Apparatus for determining mass transfer coefficients at high temperatures for exposed particulate crops, with initial results for wheat and hops

Abstract

This paper describes an apparatus for studying the through drying behaviour of thin layers of particulate crops at air temperatures of up to 300°C and air speeds of up to 1 m/s. The need for such data in the solution of mathematical models is explained. Tests with wheat grain dried with air at 70°C for up to 8 h show that the drier produces consistent weight loss data. The influence of through air flow on apparent weight is identified and removed from the data and the moisture content of the drying sample is calculated. The sensitivity of the weighing apparatus and the rate of data logging is such that the initial rise in drying rate as the sample temperature rises can be detected. Data from a test with hops is given for comparison. Results show a more complex drying behaviour than that predicted by Newton's falling rate law or a diffusion law for a spherical particle. Measurements of the surface temperature of the sample by infrared pyrometry are shown to be valid and are expected to be important in modelling crop quality.