Published 6 minutes read
Contents
Every autumn brings the same sum: how much water to take out of the grain, with how much gas, and without damaging it.
Grain drying is not just a step between the combine and the silo. It is where most money is lost without anyone noticing — on gas burned for nothing, on cracked kernels, or on tonnes of produce sold as water.
Safe storage moisture for each crop
The target is not "as dry as possible". It is the moisture at which grain sits quietly in the silo until it is sold. Below it you pay for gas to remove water you did not need to remove; above it you risk heating and mould.
| Crop | Typical at harvest | For storage | Dryer air, indicative |
| Feed maize | 20–30% | 14% (13% over summer) | 90–110 °C |
| Maize for food or starch | 20–30% | 14% | 60–70 °C |
| Milling wheat | 15–19% | 14% | 55–65 °C |
| Malting barley | 15–19% | 13–14% | 45–50 °C |
| Sunflower | 10–15% | 8–9% | 70–90 °C |
| Rapeseed | 9–15% | 8% | 60–65 °C |
| Soybeans | 14–18% | 12–13% | 50–60 °C |
| Seed, any crop | — | by species | 40–45 °C |
The temperatures are indicative. The exact ones come from the dryer manufacturer and from the buyer: a starch plant or a maltings will tell you plainly what they accept and what they reject.
Why temperature matters, not just moisture
A kernel has something alive in it — the germ, the proteins, the gluten. In wheat, too much heat damages the gluten and the flour no longer rises. In malting barley it kills germination and the maltings rejects the load. With seed it is simpler still: it can no longer be sold as seed.
Feed maize tolerates the most, which is why it is dried at the highest temperatures. But there is a limit there too, and it is about speed rather than the kernel.
Stress cracks in maize
Take the water out too fast and then cool abruptly, and the maize kernel cracks inside. You cannot see it, but you see it at the first handling: kernels break in elevators and conveyors, producing dust and broken grain.
Broken grain lowers the price, and dust in the silo is also a safety issue.
What helps: somewhat lower temperatures for very wet lots, an adequate cooling zone and, where possible, taking the last point of moisture out slowly, with aeration in the bin.
Cooling is part of drying
Grain leaves the hot zone at 50–60 °C. If it goes into the silo like that, the vapour condenses on the cold steel and the water goes back into the grain, right in the top layer.
That is why the dryer has a cooling zone at the bottom, and the grain should leave it no more than 5–8 °C above outside temperature. If it comes out warmer, either the throughput is too high or a fan is no longer pulling as it should.
What one point of moisture costs
Rule of thumb: for every point of moisture taken out of a tonne, an ordinary dryer uses around 1.3–1.6 m³ of natural gas. Dryers with heat recovery sit at the low end of that range; old or poorly tuned ones go above it.
An example with 1,000 tonnes of maize harvested at 25% and dried to 14%:
- 11 points come out, that is almost 128 tonnes of water;
- 872 tonnes of maize remain;
- around 15,000–17,000 m³ of gas are burned, plus electricity for the fans.
The mass after drying is: initial mass × (100 − initial moisture) ÷ (100 − final moisture). For the example above: 1,000 × 75 ÷ 86 = 872 tonnes.
How part of the heat that would otherwise go up the stack is recovered is covered separately, in our article on the heat recovery unit.
Over-drying: the money you do not see
Drying "to be safe", one point below target, looks prudent. In reality you pay twice.
On the same 1,000 tonnes, drying to 13% instead of 14% means about 10 more tonnes lost on the weighbridge and some 1,500 m³ more gas. The buyer pays at 14% — you get nothing for the extra point.
Over-drying usually comes from three places:
- Mixed lots. Maize at 30% and at 20% goes through the same dryer at the same speed. The first comes out wet, the second too dry, and the average looks fine on paper.
- An uncalibrated moisture meter. One point off, over a whole season, means tens of tonnes. It is worth checking against a laboratory at the start of every campaign.
- Measuring hot grain. Right after the dryer, water is still redistributing inside the kernel and the reading is misleading. Measure the sample once it has cooled.
What can be adjusted on a dryer
A continuous-flow dryer has, in practice, two controls: air temperature and discharge speed, meaning how long the grain stays in the column. Everything else — fans, burners, dampers — serves those two.
The manual way is an operator taking samples at the outlet and adjusting speed. The automatic way measures moisture continuously at the outlet and adjusts discharge by itself, keeping final moisture in a narrow band even when the incoming lot changes. We described it in our article on automating a grain dryer.
Sunflower and fire risk
Sunflower brings lint, chaff and fine dust that collect in the corners of the dryer and ignite easily. Most dryer fires of the season start here.
What keeps the risk down: pre-cleaning before the dryer, cleaning the column at set intervals rather than "when there is time", and temperature sensors that shut the burner off in time.
What to check before the season
- the burner and flame: tuning, safety devices, the flame detector;
- the fans: clean blades, belts, bearings;
- the temperature and moisture sensors, compared with a reference instrument;
- louvres and dampers: move freely and close tightly;
- the column: cleaned of last season's residue;
- the discharger: constant speed, no blockages.
A day of checks in August costs far less than a day with the dryer stopped in October and trucks at the gate.
What we do
We carry out installation and commissioning of grain dryers — column and tower types — together with their electrical work and automation, and connect them into the silo flow, from intake to the bins. We also work on existing dryers that need to be relocated, extended or automated.
If you are preparing a project or have a dryer that uses too much, write to us with the crop you dry, the capacity you need and the equipment you have now.
See also how a grain dryer works.
