WO1988008949A1

WO1988008949A1 - Method and apparatus for drying planar material, e.g., veneer sheet

Info

Publication number: WO1988008949A1
Application number: PCT/FI1988/000072
Authority: WO
Inventors: Anita Halonen
Original assignee: Imatran Voima Oy
Priority date: 1987-05-14
Filing date: 1988-05-12
Publication date: 1988-11-17
Also published as: FI84658B; AU1786688A; FI872123A0; FI84658C; FI872123A

Abstract

This publication discloses a method and an apparatus for drying planar material, e.g., wood sheet. In accordance with the method, the main drying phase is implemented by subjecting the material to heat energy exposure, and the moisture content of material to be dried is measured prior to and/or after the main drying phase. In accordance with the invention, material identified in the measurement to be of the highest moisture content is exposed to a first IR radiation with its wavelength of maximum intensity approximately coincident with the wavelength of maximum absorption in water; immediately after the first exposure the material to be dried is exposed to a second IR radiation of a shorter wavelength than the wavelength of maximum intensity in the first radiation; and the energy doses imposed on the material to be dried by both the first and the second radiation are adapted to be at least approximately equal in magnitude. With help of the invention, variations in the moisture content of the material are minimized.

Description

Method and apparatus for drying planar material , e. g. , veneer sheet

The present invention relates to a method for drying planar material, e.g., veneer sheet, in accordance with the preamble of claim 1.

The invention also concerns an apparatus for the implementa¬ tion of the method.

In the inventions of the prior art, particularly the drying of veneer sheets has been solved by using roll or screen dryers in which the sheet is dried by hot steam. The moisture content of the dried sheets was measured and the sheets of highest moisture content were sorted aside and redried.

A disadvantage of the conventional technique is that the use of a mere roller or screen dryer results in a nonhomogeneous moisture profile which increases the costs and slows down the processing of the final product.

Conventional dryers do not lend themselves to adjustments for individual sheets. Consequently, sheets with lower initial moisture content tend to become overdried, that is, subjected to excessive stay in the dryer. Due to overdrying, the quality of sheets deteriorates by embrittlement, which leads to easy breaking in handling stages following the drying process. Although a portion of the sheets is overly dry, yet still another portion may remain excessively moist. In industrial production, the proportion of excessively moist sheets exiting the dryer is about 15 %. The redrying run required by these sheets slows down the process. The aim of the present invention is to overcome the disadvant¬ ages of the prior art technology and to achieve a totally new kind of method and apparatus for drying planar material, e.g., veneer sheet.

The invention is based on the adaptation of two subsequent infrared radiating dryers of different types onto the dryer line such that the wavelength of maximum intensity in the radiation spectrum of the first radiator type coincides at least essentially with the wavelength of maximum absorption in water (and possibly also with that of the material to be dried) while the wavelength of maximum intensity of the second radiator is within the near-IR range (approx. 1 um), and energy doses from both radiator types imposed on the material to be dried are approximately equal in magnitude.

More specifically, the method in accordance with the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the apparatus in accordance with the invention is characterized by what is stated in the characterizing part of claim 3.

The invention provides outstanding benefits.

The dryer implementation in accordance with the invention has presented decreases in energy cost in some cases. Variations in moisture contents of the product has been reduced to a lower level compared with conventional solutions, allowing, together with the shortened drying time, an increase in the capacity of the dryer.

The invention is next examined in detail with help of an exemplifying embodiment illustrated in the attached drawings.

Figure 1 is a diagram of definitions of wavelength^'ranges applicable to the implementation of the invention. Figure 2 is a graph of absorption curves of wood and water, respectively, as a function of wavelength.

Figure 3 shows in an essentially diagrammatic side view a dryer apparatus in accordance with the invention.

Figure 4 shows in a cross-sectioned top view a radiator element in accordance with the invention.

Measurement results described herewithin are based on small- scale experiments that were performed to evaluate the effect of infrared radiation in the intermediate wavelength and near-IR ranges on veneer sheet in preheating and postdrying conditions. The aim of practical tests was to ind optimal solutions to the drying process and apparatus construction.

As illustrated in Fig. 1, the spectrum of IR radiation is divided into near-IR (short-wave), intermediate wavelength, and far-IR ranges. Evidenced in Fig. 2 is the coincidence of absorption maxima of water and wood in the intermediate IR range at a wavelength of about 3 μm. From this, one is apt to assume a highest efficiency of both wood heating and water evaporation to be obtained in this wavelength range. Since, however, radiation impinging on a body may experience a total or partial reflection, absorption, or guidance, a drying effect of maximum efficiency will not be achieved by a simple coincidence adaptation of radiation wavelength with the absorption maxima, but rather with a combination dryer in accordance with the invention.

The embodiment of the IR dryer apparatus is based on the subsequent use of intermediate and near-IR range radiation. The sheet is first exposed from an appropriate distance to intermediate range IR radiation and then to near-IR radiation such as to obtain an approximately equal energy doses of impinging radiation onto the sheet. In addition, a rear reflector capable of improving the IR drying efficiency is adapted to the conveyor used in the IR dryer construction.

In accordance with Fig. 3, the IR dryer apparatus in accordance with the invention can be adapted to a conventional drying process of sheets, either before or after a roll ^'- dryer 6. Further, a simultaneous use of a predryer and a postdryer is possible. Controlled by a predetermined moisture categorization, excessively moist sheets are conveyed via an IR dryer either to preheating or to postdrying. Moist sheets are brought on a band conveyor 1 to a moisture measuring system 2, and the sheets are directed by a control unit 8 to an appropriate drying line 15...18. Sheets with highest moisture content are predried by IR preheaters 5 on the line 15. The heaters 5 are controlled by a control unit 3. After predrying, the sheets are conveyed to a roll dryer 6, in which the sheets are dried by hot steam at a temperature of about 180 °C on four different levels. After steam-drying, the sheets are cooled. After exiting the roll dryer 6, the sheets are again measured for moisture content in a measurement apparatus 7, measurement information is fed to a control unit 9 which steers a sorter so that sheets still excessively moist are routed to a post-drying line 20. On the post-drying line 20 the sheets are dried by intermediate IR range radiators 11 and near-IR range radiators 12.

Conversely, sheets identified as dry after the roll heater 6 are directed to a processing line 19 for further fabrication.

Figure 4 illustrates a radiation heater construction in which a conveyor 22 carrying sheets to be dried traverses under the heater from left to right. A typical speed for the conveyor is 70...80 m/min. The sized of dried sheets is 1.5 x 1300 x 1410 mm. The heater is divided into two zones approximately equal in area called intermediate IR range zone A and near-IR range zone B. The combined length of the zones A and B in the direction of the conveyor 22 amounts to 1500 mm with a total width of the heater unit being approx. 1550 mm. The intermediate IR range zone A comprises three groups 21 of dual tubular bulb radiators mounted with approx. 55 mm spacing. The near-IR zone B has six groups of dual tubular bulb radiators mounted with approx. 65 mm spacing. Because all IR heated units must be provided with sufficient air circulation to vent fumes and improve drying, the heater unit in accordance with the figure is equipped with two^" fans (not shown) . The fans are placed atop the heater unit approximately centered to each zone. The flow rate per fan is 50 1/min. This rate maintains fanned air temperature at approx. 30...35 °C. To further improve radiation efficiency, the heater unit is provided with an aluminum back reflector (not shown), whose purpose is to reflect radiation passed through the conveyor 22 back to the sheet. The conveyor 22 moves between the radiator and the back reflector plate.

Figure 3 illustrates experiments performed in practical conditions using IR radiators as preheaters in conjunction with a roll heater 6. Material to be dried was birch sheet. Firwood sheets were also tested and found to require less energy to dry. A majority of the material had a moisture content exceeding 80 %. In approx. 15...18 % of the material, the moisture content was in excess of 90 %. Infrared predrying was performed using in turn intermediate wavelength IR energy alone, near-IR energy alone, or combinations of these so that the first combination had a first unit producing intermediate wavelength IR radiation immediately followed by a second unit producing short wavelength IR radiation while the second combination was identical except for reversed order of units. In each run, the applied power and exposure time were maintained constant.

According to the tests, the most* advantageous combination was such a configuration of intermediate and short wavelength IR radiators in which the material to be dried is first exposed to an intermediate wavelength IR radiation whose wavelength of maximum intensity at least approximately coincides with the coinciding maximum absorption wavelengths of water and wood sheet. The final drying is then performed using short wavelength IR radiation. Input power to both dryer units was approximately equal averaging to 41.5 kW per m² of sheet and the exposure time in both zones was 5 s. Due to practical arrangements of test conditions, however, the proportion of energy exposure in the intermediate wavelength unit was 46 % while the rest of the exposure was delivered in the short wavelength unit. The distance of IR radiators to the sheet was 140 mm. Radiators used were dual tubular bulb fused-quartz radiators with gold-plated back reflection layer, manufactured by Heraeus GmbH. The intermediate wavelength dual bulb radiators used were characterized as: radiating section length 1300 mm, cross-section 33/15 mm² (outer/inner), power 3250 , and emission wavelength range 1.5...6.0 μ with a maximum at about 3 urn. Respectively, the short- wavelength dual tubular bulb radiators were characterized as: radiating section length 1300 mm, cross-section 23/11 mm² (outer/inner), power 7000 , and emission wavelength range 0.5...2.5 μm with a maximum at about 1.2 μm. Wavelength specifications referred to above are applicable at maximum nominal power. The maximum exposure time used was 6 s per zone.

Application of predrying decreased moisture content in a sheet lot by 5.0...9.7 percent units. The average decrease in moisture content was 7 percent units. A combination drying procedure achieved a 10 % increase in drying capacity in comparison with conventional drying methods. The drying time was decreased by 10 %. Electric energy amounted to 14 % of energy consumption in the process. Total energy consumption was decreased by approx. 10 %.

In a second experiment the combination of a roll dryer and IR postdrying was tested. When IR radiation is used for postdrying, all sheets with a moisture content in excess of 5...6 % are to be sorted to IR drying.

The IR postdrying tests were run using same wavelength combinations as in the predrying test resulting in an equivalent advantageous combination. The average power used was 43.3 kW/m², of which 48 % was delivered in the inter¬ mediate wavelength radiation unit. The maximum exposure time used was 6 s per zone.

The postdrying process was performed on sheets with initial moisture contents of either approx. 10 % or approx. 15 %.

Application of IR drying decreased moisture content in an average by 6.0 and 7.9 percent units, respectively. The tested combination of power/speed in realistic industrial conditions sufficient to dry overly moist sheets rendering an increase of 15 % in drying capacity. Electric energy amounted to 13 % of energy consumption in the process. Total energy consumption was decreased by approx. 30 %.

Evaporative drying applied using other methods was less effective in comparison with the combination intermediate/short-wave IR drying. In addition, the combination short-wave/intermediate wavelength IR drying presented sheet bulging with equivalent process parameters.

The disclosed invention is also applicable to be used in conjunction with a screen dryer.

Claims

WHAT IS CLAIMED IS:

1. A method for drying planar material, e.g., wood sheet, in which method

- the main drying phase is implemented by exposing the material to be dried to heat energy, and

- measuring the moisture content of the material to be dried prior to and/or after the main drying phase,

c h a r a c t e r i z e d in that

_ material identified in the measurement to be of the highest moisture content is subjected to a first IR radiation, whose wavelength of maximum intensity is at least approximately coincident with the wavelength of maximum absorption in water,

- immediately after the first exposure the material to be dried is exposed to a second IR radiation of a shorter wavelength than the wavelength of maximum intensity in the first radiation, and

- the energy doses imposed by both the first and the second radiation on the material to be dried are arranged to be at least approximately equal.

2. A method in accordance with claim 1, c h a r a c t e r¬ i z e d in that the wavelength of maximum intensity in the first radiation is arranged to be approx.

3 μm and the wavelength of maximum intensity in the second radiation is adapted to be approx. 1 um. 2. An apparatus for drying planar material, e.g., wood sheet, comprising

- a roll (6) or screen dryer, in which the material to be dried can be dried with steam,

- a conveyor (1) by means of which the material to be dried can be transferred into the roll dryer

- measurement devices (2, 7) by means of which the moisture content of the material to be dried can be measured, and

_ control and sorting units (4, 8) by which material identified to be of the highest moisture content can be sorted from rest of the material,

c h a r a c t e r i z e d by

- infrared radiators (5, 11, 12) by means of which such material as identified to be of the highest moisture content can be first exposed to a first

IR radiation whose wavelength of maximum intensity is approximately coincident with the wavelength of maximum absorption in water, and immediately after the first exposure, the material is exposed to a second IR radiation of a shorter wavelength than the wavelength of maximum intensity in the first radiation so that the energy doses imposed on the material to be dried by both the first and the second radiation are arranged to be at least approximately equal in magnitude.

4. An apparatus in accordance with claim 3, c h a r a c t e r i z e d in that reflectors are adapted to oppose the IR radiators (5, 11, 12) behind the conveyor in order to improve the efficiency of exposure to radiation.

5. An apparatus in accordance with either of claims 2 or 3, c h a r a c t e r i z e d in that the IR radiator is comprised of an intermediate wavelength zone (A) comprising a plurality of fused-quartz radiators of intermediate wavelength IR energy, adapted essentially transverse with respect to the transportation direction of the conveyor (1), and a short-wavelength zone (B) comprising a plurality of fused-quartz radiators (23) of near-IR range energy, adapted essentially transverse with respect to the transportation direction of the conveyor (1).