On the falling‐rate period

A fixed bed of glass particles, wetted with water, was dried from above. The measured moisture profiles within the sample show that its surface is not completely dry during the falling‐rate period. Hence, the current view, according to which the moisture front retreats into the sample directly after reaching the critical moisture content, has to be revised. Assuming that parallel dry and wet channels form within the sample during the constant‐rate period, the onset of the falling‐rate period depends only on the diameter of these channels. If the moisture is distributed very finely, a large number of channels with a small diameter have to be considered. The falling‐rate period for this system occurs at low moisture contents. For a coarse distribution of moisture, fewer channels with a large diameter should be assumed. Their drying rates show the beginning of the falling‐rate period already at high moisture contents. A good agreement between measured and calculated drying rates could be achieved by fitting the channel diameter. Only for very low moisture contents, is the agreement poor. This is to be expected, because the measured moisture profiles show a completely dry sample surface for these moisture contents, indicating that the moisture front has already retreated into the sample.