The drying benchmark is the program table for the tempe […]
The drying benchmark is the program table for the temperature and humidity changes of the drying medium compiled according to the drying time and the change of the wood state (moisture content, pressure). In the actual drying process, if the program table is correctly executed, the drying of the wood can be reasonably controlled. process, thus ensuring the dry quality of the wood.
According to the control factors of the drying process, the drying standard is usually divided into a time drying standard and a moisture content drying standard.
Time drying benchmark: control the drying process according to the drying time, formulate the size of the medium parameters, that is, divide the time required for the drying process into several time stages, specify the weight of each time stage, and stipulate the corresponding time stage according to each time stage. media temperature and humidity.
The time drying benchmark is the empirical drying benchmark summed up on the basis of the long-term use of the moisture content benchmark. As long as the operator has a good understanding of the drying equipment used and the properties of the wood to be dried, and controls the drying process according to the drying time, qualified boards can be produced.
Moisture content drying benchmark: control the drying process according to the moisture content of the wood, formulate the size of the medium parameters, that is, divide the entire drying process into several stages according to the range of the moisture content stage, and specify the corresponding medium temperature and humidity according to the stage . The range of water content reduction in each stage is consistent, such as from 35% to 30%; from 30% to 25% and so on.
Divide the entire drying process into 2 or 3 moisture content stages, called two-stage or three-stage drying benchmarks.
For those thick boards of hard broad-leaved trees, because it is difficult to dry, a large moisture content gradient is likely to occur during the drying process. In order to speed up the drying speed and avoid a large moisture content gradient, a wave drying benchmark can be used. During the entire moisture content stage, the temperature of the medium fluctuates repeatedly, which is called a fluctuating drying benchmark; the medium temperature gradually rises in the early stage of drying and fluctuates in the later stage of drying, which is called a semi-fluctuating drying benchmark.
The fluctuating drying process is to make the drying temperature fluctuate continuously, that is, to periodically repeat the process of "heating-cooling-constant temperature". The heating process only heats the wood without drying it. In the cooling and drying stage, the temperature is lowered to a certain extent and kept constant for a certain period of time, so as to make full use of the temperature gradient of high inside and low outside. When the temperature of the central layer of the wood decreases and the temperature gradient is gentle, it needs to be heated again, and this cycle is repeated to ensure a temperature gradient of high inside and low outside. The fluctuation drying process is more obvious in the early stage of drying, but not so obvious in the later stage. However, a certain relative humidity must be ensured for early fluctuations, otherwise the wood is prone to cracking, and later fluctuations are safe.
In production, the semi-wave process is usually used, that is, the conventional drying process is used in the early stage of drying, and the wave process is used in the later stage.