About drying of saw-timbers for bearing wooden kleenyih designs

There is an opinion, that quality bearing KDK depends in certain degree on disorder of humidity of separate boards in a design.
In the report there is a speech about the reasons defining non-uniformity of final humidity of saw-timbers on volume of a stack (or chambers) at konvektivnoy to their drying in chambers of periodic action; about the actions directed on reduction of this non-uniformity, and about the standard documents regulating deviations of humidity of separate boards from the average final.
The first reason- non-uniform distribution of a moisture both on radius, and on tree trunk height. At coniferous breeds (a pine, a fur-tree) humidity zaboloni in 3-4 times above humidity of a nucleus (ripe wood). The second reason - anisotropy of properties of wood. In particular density which defines it vlagoprovodnost. Change of conditional density about 300 kg/m3 to 500 kg/m3 leads to factor change vlagoprovodnosti in 5 times [2].
The third reason - the factors connected with the organisation of circulation of the agent of drying in the chamber. It: non-uniformity of circulation in a stack, size of speed of the agent of drying, character of circulation (absence of a backspacing). Uniformity of circulation is a uniform distribution of the agent of drying on length and stack height, and also, connected with it, size of operating ratio of a stream. Research of aerodynamic characteristics of the basic types sushilnyih chambers of periodic action [3] has shown unsatisfactory uniformity of circulation of the agent of drying that is characterised by a wide scatter of speed on stack volume (1,1 - 3,5 km/s); in significant size of a root-mean-square deviation and in the big factor of a variation (to 86 %), and also low operating ratio of a stream (to 0,3 and more low). Recommendations about improvement of uniformity of circulation of the agent of drying about a stack are given in [3], [4]. The size of speed of drying in a stack defines uniformity prosyihaniya last on its width. I.V.Krechetov [4] has shown, that the above speed on an input in a stack, the more low difference of humidity of saw-timbers on its width. So at Wk = 8 %, width of a stack and absence of a backspacing humidity difference ΔW makes of 1,8 m:

Vts-1.0-1.5-2.0-4.0;

ΔW %-7.0-4.0-3.0-1.3;

Wk %-2.0-15-8-6;

ΔW % 4;

Decrease in final humidity at constant speed leads to reduction of difference of humidity. At W=2,0m/with ΔW has significances depending on the final humidity, resulted above. In the presence of a backspacing humidity difference decreases depending on duration of a cycle and width of a stack approximately in 1,5-2,0 times.

Humidity of % The majority of areas
THE USA The dry southwest
Area —

For operation in premises 8/6-10 6/4-9 11/8-13

For operation on open air 12/9-14 9/7-12 12/9-14

From here follows, that the increase in a dimension of a stack on height and on width essentially worsens uniformity of final humidity and requires realisation of additional actions for reception of positive results. The optimum size of speed accepted in domestic and foreign practice, makes, 2,0-3,5 km/s (at the first stage of process of drying). However significance of speed of the agent of drying should be co-ordinated to size of an optimum saturation of air φopt which defines safety of process and availability of internal pressure in a material. [5] with increase in speed the agent of drying in a stack φopt also it is necessary to increase.
The saw-timbers intended for bearing KDK, after the termination of drying and vlagoteploobrabotki should be exposed to air-conditioning. Modes and duration of air-conditioning are resulted in [6]. Duration of air-conditioning depends on many factors. Roughly it is equal to half of duration vlagoteploobrabotki. However it is necessary to correct this size according to real conditions of drying and use of saw-timbers.
Important correctly to establish the moment of the termination of drying, i.e. achievement of the set average final humidity. Nedosushka and redrying of saw-timbers reduce efficiency of conditioning processing.
Uniformity of final humidity of saw-timbers is rationed RTM [6]. For I category of quality of drying (on this category quality of drying of saw-timbers for bearing KDK is evaluated) at final humidity
ΔWk = 7,0-10,0 % of a deviation of humidity of separate boards from average should be no more + 2,0 %. At work KDK on open air uniformity of final humidity is not rationed. Labora -
Thorium of wood products of the USA [7] recommends following significances of final humidity of wood and permissible deviations from average humidity of saw-timbers for «layered constructional materials» depending on their use conditions.
In numerator - average humidity, in a denominator - deviations of humidity of separate boards from average. It is specified [7], that «among the layered elements collected in one design, and +5 %» should not exceed fluctuation of humidity.
Practical experience of drying in Russia and abroad shows, that humidity fluctuation in saw-timbers in a stack at average final 6-8 % cannot be less than 2,0 %-3,0 %.
To extend process of air-conditioning of badly dried saw-timbers inefficiently, considering, that the hysteresis of stable humidity sorbtsii and desorbtsii makes 2,5 % [2].

The literature:

  • B.N.Ugolev, Drevesinovedenie with bases wood tovarovedeniya, «the Wood industry», 1975.
  • P.S.Sergovsky, Hydrothermal processing and wood conservation. «The wood industry». 1975.
  • I.M.Merkushev, Aerodynamic characteristics of the basic types lesosushilnyih chambers of periodic action, «Derevoobrabatyivayuschaya the industry», # 2, 1973.
  • I.V.Krechetov, wood Drying, "Breeze", 1997.
  • P.S.Sergovsky, B.N.Ugolev, N.V.Skuratov, Internal pressure and modes of drying of wood, sb. «Actual directions of development of drying of wood», Arkhangelsk, 1980.
  • Open Societies "Nauchdrevprom" TSNIIMOD Supervising and technical materials on technology of chamber drying of saw-timbers, Arkhangelsk, 2000.
  • Laboratory of wood products of the USA, Help management on wood, «the Wood industry», 1979.

L.A.TETERIN,
The Russian engineering academy
 Source: "the Building Expert" #18

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