Features of a reciprocation sprout broadcaster in the process of granular fertilizer application

Introduction.

Since the end of the 1950's, variable bout width distributors were used for broadcast application of granular fertilizers. The bout width is a multiple of the width of the metering and distribution devices. The most important types are the spinning disc and reciprocating sprout broadcasters. The distribution device mainly transfers translation energy to the particles so that they are distributed over a large area.

The construction of the distribution and metering devices is simple; the broadcasters are relatively cheap to purchase and maintain. The combination of a high bout width (8-15 m) and a high driving speed enables high capacities at high application rates of fertilizer. Work load and operation costs are relatively low.

Objectives of this study.

Contrary to the spinning disc system, the reciprocating sprout broadcaster has been the subject of fundamental research only on a limited scale. The objective of this research work described in the previous chapters was to give a first impulse for more fundamental analysis of factors which determine the distribution process of the reciprocating sprout broadcaster. Within this framework, attention was paid to:- the kinematic aspects of the oscillating distributor device;- an analysis of the characteristics of motion of separate particles within the sprout and after dispatch;- the formulation of a theory and the development of a model for the calculation of particle trajectories and velocities;- the consequence of alterations of relevant construction parameters for both the oscillatory characteristic of the distributor device and particle motion resulting in the basic transverse distribution pattern;- the influences of fertilizer properties on the quality of the distribution pattern, which e.g. is represented by the optimum and maximum bout width and the evenness of the compound distribution pattern.

Developments.

From the outline of the development of fertilizer distributors (chapter 2), it appears that in the beginning the evenness of the transverse distribution pattern of the reciprocating sprout broadcaster was poor. The evenness of the compound transverse distribution pattern is a dominating factor for the quality of the distribution process. Maintenance of the correct ratio of overlap was difficult because of the steep slopes of the basic transverse patterns. However, comparative machine testings, experimental research work of the manufacturors, and a progressing mutual adaption of fertilizers and broadcasters resulted in a considerable increase of the quality of the spreading process.

It can be stated that in their present designs the reciprocating sprout and spinning disc broadcasters are equal to each other with respect to operation quality. Values of the coefficient of variation <10% for the transverse compound distribution patterns can be realized when these types of broadcasters are carefully used in practice.

Nevertheless, yield losses of 1-3% which were still noted in practice at the beginning of 1970, emphasize the need for continuous attention to the evenness of the transverse distribution pattern. The most serious source of failures in practice is application at an incorrect ratio of overlap.

The need for increase of the working width is supported by the timeliness of fertilizing and the desirability for a decrease of the number of wheel tracks. Depending on factors such as working speed, application rate, and hopper content, the net working times (h ha -1) can decrease by about 10 and 20% respectively, when increasing the bout width from 8 to 10 m and from 10 to 15 m.

Distribution patterns.

Chapter 3 deals with the determination, representation, and interpretation of the evenness of distribution. Determination of the size of the standard sampling area depends on fertilizing-technical, climatological, soil, and cultivation factors. The specific characteristics of the transverse distribution pattern should be taken into consideration.

It appears that for broadcast application of fertilizer, irregularities within areas of 0.5 x 0.5 m hardly influence crop yields. As a consequence, the width of the collecting trays (0.5 m) which are recommended in the O.E.C.D. standard testing procedure, can be qualified as acceptable.

The most frequently used coefficients of irregularity are the coefficient of variation C.V., the linear deviation from the mean d, and Burema's number of irregularity R. The relationships between these coefficients are presented.

The shape of the basic transverse distribution pattern determines the evenness of the compound pattern, the ratio overlap, and the optimum and maximum working widths. The optimum working width is defined by that width>5.0 m at which the values for 5.0 m at which the values for C.V. and/or R are minimal. The maximum working width is limited by values for C.V. <15% and/or R <75.

Partly based on practical considerations. a number of requirements have been formulated for the shape of the compound transverse distribution pattern. In section 3.3 a number of basic transverse distribution patterns are discussed which enable a good rate of evenness (e g. C.V. <8 %) at a low ratio of overlap (e.g. O = 0.4). Two- or more-peak basic transverse distribution patterns have to be refused.

The complexity of external factors prevents the formulation of a uniform standard for evenness of distribution which is based on agricultural requirements. Results of incidentally performed field trials tend towards more generous standards for the tolerable rate of unevenness than those used by testing departments.

Increase of the application rate of (N) fertilizers or increase of their cocentration justifies a tightening of the standards for the coefficients of irregularity. The higher costs of the progressing technical development of distributors has to be weighed against the financial advantages of increased yields.

Characteristics of oscillation.

Studies of the characteristics of the oscillation pattern of the distributor device deals with the relation between relevant construction parameters (angular velocity omega, or rotary frequency N of the driving shaft, ratio C between the length of the crank and the connecting rod, and the length of the sprout RB) and the angles of oscillation, angular velocities, and angular accelerations. Additionally, the velocity and acceleration components of the sprout are discussed (chapter 4).

Velocity and accelerations are linearly and quadraticly proportional to the rotatory velocity of the driving shaft. The influence of the ratio between crank and connecting rod is somewhat more complicated. For values of C e"0.408, the angular acceleration phiincreases from alpha=0. After reaching its maximum value phi max at a c, phi decreases to 0 at alpha= pi/2. For C <0.408, phicontinuously decreases during the period 0 d"alpha d"pi/2.

Increase of the values of omega, RB, and C creates possibilities for an increase of the level of energy of the oscillating sprout. As a consequence, occasions arise for an increase of the spreading width. Alteration of these design variables must be attended by adaption of parts of the broadcaster in order to maintain a sufficient rate of symmetry in the characteristic of oscillation. In its present design, deviations from the nominal value of the rotary frequency N of 540 min -1should be avoided. For tractor-mounted distributors, deviations can lead to asymmetry in the oscillation pattern.

Particle motion.

Particle motion was studied with the aid of a high speed movie technique (chapter 5). In the characteristic of particle motion two stages can be distinguished.

1. A stage of discontinuous contact between sprout wall and separate particles. This process of impacts can include a period during which friction and restitution properties define particle translation, and a period during which only restitution properties are defining the process of motion. Under these circumstances particle translation is defined also by the momentary value of particle rotation.

2. Successively, a stage of discontinuous contact can be distinguished during which particles slide and/or roll. The values which had to be adjusted to:a. initial particle rotation omega 1 andb. the imaginary coefficient of friction and restitution micro-*, and epsilon*- in order to obtain agreement between results of motion analyses and simulation experiments -could be explained to a reasonable extent. In addition, experimental research in a later stage supported these values (chapter 7).

It can be concluded that the simulation technique provides possibilities for the calculation of trajactories and velocities of separate particles. Increase of the application rate (which was 0.5 kg min -1during the experiments) to more practical values of 60-120 kg min -1increases the mutual affection of the particles. It can be expected that this will lead to an increase of variation in particle trajectories and velocities. The character of the process of sorting-in of the particles must be subject to further research.

Design variables.

Shape and section of the sprout influence particle motion (chapter 6). The sprout types investigated show only relatively small differences. A divergent sprout type (gamma= -4°) with a rhombic section increases the variation of the velocities of outlet of the particles and the variation of the horizontal and vertical angles of dispatch. It can be derived from simulation experiments that increase of the length of the sprout RB, the rotary frequency of the driving shaft N, and the angle of oscillation phi(C), lead to an increase of the mean velocity of outlet of the particles VK . An increase of the value C to 0.600; N to 660 min -1and RB to 0.75 m results in values for VK of about 39 m s -1. Generally speaking, an increase of the value of VK is attended by a lower mean value of the angle of dispatch (beta).

Without additional sprout accessories, the basis transverse distribution pattern tends towards a more pronounced two-peak characteristic. The relative frequency of the occurrence of extreme (high) values of betavaries. It depends on the type of fertilizer (micro-* and epsilon*) and the design properties of the broadcaster (RB. C and N). Therefore, the value of betavaries. As a result, dimensions and shape of the grooves and the bow at the sprout end must be adjusted to fertilizer and broadcaster properties, since both accessories have to change the two-peak character into the one-peak character of the transverse basic distribution pattern.

It has been proven that filling of the centre of the distribution pattern is realized by directing downwards a part (H"0 %) of the particles that impact the bow (H"0 %). In addition, the bow can increase the spreading width, since the value of P decreases significantly and the value of VK increases slightly. These effects are supported by that part of the particles which are leaving the sprout with a positive angle of elevation.

Fertilizer properties.

Experimental results in chapter 7 show that an increase of the Mass Median Diameter of the particles (and the correlated 1000 kernel weight) positively affects the quality of the transverse distribution pattern. Sieve fractions <1.6 mm must be avoided. A relatively high value for bulk density, which was noticed for the highdensity prills, supports the favourable effect of the M.M.D. mentioned above. Contrary to what is frequently stated for spinning disc fertilizer broadcasters, it was noticed that increase of the variation of particle diameters positively affects the quality of the compound transverse distribution pattern of the reciprocating sprout broadcaster.

The study which is described in this thesis has contributed to an increase of knowledge of characteristics of particle behaviour in the oscillating distributor device of the reciprocating sprout broadcaster. The characteristics of motion for both the distributor device and the particles are more complicated than for the spinning disc broadcaster. Based on present standards, both types of broadcasters can realize a good quality of the distribution pattern.

Both broadcasters create opportunities for an increase of capacity by means of increase of the bout width. For this purpose an increase of both the velocity of outlet of the particles and the value of the spreading angle A is necessary. This requires adaptions in construction of both types of broadcasters. For the reciprocating sprout broadcaster it can be stated that adaption of the design to a modified oscillation characteristic will be complicated. For the spinning disc type the necessity for the outwards transport of a higher fertilizer mass by the vanes will affect the process of sorting-in and can cause unwanted mutual affection of the particles which prevents a smooth motion. Adaption of those fertilizer properties which increase the discharge distances of the particles after dispatch will be necessary.

For the reciprocating broadcaster further studies of the processes of particle flow and sorting-in at the sprout entrance and the effects of dimensions and shapes of grooves and bow at the orifice on particle dispatch are recommended.