Vacuum drying method

Abstract

To surely vacuum-dry a drying object in a shorter time.SOLUTION: Upon storing and drying an object in a vacuum drying container, a drying process of feeding nitrogen gas to the inside of the vacuum drying container until the inside of the vacuum drying container becomes closer to the atmospheric pressure while continuing vacuum suction in a state that the inside of the vacuum drying container is put into a vacuum state by performing the vacuum suction by the vacuum pump, is intermittently repeated. Thereby, the object stored in the vacuum drying container can surely be vacuum-dried in a shorter time.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a vacuum drying method. [Background technology]

[0002] A conventional vacuum drying method involves causing cavitation in the moisture contained in the object, thereby imploding the moisture and generating shock waves, which form minute voids in the object and promote vacuum drying of the object using a vacuum pump (see Patent Document 1). In this vacuum drying method, the object is placed in a vacuum drying container that can take the object in and out, and while a vacuum is applied to the inside of the vacuum drying container using a vacuum pump, an air inlet valve that is provided in the vacuum drying container and configured to be openable and closable is opened and closed to allow and stop the supply of air into the vacuum drying container.This process is repeated at preset times to cause cavitation in the object. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-081119 Summary of the Invention [Problem to be solved by the invention]

[0004] This vacuum drying method allows the object to be vacuum dried more reliably and in a shorter time than previous vacuum drying techniques. An object of the present invention is to vacuum dry an object in a shorter time. [Means for solving the problem]

[0005] In order to solve the above problems, the invention described in claim 1 is characterized in that, when placing an object in a vacuum drying container and drying it, a drying process is intermittently repeated in which a vacuum pump is used to create a vacuum inside the vacuum drying container, and while continuing the vacuum suction, nitrogen gas is fed into the vacuum drying container until the pressure inside the vacuum drying container reaches near atmospheric pressure.

[0006] The invention described in claim 2 is the vacuum drying method described in claim 1, The vacuum drying container is provided with an electromagnetic wave generating unit that generates electromagnetic waves, and in the drying step, the object is irradiated with electromagnetic waves from the electromagnetic wave generating unit.

[0007] The invention described in claim 3 is characterized in that, in the vacuum drying method described in claim 2, the object is a piece of wood that is long in the fiber direction, and electromagnetic waves are irradiated to the center of the piece of wood in the long direction.

[0008] The invention of claim 4 is characterized in that in the vacuum drying method of claim 3, the temperature of the end grain of the wood is maintained at a temperature close to the boiling point in the drying step. [Effects of the Invention]

[0009] According to the present invention, an object placed in a vacuum drying container can be vacuum-dried reliably in a short time. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing an example of a vacuum drying apparatus according to the present invention. [Figure 2] 1A and 1B are diagrams illustrating a vacuum drying method according to the present invention. [Figure 3] This figure shows the temperature distribution trends resulting from investigating the temperature changes in the center, middle, and end grain of wood when microwaves are irradiated to the center of the wood. [Figure 4] FIG. 10 is a diagram showing experimental results. [Figure 5] FIG. 10 is a diagram showing experimental results. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, although the embodiments described below are subject to various limitations that are technically preferable for carrying out the present invention, the technical scope of the present invention is not limited to the following embodiments and illustrated examples.

[0012] [About the vacuum drying device] In Fig. 1, reference numeral 1 denotes a vacuum drying apparatus. This vacuum drying apparatus 1 is used to vacuum-dry an object 10 containing moisture (to reduce the moisture content). This vacuum drying apparatus 1 includes a vacuum drying container 2 into which the object 10 can be placed and removed, a vacuum pump 3 connected to the vacuum drying container 2, a nitrogen gas supply source 9, a nitrogen gas inlet valve 4 provided in the vacuum drying container 2, a timer control unit 5 connected to the nitrogen gas inlet valve 4, and an electromagnetic wave generation unit 6 that irradiates the object 10 with electromagnetic waves.

[0013] The vacuum drying container 2 has a cylindrical (cylindrical in this embodiment) container body 20. A tubular suction part 20a that communicates the inside and outside of the container body 20 is integrally provided on the cylindrical wall of the container body 20, and a suction pipe 3a (described later) is connected to the suction part 20a. Furthermore, a support portion 20b on which the object 10 is placed and supported is integrally provided inside the container body 20.

[0014] Both ends of the container body 20 in the length direction are open, and either opening can be used as an opening for putting in and taking out the object 10. A first flange 20c is integrally provided at one end in the length direction of the container body 20, and a second flange 20d is integrally provided at the other end in the length direction. The opening at one end of the container body 20 in the longitudinal direction can be closed by a first lid 21 set to a diameter approximately equal to that of the first flange 20c, and the opening at the other end of the longitudinal direction can be closed by a second lid 22 set to a diameter approximately equal to that of the second flange 20d. The first flange 20c and the first lid 21, and the second flange 20d and the second lid 22 are firmly connected by a plurality of bolts and nuts.

[0015] The vacuum pump 3 is a water-sealed Elmo-type vacuum pump. A water-sealed Elmo-type vacuum pump has a fan built into a casing, which is provided with a suction port and a discharge port. The cylindrical fan is incorporated into the cylindrical casing with its fan center offset by about 20 to 30 mm from the center of the casing, and a suction pipe 3a is connected to the suction port. The suction pipe 3a is also connected to the suction section 20a of the container body 20. This water-sealed Elmo-type vacuum pump sucks steam from inside the vacuum drying container 2 into the casing through the suction port via the suction pipe 3a by eccentric rolling rotation of the fan relative to the casing, and discharges it from the discharge port. Furthermore, water is sealed inside the casing. That is, a circulating water channel is connected to the bottom of the casing, and the tip of this circulating water channel is introduced into the circulating water in a large-capacity cooling water tank with excellent heat dissipation. Therefore, due to the eccentric rolling rotation of the fan relative to the casing, the circulating water in the cooling water tank is vacuum-sucked through the circulating water channel, and the water content in the nitrogen gas is discharged from the discharge port.

[0016] The nitrogen gas inlet valve 4 is an electromagnetic valve (also called an electronic valve) for feeding nitrogen gas into the inside of the container body 20, and is provided integrally with the cylindrical wall of the container body 20. The nitrogen gas inlet valve 4 includes a tubular inlet that connects the inside of the container body 20 with the nitrogen gas supply source 9 to allow the inflow of nitrogen gas, and a valve body that opens and closes the inlet.

[0017] The timer control unit 5 is connected to the nitrogen gas inlet valve 4 so as to be able to communicate with it, and is capable of transmitting, at a preset timing, a control signal that controls the opening and closing operation of the valve element in the nitrogen gas inlet valve 4. By controlling the opening and closing operation of the valve element and operating the valve element to open the inlet, external nitrogen gas is sent into the inside of the container body 20, and by operating the valve element to close the inlet, nitrogen gas can be prevented from flowing into the inside of the container body 20. The timing for controlling the opening and closing operation of the valve disc can be set to various patterns, and in this embodiment, a pattern is adopted in which the valve disc is operated to open the inlet after a predetermined time has passed, and then the valve disc is operated to close the inlet after another predetermined time has passed, repeating this operation. However, this is not limited to this, and other patterns may also be used.

[0018] The electromagnetic wave generating unit 6 is attached to the center of the container body 20. The electromagnetic wave generating unit 6 has an irradiation unit that irradiates electromagnetic waves. Note that, as the electromagnetic wave generating unit 6 in this embodiment, a magnetron that irradiates microwaves as electromagnetic waves is adopted. In order to attach the electromagnetic wave generating unit 6 to the container body 20, an opening (not shown) into which the irradiation unit is fitted is formed in the container body 20. The irradiation unit is joined to the container body 20 by welding.

[0019] [About wood drying methods] Next, a method for vacuum drying the object 10 using the vacuum drying apparatus 1 configured as above will be described. Note that the following description will be given of a case where wood is used as the object 10.

[0020] Here, the relationship between the state of wood and its moisture content will be explained using the schematic diagram in FIG. In wood, when the moisture content (U) is 30% or more, the cell wall 11 is saturated with bound water, and free water 13 exists in the cell cavity 12. In this case, the wood contains a lot of moisture, which is not an ideal state. When the moisture content (U) is approximately 30% (fiber saturation point), there is no free water 13 in the cell lumen 12, but the cell wall 11 is saturated with bound water. Even in this case, the wood still contains a lot of moisture, which is not an ideal state. When the moisture content (U) is 0% or less than 30%, bound water exists in the cell walls 11 but is not saturated. This is an ideal state for wood.

[0021] 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 period of time, electromagnetic waves are irradiated onto the center of the wood and "CVT drying" is performed using nitrogen gas. Here, "CVT drying" refers to a drying method that physically accelerates moisture movement by periodically repeating decompression and the introduction of gas at atmospheric pressure (in this example, nitrogen gas) in order to cause cavitation (CVT), which means hollowing out. The characteristics of this nitrogen gas CVT drying are: (i) creating an anhydrous steam environment, (ii) thinning the gas boundary film to promote evaporation, (iii) preventing changes in physical properties in an oxygen-free environment, and (iv) a carrier effect that sends evaporated steam to the outside.

[0022] In this CVT drying, the inside of the container body 20 containing the wood is vacuum-suctioned, and at predetermined intervals, the nitrogen gas inlet valve 4 is opened to allow nitrogen gas to flow into the inside of the container body 20. As a result, the inside of the container body 20 suddenly returns from a vacuum state (a state with a pressure lower than the atmosphere) to near atmospheric pressure. When the inside of the container body 20 is evacuated, the water boils and turns into steam, generating cavities (air bubbles). At that time, nitrogen gas is introduced into the inside of the container body 20 from the outside, causing the air bubbles to collapse, i.e., "cavitation." "Cavitation" refers to the phenomenon in which bubbles implode and then collapse immediately, generating shock waves that travel outward. In the case of water, these shock waves are equivalent to a pressure of 1,000 to 10,000 atmospheres per micron, and can even cause erosion of metals. That is, when the inside of the container body 20 is in a vacuum state, nitrogen gas is flowed into the container body 20, causing cavitation (bubble collapse) of the moisture (including water vapor) contained in the object (here, wood) 10, as shown in Figure 2, causing the moisture to implode (see arrow Ip) and generating shock waves (see arrow Sw).

[0023] The moisture contained in the object 10 includes free water 13 present in the cell lumen 12 of each of the countless cells that make up the object 10, and bound water present in the cell walls 11. The free water 13 can be vaporized and sucked up using normal vacuum suction. On the other hand, the bound water is contained in the cell walls 11, and the impact force caused by cavitation can form microvoids (holes, water paths) in the cell walls 11 where the bound water has entered. In other words, countless microvoids are formed in the cell walls 11. The formation of these microvoids allows the microvoids to function as water passageways, and the bound water that has contained the cell walls 11 can easily escape from the cell walls 11 through the voids, making it easier to vacuum suction the object 10 using the vacuum pump 3, accelerating vacuum drying of the object 10. It should be noted that even while nitrogen gas is being introduced from the outside, the inside of the container body 20 is suctioned by the vacuum pump 3, so that the moisture coming out of the target object 10 is continuously sucked.

[0024] Furthermore, in this wood drying, in CVT drying, in order to further promote vacuum drying, the electromagnetic wave generating unit 6 irradiates electromagnetic waves onto the center of the object (wood in this example) 10 inside the container body 20. When electromagnetic waves are irradiated onto the object 10, not only the free water 13 present in the cell lumen 12 but also the bound water present in the cell wall 11 are affected by the electromagnetic waves (microwaves), causing intermolecular vibration of the water molecules and generating heat. As a result, the free water 13 and bound water evaporate and are sucked into a vacuum by the vacuum pump 3.

[0025] It is preferable to provide a pressure reduction / temperature increase step before the CVT drying step. In this case, the pressure reduction / temperature increase step serves as a preparatory step for the CVT drying step. In the depressurizing and heating process, the inside of the container body 20 containing the wood is suctioned with a vacuum while electromagnetic waves are irradiated onto the center of the wood. The wood is also dried in this depressurizing and heating process. In this decompression and temperature increase step, the pressure inside the container body 20 is reduced and the temperature of the wood is increased until the timing is suitable for introducing nitrogen gas for CVT drying.

[0026] The above wood drying method can provide the same (1) low-temperature drying effect, (2) reduced pressure / nitrogen CVT effect, and (3) microwave effect as described above.

[0027] (1) Low-temperature drying effect The inside of the container body 20 can be suctioned by the vacuum pump 3, and the pressure inside the container body 20 is reduced, which lowers the boiling point and increases the evacuation effect with strong suction force. Therefore, the inside of the container body 20 can be maintained in a low-temperature environment. Furthermore, this low-temperature drying effect reduces heat energy consumption, thereby reducing power consumption.

[0028] (2) Decompression / Nitrogen Cavitation Effect Nitrogen gas can be introduced into the container body 20 while the inside of the container body 20 is evacuated (depressurized) by the vacuum pump 3, which causes CVT (Cavitation), which means cavitation, within the object 10, accelerating moisture movement. This promotes vacuum drying of the object 10. Incidentally, when air is used instead of nitrogen gas, approximately 1,000 liters of humid air flows into the drying chamber for each CVT, but when nitrogen gas is used, no moisture flows in, so the CVT time can be reduced by another quarter. As a result, the drying time is further shortened, and the amount of power consumption is reduced accordingly. Furthermore, even if the object 10 is exposed to the air after drying, the effect of the nitrogen gas makes it difficult for moisture to enter the inside of the object 10.

[0029] (3) Microwave effect Since the object 10 can be irradiated with electromagnetic waves, not only the free water 13 present in the cell lumen 12 of the object 10 evaporates, but also the bound water present in the cell walls 11 is affected by the electromagnetic waves (microwaves), causing the water molecules to vibrate intermolecularly and generate heat, causing the bound water to evaporate and be sucked up by the vacuum pump 3. This promotes vacuum drying of the object 10.

[0030] (4) Furthermore, the following effects can be obtained by irradiating the center of the wood with microwaves. Here, 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 tendency is observed. FIG. 3 shows the tendency of temperature distribution as a result of investigating temperature changes in the center, intermediate portion, and end grain portion of wood when microwaves are irradiated to the center of the wood. This figure shows that the temperature at the center where the microwave was irradiated was the lowest, and the temperature at the end grain was the highest. This is presumably because the heat generated in the center of the wood by the microwave irradiation moved along the grain direction with water vapor, creating a temperature gap. This effect minimizes uneven drying, resulting in uniformly dried, homogeneous wood in the longitudinal direction.

[0031] [Experimental Example] (conditions) - Materials: 24 pieces of fir stud lumber (52mm x 112mm x 1,700mm). Three microwave irradiation units are placed in the center of the wood at specified intervals around the circumference of the container body. CTV with heated nitrogen gas at 30°C for 5 seconds / 30 minutes. -Microwave irradiation is controlled so that the temperature of the end grain is close to the boiling point.

[0032] (result) The results are shown in FIGS. These results confirmed that drying can be carried out quickly in a low-temperature environment. In this experiment, the moisture content was reduced to about 18% by CVT drying, but by continuing to perform CVT drying, the moisture content can be reduced even further. Incidentally, in another experiment, it was confirmed that by repeating CVT drying further, the moisture content could be reduced to nearly 5%. In another experiment, the moisture content was reduced to about 20% using CVT drying, and then the material was allowed to dry naturally for about five hours, achieving the target moisture content of 15% or less.

[0033] <<Variation>> It should be noted that the embodiments to which the present invention can be applied are not limited to the above-described embodiments, and can be modified as appropriate without departing from the spirit of the present invention. Modifications will be described below. The following modifications may be combined as much as possible. In each of the following modifications, elements common to the above-described embodiments will be assigned the same reference numerals, and descriptions thereof will be omitted or simplified.

[0034] [Variation 1] The vacuum drying apparatus 1 in this modification includes a plurality of vacuum drying containers 2 and a vacuum pump 3 connected to the plurality of vacuum drying containers 2 and collectively suctioning the insides of the plurality of vacuum drying containers 2. Each vacuum drying container 2 is appropriately provided with a nitrogen gas inlet valve 4 (with a timer control unit 5) and an electromagnetic wave generator 6. That is, the suction pipe 3 a of the vacuum pump 3 is branched into a number of pipes equal to the number of vacuum drying containers 2 , and the tips of the branched suction pipes 3 a are connected to the suction parts 20 a of the container body 20 . The vacuum drying containers 2 are stored in a storage shelf having a plurality of storage spaces formed in the vertical and horizontal directions. That is, a vacuum drying container 2 is stored in each of the plurality of storage spaces.

[0035] According to this modification, the insides of a plurality of vacuum drying containers 2 are evacuated all at once by one vacuum pump 3, so that a greater number of objects 10 can be vacuum-dried at one time. Furthermore, if a plurality of vacuum drying containers 2 are stored on a storage shelf, the space required for installing the plurality of vacuum drying containers 2 is not required, thereby enabling space saving.

[0036] Furthermore, the above drying method can be applied not only to drying wood, but also to drying objects 10 that are organic substances (organic compounds) such as food, vegetables, fruits, wood, rice, grains, etc., and that are derived from living organisms (living bodies) including plants and animals. The positions at which the electromagnetic waves are irradiated onto the object 10 can be determined in accordance with the characteristics of the object 10 . [Explanation of symbols]

[0037] 1 Vacuum drying device 2 Vacuum drying container 20 Container body 20a Suction part 20b Support part 20c First flange 20d Second flange 21 First lid 22 Second lid 3. Vacuum pump 3a suction tube 4 Nitrogen gas inlet valve 5 Timer control section 6 Electromagnetic wave generator 9. Nitrogen gas supply source 10 Object 11 Cell wall 12 Cell lumen 13 free water 14 Bound water

Claims

1. A vacuum drying method characterized by intermittently repeating a drying process in which, when placing an object in a vacuum drying container and drying it, the inside of the vacuum drying container is evacuated using a vacuum pump to create a vacuum inside the vacuum drying container, and while continuing the vacuum suction, nitrogen gas is fed into the vacuum drying container until the pressure inside the vacuum drying container reaches near atmospheric pressure.

2. 2. The vacuum drying method according to claim 1, wherein the vacuum drying container is provided with an electromagnetic wave generating unit that generates electromagnetic waves, and the object is irradiated with electromagnetic waves from the electromagnetic wave generating unit in the drying step.

3. 3. The vacuum drying method according to claim 2, wherein the object is a piece of wood that is long in the direction of its fibers, and the electromagnetic waves are irradiated to the center of the piece of wood in the longitudinal direction.

4. 4. The vacuum drying method according to claim 3, wherein the temperature of the end grain of the wood is maintained at a temperature close to the boiling point in the drying step.

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