Kiln types and drying schedules

A kiln schedule is, in the words of the FPL manual, "a carefully worked-out compromise" between fast drying and avoiding defects: a series of dry-bulb and wet-bulb temperatures applied in stages and limited by drying stress.

Kiln types (FPL classes, North America)

Boundaries differ by source, so ask a supplier for actual dry-bulb values.

• High-temperature kilns can dry in half the time or less (Garrahan, Canada). Frühwald (Sweden, 2007) found bending strength fell with drying temperature (80, 120, 170 degrees C, Norway spruce and larch) while stiffness was largely unaffected. Do not assume strength neutrality without your own test data.
• Dehumidification kilns condense water on cold coils; Garrahan says they can use up to 50% less energy, but mostly electricity, which costs more than thermal energy.
• Vacuum kilns cut time sharply. In a Hungarian oak plant (Kocsis and Nemeth 2026, Europe) 28 mm oak took 20 to 25 days convective against 4 to 5 days in vacuum. Annual average thermal energy was about 1560 MJ/m3 against about 1000 MJ/m3, but electricity rose from about 45 to 176 kWh/m3. One plant; no capital cost was verified.

Setpoints and stages

The wet-bulb setpoint equals the dry-bulb setpoint minus the depression. Hardwoods are usually dried by moisture-content schedules (settings change when controlling samples reach given MC); softwoods usually by time schedules.

North American T and C codes. Each species and thickness has a temperature code T (T1 to T14) and a depression code C. For 4/4 sugar maple (T8-C3) the moisture steps are above 40, 40, 35, 30, 25 and 20% and depression schedule 3 gives 5, 7, 11, 19, 35 and 50 degrees F. Temperature changes wait until the controlling samples reach 30%. The T-series dry-bulb values were not readable and are not given, and no commercial Swedish, Finnish or German schedules were available; the T/C system is North American.

Worked example (illustrative dry bulb). At the 25% step of schedule C3 the depression is 35 degrees F. If the dry bulb were 130 degrees F, the wet bulb would be 130 - 35 = 95 degrees F.

Stage logic (FPL manual)

1. Early stage: low temperature, high humidity to protect the surface of wet wood; hold the initial temperature until controlling samples reach 30%. For softwood brown-stain control the initial dry bulb should not exceed 120 degrees F.
2. Humidity down: RH can be lowered gradually after about one-third of the green MC has been lost.
3. Temperature up, late: temperature generally cannot be raised until the average is about 30%. After the samples reach 15 to 20%, 200 degrees F or more is usable. Mid-drying dry-bulb increases are the most dangerous change because of honeycomb risk.
4. Equalise and condition (Lesson 6).

Doubling thickness raises drying time by a factor of about 3 to 3.5. Using the next higher depression schedule speeds drying at the cost of minor checks.

A European oak example

Campean and Lazarescu (Romania, 2016) recommend for conventional oak: at or below 30 to 40 degrees C until the critical MC of 27%, EMC as high as possible, air velocity up to 2 m/s; then 60 degrees C (70 degrees C up to 40 mm); equalise and condition at 50 degrees C and spray to EMC 10 to 12%. Their 40 mm Quercus robur trial dried from 50% in 700 hours with few surface checks. A schedule is calibrated to a raw material: changing species or sapwood share without recalibration invites defects (Hajian et al. found spruce and pine schedules unsuitable for western hemlock).

Key points

• A schedule sets dry bulb and depression by stage; wet bulb = dry bulb - depression.
• Hardwoods: MC-based steps; softwoods: time-based steps.
• Start gently, lower humidity after about one-third of green MC is lost, raise temperature late.
• Doubling thickness multiplies drying time by about 3 to 3.5.
• Vacuum and heat-pump systems trade thermal energy for electricity.
• T/C codes are North American; no verified commercial European schedules.