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Maderas. Ciencia y tecnología

versión On-line ISSN 0718-221X

Maderas, Cienc. tecnol. v.5 n.2 Concepción  2003

http://dx.doi.org/10.4067/S0718-221X2003000200005 

Maderas. Ciencia y tecnología. 5(2): 137-143, 2003

NOTA TÉCNICA

EVALUATION OF A KILN INCORPORATING AN OSCILLATING PLATE AIRFLOW SYSTEM

Mihaela Campean1, Ion Marinescu1, Mihai Ispas1
1Transilvania University of Brasov, Faculty of Wood Industry, Brasov. Romania.

Corresponding Author: campean@unitbv.ro


ABSTRACT

Boards of spruce (Picea abies) were dried in a pilot kiln with an oscillating plate that provides -“alternating air movement in the stack”. The paper outlines the airflow concept and provides results for drying time and quality.It is suggested that the system has certain advantages which make it suitable as an alternative to conventional drying, especially for small-sized enterprises.

Keywords: kiln drying, air circulation, quality, moisture content, drying time, Picea abies
INTRODUCTION

This paper presents the results from continuing the series of trials involving drying with fast alternating air movement in the stack (Fig. 1). This new drying system is described in a Romanian patent (Stanasila 1993), as well as in previous publications (Campean 1996; Campean 1999; Campean 2001).

Although based on conventional kiln drying, this method has a distinctive airflow feature. Instead of traditional fans, airflow in this kiln is generated by an oscillating plate which generates alternating forward and reverse flow with each of the plates 2 second movements (Fig. 2). Because the plate is placed laterally close to the stack, it ensures minimum air pressure losses. The frequent air circulation reversals lead to a high turbulence of the airflow, which is suggested to accelerate drying (swing-drying – Grau 1937; Lignomat impulse method – Welling 1987).

The oscillating plate is driven by means of a 3 kW electric motor, a quadrilateral (crank-connecting rod-balancing beam) mechanism and a horizontal steel shaft. The pilot-kiln has been built within a laboratory of the Wood Industry Faculty in Brasov-Romania.

In order to establish the performances (i.e. drying times and quality) and capabilities of the new system, several drying experiments were performed. This paper presents some of the results obtained.

Figure 1. Illustration of principle of drying with fast alternating air movement in the stack.

Figure 2. Oscillating plate.

METHODS & MATERIALS

Spruce timber (Picea abies L.) of different thicknesses (24, 38, 48, 68 and 75mm) was dried from an average initial moisture content Ui=30-40% to an average final moisture content Uf=10± 2%. The drying schedules are shown in Tables 1-3 and were established by the authors (Marinescu 1979; Campean 1997). Continuous measuring and control of the kiln conditions (air parameters) was provided by a semi-automatic control system (measured by electric psychrometer), and the moisture content of wood was monitored by means of a central kiln sample and four V2A resistance electrodes.

Table 1. Drying schedule applied to 24mm thick spruce timber.

Step

MC overall
(%)

Dry-bulb temp.
(° C)

Wet bulb temp.
(° C)

Drying
gradient

Heat-up

50

65

63

-

Drying

50 – 45

70

65

-

45 – 30

80

70

-

30 – 20

90

66

4

20 – 15

90

68

4

15 - 10

90

64

4

Cooling

10

40

-

-


Table 2. Drying schedule applied to 38 and 48mm thick spruce timber.

Step

MC overall
(%)

Dry-bulb temp.
(° C)

Wet bulb
temp.
(° C)

Drying
gradient

Heat-up

40

65

63

-

Drying

40 -30

73

64

-

30 –20

76

60

3.8

20 –15

80

58

3.8

15 -10

83

58

3.8

Equalisation

10

70

66

-

Cooling

10

40

-

-

Table 3. Drying schedule applied to 68 and 75mm thick spruce timber.

Step

MC overall
(%)

Dry-bulb temp.
(° C)

Wet bulb temp.
(° C)

Drying gradient

Heat-up

30

63

58

-

Drying

30 – 20

70

59

3.1

20 – 15

72

54

3.1

15 - 10

75

52

3.1

Equalisation

10

70

66

-

Cooling

10

40

-

-

Note:

The drying gradient is an abstract parameter, characterising the milderness degree of the drying schedule during the drying period below the fibre saturation point. It represents the ratio between the moisture content of wood and the equilibrium moisture content at a certain moment of time. The lower its value, the milder the drying conditions are.

Assessment of drying quality included determination of the final moisture content and its variation within the stack and in board thickness, the determination of number and sizes of cracks (surface checks, end splits and internal shakes), warp (bow, twist, cupping) and discoloration (surface staining and sticker marks). All data were recorded in a “quality assessment sheet”, relying on the pilot version of the EDG-Recommendation for quality assessment (Nardi 2000).

The final moisture content was determined by using the oven-dry method with 1.5m long samples cut from 9 boards, extracted from different positions within the stack (Fig. 3).

Figure 3. Positioning of boards sampled in order to determine the distribution of final moisture content within the stack.

The final moisture content gradient (ΔU) across the board thickness was determined by means of slicing-samples (3-layers for 24, 38 and 48 mm thick samples; 5-layers for 68 and 75 mm thick samples).

All the boards were visually assessed during unstacking, in order to identify splits, warp and discoloration. Discoloration and warp were not considered as a key part of the quality evaluation and were included only to have an overall image of the quality obtained.

Additionally, measurements of air speed and temperature at the same locations over stack height and width as shown in Fig. 3 have been performed by means of a resistive wire transducer, introduced at 0.5m depth in the stack. The results of these measurements (presented in detail by Campean 2001) allowed obtaining a preliminary view on airflow distribution within the stack, enhancing thus a correlation with the distribution of the final moisture content.

EXPERIMENTAL RESULTS & DISCUSSION

Table 4 presents a synthesis concerning the drying times and average drying rates obtained when drying in the oscillating plate kiln, compared to a conventional kiln, while Table 5 synthesises the results of the quality control performed according to the criteria described above.

Table 4. Drying time and rate of oscillating and conventional kiln (conventional kiln values in parenthises), as function of timber thickness, initial and final moisture content.

Timber thickness, mm

Moisture content of samples

Drying

Average initial
(%)

Average final
(%)

Time
(h)

Average rate
(%/h)

osc.

conv.

osc.

conv.

24

37

10

14

25

1.92

1.08

38

40

8

40

64

0.80

0.50

48

40

9

40

68

0.77

0.46

68

30

11

46

86

0.41

0.22

75

30

12

44

150

0.40

0.12

Table 5. Results of quality control for experimental drying trials with spruce timber in the oscillating plate kiln.

Assessment criteria

Timber thickness

24mm

38mm

48mm

68mm

75mm

Distribution of final moisture content within the stack:

range of values

8.5-12.4

7.5-9.2

8.0-9.9 13.6-8.1 14.9-6.9

•average

10.3 8.2 9.2 15.2 0.77

•standard deviation

1.57

0.48

0.58

1.35

15.8

Average moisture gradient across the timber thickness (ΔU)

1.6 1.5 2.2 7.3 7.3

Surface checking

very low

absent

very low

low

moderate

Internal checking

absent

absent

absent

absent

absent

The results of the quality control revealed satisfactory drying uniformity for all timber thicknesses, but especially for grades thinner than 50mm. The best results have been recorded for 38mm thick timber, where the individual values of the final moisture content differed with less than 1% from the average final moisture content and the difference between the maximum and the minimum value was less than 2%. Along with the low initial moisture contents, the equalisation period used during the drying of batches with 38 and 48mm thick timber has probably contributed to the evenness of drying.

The quality control also revealed moderate casehardening, very low checking tendency, even with the thick grades, no discoloration and very low proportion of deformations.

Fig. 4 and Fig. 5 present the distribution of air parameters (air temperature and air speed) across the width and respectively the height of the timber stack.

a

b

Figure 4. Distribution of air speed (a) and air temperature (b) over the stack width.


a

b

Figure 5. Distribution of air speed (a) and air temperature (b) over the stack height.

A remarkable uniformity of both air speed (Fig. 4, a) and air temperature (Fig. 4, b) can be noticed over the stack width. This statement is supported by the results of measurements performed with the same apparatus in a one-stack conventional kiln, which revealed air speed differences up to 1.2m/s between the air entering and air exit side (over the stack width) and temperature drops in the middle of the stack up to 4.5° C, which are values twice higher than obtained in the oscillating plate kiln.

The more uniform distribution of the airflow over the stack width in case of the oscillating plate kiln is attributed to the rapid reversal of the air direction, as well as to the placement of the heaters on both lateral sides of the stack (see Fig. 1).

As far as the drying uniformity over the stack height is concerned (Fig. 5), an other advantage of the system could be noticed: air speed tends to decrease from the bottom to the top of the stack, while temperature has an inverse evolution, as in any kiln, due to the rising tendency of warm air. Thus, top boards are subject to 8-10° C higher temperature than bottom boards, but higher air speed at the bottom partly compensates this difference, helping bottom boards to dry almost in the same rhythm as top boards.

CONCLUSIONS

The results of drying with alternating air movement in the stack are comparable to those of small-sized conventional kilns and can be considered appropriate for small-scale industries for the following reasons:

  • up to 50% lower drying times, due to high air turbulence within the stack;
  • acceptable drying uniformity;

  • lower electric energy consumption (Campean 2000), due to time reduction and diminution of air pressure losses within the kiln.
  • The authors intend in future to extend the research to other wood species as well and to implement the system at industrial scale.

    REFERENCES

    CAMPEAN, M.; STANASILA, C.; STANASILA, O.; MARINESCU, I.; BOAC, V. 1996.
    Drying Method for Softwood Lumber Under the Effect of the Fastly Alternating Air Movement Through the Stack. In: Proceedings of the “5-th International IUFRO Wood Drying Conference”, Quebec, pp. 345-352.         [ Links ]

    CAMPEAN, M. 1997. Thermal treatments of wood. Timber drying. Publishing House of the “Transilvania” University, Brasov.         [ Links ]

    CAMPEAN, M.; MIHAI, D.; ISPAS, M.; MARINESCU, I. 1999. Possibilities of achieving fast alternating air movement in a timber drying kiln. In: Bulletin of the “Transilvania” University of Brasov. Vol 6(41), pp. 179-186.         [ Links ]

    CAMPEAN, M.; MIHAI, D.; MARINESCU, I.; ISPAS, M. 2000. Timber drying method with reduced electric energy consumption. In: Proceedings of the International Scientific Conference “Forest and wood technology vs. environment”, Brno, pp. 50-66.         [ Links ]

    CAMPEAN, M.; MIHAI, D.; MARINESCU, I.; ISPAS, M. 2001. The distribution of air parameters in a drying kiln with fast alternating air movement in the stack. In: Proceedings of the “7-th International IUFRO Wood Drying Conference”, Tsukuba, pp. 72-77.         [ Links ]

    CAMPEAN, M. 2002. Study of the drying process with fast alternating air movement in the stack applied to resinous timber. PhD Thesis, “Transilvania” University of Brasov.         [ Links ]

    GRAU, E. 1937. Die Schaukeltrocknung.

    MARINESCU, I. 1979. Wood drying. Volume 1. Editura Tehnica, Bucuresti.         [ Links ]

    NARDI. 2000. Assessment of drying quality of timber.

    STANASILA, C.; STANASILA, O. 1993. Wood drying kiln. Patent Nr. 10691BI/1993.

    WELLING, J. 1987. Technische Neuerungen fuer die Schnittholztrocknung. IHM-Holztrocknung. HOB 3/87, pp. 50-52.         [ Links ]

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