Modelling the performance of a cross-flow grain drier

Abstract

A computer model of a cross-flow grain drier was developed in which the equations of heat and mass transfer were solved by Euler integration and the drying rate was defined as the product of a drying constant, k , and the excess of the instantaneous moisture, M , above an equilibrium value, M e . To validate the model, five different combinations of expressions for k and M e were used to predict the performance of three cross-flow driers for which test data were available. Comparisons were made of the observed and predicted values of grain throughput, specific heat energy consumption, temperatures of output grain and grain germination. That combination of expressions for k and M e , which gave the best correlation of throughput of wet grain and specific heat consumption, was used in the computer model to correct the test results to standard conditions and thereby to separate the effect of design from that of operating conditions. A short study of the effect of bed depth, drying air temperature and ratio of drying area to cooling area was used to define a specification for an example cross-flow drier which, except for its having a larger cooling area, could be regarded as typical of contemporary models. The performance of this example drier was then calculated for initial moisture contents of from 16–26% w.b. and drying air temperatures from 40 to 100°C when drying wheat to 15% w.b. At the normal rating conditions of drying air temperature and initial moisture content of 65.5°C and 20% w.b. respectively, the drier had a predicted throughput of 2·2 t/h and specific heat energy consumption of 4·99 MJ/kg water evaporated. However, across the range of temperatures and moisture contents examined, throughput varied from 0·45 to 12·5 t/h and specific heat consumption from 8·74 to 3·74 MJ/kg water evaporated. Grain cooling was shown to depend upon the residence time determined by the interaction of drying air temperature and initial moisture content. In the example drier, total residence times greater than 0·6 h were necessary for output grain to be cooled to within 5°C of ambient temperature.