Using the air-drying estimator to predict the air-drying time for lumber

WOOD PRODUCTS NOTES Department of Forest Biomaterials Campus Box 8003 North Carolina State University Raleigh, North Carolina 27695-8003 Summary: Lumber is often air-dried for months prior to being kiln dried. The air¬ drying yard manager must decide when to end air¬ drying and move the lumber to the dry kiln. This issue of Wood Products Notes presents the Air-Drying Estimator Excel spreadsheet which is based on previous air-drying of lumber research. The intent of the program is to help the air-drying yard manager estimate how long to air-dry lumber prior to kiln drying based on species group, U.S. location, the start date, initial moisture content, and the target or ending moisture content. The Air- Drying Estimator will estimate the date the lumber dries to the target moisture content, the number of air-drying days required for the base case and for lumber placed on the yard each month, and displays drying curves. Phil Mitchell Wood Products Extension Campus Box 8003 Raleigh, NC 27695 (919) 515-5581 phil_mitchell@ncsu.edu https://research.cnr.ncsu.edu/bloes/wp Distributed In furtherance of the acts of Congress of May 8 and June 30. 1914 North Carolina State University and North Carotma A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national onpn. religion, sex. age. or disability In addition, the two Universities welcome all persons without regard to sexual orientation North Carolina Stale University. North Carolina A&T State University. U.S Department of Agriculture, and local governments cooperating Using the Air-Drying Estimator to Predict the Air-Drying Time for Lumber By Phil Mitchell North Carolina State University Introduction For many applications, hardwood lumber must be dried prior to use. Because some species (such as oak) must be dried slowly, using a limited resource such as a dry kiln is too time consuming and would result in low productivity of dried lumber. As a result, air-drying is typically employed prior to kiln drying for many slow drying species. As Denig et at. point out (2000), the air-drying process is incorrectly thought to be efficient and cost effective. First, air-drying time for some locations and seasons are simply too long and hence costly to be either efficient or cost effective. Secondly, air-drying beyond about 28% moisture content (MC) usually increases degrade. When these two facts are coupled with the knowledge that most drying degrade begins during the early stages of drying, the importance of being able to model the rate of moisture loss during air¬ drying becomes evident. The problem is complicated by the fact that the rate of air-drying for lumber varies based on geographic location and the time of year. Air-Drying Models There have been studies to estimate air-drying times for lumber. Denig and Wengert (1 982) published an air-drying calendar for red oak and yellow poplar 4/4 lumber drying in the Roanoke, VA area based on weather data and measured moisture loss. The equations developed predicted daily MC loss and included the variables of average daily temperature and relative humidity and the average daily moisture content. Other weather variables such as wind speed and precipitation were not found to be significant. The resulting models that they found to best describe the air-drying process were as follows: NC STATE EXTENSION