Natural air can be used for drying grain if the conditions are right. This calculator will determine if the ambient outside air is suitable for drying. The inputs are grain moisture content, %, grain temperature, C, and the outside air temperature, C. The output is the threshold relative humidity for several different types of grain. If the outside air's relative humidity is less than this threshold, drying will occur. A larger difference is indicative of better drying conditions.
When grain at a specific moisture content is allowed to equalize with the surrounding air, it will approach a relative humidity as determined by its moisture content. Equations relating the relative humidity, the temperature, and the Moisture Content at Equilibrium have been developed (EMC) and can be found: ASAE D245.5 'Moisture Relationships of Plant-based Agricultural Products'. These equations will give the relative humidity of a specific grain, that is, at a specific moisture content and temperature. If we blow air into the grain at the grain temperature but with relative humidity below this EMC relative humidity, then drying will occur.
However, the outside air temperature is not the same as the grain temperature as it swings up and down in its daily cycle, and the grain temperature reluctantly chases. We must find this threshold relative humidity for the air at ambient temperature, not grain temperature. We will use psychrometric saturation charts and equations to do this.
The EMC equations gave us the threshold relative humidity for air at the temperature of the grain. So, first, calculate the maximum amount of water (saturation) that water could hold at this temperature (grams of water per cubic meter of air). Multiply this by the EMC threshold relative humidity as determined by the EMC equation, and this then will be the amount of water in the air for the grain at that temperature.
When the outside air hits the grain in question, it will become the same temperature as the grain because it is much, much denser. It will have changed temperature but will contain the same amount of water (absolute humidity). And we just calculated this absolute humidity for the air at EMC. Now calculate the saturation absolute humidity for air that is at ambient outside temperature. The ratio of the EMC absolute humidity over the saturation absolute humidity is the Threshold Relative Humidity for drying for air at ambient temperature. More details can be found at Ron Palmer
If the outside relative humidity is the same as this threshold relative humidity, then no drying or wetting will occur. If the outside relative humidity is greater, then wetting will occur. And if the outside relative humidity is less than this calculated threshold relative humidity, then drying will occur.
This calculator can also be used to determine when condensation will occur on the interior of the walls and roof. This is the case if the calculated threshold relative humidity is greater than 100. This happens when the air temperature is much less than the grain temperature. The cold outside air goes through the grain, is warmed, and moisture is added, but when it hits the cold exterior walls of the bin that are the same temperature as the outside air, this discharged air becomes over-saturated, and water is expelled into the form of condensation. This condensation can run down and form pockets of grain. It is not recommended to run the fans if conditions have a threshold humidity > 100.