How to Formulate Dry-Kiln Schedules

The efficient operation of a lumber dry kiln depends to a considerable extent on the use of drying schedules that reduce over-all kiln time and minimize seasoning degrade. Forced-air-circulation dry kilns, susceptible to good control of dry- and wet-bulb temperatures, can be operated to increase production at reduced costs if the kiln operator is familiar with the drying characteristics of the wood and uses severe drying conditions when the likelihood of damaging the stock no longer exists. General experience has indicated that, in drying hardwoods green from the saw, moderately low temperatures must be used in the beginning, but as the stock dries, the dry-bulb temperature can be increased step by step. A similar pattern of reduction in relative humidity as the stock dries is generally known to be effective. The problem is to formulate a schedule which involves temperature and relative humidity changes that appear to be adapted to practical kiln operation, yet are based on factors related to the causes of seasoning degrade. In kiln-drying green hardwood, high initial relative humidities must be used to prevent the development of an excessive amount of end checking and splitting or surface checking. In developing a fast schedule, the problem is to set up a technique to determine when it is safe to reduce the relative humidity. These concepts of accelerating the drying of wood by elevating dry-bulb temperatures and reducing the relative humidity when damage through honeycomb or surface checking is no longer likely are incorporated in the Forest Products Laboratory’s recommendations for drying schedules, Report No. D1791.

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