GRAIN DRYING EXPLAINED
GRAIN DRYING EXPLAINED
How does it dry?
Drying grain using gas or diesel is not about pre drying the air but about expanding the air so it can hold more moisture.
In the natural air there is always a certain amount of water and this is referred to as RH % (relative humidity) when air is 100% humidity it is probable misty rain or fog, in hot weather and high RH you will feel quite hot because the air is so wet there is no evaporation off your skin and your shirt will be saturated. Evaporative coolers do not work well in this situation because the air is already wet. On the other hand if it is low humidity the air wants to take your sweat away and you feel cooled. The % of RH is relative to temperature and density of air. The colder the air the less water it can hold therefore we get dew forming. Drying is a bit like a balloon that I have filled with a cubic meter of air with a litre of water inside it. If I now blow up the balloon to twice the size it is still the same amount of water but instead of being 100% RH it is now 50% I can now fit more water into it. If we heat air or expand it, the RH falls and the air will hold more moisture. So once we heat the air it is about how much air we can get through the product to take as much water as possible. This is often measured in litres or cubic meters of air per sec. Normally taking air beyond 100 Deg c is not achieving much more. It is also dependant as to how well the product gives up its moisture a small seed may give it up willingly but a large seed as with corn, the surface quickly gives up the moisture but around the heart will have its own natural timing and you can’t hurry it.
In silo drying
Many have asked about in silo drying. This is a high risk practice and it requires very good air flow, about 10 times what you would use for aeration. In silos you may have a bed depth not, 600mm but 6000mm or more and the moisture needs to be pushed up through the grain adding the risk of rotten grain if the air flow stalls for any reason. In a 1000 tonne silo with 3% to be disposed of, you have approx. 30 tonne of water to move, try blowing air through your rain water tank and see how long it takes to empty it. There are specially designed silos for drying with high air flow and reduced bed depth. This process is based on favourable weather conditions, if you are looking for 12.5% moisture in the grain you will need a lot of natural air at about 55% RH. Remember if you can’t dry it in the paddock it won’t dry it in the silo. Some have tried to speed the process up by a heater at the fan entry which tends to kill the grain near the fan entry and push more water into other areas. As water moves it chills the next grain via evaporative cooling, so high energy at a small entry in a large volume of grain is very risky. You need a lot of energy to dry grain with heat and high air flow, a bit like a 1 kw bar heater in the kitchen trying to heat the whole house. What we are looking for in silo drying is low RH air that wants to take the moisture with it. Many of the big dryers will be using anywhere between 340 and 2000 bar heaters to do the job depending on air flow
There a number of factors to consider when drying grain.
The type of product
The type of product will determine the expected result, how well does the product breathe? If it is canola or millet for example the air flow will be severely reduced due to the density of the product. Air flow is king and without it to carry away the moisture the rate of drying is very much slowed down. A bit like using a box trailer to empty a 1000 tonnes silo rather than a road train.
The permissible operating temperature
Temperature needs to be considered carefully, so as not to damage the grain particularly with regard to germination. A lot of dryers are limited to approx. 40 deg c. because in a batch situation the grain will see the temperature you have the dryer set to. The grain closest to the heat source could become over dried while wanting for the moisture to completely move through the depth of the product. It is always best to have a bed depth of not more than 600mm.
Grain dried in a cascading dryer can have a much higher operating temperature because the grain is constantly moving in and out of the hot zones and a dryer operating at 60 to 70 deg c may only produce 48deg c in the grain.