Aeration Airflow Requirement

Aeration is not drying. Its purpose is to move a temperature front through stored grain so the whole mass sits cool and uniform, and it works at airflows one or two orders of magnitude below what drying needs. Cool grain keeps: insect activity falls away below about 15 °C and stops near 10 °C, mould growth slows sharply, and — as the safe storage calculation shows — every 10 F° of cooling roughly doubles the time the grain will hold.

The rate is quoted per unit of grain, and the two trades use different units for the same thing. North America says cubic feet per minute per bushel; the rest of the world says litres per second per tonne. Useful anchors: maintenance aeration runs 0.5–1 L/s per tonne, cooling a warm bin wants 1–2 (roughly 0.1–0.2 cfm/bu), and in-bin near-ambient DRYING needs 10–20 or more, which is a different class of fan entirely. The cooling front moves through the grain at a speed set by the airflow, and a rule worth remembering is that a full cooling cycle takes on the order of 100–150 hours at 1 L/s per tonne — days, not hours, and the fan must be left running through them rather than cycled.

The trap in fan selection is that airflow is the easy half. Grain resists airflow, and the resistance rises much faster than linearly: doubling the airflow through a given depth can roughly quadruple the static pressure required, and deepening the bed raises it again. A fan that delivers its rated airflow against 250 Pa may deliver a fraction of that against the 1.5 kPa a deep bin of canola presents. Canola, flax and other small seeds are far more resistant than corn or wheat at the same depth, and a fan sized from a corn table will disappoint badly on canola. The airflow figure this page gives is the starting requirement; matching it to a fan curve at the actual static pressure is the rest of the job.

Two operating cautions. Air carries moisture as well as heat, so running a fan in warm humid weather can rewet the top of the bin — aeration is run when ambient air is cooler than the grain, which usually means at night and in autumn. And the cooling front is a front: it must be pushed all the way through and out, because a fan stopped halfway leaves a warm zone with a moisture band at its boundary, which is worse than not having started.