A Conventional Kiln Schedule

Accelerated schedules for fast drying softwoods - a single step 90/60°C -compared to that offered in Table 8.4a produce evenly dried, stress and strain free wood in two days rather than 4-5 days. The lumber will also be free of kiln brown stain (see later). 

Moisture content of Dry-bulb Wet-bulb Relative Equilibrium moisture 

In one sense, kilns maintain a relatively high humidity in order to slow down the rate of evaporation, particularly early in the sehedule when drying impermeable speeies. This means a high equilibrium moisture eontent, less surfaee shrinkage and differential strains early in the schedule - less risk of eheeking. 

The kiln operator selects the appropriate sehedule and adjusts the temperature and humidity as necessary by manipulation of the heating eoils (dry heat) and spray valves (steam), with occasional adjustments to the air inlet and outlet vents, which exchange hot, moist air from the kiln for cold, drier air from outside. The steam line raises and maintains the humidity while the kiln warms up. It should be used only infrequently thereafter as economy in operation depends on making maximum use of the moisture extracted from the lumber to maintain the required humidity in the kiln. As moisture evaporates the humidity rises above the schedule value and the vents open a little to bleed off some of the hot damp air. 

---