Water Removed in Drying Grain

This is the same relation as the shrink calculation looked at from the other side. Instead of asking what the lot weighs afterward, it asks how much water had to leave, which is the quantity that a dryer is sized against, fuelled against and billed against. Taking 30 t of corn from 24% to 15% removes $30 \times (24-15)/(100-15) = 3.18$ t of water.

The denominator surprises people, and it is worth understanding rather than memorising. It is $100 - M_2$, the FINAL moisture, not the starting one. The reason is that the dry matter is the fixed quantity, and the finished weight is that dry matter divided by its final share. Using $100 - M_1$ instead — a very common slip — understates the water by roughly the ratio of the two brackets, which is about 10% on a typical corn drying job.

The energy behind that figure is the part nobody sees on the invoice. Evaporating a kilogram of water takes about 2.26 MJ of latent heat, and a real dryer achieves perhaps 40–60% of the theoretical minimum, so 3.18 t of water needs somewhere near 12–18 GJ — several hundred litres of propane for a single truckload. That is why drying charges are quoted per bushel per point and why the difference between harvesting at 24% and waiting for 20% is measured in fuel bills rather than convenience.

Two practical footnotes. Overdrying is a straight financial loss on both sides: the grower pays fuel to remove water and then sells fewer kilograms, so a lot dried to 13% against a 15% contract has given away roughly 2.3% of its saleable weight for the privilege. And equilibrium moisture content means the grain will re-absorb water from humid air in storage, so "drying to 15%" in a September prairie bin and in a coastal warehouse are not the same operation.