United States

Department of

Agriculture Radio-Frequency Forest Service

Forest

Products

Laboratory Dielectric Drying

Research

Paper of Short Lengths

FPL 377

December 1980 of Northern RedOak
Abstract

For most uses hardwoods are dried

as entire boards that include all

defective portions discarded after drying. The United States has a large

resource of low-quality hardwoods

and the potential exists for significant savings in energy and in dryer

capacity by cutting out defects before

drying. One approach could use radiofrequency drying. In this investigation

short lengths of northern red oak

were dried rapidly, but energy consumption and honeycomb were excessive in drying to low moisture

levels. However, rapid radio-frequency

dielectric drying to 25 to 30 percent

moisture content followed by more

conventional finish drying may hold

technical promise.
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory1

Research

Paper

FPL 377

Introduction

Radio-Frequency

Dielectric Drying

of Short Lengths

of Northern Red Oak

BY

WILLIAM T. SIMPSON, Forest Products Technologist

Drying Hardwood Cuttings

Hardwoods for use in furniture

stock, millwork, flooring, and similar

uses, are traditionally dried as entire

boards that range from 8 to 16 feet

long and that vary in width. Depending on grade, a certain percentage of

the volume of these boards is defective and unsuitable for a high-quality

end product. The eastern part of the

United States has an abundance of

low-grade hardwoods, and advances

in processing efficiency could lead to

increased utilization of this resource.

Low-grade boards have a significant

percentage of fiber unusable for a

final furniture-type product. To dry

this unusable wood only to discard

later is a waste of energy and of dryer

capacity. If the defective volume

could be cut out before drying, significant savings may be possible.

The idea of drying clear hardwood

cuttings instead of entire boards is

not new. Torgeson (14)2, Rice (11), Catterick (4), and Bingham and Schroeder

(1) considered the idea. Rice (11) investigated the idea experimentally

and concluded drying dimension

parts instead of lumber is technically

and economically feasible. Despite

Rice’s conclusions, drying dimension

parts is not practiced commercially to

any great extent, probably because industry is not convinced the advantages outweigh the disadvantages.

The advantages of drying dimension parts instead of whole boards

are:

Reduced energy consumption

Increased dryer capacity

Possible reduction in transportation costs

Disadvantages and potential

problems are:

More pieces to handle

More end checking

Increased warp

More wet residue to handle

These major advantages of reduced

drying energy and increased dryer

capacity are illustrated in table 1,

which shows estimates of these savings for hardwood lumber grades

(assumes 4/4 red oak). If, for example,

No. 2C were used, energy consumption per usable thousand board foot

(Mfbm) increases from 4.51 to

7.73 million British thermal units

(Btu); required dryer capacity per

usable Mfbm increases from 1 to 1.71

Mfbm; and at a rate of $2.50 per

million Btu, energy savings by drying

only usable cuttings is $8.05 per

Mfbm.

The major disadvantages of drying

only usable parts of boards are the

much greater number of individual

pieces that must be handled, the increased number of ends exposed and

susceptible to end checking, and the

possible presence of warp in pieces

for which only small machining

allowances remain.

This investigation attempted to

determine if one particular approach,

radio-frequency dielectric drying, has

sufficient technical promise for drying

hardwood cuttings to be considered a

potential drying process of the future

when cost, availability, and sources

of energy may be considerably different than they are today.

Radio-frequency drying requires

electrical energy, which currently is

more expensive than most other

sources (including oil, gas, and wood

residues). Whether the use of electrical energy will increase or decline

in the future when the mix of our

energy sources may change is

unknown. However, a system that

could rapidly dry hardwood cuttings.

such as radio-frequency drying, has

sufficient potential payoff so that

technical feasibility should be investigated now in anticipation of the time

economic feasibility might be a

reality.

1

 Maintained at Madison. WiS in cooperation

with the University of Wisconsin.

2

 ltalicized numbers in parentheses refer to

literature cited at end of report.
Table 1 .–Potential savings in energy and dryer capacity by drying 1-inch northern red oak cuttings instead of entire boards

Energy required per Required dryer Savings in energy

Grade Clear surface usable 1,000 board costs ($2.50 per

required feet in drying

capacity

per usable million Btu) by

from 50 to 7 percent11,000 board feet drying only cuttings

-------------------------------------------------------------------------------------------------------------------------------

Pct Million Mfbm/Mfbm $/Mfbm

Btu/Mfbm

All clear 100 4.51 1.00 0

Firsts 91-2/3 - 100 4.71 1.04 .50

Seconds 83-1/3 - 91-2/3 5.15 1.14 1.60

Selects 91-2/3 - 100 4.71 1.04 .50

No. 1C 66-2/3 - 83-1/3 6.01 1.33 3.75

No. 2C 50 - 66-2/3 7.73 1.71 8.05

No. 3A 33-1/3 - 50 10.83 2.40 15.80

No. 3B225 - 33-1/3 15.46 3.43 27.38

1

 Simpson and Tschernitz (12)

2

 Requires sound cuttings, not clear.

Radio-Frequency Drying

For many years, wood drying by

radio-frequency electrical energy has

been investigated, but has not been

accepted industrially except for a few

specialty items. Some of the past

research is described in (2,5,6-9,15,16).

There are two general methods of drying wood with radio-frequency electrical energy. In one, the boiling

method, the surrounding atmosphere

is not controlled and a high level of

power is applied to the wood. The

free water is driven out of the wood

both as vaporized water and as liquid

water forced out by steam pressure.

Of the two methods, this is the faster;

it is generally considered applicable

only to short lengths of permeable

species that do not permit the build

up of high steam pressures that rupture the wood. The other method, the

temperature gradient, is slower than

the boiling method, but in sensitive

species causes less degrade. A lower

level of power is applied-just

enough to keep the interior of the

wood slightly below 100° C. The surrounding atmosphere is maintained

cool and humid. The temperature gradient from the inside to the outside

moves moisture out of the wood,

because of the associated vapor

pressure gradient.

Ward and Anderson (16) report that

northern red oak is tolerant to the

boiling method. They present a drying

curve for 1- 3/4- by 2- 1/4- by 24- inch

red oak that shows drying from about

65 to 10 percent moisture content in

about 14 minutes produced no

serious drying defects. An objective

of the present work is to confirm this

tolerance and to take advantage of

the high longitudinal permeability of

red oak to rapidly remove water from

the end grain of short lengths by the

boiling method of radio-frequency drying. Further objectives are to

establish a drying-rate curve, to

assess any drying defects that occur,

and to determine power and energy

consumption characteristics.

Experimental

Material for this investigation was

taken from freshly sawed boards of

northern red oak. Clear specimens

4 inches wide by 24 inches long were

cut from the boards, and surfaced on

both sides to 1 inch thickness. The

specimens were then randomly

assigned to different treatments,

13 drying times (0, 4, 6.5, 9, 12, 15, 18,

24, 30, 45, 75, 120, and 300 min).

Seven replicates were dried at each

drying time. The lengthwise moisture

gradient was determined for each

specimen by cutting moisture sections 1/4 inch along the grain at

3-inch intervals over the 24-inch

length of the specimen. The average

moisture content of the whole piece

was estimated from this gradient.

These moisture sections also served

as crosscut samples to determine

presence or absence of honeycomb.

In addition to drying rate,

lengthwise moisture gradient, and

degrade assessment, other pertinent

information was gathered. The power

absorbed by the wood was monitored

during the 300-minute drying run. The

internal temperatures were measured

in four specimens over a 3-hour

period. An estimate of the moisture

gradient across the l-inch thickness

was also determined, based on six

replicates, each dried for 12 minutes.

These were obtained from 1/4-inchlong moisture sections similar to the

lengthwise moisture gradient sections

but cut from the center of the 24-inchlong pieces. Three smaller moisture

sections (1116 in. wide) were cut along

the l-inch thickness of each of these

1/4-inch sections, one section was

taken from each face and one from

the center (fig 1).

The radio-frequency generator was

a 5-kilowatt-capacity unit with the

load circuit containing the drying

wood electromagnetically coupled to

the generator circuit. A variable tuning coil was built into the load circuit

to maintain tuning for transfer of

power as the dielectric properties of

the wood decreased during drying

(fig. 2). The frequency was

27.12 megahertz (MHz). Aluminum

electrodes were held in contact with

the wood by a slight pressure. Each

sample was dried individually.

2
Figure 1.–Method of preparing specimens for moisture gradient in thickness direction. M 149 046

Figure 2.–Schematic of radiofrequency generator and load circuit

for drying lumber.

M 148 992

Results and Discussion

Drying Rate

The average lengthwise moisture

gradients at the various drying times

are shown in figure 3. The figure illustrates the statement often made

for radio-frequency drying-it dries

wood from the inside out. The 0- and

4-minute gradients show the effects

of normal end drying. The 6.5-minute

gradient is almost flat; from then up

to 30 minutes, the gradient is reversed because steam pressure

forces free water to the ends of the

boards. At 30 minutes, the moisture

content has dropped below the fiber

saturation point; from this point on,

diffusion takes over, and the moisture

gradient again reverses. The average

moisture content of the boards as a

function of time is shown in figure 4;

data for moisture content and drying

time are listed in table 2, as are

statistics showing variation in

moisture content. The average

moisture content for each specimen

was estimated by integrating the

moisture-gradient curve. Figure 4

shows that drying is very rapid from

the initial 79.2 percent moisture content down to the fiber saturation

point of approximately 25 percent; at

fiber saturation, the much slower process of diffusion takes over.

The moisture gradient through the

thickness, measured when the

average moisture content was

38.9 percent, showed the center considerably dryer than the shell. The

average moisture content of the outer

moisture sections was 41.5 percent;

the average of the center sections,

11.7 percent.

The internal temperature is shown

as a function of time in figure 5. Initial temperature rise is rapid; in

3 minutes the temperature is up to

100° C, after which it gradually increases to a maximum of approximately 135° C.

Power and Energy

Consumption

Any data on power and energy consumption apply only to the radiofrequency generator load system

developed for this investigation, and

do not apply to other systems except

to point out general characteristics.

The power absorbed by the load during drying from green to 20 percent

moisture content is shown in figure 6.

3
Figure 3.–Lengthwise moisture gradients for 1- by 4- by 24-inch-long red oak

lumber during radio-frequency dielectric drying.

M 148 993

The striking feature shown by figure 6

is the rapid decrease in power consumption as moisture content decreases below 50 percent moisture

content. Electric power supplied to a

load can be expressed as (2):

P = 0.55 f E2 e tan d · 10-12

where

P = power absorbed (watts/cm3

)

f = frequency (hertz)

E = field strength (volts)

e = dielectric constant of load

d = dielectric loss angle

4

Skaar (13) has shown that the dielectric constant decreases as moisture

content decreases. The decrease in

dielectric constant is severe, dropping

from 30 to 40 for green wood to approximately 2 for ovendry wood. Thus,

as wood dries it can absorb less and

less radio-frequency power. The data

on thickness-moisture gradient show

that, even at fairly high average

moisture contents (40 to 50 pct) the

center of wood is considerably drier

than the outer wood, and can account

for the rapid decrease in power

absorbed.

Another factor of practical

significance in the design of any

radio-frequency system is the impedance matching between generator

and load circuit (10). Without proper

“tuning,” maximum power cannot be

transferred between generator and

load. As the dielectric properties of

the wood decrease during drying, the

impedance of the load circuit

changes, and some adjustments must

be made in the load circuit to maximize power transfer (fig. 2). The

variable tuning coil used in this work

was an attempt to accomplish this

tuning and was at least partially successful in maintaining maximum

power transfer during drying. Any

practical radio-frequency drying

system will require considerable

engineering analysis to maintain optimum power transfer during drying.

The energy required to dry the red

oak of this investigation with this

radio-frequency system is shown in

figure 7 as a function of final

moisture content. The actual energy

used and the percent efficiency are

relevant only to this system and are

not as important as the shape of the

curve in figure 7. The curve shows a

sharp increase in energy required at a

final moisture content of about

25 percent. This corresponds to the

point in drying at which mass flow of

water no longer contributes much to

drying, and diffusion, which is much

slower, has taken over as the controlling mechanism. The much slower drying rate requires maintaining power

to the load for lengthy periods of

time, during which energy consumption continues to add up.

Wood Quality

Drying defects are summarized in

table 3. Some honeycomb and surface checking occurred in 54 percent

of the boards dried 15 minutes or

longer. Because of the moisture gradient in the thickness direction (41.5

to 11.7 pct at an average moisture

content of 38.9 pct) it is understandable that honeycomb could occur.

The drier center was restrained from

shrinking by a still-wet shell, and tensile failures occurred.

Severity of honeycomb varied;

whether or not it would cause rejection of the piece would depend on its

severity and the requirements of the

end product. Nevertheless, the

honeycomb present is probably too

severe for many oak products. As

McAlister and Resch (6) noted, one

means to overcome the problem of

honeycomb is to reduce power input
Figure 4.–Relationship of average moisture content to drying time for 1- by 4- by

24-inch-long red oak dried by radio-frequency energy.

M 148 997

Figure L-Relationship of internal temperature in 1- by 4- by 24-inch-long red oak

to time during radio-frequency dielectric drying.

M 148 996

to the boards. Although this reduces

drying rate, it may be possible to

develop procedures that will minimize

honeycomb yet retain short drying

times.

Feasibility of

Radio-Frequency Drying

Although the technical feasibility of

radio-frequency drying short lengths

of red oak has not been established

by this investigation, some

possibilities for potential applications

have resulted. The results of analyzing drying time, energy consumption,

and defect development point to the

impracticality of using continuous

high-power levels in radio-frequency

drying oak to low moisture-content

levels. The results, however, suggest

an application of radio-frequency drying that, if successfully developed,

would retain some of the advantages

of rapid drying without resulting in excessive defects. Air drying is very

slow, and it may be possible to

replace months of air drying by short

high-frequency cycles of only minutes

in drying to moisture contents in the

30 percent range. Conventional kiln

drying could then be used in drying to

final moisturecontent levels of 6 to

8 percent. It would be necessary to

develop moderated radio-frequency

cycles that might take longer than the

12 to 15 minutes used in this work,

but they would still be relatively short

and would upgrade final quality.

This investigation was limited to

the response of oak to rapid radiofrequency drying. Many other hardwood species are much more tolerant

to rapid high-temperature drying than

oak. Since this study was completed,

Boone (3) has shown that some hardwood species, particularly those of

low density, are tolerant to rapid hightemperature drying. Thus, there are

probably species more suitable than

oak for which the advantages of rapid

radio-frequency drying could be successfully realized. This rapid drying, if

coupled with the potential energy and

dryer-capacity savings made possible

by drying only usable parts instead of

entire low-grade boards, could result

in a drying process more efficient

than the processes now used.

Research should be encouraged to

determine optimum radio-frequency

drying schedules that will maximize

drying rate and minimize drying

defects.

5
Figure 6.–Relationship of power absorbed during radio-frequency dielectric drying of 1- by 4- by 24-inch red oak from green to 20 percent moisture content.

M 148 995

Figure 7.–Relationship of energy required to dry 1- by 4- by 24-inch red

oak to final moisture content.

M 148 994

6

Summary and

Conclusions

Northern red oak boards, 1 inch

thick by 4 inches wide by 24 inches

long, were dried from an initial

moisture content of approximately

80 percent to various final moisture

contents by radio-frequency electrical

energy. Drying occurred rapidly above

the fiber saturation point, but slowed

down considerably below this

moisture content. With the drying

system used in this investigation, the

average moisture content could be

reduced in 15 minutes from 80 to

25 percent. However, the level of

degrade in boards dried to 25 percent

or below was probably too high for

many oak products in which

honeycomb is an important defect.

The results did suggest that radiofrequency drying has some potential

as a predrying process to be followed

by lower temperature drying.
Table P.-Average moisture content after 13 radio-frequency drying times for 1- by 4- by

24-inch red oak

Drying Average Standard 95 percent

time moisture confidence

content deviation limits

-------------------------------------------------------------------------------------------------------

Min Pct Pct Pct

0

4 68.5

79.2 10.5

8.5

69.5 - 88.9

60.6 - 76.4

6.5 57.2 13.9 44.4 - 70.1

9

12

43.9

38.9 12.6

7.7 36.8 - 51.0

27.3 - 50.5

15 24.1 8.4 16.1 - 31.9

18 22.1 5.3 17.2 - 27.0

24

30

21.7

20.0

3.0

3.5

18.9 - 24.4

16.8 - 23.2

45 20.4 3.5 17.2 - 23.6

75 15.5 1.7 14.0 - 17.1

120 14.6 2.2 12.6 - 16.6

300 11.5 2.0 9.7 - 13.3

Table 3.–Summary of drying defects associated with radio-frequency drying 1- by 4- by

24-inch red oak

Average Number-of boards

moisture

Drying content Honey- Surface- Either honeycombed

time at end of combed checked or surface checked

drying time

----------------------------------------------------------------------------------

Min Pct

15 24.1

18

24

22.1

21.7

6

3

2

1

2

0

6

3

2

30 20.0 3 1 3

45 20.4 4 4 4

75 15.5

120 14.6

5

5

4

4

5

5

300 11.5 2 2 2
Literature Cited

1. Bingham, S. A., and J. G. Schroeder.

1977. Short lumber in furniture manufacture. Wood and Wood Prod.

82(11):78-79.

2. Biryukov, V. A.

1968. Dielectric heating and drying of wood. Translated from book published in Russia, 1961, by Israel Program for Scientific Translations.

Jerusalem, Israel. 117 p.

3. Boone, R. S.

1979. High-temperature drying of hardwoods. Proc. Symposium, High

Temperature Drying of Hardwoods, New Albany, Ind. Mar.

4. Catterick, J.

1973. Seasoning short boards and cuttings. Proc. First Annual Hardwood

Symposium, Hardwood Research Council, Cashiers, N.C. 117 p.

5. Dean, A. R.

1970. System for drying timber using a combination of radio frequency and

steam heating. Wood 35(3):9-11.

6. McAlister, W. R., and H. Resch.

1971. Drying l-inch ponderosa pine lumber with a combination of microwave power and hot air. Forest Prod. J. 21(3):26-34.

7. Miller, D. G.

1948. Application of dielectric heating to the seasoning of wood. Proc.

Forest Prod. Res. Soc. 2:235-243.

8. Miller, D. G.

1969. The state of the art in electronic drying of wood. Canadian Forest Ind.

89(12):42-43.

9. Pound, J.

1966. RF Drying of timber. Wood 31(12):43-45.

10. Pound, J.

1974. Radio frequency heating in the timber industry. E. & F. N. Spon, Ltd.,

London, England. 191 p.

11. Rice, W. W.

1964. Kiln-dry dimension parts instead of boards and save money. South.

Lbrmn. 209(2609):141-145.

12. Simpson, W. T., and J. L. Tschernitz.

1978. Does thickness variation in sawing affect kiln drying? South. Lbrmn.

236(2933):15-17.

13. Skaar, Christen.

1948. The dielectric properties of wood at several radio frequencies. N.Y.

State Coll. For. Tech. Publ. 69, illus., Syracuse, N.Y. 36 p.

14. Torgeson, O. W.

1956. Seasoning dimension stock. U.S. For. Serv. Res. Rep. 1242. Forest

Prod. Lab., Forest Serv., U.S. Dep. Agric., Madison, Wis.

15. Vermaas, H. F.

1978. The combination of high-frequency and high-temperature convective

drying of wood. Holzforschung. 32(21):55-59.

16. Ward, Darrell, and R. C. Anderson.

1964. Dry wood parts in minutes. Woodworking Digest 66(9):45-48.

8

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