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Volume: 1996/3


Title: Establishment of specific drying energy of oak-wood flooring deal and thermodynamic model of drying
Author: Miljenko Primorac, Stjepan Risović

Total heat consumption per unit of drying evaporation in spring season Cu = 119 MJ/kg has been established by measurement of heat consumption and computation for oak-wood (Quercus robur & petraea) flooring deal 25 mm thick in kiln drying chamber (166.685 m3 of green lumber). The energy consumption per unit of drying evaporation without dissipation is the specific energy of oak-wood flooring deal that for this case amounts to C = 8.6MJ/kg. That total electrical power consumption per unit of drying evaporation has been established in the amount of Cel =0.355 kWh/kg. The above mentioned values are average values through the whole drying cycles. The average inital value of the moisture content was 30% and the final average value of the moisture content was 6.5%. The measurement error of the average result values was around 12%. The cycles were managed by the usual drying proces. The established specific drying energy is about 3.5 times greater than the evaporation heat of pure water. Therefore, the general theoretic model has been introduced for explanation of measuring results. This model is a wet air irreversible heat engine considering its thermodynamic sense. The results of such an introduced model are shown as examples in two tables. These results indicate that such a model can be used for a wood drying simulation process with the optional regimes in a technological, energetic and timing sense. The parameter of heat consumption can be an important indication in managing the drying process. Specific drying energy is a function of wood moisture in a determined climate as well as a function of the structural openings from which the water evaporates. The introduced model connects those two parameters.

Title: Investigation into the changes of wood surface layer by measurement of pH-value
Author: Boris Ljuljka, Vladimir Sertić, Ivica Grbac, Vlatka Jirouš-Rajković

Wood pH has been studied with regard to its affecting some change on wood surface. The change on wood surface was analyzed after the following: - chemical treatment; - aging in room climate;- exposure to the sun, rain and surrounding atmosphere; - exposure to ultraviolet radation. Each treatment was followed by measurements of the pH and the adhesion of the NC varnish. The procedure of pH measurement on the wood surface with a combined surface electrode was devised. A 25 mm circle was drawn on the wood surface with NC varnish to prevent water spreading along the fibers. Into the circle 0,75 ml of water was applied, the electrode plunged in. After 5 minutes, the pH was measured. Within this time, no negative effects of carbon dioxide on the water pH were established. The research was done on fir, beech, oak and poplar. After chemical treatment, the pH on the wood surface varied depending on the chemical used but some general conclusions about the effects of pH on the followmg adhesion may be drawn. Summer sunlight quickly reduces the adhesion of varnish on beechwood, increasing the pH of the wood surface. Sunlight, and in combination with the rain in February, lasting 10 days, does not affect adhesion significantly. Instead, the surrounding atmosphere seems to have a greater influence in this time period. Varnish adhesion may be enhanced with a certain dose of UV-light. It is likely that maximum adhesion is achieved by the highest pH. Wood pH should be regarded as an indicator of chemical changes rather than cause of change.

Title: Methods for Testing the Resistance of Wood Hotmelt Adhesives to Temperature Changes and Weathering
Author: Andrija Bogner, Boris Ljuljka, Ivica Grbac

The paper presents a research on various types of hotmelt adhesives with regard to their temperature resistance and durability. The tested adhesives were based on ethylene/vinyl acetate (EVA); polyamide (PA); polyurethane (PU); and polyolephine (PO), while the adherents were solid beech, 5 mm thick (T-shaped samples); beech veneer, 0.6 mm; PVC strip, 3 mm; paper strip, 0.6 mm; HPL laminate strip, 0.9 mm. The adhesives were comparatively tested with various testing methods for their resistance to weather and temperature change. All adhesives exhibited different characteristics under temperature changes; prolonged higher temperature; and cyclic climatic changes. The aim of the research was to study the bond strength of various edging materials glued with four different hotmelts, with joints exposed to different climatic conditions, and to establish the most suitable methods for the bond strength assessment.

Title: Optimisation of pneumatic conveyors working conditions
Author: Vlado Goglia, Ankica Kos-Pervan

Since the pneumatic conveyors are great users of electricity in woodworking companies, especially in Croatia, there are some possibilities for improving energy consumption, by means of controlling the fan rpm. During the work of the a conveyor, the changing of air pressure and velocity will be registered by means of the measuring device. The incoming value of the voltage will be converged in regulating the voltage of the frequency converter. In that way, by changing the rpm of the electric motor, it is possible to control the value of the necessary air power.





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