In order to ensure dimensional stability for construction, SPF $($spruce$,$ pine, fir$)$ lumber is kiln dried from an initial moisture content of $28wt\%$ to a maximum of $19wt\%$ on a dry wood basis. Assuming that the kiln maintains the surface moisture of the wood at $15wt\%,$ use the general transient charts to determine how long it will take to meet the maximum concentration of $19wt\%$ at the center of the board for a standard $212($actual dimensions are $1\mathrm{.}5in11\mathrm{.}25in)$ that is $10ft$ long,

a$)$ Assuming that mass transfer through the end sides is negligible, that is$,$ only the $2$ long flat faces are important for mass transfer.

b$)$ Assuming all faces are important. This will require an iterative solution.

Data: $D_{H20,SPF}=D_{AB}=1\mathrm{.}0{10}^{-6}c{m}^2{s}^{-1}.$