Innovative technologies and technical means of drying which allow to dry the grain efficiently and with minimal costs are very important for development and improvement of grain industry. These technologies include the oscillating modes, the essence of which is that the material is periodically affected by heated and unheated drying agent. These modes provide periodical grain cooling, that allows to increase drying agent temperature and decrease heat losses. However, periodical grain cooling with unheated air often lowers the effect of oscillating mode. Pulse mode, unlike the oscillating one, is characterized only by decreased liquid fuel supply in the dryer, and grain cooling on a value close to theoretical one, which allows to decrease heat losses and increase dryer efficiency. The mathematical methods of calculation of temperature and duration of impact of low-heated drying agent on the grain are suggested. To determine the temperature of low-heated drying agent and its impact duration, the heat and mass transfer equations in layer and caryopsis were composed and solved; the calculated expressions were obtained from them. During acceptance testing of the S-30 dryer, the calculated dependences for determining these parameters were checked. The wet grain was continuously sent into grain dryer, the dried and cooled grain was withdrawn at the same time. The «big fire» nozzle in firebox was periodically enabled and disabled, and the «low fire» nozzle remained in working mode, thus a pulse mode of drying was created. The maximum temperature of heated drying agent was 105 degrees С, while the temperature of low-heated drying agent was 46 degrees С. It was established that curves of changing in temperature and moisture of grain did not differ from the classic curves under oscillating drying mode. The use of pulse drying mode allows to shorten by two times the amplitude of temperature oscillations of drying agent and to reduce heat losses by 10-12%. The temperature of low-heated drying agent should not be below the maximum permissible heating temperature of grain by more than 8-10 degrees C.
In seed-production farms, the temperature of drying agent is often limited by the maximum permissible temperature of heating; the simultaneous moisture removal is 3-6%; seeds are dried to a moisture content that exceeds the normal one by 2-3%; the layer is stirred several times during drying process. All of this increases the process cost and could lead to crop losses. The paper analyzes the essential factors influencing the safe drying of seeds in dense layer. It is found that along with the maximum permissible temperature of heating, account must be taken of the rate of dehydration, the specific surface of seeds, the heat transfer coefficient and the unevenness of moisture of basic material and of a dried-up one. The estimation of parameters for safe drying of seeds is carried out on the basis of mathematical models and well-known empirical expressions of the heat resistance and maximum permissible moisture removal. The article proposes the calculation of drying agent temperature that ensures the complete safety of the process, for breeder seeds also. Permissible temperature of drying agent is calculated by the expression that takes into account the specific surface of grain, the heat transfer coefficient, the heat of vaporization and the coefficient that depends on the permissible rate of dehydration, as well as the maximum permissible temperature of seeds heating. The maximum height of the layer of seeds being dried without stirring depends on the speed of drying agent, the diameter of grain, the porosity of layer, the mass transfer coefficient and the moisture content of drying agent before and after the layer. The maximum permissible temperature of seeds heating should be calculated according to the formula of S.D. Ptitsyn, but the maximum humidity of 10% of weighed portion should be used as a basis.
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