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In the dynamic process of steam injection and hot press, the steam flow is not very stable. The steam ejected from the upper and lower spray holes will impact the slab, which will inevitably cause changes in the voidage of the slab. In addition, the steam emitted from the spray hole may not directly pass through the slab, but is scattered. Therefore, it is particularly important to study the temperature field inside the slab during steam-vacuum hot-pressing. Through experiments, the authors have investigated the temperature variation of the center layer of poplar slabs parallel to the hot press plate and the temperature variation of each layer along the thickness direction of the slab during steam jet vacuum hot pressing.
The temperature variation inside the poplar slabs of poplar wood was parallel to the temperature change in the slab center plane of the hot platens during steam injection vacuum hot pressing. Experimental materials Poplar wood shavings were taken from an oriented particleboard factory in Xuzhou, Jiangsu. The composition of the shavings was determined as shown. The experimental method uses Kang-copper thermocouple to measure the internal temperature of the slab. The diameter of the copper wire is 0.2n2.
In order to reduce the existing moisture in the slab and the interference of the moisture in the resin glue on the measurement accuracy, the slab is not sizing, and the initial moisture content of the shavings is taken as low as possible, subject to the experimental conditions, taking 2% in the experiment. Paving slabs with a 30x3000 single-layer structure. The density and nominal thickness of the slab are 0.69/en13 and 19nlm, respectively. In order to reduce the velocity of steam escaping from the spray hole, the steam is evenly distributed on the upper and lower surfaces of the slab and prevents sticking. Each of the upper and lower surfaces of the slab is covered with a Zhang copper network.
Taking into account the symmetry of the center plane of the slab, the placement of thermocouples in the center plane of the slab is as follows. The slab was pressed using a BYS602X4 test press. The orifice diameter of the spray hole of the steam jet vacuum hot press is 1.5, and the pitch of the holes is 3. In order to ensure that the steam can be uniformly injected into the slab during steaming, the spray holes are staggered.
Hot press platen temperature is 170 °C, the maximum gauge pressure is 3.2MPa, saturated steam pressure is 0.8MI] a. Vacuum pump vacuum degree is 3104 to 4x1o. 20-50 a steaming time, 50-1 1 steam holding time, 150-360 a vacuum time, 360-375 a step-down, open the press, remove the slab. Record the reading on the potentiometer at regular intervals. The platen was repeated once and the temperature was averaged.
The experimental results and discussion are the temperature changes of the measuring points in the center plane of the slab parallel to the hot platen during the hot pressing process. The temperature of each measuring point in the figure is very close to each other. It can be seen that in the different stages of the jet vacuum hot pressing, The difference in temperature at each measurement point is very small. Before steaming (O-205), the heat of the heated slab mainly comes from the contact heat transfer of the hot platen.
The wood itself is a non-heat-conducting body; the initial water content of the slab is minimal; the water-soluble phenolic resin is not contained in the slab; and the slab is compressed to a small extent, and the contact between the shavings is not very close by the theory of heat conduction. Under such conditions, it is impossible to form a sufficiently large temperature difference inside the slab within such a short period of time. Therefore, the heat transfer rate within the slab is extremely small, and the temperature in the center level of the slab cannot be significantly increased. At the 13th hour, the upper platen hits the surface of the slab blank. At the 20th hour, the compression ratio of the slab bulk density reaches 1.2t 2 ], and then the spraying starts.
Between 0 and 13, the temperature at each measurement point varies very little; between 13 and 20, the temperature at each measurement point slightly increases (only 2 to 4°C). This may be due to the fact that the air remaining inside the slab is compressed and the temperature _L is increased by one. Time (s); temperature distribution in the horizontal plane of the center of the slab l The position of the thermocouple in the center plane of the slab is in the steam injection phase. The temperature of each measuring point rises quickly. By 305 o'clock, the temperature at each measuring point has reached 120°C or more.
This is determined by the unique heat transfer within the slab during the steam injection stage. At this time, although there is a considerable degree of contact between the shavings inside the slab, there is still a certain gap, and the steam does not encounter a great resistance when passing through the gap between the shavings. The high-temperature saturated steam is rapidly pushed through the gap under the external pressure and diffuses and convects toward the center of the slab. In a very short time, the slab can be heated to above 100°C. Therefore, if only from the point that the center of the slab reaches 100°C or more, it is sufficient that the steaming time is 105 when the saturated steam pressure is 0.SMPa. When the steam injection reaches a certain degree, the center of the slab approximates a relatively saturated environment.
If the inside of the slab approximates a pure steam environment and there is a sufficient amount of condensate (free or bound water), the temperature rise is completely controlled by the total pressure of the system. When the slab is heated and its central temperature reaches the boiling temperature of its environment, the water in the slab rapidly evaporates and consumes extra heat, so that the center temperature of the slab remains stable. However, in practice, there is steam leakage around the slab.
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