Air drying and pre-drying before the kiln

Air drying removes the easy water cheaply. Most Northern European sawmills kiln-dry (Swedish Wood: about 95% of Swedish sawn wood is kiln-dried at the sawmill), so the yard is a niche, but a valuable one for thick, slow-drying or high-value stock.

Why the yard saves energy

Above FSP the water is free and moves fast, so the first 25 to 30 moisture points are the cheapest to remove. An old US figure (McMillen and Wengert, 1978) puts the saving at roughly 50 to 85 Btu of kiln heat per board foot for each percentage point removed outdoors; These are old, case-specific figures; use them for order of magnitude only.

A board never dries below the EMC of the surrounding air. In a humid maritime climate that is 15 to 18% in autumn and winter, so the yard cannot replace the kiln for interior products. The Wood Handbook defines "air dry" as typically 20 to 25% for dense hardwoods and 15 to 20% for softwoods and low-density hardwoods. Fungi do not grow at 20% or less, so the board must pass through the 30% to 20% band quickly.

Drying time: thickness and season

Air-drying time rises roughly with thickness to the 1.5 power (FPL-GTR-117, North America). So 50 mm lumber needs about three times as long as 25 mm: 2^1.5 = 2.83.

Worked example. If 25 mm stock takes 60 days, then 75 mm stock (three times as thick) takes about 60 x 3^1.5 = 60 x 5.2, about 312 days. The Missouri Extension figures (1 inch red oak 2 to 3 months, 2 inch 6 to 8 months) show the same direction.

Season matters as much as species. Simpson and Hart (2001, USA, experimental days to reach 20%, 4/4 lumber):

The Madison autumn stack took 188 days against 65 for mid-May, a factor of 2.9. After a "critical stacking date" in late summer or early autumn, drying time rises quickly, more so farther north. No European series of air-drying times was verified; the northern US rows are only an analogue for Central European conditions.

Stacking basics (US figures; no European norm was verified)

• Stickers 19 to 25 mm thick, usually 16 to 24 in apart (12 in for valuable or warp-prone stock), vertically aligned; top weight of 50 to 60 lb/sq ft or more to restrain warp.
• End-coat green ends quickly with a thick coating; roof every pile; stack within 2 to 3 days of sawing.

Defects in the yard

Summer sun produces checks and warp; autumn and winter produce slow drying and stain. Sticker stain, blue stain and mould follow warm, humid periods with little air movement.

Pre-dryers and the beech example

A pre-dryer is an enclosed, ventilated compartment that takes green lumber to about 25% or lower before the kiln, at 27 to 38 degrees C and 65 to 85% RH (Wood Handbook). A Polish trial (Kowal et al. 2013, 28 mm beech) air-dried for five months to about 30% MC, then needed 222 hours in the kiln, against 279 hours for green beech loaded at 42%. That is 279 - 222 = 57 hours less, 57 / 279 = about 20%. The authors found no significant defects if stacking was correct, but report no energy or final MC.

Does waiting pay?

Capital tied up in the yard costs: capital cost = value x interest rate x days / 365.

Worked example (portal's illustration with assumed values). 300 euro/m3, 6% per year, 150 days: 300 x 0.06 x 150 / 365 = 7.4 euro/m3. The portal's assumed heat saving was 3 to 12 euro/m3, so the margin is thin. Yard drying pays with expensive stock, cheap land and dear heat; for cheap softwood it usually does not.

Key points

• Air drying is cheapest above FSP; it cannot reach interior targets in humid climates.
• Time rises about with thickness^1.5, so doubling thickness roughly triples the time.
• Stacking date is the strongest lever (Madison: 65 days in May against 188 in October).
• Pre-drying the Polish beech to about 30% cut kiln time by about 20% (222 against 279 hours).
• Interest on stock value x rate x days / 365 must be set against kiln-heat savings.