Time from Start of the Constant-Drying Rate Period, minutes X avg 67 0.165 (critical value) 87 0.145 102 0.134 119 0.124 138 0.114 162 0.106 183 0.099 205 0.095 216 0.090 a? -In X - X* tf 3DAB"[Xavg – X*
Gilliland and Sherwood obtained data of the drying of waterwet 7x7x 2.54-cm slabs of 193.9 g (bone-dry) brick clay mix for direct-heat convective air drying from the two faces in both the constant- and falling-rate periods. For Xo=0.273, X*=0.03, the rate of drying in the constant-rate period to Xc =0.165 was 0.157 g/h-cm2. The air velocity past the two faces was 15.2 m/s, with Td = 25oC and Tw = 17oC. During the falling-rate period, experimental average slab moisture contents were as attached. At the end of the constant-drying-rate period, the moisture profile is assumed parabolic. Other experiments give DAB = 0.72x10-4 cm2/s. Use tf=a2/3DABln[ (Xc-X*)/ (Xavg-X*)] to predict values of Xavg during the falling-rate period and compare predicted values to experimental values.
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