INVESTIGATION OF RELATION BETWEEN MOISTURE CONTENT DECREASE AND WATER MASS FLOW REMOVED FROM A KILN

Henryk Widłak, Jerzy Majka

ABSTRACT

Currently, the basic way of wood moisture content decrease control during drying process is measurement of that moisture content with resistance meters on a random selection of timber items. The results of that measurements are particularly unreliable, especially at wood moisture content below fibre saturation point and, on the other hand, they are influenced by accuracy of sample’s choice. Research considered the problem of monitoring of water mass flow that is removed with air in order to control the course of drying process. As a results of experiments, during drying of pinewood timber, it was stated that the monitoring of outlet air state allows for the verification of measurements of moisture content with resistance meters. The monitoring of air state at inlet and outlet of a kiln creates the possibilities of a more complex control of wood drying process.

Key words: drying, wood moisture content decrease, water mass flow, outlet air volume flow.

INTRODUCTION

The state of air as a drying medium is characterised by such parameters as: temperature, humidity and speed. They determine the air drying ability i.e.: the ability to transfer thermal energy and pick up water vapour. The accurate choice of values of air parameters, i.e. process parameters, depending on wood features, is a the most important factor during drying. The air drying ability influences the speed of water pick-up from the surface of a material being dried. That speed must remain in a strict relation with a water mass stream that flows at the timber’s cross section from the deeper layers towards the surface. The speed of water removal from wood surface, i.e.: water mass stream taken by drying air, is a drying speed, that results in drying quality.

Currently the basic way of drying process monitoring is a determination of timber’s moisture content with resistance meters. The results of measurements of wood with such method are particularly unreliable. Apart form the errors connected with a limited random sample size, these results are subjected to deviations that come from the characteristic of function that describes the relation between resistance of wood (below fibres saturation point) and moisture content. Depending of the placement of probes in case of wood of moisture content above 30% the arbitrary error of measurements may reach 20% [7]. The need for the universal drying process monitoring method, that would even eliminate the necessity of knowledge and respecting of drying schedule, resulted in investigation with the use of optimisation methods [1]. The results of that papers are still unsatisfactory due to many simplifying assumption connected with a selected model of timber’s drying [3], and resistance meters measurements o f wood moisture content. Therefore, currently in the first stage of drying process, its control is only approximate. The rational process flow in that period, i.e.: its precision control, are especially important, because most drying defects are results of improper course in this phase of the process. It is worth mentioning that in the face of the risk connected with described problems in programs of computer-controlled kilns there is tendency to excessively lowering of drying speed and increasing of drying time in a preliminary stage. A prove for that are remarks from kiln users [2].

The need and the possibility of analysis of moisture mass stream taken by drying in order to correct a drying process was expressed in literature yet in nineties e.g.: Widłak [8]. At the time an opinion was also formulated that the value of moisture mass stream taken by air may be a better indicator of moisture content decrease than mentioned measurements with resistance method. In further papers concerning that problem e.g.: Widłak [9, 10] it was proved that moisture mass stream taken by drying air determined threw consideration of chemical potential of water vapour in air [5, 6] may be successfully used for the evaluation of effectiveness of drying process. Therefore it seems possible, that in case of contemporary kilns, that are produced with essential solitude about tightness, the monitoring of water mass flow removed with the outlet air may ensure the more precise control of drying process.

METHODS AND EXPERIMENTS

In the investigation a procedure that relays on measurements of air parameters at the inlet and the outlet of a kiln and on controlling of volume stream of air removed from the kiln was used. At this time a moisture content measurements were carried out with the resistance meters on three timber items selected at random. Investigation were performed in a double-segmented kiln DQKC-140 equipped with a controller that allowed for the supervision of the process with a computer. In the outlet vent of the kiln a fan was installed with the electric motor powered with frequency converter that allowed the adjustment of air volume stream removed from kiln. Moreover the outlet of the kiln an inspection converter was installed in order to carry out measurements of humidity and temperature of air.

The parameters of inlet air were determined using a psychrometer, that, like other devices, was connected to the computer. In research a processes of drying of 25 and 29 mm thick dressed timber of various moisture content were carried out.

In drying processes that were controlled threw the adjustment of outlet flow of air volume, a previously described by Widłak and Olek [11] procedure was performed; and in order to determine the air flow removed by the fan the following relation was used:

	- air flow removed from the kiln, m3/h,
	- water mass flow, that should be removed from the kiln, kg/h,
	- specific volume of moist air per unit mass of dry air and water vapor [4], m3/kg,

From practical reasons (small kiln’s volume) in the research a procedure was assumed, that based on periodic opening of a throttle in kiln’s outlet (also inlet) vent and starting of the exhausting fan. Three different periods between succeeding throttle openings were assumed: 5 minutes, 10 min and 15 min.

RESULTS

In order to determine the values of observed and analysed parameters some necessary calculations were carried out; the result of which are presented in Tables 1 and 2 and additionally in Figures 1 and 2 (for pinewood).

Tables 1 and 2 present the results of analysis that concern the drying of pine beech and oak timber of various initial moisture content, of thickness 25, 32 and 45 mm, placed on distance sticks 20 or 25 mm thick in used in the research drying kiln DQKC-140 that is capable to contain timber stack of gabarite dimensions of B = 1.4 m, H = 1.7 m and L = 2.8 m.

The analysis employed the method of general balances that relays on division of the total drying time on periods at which moisture content decrease 10%. Then, for each period the mean drying intensity was calculated for the centre of each wood moisture content range. The results of the analysis evidence the intuitively expected rule that characterises the timber’s drying. The highest values of water mass flow are observed at the preliminary stage of drying of the thinnest pine timber. The value air volume flow needed to remove the water picked up from the timber in the range of moisture content 70…60% amounts to 102.7 m³/h. In the last stage, in the range of moisture content of 20…10%, the air volume flow is 75.4 m³/h. It is interesting that in case of the thinnest oak timber analogical values of air flow that is removed from the kiln should be 130.5 m³/h and 49.9 m³/h, i.e. in the first stage this flow is higher that in case of pinewood, though when drying oak the water mass flow is near twice as lower. It is a result of the variety of process parameters that were applied when drying pine and oak timber and also suggests the need of particular attention in case of hard wood drying using the adjustment system based on water mass flow picked up by air.

In the experiments a moisture content decrease of pinewood 25mm thick placed on distance sticks of thickness 25mm was observed. Figures 3 and 4 presents two 8-hour fragments of one of drying processes that illustrate wood moisture content decrease measured with the resistance meters as well as determined basing on parameters of outlet air.

The first fragment that includes the moisture content decrease from 24.3% to 22.8% shows the variety of wood moisture content measured with resistance method. That variety, characteristic to that method amounted from 30.3% to 28.8% at the beginning of observed fragment and from 19.0% to 28.8% at the end of it. This variety is a result of moisture content differences between 4 timber items that measuring electrodes were placed on. Drying curve obtained from these measurements includes moisture content decrease from 24.3% to 22.8 at the end of the section. Almost the same wood moisture content decrease was calculated basing on the measurements of air humidity at the inlet and the outlet of the kiln. The second illustrated fragment of the process present drying curves which were determined with both methods and include the mean wood moisture content decrease from 18.8% to 17.4% (17.9%). In this case the calculated wood moisture content was lower than the mean value that resulted from measureme nt with the resistance method. Presented observations do not allow for the formulation of final conclusions that determine the effects of use and possible superiority of the method of air humidity decrease control at the inlet and the outlet of the kiln. The conclusions will be possible after equipping the kiln with a controller that will realise the elaborated technique of drying process adjustment. Basing on above discussed remarks it can be stated that the control of outlet air state allows for the verification of the result of measurements of wood moisture content with a resistance method. This verification concerns both the propriety of the measurements itself and the accuracy of timber item selection for measurement electrode placement. Basing as an example on discussed above results it can be stated that in both ceases electrodes were placed on timber items that were representative of the whole group of dried material. It is worth also paying attention to other circumstances that prove the purposefulne ss and the signification of discussed verification. These circumstances are: elimination of some measurements probes and also, in case of excessive moisture content differences, the process' control basing on the highest measurements. The result of it is an impossible to avoid process lengthening and so called overdrying of a part of the material.

ACKNOWLEDGEMENTS

This study is a part of researches that was supported by the State Committee for Scientific Research, as research project 3P06L00122.

REFERENCES

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