Grain Drying Energy

Drying grain means boiling water off it, and the energy that takes is set by the latent heat of vaporisation — about 2,257 kJ per kilogram for free water at atmospheric pressure. Water held in grain takes rather more, because it is bound to the starch and protein and must be released before it can evaporate: 2,400–2,600 kJ/kg is the practical range, rising as the grain gets drier and the remaining water is held more tightly. That rise is why the last two points of moisture always cost more than the first two.

The efficiency term is what turns that thermodynamic minimum into a fuel bill. Continuous-flow dryers without heat recovery typically deliver 50–65% of their fuel energy to the water; designs with vacuum cooling or exhaust recovery reach 70–80%. The losses are not simply waste heat — a large share leaves as sensible heat in the exhaust air and in the grain itself, which comes out of the dryer hot. Recovering that heat by cooling the grain in a separate pass, or recirculating exhaust to the burner, is where the better efficiencies come from.

The mass of water is the input most often got wrong, because removing moisture also removes weight from the lot. Drying $M$ tonnes from $m_1$ to $m_2$ percent wet basis removes $M(m_1 - m_2)/(100 - m_2)$ tonnes of water — note the denominator, which is the dry-matter fraction that remains and NOT 100. Taking 20 tonnes of wheat from 18% to 14% removes $20(4)/86 = 0.93$ tonnes of water, not 0.8. The difference is the shrink, and it is the same arithmetic that decides what the load weighs when it is sold.

Two practical notes for costing. Propane carries about 25.3 MJ per litre and natural gas about 37.5 MJ per cubic metre, so the energy figure converts to fuel directly. And the fan is not free: a high-temperature dryer's electrical load is a real cost, and for near-ambient drying, where the fuel is mostly electricity, it is essentially the whole cost. Near-ambient drying trades money for time and weather risk, which makes it cheap in a kind autumn and expensive in a wet one.