Porous glass body heating device

Abstract

To provide a device for heating a porous glass body, capable of appropriately heating the porous glass body.SOLUTION: A device 1 for heating a porous glass body having an internal space 31S capable of storing a porous glass body 20, an air inlet S1 communicated to the internal space 31S and an air outlet E1 communicated to the internal space 31S above from the air inlet S1 includes: a furnace core tube 31 extended in a vertical direction; a heat insulation part 40 surrounding an outer peripheral surface of the furnace core tube 31 between the air inlet S1 and the air outlet E1; a heater 35 surrounded by the heat insulation part 40 between the furnace core tube 31 and the heat insulation part 40 and heating the porous glass body 20; and a gas supply part 65 for supplying a gas to the internal space 31S from the air inlet S1. A quantity of heat emitted from the outer peripheral surface of the heat insulation parts 40 on an upper side from the center of the heater 35 in the state of heating the porous glass body 20 by the heater 35 is equal to or less than that emitted from the outer peripheral surface on a lower side from the center of the heater 35.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heating device for a porous glass body. [Background technology]

[0002] Known methods for producing optical fiber preforms used in the manufacture of optical fibers include depositing glass particles using the OVD (Outside Vapor Deposition) method, the VAD (Vapor Phase Axial Deposition) method, or the like to form a porous glass body, and then heating the porous glass body to dehydrate and sinter it.

[0003] Patent Document 1 below discloses a heating device for a porous glass body that heats the porous glass body to perform dehydration or sintering. This heating device includes a furnace tube that extends vertically and can accommodate the porous glass body, a heater that is arranged outside the furnace tube and heats the porous glass body, and a furnace body that surrounds part of the outer circumferential surface of the furnace tube and the heater. The heating device also includes an upper insulation that surrounds the outer circumferential surface of the furnace tube above the furnace body, and a lower insulation that surrounds the outer circumferential surface of the furnace tube below the furnace body. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-339024 Summary of the Invention [Problem to be solved by the invention]

[0005] In the heating apparatus described above, the furnace body, upper insulation, and lower insulation surround the outer periphery of the muffle tube, and these components suppress heat radiation from the muffle tube. In addition, in this heating apparatus, dehydration gas is supplied into the muffle tube through an air inlet at the bottom of the muffle tube and exhausted through an opening at the top. Therefore, gas flows from bottom to top within the muffle tube. This gas flow transports heat from the heater from bottom to top, and the temperature inside the muffle tube tends to be higher above the heater than below. However, this heating apparatus does not take such heat distribution into consideration, and the above-mentioned sections that suppress heat radiation from the muffle tube are generally symmetrical above and below the center of the heater. Therefore, the amount of heat radiated from the outer periphery above the center of the heater is greater than the amount of heat radiated from the outer periphery below the center of the heater. In other words, a larger amount of heat is radiated from the upper section of the muffle tube, where the temperature is higher. Therefore, there is room for improvement in the member for suppressing heat radiation from the furnace tube in order to heat the porous glass body appropriately.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a heating device for a porous glass body that can appropriately heat the porous glass body. [Means for solving the problem]

[0007] In order to achieve the above object, the heating device for a porous glass body of the present invention comprises: a furnace tube extending in a vertical direction and having an internal space capable of accommodating a porous glass body, an air inlet communicating with the internal space, and an exhaust port above the air inlet communicating with the space between the accommodating portions; a heat insulating section surrounding the outer circumferential surface of the furnace tube between the air inlet and the exhaust port; a heater surrounded by the heat insulating section between the furnace tube and the heat insulating section and for heating the porous glass body; and a gas supply section for supplying gas from the air inlet to the internal space, wherein, when the porous glass body is heated by the heater, the amount of heat emitted from the outer circumferential surface of the heat insulating section above the center of the heater is equal to or less than the amount of heat emitted from the outer circumferential surface of the heater below the center.

[0008] In this heating device for porous glass bodies, gas is supplied to the internal space through the air inlet, as described above. This causes gas to flow from the bottom to the top between the air inlet and the exhaust port located above the air inlet. The outer circumferential surface of the muffle tube where this gas flow occurs is surrounded by an insulating section. Therefore, in the area of ​​the internal space surrounded by the insulating section, the upper side relative to the heater is hotter than the lower side. However, as described above, the amount of heat released from the outer circumferential surface of the insulating section above the center of the heater is less than the amount of heat released from the outer circumferential surface below. Therefore, even if the amount of heat released from the outer circumferential surface of the muffle tube is the same as that of the heating device in Patent Document 1, the amount of heat released from the outer circumferential surface of the muffle tube above the heater can be reduced compared to the heating device in Patent Document 1, thereby widening the high-temperature region where the temperature exceeds a predetermined temperature. Therefore, this heating device for porous glass bodies can appropriately heat the porous glass body.

[0009] The insulation section may comprise a central insulation section surrounding the heater, an upper insulation section extending from the upper end of the central insulation section along the core tube and surrounding the outer peripheral surface of the core tube, and a lower insulation section extending from the lower end of the central insulation section along the core tube and surrounding the outer peripheral surface of the core tube, and the thickness of the upper insulation section in the radial direction of the core tube may be greater than the thickness of the lower insulation section.

[0010] With this configuration, even if the thermal conductivity of the upper heat insulating portion is the same as that of the lower heat insulating portion, the high temperature region can be easily widened.

[0011] In this case, the upper insulation section and the lower insulation section are cylindrical, the outer diameter and inner diameter of each of the upper insulation section and the lower insulation section are constant in the extension direction, the distance from the center of the heater to the lower end of the upper insulation section is the same as the distance from the center of the heater to the upper end of the lower insulation section, the length of the upper insulation section is the same as the length of the lower insulation section, the thermal conductivity of the upper insulation section is the same as the thermal conductivity of the lower insulation section, and the ratio of the volume of the lower insulation section to the sum of the volumes of the upper insulation section and the lower insulation section may be greater than 0.36 and not more than 0.49.

[0012] The inventors have found that by setting the ratio within the above range, it is possible to reduce the total amount of heat released from the upper and lower heat insulating sections under typical conditions for heating a porous glass body that will become a part of an optical fiber preform, while preventing the device from becoming too large. Therefore, with this heating device for a porous glass body, it is possible to reduce the amount of heat released from the furnace tube, while preventing the device from becoming too large.

[0013] Alternatively, the thickness of the upper heat insulation section on the side of the central heat insulation section may be greater than the thickness of the upper heat insulation section on the side opposite to the central heat insulation section.

[0014] In the region of the furnace tube surrounded by the upper insulation, the temperature on the central insulation side is higher than the temperature on the opposite side of the central insulation side, and this configuration makes it easier to widen the high-temperature region compared to when the thickness of the upper insulation on the central insulation side is smaller than the thickness of the opposite side of the central insulation side. [Effects of the Invention]

[0015] As described above, the present invention provides a heating device for a porous glass body that can appropriately heat a porous glass body. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a diagram schematically illustrating a heating device for a porous glass body according to an embodiment of the present invention. [Figure 2] 10 is a graph showing an example of the relationship between the temperature of the inner space of the furnace tube and the position along the extension direction of the furnace tube. [Figure 3] 10 is a graph showing the amount of heat released from the outer peripheral surface of the heat insulating portion. [Figure 4] 10 is a graph showing the measurement results. DETAILED DESCRIPTION OF THE INVENTION

[0017] The heating device for a porous glass body according to the present invention will be exemplified below with reference to the accompanying drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved without departing from the spirit and scope of the present invention. In the drawings referred to below, the dimensions of each component may be changed to facilitate understanding.

[0018] Fig. 1 is a schematic diagram showing a heating apparatus for a porous glass body according to an embodiment of the present invention. As shown in Fig. 1, the heating apparatus 1 for a porous glass body according to this embodiment mainly comprises a heating furnace 30, an elevating unit 60, and a gas supply unit 65, and is configured to heat a porous glass body 20 to dehydrate and sinter the porous glass body 20. The porous glass body 20 shown in Fig. 1 is a porous glass body that will become a core glass rod for forming the core of an optical fiber. However, the porous glass body is not particularly limited, and may be, for example, a cladding glass body for forming a cladding surrounding the core.

[0019] The heating furnace 30 of this embodiment mainly comprises a furnace tube 31, a heat insulating section 40, and a heater .

[0020] The muffle tube 31 is a cylindrical member extending in the vertical direction, and can accommodate the porous glass body 20 in its internal space 31S. The inner and outer diameters of the muffle tube 31 are generally constant in the extension direction. The lower opening of the muffle tube 31 is closed by a lower lid 32, and the upper opening is closed by an upper lid 33. In this embodiment, the muffle tube 31 and the lower lid 32 are integrally formed, but the muffle tube 31 and the lower lid 32 may also be formed separately. A through-hole is formed in the upper lid 33, through which a support rod 22 for suspending the porous glass body 20 is inserted. A connecting portion 23 is provided at the lower end of the supporting rod 22, and a glass rod 24 on which the porous glass body 20 is deposited is connected to this connecting portion 23. The furnace core tube 31 is also formed with an air inlet S1 that communicates with the internal space 31S, and an exhaust port E1 that is located above the air inlet S1 and communicates with the internal space 31S. In this embodiment, the air inlet S1 is formed near the lower end of the furnace core tube 31, and the exhaust port E1 is formed near the upper end of the furnace core tube 31. Examples of materials that form the furnace core tube 31, the lower lid 32, and the upper lid 33 include quartz and carbon.

[0021] The heat insulating section 40 is configured to surround the outer periphery of the furnace core tube 31 between the air inlet S1 and the exhaust outlet E1, and the heater 35 is surrounded by the heat insulating section 40 between the furnace core tube 31 and the heat insulating section 40.

[0022] The heat insulating section 40 of this embodiment is made up of a central heat insulating section 41 , a lower heat insulating section 51 , and an upper heat insulating section 52 .

[0023] The central insulation section 41 of this embodiment is composed of a housing 42 and an internal insulation section 43, and has a generally vertically symmetrical configuration. The housing 42 of this embodiment is formed into a generally vertically symmetrical hollow box shape, with a cylindrical peripheral wall 42a extending in the vertical direction, an upper wall 42b closing the upper opening of the peripheral wall 42a, and a lower wall 42c closing the lower opening of the peripheral wall 42a. The housing 42 of this embodiment also has a cooling function. Specifically, a flow path (not shown) is formed inside each of the peripheral wall 42a, the upper wall 42b, and the lower wall 42c, and cooling water supplied from a cooling water supply unit (not shown) flows through this flow path. The flow of cooling water through the flow path cools the housing 42, thereby suppressing damage to the housing 42 due to heat. Note that the housing 42 does not necessarily have to have a cooling function. A through-hole penetrating the upper and lower walls 42b and 42c is formed in the center of each wall, and the furnace tube 31 is inserted into this through-hole. The upper and lower ends of the furnace core tube 31 protrude from the housing 42, and the housing 42 surrounds the center of the furnace core tube 31 in the vertical direction, forming a space surrounded by the furnace core tube 31 and the housing 42.

[0024] The internal insulation section 43 is made of a heat insulating material and is formed in a cylindrical shape surrounding the muffle tube 31 in the space surrounded by the muffle tube 31 and the housing 42. In this embodiment, an annular recess 44 is formed on the inner circumferential surface of the internal insulation section 43. The center of the recess 44 in the vertical direction generally coincides with the center of the central insulation section 41 in the vertical direction, and the shape of the internal insulation section 43 is generally symmetrical about the center of the recess 44. Therefore, the central insulation section 41 in this embodiment is generally symmetrical about the recess 44. The outer circumferential surface of the internal insulation section 43 is in contact with the peripheral wall 42a, and the portion of the inner circumferential surface other than the recess 44 is in contact with the muffle tube 31. The upper end surface is in contact with the upper wall 42b, and the lower end surface is in contact with the lower wall 42c. Note that the internal insulation section 43 and the housing 42 may be separated from each other, and the internal insulation section 43 and the muffle tube 31 may also be separated from each other. In this case, the gap between these members is preferably, for example, 2 mm or less. The recess 44 may not be formed on the inner peripheral surface of the internal heat insulating portion 43. The thermal conductivity of the internal heat insulating portion 43 is constant in the length direction and the radial direction. Examples of materials that form the internal heat insulating portion 43 include carbon.

[0025] The heater 35 in this embodiment is formed in a ring shape and is disposed in the recess 44 of the internal insulation 43 to surround the muffle tube 31. The vertical center of the heater 35 coincides with the vertical center of the recess 44. Therefore, the central insulation 41 is configured symmetrically about the center of the heater 35. Note that FIG. 1 shows a dashed line DL extending horizontally through the vertical center of the heater 35. The heat generated by the heater 35 heats the muffle tube 31, thereby heating the porous glass body 20 contained in the internal space 31S of the muffle tube 31. The heater 35 is located between the muffle tube 31 and the central insulation 41 of the insulation unit 40 and is surrounded by the internal insulation 43 of the central insulation 41. Therefore, the internal insulation 43 allows the heat of the heater 35 to be effectively transferred to the muffle tube 31. Furthermore, heat release from the portion of the muffle tube 31 surrounded by the internal insulation 43 is suppressed. The heater 35 may be divided into multiple heating sections, which may be discontinuously arranged in the recess 44 so as to surround the muffle tube 31. If the internal insulation section 43 does not have a recess 44, the heater 35 may be arranged between the outer circumferential surface of the muffle tube 31 and the internal insulation section 43. The heater 35 may be, for example, a heater made of carbon.

[0026] The lower insulation 51 is made of a thermal insulating material and is formed into a cylindrical shape that extends from the lower end of the central insulation 41 along the muffle tube 31 and surrounds the outer circumferential surface of the muffle tube 31. This suppresses heat radiation from the portion of the muffle tube 31 surrounded by the lower insulation 51. In this embodiment, the lower insulation 51 is formed into a cylindrical shape with constant outer and inner diameters in the longitudinal direction. The inner circumferential surface of the lower insulation 51 contacts the muffle tube 31, and the upper end contacts the upper end of the central insulation 41. The lower insulation 51 and the muffle tube 31 may be spaced apart. In this case, the gap between the lower insulation 51 and the muffle tube 31 is preferably, for example, 2 mm or less. In this embodiment, the lower end of the lower insulation 51 is located above the lower end of the muffle tube 31. The thermal conductivity of the lower insulation 51 is constant in the longitudinal and radial directions. The lower heat insulating portion 51 may be made of, for example, porous ceramic.

[0027] The upper insulation section 52 is made of a thermal insulating material and has a cylindrical shape that extends from the upper end of the central insulation section 41 along the muffle tube 31 and surrounds the outer periphery of the muffle tube 31. This suppresses heat release from the portion of the muffle tube 31 surrounded by the upper insulation section 52. In this embodiment, the upper insulation section 52 has a cylindrical shape that is larger than the shape of the lower insulation section 51 only in outer diameter. Therefore, the outer and inner diameters of the upper insulation section 52 are constant in the length direction, the inner diameter of the upper insulation section 52 is the same as the inner diameter of the lower insulation section 51, the length of the upper insulation section 52 is the same as the length of the lower insulation section 51, and the radial thickness of the upper insulation section 52 is greater than the radial thickness of the lower insulation section 51. Furthermore, the thermal conductivity of the upper insulation section 52 is constant in the length direction and radial direction and is the same as the thermal conductivity of the lower insulation section 51. The inner circumferential surface of the upper insulation 52 contacts the muffle tube 31, and its upper end contacts the upper end of the central insulation 41. The upper insulation 52 and the muffle tube 31 may be spaced apart. In this case, the gap between the upper insulation 52 and the muffle tube 31 is preferably, for example, 2 mm or less. In this embodiment, the distance from the center of the heater 35 to the lower end of the upper insulation 52 is the same as the distance from the center of the heater 35 to the upper end of the lower insulation 51. This distance is measured in the vertical direction. The upper end of the upper insulation 52 is located below the upper end of the muffle tube 31. The lower insulation 51 may be made of a material such as porous ceramic.

[0028] The center of the insulating section 40 of this embodiment, which consists of the central insulating section 41, the lower insulating section 51, and the upper insulating section 52, in the vertical direction is the center of the recessed section 44 in the vertical direction, and the center of the insulating section 40 roughly coincides with the center of the heater 35.

[0029] The lifting unit 60 is configured to lift and lower the supporting rod 22 that it holds, thereby moving the porous glass body 20 up and down. The configuration of the lifting unit 60 is not particularly limited.

[0030] The gas supply unit 65 supplies gas containing a chlorine-based gas to the internal space 31S via a pipe 66 connected to the gas inlet S1. The gas supplied to the internal space 31S is exhausted from the exhaust port E1 to an exhaust pipe 67. The vertical distance between the gas inlet S1 and the exhaust port E1 is greater than the vertical length of the porous glass body 20. In this embodiment, the gas is a mixed gas of a chlorine-based gas and an inert gas. Examples of the chlorine-based gas include chlorine, SiCl4, and thionyl chloride (SOCl2), and examples of the inert gas include He, Ne, Ar, and N2.

[0031] Next, a method for heating the porous glass body 20 using the heating device 1 for a porous glass body will be described.

[0032] First, a porous glass body 20 is prepared. The porous glass body 20 can be formed by a soot method such as an OVD method or a VAD method. In this embodiment, the porous glass body 20 is obtained by depositing glass particles from one end of a prepared glass rod 24 along the axial direction of the glass rod 24 using the VAD method.

[0033] Next, the porous glass body 20 is heated to perform a dehydration process. The obtained porous glass body 20 is suspended from a support rod 22 and placed in the internal space 31S of the furnace tube 31. The gas supply unit 65 supplies gas to the internal space 31S through the gas inlet S1 to fill the internal space 31S, and the gas in the internal space 31S is exhausted through the exhaust outlet E1 to the exhaust pipe 67. Therefore, a gas flow from below to above is generated between the gas inlet S1 and the exhaust outlet E1 in the internal space 31S.

[0034] With the gas being supplied in this manner, the heater 35 is activated to heat the muffle tube 31. FIG. 2 is a graph showing an example of the relationship between the temperature of the internal space 31S of the muffle tube 31 and the position along the extension direction of the muffle tube 31, and is a graph showing the relationship between temperature and position when dehydrating the porous glass body 20. The vertical axis of the graph in FIG. 2 represents the position in the extension direction based on the upper end of the muffle tube 31. Position p1 is the upper end of the upper insulation section 52, position p2 is the lower end of the upper insulation section 52, position p3 is the center of the heater 35, position p4 is the upper end of the lower insulation section 51, and position p5 is the lower end of the lower insulation section 51. As shown in FIG. 2, the temperature of the internal space 31S is highest near the center of the heater 35 and decreases with increasing distance from the heater 35. The temperature of the internal space 31S is higher on the upper side than on the lower side based on the heater 35. Such uneven distribution of heat is caused by the flow of gas supplied from the gas supply unit 65.

[0035] While the furnace tube 31 is heated in this manner, the lifting unit 60 moves the porous glass body 20 downward at a predetermined speed so that the entire porous glass body 20 crosses the heater 35. This causes the porous glass body 20 to be heated by the heater 35. This heating removes OH groups and moisture adhering to the surface of the porous glass body 20 by the chlorine-based gas contained in the gas from the gas supply unit 65. The porous glass body 20 may be moved back and forth in the vertical direction so that it crosses the heater 35 multiple times. The heating temperature may be any temperature lower than the sintering temperature of the porous glass body 20 and capable of removing moisture from the porous glass body 20, and is preferably, for example, 1000°C to 1200°C. This heating temperature is the temperature in the vicinity of the heater 35 in the internal space 31S. By setting the heating temperature to 1000°C or higher, the time required to remove moisture from the porous glass body 20 can be shortened, and by setting the heating temperature to 1200°C or lower, softening of the porous glass body 20 can be sufficiently suppressed.

[0036] Next, the porous glass body 20 is heated for sintering. In this embodiment, this is performed consecutively with the dehydration treatment. The lifting unit 60 positions the porous glass body 20 above the heater 35. The heater 35 is activated so that the temperature in the internal space 31S near the heater 35 reaches a temperature at which the porous glass body 20 is sintered into transparent glass, for example, 1300°C to 1600°C. During this process, gas is supplied to the internal space 31S by the gas supply unit 65, as in the dehydration treatment. The temperature of the internal space 31S is highest near the center of the heater 35 and decreases with increasing distance from the heater 35, as in the dehydration treatment, with the upper side relative to the heater 35 being hotter than the lower side. In this state, the lifting unit 60 moves the porous glass body 20 downward at a predetermined speed so that the entire porous glass body 20 crosses the heater 35. As a result, the porous glass body 20 is heated by the heater 35 and sintered. The porous glass body 20 may be moved back and forth in the vertical direction so that it crosses the heater 35 multiple times. In this way, the porous glass body 20 is vitrified into a transparent glass and becomes a core glass rod for forming the core of an optical fiber.

[0037] From the core glass rod thus obtained, an optical fiber preform for forming an optical fiber can be produced. For example, glass particles are deposited on the obtained core glass rod by the OVD method to form a porous glass body that will serve as a cladding glass body for forming the cladding of the optical fiber. Next, similar to the dehydration and sintering processes described above, the porous glass body is heated to undergo dehydration and sintering treatments. The core glass rod remains almost unchanged by this heating and becomes the core glass body that will serve as the core of the optical fiber, while the porous glass body is vitrified into a transparent glass to become the cladding glass body that will serve as the cladding of the optical fiber. This results in an optical fiber preform having a core glass body and the outer peripheral surface of the core glass body surrounded by a cladding glass body. By heating and drawing this optical fiber preform, an optical fiber can be obtained in which the outer peripheral surface of the core is surrounded by a cladding.

[0038] As described above, the furnace tube 31 in the heating apparatus 1 for a porous glass body is surrounded by the insulating section 40. Therefore, when the heater 35 is used to heat the porous glass body 20 and perform the dehydration and sintering processes, heat is released from the outer peripheral surface of the portion of the furnace tube 31 surrounded by the insulating section 40 through the insulating section 40. The outer peripheral surface of the insulating section 40 above the center of the heater 35 consists of the entire outer peripheral surface of the central insulating section 41 above the center of the heater 35 and the entire outer peripheral surface of the upper insulating section 52. On the other hand, the outer peripheral surface of the insulating section 40 below the center of the heater 35 consists of the entire outer peripheral surface of the central insulating section 41 below the center of the heater 35 and the entire outer peripheral surface of the lower insulating section 51. In this embodiment, the outer diameter of the upper insulating section 52 is made larger than that of the lower insulating section 51, thereby making the insulating performance of the upper insulating section 52 higher than that of the lower insulating section 51. The amount of heat radiated from the outer peripheral surface of the insulation unit 40 above the center of the heater 35 is set to be equal to or less than the amount of heat radiated from the outer peripheral surface below the center of the heater 35. In this embodiment, the central insulation unit 41 of the insulation unit 40 is configured to be approximately vertically symmetrical with respect to the center of the heater 35. The area of ​​the outer peripheral surface of the muffle tube 31 surrounded by the upper insulation unit 52 and the area of ​​the outer peripheral surface of the muffle tube 31 surrounded by the lower insulation unit 51 are also approximately vertically symmetrical with respect to the center of the heater 35. However, as shown in FIG. 2 , the area above the heater 35 in the internal space 31S of the muffle tube 31 is hotter than the area below. Therefore, taking this heat bias into consideration, the vertical lengths and outer diameters of the central insulation unit 41, lower insulation unit 51, and upper insulation unit 52 are adjusted so that the amount of heat radiated from the outer peripheral surface of the insulation unit 40 is as described above.

[0039] In this embodiment, the heat insulating unit 40 includes a housing 42 having a cooling function. When the heat insulating unit 40 includes a member having a cooling function in this manner, the amount of heat radiated from the outer peripheral surface of the heat insulating unit 40 above the center of the heater 35 includes the amount of heat radiated from the outer peripheral surface and the amount of heat radiated by the cooling function from a portion of the member having the cooling function above the center of the heater 35. Therefore, in this embodiment, the amount of heat radiated from the outer peripheral surface of the heat insulating unit 40 above the center of the heater 35 includes the amount of heat radiated by the cooling function from a portion of the housing 42 above the center of the heater 35. Furthermore, the amount of heat radiated from the outer peripheral surface of the heat insulating unit 40 below the center of the heater 35 includes the amount of heat radiated by the cooling function from a portion of the housing 42 below the center of the heater 35, as well as the amount of heat radiated from the outer peripheral surface on the upper side.

[0040] In the heat insulation unit 40 of this embodiment, the inner heat insulation unit 43, the lower heat insulation unit 51, and the upper heat insulation unit 52 are cylindrical members that do not have a cooling function. In such a cylindrical member, the amount of heat Q that is transferred from the inner peripheral surface to the outer peripheral surface and released from the outer peripheral surface can be expressed by the following equation (1) according to Fourier's law. TIFF0007777404000001.tif27170 Note that T(x) is the difference between the temperature of the inner circumferential surface and the temperature of the outer circumferential surface at position x in the extension direction based on one end of the component, and D in(x) is the inner diameter at position x, Dout(x) is the outer diameter at position x, k(x) is the average value of the thermal conductivity in the radial direction at position x, and L is the length of the member. The upper end surface of the lower heat insulating member 51 and the lower end surface of the internal heat insulating member 43 are in contact with the lower wall 42c of the housing 42, which has a cooling function, while the lower end surface of the upper heat insulating member 52 and the upper end surface of the internal heat insulating member 43 are in contact with the upper wall 42b of the housing 42. Therefore, heat is released from the upper end surface of the lower heat insulating member 51, the lower and upper end surfaces of the internal heat insulating member 43, and the lower end surface of the upper heat insulating member 52 to the cooling function of the housing 42. However, during the dehydration and sintering processes described above, the amount of heat released in this manner is negligibly small compared to the amount of heat released from the outer peripheral surfaces of each member. Therefore, the amount of heat released from the outer peripheral surfaces of the internal heat insulating member 43, lower heat insulating member 51, and upper heat insulating member 52 can be roughly approximated by the above formula (1). Here, the outer surfaces of the lower insulating section 51 and the upper insulating section 52 are part of the outer surface of the insulating section 40, and below, the amount of heat released from the outer surfaces of the lower insulating section 51 and the upper insulating section 52 will be explained in detail.

[0041] In this embodiment, as described above, the outer diameter of the furnace core tube 31 is constant in the length direction, and the inner circumferential surfaces of the lower insulation section 51 and the upper insulation section 52 are in contact with the furnace core tube 31. In addition, the outer diameters of the lower insulation section 51 and the upper insulation section 52 are constant in the extension direction, the length of the lower insulation section 51 is the same as the length of the upper insulation section 52, and the thermal conductivity of the lower insulation section 51 is the same as the thermal conductivity of the upper insulation section 52. Therefore, if the difference in temperature between the inner and outer surfaces of the lower insulation section 51 is ΔT1, the difference in temperature between the inner and outer surfaces of the upper insulation section 52 is ΔT2, the outer diameter of the furnace core tube 31 is D0, the outer diameter of the lower insulation section 51 is D1, the outer diameter of the upper insulation section 52 is D2, the thermal conductivity of the lower insulation section 51 and the upper insulation section 52 is k, and the length of the lower insulation section 51 and the upper insulation section 52 is L, the amount of heat Q1 released from the outer surface of the lower insulation section 51 is expressed by the following equation (2), and the amount of heat Q2 released from the outer surface of the upper insulation section 52 is expressed by the following equation (3). TIFF0007777404000002.tif22170TIFF0007777404000003.tif22170Further, the cross-sectional area A1 of the lower heat insulating section 51 is expressed by the following formula (4), and the cross-sectional area A2 of the upper heat insulating section 52 is expressed by the following formula (5). TIFF0007777404000004.tif30170Furthermore, if the sum of the cross-sectional area A1 and the cross-sectional area A2 is A and the ratio of the volume of the lower insulation section 51 to the sum of the volumes of the lower insulation section 51 and the upper insulation section 52 is β, then the cross-sectional area A1 is expressed by the following equation (6), and the cross-sectional area A2 is expressed by the following equation (7). TIFF0007777404000005.tif17170Then, based on the above equations (4) and (6), the outer diameter D1 of the lower insulating section 51 is expressed by the following equation (8), and based on the above equations (5) and (7), the outer diameter D2 of the upper insulating section 52 is expressed by the following equation (9). TIFF0007777404000006.tif33170

[0042] Here, if the ratio of ΔT2 to ΔT1 is α and the area of ​​a circle whose diameter is the outer diameter of the furnace core tube 31 is A0, then based on the above equations (2) and (8), the heat quantity Q1 is expressed by the following equation (10), and based on the above equations (3) and (9), the heat quantity Q2 is expressed by the following equation (11), and the total heat quantity Q of the heat quantities Q1 and Q2 is expressed by the following equation (12). TIFF0007777404000007.tif23170TIFF0007777404000008.tif22170TIFF0007777404000009.tif27170If the ratio of A to area A0 is C, the part f(α, β) that depends on β in the above equation (12) is expressed by the following equation (13). TIFF0007777404000010.tif23170

[0043] FIG. 3 is a graph showing the amount of heat released from the outer peripheral surface of the heat insulating section. Specifically, the graph in FIG. 3 is an example of a graph showing the relationship between f(α,β) and the ratio β, where the ratio C is 1 and the ratio α is 1.5, 2, 3, and 5. If both the lower heat insulating section 51 and the upper heat insulating section 52 are present, the range of the ratio β is greater than zero and less than 1. The graph in FIG. 3 shows that f(α,β) reaches its minimum value within this range, and the value of the ratio β at which f(α,β) is minimum decreases as the ratio α increases. Note that a circle is marked in FIG. 3 at the position where f(α,β) is minimum.

[0044] As described above, due to the flow of gas supplied to the internal space 31S, the upper side of the heater 35 becomes hotter than the lower side in the portion of the internal space 31S surrounded by the thermal insulation section 40. When the condition that the ratio α is greater than 1 is applied to f(α, β), the ratio β at which f(α, β) is minimized becomes less than 0.5. This indicates that there is an optimal ratio β that minimizes the total heat quantity Q, which is the sum of the heat quantity Q1 radiated from the outer peripheral surface of the lower thermal insulation section 51 and the heat quantity Q2 radiated from the outer peripheral surface of the upper thermal insulation section 52. This provides a guideline that the larger the ratio α, the smaller the ratio β at which the heat quantity Q is minimized.

[0045] The ratio α varies depending on conditions such as the outer diameter D0 of the furnace core tube 31, the type of gas supplied to the internal space 31S of the furnace core tube 31, the flow rate of the gas, and the heating temperature. Therefore, the difference in temperature between the outer surface of the furnace core tube 31 and the outer surface of the heat insulating section 40 was measured under the following four conditions a, b, c, and d. The results are shown in FIG. 4. Under condition a, the type of gas was helium, the gas flow rate was 3 SLM, and the temperature near the heater 35 in the internal space 31S was approximately 1400°C. Under condition b, the type of gas was helium, the gas flow rate was 3 SLM, and the temperature near the heater 35 in the internal space 31S was 1100°C. Under condition c, the type of gas was helium, the gas flow rate was 3 SLM, and the temperature near the heater 35 in the internal space 31S was approximately 1400°C. Under condition d, the type of gas was helium, the gas flow rate was 3 SLM, and the temperature near the heater 35 in the internal space 31S was approximately 1000°C. These conditions are typical conditions for heating a porous glass body that will become a part of an optical fiber preform. Under these conditions, the outer diameter D1 of the lower insulating part 51 and the outer diameter D2 of the upper insulating part 52 were 500 mm.

[0046] In the graph of FIG. 4, the horizontal axis represents the relative vertical position relative to the center of the heater 35, with 1.0 representing the lower end of the lower insulation section 51 and −1.0 representing the upper end of the upper insulation section 52. Near 0.2 represents the lower end of the central insulation section 41, and near −0.2 represents the upper end of the central insulation section 41. The vertical axis represents the difference between the temperature of the outer surface of the furnace core tube 31 and the temperature of the outer surface of the insulation section 40, normalized by dividing the difference by the maximum value. From the results shown in FIG. 3, the ratio α was calculated as the ratio of the average temperature of the outer surface of the upper insulation section 52 to the average temperature of the outer surface of the lower insulation section 51. The ratio α under condition a was 2.4, the ratio α under condition b was 1.6, the ratio α under condition c was 1.9, and the ratio α under condition d was 1.1. Therefore, it was found that under typical conditions for heating a porous glass body that will become a part of an optical fiber preform, α is 1.1 or more and 2.4 or less.

[0047] Here, the ratio C is the ratio of the total A, which is the cross-sectional area A1 of the lower insulating section 51 and the cross-sectional area A2 of the upper insulating section 52, to the area A0 of a circle whose diameter is the outer diameter of the furnace tube 31. Therefore, as C increases, the lower insulating section 51 and the upper insulating section 52 become larger, and the heating apparatus 1 for the porous glass body becomes larger. Therefore, from the viewpoint of preventing the apparatus from becoming large-sized, C is preferably 30 or less. Furthermore, when the ratio C is set to 30 or less and the ratio α is set to 1.1 or more and 2.4 or less in the f(α,β) shown in the above formula (13), the ratio β at which f(α,β) becomes the minimum is found to be greater than 0.36 and less than 0.49. Thus, the present inventors have found that by setting the ratio β within this range, it is possible to reduce the total amount of heat released from the upper insulating section 52 and the lower insulating section 51 while preventing the apparatus from becoming large-sized under typical conditions for heating a porous glass body that will become a part of an optical fiber preform. In the heating device 1 for a porous glass body of this embodiment, the outer diameters D1, D2 of the lower heat insulating section 51 and the upper heat insulating section 52 are set so that the ratio β falls within the above range.

[0048] As described above, the heating apparatus 1 for a porous glass body of this embodiment includes the furnace tube 31 extending in the vertical direction, the heat insulating unit 40, the heater 35, and the gas supply unit 65. The furnace tube 31 has an internal space 31S capable of accommodating the porous glass body 20, an air inlet S1 communicating with the internal space 31S, and an exhaust port E1 above the air inlet S1 and communicating with the internal space 31S. The heat insulating unit 40 surrounds the outer periphery of the furnace tube 31 between the air inlet S1 and the exhaust port E1. The heater 35 is surrounded by the heat insulating unit 40 between the furnace tube 31 and the heat insulating unit 40 and heats the porous glass body 20. The gas supply unit 65 supplies gas from the air inlet S1 to the internal space 31S. When the heater 35 heats the porous glass body 20, the gas causes the temperature of the upper side of the heater 35 in the portion of the internal space 31S surrounded by the insulating section 40 to be higher than that of the lower side. However, in this state, the amount of heat radiated from the outer peripheral surface of the insulating section 40 above the center of the heater 35 is less than the amount of heat radiated from the outer peripheral surface below the center of the heater 35. On the other hand, in the heating device of Patent Document 1, the amount of heat radiated from the outer peripheral surface above the center of the heater in the portion consisting of the furnace body, upper insulating section, and lower insulating section, which corresponds to the insulating section 40, is greater than the amount of heat radiated from the outer peripheral surface below the center of the heater. Therefore, even if the amount of heat radiated from the outer peripheral surface of the furnace core tube 31 is the same as in Patent Document 1, the amount of heat radiated from the outer peripheral surface above the heater 35 in the heating device of Patent Document 1 can be reduced compared to the heating device of Patent Document 1, and as a result, the high-temperature region where the temperature exceeds a predetermined temperature can be expanded. Therefore, the heating device 1 for a porous glass body of this embodiment can appropriately heat a porous glass body. The predetermined temperature may be, for example, 0.9 times the maximum temperature in the internal space 31S.

[0049] In this embodiment, the insulation section 40 is made up of a central insulation section 41 that surrounds the heater 35, an upper insulation section 52 that extends from the upper end of the central insulation section 41 along the muffle tube 31 and surrounds the outer circumferential surface of the muffle tube 31, and a lower insulation section 51 that extends from the lower end of the central insulation section 41 along the muffle tube 31 and surrounds the outer circumferential surface of the muffle tube 31. The thickness of the upper insulation section 52 in the radial direction of the muffle tube 31 is greater than the thickness of the lower insulation section 51. Therefore, even if the thermal conductivity of the upper insulation section 52 is the same as that of the lower insulation section 51, it is easy to widen the high-temperature region.

[0050] In the heating apparatus 1 for a porous glass body of this embodiment, the upper and lower insulating sections 52 and 51 are cylindrical, the outer diameters of the upper and lower insulating sections 52 and 51 are constant in the direction of extension, the distance from the center of the heater 35 to the lower end of the upper insulating section 52 is the same as the distance from the center of the heater 35 to the upper end of the lower insulating section 51, the length of the upper insulating section 52 is the same as the length of the lower insulating section 51, and the thermal conductivity of the upper insulating section 52 is the same as the thermal conductivity of the lower insulating section 51. Furthermore, the ratio β is greater than 0.36 and equal to or less than 0.49. Therefore, the heating apparatus 1 for a porous glass body of this embodiment can reduce the amount of heat radiation from the furnace tube 31 while preventing the apparatus from becoming large.

[0051] Although the present invention has been described above using the above-mentioned embodiments as examples, the present invention is not limited to these.

[0052] For example, in the above embodiment, the insulating section 40 is described as having a cylindrical shape in which the lower insulating section 51 and the upper insulating section 52 have constant outer and inner diameters in the length direction. However, the shapes of the lower insulating section 51 and the upper insulating section 52 are not limited as long as the amount of heat emitted from the outer peripheral surface of the insulating section 40 above the center of the heater 35 is equal to or less than the amount of heat emitted from the outer peripheral surface of the insulating section 40 below the center of the heater 35 when the porous glass body 20 is heated by the heater 35. For example, the outer and inner diameters of the lower insulating section 51 and the upper insulating section 52 do not have to be constant in the length direction, and the thicknesses of the upper insulating section 52 and the lower insulating section 51 do not have to be constant in the length direction. Although not illustrated, for example, the thickness of the upper insulating section 52 on the side of the central insulating section 41 may be greater than the thickness of the upper insulating section 52 on the side opposite to the central insulating section 41. In this case, the thickness of the upper insulation 52 may increase stepwise or gradually as it approaches the central insulation 41. As shown in Fig. 2, in the part of the furnace muffle tube 31 surrounded by the upper insulation 52, the temperature on the central insulation 41 side is higher than the temperature on the opposite side of the central insulation 41. For this reason, with this configuration, it is easier to widen the high-temperature region compared to when the thickness of the upper insulation 52 on the central insulation 41 side is thinner than the thickness on the opposite side of the central insulation 41.

[0053] Furthermore, in the above embodiment, the thermal conductivity of the lower heat insulating section 51 and the upper heat insulating section 52 was constant in the length direction and the radial direction, but it may vary in at least one of these two directions. Furthermore, in the above embodiment, the length of the upper heat insulating section 52 and the length of the lower heat insulating section 51 were the same, but they may be different. Furthermore, in the above embodiment, the distance from the center of the heater 35 to the lower end of the upper heat insulating section 52 was the same as the distance from the center of the heater 35 to the upper end of the lower heat insulating section 51. However, these distances may be different, and the configuration of the central heat insulating section 41 does not have to be symmetrical above and below the center of the heater 35.

[0054] In the above embodiment, the central insulation 41 including the housing 42 and the internal insulation 43 has been described as an example. However, the central insulation 41 only needs to surround the heater 35, and may be configured without the housing 42, for example. In the above embodiment, the insulation 40 surrounding a portion of the outer circumferential surface of the muffle tube 31 has been described as an example. However, the insulation 40 only needs to surround the outer circumferential surface of the muffle tube 31 between the inlet S1 and the outlet E1. For example, if the inlet S1 is formed in the lower lid 32 and the outlet E1 is formed in the upper lid 33, the insulation 40 may surround the entire outer circumferential surface of the muffle tube 31. In addition to the insulation 40, the heating device 1 for a porous glass body may further include another insulation that surrounds the outer circumferential surface of the muffle tube 31 above or below the insulation 40, and the insulation 40 and the other insulation may be connected to each other.

[0055] In the above embodiment, the gas supply unit 65 supplies a gas containing a chlorine-based gas. However, the gas supply unit 65 may be configured to be able to switch the gas to be supplied between a gas containing a chlorine-based gas and only an inert gas. In this case, for example, the gas supply unit 65 supplies a gas containing a chlorine-based gas to the internal space 31S when dehydrating the porous glass body 20, and supplies only an inert gas to the internal space 31S when sintering the porous glass body 20. Furthermore, the heating device 1 for a porous glass body may be one that only dehydrates the porous glass body 20, or one that only sinters the porous glass body 20. [Industrial Applicability]

[0056] As explained above, a heating device for a porous glass body that can appropriately heat a porous glass body has been provided, and it is expected to be used in fields such as optical fiber communications. [Explanation of symbols]

[0057] 1. Porous glass body heating device 20. Porous glass body 31. Furnace tube 35 Heater 40....Insulation section 41 Central insulation section 51 Lower insulation section 52 Upper insulation section

Claims

1. a furnace tube extending in a vertical direction, the furnace tube having an internal space capable of accommodating a porous glass body, an air inlet port communicating with the internal space, and an exhaust port above the air inlet port communicating with the internal space; a heat insulating section that surrounds the outer circumferential surface of the furnace tube between the air inlet and the exhaust port; a heater that is surrounded by the heat insulating section and is located between the furnace tube and the heat insulating section, and that heats the porous glass body; a gas supply unit that supplies gas from the air supply port to the internal space; Equipped with the heat insulating section comprises a central heat insulating section surrounding the heater, an upper heat insulating section extending from an upper end of the central heat insulating section along the furnace tube and surrounding an outer peripheral surface of the furnace tube, and a lower heat insulating section extending from a lower end of the central heat insulating section along the furnace tube and surrounding an outer peripheral surface of the furnace tube, If the difference in temperature between the inner peripheral surface and the outer peripheral surface of the lower insulation section is ΔT 1 , the difference in temperature between the inner peripheral surface and the outer peripheral surface of the upper insulation section is ΔT 2 , the thermal conductivity of the lower insulation section and the upper insulation section is k, the length of the lower insulation section and the upper insulation section is L, the sum of the cross-sectional area of ​​the lower insulation section and the cross-sectional area of ​​the upper insulation section is A, the area of ​​a circle having the outer diameter of the furnace tube as its diameter is A 0, the ratio of the volume of the lower insulation section to the sum of the volumes of the lower insulation section and the upper insulation section is β, and the ratio of the difference ΔT 2 to the difference ΔT 1 is α, the amount of heat Q 1 released from the outer peripheral surface of the lower insulation section is expressed by the following equation (10), and the amount of heat Q 2 released from the outer peripheral surface of the upper insulation section is expressed by the following equation (11): When the porous glass body is heated by the heater, the sum of the heat quantity Q2 and the heat quantity emitted from the outer peripheral surface of the central heat insulating part above the center of the heater is equal to or less than the sum of the heat quantity Q1 and the heat quantity emitted from the outer peripheral surface of the central heat insulating part below the center of the heater. A heating device for a porous glass body characterized by:

2. a furnace tube extending in a vertical direction, the furnace tube having an internal space capable of accommodating a porous glass body, an air inlet port communicating with the internal space, and an exhaust port above the air inlet port communicating with the internal space; a heat insulating section that surrounds the outer circumferential surface of the furnace tube between the air inlet and the exhaust port; a heater that is surrounded by the heat insulating section and is located between the furnace tube and the heat insulating section, and that heats the porous glass body; a gas supply unit that supplies gas from the air supply port to the internal space; Equipped with the heat insulating section comprises a central heat insulating section surrounding the heater, an upper heat insulating section extending from an upper end of the central heat insulating section along the furnace tube and surrounding an outer peripheral surface of the furnace tube, and a lower heat insulating section extending from a lower end of the central heat insulating section along the furnace tube and surrounding an outer peripheral surface of the furnace tube, a thickness of the upper insulation part in a radial direction of the furnace tube is greater than a thickness of the lower insulation part; The upper heat insulating section and the lower heat insulating section are cylindrical, The outer diameter and the inner diameter of each of the upper heat insulating section and the lower heat insulating section are constant in the extension direction, the distance from the center of the heater to the lower end of the upper heat insulating section is the same as the distance from the center of the heater to the upper end of the lower heat insulating section, The length of the upper insulation section is the same as the length of the lower insulation section, When the porous glass body is heated by the heater, the amount of heat released from the outer peripheral surface of the heat insulating part above the center of the heater is equal to or less than the amount of heat released from the outer peripheral surface of the heater below the center. A heating device for a porous glass body characterized by:

3. The thermal conductivity of the upper insulating section is the same as the thermal conductivity of the lower insulating section, The ratio of the volume of the lower insulation section to the sum of the volumes of the upper insulation section and the lower insulation section is greater than 0.36 and is equal to or less than 0.

49.

3. The heating device for a porous glass body according to claim 2.

4. a furnace tube extending in a vertical direction, the furnace tube having an internal space capable of accommodating a porous glass body, an air inlet port communicating with the internal space, and an exhaust port above the air inlet port communicating with the internal space; a heat insulating section that surrounds the outer circumferential surface of the furnace tube between the air inlet and the exhaust port; a heater that is surrounded by the heat insulating section and is located between the furnace tube and the heat insulating section, and that heats the porous glass body; a gas supply unit that supplies gas from the air supply port to the internal space; Equipped with the heat insulating section comprises a central heat insulating section surrounding the heater, an upper heat insulating section extending from an upper end of the central heat insulating section along the furnace tube and surrounding an outer peripheral surface of the furnace tube, and a lower heat insulating section extending from a lower end of the central heat insulating section along the furnace tube and surrounding an outer peripheral surface of the furnace tube, a thickness of the upper insulation part in a radial direction of the furnace tube is greater than a thickness of the lower insulation part; a thickness of the upper heat insulation section on the side of the central heat insulation section is greater than a thickness of the upper heat insulation section on the side opposite to the central heat insulation section, When the porous glass body is heated by the heater, the amount of heat released from the outer peripheral surface of the heat insulating part above the center of the heater is equal to or less than the amount of heat released from the outer peripheral surface of the heater below the center. A heating device for a porous glass body characterized by:

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