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Title: Chapter 07 Kiln Schedules

Source: USDA Agricultural Handbook AH-188: Dry Kiln Operator's Manual

Author(s): Boone, Simpson

Publication Year: 2001  View PDF »

Related Publications: view

Category: Agricultural Handbook

Abstract: A kiln schedule is a carefully worked-out compromise between the need to dry lumber as fast as possible and, at the same time, to avoid severe drying conditions that ill cause drying defects (ch. 8). It is a series of dry and wet-bulb temperatures that establish the temperatureand relative humidity in the kiln and are applied at various stages of the drying process. Temperatures are chosen to strike this compromise of a satisfactorydrying rate and avoidance of objectionable drying defects. The stresses that develop during drying (ch. 1) constitute the limiting factor that determines the kiln schedule. The schedules must be developed so that the drying stresses do not exceed the strength of the wood at any given temperature and moisture content. Otherwise, the wood will crack either on the surface or internally, or be crushed by forces that collapse the wood cells. Wood generally becomes stronger as moisture content decreases, and, to a lesser extent, it becomes weaker as temperature increases. The net result is that as wood dries, it becomes stronger because of the decreasing moisture content and can tolerate higher drying temperatures and lower relative humidities without racking. This is a fortunate circumstance because as wood dries, its drying rate decreases at any given temperature, and the ability to raise the drying temperature helps maintain a reasonably fast drying rate. Thus, rapid drying is achieved in kilns by the use of temperatures as high as possible and relative humidities as low as possible. For hardwoods, relative humidity can generally be reduced substantially before temperature can be raised substantially.

Keywords: Dry Kiln Operator's Manual Dry Kiln Operators Manual

File size: 490 kb(s)

Date posted: 06/10/2009

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