Heated air versus low-temperature drying


Heated air drying and low-temperature drying (also referred as in-store dyring or near-ambient drying) employ two fundamentally different drying principles. Both have their own advantages and disadvantages and are sometimes used in combination e.g., in two-stage drying.

Heated air drying employs high temperatures for rapid drying and the drying process is terminated when the average moisture content (MC) reaches the desired final MC. While, in low-temperature drying the objective is to control the relative humidity (RH) rather than the temperature of the drying air so that all grain layers in the deep bed reach equilibrium moisture content (EMC). The figure on the right shows the major differences.

Heated-air drying

In heated-air fixed-bed batch dryers the hot drying air enters the grain bulk at the inlet, moves through the grain while absorbing water and exits the grain bulk at the outlet. The grain at the inlet dries faster because in there the drying air has the highest water absorbing capacity. Because of the shallow bed and relatively high airflow rates, drying occurs in all layers of the grain bulk, but fastest at the inlet and slowest at the outlet (Refer to drying curves in the table).

As a result a moisture gradient develops, which is still present at the end of drying. The drying process is stopped when the average moisture content of the grain (samples taken at drying air inlet and drying air outlet) is equal with the desired final moisture content. When the grain is unloaded and filled in bags the individual grains equilibrates meaning that wetter grains release water which the dryer grains absorb so that after some time all grains have the same MC.

The re-wetting of the dryer grains, however, leads to fissuring causing the grains to break in the milling process. This explains why the milling recoveries and head rice recoveries of grains dried in fixed bed batch dryers is not optimal. One way to minimize the moisture gradient during drying is to mix the grains in the drying bin after around 60-80% of the drying time has passed.