Brief summary of the drying process
The drying process is composed of three well-separated phases in the tower dryers.
(Figure 1) In phase I the material mainly warms up with a slight (approximately 1%) loss of moisture. That takes about half an hour. In phase II intensive and rather uniform drying takes place with significant loss of water content per unit of time, with constant grain temperature. The next phase which comes one and a half hours after the housing of the grain is (from the aspect of the loss of water) a decelerating process and a critical one at that. Water content is decreasing in the shape of a flattening curve while the temperature of grain is rising. Over-drying can take place obviously at that phase, which is the asymptotic part after the 13% value, where 1% loss of water requires increasing time and grain temperature increases following an exponential curve.
The effects of over-drying
When considering the content of the above section it is obvious that the consequences of storage, primarily those of over-drying performed for reasons of safety are as follows:
- the nominal performance of the tower decreases, meaning decrease in capacity
- in the given phase the energy cost of the elimination of 1% water increases
- the grain can become overheated with damaging effect, causing deterioration of its composition
- over-drying means loss in the weight of grain
The economic effects of over-drying
Considering the facts described above it is possible to realistically estimate the surplus costs arising when the grain is over-dried during the operation of tower dryers.
The drop in nominal performance leads to about 11-14% capacity decrease when 2% over-drying is performed and that occurs when surplus drying takes place for 2-3 hours compared to the same performance in 24 hours
2% over-drying consumes 12% more energy
the deterioration of the composition (quality) of the grain as a consequence of over-heating leads to a drop in sales price, which is difficult to foresee and can be calculated only on the case by case basis
The following lessons can be drawn from the consequences of over-drying – which are easy to estimate:
cost increases by some 11-14% for the entire drying process due to capacity decrease
surplus cost is approximately 12% considering the energy costs alone
For reasons discussed above a regulatory device is needed which is suited to avoid to the highest possible extent the subjective mistakes caused by the operator as well as the sudden change in water content so the dryer can operate with the least possible loss.
For the operation of the regulator one must always keep in mind the fact that:
the engineering systems of the dryers permit regulation only to a limited extent
because of the unusually big time constants regulation is of specific character
due to the large masses moving together the control measurements at the measuring points are less exact than the measuring intended to check
- the setting of the regulator is always linked to the given place, so calibration on the site could not be avoided and it is also specific