In the continuous drying of veneer sheets, a specific problem occurs which is not usually encountered in other drying processes. Two factors which determine the amount of water evaporated are subject to wide variations, namely the starting moisture level of the veneer sheets and the density of the load on the conveyer (i.e. the number of panels per unit time or per unit length of conveyer). Correspondingly, the drying efficiency must be varied over a wide range in order to match the constantly varying demand.
As a solution to this problem, it was proposed in German Patent DE-PS No. 27 21 965 to use a control process wherein the local heat consumption in at least two zones of the dryer was to be continually measured by measurement of the drop in temperature of the ambient air stream. From this measurement, determination could be made of the local heat demand in reference to the loading density and the initial moisture level. With the aid of a computer, the temperature and/or the throughput velocity is calculated and the appropriate control signal is generated for the heating regulator and/or for the drive motor of the conveyer.
Since the temperature can be varied only over a narrow range for any given kind of wood, and since the temperature changes occur only very slowly due to the large heat capacity of the apparatus, only very rough control of temperature is actually practicable. Control of the conveyer speed has a major influence on the total output of the factory, and in a continuous production line, such control can only be done in close coordination with the work stations located before and after the dryer. Therefore, it is preferable to avoid unnecessary changes of the conveyer speed. For this reason, it has been proposed in German Patent Application DE-OS No. 31 21 348 that the rotation speed of the blowers be controlled in accordance with the measured heat demand.
By means of the aforementioned process, it is possible to hold the final moisture level within a small tolerance range even where large variations occur in the initial moisture level and in the operating load. However, intolerable variations of the final moisture level occasionally occur due to disturbing influences. One of these disturbing influences relates to the fact that the thickness of the veneer sheets varies to a certain extent. These thickness variations not only influence the amount of moisture to be vaporized but they also affect the rate of drying as the diffusion distance increases or decreases. By experience, it is found that an increase in thickness of 10% leads to nearly a 20% slowing of the drying speed. Since the tolerated thickness variations cannot be measured in any practical way and indeed are practically unmeasurable, these variations cause deviation of the final moisture level from the initially computed level. Another disturbing influence is the differing density of the raw wood, which also leads to practically unavoidable variations of the final moisture level away from the desired value.
From U.S. Pat. No. 3,350,789 it is known that the moisture level of thin porous goods, for example veneer sheet, can be determined from the emitted radiant heat, taking account of the drying conditions, by means of a computation using an empirically determined formula. Moreover, it is known to use a dryer for continuous web goods which has two successive zones. In the first zone, the temperature and gas velocity are held constant. A radiation measuring device is placed at the end of the first zone. The measured value is compared to a desired setting, and the difference serves as a control signal for the heater or the blower of the second zone.
The aforementioned dryer is perhaps suited for the drying to constant moisture of continuous web goods, the initial moisture level of which varies within certain limits of tolerance. However, as a rule, it is practically impossible to get the moisture content of veneer sheets to approach close enough to a preset value under constant drying conditions in the first zone, in order that remaining differences can be overcome in the second zone, using the control process just described. The varying load is not measured by this radiation measuring device, and therefore is not compensated for. Moreover, in the patent referred to, no reference is made to the fact that the functional relationship between surface temperature and residual moisture is only valid below the fiber saturation point, which for most types of wood is in the range of 25 to 40%. If one attempts to use this prior art drying method for the drying of veneer, it would be impossible to assure that at the end of the first zone, all of the veneer sheets would have reached the fiber saturation point, without somehow taking into account the starting moisture level and the load. Above the fiber saturation level, the material temperature is practically independent of its moisture level; the drying process runs in accordance with the cooling boundary temperature.