Industrial furnace and methods for utilizing its heat

Abstract

Continuous industrial furnace comprising: an inlet (11); a heating zone (12); a cooling zone (13); and an outlet (14) in that order, wherein the continuous industrial furnace is configured to subject a workpiece to heat treatment while the workpiece is conveyed from the inlet (11) to the outlet (14), wherein at least a part of the cooling zone (13) has a furnace wall thermal insulation assembly comprising: an outer wall (21) having one or more gas inlet openings (24b); and a porous thermal insulation layer (23) arranged with a gap (22) on the inside of the outer wall (21); and wherein the cooling zone (13) further comprises one or more exhaust openings (26b) for drawing in and expelling a gas after the gas has flowed from the gas inlet openings (24b) through the gap (22) and the porous thermal insulation layer (23) in that order into the cooling zone (13) and has subsequently been used to cool the workpiece.

Description

TECHNICAL AREA

[0001] The present invention relates to an industrial furnace. The present invention further relates to a method for utilizing the heat of the industrial furnace. STATE OF THE ART

[0002] Previously, intensive efforts were made to improve the thermal efficiency of industrial furnaces with a view to energy conservation. Today, the need for improved thermal efficiency is increasing due to the problem of global warming. To improve the thermal efficiency of an industrial furnace, it is important to reduce heat loss from the furnace walls and heat dissipated through exhaust gases, which are the two main sources of heat loss.Currently, so-called high-performance industrial furnaces are increasingly used in practice, in which a heat insulation material made of inorganic fiber with low thermal conductivity is used as a countermeasure for heat loss from the furnace wall (for example, Japanese patent JP 3 517 372 B2), a heat exchanger-type burner (Japanese patent application publication JP H10 - 238 757 A) or a regenerative burner (for example, Japanese patent JP 5 051 828 B2) is used as a countermeasure for heat emitted by exhaust gas.

[0003] Furthermore, the heat released by exhaust gases is conventionally recovered routinely by a boiler or heat exchanger and used as a heat source for the furnace itself or other equipment (for example, Japanese patent application publication JP 2010 - 48 440 A). More recently, the development of further utilizing this unused heat for heat storage, cooling, power generation, and the like has made further progress and has been partially implemented in practice. In other words, there has been steady progress in reducing the heat released by exhaust gases and utilizing exhaust heat.

[0004] On the other hand, the heat loss from the furnace wall can hardly be reduced further. When considering heat loss from the furnace wall, it is assumed that the outer wall is a double wall and that air or water is passed through the space between the walls to recover heat. However, the temperature of the heat is generally around 100 °C, which is considered low for a heat source, and the heat is widely distributed over the surface area, resulting in lower exergy and making the production costs for heat recovery unacceptable. Therefore, practical use of effective heat recovery has not been achieved. Furthermore, thermoelectric power generation, thermoacoustic power generation, and cold extraction using heat loss from the furnace wall have been developed, but the efficiency of these conversions is still low and they remain in the developmental stage.

[0005] Regarding the reduction of heat emitted from the furnace wall, a NEDO research report from fiscal year 2009, "Research for Extracting Theme of Thermal Radiation Control Technology Development Aiming at Energy Saving of High Temperature Equipment / Plants, etc.", introduces an active thermal insulation method as an advanced thermal insulation technique. In this method, a low-temperature gas flowing in the opposite direction to heat transfer is passed through an optically semi-transparent porous layer. Details of the active thermal insulation method are disclosed in Japanese patent application JP H03-41295A, which teaches that the method is used, in particular, for thermal insulation during the reentry of a rocket nozzle or spacecraft, as well as for thermal protection of a furnace for the development of new materials or a wall of a nuclear fusion furnace.Furthermore, the document reveals that the throughput time of a blast furnace or the furnace for the development of new materials can be shortened because the thermal insulation layer can be made extremely thin and the time to reach a stable state is extremely short, thus enabling effective use of the facilities and energy savings.

[0006] The NEDO research report concludes, however, that "although this technique is excellent for thermal insulation, it utilizes heat transfer due to the free heat of a gas flowing in the opposite direction to the heat input, making it difficult to combine this technique with energy saving in high-temperature devices or systems, etc." Currently, there is no actual application of this technique.

[0007] Furthermore, Japanese patent application JP 2005-48984A proposes a heat treatment furnace in which gas-permeable refractory material is arranged within a furnace wall along a surface of the furnace wall. The heat treatment furnace is characterized by a gap between the furnace wall and the refractory material, and by the fact that, when an atmosphere is set in the furnace, an atmosphere-setting gas with a predetermined composition is introduced into the interior of the furnace after being introduced into the gap and passing through the interior of the refractory material. This document discloses that, according to the heat treatment furnace, the time required to replace the initial atmosphere in the furnace with a desired atmosphere can be significantly reduced when the furnace atmosphere is set, and atmosphere control can be further facilitated.Furthermore, it discloses that by introducing the atmospheric control gas between the furnace wall and the refractory material, the furnace wall is cooled by the atmospheric control gas, and the temperature of the furnace wall surface is reduced compared to conventional techniques, thus improving the thermal efficiency of the furnace and occupational safety. Further prior art is cited in the following documents: US 4,477,718 A and JP 2004-225,995 A. DIRECTORY OF PRINTED PUBLICATIONS Patent Literature Patent document 1: Japanese patent JP 3 517 372 B2 Patent document 2: Japanese patent application publication JP H10 - 238 757 A Patent document 3: Japanese patent JP 5 051 828 B2 Patent document 4: Japanese patent application publication JP 2010 - 48 440 A Patent document 5: Japanese patent application publication JP H03 - 41 295 A Patent document 6: Japanese patent application publication JP 2005 - 48 984 A Non-patented literature

[0008] NEDO research report in fiscal year 2009, “Research for Extracting Theme of Thermal Radiation Control Technology Development Aiming at Energy Saving of High Temperature Equipment / Plants, etc.,” New Energy and Industrial Technology Development Organization, March 2010, p. 9 BRIEF DESCRIPTION OF THE INVENTION Technical Problem

[0009] The Japanese patent application publication JP H03-41295A suggests that the active thermal insulation method leads to energy savings, but does not discuss in detail how these savings are achieved. In fact, the NEDO research report teaches that the active thermal insulation method is difficult to correlate with energy savings. The aforementioned patent document discloses that the flow velocity of the working gas is preferably as high as possible and can range from 0.1 to 1.0 m / s. Example 1 of the patent document presents a numerical analysis of the effect of active thermal insulation at a specific calorific value of 1 MW / m². 2The tests were carried out with a thermal insulation thickness of 10 mm and gas flow velocities of 0.08 and 0.8 m / s. However, under these conditions, very high exhaust gas losses occur during the operating conditions of general industrial furnaces, making it difficult to achieve energy savings in industrial furnaces.

[0010] Furthermore, Japanese patent application publication JP 2005-48984A discloses that the technique described in the document contributes to improving the thermal efficiency of the heat treatment furnace. However, it does not disclose specific structures and mechanisms that contribute to improving the thermal efficiency. Rather, the technique described in the patent document is based on the principle of active thermal insulation, and a considerable amount of exhaust heat is generated when the atmosphere control gas is vented outside the furnace, making it difficult to improve the overall thermal efficiency of the furnace.

[0011] The present invention was made in light of the foregoing circumstances. One of the objects of the present invention is to provide an industrial furnace that makes it possible to combine the reduction of heat emission from a furnace wall with energy savings. A further object of the present invention is to provide a method for utilizing heat from an industrial furnace that makes it possible to combine the reduction of heat emission from the furnace wall with energy savings. Solution to the problem

[0012] When the active thermal insulation method described above is applied to the insulation of the furnace wall of an industrial furnace, heat loss from the furnace wall is considerably reduced by directing gas from the outside of the insulating layer, which is made of a porous material, towards the interior of the furnace. Conversely, the incoming gas enters the furnace. Therefore, as described above, a considerable amount of heat is generated and dissipated by the exhaust gas when the gas is released outside the furnace, making it difficult to achieve a high overall furnace efficiency.

[0013] From another perspective, however, the active thermal insulation method can be a technique in which the heat emitted by the furnace wall, from which heat recovery / utilization is considered difficult, can be converted into free gas heat, with which heat recovery and utilization can be achieved relatively efficiently. The inventors of the present invention have focused on this point and carried out detailed investigations for its applicability to industrial furnaces.As a result, the inventors of the present invention found that the application on the furnace wall in a heating zone and a cooling zone of a continuous industrial furnace at elevated temperature enables the heat utilization in the furnace, so that energy savings for the entire system can be achieved by taking into account the use of free gas heat inside and outside the furnace, and the inventors of the present invention then completed the present invention.

[0014] To solve the aforementioned problem, a continuous industrial furnace with the features of claim 1 is described. Furthermore, a continuous industrial furnace with the features of claim 4 is described. Additionally, a method for utilizing the heat of a continuous industrial furnace with the features of claim 3 is specified. Furthermore, a method with the features of claim 8 is specified. Further advantageous embodiments are defined in the dependent claims. Advantageous effects of the invention

[0015] The operation of the continuous industrial furnace according to the present invention allows for energy savings by reducing heat emission from the furnace wall, which is effective in reducing the operating costs of the continuous industrial furnace and in counteracting global warming. The present invention may be an epochal invention that has successfully combined the reduction of heat emission from the furnace wall and energy savings, which has previously been considered a difficult problem. An exemplary embodiment is described below, insofar as the exhaust gas opening for intake and exhaust in the heating zone is subsequently referred to, and the gas inlet opening in the heating zone are only the subject of the independent claims insofar as they are combined with the corresponding exhaust gas openings and gas inlet opening in the cooling zone. BRIEF DESCRIPTION OF THE DRAWINGS Fig.Figure 1 is a representation showing a basic configuration and a heat curve along a workpiece transport direction in an oven according to an embodiment of a continuous industrial oven according to the present invention. Fig. Figure 2 is a representation that schematically depicts a thermal insulation structure of an oven wall and its thermal insulation principle according to the present invention. Fig. Figure 3 is a diagram showing experimental calculation results of changes in the heat output of the furnace wall and the free gas heat when the flow rate per unit area of ​​the gas flowing through a porous thermal insulation layer is changed. Fig. 4(a) is a diagram showing the effect of reducing the heat output of the furnace wall by supplying a furnace wall heat insulating gas; Fig. 4(b) is a graph showing a minimum heat utilization ratio η minrepresents the free heat of the furnace wall insulating gas, which is required for energy saving compared to a case in which no furnace wall insulating gas is supplied. Fig. Figure 5 is a representation showing the conditions of a model of a continuous furnace in which experimental calculations for the effects of the present invention were carried out. DESCRIPTION OF PREFERRED EXECUTION FORMS

[0016] In the following, embodiments of the present invention are described in detail with reference to the drawings. Fig.Figure 1 is a representation showing a basic configuration and a heat curve (temperature profile) along a workpiece transport direction in a furnace according to an embodiment of a continuous industrial furnace according to the present invention. The continuous industrial furnace according to the present invention comprises: an inlet 11, a heating zone 12, a cooling zone 13, and an outlet 14 in that order, and it can perform heat treatment while a workpiece (not shown) is transported from the inlet 11 to the outlet 14.In the usage herein, the heating zone refers to an area in the workpiece transport direction from the inlet of the continuous industrial furnace to a heating device for heating the furnace interior, which is installed at a position closest to the outlet side, and the cooling zone refers to an area in the workpiece transport direction from a position immediately after the installation position of the heating device at the position closest to the outlet side to the outlet of the continuous furnace. A furnace wall thermal insulation gas supply line 15 and an exhaust gas line 16 for the heating zone, as well as a furnace wall thermal insulation gas supply line 17 and an exhaust gas line 18 for the cooling zone, are connected to the continuous industrial furnace of the embodiment. At least one, and preferably both, of the exhaust gas line 16 for the heating zone and the exhaust gas line 18 for the cooling zone can be connected to a heat recovery device outside the furnace.If hot gas is allowed to flow through exhaust pipe 16 and exhaust pipe 18, the heat can be used outside the oven.

[0017] In the present embodiment, the furnace wall insulating gas supply lines for the heating zone and the cooling zone are fed separately. Since the heating zone and the cooling zone have different optimal flow rates of the furnace wall insulating gas, as described below, the use of such a configuration makes it possible to easily adjust the gas flow rate for the heating zone and the cooling zone separately. However, it is also possible to branch the same gas supply line into lines for the heating zone and the cooling zone, and it is also possible to install a flow control valve to adjust the flow rate in the gas supply line if required. It should be noted that in addition to the air supply and exhaust gas lines shown in the figure, the continuous industrial furnace usually has further air supply and exhaust gas lines, which are omitted here.

[0018] The workpiece is an item that undergoes heat treatment, including, without particular limitation, electronic components such as ferrite and ceramic capacitors, semiconductor products, ceramic products, pottery, oxide-based refractories, glass products, metal products, and carbon-based refractories such as aluminum oxide / graphite and magnesium oxide / graphite. The workpiece also includes a kiln tooling. The heating temperature of the workpiece varies depending on the purpose of the heating, but the continuous industrial furnace according to the present invention can be used appropriately, with a view to achieving an effective energy-saving effect, when the workpiece is heated to 1000 °C or more, and typically to 1200 °C or more, and even more typically to 1400 °C or more, for example, from 1000 to 2000 °C. It should be noted that the concept of "heating" also includes "firing."By applying it to an oven with an elevated temperature, such as a kiln, the heat utilization ratio is further improved.

[0019] The workpiece entering the furnace through inlet 11 is heated in the heating zone and cooled in the cooling zone according to a predetermined temperature curve while being transported towards outlet 14. Although the in Fig.While the exemplary heat curve shown in Figure 1 is a simple trapezoidal curve, it can also be a complex curve, for example, one with multiple temperature retention zones. The method for transporting the workpiece in the furnace is not subject to any particular restrictions and can, for example, be a carriage type, a slide type, a roller hearth type, or the like. The workpiece, having undergone the predetermined heat treatment, is removed from outlet 14. Examples of heating methods in heating zone 12 include, without limitation, heating methods using electrical power, such as resistance heating, induction heating, dielectric heating, arc heating, and radiant heating, as well as heating methods involving the combustion of a fuel with a burner (including a heat exchanger burner and a regenerative burner).The cooling process in the cooling zone, which can be adapted as needed, includes gas cooling methods carried out by supplying a cooling gas to the furnace. In the cooling zone, the workpiece is cooled by convection heat transfer to the cooling gas flowing into the furnace and by radiant heat transfer to the inner surface of the furnace wall.

[0020] In the continuous industrial furnace according to the present embodiment, the heating zone 12 and the cooling zone 13 can have a furnace wall thermal insulation assembly comprising: an outer wall 21 having one, two, or more gas inlet openings 24a, 24b; and a porous thermal insulation layer 23a arranged with a gap 22 on the inside of the outer wall 21. Since the heating zone and the cooling zone in the continuous industrial furnace have a predetermined length in the direction of workpiece transport, the heating zone and the cooling zone are preferably each provided with two or more gas inlet openings 24a and 24b, corresponding to the length of the heating zone and the cooling zone where the furnace wall thermal insulation assembly is to be installed, so that the furnace wall thermal insulation gas is supplied uniformly into the furnace.In this case, each of the gas supply lines for the two or more gas inlet openings 24a, 24b can be branched from a common gas supply line or can be provided as an assigned line by individually preparing gas supply equipment. If there is no difference with regard to the type of furnace wall insulating gas to be supplied, the gas supply lines are preferably branched from the same gas supply line in order to reduce gas piping costs.

[0021] When the furnace wall insulating gas is supplied from the gas inlet openings 24a via the furnace wall insulating gas supply line 15 to the heating zone, the gas successively passes through the gap 22 and the porous insulating layer 24 and then flows into the heating zone 12 of the furnace. As the gas passes through the porous insulating layer 23 of the heating zone 12, heat is exchanged between the gas and the porous insulating layer 23, thereby increasing the temperature of the gas and reducing heat loss from the porous insulating layer 23 to the outside of the furnace. Furthermore, when the furnace wall thermal insulation gas is supplied from the gas inlet openings 24b from the furnace wall thermal insulation gas supply line 17 for the cooling zone, the gas successively passes through the gap 22 and the porous thermal insulation layer 23 and then flows into the cooling zone 13 of the furnace.As the gas passes through the porous thermal insulation layer 23 of the cooling zone 13, heat is exchanged between the gas and the porous thermal insulation layer 23, thereby reducing the heat loss from the porous thermal insulation layer 23 to the outside of the furnace and lowering the surface temperature of the porous thermal insulation layer 23 on the inside of the furnace.

[0022] Thus, in the present embodiment, both the heating zone 12 and the cooling zone 13 have the furnace wall thermal insulation structure according to the present invention, and this embodiment is preferred from the point of view of energy saving. However, it is also possible for only either the heating zone 12 or the cooling zone 13 to have the furnace wall thermal insulation structure according to the present invention.

[0023] Without wishing to be bound by any theory, the furnace wall thermal insulation structure and its thermal insulation principle according to the present invention are schematically shown in Fig.Figure 2 illustrates the principle of reducing heat loss from the furnace wall. The principle is simple: since heat is exchanged between the gas and the porous thermal insulation layer 23 as the furnace wall insulating gas passes through the porous thermal insulation layer 23 from the outside of the furnace to the inside, the heat transferred from the thermal insulation layer 23 to the outside of the furnace is reduced. In the thermally stable state of the porous thermal insulation layer 23, a change in heat transfer through the layer (solid line), an increase in gas temperature (change in free heat), the heat exchange between the thermal insulation layer and the gas are balanced, and the temperature of the thermal insulation layer Ts and the temperature of the gas Tg are related by a fundamental equation given below.

[0024] Change in heat transfer through the thermal insulation layer = increase in gas temperature = heat exchange between insulation layer / gas: λ⋅(∂2Ts / ∂x2)=m⋅Cp⋅(∂Tg / ∂x)=Ae⋅he⋅(Ts−Tg) wherein: Ts: Temperature of the thermal insulation layer [K] (Ts': at the time when no gas is supplied); Tg: Gas temperature [K]; λ: Thermal conductivity in the thermal insulation layer (including radiative heat transfer effect) [W / (m·K)]; m: Mass flow rate of the gas per unit area [kg / (m²] 2 ·s)]; Cp: ​​specific heat of the gas [J / (kg·K)]; Ae: Surface area of ​​the thermal insulation layer per unit volume [m²] 2 / m 3 ]; and he: Heat transfer coefficient of the thermal insulation layer [W / (m²) 2 ·K)].

[0025] Comparing the temperature of the porous thermal insulation layer 23 in the direction of its thickness under normal conditions (Ts'), in which no gas is supplied, with that under the conditions of the present invention (Ts), in which gas is supplied, a roughly Fig. The difference shown in Figure 2 is as follows. Since the amount of heat transferred from the furnace wall is a function of the temperature of the outer surface, the amount of heat transferred from the furnace wall decreases depending on a reduced proportion of the temperature of the outer surface.

[0026] By providing the gap 22 between the outer wall 21 and the porous thermal insulation layer 23, the incoming gas fills the gap 22 and forms a gas layer. This allows the gap 22 to function as a pressure reservoir, enabling the gas to spread over the entire surface of the porous thermal insulation layer 23. This ensures a uniform gas flow through the layer, thus improving the heat loss suppression effect. Furthermore, controlling the pressure differential between the pressure reservoir and the interior of the furnace allows for a stable gas flow at a predetermined velocity. The gas flow velocity in the gap should be between 0.1 and 1 m / s to ensure the gap functions as a reservoir with a uniform pressure. For this purpose, the thickness of the gap 22 can preferably be between 5 and 50 mm, and more preferably between 10 and 30 mm.

[0027] A method for maintaining the porous thermal insulation layer 23 in a state in which the gap 22 exists between the outer wall 21 and the porous thermal insulation layer 23 comprises, without limitation, a method for fastening the porous thermal insulation layer 23 over a spacer using fasteners such as studs, ceramic pins, or bolts attached to the outer wall, and a method for fastening the porous thermal insulation layer 23 over a spacer in which the outer wall is provided with a hole through which a fastener such as a stud, ceramic pin, or bolt is passed. To improve the controllability of the uniform flow rate, a perforated plate can be installed on the surface of the porous thermal insulation layer 23 facing the outside of the furnace.Since the perforated plate acts as a resistor for rectification, the uniformity of the gas flow through the porous thermal insulation layer 23 is increased.

[0028] The furnace wall insulating gas can be selected appropriately, taking into account its reactivity with the workpiece, the atmosphere inside the furnace, cost, specific heat capacity, and other factors. Examples include oxidizing gases (air, O2, etc.), inert gases (N2, Ar, He, etc.), and reducing gases (H2, CO, etc.). Generally, air can be used for cost reasons. To save energy, the furnace wall insulating gas does not need to be heated or cooled and can be at ambient temperature (e.g., 5 to 40 °C).

[0029] In some cases, gas can be supplied to the industrial furnace to adjust the furnace atmosphere, including, for example, when oxygen is required as the atmosphere in the furnace for heat-treating the workpiece in a combustion furnace, when an inert gas is supplied to an electric furnace that requires an inert gas atmosphere, when a gas such as air is supplied to purge volatile components from the workpiece, and the like. Such an atmosphere-adjusting gas is not originally supplied as the thermal insulation gas, but the atmosphere-adjusting gas can be supplied through the porous thermal insulation layer 23 so that it also acts as the furnace wall thermal insulation gas.In this case, a reduced proportion of the heat output from the furnace wall is converted into an increased proportion of the free heat of the gas, and the use of the free heat of the gas inside or outside the furnace results in an energy-saving effect. If the atmospheric gas is supplied through the porous thermal insulation layer 23 in the heating zone, the heat output from the furnace wall can be reduced without increasing the heat loss of the exhaust gas, and consequently, a reduction in the amount of fuel used (heating value) is achieved.

[0030] The material and shape of the porous thermal insulation layer 23 are not subject to any particular restrictions, as long as they generally exhibit thermal insulation performance. Examples of suitable materials for the porous thermal insulation layer 23 include fibrous materials with high gas permeability, such as ceramic fiber, aluminum oxide fiber, and carbon fiber. Since the porous thermal insulation layer itself is soft, the outer wall 21 is suitably made of a metal such as iron, an iron alloy, aluminum, a nickel / chromium-based metal, stainless steel, or the like to maintain the strength of the furnace body. Examples of shapes for the porous thermal insulation layer 23 include a cloth shape and a sheet shape, and a required number of these layers can be stacked. Alternatively, the cloth can be folded into a block shape.Furthermore, these forms can be used in combination with each other. Additionally, with regard to the balance between gas permeability (pressure loss) and thermal insulation performance, the porous thermal insulation layer 23 can have a filling density of approximately 100 to 500 kg / m³. 3 and have a porosity of approximately 0.8 to 0.95. The filling density can be measured according to JIS R 3311: 1991. The porosity can be calculated using the following equation: Porosity = 1 − solid volume fraction = 1 − (fill density / true density)

[0031] Even if the thermal conductivity of the porous thermal insulation layer 23 is high, the effect according to the present invention occurs. However, to minimize heat loss, a porous thermal insulation layer 23 with a thermal conductivity of approximately 0.1 to 1 W / mK (according to JIS A 1412-1: 1999) can be used. The thickness of the porous thermal insulation layer 23 can be adjusted according to the required thermal insulation performance and can range from approximately 100 to 500 mm.

[0032] The portion of the furnace wall thermal insulation assembly used in the heating zone 12 according to the present invention can be adjusted according to the temperature curve and can encompass the entire area or a portion thereof in the workpiece transport direction. When gas is supplied to the porous thermal insulation layer 23 from a plurality of gas inlet openings 24a in the heating zone 12, the gas flow rate at all gas inlet openings 24a can be the same or can be varied depending on the temperature curve. To improve the efficiency of free heat utilization, the furnace wall thermal insulation assembly is preferably used in an area where the furnace reaches its highest temperature.For example, the furnace wall thermal insulation assembly can be used in an area that has an elevated temperature of 1000 °C or more, and the furnace wall thermal insulation gas can be supplied to this area, thereby improving the energy saving effect.

[0033] Similarly, the position for using the furnace wall thermal insulation assembly according to the present invention in the cooling zone 13 can be adjusted according to the thermal curve, and it can be the entire area or a portion of the cooling zone 13 in the direction of workpiece transport. When gas is supplied to the porous thermal insulation layer 23 from a plurality of gas inlet openings 24b in the cooling zone 13, the gas flow rate at all gas inlet openings 24b can be the same or can be varied depending on the thermal curve. Conventionally, an operating sequence is carried out in the cooling zone to reduce the temperature of a workpiece to be heated by supplying a cooling gas at a lower temperature than that of the workpiece from an inlet opening built into a furnace wall, thereby exchanging heat between the workpiece and the cooling gas, after which the gas is expelled.In this case, the cooling gas flows locally into the furnace without significant heat exchange between the cooling gas and the furnace wall. In contrast, according to the present invention, all or part of the gas supplied through the inlet opening can be fed through the porous thermal insulation layer 23. The use of the furnace wall thermal insulation structure according to the present invention in the cooling zone does not contribute to a reduction in fuel consumption. However, the reduced proportion of heat dissipated from the furnace wall is converted into an increased proportion of the free heat of the gas, so that energy savings can be achieved by utilizing the free heat of the gas inside or outside the furnace. Generally, the required gas flow rate per unit area for the cooling zone 13 is greater than for the heating zone, although this depends on conditions such as the thermal curve.Therefore, the cooling zone can be calculated according to the experimental calculation in . Fig. 3, which is explained below, have a heat emission ratio of 0.1 or less, which, compared to heating zone 12, allows for a further efficient reduction of heat loss due to heat emission from the furnace wall.

[0034] It is preferred that the furnace wall thermal insulation assembly according to the present invention is arranged in the heating zone 12 and also the cooling zone 13 such that, when considering the furnace in a cross-section perpendicular to the workpiece transport direction, it surrounds the entire circumference of the furnace chamber in order to equalize the temperature distribution in the furnace and to reduce heat loss from the furnace wall. That is to say, the furnace wall in this context is a concept that includes a side wall of the furnace chamber, a ceiling of the furnace, and a floor of the furnace.

[0035] Fig. 3(a) and Fig.Figure 3(b) shows diagrams of the results obtained by experimental calculations using the basic equation described above for changes in heat dissipation from the furnace wall and free heat of the gas when the gas flow rate per unit area through the porous thermal insulation layer is varied. The internal temperature of the furnace is 1400 °C (in the case of Fig. 4(a)) and 1000 °C (in the case of Fig. 3(b)) and the thickness of the porous thermal insulation layer is 400 mm (in the case of Fig. 3(a)) and 300 mm (in the case of Fig. 3(b)) and the thermal conductivity is from 0.1 to 0.6 W / (m·K) (depending on the temperature), starting from ceramic fiber with a filling density of about 130 kg / m³ 3 Compared to the heat output of the oven (905 W / m²) 2 in the case of Fig. 3 (a); Fig. 576 W / m 2 in the case of Fig.3(b)) Under normal conditions (without the addition of furnace wall insulating gas), the heat output of the furnace wall decreases with increasing gas flow when the furnace wall insulating gas is added. Conversely, the free heat of the added gas increases with the gas flow rate. As a result, it is found that the combined calorific value of the furnace wall heat output and the free heat of the gas becomes greater than the heat output of the furnace wall under normal conditions, and that the overall calorific value increases with increasing gas flow rate. Therefore, to improve the thermal efficiency of the entire heat utilization system, considering heat utilization both inside and outside the furnace by adding the furnace wall insulating gas, it is desirable to utilize approximately half or more of the free heat of the gas simultaneously generated, either inside or outside the furnace.

[0036] Fig.Section 4 explains the findings described above in detail. Fig. Figure 4(a) shows a diagram of the relationship between a dimensionless gas flow rate (g) and a furnace wall heat dissipation ratio (r), where the furnace temperature and the thickness of the thermal insulation layer (d) are used as parameters. The effect of reducing the furnace wall heat dissipation by supplying the furnace wall insulating gas is readily apparent to those skilled in the art. Here, the horizontal axis of the diagram represents a dimensionless flow rate, which is obtained by dividing a heat capacity ratio (Cp × G / 3.6 [W / (m²)]) by the r. 2 ·K)]) of the supplied gas by a heat transfer coefficient in the thermal insulation layer (λ / d [W / (m 2 ·K)]) is obtained because the flow rate of the gas to be supplied for the thermal insulation performance of the thermal insulation layer is determined from the in Fig.The basic equation shown in Figure 2 is relevant. In its use herein, the flow rate is referred to as "a dimensionless gas flow rate." It is understood that the relationship between the dimensionless gas flow rate and the furnace wall heat transfer ratio is independent of the thermal insulation performance (thickness) of the insulation layer. Furthermore, it is understood that the higher the furnace temperature, the lower the furnace wall heat transfer ratio for the same dimensionless gas flow rate. The results indicate that if the furnace wall heat transfer is to be reduced to at least approximately 30%, the dimensionless gas flow rate should be between 1 and 2, although this is dependent on the furnace temperature.

[0037] How Fig.Figure 3 shows that the free heat of the exhaust gas is generated depending on the amount of furnace wall insulating gas supplied, while the furnace wall heat emission is reduced. Therefore, in order to reduce the extent of combustion utilized in the furnace through this process, or to achieve energy savings for the entire system, including heat utilization outside the furnace, it is necessary to utilize a certain proportion of the generated free heat of the gas, both inside and outside the furnace. Thus, a heat utilization ratio of the free gas heat, in the case where the calorific value is exactly identical to that under normal conditions without furnace wall insulating gas supply, is shown in the diagram. Fig. 4 (b) as a smallest utilization ratio η minThe free heat of the gas is represented. In this case, too, the tendency exists that the minimum utilization ratio of the free heat decreases with increasing internal temperature of the furnace, without any dependence on the thermal insulation performance. Furthermore, it is understood that under all conditions, if the flow rate of the furnace wall insulating gas increases, the utilization ratio of the free heat of the gas must be increased to achieve energy savings.

[0038] For example, if the furnace wall insulating gas is supplied to a section with a furnace temperature of 1400 °C and an insulation thickness of 0.4 m, such that the furnace wall heat transfer ratio in this section is set to 30%, the dimensionless gas flow rate g is 1. Furthermore, in order to achieve an energy-saving effect, the heat utilization ratio η of the generated free gas heat should be greater than at least 43%.

[0039] The conditions of the calculation, which are used to create the curves in the Fig. The methods used in 4(a) and (b) are shown below: <(g) dimensionless supplied gas flow> g=Cp⋅G / (λ / d) / 3,6. where Cp: ​​Gas heat capacity [J / (kg·K)] (here, Cp = 1.34, a constant value); G: Gas flow rate per unit area [Nm³] 3 / (Hm 2 )] (Nm 3 refers to a volume (m³) 3 ) when converted to a reference state (0 °C, 1 atm); λ: Thermal conductivity in the thermal insulation layer; λ=A⋅ρ+(B / ρ)⋅Ts3+(C⋅T+D)⋅λf[W / (m⋅K)] where Ts: Temperature in the thermal insulation layer [K]; ρ: Filling density: 130 [kg / m³] 3 ]; λf: Thermal conductivity in stationary gas 0.05 [W / (m·K)]; A: 6.9 × 10 -5 , B: 1.5 × 10 -8 , C: -2.1 × 10 -5 , D: 2.0.

[0040] <r: Verhältnis der Ofenwand-Wärmeabgabemenge mit Zufuhr von Ofenwand-Isoliergas zur Ofenwand-Wärmeabgabemenge unter Normalbedingungen> r=Qw / Qw0; Q w0 Oven wall heat output under normal conditions [W / m²] 2 ]; Q w : Oven wall heat output under conditions where oven wall insulating gas is supplied [W / m²] 2 ].

[0041] <η min : Smallest required heat utilization ratio of the free heat of the furnace wall insulating gas to achieve energy savings of the system> η: Heat utilization ratio of the free heat of the furnace wall supply gas in the system; ηmin=1−(Qw0−Qw) / Qg, where Q g : free heat that the gas possesses in a state in which the furnace wall insulating gas reaches the furnace temperature at the supply position; Qg=Cp×G×(Ti−T0)[W / m2]; Ti: Temperature in the furnace at a position where the furnace wall heat insulating gas is supplied [°C]; T0: Reference temperature of 20 °C.

[0042] Simply put, the heat utilization ratio η of the generated free gas heat is determined by the temperature of the free heat ultimately released after the free heat of the gas supplied by the furnace wall has been utilized both inside and outside the furnace. If the temperature is simply reduced during this process by dilution with a cooling gas, the heat utilization ratio should be calculated by subtracting the reduced temperature fraction. In this context, the cooling gas refers to a cooling gas required for additional supply from the special opening into the interior of the furnace, without passing through the porous thermal insulation layer, to create a desired temperature curve, accompanied by the furnace wall thermal insulation gas supply.Therefore, the gas that was originally required to be supplied to the interior of the furnace, even if the furnace wall insulating gas is not supplied, does not correspond to the cooling gas as used herein. For example, in cases where cooling air is required for a predetermined heating curve, even if the furnace wall insulating gas is not supplied, when excess oxygen is required as atmosphere in the furnace, and when excess air is required for turbulence in the furnace, this oxygen and this air do not correspond to the cooling gas as used herein. Furthermore, it is assumed in the present invention that when a burner is used for heating, the minimum air-fuel ratio required for stable combustion is 1.05, and of the portion of air exceeding this ratio, a portion of the air, excluding a portion that must originally be supplied to the interior of the furnace, is considered to be the cooling gas.

[0043] In general, the heat utilization ratio of the generated free gas heat is calculated by the following equation: η=[1−∑jQbj / ∑jQai]×100[%], where: Qbj=Cp⋅Gbj⋅(Tbj−T0) / 3600; Qai=Cp⋅Gai⋅(Tai−T0) / 3600; ∑jGbj=∑iGai−∑kGck; Qb j : free gas heat [kW] after the expulsion of the furnace wall heat insulation gas at position j (in the case of heat utilization outside the furnace after heat utilization); Qa i : free gas heat [kW] immediately after the supply of the furnace wall insulating gas into the furnace at position i; Cp: ​​specific heat of gas of the furnace wall insulating gas [kJ / (Nm³) 3 ·K)] (for simplification Cp = 1.34, which is a constant value; Nm 3 refers to a volume (m³) 3 ) when converted to a reference state (0 °C, 1 atm); T0: Reference temperature [°C] (the reference temperature is a temperature of an external environment of the oven, but for the sake of simplicity is defined in the present invention as T0 = 20 °C); GB j : Exhaust gas flow rate of the furnace wall insulating gas at position j [Nm²] 3 / h]; Ga i : Flow rate of the furnace wall heat insulation gas at position i [Nm²] 3 / h]; Gc k : Flow rate of the cooling gas supplied to position k via the furnace wall thermal insulation gas supply [Nm³] 3 / h]; Tb j : Temperature of the furnace wall insulating gas at position j [°C]; Ta i : Temperature of the furnace wall heat insulating gas at position i [°C].

[0044] Here, if the heat utilization ratio η is only considered within the furnace, the free gas heat Qb is calculated. j after the gas is expelled, it is considered the free heat of the gas at the exhaust opening of the furnace.

[0045] If the furnace wall insulating gas is supplied in a temperature zone of 1400 °C, the heat inside the furnace is utilized without the supply of cooling air along its path, and the gas is expelled in a temperature zone of 500 °C, and then Ga1 = Gb1 and the heat utilization ratio η f1 Inside the oven is: ηf1=1−(500−20) / (1400−20)=65%.

[0046] In this case, the calorific value, which is 22% (65 - 43) of the generated free gas heat, represents the fuel reduction within the furnace. Furthermore, if the exhaust gas, with a furnace exhaust temperature of 500 °C and a heat utilization ratio of 50%, were utilized outside the furnace, then the final exhaust gas temperature after heat utilization outside the furnace would be 260 °C ((500 - 20) × 0.5 + 20). Therefore, the heat utilization ratio η t1 in the entire system: ηt1=1−(260−20) / (1400−20)=83%

[0047] In this case, the calorific value, which is 40% (83 - 43) of the generated free gas heat, is the energy saving effect of the entire system.

[0048] Furthermore, if, for example, in the example described above, excess heat remains in the furnace before exiting, and dilution and cooling are carried out with gas that has the same flow rate as the furnace wall gas supply flow rate in order to control the interior of the furnace to a predetermined temperature, then: Gb2=Ga+Gc2 =2⋅Ga2.

[0049] The heat utilization ratio η f2 Inside the oven is: ηf2=1−2×(500−20) / (1400−20)=30%.

[0050] Since in this case the smallest utilization ratio of the free gas heat (η minIf the heat utilization ratio is lower than 43%, the amount of fuel used in the furnace is increased. Furthermore, if heat is used outside the furnace under the same conditions as in the preceding example, then the heat utilization ratio η is... t2 of the entire system: ηt2=1−2×(260−20) / (1400−20)=65%.

[0051] In this case, the calorific value, which corresponds to 22% (65 - 43) of the generated free gas heat, is the energy saving effect of the entire system.

[0052] In a batch furnace, it is difficult to utilize the free heat of the gas within the furnace, and the heat is recovered outside the furnace. In particular, the exhaust gas, with an elevated temperature of 1000 °C or more, generally needs to be cooled to around 500 °C due to limitations regarding the heat resistance of the heat recovery system, such as ductwork like dampers and heat exchangers. This operating procedure reduces the efficiency of heat recovery. For example, if, compared to the continuous furnace example described above, the free heat of 1400 °C is simply diluted with air to produce a gas with a temperature of 500 °C in the batch furnace, and the heat is then utilized outside the furnace, the heat recovery efficiency η t3 of the entire system η t3= 50%, since this is the heat utilization ratio (50%) even outside the furnace. In this case, the calorific value, which corresponds to 7% (50 - 43) of the generated free gas heat, is the energy-saving effect of the entire system. Even in the case of a furnace where the heat treatment is carried out at a temperature of 1400 °C, the time during which the furnace temperature reaches 1400 °C is limited, and for most of the time it is 1400 °C or less. If the furnace temperature is lower, the smallest utilization ratio of the free gas heat η exceeds min according to the results of Fig. 4(b) 50%. Therefore, if the evaluation is carried out in a cycle of batch operation, the energy saving effect of the entire system cannot be expected and, in fact, energy consumption is often increased.

[0053] On the other hand, in the continuous furnace, the free gas heat generated in the high-temperature section of heating zone 12 can be utilized in the low-temperature section of heating zone 12. For example, if the furnace wall heat-insulating gas flowing from gas inlet 24a through gap 22 and then through the porous heat-insulating layer 23 into heating zone 12 can flow towards inlet 11, heat is exchanged between the gas and the workpiece as the gas flows through the furnace, thereby reducing the temperature of the gas and increasing the temperature of the workpiece. This allows for the effective utilization of the free gas heat in heating zone 12. After flowing through the furnace, the gas can be drawn in and expelled from one or more exhaust ports 26a located in heating zone 12.The flow of furnace wall insulating gas into the furnace can be regulated by adjusting the furnace pressure in the longitudinal direction by setting a supply / output quantity.

[0054] The position for installing the exhaust port 26a in the heating zone 12 can be determined in accordance with the heat curve. With regard to the effective utilization of the free gas heat within the furnace, for example, the entire furnace, preferably 50% or more, and more preferably 60% or more, of the free gas heat is used to heat the workpiece before the gas is exhausted. Furthermore, to provide a temperature at which the heat can be readily utilized outside the furnace, the temperature of the gas exhausted from the heating zone 12 can preferably be from 100 to 600 °C and even more preferably from 250 to 500 °C. Therefore, the exhaust port 26a of the heating zone is preferably located at a position where the gas in the furnace is within this temperature range. This enables the utilization of the heat with a heat recovery rate of 50% or more outside the furnace.Non-restrictive examples of heat utilization outside the furnace include, in addition to the direct use of elevated free heat to heat other workpieces, utilization by conversion into steam, hot water, hot air, or the like in heat recovery devices such as boilers or heat exchangers (water boilers, air preheaters, and the like). It should be noted that if no equipment or similar is available for use as a heat source, the utilization efficiency is reduced by 5 to 20%, but the heat can still be converted into electricity for use.

[0055] Furthermore, when the furnace wall insulating gas flows into the cooling zone 13 by successively exiting the gas inlet 24b through the gap 22 and the porous insulating layer 23, the workpiece is cooled by convection heat transfer through the gas. The temperature of the gas flowing into the furnace is increased by heat exchange between the gas and the workpiece as it flows through the furnace. The workpiece is also cooled by radiative heat transfer to the inner surface of the furnace wall. After flowing through the furnace, the gas can be drawn in and expelled from one or more exhaust ports 26b located in the cooling zone 13. The position for installing the exhaust port 26b in the cooling zone 13 can also be determined according to the thermal curve.To ensure easy heat utilization outside the furnace, the temperature of the exhaust gas from the cooling zone 13, like that of the exhaust gas from the heating zone 12, can preferably be from 100 to 600 °C and more preferably from 250 to 500 °C. Therefore, the exhaust gas opening 26b of the cooling zone 13 can preferably be located at a position where the gas in the furnace lies within such a temperature range.

[0056] According to the in Fig. The calculations shown in the 3 experiments also apply when the furnace wall heat insulating gas has a density of approximately 4.7 Nm³. 3 / (hr·m 2With regard to the use of the cooling zone, the temperature of the gas supplied from the surface of the inner surface of the furnace wall's thermal insulation layer into the furnace interior is only about 30 °C lower than the furnace interior temperature, and the temperature of the inner surface of the furnace wall's thermal insulation layer is only about 10 °C lower than the furnace interior temperature. In other words, the workpiece passing through this temperature zone (which in this case is being cooled) can undergo gentle cooling by convection heat transfer through the gas, which is heated to nearly the furnace interior temperature, and by radiative heat transfer to the inner surface of the furnace wall's thermal insulation layer, which has a slightly lower temperature than the furnace interior temperature.In general, so-called "cold cracking" can occur in the workpiece (which in this case is being cooled) due to a rapid cooling process, such as in a process where the workpiece is exposed to gases with a large local temperature difference. However, if the workpiece is cooled using the furnace wall thermal insulation assembly according to the present invention, the cooling process is gentler, which has the advantage that these problems can be easily avoided.

[0057] To improve the thermal efficiency of the entire heat utilization system, including heat utilization inside and outside the furnace, the gas flow rate supplied to the heating and cooling zones is ideally determined by considering a gas flow rate that allows for effective heat utilization inside or outside the furnace. As shown in the diagram of Fig.As can be seen in Figure 3, if the energy-saving effect generated by supplying the furnace wall insulating gas is to be achieved only within the furnace, then, by increasing the flow rate of the supplied gas, the generated free gas heat should be utilized to a greater extent. Even if the free gas heat is effectively used to heat the workpiece inside the furnace, excess free gas heat remains, although this depends on the heat curve. Therefore, from the perspective of the thermal efficiency of heat utilization outside the furnace, it is not desirable to generate a large amount of free gas heat. Compared to the cooling zone, the optimal gas flow rate of the furnace wall insulating gas supplied to the heating zone is, from this perspective, limited to a relatively low flow rate and depends on the furnace's technical specifications, such as the calorific value of the workpiece, the area of ​​the furnace wall, and the heat curve.For example, a suitable gas flow rate per unit area can be 1 to 3 Nm³. 3 / (Hm 2The dimensionless gas flow rate can suitably be in the range of 0.5 to 3, and preferably in the range of 1 to 2. If the flow rate is lower than the lower limit, the minimum utilization ratio of the free gas heat is lower, so heat utilization can still be achieved, but the quantitative effect is smaller in terms of energy. Conversely, if the flow rate is higher than the upper limit, the minimum utilization ratio of the free gas heat is higher, which is not practical. An optimal gas flow rate of the furnace wall insulating gas, which is supplied through the cooling zone, depends on the furnace's technical specifications, such as the calorific value of the workpiece, the area of ​​the furnace wall, and the thermal curve.Considering the purpose of cooling the workpiece, the optimal gas flow rate is generally greater than the optimal gas flow rate of the furnace wall heat insulation gas supplied to the heating zone, and, for example, an optimal gas flow rate is 3 to 6 Nm. 3 / (Hm 2 ) appropriate.

[0058] This makes it possible to achieve both a reduction in heat loss from the furnace wall and an improvement in the heat utilization ratio, thereby enabling energy savings for the entire system, including areas outside the furnace, by successfully utilizing the free heat of the furnace wall insulating gas, either inside or outside the furnace. Furthermore, it is also possible to achieve the energy-saving effect solely within the furnace by optimizing various conditions, such as the flow rate of the respective furnace wall insulating gas supplied according to the thermal curve of the heating or cooling zone, the position for introducing the furnace wall insulating gas, and the exhaust gas position. EXAMPLES

[0059] Examples of experimental calculations of the reduction effect of furnace wall heat emission and the energy saving effect according to the present invention are given below, but the present invention is not intended to be limited to these examples. <Beispiele 1-1 und 2-1, Vergleichsbeispiele 1 und 2>

[0060] Experimental calculations for the effects of the present invention were performed on the continuous furnace model as described in Fig. The process is shown in Figure 5 and Table 1. The furnace type is a gas-burning continuous furnace with internal dimensions of 90 m in length, 2.8 m in width, and 2.1 m in height. Fig.Figure 5 shows that the continuous furnace, from the furnace inlet to the furnace outlet, consisted of a low-temperature heating zone, a medium-temperature heating zone, a high-temperature heating zone, and a cooling zone. The furnace cycle time was 30 hours, and the internal temperature of the furnace corresponded to the temperature conditions in the table in the heat curve diagram. Fig. Figure 5 shows the maximum temperature of the heating zones, which was 1400 °C and a residence time of 4 hours. The heat capacity of the workpiece was defined as a heat capacity rate taking into account its processing rate and was 0.465 kW / K, the sum of the heat capacities of the product and the kiln fixtures. The furnace was divided into 30 sections along its length, and a thermal equilibrium calculation was performed for each 3-meter section based on the described calculation conditions. The furnace wall area per section was 29.4 m². 2It should be noted that, for the sake of simplicity, the specific heat of the gas in the furnace, which was used to calculate the thermal equilibrium, is a constant value of 1.34 kJ / Nm³. 3 regardless of temperature and composition. Furthermore, this experimental calculation was performed under the condition that one burner was installed per element. However, since each element has a length of 3 m, in practice a multiple burner is installed per element in a continuous furnace. Table 1 Experimental calculation conditions for continuous furnace model type Gas burner continuous oven fuel Class Town gas (13 A) Lower heating value 41,7 MJ / Nm 3 Combustion gas Specific heat (constant regardless of temperature and gas composition) 1.34 KJ / Nm 3 Oven dimensions Full length 90 m Number of subdivided elements in the direction of the oven length 30 Length of each element: 3 m Inside width 2.8 m Interior height 2,1 m Oven wall surface per element 29,4 m 2 / Element Heat curve Transit time 30 h (See Fig. 5) Maximum temperature 1400 °C Maximum temperature holding time 4 h Heat capacity of the workpiece (product and blast furnace tool) 0.465 kW / K Oven wall thermal insulation performance Oven wall outer surface temperature (e.g. in the 1400 °C zone) 130 °C (Normal) *1 Oven wall heat output amount (e.g. in the 1400 °C zone) 1245 W / m 2 Furnace wall gas supply conditions (see Fig. 3) Gas type Air Flow rate per unit area Heating zone 2,2 Nm 3 / (h · m 2 ) Cooling zone 4,7 Nm 3 / (h · m 2 ) flow rate per element Heating zone 64,7 Nm 3 / (h · Element) Cooling zone 138,2 Nm 3 / (h · Element) heat output ratio Heating zone < 700 °C 0,40 Heating zone > 700 °C 0,30 (vs. Normal) Cooling zone 0,15 burner combustion Low temperature zone Combustion gas flow 100 Nm 3 / (h · Element) Medium / High Temperature Zone air ratio 1,05 Note: Minimum airflow setting 20 Nm 3 / (h · Element) Performance of regenerative burners Exhaust gas rate (vs burner combustion gas flow rate) 90 % *2 Heat recovery rate (vs. burner combustion gas calorific value) 72 % Exhaust gas temperature (e.g. in the 1400 °C zone) 307 °C *1: The following equations were used to calculate the outer surface temperature and the amount of heat loss: - Temperature of the outer surface Te (°C) = (100 / 1370) × (internal temperature of the oven (°C) - 30) + 30; - Amount of heat released (W / m²) 2 ) = (0.5 × 5.67 × (((Te + 273) / 100) 4 - (303 / 100) 4 ) + 2.3 ×(Te - 30)( 5 / 4) . *2: The following equations were used to calculate the regenerative exhaust gas flow rate and exhaust gas temperature: - regenerative exhaust gas flow (Nm) 3 / h) = combustion gas flow rate (Nm³ / h) 3 / h) × exhaust rate (%) / 100; - Regenerative exhaust gas temperature (°C) = Internal temperature of the furnace (°C) × (1 - heat recovery rate / exhaust gas rate). *3: Radiant heat transfer in the oven

[0061] The amount of radiative heat transfer between an element inside the oven (i) and an element inside the oven (i+1) was calculated using: - Radiative heat transfer (i→i+1)(W / m2)=Ai×5.67×108×{(T(i+1)+273)4−(T(i)+273)4} where: T(i) and T(i+1) are each an internal temperature of the oven (°C); Ai (m 2) an apparent radiative heat transfer surface in the oven (which is 1 m² in the experimental calculation) 2 fraud).

[0062] For heat dissipation from the furnace wall under normal conditions without the addition of furnace wall insulating gas, a porous ceramic fiber insulation layer with relatively good thermal insulation properties was assumed. In the experimental calculation, the amount of heat dissipation was specified by the equation shown in "*1" of Table 1. For example, if the furnace internal temperature is 1400 °C, the furnace outer surface temperature of the porous insulation layer is 130 °C, and the furnace wall heat dissipation is 1245 W / m². 2 The furnace wall insulating gas supplied was air at a temperature of 20 °C, which was fed to both the heating and cooling zones. The optimal conditions for the gas flow rate differ between the heating and cooling zones, as described above. In the Fig. In the 3 experimental calculations shown, the supply flow rate per unit area was 2.2 Nm³. 3 / (Hm 2 ) or 4.7 Nm 3 / (Hm 2 The heat transfer ratios for the heating zone and cooling zone, when furnace wall insulating gas was supplied versus when no insulating gas was supplied, were 0.40, 0.30, and 0.15, respectively, for the heating zone with a temperature of less than 700 °C, the heating zone with a temperature of 700 °C or more, and the cooling zone. The heat transfer ratio was adjusted depending on the temperature zone within the heating zone, taking into account that the furnace wall thickness varied depending on the temperature zone and that the heat transfer ratio was slightly reduced even when the forced gas flow remained the same, provided the insulation thickness was the same.

[0063] The burner for the low-temperature heating zone introduces atmospheric air to safely remove volatile substances from the workpiece. The air supply flow rate was set to produce a combustion gas with a density of 100 Nm. 3 The test conditions were carried out for two cases: a conventional burner and a regenerative burner for the medium-temperature and high-temperature heating zones, respectively. The air-fuel ratio during combustion (when preheating was required) was approximately 1.05 for both the medium-temperature and high-temperature heating zones. The minimum air flow rate (20 Nm³) was... 3However, the / h) setting was adjusted to prevent damage from the combustion of metal parts, such as the burner nozzle, during combustion. Furthermore, when the heating zone did not require heating to achieve a target heat curve, but rather cooling, a required volume of air at a reference temperature (20 °C) was supplied from the burner to maintain a predetermined temperature. In the cooling zone, a required volume of air at a reference temperature was supplied from a cooling vent to maintain a predetermined temperature.

[0064] When the furnace wall thermal insulation air was supplied (example), 64.7 Nm were measured. 3 Air was supplied to the porous thermal insulation layer for each element in the heating zone at a rate of [per hour]. In the low-temperature heating zone, approximately 100 Nm [per centimeter] was added. 3To generate combustion gas for each element, including the furnace wall's thermal insulation air, an air flow corresponding to this difference is supplied by the burner. In the cooling zone, 138.2 Nm³ were generated. 3 / h of the porous thermal insulation layer for each element was supplied and in this state a required amount of air was also supplied from the cooling opening so that a predetermined temperature was maintained.

[0065] The exhaust gas was discharged from the same location in both cases where the furnace wall insulating air was not supplied (comparative example) and in cases where the furnace wall insulating air was supplied (example). More precisely, the exhaust gas was discharged from an exhaust vent in the low-temperature heating zone (furnace temperature: 296 °C); from an exhaust vent in the medium-temperature heating zone (furnace temperature: 448 °C); from an exhaust vent in the cooling zone (furnace temperature: 435 °C); and from a burner exhaust outlet during the use of the regenerative burner (the exhaust gas temperature ranged from approximately 100 to 300 °C depending on the burner position).

[0066] In the heating zone, when using a standard burner, direct exhaust gas discharge in this temperature zone results in significant heat loss due to the elevated exhaust temperature, and designing the exhaust port becomes complex. Therefore, direct exhaust gas discharge is generally avoided. Instead, the combustion gas generated in the high-temperature zone is routed through the low- and medium-temperature zones and discharged after heat exchange between the gas and the workpiece. In contrast, a regenerative burner alternates combustion and exhaust gas discharge, allowing the burner to recover waste heat. Even with an internal furnace temperature of 1000°C or higher, the exhaust gas temperature from the burner remains approximately 100 to 300°C due to heat exchange within the burner itself.Therefore, the use of a regenerative burner for the heating zone allows for direct exhaust gas discharge from the heating zone. The exhaust gas flow rate and exhaust gas temperature of the regenerative burner were calculated using the equation shown in “*2” of Table 1.

[0067] Under these conditions, the free heat of the workpiece, the heat dissipated from the furnace wall, the heat carried away by the exhaust gas, and the radiant heat transfer in the furnace were calculated for each element, and a required calorific value of the fuel was determined. The heat carried away by the exhaust gas was calculated from the amount of gas supplied to and expelled from each element and the amounts of combustion gas flowing into and out of an adjacent element. The radiant heat transfer in the furnace was calculated using the equation shown in “*3” of Table 1.

[0068] The fuel reduction effect according to the present invention was determined by performing a thermal equilibrium calculation for the conditions under which no furnace wall insulating gas was supplied (comparative example) and under which the furnace wall insulating gas was supplied (example), in two cases: using the normal burner and using the regenerative burner. The most efficient combustion, supply, and exhaust gas discharge conditions were determined for each of these cases, and the required calorific values ​​and the heat discharged by the exhaust gas were compared. The results are listed in Tables 2 and 3. In the continuous furnace model, the heat loss due to gas leakage from the inlet and outlet of the continuous furnace is not taken into account. However, even if, for example, 100 Nm 3 If gas flows out at 100 °C per hour, the heat loss is only about 3 kW, which is a negligible heating value.

[0069] Table 2 is explained below. Table 2 shows the results of the heat input and heat output, a heat utilization ratio of the furnace wall insulating gas inside the furnace, the heat of the exhaust gas from the flue outlet, the utilization of the heat of the exhaust gas outside the furnace, and a true calorific value of the entire system as a result of the thermal equilibrium. Table 2 further shows a reduction effect of the fuel calorific value inside the furnace, an effect of increasing the heat utilization of the exhaust gas outside the furnace, and an effect of reducing the true calorific value of the entire system as the effects of examples compared to comparative examples based on these results.

[0070] First, the heat input and heat output in kW units for the entire furnace and each temperature zone are shown as the thermal equilibrium results. The heat input is simply the fuel calorific value A, and the details of the heat output show the free heat of the workpiece, the heat output from the furnace wall, the heat carried away by the exhaust gas, and radiation loss. Some of the details of the heat output in each temperature zone could be heat inputs, but in this case, they are indicated with a minus sign for simplicity. For example, if the heat carried away by the exhaust gas is marked with a minus sign, this means that the heat from the exhaust gas has been added. It should be noted that the heat carried away by the exhaust gas in each temperature zone includes not only the heat released from the exhaust port but also any increase and decrease in the free heat of the gas associated with the flow of gas into and out of the adjacent zone.

[0071] The heat utilization ratio of the furnace wall insulating gas within the furnace represents the ratio for which the free heat of the gas was used as a heat source for the heating zone until the gas is expelled from the furnace to the outside, after the gas has been supplied to the furnace wall in the medium-temperature and high-temperature zones and the temperature has been raised to the internal temperature of the furnace within the furnace wall. More precisely, the heat utilization ratio was calculated using the equation shown in Table 2. When calculating the free heat of the furnace wall at the time the furnace wall insulating gas supplied in each element is expelled at the furnace exhaust port, it is necessary, if cooling air is supplied at the same time, to add the flow rate of the cooling air to the flow rate of the furnace exhaust gas.Since the flow rate of the furnace wall gas supply from the respective elements of the medium temperature zone and the high temperature zone was set to the same, the flow rate of the furnace exhaust gas was calculated such that, if one or more elements were present in which the cooling air was supplied, the gas flow rate of the cooling air was distributed evenly to all elements in which the furnace wall thermal insulation gas was supplied, including the element(s) in question and those that are closer to the furnace outlet side than the element(s), and the evenly distributed cooling gas flow rate was added to the supply flow rate of the furnace wall thermal insulation gas from each element.It should be noted that for the low-temperature zone and the cooling zone, the heat utilization ratio of the furnace wall thermal insulation gas within the furnace has been omitted, as it is not directly related to the fuel reduction or the energy saving effect of the entire furnace.

[0072] The table details the heat output carried by the exhaust gas in each temperature zone. It also shows the heat carried by the exhaust gas exiting the flue (including the burner exhaust) to indicate the heat dissipated outside the furnace in each temperature zone. It should be noted that if the dissipated heat is utilized by a heat recovery system outside the furnace, the heating value should be evaluated not only by the enthalpy of the total heating value but also by the exergy, which represents the effective energy. Therefore, these values ​​are also included in the table.

[0073] The utilization of waste heat outside the furnace (B) was defined as being capable of using 50% or more of the heat discharged by the furnace's exhaust gas for other processes. The table also shows a true calorific value (A - B) obtained by subtracting the utilization of waste heat (B) outside the furnace from the calorific value (A) of the fuel input within the furnace. 1. Case of using a normal burner (Example 1-1, Comparative Example 1)

[0074] The required calorific value when using the standard burner was 1251 kW for the example, whereas it was 1336 kW for the comparison example. The fuel reduction rate was 6%.

[0075] Comparing the details of the heat output for the entire furnace, it is first noted that in this example, the heat output from the furnace wall is significantly reduced due to the effect of the furnace wall heat insulation gas supply, while the heat carried away by the exhaust gas is increased.

[0076] A comparison was then carried out in the respective temperature zones. In the low-temperature zone, the fuel calorific value is reduced from 268 kW (comparison example) to 253 kW (example), although the difference between these values ​​is only 15 kW. The details of the heat input make it clear that this is due to the reduced proportion of heat dissipated from the furnace wall. The reason for this is that the air supplied from the furnace wall also contributes to purging volatile components from the workpiece. Therefore, in this example, the amount of air supplied by the burner is reduced accordingly, so that it does not increase the total amount of air supplied to the low-temperature zone. This is because the portion of air supplied from the furnace wall is instead preheated by heat exchange between this portion and the thermal insulation layer of the furnace wall.

[0077] In the medium-temperature zone, the calorific value is reduced from 129 kW (comparative example) to 11 kW (example), and the substantial reduction in calorific value is 118 kW. A closer look reveals that the reason for this is that the heat transfer from the furnace wall has been significantly reduced, and the heat transferred by the exhaust gas (a negative value for the heat transferred) has also been slightly increased. A more detailed analysis shows that the heat utilization ratio of the furnace wall insulating gas inside the furnace is only 26% in the medium-temperature zone, indicating that, as in Fig.As described in section 4, the supply of furnace wall insulating gas to the medium-temperature zone does not lead to any fuel reduction within the furnace. Furthermore, the fact that the heat input into the medium-temperature zone via the exhaust gas is greater than the heat output (a negative value for the heat output) and that the exhaust gas heat from the exhaust opening is significantly increased indicates that the heat input from the adjacent high-temperature zone is significantly increased, which is the true reason for the fuel reduction in the medium-temperature zone.

[0078] In the high-temperature zone, the fuel calorific value is increased from 938 kW (comparative example) to 987 kW (example), representing an increase of 49 kW. Looking at the details of the heat output, the reason for this is that the heat loss from the furnace wall has been significantly reduced, while the heat carried away by the exhaust gas has been further increased. This is a natural consequence of the principle of furnace wall heat-insulating gas supply, as described in Fig.3. Extracting the thermal equilibrium of only the high-temperature zone of the continuous furnace is essentially the same as considering the thermal equilibrium of the batch furnace, indicating that in the batch furnace, the furnace wall insulating gas supply method is difficult to reconcile with furnace energy savings. In the continuous furnace, the heat carried away by the exhaust gas in the high-temperature zone is transferred to the adjacent medium-temperature zone, and the heat is utilized in the medium-temperature zone, thus enabling energy savings for the entire furnace. The heat utilization ratio within the furnace of the furnace wall insulating gas in the high-temperature zone alone is 55%, indicating that heat recovery is only possible within the furnace itself.

[0079] Additionally, the average heat utilization ratio within the furnace of the furnace wall insulating gas in the medium temperature zone and the high temperature zone is 45%, which, with regard to the diagram in Fig. 4 close to the smallest utilization ratio of free gas heat η min However, such a fuel reduction effect was achieved because the combustion furnace, which provides a required calorific value through combustion, achieves a synergistic effect of reducing the combustion air when the required calorific value is reduced.

[0080] Since combustion was not carried out in the cooling zone, the heat carried away by the exhaust gas was simply increased by an amount corresponding to the reduction in heat loss from the furnace wall due to the supply of furnace wall insulating gas, and this increase amounted to 80 kW. For the sake of simplicity, the supply flow rate of the furnace wall insulating gas was kept constant throughout the entire cooling zone in this example of the experimental calculation. However, the heat loss from the furnace wall can be further reduced by optimizing the gas supply rate of the furnace wall insulating gas according to the cooling heat curve, thereby increasing the heat carried away by the exhaust gas.

[0081] The results are summed. The fuel reduction effect within the furnace was 6%, the effect of increasing the heat utilization of the exhaust gas outside the furnace was 46%, assuming that 50% of the heat carried away by the exhaust gas can be recovered in other steps, and based on these results, the actual reduction effect of the calorific value was 25%. A considerable energy saving effect was achieved. 2. Case of using a regenerative burner (Example 2-1, Comparative Example 2)

[0082] The required fuel calorific value when using the regenerative burner was 1081 kW for the example, whereas it was 1244 kW for the comparison example. The fuel reduction rate was 13%, which was more effective than when using the conventional burner. Comparing the details of the heat output for the entire furnace, it can also be seen that the heat loss from the furnace wall is significantly reduced in this example due to the effect of the furnace wall insulating gas supply, while the heat carried away by the exhaust gas is increased. Compared to the case where the conventional burner is used, the increased proportion of heat carried away by the exhaust gas is smaller, thus resulting in a greater overall fuel reduction rate for the furnace.

[0083] A comparison is then carried out in the respective temperature zones. For the low-temperature zone and the cooling zone, the experimental calculation conditions are the same as those in the case where the normal burner is used, and the results are also the same.

[0084] In the medium-temperature zone, the fuel heating value is reduced from 388 kW (comparative example) to 210 kW (example), resulting in a difference of 178 kW, which is significantly lower than when using the standard burner (118 kW). This is because, compared to the standard burner, the proportion of heat transferred via the exhaust gas (a negative value for heat carried away by the exhaust gas) is greater, even though the reduction in heat loss from the furnace wall remains the same. Consequently, the heat utilization ratio within the furnace of the furnace wall insulating gas increases from 26% to 44% for the medium-temperature zone, from 55% to 68% for the high-temperature zone, and from 45% to 60% on average for both.Furthermore, the main reason is that with the regenerative burner, 90% of the combustion gas produced by the burner combustion can be expelled by the burner itself. Therefore, the flow rate of the exhaust gas expelled from the exhaust port located at the beginning of the medium-temperature zone, after passing through the medium and high-temperature zones to the inlet side of the furnace, is significantly lower than with a conventional burner. In other words, under the conditions of a conventional burner, the combustion gas produced in the high-temperature zone flows into the medium-temperature zone and is used as a heat source for the medium-temperature zone even when the furnace wall insulating gas is not supplied. Consequently, the free heat newly generated within the furnace by the supply of the furnace wall insulating gas cannot be fully utilized and remains unused.As a result, the supply of simple cooling air was required to create a predetermined heat curve and to lower the exhaust gas temperature with the cooling air before it is expelled from the furnace. On the other hand, in the case of the regenerative burner, the heat utilization ratio within the furnace was improved because, since the gas flow from the high-temperature zone to the intermediate-temperature zone is inherently lower, the free heat generated by the supply of furnace wall insulating gas can be effectively and completely utilized in the intermediate-temperature zone. However, since Example 1-1 and Example 2-1 have the same calorific value generated by the supply of furnace wall insulating gas, in Example 2-1 the exhaust gas heat discharged outside the furnace is reduced, as more heat can be utilized within the furnace.

[0085] In the high-temperature zone, the fuel calorific value was increased from 587 kW (comparative example) to 619 kW (example), representing an increase of 32 kW. The extent of the increase was lower than under normal burner conditions, due to the fact that the fuel calorific value is initially lower because of the regenerative burner's effect.

[0086] The results are summed. The fuel reduction effect inside the furnace was 13%, the effect of increasing the heat utilization of the exhaust gas outside the furnace was 40%, assuming that 50% of the heat carried away by the exhaust gas can be recovered in other steps, and based on the results, the actual reduction effect of the calorific value was 30%, which was more efficient than in the case of using the normal burner. <Beispiele 1-2 bis 1-5, Beispiele 2-2 bis 2-5>

[0087] In Examples 1-1 and 2-1, the experimental calculations of the energy-saving effect according to the present invention were carried out in the continuous furnace under the conditions that the furnace wall heat-insulating gas was supplied to all of the low-temperature heating zone, the medium-temperature heating zone, the high-temperature heating zone, and the cooling zone. The furnace wall heat-insulating gas supply positions were selected in combination as specified in Table 3, and the thermal equilibrium was calculated. The other conditions were the same as those in Example 1-1 for the case of using the conventional burner and those in Example 2-1 for the case of using the regenerative burner. Based on the thermal equilibrium calculation, the fuel reduction effect according to the present invention was determined. The results are listed in Table 3. A similar trend was observed in both cases, where the normal burner was used and the regenerative burner was used.In both cases, the energy-saving effect was achieved, but the energy-saving effect of the entire system was achieved in the following order: in the case where the furnace wall heat-insulating gas was supplied to the medium-temperature zone and the cooling zone (Example 1-5, Example 2-5); in the case where the furnace wall heat-insulating gas was supplied to the high-temperature zone and the cooling zone (Example 1-2, Example 2-2); in the case where the furnace wall heat-insulating gas was supplied to the low-temperature zone, the high-temperature zone, and the cooling zone (Example 1-4, Example 2-4); in the case where the furnace wall heat-insulating gas was supplied to the medium-temperature zone, the high-temperature zone, and the cooling zone (Example 1-3, Example 2-3); and in the case where the furnace wall heat-insulating gas was supplied to the low-temperature heating zone, the medium-temperature heating zone, the high-temperature heating zone, and the cooling zone. (Example 1-1, Example 2-1), improved.

[0088] The results show that the energy saving effect can be achieved for each of the low-temperature heating zone, the medium-temperature heating zone and the high-temperature heating zone by supplying the furnace wall heat insulating gas, and the contribution of the effect is increased in the order of the low-temperature heating zone, the medium-temperature heating zone and the high-temperature heating zone. INDUSTRIAL APPLICATIONS

[0089] The continuous industrial furnace according to the present invention can be used effectively in industrial fields that utilize a continuous furnace at a high temperature of more than 1000 °C, such as the ceramics industry, the electronics parts manufacturing industry, the ceramics manufacturing industry, the glass manufacturing industry, the refractory materials manufacturing industry and the steel industry. REFERENCE MARK LIST 11 Admission 12 heating zones 13 Cooling zone 14 Outlet 15 Gas supply line for furnace wall insulating gas for the heating zone 16 Exhaust pipe for the heating zone 17 Gas supply line for furnace wall insulating gas for the cooling zone 18 Exhaust pipe for the cooling zone 21 Exterior wall 22 columns 23 porous thermal insulation layer 24a, 24b Gas inlet 25 blowers 26a, 26b Exhaust opening

Claims

[1] Continuous industrial furnace comprising: an inlet (11); a heating zone (12); a cooling zone (13); and an outlet (14) in that order, wherein the continuous industrial furnace is configured to subject a workpiece to heat treatment while the workpiece is conveyed from the inlet (11) to the outlet (14), wherein at least a part of the cooling zone (13) has a furnace wall thermal insulation assembly comprising: an outer wall (21) having one or more gas inlet openings (24b); and a porous thermal insulation layer (23) arranged with a gap (22) on the inside of the outer wall (21); and wherein the cooling zone (13) further comprises one or more exhaust openings (26b) for drawing in and expelling a gas after the gas has flowed from the gas inlet openings (24b) through the gap (22) and the porous thermal insulation layer (23) in that order into the cooling zone (13) and has subsequently been used to cool the workpiece. [2] Continuous industrial furnace according to claim 1, wherein the gas emitted from the exhaust openings (26b) has a temperature of 100 to 600 °C. [3] Method for utilizing the heat of a continuous industrial furnace according to one of claims 1 or 2, comprising: Supply of gas through the gas inlet openings (24b), wherein the gas successively passes through the gap (22) and the porous thermal insulation layer (23) and subsequently flows into the cooling zone (13) of the furnace, whereby heat is exchanged between the gas and the porous thermal insulation layer (23) as the gas passes through the porous thermal insulation layer (23), thereby reducing the heat transfer from the porous thermal insulation layer (23) to the outside of the furnace and reducing the temperature of a surface of the porous thermal insulation layer (23) on the inside of the furnace; Cooling of the workpiece by convection heat transfer through the gas flowing into the furnace and by radiative heat transfer to an inner surface of the furnace wall, and increasing the temperature of the gas flowing into the furnace by heat exchange between the gas and the workpiece, while allowing the gas to flow through the furnace; Intake and expulsion of the gas after the gas flowing into the furnace has been used to cool the workpiece; and Utilizing the free heat of the drawn-in and expelled gas outside the furnace. [4] Continuous industrial furnace comprising: an inlet (11); a heating zone (12); a cooling zone (13); and an outlet (14) in that order, wherein the continuous industrial furnace is configured to subject a workpiece to heat treatment while the workpiece is conveyed from the inlet (11) to the outlet (14), wherein at least a part of the heating zone (12) has a furnace wall thermal insulation assembly comprising: an outer wall (21) having one or more gas inlet openings (24a); and a porous thermal insulation layer (23) arranged with a gap (22) on the inside of the outer wall (21); and wherein the heating zone (12) further comprises one or more exhaust gas openings (26a) for drawing in and expelling a gas, after the gas flows from the gas inlet openings (24a) through the gap (22) and the porous thermal insulation layer (23) in this order into the heating zone (12) and subsequently flows towards the inlet side, wherein at least a part of the cooling zone (13) has a furnace wall thermal insulation assembly comprising: an outer wall (21) having one or more gas inlet openings (24b); and a porous thermal insulation layer (23) arranged with a gap (22) on the inside of the outer wall (21); and wherein the cooling zone (13) further comprises one or more exhaust openings (26b) for drawing in and expelling the gas after the gas has flowed from the gas inlet openings (24b) through the gap (22) and the porous thermal insulation layer (23) in that order into the cooling zone (13) and has subsequently been used to cool the workpiece. [5] Continuous industrial furnace according to claim 4, wherein the gas emitted from each exhaust gas opening (26a, 26b) of the heating zone (12) and the cooling zone (13) has a temperature of 100 to 600 °C. [6] Continuous industrial furnace according to claim 4 or 5, wherein the furnace has a part in which the internal temperature of the furnace in the heating zone (12), into which gas flows through the porous thermal insulation layer (23), is 1000 °C or more. [7] Continuous industrial furnace according to one of claims 4 to 6, wherein the gas flowing into the heating zone (12) of the furnace comprises a furnace atmosphere setting gas. [8] Method for utilizing the heat of a continuous industrial furnace according to any one of claims 4 to 7, comprising: Supplying gas from the gas inlet openings (24a) of the heating zone (12), wherein the gas successively passes through the gap (22) and the porous thermal insulation layer (23) in the heating zone (12) and subsequently flows into the heating zone (12) of the furnace, whereby heat is exchanged between the gas and the porous thermal insulation layer (23) in the heating zone (12) as the gas passes through the porous thermal insulation layer (23) in the heating zone (12), thereby increasing the temperature of the gas and reducing the heat loss from the porous thermal insulation layer (23) in the heating zone (12) to the outside of the furnace; Allowing the gas flowing into the heating zone (12) of the furnace to flow towards the inlet side, whereby heat is exchanged between the gas and the workpiece as the gas flows through the furnace towards the inlet side, thereby reducing the temperature of the gas and increasing the temperature of the workpiece; Intake and exhaust of the gas after the flow of gas into the heating zone (12) of the furnace has been permitted towards the inlet side; and Utilizing the free heat of the gas drawn in and expelled from the heating zone (12) outside the furnace, Supplying gas from the gas inlet openings (24b) of the cooling zone (13), wherein the gas successively passes through the gap (22) and the porous thermal insulation layer (23) in the cooling zone (13) and subsequently flows into the cooling zone (13) of the furnace, whereby heat is exchanged between the gas and the porous thermal insulation layer (23) in the cooling zone (13) as the gas passes through the porous thermal insulation layer (23) at the cooling zone (13), thereby reducing the heat transfer from the porous thermal insulation layer (23) in the cooling zone (13) to the outside of the furnace and reducing the temperature of a surface of the porous thermal insulation layer (23) on the inside of the furnace at the cooling zone (13); Cooling of the workpiece by convection heat transfer through the gas flowing into the cooling zone (13) of the furnace and by radiative heat transfer to an inner surface of the furnace wall and increasing the temperature of the gas flowing into the cooling zone (13) of the furnace by heat exchange between the gas and the workpiece, while allowing the gas to flow through the furnace; Intake and exhaust of the gas after the gas flowing into the cooling zone (13) of the furnace has been used to cool the workpiece; and Utilizing the free heat of the gas drawn in and expelled from the cooling zone (13) outside the furnace. [9] Method according to claim 8, wherein the gas is expelled outside the furnace after the gas has flowed through the porous thermal insulation layer (23) into the furnace at a position where the internal temperature of the furnace in the heating zone is 400 °C or more and an average of 40% or more of the free heat of the gas inside the furnace has been utilized.

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