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Maderas. Ciencia y tecnología

versión On-line ISSN 0718-221X

Maderas, Cienc. tecnol. v.7 n.1 Concepción  2005

http://dx.doi.org/10.4067/S0718-221X2005000100006 

Maderas. Ciencia y tecnología, 7 (1): 49-56, 2005

NOTAS TECNICAS

VENEER BLOCK CONDITIONING MANUAL FOR VENEER AND PLYWOOD PRODUCTION

H. Peter Steinhagen†
†Past visiting Professor at the Wood Engineering Department, University of Bío-Bío, Concepción. Chile.

Received: 30.12.2004. Accepted: 05.04.2005.

ABSTRACT

Veneer blocks are heat-conditioned in water or steam in an effort to plasticize (“soften”) the wood. When a sufficiently heated block is cut into veneer, the veneer will bend over the lathe's knife without splitting. This leads to improved volume recovery as the greatest conditioning benefit. Since conditioning adds to the production cost, a site-specific economic analysis will be necessary to determine profit margins. This manual is based on selected literature sources. It briefly addresses the cost/benefit of block conditioning, heat-conditioning systems, energy demand, target temperatures, and conditioning times.

Keywords: veneer, heat conditioned, conditioning times

COST/BENEFIT OF BLOCK CONDITIONING

Advantages and disadvantages of block conditioning (Resch 1988) are given in Table 1. Based on mill studies and industrial observations, conditioning reportedly increased the volume recovery by 3 to 25 percent. Payback of increased production cost, on the other hand, may often occur at a less-than-10-percent additional volume recovery, assuming fixed capital investment (Resch 1988, Steinhagen et al. 1989). If boiler capacity must be added, payback and return on investment should be carefully calculated. A rigorous cost/benefit analysis of block conditioning has not been published.

Penalties of insufficient block conditioning have been studied in two mills (Steinhagen et al. 1989, Sim et al. 1989). Underheating the blocks by a given amount of time appeared more costly than overheating them. Maximum economic benefit coincided closely with maximum veneer recovery (Fig. 1, where “a” denotes differences between adjacent data points as statistically significant at the 0.05 level, using Scheffé's method)

 

Table 1. Advantages and disadvantages of block conditioning (Resch 1988)


Advantages

Reasons

Increased volume of recovered veneer There is less splitting and breakage in handling
Increased quality of recovered veneer from high-quality and frozen blocks

There is decreased splitting, reduced degrade from surface roughness

Reduced knife wear Knots are softened
Reduced glue spread

Peel is smoother

Tighter veneer with finer checks and reduced nosebar pressure, especially for thickness above 1 /8 inch Wood is more plastic and less resistant to fine checking, thus reducing deep splits
Greater tensile strength of veneer perpendicular to the grain Veneer is tighter and fine, checks are shallower
Reduced power required for peeling Softened wood offers less resistance to Peeling.
Increased production Faster peeling of softer wood
Reduced drying time with in-line dryers Some heat is stored in wood, and steamed wood is more permeable
Decreased spinouts Thoroughly softened wood requires a Iower turning force

Disadvantages

Reasons

Increased spinouts Main block remains cold despite heat- softened ends
Fuzzy veneer surface Blocks are overheated
End-checking of blocks arid veneer Blocks are heated in dry steam

Fig. 1.- Cost and benefit of block conditioning as a function of heating time (Steinhagen et al. 1989)

HEAT-CONDITIONING SYSTEMS

Blocks are usually heat-conditioned via (a) steaming in drive-in chambers which is a batch process, or (b) hot-water spraying (deluging) in drive-in chambers, also a batch process, or (c) feeding through hot water vats which is a continuous process (Resch 1988). An overview is given in Table 2.

Steam chambers (a) are relatively inexpensive to build and maintain. But the condensate from steaming is "dirty" and must be handled in accordance with governmental guidelines on effluent discharge. Also, the steam must be saturated for conditioning, or blocks will dry and check.

In the deluge system (b), the water can be reused in a closed loop. Blocks may not heat evenly by this method.

Feed-through in hot water (c) will heat submerged blocks evenly. This method is very capital-intensive initially.

To help achieve temperature uniformity between blocks, the blocks must be sorted into diameter classes and classes conditioned for various lengths of time. In addition, doors rather than curtains must be used with chambers (a and b) to avoid heat leakage. Also, blocks must be fully submerged in water and the water agitated (c).

Table 2. Methods for block conditioning (Resch 1988).


 

Process

Method

Batch Continuous

Steam sprayed under Aboveground Aboveground chamber
  low pressure chambers (drive-in vaults) (conveyors)
  high pressure    
Spray or deluge with hot water: Aboveground or Aboveground chamber
  below 90 °C belowground chambers (conveyors)
  super heated    
  mixed with steam    
Immersion in water heated by: Submerged, covered Feed-through soaking vats,
  steam coils soaking vat above or below ground
  live steam    
  external heat exchanger    

CONDITIONING ENERGY DEMAND

Examples of net energy required to heat green wood (Steinhagen 1977) are given in Table 3. The table values, reflecting unit energy demand in terms of kJ/m3.°C, vary strongly with moisture content: wood low in moisture content (Pseudotsuga menziesii heartwood, etc.) demands relatively little unit energy, and wood high in moisture content (Quercus sp., Populus sp., etc.) demands much unit energy. Thawing also has an important effect on unit energy demand.

Unit energy demand values must be multiplied by the total volume input and the total temperature increase over the heating range to estimate the total net energy demand.

The gross energy demand is the sum of the net energy demand and energy losses to the surroundings. Losses occur by warming up the construction, and by leakage, and reach a peak during winter. Losses may account for 95 percent, and leakage alone for 60% percent, of actual gross energy consumption (Kuhlmann 1962). This should offer substantial opportunities for improvement.

Table 3. Net energy required to heat green wood (Steinhagen 1977).


 

Initially nonfrozen

Initially frozen

 

(kJ/m3.°C)

(kJ/m3.°C)


Pseudotsuga mensiezii

1283

1540

(heartwood)

Quercus sp.

2566

3079

Populus sp.

2566

3079


CONDITIONING TEMPERATURES

Softwood blocks temperatures suggested for rotary peeling (Resch 1988) are often between 50 and 60 °C or above, measured at the core limit (Table 4). Using the upper-range table values appears economically beneficial (Resch 1988, Sim et al. 1989).

Hardwood block temperatures suggested for peeling (Fleischer 1959) are strongly correlated with the wood's specific gravity. For example light wood species (Tilia sp., Populus sp., etc.) peel well at 20 °C, but dense wood species (Quercus sp., Carya sp., etc.) may require 90 °C, measured at the core limit (Fig. 2).

Temperatures recommended for slicing are often 6 to 12 °C higher than for peeling (Lutz l972).

Table 4. Conditioning temperatures suggested for softwood peeler blocks (Resch 1988).


Species T(°C) Species T (°C)

Western

Western

Chamaecyparis nootkatensis

50-60

Pinus sabiniana

60-80

Calocedrus decurrens

20-50

Pinus contorta

60-80

Chamaecyparis lawsoniana

50-70

Pinus ponderosa

60-80

Thuja plicata

60-70

Pinus lambertiana

50-60

 

Pinus monticola

50-60

Pseudotsuga mensiezii

15-60

Sequoia sempervirens

70-80

Abies balsamea

20-55

 

Abies magnifica

20-65

Picea engelmannii

50-60

Abies grandis

20-65

Picea sitchensis

50-60

Abies procera

20-65

 

Abies sp.

20-65

Taxus brevifolia

80-90

Abies concolor

20-65

 

 

 

 

Southern

Tsuga heterophylla

50-70

Pinus tadea

50-70

Juniperis occidentalis

60-70

Pinus palustris

50-70

 

Pinus serotina

50-70

Larix occidentalis

60-65

Pinus echinata

50-70

 

Pinus elliottii

50-70

 

Picea sp.

50-60


Fig. 2.- Conditioning temperatures suggested for hardwood peeler blocks (Fleischer 1959).

 

Conditioning times

Conditioning periods necessary to meet target temperatures (Steinhagen 1989) are shown in Figures 3 and 4, respectively, for nonfrozen and frozen peeler blocks 8 feet long and up to 25 inches, in diameter. The graphs apply to a target core diameter of 5 inches, a specific gravity of 0.5, and a moisture content of 100 percent. (Specific gravity and moisture content are used here as a key to the wood species effect, and data for many wood species may be looked up in the USDA Wood Handbook 1987). Also, the steam or agitated water bath temperature must be known or estimated, as well as the block's initial temperature and its target temperature.

As an example of how to use the heating time graphs, let us make assumptions as follows: the block under consideration is nonfrozen; block diameter = 18 inches; the initial temperature of the block (Tinitial) = 21.1 °C; the target temperature of the block (Tfinal) = 60 °C; and the water bath temperature (Tbath) = 82.2 °C. Then, (Tbath – Tfinal)/(Tbath – Tinitial)=(82.2 °C – 60 °C)/(82.2 °C – 21.1 °C) = 0.36. This value, together with 18 inches of block diameter, gives a heating time estimate of 25 hours (Fig. 3, dashed line). Adjustments, if necessary, can be made as follows;

If the target core diameter = 4 inches, add 1 hour to the hours given by the figure.

If the target core diameter = 6 inches, subtract 1 hour from the hours given by the figure.

If the specific gravity = 0.3, subtract 5 percent from the hours given by the figure.

If the specific gravity = 0.7, add 5 percent from the hours given by the figure.

If the moisture content = 50 percent, subtract 10 percent from the hours given by the figure.

If the moisture content = 150 percent, add 10 percent from the hours given by the figure.

If the water bath is not agitated, add 10 percent from the hours given by the figure.

If the wood species contains much ray volume (Quercus sp., Carya sp., etc), subtract 10 percent from the hours given by the figure.

The user may perform linear interpolations between these values to find the proper adjustment to the hours given by the figure.

The significance of block diameter on heating time should be noted. For example, if block A has twice the diameter of block B, block A will require about five times as much heating time as block B, given equal core diameters. Therefore, small diameter and large-diameter blocks should not be conditioned together and for the same number of hours.

Fig. 3.- Conditioning time to reach target temperature in a nonfrozen block, given a specific gravity of 0.5, a moisture content of 100 percent, and a target core diameter of 5 inches. The dashed line and data point refer to the example given in the text (Steinhagen 1989).

Let us now reconsider the previous example but assume that the block is frozen. Then, Tbath – Tfinal = 82.2 °C – 60 °C = 22.2 °C. This value, together with the assumed 18 inches of block diameter, gives a heatíng time estimate of 42 hours (Figure 4). For frozen wood, it is not important to know the initial temperature precisely, as long as it is safely below 0 °C.

Adjustments, if necessary, can be made as stated for nonfrozen blocks, with the following exceptions:

If the moisture content = 50 percent, subtract 20 percent from the hours given by the figure.

If the moisture content = 150 percent, add 20 percent from the hours given by the figure.

The effect of block diameter on heating time is the same as mentioned for nonfrozen wood.

Heated blocks cool rapidly during transfer from the conditioning facility to the lathe, particularly in winter. It is advisable to install, for process control, an infrared temperature sensor at the lathe so that veneer temperature can be monitored continuously while the block is peeled (Resch 1988).

 

Fig. 4.- Conditioning time to reach target temperature in a frozen block, given a specific gravity of 0.5, a moisture content of 100 percent, and a target core diameter of 5 inches. The dashed line and data point refer to the example given in the text (Steinhagen 1989).

NOTE

♣Technical note invited in memoriam of  Professor Dr. H. Peter STEINHAGEN. First published as Technical Report N° 23 of the Idaho College of Forestry, Wildlife and Range Sciences, University of Idaho, Moscow, Idaho. September 1991.

LITERATURE CITED

FLEISCHER, H.O. 1959. Heating rates for logs, bolts, and flitchs be cut into veneer. USDA Forest Service, Forest Products Lab. Report 2149, Madison, Wis. 18 pp.        [ Links ]

KUHLMANN, A. 1962. Heat consumption and heat balances for steaming of gaboon peeler logs. Holz als Roh- und Werkstoff 20(6):224-235. (In German.)        [ Links ]

LUTZ, J.F. 1972. Veneer species that grow in the united states. USDA Forest service, Forest Products Lab. Research Paper 167, Madison, Wis. 129 pp.        [ Links ]

RESCH, H. 1988. Heat conditioning of veneer blocks. Forest Industries, April 1988: 22-23         [ Links ]

SIM, H.C.; STEINHAGEN, H.P.; GOVETT, R.L. 1989. Effect of heat conditioning time on veneer recovery from grand fir peeler blocks. Forest Products Journal 39 (7-8);25-21.        [ Links ]

STEINHAGEN, H.P. 1977. Note on energy requirements for heating veneer logs. Proceedings, Practical Application of Solar Energy wood processing (Workshop at Virginia Polytechnic institute and State University, Blacksburg, Va.) 1977: 80-81, Forest Products Research Society, Madison, Wis.        [ Links ]

STEINHAGEN, H.P. 1989. Graphic method to estimate heat-conditioning periods of frozen and nonfrozen peeler blocks. Forest Products Journal 39(11-12):21-22.        [ Links ]

STEINHAGEN, H.P.; SIM, H.C.; GOVETT, R.L. 1989. Penalty of insufficient conditioning of grand fir and Douglas-fir veneer blocks. Forest Products Journal 39(3):51-52        [ Links ]

USDA FOREST SERVICE, FOREST PRODUCTS LAB. 1987. Wood Handbook: Wood as an engineering material. Agriculture Handbook 72, Washington, D.C.         [ Links ]

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