The Effect of Moisture Content of Black Locust Wood on the Heating in the Saturated Water Steam during the Process of Colour Modification

The effect of moisture content of black locust wood on the heating of sawn timber with the thickness of h = 32 mm in saturated water steam during the processes of colour modifications by modes: mode I – temperature of saturated water steam t = 112 ± 2.5 °C for τ = 5.5 hours, mode II – temperature of saturated water steam t = 127 ± 2.5 °C for τ = 6.5 hours, and mode III – temperature of saturated water steam t = 137 ± 2.5 °C for τ = 7.5 hours is analysed in the paper. The increase in moisture content affects the heating time, it lasts longer. When the moisture content of black locust wood ranges from W = 60 – 65 %, the heating time by modes I – III did not last longer than the time ∆τ ≤ 0.5 min. and no negative impact on the final colour modification is observed.


Introduction
Wood placed in the environment of hot water, saturated water steam or saturated humid air is getting warmer and its physical, mechanical and chemical properties changes.Mentioned facts come in useful in the technology of wood steaming and boiling used in the producing veneers and plywood, bent furniture or pressed wood.Processes of wood heat treatment in saturated water steam result not only in changes of physical, mechanical and chemical wood properties but also in changes of colour.In the past, colour modification, especially wood darkening, was used to remove undesirable differences in colour of lighter sapwood and darker heartwood, or to remove wood stains resulting from steaming or moulding.Recently, the issue of colour modification of various wood species has become of great importance [1][2][3][4][5][6].
Firstly, the wood is heated to the required temperature in the shortest time.Initial moisture content of modified wood must be taken into account during manufacturing processes.Various moisture contents of analysed wood species can affect physical properties of wood, such as density, heat capacity, thermal conductivity.Process and the length of non-stationary temperature distribution during heating are affected by mentioned physical properties directly.
The aim of the paper is to analyse the effect of initial moisture content of black locust wood ranged from w = 40 -100 % on the heating time of sawn timber with the thickness of h = 32 mm in saturated water steam during the processes of colour modification by modes: mode I -temperature of saturated water steam t = 112 ± 2.5 °C for τ = 5.5 hours, mode IItemperature of saturated water steam t = 127 ± 2.5 °C, for τ = 6.5 hours, and mode IIItemperature of saturated water steam t = 137 ± 2.5 °C for τ = 7.5 hours.

Model to Calculate the Heating Time of Sawn Timber in Saturated Water Steam
Sawn timber heating by saturated water steam in an autoclave is described by Fourier's equation of heat conduction, one-dimensional, steady-state conduction, i.e. amount of heat transmitted under the given temperature and boundary conditions.

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Lykov [7] published a nomogram of the relation θ = f(x/b, Fo) to determine dimensionless heat θ of the infinite long and wide sawn timber b << y<< z for wood heating at the high value of heat transfer coefficient α→∞.Heating of both surfaces x/b = 1 is as follows: Time necessary for heating the sawn timber can be defined following the Fourier number by the equation: Fo is Fourier number, b is 1/2 of the thickness of a board, a is a heat transfer coefficient.

Technological Parameters of Wood Heating by Saturated Water Steam
Heating time of black locust wood with the thickness of h = 32 mm in saturated water steam by individual modes of colour modification process is analysed for initial values of moisture content ranged: W = 40 ÷ 100 %.Physical properties of heated black locust wood: wood density ρw, specific wood heat capacity cw, heat transfer coefficient perpendicular to the fibres in tangential direction λwꓕ and thermal conductivity of wood aw for individual moisture content W and the temperature calculated tvyp are shown in Table 1.Temperature calculated in the process of non-stationary temperature distribution during heating by individual modes of colour modification are determined using the equation: t0 is a temperature of sawn timber at the beginning of heating, tx temperature of heated sawn timber in the 1/2 of thickness, tp is a temperature on the sawn timber surface.
Temperature in the centre of the sawn timber thickness tx is lower ∆t = 2.5 °C in comparison to the temperature on the sawn timber surface.tx = tp -2.5 (5)   the temperature of t´´ = 127.5 °C by mode II is τ = 39.85 min and in saturated water steam with the temperature of t´´ = 137.5 °C by mode III is τ = 40.57min.When the moisture content of black locust wood ranges from W = 60 -65 % the heating time by any mode of colour modification did not prolong the time ∆τ ≤ 0.5 min and did not have a negative impact on colour modification.
This paper was prepared within the grant project: VEGA-SR No: 1/0563/16, as the result of work of author and the considerable assistance of the VEGA-SR agency.
Time of non-stationary temperature distribution during heating of black locust sawn timber with the thickness of h = 32 mm in saturated water steam by individual modes of colour modification is illustrated in Figure1.

Fig. 1 .
Fig. 1.Heating time of black locust sawn timber depending upon the moisture content of wood and mode of colour modification.

Table 2 .
Temperatures of sawn timber during the process of heat treatment.