general layout of the temperature control system TCS, for any heating process, must have cascade control, in which an external control loop will control the temperature of the body or of the process gas PG, and an internal control loop will control the internal temperature of the Heaters HEX. Further, it is necessary to provide a certain number of safety sensors.

[0208] As already mentioned, both the external control loop and the internal one may be based on different types of control systems, such as Proportional-Integral-Derivative control - PID, or Model Predictive Control - MPC, or fuzzy logic algorithms, or a combination of control algorithms, but, in any case, these controllers will generate a reference output signal “Y”, which will guide the heating process. This signal may be digitized and converted into an integer from 0% to 100% (see point D) calculation of the required power, P).

[0209] Defining P% the output of the external controller Y and Yj, j=1 ...m the output of the m internal controllers associated with the m Heaters HEXj. Hereinafter, the interface and the interconnections between the load management algorithm and the temperature control system TCS, as well as the function of each functional block shown in Figure 5, are described, with specific reference to the temperature control system TCS of an electric Process Gas Heater e_PGH.

[0210] The diagram of Figure 5 shows, as Example 2, a group of three Heaters HEX1 , HEX2 and HEX3, but is applicable to a group composed of any number of Heaters HEXj, each of which has any number “n” of SCR-controlled switches.

[0211] Similarly, the illustrated temperature control system TCS is a combination of Proportional-Integral-Derivative control and “Override”, based on temperature feedback and flow rate feed-forward.

[0212] However, it is applicable, with the appropriate modifications, to any control system that produces output signals, P% and Yj, that control the power delivered to heat a body by means of heating elements operated by SCR-controlled switches. In the Example 2 illustrated in Figure 5:

[0213] - the external control loop, based on Proportional-Integral-Derivative PID controllers, which process the temperature measurements TTg of the gas inlet into the heater and of the gas outlet from the heater, and the gas flow measurement, FTg, originating from the field instrumentation, and generate an output Yg, digitized and filtered so as to have as output only integers from 0 to 100. The value Yg is exactly the parameter P% indicated in the preceding paragraphs;

[0214] - the internal control loop, which controls the internal temperature of each Heater HEX, is also based on a Proportional-Integral-Derivative PID controller. Its main object is to prevent the Heater HEX from incurring overtemperatures, ensuring that, in any condition, it operates at temperatures lower than those set, Tjset, which are the maximum allowable temperatures defined in the design step for each Heater HEXj. The output Yj of the internal controller is further digitized and filtered so as to have, as output, only integers from 0 to 100.

[0215] With reference to the system of Example 2 shown in Figure 5, the operations performed in the functional blocks are the following:

[0216] - BLOCK 1 : based on the described criteria, on the basis of P% a set of (m-1 ) values of percentage powers without fluctuations P'% ,j is selected, choosing them from the table of the percentage powers without fluctuations P'%,j. This means that P% is the pointer to a numerical table containing all the values of the percentage powers without fluctuations P'%,j applicable to the first (m-1 ) Heaters HEXj.

[0217] It should be recalled that the percentage powers without fluctuations P'%,j are calculated according to equation (6) by selecting an integer k=1...nSCRj and some selection criteria, such as those listed previously and identified by letters a) to g).

[0218] Therefore, for Example 1 , a correct sequence is one of those reported in Table 4 for P% = 81 %, i.e. , the percentage powers without fluctuations of Table 5, applying the selection criteria, which generates a sequence of the first 4 values of percentage powers without fluctuations for the first 4 Heaters HEX1 , HEX2, HEX3 and HEX4:

[0219] Instead, with reference to Example 2, which considers only 3 Heaters HEX1 , HEX2 and HEX3, there will only be two selected values: P'%,1 and P'%,2.

[0220] - BLOCK 2: The values of the percentage powers without fluctuations P'% ,j selected (P'%,1 and P'%,2 in Figure 5) by BLOCK 1 are each compared with the corresponding Yj (Y1 and Y2 in Figure 5): if P'%,j is < Yj, then P'%,j is taken, otherwise the value P'%,j*, calculated with a lower k, is taken, such as to satisfy this condition.

[0221] Thus, for example, if for HEX2 P'%,2 = 100% (from Table 3, with k=6) and Y2 = 71 %, then P'%,2 = 66,67% (with k=4) will be selected.

[0222] This condition must be verified for any HEXj, j=1 ...(m-1 ), so as to obtain a set of (m- 1 ) values of percentage powers without fluctuations P'%,j that are all less than or equal to the corresponding Yj.

[0223] - BLOCK 3: the percentage power value without fluctuations P%,m (P%,3) is calculated from equation (8), on the basis of the (m-1 ) percentage power values without fluctuations P'%,j selected in BLOCK 2 (P'%,1 and P'%,2), with the limitation P%,m < 100%.

[0224] - BLOCK 4: obviously, also in this case P%,m (P%,3) calculated from equation (8), at BLOCK 3, must be less than or equal to Ym (Y3), generated by the internal control loop, otherwise the lower value, between P%,m and Ym, will be selected to drive the SCRi switches of the last Heater HEXm (HEX3).

[0225] - BLOCK 5: the values of AtdJ, AtonJ and AtoffJ are calculated for each HEXj, according to equations (2), (3) and (4), and the on / off sequence for each Heater HEXj is transmitted to the corresponding board SCRjBOARD for the performance of the on / off cycles.

[0226] Considering the thermal inertia of the system and the predictable transients, the operations of BLOCKS 1 to 5 should be executed within a time interval of about 200 ms and not greater than Atcycle, so as to process the new absorbed power demand, P%, with a time lower than Atcycle.

[0227] Obviously, the shorter this time interval is, the more responsive the control response will be.

[0228] The above description of embodiments of the invention 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 in various applications such specific embodiments without further research and without departing from the inventive concept, and thus it is understood that such adaptations and modifications will be considered as equivalents of the specific embodiments.

[0229] The means and materials for achieving the various functions described can be of a various nature, without thereby departing from the scope of the invention.

[0230] The terminology or expressions used are intended to be only descriptive and therefore non-limiting.

[0231] Obviously, without prejudice to the principle of the invention, the construction details and the embodiments may vary widely with respect to what is described and illustrated above by way of example, without departing from the scope of the present invention.

[0232] When the constructional features and techniques mentioned in the claims below are followed by references signs or numerals, such reference signs were introduced for the sole purpose of increasing the intelligibility of the claims, and therefore have no limiting effect on the interpretation of each element identified, merely by way of example, by such reference signs.

Claims

CLAIMS1 ) A control system, for temperature control, power management and distribution over time, as a function of the thermal demands of an electric heating system, wherein said electric heating system comprises a plurality “m” of heaters (HEX), wherein each heater (HEX) in turn comprises a plurality “n” of SCRs, wherein said control system comprises a load switch device for distributing power to the load so as to achieve the desired process temperature (Tg),- wherein said SCRs are subdivided into “j” groups, with j = 1 , ... m, and for groups “j” from 1 to “m-1” the control system is configured to sequentially switch the different groups on and off with a delay between each group,- wherein for each group “j” of SCRs, with “j” from 1 to “m-1”, the control system is configured to command switch-on with a delay with respect to the previous group “j- 1”,- wherein the control system is configured to select, for each group “j”, a required power value P’ %,j from those calculated according to the equation:100 * kP'%‘i =^CRj where k is an integer number selected from the range from 1 to nSCRj and where nSCRj is the number of active SCRs present in the group “j”,- and wherein the last group “m” of SCRs is controlled with a power P%,m selected from those calculated according to the equation:wherePtot = ^i Pj, where Pj is the rated power of the “j-th” heater (HEXj), wherePm is the rated power of the “m-th” heater (HEXm), where P% =100*AtowwhereAton is the time interval in which the SCRms of said last group “m” switch on at the rated power, and where Atoff is the time interval in which the SCRms switch off.2) The control system according to claim 1 , wherein said control system is configured to calculate for each group “j” of SCRs the switch-on instant Atonj p% calculated according a to the equation Aton = - Atoff, the relative switch-off 100-P% instant AtoffJ calculated according to the equation Atoff =10°o / P%Aton and theswitch-on delay with respect to the previous group “j-1 ” Atdj calculated according to the equation Atdj =, where Atcycle is calculated as Atcycle = (Aton + Atoff).3) The control system according to claim 1 , wherein said load switch device is adapted to distribute power to the load according to predetermined selection criteria, wherein said selection criteria depend on the constructional and functional characteristics of the Heaters (HEXj) and the specific heating process.4) The control system according to claim 3, wherein said selection criteria are selected from:A) Defining a minimum P% value for automatic control, depending on the capacity of the system and / or the installed power of the heater HEX; and / orB) Maximizing the power of the Heaters HEXj powered with Medium Voltage, which have higher energy efficiency; and / orC) Maximizing the percentage power without fluctuations P’%,j of heaters HEXj working at lower temperatures, and / orD) Equalizing the percentage powers without fluctuations P’%,j of the first two heaters HEX1 and HEX2, and / orE) Minimizing the value P’%,m of the last Heater HEXm, which works at a higher temperature, orF) Selecting the value P’%,m of the last Heater HEXm within a predetermined range.5) The control system according to claim 1 , wherein the rated power (Pj) of each Heater (HEXj) varies according to the requirements of the thermal process and said rated power (Pj) is to be equally divided between the “n” SCRs of the “j-th” Heater (HEXj) defining a unit power P1 ,1 = P1 ,2 = ... P1 ,n equal for the number nSCRj of active SCRs present in the group “j”.6) The control system according to any one of the preceding claims, wherein said system comprises an external control loop, based on Proportional-Integrative- Derivative controllers PID, adapted to process the temperature measurements TTg of gas input into the heater and gas output from the heater and the gas flow rate measurement, FTg, from the field instrumentation, and generate an output Yg, digitized and filtered with integers from 0 to 100, wherein said value Yg is exactly the parameter P%.7) The control system according to claim 6, wherein said system comprises an internal control loop, adapted to control the internal temperature of each “j-th” Heater (HEXj), and also based on a respective Proportional-Integrative-Derivative PID controller, with the main purpose of preventing the “j-th” Heater (HEXj) from incurring overtemperatures, ensuring that, under any condition, it works at temperatures below those set, Tjset, which are the maximum permissible temperatures defined at the design stage for each “j-th” Heater (HEXj), and wherein the output Yj of the internal control loop is digitized and filtered with integers from 0 to 100.8) The control system according to claim 7, whose functional blocks are adapted to perform the following respective operations:- selecting (BLOCK 1 ) on the basis of P% a set of (m-1 ) percentage power values without fluctuations P’%,j, applicable to the first (m-1 ) Heaters (HEXj - j=1 ...m-1 );- comparing (BLOCK 2) the values of the percentage powers without fluctuations P’%,j selected in the previous step with the corresponding value Yj calculated from the internal control loop; if the condition P’%,j < Yj is met, then P’%,j is used as the percentage power value without fluctuations for the “j-th” Heater (HEXj), otherwise the value P’%,j* is used, calculated with a lower number k, such as to satisfy this condition,- calculating (BLOCK 3) the percentage power value without fluctuationsP%,m according to the equation P%,m, based on the (m-1 )percentage power values without fluctuations P’%,j selected in the previous step, with the limitation P%,m < 100%,- checking (BLOCK 4) that the value P%,m calculated in the previous step is less than or equal to the value of the output Ym, generated by the internal control loop, otherwise the lower value, between P%,m and Ym, is selected to drive the SCRs of the last “m-th” Heater (HEXm);- calculating (BLOCK 5) for each group “j” of SCR the switch-on instant Atonj p% calculated according a to the equation Aton = - Atoff, the relative switch-off 100-P% instant AtoffJ calculated according to the equation Atoff =10°~P%Atonand theswitch-on delay with respect to the previous group “j-1 ” AtdJ calculated according to the equation Atd =each HEXj, and the on-off sequence for eachHeater (HEXj) is transmitted to a corresponding board (SCRj_BOARD) for performing the on / off cycles.9) Electric heating system of a process gas, suitable for heating the process gas prior to use thereof in an industrial process, wherein said electric heating system is traversed by at least one pipe for conveying the process gas and comprises a plurality “m” of heaters (HEX), wherein each heater (HEX) further comprises a plurality “n” of SCRs, wherein said heating system provides a control system according to any one of the preceding claims.10) Control method for controlling temperature, managing and distributing power over time as a function of the thermal demands of an electric heating system, performed by means of a control system according to any one of claims 1 to 9.11 ) The control method according to claim 10, wherein the method comprises the steps of: defining an external control loop, based on Proportional-Integrative- Derivative PID controllers, which process the temperature measurements TTgdi of gas input into the heater and gas output from the heater and the gas flow measurement, FTg, from the field instrumentation, and generate a digitized and filtered output Yg with integers from 0 to 100, defining an internal control loop of the internal temperature of each “j- th” Heater (HEXj), based on a respective Proportional-Integrative-Derivative PID controller, with the purpose of avoiding overtemperatures, ensuring that each “j-th” Heater (HEXj) works at temperatures below those set, Tjset, and calculating a digitized and filtered Yj output with integers from 0 to 100,- selecting (BLOCK 1 ) on the basis of P% a set of (m-1 ) percentage power values without fluctuations P’%,j, applicable to the first (m-1 ) Heaters (HEXj - j=1 ...m-1 );- comparing (BLOCK 2) the values of the percentage powers without fluctuations P’%,j selected in the previous step with the corresponding value Yj calculated from the internal control loop; if the condition P'%,j < Yj is met, then P'%,j is used as the percentage power value without fluctuations for the “j-th” Heater(HEXj), otherwise the value P’%,j* is used, calculated with a lower number k, such as to satisfy this condition,- calculating (BLOCK 3) the percentage power value without fluctuationsPtOt*P%- ;™-1)P'%,j*PjP%,m according to the equation P%,m = - - , based on the (m-1 ) percentage power values without fluctuations P’%,j selected in the previous step, with the limitation P%,m < 100%,- checking (BLOCK 4) that the value P%,m calculated in the previous step is less than or equal to the value of the output Ym, generated by the internal control loop, otherwise the lower value, between P%,m and Ym, is selected to drive the SCRs of the last “m-th” Heater (HEXm);- calculating (BLOCK 5) for each group “j” of SCRs the switch-on instant p%Atoni calculated according to the equation Aton = - Atoff, the relative switch-off instant Atoff J calculated according to the equation Atoff =10°~P%Atonand theswitch-on delay with respect to the previous group “j-1 ” AtdJ calculated according to the equation Atd =, and the on / off sequence for each “j-th” heater (HEXj)is transmitted to the control module (SCRj_BOARDS) for performing the on / off cycles.

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