Vacuum microwave dryer

Abstract

To provide a vacuum microwave dryer with high drying efficiency. [Solution] The invention provides a microwave irradiation chamber for irradiating an object with electromagnetic waves and a vacuum container for applying reduced pressure to the object. The vacuum container is installed inside the microwave irradiation chamber, and the vacuum container is made of a sealed, flexible resin bag. A vacuum pump, located outside the microwave irradiation chamber, is connected to the resin bag via a first conduit. This makes it possible to realize a vacuum microwave dryer that is highly efficient and inexpensive.

Description

Technical Field

[0001] The present invention relates to a vacuum microwave dryer.

Background Art

[0002] Conventionally, in a vacuum microwave dryer, a cylindrical drying furnace that also serves as a microwave irradiation chamber and a vacuum container is used. An object is placed inside the drying furnace, and the drying furnace is depressurized and the object is irradiated with microwaves (see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the above-described vacuum microwave dryer, since it is necessary to depressurize the entire drying furnace during depressurization, it takes time for depressurization and the drying efficiency is low. The present invention has been made in view of such circumstances, and an object thereof is to provide a vacuum microwave dryer with high drying efficiency.

Means for Solving the Problems

[0005] The vacuum microwave dryer according to claim 1 includes a microwave irradiation chamber that irradiates an object with electromagnetic waves and a vacuum container that applies a vacuum to the object, wherein the vacuum container is installed inside the microwave irradiation chamber, and the vacuum container is composed of a sealed soft resin bag, and a vacuum pump disposed outside the microwave irradiation chamber is connected to the resin bag via a first pipeline.

[0006] The vacuum microwave dryer of claim 2 is the vacuum microwave dryer of claim 1, characterized in that the object to be dried is rectangular or rectangular timber, and the microwave irradiation chamber has a rectangular parallelepiped structure.

[0007] The vacuum microwave dryer of claim 3 is a vacuum microwave dryer of claim 1 or claim 2, characterized in that a second conduit is connected to the resin bag, allowing air or nitrogen gas to be introduced from outside the microwave irradiation chamber, a solenoid valve is provided in the portion of the second conduit that is led out of the microwave irradiation chamber, the solenoid valve is configured to be opened and closed by a control unit, and cavitation is caused inside the wood by introducing air or nitrogen gas under reduced pressure.

[0008] The vacuum microwave dryer of claim 4 is the vacuum microwave dryer of claim 3, characterized in that, in addition to the object to be dried, a drain material that is electromagnetically permeable and breathable is placed in the resin bag. [Effects of the Invention]

[0009] According to the vacuum microwave dryer of claim 1, the microwave irradiation chamber for irradiating the object with microwaves and the vacuum container for applying reduced pressure to the object are made into a double structure, the vacuum container is placed inside the microwave irradiation chamber, the vacuum container is made into a flexible, sealed resin bag, and the resin bag can be vacuumed, thus enabling a vacuum microwave dryer with high drying efficiency and low cost.

[0010] According to the vacuum microwave dryer of claim 2, since the microwave irradiation chamber has a rectangular parallelepiped structure, for example, when drying rectangular or flat rectangular lumber, the lumber storage rate can be improved compared to the case of a cylindrical structure.

[0011] According to the vacuum microwave dryer of claim 3, since air or nitrogen gas can be introduced into the resin bag from outside the microwave irradiation chamber under reduced pressure via a flow path, cavitation can be caused inside the wood, thereby increasing the drying efficiency.

[0012] According to the vacuum microwave dryer of claim 4, in addition to the object to be dried, a drain material that is electromagnetically permeable and breathable is placed in the resin bag, which prevents the resin bag from sticking to the object, allowing for effective vacuuming and preventing water droplets from condensation from coming into contact with the object, thus enabling even drying. [Brief explanation of the drawing]

[0013] [Figure 1] This is an illustrative diagram of a vacuum microwave dryer according to an embodiment. [Figure 2] This diagram shows how to seal wooden blocks into plastic bags. [Figure 3] This is a diagram illustrating the drying state of wood. [Figure 4] This figure shows the trend of temperature distribution when microwaves are applied. [Modes for carrying out the invention]

[0014] Hereinafter, a vacuum microwave dryer according to an embodiment of the present invention will be described based on the drawings.

[0015] Figure 1 is an illustrative diagram of a vacuum microwave dryer 100 according to an embodiment. The vacuum microwave dryer 100 has a double structure consisting of a microwave irradiation chamber 2 for irradiating the object 1 with electromagnetic waves and a vacuum container 3 for applying reduced pressure to the object 1, with the vacuum container 3 being located inside the microwave irradiation chamber 2.

[0016] (Microwave irradiation room 2) The microwave irradiation chamber 2 has a rectangular parallelepiped structure, and the wall itself or the inner surface of the wall is composed of a microwave leakage prevention material. A microwave irradiation device 21 is provided at a predetermined position on the wall of the microwave irradiation chamber 2. A door (not shown) is provided in the microwave irradiation chamber 2, and by opening and closing this door, the object 1 can be carried in and out of the microwave irradiation chamber 2. In the embodiment, a cart 6 is used for carrying in and out the object 1.

[0017] (Vacuum container 3) In the embodiment, the vacuum container 3 is composed of a resin bag 31 having electromagnetic wave (microwave) permeability and flexibility. As the material of the resin bag 31, a highly airtight material such as vinyl-based, rubber-based, or carbon-based material that allows easy passage of the magnetron is used. A zipper (not shown) is attached to the resin bag 31, and the object 1 is put in and sealed in a predetermined unit. As shown in FIG. 2, two pipeline connection parts 32 and 33 are attached to the resin bag 31. One end of the first pipeline 41 can be connected to the pipeline connection part 32. The first pipeline 41 is composed of a material having electromagnetic wave permeability and flexibility. The first pipeline 41 penetrates the wall of the microwave irradiation chamber 2, and a part of it is led out of the microwave irradiation chamber 2 and connected to the vacuum pump 4. On the other hand, one end of the second pipeline 51 can be connected to the pipeline connection part 33. The second pipeline 51 is composed of a material having electromagnetic wave permeability and flexibility. The second pipeline 51 penetrates the wall of the microwave irradiation chamber 2, and a part of it is led out of the microwave irradiation chamber 2 and connected to, for example, a PSA type nitrogen gas generator 5. Also, a solenoid valve 52 is provided in the part of the second pipeline 51 led out of the microwave irradiation chamber 2. The solenoid valve 52 can be controlled by a control unit (not shown).

[0018] Note that the first pipeline 51 and the second pipeline 52 can be detached from the corresponding pipeline connection parts 32 and 33.

[0019] (Object 1) The object 1 is, for example, wood 10. The wood 10 is laminated via a resin-made crossbar 10a having electromagnetic wave permeability, and a wood block formed by laminating the wood 10 into a single piece is enclosed in a resin bag 31. At that time, between the wood block and the resin bag 31, a drain material 37 made of, for example, PVC (polyvinyl chloride) having electromagnetic wave permeability is interposed so that a clearance is formed. A large number of holes are formed in the drain material 37. A bamboo blind or the like can also be used as the drain material. The drain material 37 prevents the wood block and the resin bag 31 from adhering during the vacuum suction of the resin bag 31 and forms a decompression zone therebetween. Also, it prevents water droplets due to condensation from adhering to the wood block.

[0020] (Cart 6) The cart 6 is composed of a material having electromagnetic wave permeability. A shelf 61 is provided on the loading platform 60 of the cart 6, and the object 1 is placed on this shelf 61. The cart 6 can be moved in and out of the microwave irradiation chamber 2 with the object 1 placed on the shelf 61.

[0021] [Regarding the method of drying wood] Next, a method of vacuum drying the object 1 by the decompression microwave dryer 100 having the above configuration will be described. Here, the case where the wood 10 is adopted as the object 1 will be described.

[0022] FIG. 3 is a schematic diagram showing the relationship between the state of the wood 10 and the moisture content. In the wood 10, when the moisture content (U) ≥ 30%, the cell wall 11 is in a state where the bound water is saturated, and free water 13 exists in the cell lumen 12. In this case, the wood 10 contains a lot of moisture and is not in a suitable state. When the moisture content (U) ≈ 30% (fiber saturation point), although there is no free water 13 in the cell lumen 12, the cell wall 11 is in a state where the bound water is saturated. Also in this case, the wood 10 contains a lot of moisture and is not in a suitable state. When 0 < moisture content (U) ≤ 30%, bound water is present in the cell wall 11, but it is not saturated. In this case, it can be said that the material is in a suitable state for wood 10.

[0023] Therefore, in the wood drying method of this embodiment, in order to reduce the moisture content of the wood to 0 < moisture content (U) ≤ 30% in a short time, electromagnetic waves are irradiated onto the center of the wood 10 and "CVT drying" of nitrogen gas is performed. Here, "CVT drying" refers to a drying method that physically accelerates moisture movement by repeatedly reducing the pressure and introducing a gas at atmospheric pressure (in this example, nitrogen gas) at regular intervals in order to cause cavitation (CVT), which means the formation of cavities. The characteristics of this nitrogen gas CVT drying are (i) creating an anhydrous vapor environment, (ii) thinning the gas boundary film to promote evaporation, (iii) preventing changes in physical properties in an oxygen-free state, and (iv) the carrier effect that sends the evaporated vapor to the outside.

[0024] In CVT drying, nitrogen gas is introduced into the resin bag 31 containing the wood 10 by opening the solenoid valve 52 at predetermined intervals while the inside of the resin bag 31 is vacuum-suctioned. As a result, the inside of the resin bag 31 rapidly returns from a vacuum state (a state with a pressure lower than atmospheric pressure) to near atmospheric pressure.

[0025] When the inside of the resin bag 31 is evacuated, the water boils and turns into steam, creating cavities (bubbles). At that time, introducing nitrogen gas from the outside into the resin bag 31 causes the bubbles to collapse, or "cavitation." "Cavitation" refers to the phenomenon where bubbles collapse immediately after implosion, generating shock waves that move outward. In the case of water, these shock waves are equivalent to pressures of 1,000 to 10,000 atmospheres per micron, and can cause erosion even in metals. In other words, when the inside of the resin bag 31 is in a vacuum state, introducing nitrogen gas into the resin bag 31 causes cavitation (bubble collapse) of the moisture (including water vapor) contained within the object (in this case, wood) 10, as shown in Figure 3, causing the moisture to implode (see arrow Ip) and generating a shock wave (see arrow Sw).

[0026] The moisture contained within the wood 10 consists of free water 13 present in the intracellular lumen 12 of each of the countless cells that make up the wood 10, and bound water present in the cell walls 11. Free water 13 can be vaporized and aspirated by normal vacuum suction. On the other hand, bound water is embedded in the cell walls 11, and the impact force caused by cavitation can form tiny voids (holes, water channels) in the cell walls 11 where the bound water has entered. In other words, countless tiny voids are formed in the cell walls 11. As a result of the formation of these tiny voids, these voids function as pathways for water, and the bound water that had entered the cell walls 11 can easily escape from the cell walls 11 through these voids, making it easier to vacuum aspirate with the vacuum pump 4, and thus promoting the vacuum drying of the wood 10. Furthermore, even while nitrogen gas is being introduced from the outside, the vacuum pump 4 performs vacuum suction inside the resin bag 31, so moisture released from the wood 10 is continuously sucked out.

[0027] Furthermore, in order to further promote vacuum drying of the wood 10, electromagnetic waves are irradiated onto the center of the wood 10 inside the resin bag 31. When electromagnetic waves are irradiated onto the wood 10, not only the free water 13 present in the intracellular lumen 12 but also the bound water present in the cell wall 11 are affected by the electromagnetic waves (microwaves), causing the water molecules to undergo intermolecular vibrations and generate heat. As a result, the free water 13 and bound water evaporate, flow along the fibers toward the end grain, and are then vacuum-suctioned by the vacuum pump 4.

[0028] (effect) The following effects can be obtained using the above 10 wood drying techniques.

[0029] By making the vacuum microwave dryer 100 a double-layered structure, it becomes unnecessary to apply vacuum pressure to the microwave irradiation chamber 2 that emits microwaves, while it is sufficient to apply vacuum pressure to the resin bag 31 installed inside the microwave irradiation chamber 2, thus significantly reducing the area to which pressure needs to be applied.

[0030] Furthermore, by changing the microwave irradiation chamber 2 from a cylindrical shape to a rectangular parallelepiped structure, the wood storage rate can be improved from 43.5% to 90%. This increases the number of pieces of wood 10 that can be dried at one time.

[0031] Furthermore, by changing the microwave irradiation chamber 2 from a cylindrical shape to a rectangular parallelepiped structure, microwaves can be irradiated almost perpendicularly to the surface of the loaded wood 10, thus reducing the possibility of edge effects and maintaining a uniform heating effect. Here, "edge effect" refers to the phenomenon in which microwave energy is concentrated at the pointed parts of the object (in this case, the wood 10), causing heating.

[0032] Furthermore, since the outer periphery of the wood 10 is covered with a resin bag 31, the steam exhausted from the outer periphery of the wood 10 clings to the outer periphery, and the steam effect, which causes heating to occur simultaneously inside and outside, can prevent cracking.

[0033] Furthermore, by reducing the size of the vacuum vessel 3 being evacuated, one vacuum pump 4 can reduce the pressure of several vacuum vessels 3. In other words, the size of the vacuum vessel 3 affects the generation of vacuum and the degree of vacuum. That is, the larger the vacuum vessel 3, the longer it takes for the vacuum pump 4 to expel the gas, but conversely, if the vacuum vessel 3 is small, the time it takes for the vacuum pump 4 to expel the gas can be shortened, or the load on the vacuum pump 4 can be reduced.

[0034] Furthermore, the following wood drying methods 10 described above can be obtained: (1) low-temperature drying effect, (2) reduced pressure / nitrogen CVT effect, and (3) microwave effect.

[0035] (1) Low-temperature drying effect Since the inside of the resin bag 31 can be vacuum-suctioned by the vacuum pump 4, the pressure inside the resin bag 31 is reduced, which lowers the boiling point and enhances the exhaust effect with strong suction. Therefore, the inside of the resin bag 31 can be kept in a low-temperature environment. Furthermore, this low-temperature drying effect reduces thermal energy consumption, thus lowering power consumption.

[0036] (2) Decompression / Nitrogen cavitation effect By introducing nitrogen gas into the resin bag 31 while simultaneously creating a vacuum (reducing pressure) inside the resin bag 31 using the vacuum pump 4, cavitation (CVT), which means the formation of cavities, can be created within the wood 10, accelerating moisture movement. This promotes vacuum drying of the wood 10. Incidentally, compared to systems that use atmospheric air instead of nitrogen gas, this can further reduce the CVT time by another quarter. As a result, the drying time is further reduced, which in turn reduces power consumption. Furthermore, even when exposed to air after drying, the effect of nitrogen gas makes it difficult for moisture to penetrate the interior of the wood 10.

[0037] (3) Microwave effect Since electromagnetic waves can be irradiated onto the wood 10, not only is the free water 13 present in the intracellular lumen 12 of the wood 10 evaporated, but the bound water present in the cell wall 11 is also affected by the electromagnetic waves (microwaves), causing the water molecules to undergo intermolecular vibrations and generate heat, which leads to the evaporation of the bound water, which is then sucked up by the vacuum pump 4. This promotes the vacuum drying of the wood 10.

[0038] (4) Furthermore, by irradiating the center of the wood 10 with microwaves, the following effects can be obtained. To explain the relationship between the microwave irradiation position and the wood temperature, when microwaves are irradiated to the center of the wood, the following trend can be observed. Figure 4 shows the temperature distribution trends obtained by investigating the temperature changes in the central, middle, and end grain of wood when microwaves are irradiated to the center of the wood. This diagram shows that the temperature is lowest at the center where microwaves were irradiated, and highest at the end grain. It is presumed that this is because the heat generated in the center of the wood by electromagnetic wave irradiation moves along the grain towards the end grain along with water vapor, creating a temperature gap. This effect minimizes uneven drying. As a result, uniformly dried, homogeneous lumber can be obtained along its length.

[0039] (modified version) In the above embodiment, nitrogen gas is supplied after depressurization, but instead of nitrogen gas, air may be supplied, although the drying rate will be slower compared to nitrogen gas. By supplying air, the drying rate can be significantly faster than when air is not supplied.

[0040] Furthermore, although nitrogen gas is supplied after depressurization in the above embodiment, it is also possible to simply perform vacuum suction without supplying nitrogen gas.

[0041] Furthermore, in the above embodiment, a wooden block made of stacked wood 10 is sealed in a single resin bag 31, but the entire wood 10 to be stored in the microwave irradiation chamber 2 may also be sealed in the resin bag 31.

[0042] Furthermore, although the above embodiment described the case of drying wood 10, it goes without saying that it can also be applied to drying vegetables and other things. [Explanation of Symbols]

[0043] 1. Object 2. Microwave irradiation room 3 Vacuum container 4. Vacuum pump 5. Nitrogen gas generator 6 carts 10 wood 10a Pier 11 Cell wall 12 Cell lumen 13 free water 21 Microwave irradiation device 31 Plastic bags 32,33 Pipe connection 37 Drain material 52 Solenoid valve 60 cargo bed 61 shelves 100 Reduced-pressure microwave dryer

Claims

1. The system comprises a microwave irradiation chamber for irradiating an object with electromagnetic waves, and a vacuum container for applying reduced pressure to the object. A vacuum microwave dryer characterized by having a vacuum container installed inside a microwave irradiation chamber, the vacuum container being made of a sealed, flexible resin bag, and a vacuum pump located outside the microwave irradiation chamber being connected to the resin bag via a first conduit.

2. The vacuum microwave dryer according to claim 1, characterized in that the object is a rectangular or rectangular piece of wood, and the microwave irradiation chamber has a rectangular parallelepiped structure.

3. The vacuum microwave dryer according to claim 1 or 2, characterized in that a second conduit is connected to the resin bag, allowing air or nitrogen gas to be introduced from outside the microwave irradiation chamber, a solenoid valve is provided in the portion of the second conduit that is led out of the microwave irradiation chamber, the solenoid valve is configured to be opened and closed by a control unit, and cavitation is caused inside the wood by introducing air or nitrogen gas under reduced pressure.

4. The vacuum microwave dryer according to claim 3, characterized in that, in addition to the object to be dried, a drain material that is electromagnetically permeable and breathable is placed in the resin bag.

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