Pneumatic fertilizer spreaders : a review of the literature
Svensson, Jan E.T.
Swedish agricultural soils are reported to receive an annual application of plant nutrients as fertilizer amounting to a value almost as large the total new-investment made during one year. Poor precision in nutrient supply results in yield decreases and environmental loading. The purpose of this work is to summarize the knowledge available today on factors that influence fertilizer distribution achieved by pneumatic fertilizer spreaders. The work is mainly concentrated on technical design parameters that influences the spreading uniformity parallel to and at right angles to the direction of travel. The average coefficient of variation for the evenness of spreading of today’s fertilizer spreaders is 25% in the field. In practise, there is no significant difference between centrifugal spreaders and pneumatic spreaders. During the work of which this report accounts for, 87 Swedish and international references have been studied in order to find factors influencing the result of fertilizer spreading. Factors influencing the spreading result can be divided into: the fertilizer, the machine, the field the weather and the operator. The flow properties of the fertilizer and its tendency to deposit on working mechanism are affected by its physical properties. The particle size distribution influences length of throw, flow properties and tendency of separation. The durability of the fertilizer will affect the particle size distribution. Particle shape and surface texture affect the flow properties. Density influences throw properties. Hygroscopicity influences water uptake and thereby flow properties and tendency to deposit within the machine. Thixotropic fertilizers lead to hard deposits being placed on working mechanisms and transport ducts against which the fertilizer glides. The machine can be divided into fertilizer hopper, feeder mechanism, fan, piping, spreader boom and spreader mechanism. When filling a hopper separation generally occurs. Severe separation influences the resulting distribution. When emptying the hopper the slope of the floor will influence the amount of fertilizer remaining in the hopper. 60—65° slope of the bottom of the hopper will ensure complete emptying. Metering mechanisms can be divided into active and passive mechanisms. The function of passive mechanisms depend on the flow properties of the fertilizer. The studded roller is a semi-active metering mechanism. Cyclic variations in out-flow may occur in metering devices that measure out the fertilizer in portions, e.g. cell wheels, star wheels and augers. Shaking and slope of hopper influences the mass flow from passive metering mechanisms. The fan is used to transport the fertilizer within the machine. The flow of air can be used to reduce drift caused by wind if both the flow of fertilizer and the flow of air are directed downwards at the spreading mechanism. At high fertilizer application rates the fan-pipe system in modern fertilizer spreaders tend to overload, with pulsating flows as a result. These pulsating flows are reflected in a severe deterioration of the spreading uniformity. The piping functions as a way of transport of the fertilizer. Piping with a high coefficient of friction leads to a rapidly attained even radial distribution of granules in the pipe. Curves induces pressure losses and deteriorated radial distribution in the pipe. Bends immediately before the spreading mechanism should be avoided. Movements of the spreader boom influences the final result of fertilizer spreading. Boom movements occur largely as a result of the torque of the fertilizer vehicle along its respective longitudinal and vertical axles. The response of a boom suspension to the rolling motion of the vehicle to which it is attached should be such that the boom is insulated from the high frequency random rolling movements but is able to follow the low frequency undulations of the ground. An example of such a boom suspension is found in the literature. In the literature, spreader mechanisms are found in many different shapes. Significant for the majority of them is that their evenness of distribution is sensitive to the radial distribution of granules in the pipe immediately before entering the spreader mechanism. The evenness of distribution is also sensitive to the working height above the ground. The field will through its geometry (shape, slope and bumpiness) affect the working result in form of unevennesses in the spreading pattern, gaps and overlapping. Soil type variations and varying cultivation capacity demand variable application rates. The weather factors that mainly influence the spreading accuracy are wind speed, air temperature and relative humidity. The wind speed influences the length and direction of throw. Air temperature and moisture are together with the temperature of the fertilizer decisive for the water uptake of the fertilizer. Large water uptake will change the flow properties of the fertilizer in a negative direction. The operator is the overall regulator in the "granule spreading system". The operator adjusts and maintain the machine. It is the operator who choses driving technique and the way the work is done. A technically very good fertilizer spreader cannot compensate fully for a bad operator. The operator should have assistance in steering the tractor and information of and possibility to adjust the application rate. What requirements can be placed on the future fertilizer spreader? The requirements placed on the fertilizer spreader of the future involve large control ability, i.e. high spreading accuracy and possibility to continuously vary the application rate. Furthermore, some kind of control over the mass flow of fertilizer will be needed. This review of the literature is concluded with a presentation of a new type of fertilizer spreader. The concept gives precedence to low manufacturing cost, active metering mechanism, variable mass flow and even distribution of fertilizer to the spreader mechanisms.
Fertilizer; spreaders; eveness of distribution
Rapport - Sveriges lantbruksuniversitet, Institutionen för lantbruksteknik
Utgivare: Institutionen för lantbruksteknik, Sveriges lantbruksuniversitet
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