Maderas. Ciencia y tecnología. 4(1):15-25, 2002

ARTICULO

FUNDAMENTAL PHENOMENA IN WOOD RFV DRYING WITH 50-Ohm AMPLIFIER TECHNOLOGY

Anastasios Koumoutsakos¹, Stavros Avramidis¹ and Savvas G. Hatzikiriakos^2
1Department of Wood Science, The University of British Columbia, Vancouver, BC, V6T 1Z4, Canada.
²Department of Chemical and Biological EngineeringThe University of British Columbia, Vancouver, BC, V6T 1Z4, Canada

Corresponding author: stavros@interchange.ubc.ca

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ABSTRACT

Radio frequency/vacuum drying (RFV) experiments were conducted on thick (200-250 mm) softwoods by using a 50-Ohm amplifier as the radio frequency (RF) source. The power densities selected were low to ensure highest quality of the final product and that no internal checking will occur. The effect of dryer pressures and power densities on the drying kinetics and product quality was examined for two different wood species. A number of experimental observations and drying characteristics are analyzed and discussed in detail. The concept of the identity drying card is applied and useful information concerning the drying mechanisms and the relevant phase transitions is obtained.

Key Words: identity-drying card, transport mechanisms, permeability, western hemlock, western red cedar, field uniformity

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INTRODUCTION

In dielectric heating and drying 50-Ohm systems have been characterized as the next step forward (Jones, 1996). These are based on a fixed frequency where a crystal oscillator amplifies the required power in several stages. Typically the first stage, up to a few kilowatts, is done by solid-state devices and thereafter by a thermionic valve circuit. Such an arrangement allows much better control of generation, transmission and application. It brings to RF heating the opportunities offered by the separation of the generators from the process environment, which up to now have only been possible at microwave ovens.

Because of the fixed frequency of the signal, it is quite straightforward to ensure that the equilibrium moisture content (M_e) regulations on stability are met at the more tightly controlled frequencies (f) of 13.56 and 40.68MHz that every so often have process advantages over the more usual 27.12MHz option. Furthermore, since the output is controlled from the crystal driver and every section of the system is made to look like 50Ohms, it is not necessary to use variable inductors or moving electrodes as a means of energy transfer control. Once the latter is fixed, it becomes possible to set up a truly uniform electric field over the whole of a large area electrode with implications on the temperature distribution within a product.

A number of experiments were conducted using a 50-Ohm technology amplifier. This is a first in obtaining RFV drying data where the power absorbed by the material can be controlled and quantified very accurately. Another unique aspect of these experiments is that they were conducted in low power densities in order to ensure that no internal or surface checking will occur.

When modeling with a constant permeability factor throughout the piece, the above is crucial since internal checking can increase the local permeability by orders of magnitude. Since permeability has such a central role in modeling drying behavior of porous materials, meaningful comparisons of experimental and simulation results can only be obtained if such inaccuracies are avoided. This type of degrade can only be monitored at the end of a drying experiment. Modeling involving the increase of permeability due to internal cracks is possible with the use of optimization parameters and functions. A direct consequence of such a choice will be the significant increase of the simulation times in a case where the permeability function is utilized as an optimization factor in order to fit the experimental data. The complexity to locate the proper time points when a number of internal checks develop increases considerably with their number. Increase in the duration of the drying due to low power densities is then well justified.

It has to be reminded here that there are two possible reasons that cause internal checking. One is due to non-uniform shrinking below the fiber saturation point (FSP) and the other is due to extremely rapid local heating and vaporization. Given that wood has low permeability, the internal pressures that evolve can cause fracture of the cell wall. Because local moisture contents during a drying run are not measured, there is no means to clarify which is the mechanism responsible for each specific case since these phenomena cannot be quantified.

MATERIALS AND METHODS

The drying experiments were carried out in a laboratory RFV dryer. A detailed description of the RFV dryer can be found in Koumoutsakos et al. (2001b) and the literature therein. A major difference in the configuration of the kiln is the aluminum extension that is attached to the lower ground plate so that uniformity of the field to be achieved. This arrangement can be observed in Fig.1. A compression loading system on the top electrode capable of supplying a maximum of 11kN onto the wood was also added.

Figure 1. New electrode plate configuration of the laboratory RFV dryer.

Specimens of green western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata), were dried in a series of experimental runs. Two experiments were conducted with hemlock that had an initial average moisture content (M_i) very close to FSP. Another eight experiments covered both regions above and below FSP where four pieces were cedar and the rest were hemlock. The frequencies during runs #5 and #6 were 8.4 and 8.5MHz, respectively. The rest of the experiments were carried out with a fixed frequency of 6.78 MHz. Table 1 lists the conditions of ambient pressure (P_a), power density (PD), initial and final moisture contents (M_f), drying time, and maximum temperature (T_max) for all the experiments.

All specimens were 2m long with cross sections of 200mm by 200mm except in the case of the last two hemlock runs where the specimens were 250mm by 250mm in cross section. Each piece of green timber came at about 3m in length from a local sawmill. They were immediately weighted and wrapped with a plastic cover and stored in a cold room (10^oC) until it was time to be dried. The loss of weight during storage was less than 1% in all cases. Before drying, end sections of about 300mm were removed and then, samples (cookies), each 30mm thick were cut for the gravimetric calculation of M_i.

Four fiber optic temperature sensors, monitored the temperature rise inside the heated specimens and in the surrounding partial pressure environment. Vapor pressures were measured by 2.5mm diameter teflon tubes connected to electronic pressure transducers located outside the dryer (details in Koumoutsakos et al., 2001b). Three different locations inside each sample were chosen for temperature and pressure monitoring. The first was in the center of the sample; the second on the central longitudinal line, but in quarter length from one end; and the third close to the center, but in quarter width. This way, temperature and pressure data could be obtained for both longitudinal and transverse directions. At the end of each run, the M_f distribution within each dried specimen was measured gravimetrically by slicing and oven drying sections at various locations along its length.

Table 1. Experimental conditions and data for RFV drying runs.

WESTERN HEMLOCK
RUN	Time	Mi	Mf	Tmax	Pa	Voltage	Target PD
#	hrs	%	%	C	kPa	V	(kW/m3)
1	201.16	63.32	10.39	62	3.3	0.04-0.18	1.1
2	97.33	68.11	10.64	61	3.3	0.03-0.16	2.2
3	500.00	51.22	14.17	57	13.3	0.04-0.13	1.1
4	88.33	42.40	15.43	65	13.3	0.05-0.2	2.2
5	57.75	32.93	10.56	50	3.3	0.08-0.35	2.4
6	68.94	29.84	12.79	69	13.3	0.09-0.42	2.4
WESTERN RED CEDAR
7	155	51.78	14.95	88	3.3	0.05-0.15	2.2
8	220	54.98	14.88	70	3.3	0.04-0.17	1.1
9	195	55.44	14.76	98	13.3	0.05-0.26	2.2
10	340	66.24	15.12	91	13.3	0.04-0.20	1.1

 

RESULTS AND DISCUSSION

At first, it can be observed from Table 1 that the timber T_max for the same PD level is linearly related to M_i. This is expected since the power absorbed is proportional to the moisture content (Koumoutsakos et al., 2001a). Another important factor affecting T_max is P_a. For similar M_i, as in hemlock runs #5 and #6, T_max is reached when P_a is higher. This is a strong indication that moisture transport occurs mainly in the vapor phase. Lower T_max means that lower boundary temperatures are sufficient for vaporization. Assuming that the temperature and pressure gradients are similar for the same internal mass transfer, due to the volumetric nature of the heating the T_max that usually develops in the center will be lower when P_a is lower. It has to be noted here that T_max might not appear in the geometric center of the drying timber if the M_i profile is extremely non-uniform with high local moisture gradients and at the same time, the heating is very rapid due to high PD levels.

Comparing the drying times for both species we observe that the hemlock timbers dry faster though their M_i levels are higher than those of cedar. The latter were almost pure heartwood, whereas hemlock was mostly sapwood. A reasonable explanation can then be found if the relevant properties of these species are examined where the permeability of hemlock sapwood is higher than that of cedar heartwood (Koumoutsakos and Avramidis, 2002).

The ranges of voltage (V) between the plates, as presented in Table 1, provide some further insight. The lower V values are the initial heating period ones and during this period the total moisture was assumed constant. The same is true for PD, f and the distance between the electrode plates. Examination of the relation between PD and the loss factor (e² ), as was given by Eq. 12 in Koumoutsakos et al. (2001a), indicates that e ² should increase to counterbalance this decrease in V during the heating stage. In other words, temperature increases yield an amplification of e² which is consistent with the literature (Torgovnikov, 1993; Zhou and Avramidis, 1999). The V_max values are obtained at the end of the runs when the total moisture content is the lowest. The field frequency in runs #5 and #6 was the highest (about 25%), but the timber dimensions and therefore the distance between the electrode plates was also 25% higher. The effect of the plate distance change is of second order and the effect of f change is of first order, as it is evident by Eq. 12 in Koumoutsakos et al. (2001a). This is why runs #5 and #6 exhibit higher V_max values compared to the #1, #2, #3 and #4 ones. The effect of temperature through e ² is negligible because of its low value at the end of the drying runs.

Figures 2 and 3 display drying curves of hemlock and cedar runs at different T_a and PD levels. Assuming that M_i is 42%, FSP is about 25% and M_f is 15% for hemlock, we observe that the drying time increased 3 times and 6 times, above and below FSP, respectively, when PD is reduced by half at P_a of 13.3kPa. At P_a of 3.3kPa, the drying time increased 3 times in both areas above and below FSP. Above and below FSP, an increase in the drying time by 2 and 2.5, respectively is observed, when P_a increases 4 times and PD is equal to 2.2kW/m³. At PD of 2.2kW/m³ the drying increases by 2.5 and 5 times above and below FSP, respectively, when P_a increases 4 times.

Figure 2. Drying curves of Western hemlock at different power densities and ambient pressures.

Figure 3. Drying curves of Western red cedar at different power densities and ambient pressures.

At the moisture range of 25-15% another interesting comparison can be made between #5 and #2, and between #6 and #4 specimens (the latter having 25% larger thickness and width and dried at 10% higher PD, but at similar ambient pressures). At P_a of 3.3kPa (experiments #2 and #5), the drying time was by 50% longer for the case (#5) dried under higher PD though the piece was thicker as compared to the case (#2) where the piece was thinner but was dried in lower PD. At P_a of 13.3kPa, the drying time was longer by 10% for the case (#4) dried under lower PD though this piece was thinner as compared to the case (#6) where the piece was thicker but was dried in higher PD. Assuming M_i = 52%, FSP ≈ 25% and M_f = 15% for cedar, the drying time increased by 100% and 30% above and below FSP, respectively, when the PD decreased by half at P_a of 13.3kPa. At P_a of 3.3kPa, the respective increases were only 20% and 30%. Comparing the runs at similar PD, we observe an increase in the drying time by 20% below FSP, when the ambient pressure increases four times and similarly, above FSP the drying time increases by 10% and 50% at the high and low PD levels, respectively.

The above results are indications of the trends that arise during a drying run. More specifically, there are cases where the effect of PD change is stronger than the effect of changing P_a on drying time. However, there is a maximum limit of PD due to internal checking (honeycomb) and significant degrade of the product. There is also a limit to the reduction of P_a, which is related to the ability and costs of achieving and maintaining such low levels in industrial size dryers. It is believed that the above observations can provide a direction to the levels of PD and P_a that have to be investigated in the future. Unfortunately, these results cannot be generalized due to lack of repetitions and the short range of this investigation. The total power per mass of water removed can also be an important parameter to optimize future RFV drying schedules of wood with the 50-Ohm technology.

Cross sections (cookies) were taken in the middle and quarter-points along all dried timber specimens. No internal checking (honeycomb and shake) was observed for both cedar and hemlock even at both low and high PD levels. However, some superficial end-checking was observed in the form of honeycomb and shake. The end-checks were actually observed to develop at the very early stages of the drying when the total average moisture content was well above the FSP. No top and side surface checks were observed. A possible explanation for the appearance of the end-checks will be given. Lowering P_a and heating the pieces volumetrically leads to development of local internal pressure gradients. The stresses that are thus developed have similar magnitude towards all directions. The strength of the cell wall is higher in the longitudinal direction so cracks appear in the weaker directions, i.e., radial and tangential. These are evident at the surface that is perpendicular to the longitudinal direction, which is the end surface. The reason that only end-checking and no internal checking is observed, is that the magnitude of the pressure gradient is much greater in the ends in the case of vacuum drying. Also, the mild drying conditions due to lower PD made it possible so any internal pressure gradients that could develop due to possible unevenness in the moisture profile, to remain low due to extended time for internal moisture transport and redistribution. Another possible explanation is that the small surface layers reach moisture contents below the FSP rapidly due to vacuum. These layers being in a dry environment on the side of the vacuum below FSP begin to shrink so that strong drying stresses evolve leading in cracks. The concept of the “transition layer” under vacuum drying has already been proposed in the literature (Sebastian et al., 1996), and completely agrees with this explanation.

An additional important observation is that due to the configuration of the dryer with the lower plate grounded; there is a temperature gradient between the plates. This was never the case with the old oscillator (Koumoutsakos et al., 2001b), where none of the plates was ever grounded. The temperature gradient is usually mild at about 5^oC, but in some cases it can reach 20^oC as in run #9. In such a case, a high moisture difference is observed in the direction of the plates. Specifically, in this run, the moisture content was measured to be at about 7% close to the top surface and above 40% close to the bottom one. Though all the temperature and pressure probes during the experiments were in the middle of the distance between the plates, future modeling and experimentation could account for such phenomena and give better and more realistic predictions. Such a phenomenon should also be of primary interest to industrial dryer manufacturers and users where the quality of the product is a strong function of the final moisture distribution. In order to avoid such non-uniformities, a low thermal conducting material could be introduced between the lower plate and the specimen, such as a polyethylene plate that was utilized previously to stabilize the electric field in the case of an oscillator.

Since moisture uniformity is considered an important quality factor of wood products, the M_i profiles were assumed to be linear interpolation of the end profiles that were measured. The final profiles were measured and no assumptions are necessary. A total of six specimens will be compared. These are the first four hemlock (#1 - #4) pieces and the last two cedar (#9 and #10) ones. The selection was done so that the average M_i values are the highest and so that all possible comparison cases can be covered adequately.

Hemlock run #1 had a M_i on the front end (by the dryer’s door), 39% on the left side to 63% on the right side, an average of 53%. On the rear side the moisture content ranged from 64% on the left to 91% on the right. The horizontal M_f was highly uniform with 9-12% moisture content in all pieces. The final longitudinal moisture content was 10-11% all over the piece. As it will be shown later, this is the best uniformity and was achieved at the lowest ambient pressure and low power density. The results are even more significant if we consider the high initial variation that was recorded.

The longitudinal M_i profile for hemlock run #2 can be considered very uniform since the ends moisture content was 72% and 69 %. Large non-uniformity was obtained for the M_i horizontal profiles. On the front end a range of 45-40-143% was recorded from the left to the right as the piece can be seen from the front side when it is in the dryer. Similarly, for the rear side we had a range of 45-40-124%. The longitudinal M_f profile was uniform at 12% moisture content and the horizontal was in the range of 11-15%. The latter resembled the initial one, where the inner parts were drier than the outer ones.

Hemlock run #3 initially exhibited an even longitudinal moisture profile of 44 and 46 % for the front and the rear part respectively. This was a major factor in obtaining a very uniform longitudinal final profile in the range of 10-13%. The horizontal M_i profile at the front was ranging from 50-40%. The rear side exhibited a wavy M_i pattern with minima and maxima of 53, 41, 51 and 41% from left to the right. The M_f range was 8-15%. This is a significant sign that in order to achieve maximum uniformity, P_a must be kept as low as possible. In both the longitudinal and the horizontal direction, the inner parts were wetter than the outer ones.

The longitudinal M_i patterns for hemlock run #4 can be considered as rather uniform, since the front and back M_i were 49 and 41%, respectively. This is probably the major factor for obtaining longitudinal M_f uniformity in the range of 15-17%. The M_i recorded horizontally was for the front side, 40-74%, and the rear side 38-45%. The horizontal M_f was in the range of 14-19%.

The horizontal M_i profile at the front and the back end of run #9 had drier edges. It was 34-56-31% and 45-63-39% from left to right respectively. Although the total moisture range of the pieces for the horizontal M_f ranged between 13-18%, high horizontal uniformity was observed for each individual piece measured along the length. The maximum difference was always within 1%. The average M_i at the ends was 43 and 50% at the front and the back correspondingly. The longitudinal M_f range was narrow at 13-16% with the ends been drier, following the initial horizontal trend.

Run #10 exhibited a rather non-uniform longitudinal M_i profile where 79 and 53% was recorded at the front and the back end respectively. The longitudinal M_f range was 11-16% and the ends were drier. On the front end the M_i was, from left to right, 61-103-47%. Similarly on the rear end M_i of 40-67-32% were respectively measured. The horizontal M_f was 8-18%, the highest so far. Both factors, the initial non-uniformity and the high ambient pressure, can explain these results.

Comparing runs #1 and #10, we observe that for similar drying conditions, M_f is more uniform in the former. A possible explanation is that the hemlock timber was mostly sapwood and the cedar timber was mostly heartwood. The transport properties are enhanced in the sapwood so a condition of equilibrium and uniformity is more probable. When all cases were compared with only the PD changing, no clear trend could be identified with regard to the effect of PD on moisture uniformity enhancements.

Finally, the shrinkage was minimal in all directions similar to what was obtained with an oscillator (Koumoutsakos et al., 2001b). The compression system introduced had an effect on the shrinkage where an average of 1.55% for the width, and 2.02% for the thickness was recorded.

The concept of the Identity Drying Card (IDC) as was first developed by Perre (1995) in order to extract useful information from drying experiments where internal vaporization occurred. Here, this concept will be applied to RFV drying experiments so that some information about the drying mechanisms and the relevant phase transitions is revealed. Figures 4 and 5 are pressure-temperature diagrams for the cases of run #9 and run #2. The saturated vapor pressure lines, which separate the liquid phase areas from the vapor phase areas, are denoted as P_sat. Above the line is the liquid phase and below the line is the vapor phase. The three points of comparison are: the core which is right at the geometric center of the piece; the quarter length from the front face and at the center of this face; and the quarter width away from the left side at half length and height. Initially, a pressure drop is observed in all cases due to the externally induced vacuum, until a local minimum of pressure is obtained. This local minimum is a factor of the local parameters of permeability, PD, porosity and moisture content. At this stage the vapor generated inside the material at the position of measurement and the vapor removed from this position towards neighbor areas of lower pressure are equal.

As the heating stage proceeds, more liquid turns into vapor because of the continuous dielectric heating. The creation of these new vapor molecules is faster than the removal of water from the neighbor areas. This is enhanced in areas where there is a temperature gradient too as are the outer layers. There due to lower outer temperatures the vapor can turn into liquid and more energy and time will be required for further transfer of this water mass. At some time point, there is no more void space in the neighborhood for water to be removed. As a consequence, local heat removal is reduced and the temperature increases. Increase in temperature generates a new local equilibrium between liquid and vapor, favoring more vapor generation. More vapor generation results in higher pressures. This is the loop leading in higher temperatures and pressures as is observed in all cases in both figures. The difference in the rate of increase of these two parameters is now clear that is a function of the local power density, permeability, moisture content and porosity.

The existence of the dynamic equilibrium between liquid and vapor is apparent from the shape of the Pressure-Temperature (P-T) curve that is obtained experimentally. When the majority of water is removed from the area of measurement, the P-T line crosses the saturation vapor pressure line (as shown in Figures 4 and 5) and the conditions describing the area of measurement are characteristic of a mixture rich in vapor phase. A pressure drop follows in this state, which is more obvious in Figure 4. This is probably due to the higher ambient pressure in this run. It can be noted here that in both Figures 4 and 5, for runs of the same PD and dimensions, the core pressures reached a maximum value, which is about four times higher than the ambient pressure before crossing the saturation vapor pressure curve.

In both above cases, the core curve crosses the saturation curve at about 40 hours. For cedar the quarter length crosses at about 35 hours and the quarter width at about 45 hours. For hemlock, 50 and 55 hours were required to cross the saturation line respectively. In all cases, the longer time was required for the quarter length position. This is a considerable indication that during RFV drying, water migrates towards the outer layers and due to lower temperatures at the boundaries, liquid transformation of the vapor arriving from the interior occurs, requiring further heat absorption for evaporation. The fact that in only one of the cases, conditions characterizing vapor phase, appeared faster in the core compared to the quarter length, can be explained by the longitudinal moisture non-uniformity in wood. If the core and the neighbor areas are dryer, they are expected to cross the saturation curve faster than other points of the geometric center line across the length, as is here the quarter length point of measurement. On this basis, non-destructive methods for moisture measurement could be very useful in the future, giving a clearer picture of the phenomena inside wood during RFV drying. Measurements of more points, mainly closer to the boundaries could also enlighten the subject of the existence or not of an evaporating front during RFV drying and its possible behavior and extent.

Figure 4. Identity Drying Card for the case of cedar, run #9 where sat = saturated, c=core, ql= at the center and quarter length, qw = at the center and quarter width.

Figure 5. Identity Drying Card for the case of hemlock, run #2 where sat = saturated, c=core, ql= at the center and quarter length, qw = at the center and quarter width.

AKCNOWLEDGEMENTS

This research was financially supported by a Natural Sciences and Engineering Research Council Strategic Research Grant, a BC Science Council GREAT Award and by Heatwave Technologies Inc.

REFERENCES

Jones, P.L. 1996. RF Heating, an old technology with a future. Proceedings of Microwave and High Frequency Heating. L1.1-L1.9.

Koumoutsakos, A.; Avramidis, S.; and S. G. Hatzikiriakos. 2001a. Radio frequency vacuum drying of wood. I: theoretical model. Drying Technol. 19(1): 65-84.

Koumoutsakos, A.; Avramidis, S.; and S. G. Hatzikiriakos. 2001b. Radio frequency vacuum drying of wood. II: Experimental model validation. Drying Technol. 19(1): 85-98.

Koumoutsakos, A. and S. Avramidis. 2002. Mass Transfer Characteristics of Western Hemlock and Western Red Cedar. Holzforschung. 53(2): 185-190.

Perre, P. 1995. Drying with internal vaporization: introducing the concept of identity drying card (IDC). Drying Technol. 13(5-7): 1077-1097.

Sebastian, P.; Jomaa, W. and I.W. Turner. 1996. A New Model for the Vacuum Drying of Wood based on the Concept of the Transition Layer. In Proceedings of 6^th IUFRO Wood Drying Conference. 135-138.

Torgovnikov, G.I. 1993. Dielectric Properties of Wood and Wood-Based Materials. Springer-Verlag, Berlin, Germany, pp. 196

Zhou, B. and S. Avramidis. 1999. On the loss factor of wood during radio frequency heating. Wood Sci. and Technol. 33(4): 299-310.