The effect air velocity has on the possible drying rate is simple. The air flow in the kiln chamber can be seen as a transportation system which lifts moisture from the surface of the lumber and carries it out of the chamber through vents (or to a DH unit). Providing adequate vent capacity; if you double the air velocity, you double the potential removal of moisture. We use the word "potential" to describe moisture loss because removal is dependant upon the surface moisture available.
There are generally two contradicting conditions we like to achieve with air flow.
- We try to minimize electrical cost (this can be done by lowering air flow).
- We try to achieve even air flow throughout the kiln load (higher air flow produces more even air flow).
Though lower air flow saves electricity, it decreases uniform drying across the load. Hence the question "why does high air flow produce more even air flow throughout the kiln load?" In short, the air chooses the path of least resistance. This normally occurs when air escapes between the lumber packs, between the 4x4 kiln blocks (runners), over the load, etc. As you increase the air volume, you increase the static pressure on both sides of the kiln load. As static pressure increases, the air will start utilizing more pathways until we achieve nearly uniform air flow through all pathways. This equalizing effect is due to the exponential resistance when velocity is increased through a pathway.
It is normally said that higher air flow is required when the lumber is above fiber saturation point (approx 25% MC) and lower air flow below fiber saturation point. This is not because the drying slows down in the second half of the drying process, but rather because higher temperature and lower relative humidity is used to make each cubic foot of air capable of removing more moisture. The resulting condition is one in which less air volume is required to achieve the same drying rate.
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