Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2002-12-09
2004-05-04
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S451000, C034S487000
Reexamination Certificate
active
06729043
ABSTRACT:
1. Field of the Invention
The invention relates to a method for drying stacked wood with the help of a drying gas guided in a circulatory flow, with the stacks of wood being charged zone by zone depending on the average moisture of the wood in the respective zone with partial streams of the drying gas which differ with respect to their drying power, and to an apparatus for performing the method.
2. Description of the Prior Art
Sawn timber is dried to a large part in wood drying systems to a desired final moisture. It is always a goal, after the completion of the drying process, to obtain the most uniform final moisture of the batch of wood. A batch of sawn timber is introduced into a treatment chamber for drying purposes and then dried at predetermined drying temperatures and at high air humidities. In the interior of the treatment chamber a circulatory flow for heated drying gas through the sawn timber is produced with fans. In order to ensure an even temperature distribution in the treatment chamber it is necessary to introduce any ambient air that may have been introduced into the treatment chamber as evenly as possible over the cross section of the treatment chamber, which on the other hand is a necessary precondition for an even distribution of the air moisture within the chamber. Since temperature differences in the treatment chamber lead to uneven final humidities it is known to divide the treatment chambers into several zones in order to regulate in these zones the chamber temperature separately per se to the common setpoint value applicable for the entire treatment chamber. Every zone comprises its own temperature sensors and has its own actuators for associated heating registers or groups of heating registers through which the temperature of the drying gas for each zone is regulated. Usually, the regulation of the heating register temperature occurs in such a way that the drying gas circulated in the treatment chamber and emerging from the stack is held at a constant temperature, as a result of which an evening of the transmitted thermal output occurs between the individual zones. The evaporation of the moisture supplied by the wood leads in each zone to a temperature drop which is inversely proportional to the circulated air quantity. It may occur with this type of regulation that the inlet temperature exceeds a permissible limit temperature and thus damage to the wood (formation of cracks, etc.) occurs.
Known apparatuses for drying stacked timber in a drying gas (DE 297 23 003 U1, DE 37 38 806 A1) comprise, among other things, a drying chamber, fans for circulating the drying gas and heating registers for heating the drying gas. For exchanging the drying gas in the heating chamber it is provided with a feed and discharge line for the drying gas, with valves being provided in the feed and discharge lines for regulating the exchange routes. These known apparatuses comprise only one zone however, so that they can hardly be used to achieve even wood humidities after a drying process when the treatment chamber is charged with wood batches of different moisture.
The goal in the known drying methods is always providing the drying power in a treatment chamber as evenly as possible over the entire cross section. As a result of the even drying power in the individual zones it is always noticed that the final moisture is not within the desired narrow margin. Such differences in final moisture are especially clear when the wood is dried so as to achieve relatively high final moisture values. This leads to the consequence that a batch of wood simultaneously contains considerable shares of wood that is insufficiently dried and wood that is too humid, which leads to a considerable share of rejects.
In the case of an even distribution of the mean initial humidities per zone, a favorable, even final moisture of the wood batch is achieved with the known method. However, if the initial moisture distribution of the individual zones is different, this distribution will have a direct influence on the final result due to the desired even drying power. Differences in initial moisture can be felt in particular which are present in the longitudinal direction of the chamber because there is no considerable exchange of air transversally to the direction of flow of the circulated drying gas in the treatment chamber.
It is known from DE 19522028 A1 to check the drying by means of individual wood moisture measuring points within each zone. This requires a plurality of measuring points which on the one hand require a high amount of investments and maintenance in practice and on the other hand are time-consuming in mounting them and finally lead to a far from inconsiderable risk concerning the correct assignment of the measuring points to the individual zones. These methods are hardly used in practice for such reasons and because the known wood moisture sensors supply especially imprecise measurement results in the region of high wood humidities.
A connection that can be formulated mathematically between the respective moisture content and the drying rate can only be obtained in wood, and such of coniferous trees in particular, below the fiber saturation point. The fiber saturation point represents the moisture content below which no free water is any longer contained in the wood. Since above the fiber saturation point the drying rate is substantially only dependent on the supplied heat quantity, but not on the moisture content, a moisture balance can only be produced in a moisture region between the fiber saturation point and the desired final moisture according to a connection that can be formulated mathematically, i.e. in a comparably low moisture range of 12 to 25%, assuming an average fiber saturation point of 30 to 35% and a final moisture of 10 to 18%. The usual wood humidities at the beginning of the drying process lie in the range of between 50% and 150%. The differences in the moisture content that still exist after the drying above the fiber saturation point would thus have to be compensated in relatively short intervals, which would lead to extremely small drying rates and, consequently, small temporal temperature differences with the disadvantage of relatively high measurement errors.
SUMMARY OF THE INVENTION
The invention is therefore based on the object of providing a method of the kind mentioned above in such a way that a substantial moisture balance can be ensured in comparatively short time intervals, namely by maintaining a predetermined final moisture. Moreover, an apparatus is to be created with which it is possible to stack wood of different initial moisture without prior sorting into a treatment chamber and still achieve an even mean final moisture of the wood batch.
This object is achieved in accordance with the invention in such a way that the stacks of wood are dried in the zone with the on average highest wood moisture with a permissible maximum speed up to the fiber saturation point and the other zones are dried within the terms of achieving the fiber saturation point with different speeds in the time interval as predetermined by the zone with most moisture before the stacks of wood are dried in an even circulatory flow to the predetermined final moisture, and that the average moisture of the wood used for determining the drying speeds is determined in a heating phase from the heat quantity supplied to the wood.
A simple method is created by the invention which allows balancing average differences in moisture in the individual zones above the fiber saturation point, i.e. at a time at which there is still an at least approximately linear drying behavior of the wood. The zone in which the on average largest wood moisture is present is dried with a still permissible speed up to the fiber saturation point, at which speed there is still no damage to the wood. The drying speed in the other zones is reduced relatively to this in such a way that each zone achieves the fiber saturation point at approximately the same time. According to the invention it is relevant that the mean
Collard & Roe P.C.
Lazarus Ira S.
O'Malley Kathryn S.
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