SE532567C2 - Procedure for heating in an industrial oven - Google Patents

Description

25 30 532 5G? The disadvantage of such an approach is that a very good margin often has to be used to ensure that the temperature differences inside the material do not become too large, the heating time often becomes unnecessarily long. why The reason for this is, among other things, that the actual implementation of the process entails several sources of uncertainty and that one often does not fully know the original temperature distribution of the material. This leads to increased energy and time consumption, with the consequent increased costs and environmental impact. In addition, such long heating times can result in the material being damaged, for example by excessive oxidation, which of course makes the process more expensive and complicated.

The present invention solves the above problems.

Thus, the present invention relates to a process for batch heating of blanks or ingots in an industrial furnace by means of at least one burner, in which furnace the heating effect continuously introduced into the furnace is regulated by means of a regulator which has the temperature of the furnace atmosphere as input parameter. which temperature is measured by means of a temperature measuring device, where the controller is caused to regulate the heating in at least a first heating step and a subsequent second temperature equalization step, and is characterized in that the instantaneous heating effect is measured continuously, in that the heating effect derivatives with respect to over time is calculated continuously, by the fact that the heating is interrupted when a main condition, namely that this derivative falls within. a predetermined interval, is met, provided that the heating at this time is in the subsequent second temperature equalization step, and that the interval includes the value 0 and 10 15 20 25 30 532 567 is selected so that the ingot is not heated longer than necessary but at the same time achieves sufficient temperature homogeneity.

The invention will now be described in detail, with reference to an exemplary embodiment of the invention and the accompanying drawings, of which: Figure 1 shows a general overview of an industrial furnace in which the method of the invention is applied; and - Figure 2 shows a collection of four principal and simplified graphs, which explain the method of the invention.

Figure 1 shows an industrial furnace 1, in which an ingot 2 is arranged to be heated batchwise. However, it should be understood that the method of the invention can also be used in furnaces, where slabs and blanks of various kinds are heated. The process of the invention is advantageously also used for batch heating of several ingots, blanks or slabs simultaneously.

The industrial furnace 1 comprises at least one burner 3, up the atmosphere of the furnace 1. which heats The burner or burners 3 can, for example, be operated using gaseous or liquid fuel and air or oxygen as oxidant, but other operating configurations can also be used. An especially preferred burner is a so-called oxyfuel burner, whose oxidant consists of at least 80% oxygen.

In the furnace 1 at least one temperature measuring device 4 is also arranged, which measures the temperature of the furnace atmosphere and feeds it as an input parameter to a regulator 5 of the PID type Integrating, (Proportional, Derivating). This regulator 5 generates an output signal in the form of a desired heating effect of the burner or burners 3. The output signal is fed to a flow regulator 10, which translates the desired heating effect into a certain flow of fuel and oxidant, which flow is fed from the flow regulator 6 to the burner or burners 3.

The output signal from the regulator 5 is generally less volatile than the resulting flow from the flow regulator 6, the flow regulator 6, because unlike the regulator 5, among other things, the operating limitations of the burner or burners 3 must be taken into account. Thus, the flow resulting from the flow controller 6 constitutes an approximation to the ideal output of the controller 5.

The burner or burners 3 heat the furnace atmosphere, luckily the ingot warms 2. which in its Figure 2 shows four principal and simplified graphs, explaining where the scale of the X-axis is identical for all four graphs and indicates the time. At time To, the ingot 2 is introduced into the furnace 1.

The heat treatment is divided into two steps, herein referred to as a first heating step and a second heating step, respectively, or a second temperature equalizing step. During the heating step, the power of the burner 3 is relatively high, which aims to quickly heat up the oven atmosphere.

During the temperature equalization step, the power is continuously regulated, with the aid of the PID controller 5, in order to achieve a sufficiently even temperature distribution in the ingot 2 as quickly as possible.

The top graph shows the time development for the temperature Tug »in the oven's 1 atmosphere, the temperature 'Change the surface of the ingot 2 in the center of the ingot 2. In addition, as a dashed horizontal line, the desired final temperature and temperature Tmxt 10 15 20 25 30 532 5G? pure Tbör. In the present embodiment, the temperature of the furnace atmosphere is Tug, higher than the surface temperature of the ingot 2 Tyra when the ingot 2 is introduced into the furnace 1. , the effect of the burner or burners 3.

The second uppermost graph shows the time evolution of the power P momentarily emitted by the burner or burners 3, as well as a straight, horizontal line Pvillko, which represents an interrupt condition. As shown in the graph, the heating power P of the burner or burners 3 during the heating step is set at a relatively high, constant position. The purpose of this is to quickly raise the temperature The furnace and the ingots 2 temperatures before the temperature equalization step, start at time 131. which on- During the temperature equalization step, P is regulated by the PID controller 5, with TW, as input parameter, via its output signal, regulates the flow regulator 6, which in turn regulates the flow of fuel and oxidant to the burner or burners 3, in order to reach the desired final temperature Thor as quickly as possible. When Tugn has stabilized sufficiently close to Tbö, it can be concluded that the ingot's 2 surface temperature Tyra, as well as its center temperature Tmitt, have also reached sufficient and sufficiently homogeneous temperatures.

Thus, TW is measured, and fed as an input parameter to the PID controller 5, which in turn, by regulating the flow regulator 6, indirectly regulates the heating effect P. The control aims, in a conventional manner, to obtain Tug as efficiently as possible. to swing in towards the desired final temperature Thör- 10 15 20 25 532 56? The bottom graph shows the time development of the heat energy PAck emitted by the burner or burners 3, as well as a straight, horizontal line PAck fi¿ Uw, which represents an interruption condition.

The top graph shows that the surface temperature Twa of the ingot 2 and its center temperature Tmxt increase at different speeds during the heating. Due to the heat conduction, Tys increases faster than Tmxt.

Depending, among other things, on different oven configurations and different dimensions of the heated material, heating differs markedly. time T2, is this difference in Common is, however, that when the ingot 2 is taken out of the furnace 1, it is desirable that the difference between Tyu and Tmnt should be less than a certain predetermined value, in order to avoid the problems described above in subsequent processing steps.

Instead of allowing the heating process to run for so long as to ensure that sufficient temperature equalization is present, the present invention solves this problem by examining the behavior of curve P.

Accordingly, the value P is continuously observed and its time derivative dP / dt is calculated.

The instantaneous output P of the burner or burners 3 can be measured in several different ways. First, P can be measured indirectly by observing the output signal of the controller 5. In this case, an ideal, desired value for P is obtained, which does not necessarily correspond to the momentarily actually delivered effect at a given moment. On the other hand, this ideal value is a measure of the desired burner effect given the current development of the oven temperature Tug. Therefore, for reasons described in detail below, this method of measuring P in certain embodiments is preferred in accordance with the present invention.

Second, the power P can be measured by measuring the flow from the flow regulator 6, either by means of a flow meter or by reading the flow directly from the flow regulator 6.

This alternative measurement method gives a measurement value that better corresponds to the actual momentarily delivered power from the burner or burners 3, but on the other hand contains more noise in relation to the desired effect calculated by the controller 5.

The next lowest graph shows the time development for dP / dt, and two straight, dP / dt conditions, which together represent an interrupt condition. horizontal lines dP / dtw¿DwÉ, As can be seen from the combination of the second top and second bottom graphs, in the present embodiment the dP / dt is initially approximately zero, in order to pass from the first heating time through a not necessarily continuous transition at time T1. and into the phase. with PID control, second heating step. During this phase, the derivative dP / dt will change in accordance with the PID regulation regarding P. i.e.

It has surprisingly been found that it is possible to design empirically based conditions regarding dP / dt and thus estimate with good accuracy at what time T2 the heating process can be interrupted and the ingot 2 taken out of the furnace 1. for subsequent processing steps, without heating ingot 2 unnecessarily long and at the same time achieve sufficient temperature homogeneity in ingot 2.

Accordingly, according to the present invention, the time T is selected; as the time when a main condition is met, in other words the time when dP / dt falls within a certain predetermined range {dP / dtW¿nmQ, dP / dtw¿nwf], determined on the basis of experience and dependence on a variety of factors, such as the dimensions and shape of the heated material, the number of oven types, the properties of the regulator 5 and the flow regulator 6, the desired maximum temperature difference between Twa and Tmüt, and so on. [dP / dtvillkorl; burner, Preferably, the interval dP / dtvn¿mm2] is symmetrical about dP / dt = 0.

In addition, in order to avoid that the heating process is interrupted prematurely, it must be ensured that the process has reached the temperature-equalizing second heating stage. This according to an exemplary embodiment is done by ensuring that two side conditions are met at the same time.

A first side condition is that the absolute value of P must be less than a certain first predetermined value Pw¿nm ,. It is preferred that this first predetermined value Pwlumr be selected so that it is less than the value of P during the initial phase or an early phase, between TO and T1, in order to avoid that the heating process is interrupted already during the first heating step. the total heating energy PAck must exceed a certain second predetermined value PAckvU¿mu. This second predetermined value PAck ~ vnlmm is selected, in order to avoid premature interruption of the heating process, preferably so that it exceeds the minimum amount of energy theoretically required to heat the ingot 2 to the desired final temperature distribution with the desired minimum temperature homogeneity and with one for the system. appropriate efficiency, 10 15 20 25 532 SE? It will be appreciated that the purpose of the second ancillary condition is to avoid premature interruption of the heating process and that this object is achieved by not allowing such interruption before a minimum amount of heat has been applied. Thus, approximate and / or indirect measures of PAck are used to determine whether the second adverse condition is met or not. For example, the time from the beginning of the heating process can be measured and, given that P as a function of time is approximately known, this time can be used sonx approximately and indirectly. dimensions of PAck. Similarly, many other metrics can be used to approximate and / or the other side condition is met, indirectly determining whether it such as, for example, the position of fuel valves, measured fuel pressures, etc.

The total supplied heating energy PAck is preferably measured indirectly, by wetting the total amount of fuel introduced into the furnace l since the beginning of the heating.

When the two adverse conditions are met, it is known that the heating process is in the temperature equalization phase between T1 and T2, i.e. in it During this phase is thermal equilibrium. Thus, the temperature of the furnace wall has been largely stabilized, the second temperature equalization step. the furnace 1 in a form of dynamic as well as the degree of heat loss through the wall. The flue gases basically maintain a constant temperature and flow. In practice, it is mainly the ingot 2 that changes temperature during this temperature equalization phase. In other words, the dependence between P and Tmxt is relatively direct, which is why the PID regulation of P has a relatively unambiguous and delayed effect on the change of Tmut. Conversely, because the PID control is a predictable process under uniform conditions, the change of P induced by the PID control will be dependent on the change over time of Tmxt, and further also of the instantaneous value of Twxt. Due to these relationships, it has been found that it is possible to use the value of dP / dt to predict when the value of Tmnt exceeds a certain value.

This can be understood by considering a somewhat simplified expression of the total applied power Pmt expressed as a function of the surface area A of a heated ingot, the thermal conductivity of the heated material K, the distance AD between the surface of the material and its center and the temperature difference AT between - lan surface and. middle and wide. the dynamic equilibrium described above, i.e. when the surface temperature of the heated material has reached the final temperature in the oven: AÄAT Pm! : Pmaáeríal + Pförhß1er = * __- + f} wmwf where Pme fl ßl and P fi uhwu are the parts of the total applied power that are transferred to the material in the form of heat energy or disappear from the system in the form of losses.

At dynamic equilibrium, P fl ßhmæ is built up of two components.

On the one hand, there are losses in the form of heat energy in the outgoing flue gases.

Partly These are approximately linearly dependent on Pmt. there are other losses, which are approximately constant over time at dynamic equilibrium because the temperature conditions are approximately constant over time in the furnace except inside the heated material itself, described above. as Furthermore, A, Ä and AD are approximately constant throughout the heating process, so Emwnnl is essentially a function of AT. In fact, it has been found that in the dynamic equilibrium described above, the derivative of Pmterial with respect to time is approximately a linear function of AT, hereinafter referred to as F (AT).

In other words, in the case of dynamic equilibrium: dPnu ___ dpmazeriat dpfö fiflfl "dPux dPm: T NT ___ 1_% d! _ D! + Dt ~ F (AT) + k, d: => d: sia / ir» m ,,, where k; and k; are constants and since F (A'I ') is an approximately linear function, since Tyta is known and constant when the dynamic equilibrium described above prevails, thus the time derivative of Ptot is used to approximately calculate the temperature Tmitt in the middle of the heated material.

Finally, because the specific operating conditions can be so different, it turns out that it is easier to use experience-based interruption conditions than mathematically calculated ditto. Therefore, the above derivation is primarily intended to explain the principle behind the present invention.

In Figure 2, the dashed area illustrates the area where the two adverse conditions are met. At time T; the main condition is also met and the heating can thus be interrupted.

Due to the actually varying operating conditions and generally varying conditions that occur during heating of materials in industrial furnaces, the PID regulation of P will have a relatively high variance. As described above, this variance will be slightly higher in the case P is measured at the flow regulator 6 rather than at the regulator 5. 10 15 20 25 30 582 56? As a whole, in other words, P will vary as a relatively even function with increasing, negative derivatives during the time period between T; and T2, but with a more or less strong noise component. This noise component can, if applicable, be filtered out by averaging P over a number of measuring points, depending on the current application, 10 measuring points. preferably least and preferably, to let the number of points used in the averaging It is also possible, era as a function of how strong the variance of P is momentum. Thus, more points are used in the averaging at high instantaneous variance and conversely fewer points at low instantaneous variance. For example, the variance may be lower during the beginning of the temperature equalizing second heating step and higher during its final stage. In the second lowest graph of dP / dt, for the sake of clarity, purpose, and i. Exemplifying function, averaged over 10 historical measuring points for P.

In addition, for example, there may be greater reason to average more points in the case P is measured at the flow regulator 6. The flow of fuel and oxidant regulated by the flow regulator 6 contains, as described above, an additional noise component in relation to that of the regulator - tower 5 generated the output signal. One reason for this is that the flow regulator 6 in certain applications regulates the burner or burners 3 to heat the furnace 1 by an on / off procedure. This may, for example, be due to the fact that the burner or burners 3 have a minimum power which exceeds the heating power prescribed by the PID controller 5, which is emitted to the furnace atmosphere at a certain given time. In this case, for example, the flow regulator 6 regulates the burner or burners 3 to be on or off for a certain period of time, then to be switched off or 10 15 25 30 533 55? 13 off and then repeats this procedure so that the average output power becomes that prescribed by the PID controller 5 for any given period of time. This method is called pulse width modulation.

It is also possible that the flow regulator 6 uses constant times for the on and off position and instead the power of the burner or burners 3 varies during the on position and thus achieves the same correct average heating effect. This is called pulse height modulation.

Other modulation techniques are also possible, for example a combination of pulse width modulation and pulse height modulation, to achieve an average torque heating effect which corresponds to that prescribed by the PID controller 5 for a certain given period of time.

When using such modulation, in combination with P being measured at the flow controller 6, value formation can also be advantageously the mean curve P over a number of measuring points, achieve a corresponding continuous function, dP / dt can be calculated. in which, in the same way as above, at least 10 measuring points are preferably used in the averaging, but it is also preferred to vary the number of measuring points as a function of the variance of P. For example, the modulation will most likely be stronger or more marked during the final stage of the heating process, when Tmitt is approaching its desired final temperature.

The controller 5 is in the present exemplary embodiment of the PID type, but it should be understood that other types of controllers may also be useful in the application of the method of the present invention. In such cases, the criteria of the present invention for interrupting the heating may need to be modified to suit the actual conditions. For example, it may be necessary to also examine the second derivative with respect to the time of the heating power curve in the case where a used regulator can cause a swing in the oven temperature, where the heating power derivative is zero one or more times before the oven temperature finally swings towards its final value.

Thus, the present invention provides a process which makes it possible to interrupt a heating process in an industrial furnace with greater certainty at a sufficient temperature uniformity in the material, without the need to continue the heating process unnecessarily long with the associated disadvantages in terms of long heating times, high energy costs and material damage. In addition, the investment cost of applying the present invention to existing industrial furnaces is low, as necessary sensors or similar devices for measuring or observing the output power and furnace temperature are often already installed. In these cases, it is therefore sufficient to implement the interruption condition itself, which can be achieved at a relatively low cost.

Exemplary embodiments have been described above. However, the invention may be varied without departing from the invention.

Therefore, the present invention is not to be construed as limited by these exemplary embodiments, but may be varied within the scope of the appended claims.

Claims (1)

1. 0 15 20 25 30 532 55? 15 P 1! TENT 'K IR lay V 10 Process for batch heating of blanks or ingots (2) in an industrial furnace (1) (3), introduced ~ heating effect (P) by means of at least one burner in which furnace (1) it into the furnace (1) is continuously regulated by means of a (Tag) measured by means of regulator (5) which has the temperature of the furnace atmosphere as input parameter, which temperature (13%) (4) is brought to regulate the heating in at least one first heating stage and a subsequent second temperature equalization steps, by a temperature measuring device the controller (5) is characterized in that the instantaneous heating effect (P) is measured continuously, by the derivatives of the heating effect (P) with respect to time (dP / dt) being calculated continuously, by that the heating is interrupted when a main (dP / dt) falls <[dP / dt condition, provided that the heating condition, namely that this derivative within a predetermined CïP / dflviiikorz]) f interval is met, is at this time in the subsequent second temperature out the leveling step, and in that the interval (Idp / dc condition, dP / dcvinkoal) is chosen so that the ingot (2) comprises the value 0 and is not heated longer than necessary but at the same time achieves sufficient temperature homogeneity. Process according to Claim 1, characterized in that the heating is considered to be present in the subsequent second temperature equalization step in the case of both a first secondary condition, namely that the absolute instantaneous heating effect (P) is below a first predetermined value (Pv fl¿ mn), and that a other secondary conditions, namely that the total heat energy (PAck) (pAck conditions) introduced into the furnace (1) since the beginning of the heating is the main condition. exceeds a second predetermined value are fulfilled at the same time as said hu- 10 15 20 25 30 35 533 5G? Method according to Claim 1 or 2, characterized in that the instantaneous heating effect (P) at each time point is determined indirectly by continuously measuring the amount of fuel introduced into the industrial furnace (1) per unit time. Method according to Claim 1 or 2, characterized in that the instantaneous heating effect (P) is determined at each time point as an ideal and desired heating effect calculated by the regulator (5). Method according to claim 1 or 2, characterized in that the regulator (5) is caused to control a flow regulator (6), which in turn is caused to control the heating power supplied to the industrial furnace (1), and in that the instantaneous heating effect (P) at each time point is determined as the heating effect provided by the flow regulator (6). Method according to claim 2, 3, 4 or 5, that the first predetermined value consists of the heating effect emitted in the furnace (1) by (Pvnlkor) during an early stage or a lower value. Method according to one of Claims 2 to 6, characterized in that the second predetermined value (PAckvimm) consists of the energy which is theoretically the least required to heat the material (2) to the desired final temperature distribution or higher with a appropriate efficiency. Method according to one of the preceding claims, characterized in that the variance over time of the heating effect (P) is caused to be reduced by means of a first averaging over several consecutive sequences. . lz fi 13. 532 567 17 the measuring points before the calculation of the heating power derivative with respect to time (dP / dt). Method according to one of the preceding claims, characterized in that, when pulse width modulation, pulse height modulation or combinations thereof are used to regulate it into the furnace (1) (P), a corresponding continuous heating power function is calculated, continuously introduced heating effect by means of a second average formation over several successive 'measuring points of the heating effect (P), and by the fact that the corresponding continuous heating effect function is used for the calculation of the derivative (dP / dt). Method according to Claim 8 or 9, characterized in that the first and / or second averaging over several successive measuring points uses at least 10 consecutive points. Method according to claim 8 or 9, characterized in that the first and / or second averaging over several consecutive measuring points uses a number of consecutive points, where the number of points varies depending on the instantaneous variance of the heating effect over time. Method according to one of the preceding claims, characterized in that the interval ([dP / dtv fl¿ m ”fi dP / dtVn¿mM2]) is symmetrical about 0. Method according to one of the preceding claims, characterized in that the burner ( 3) is an oxyfuel burner.