Optimising the use of grain stirrers to enhance on-floor drying

Abstract

Grain stirring by vertical augers is a method of mixing grain in bulk stores. It is used to remove the gradients in moisture content (m.c.) that develop when drying a deep bed, and before or after drying. The stirring process adds an extra dimension to bulk drying because it can be used in various ways, e.g. speeding up the drying of the upper layers to reduce risk from fungi, reducing over-drying of the lower layers, and allowing higher temperature air to be used that can increase drying rate. The aim of this project was to examine how stirring influences drying, and how the potential advantages can be best exploited, with a particular focus on potential to save costs for users. A simulation model of bulk drying was used for the study, plus a new element to simulate the action of stirring on the grain bed. This model was validated successfully against a 25t wheat drying experiment at The Food and Environment Research Agency (Fera) in 2010, drying of two such bins in 2012, one stirred and one static, a published drying test for an 86t bin using higher air temperature, and data from a farm bulk store in 2012, one block of which was stirred, one static.

The simulation model generated performance data for drying wheat from m.c. values of 24 - 16% to a target of 14.5%, at bed depths of 4, 3 and 2m using 20 years of weather data from Lincs. Risk of fungal toxin was calculated, and only treatments giving 19 or 20 years without risk were counted as successful. A range of drying approaches was tested.

When using the drier with r.h. in the plenum regulated to 62%, stirring the bed continuously reduced the risk of fungal toxin Ochratoxin A (OA) substantially, compared with a static bed, and hence allowed grain at 2% higher m.c. or 1m higher depth to be dried. But drying with stirring took substantially longer and the fuel and electricity costs were higher. So, when using near ambient air temperature, stirring was only helpful if otherwise there was a risk of OA as judged by the HGCA Safe Storage Time Calculator.

Stirring allowed use of higher plenum air temperatures, within constraints of component design and safety. Use of higher air temperature when stirring, e.g. 30°C, reduced drying time substantially compared with using near ambient air temperature, Electricity cost was reduced because the fan and stirrers were used for a shorter time but fuel use was generally increased.

Guidelines set out which drying problems would, and would not, be addressed by stirring, and how to make best use of a stirring system.