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 Main subject - mar/apr 2007
continuation: Seed Processing Units
A system to efficiently dispose of dust and undesirable material, released and separated along the processing sequence, should also be part of the planning process. When financially possible, it is best to plan the SPU from scratch in a new site, since few are the cases in which an already working site is properly located and suitable to changes. Likewise, the availability of a site does not imply success by itself.
Key issues such as placement of the dryer, site for the reception of seed lots and the sequence arrangement for the equipment evaluated. Thus, the staff in charge of planning the SPU should be familiar with the proposed site before any effective steps are taken to mount the different equipment necessary to the processing, as well as considering the open spaces for transport and storage of seeds.
It is a fairly known fact that the equipment needed to clean wheat and rice seeds is essentially the same, which applies also to the drying process. However, for seeds of crops such as soybean and corn, forage species and horticulture vegetables, changes in the equipment may be necessary.
Equipment selection
As seen in the previous item, planning involves the definition of the desired volume of clean, graded seed, bearing in mind the possibility of future expansion and the species and their required processing flow. The appropriate equipment will then be selected with the help of catalogues, brochures, information fed by fellow seed producers, visits to other SPU and previous knowledge.
Before the acquisition of equipment, some specific details pertaining to each machine must be considered. Feeding control, ease of mounting and replacement of screens, adjustment of the air flow, cleaning of the machine, type, size and ease of replacement of the cylinder perforations, cylinder rotation, energy consumption, machine size, potential sources of mechanic damage to seeds, dry air and seed mass temperature control and moisture control, are examples of the many details to bear in mind at the time of purchasing equipment.
Another important issue has to do with the capacity of the different machines that are part of the equipment, which should be equivalent to ensure that the operational flow is not disrupted. If eventually one of the machines would yield a lower performance than others, consideration should be given to mount two of them in parallel operation, such as the cylinder separator for each air screen cleaner with four screens.
It is worth to note that the equipment specifications, as established by the manufacturer, have to comply with specific international norms. However, when already working at the SPU, caution should be exercised on its yield estimate and it's prudent to consider half the established yield. If a manufacturer declares that his air screen cleaner (ASC) with four screens cleans batches of 8 tons per hour, most likely the machine will perform at 4 t per hour, depending on the species.
ASC for soybean seeds are more efficient than those for rice seeds. The newest ASC are delivered with a lower screen box (third and fourth screens) that is duplicated such that half the flow goes to the second box and the other half to the third box, which doubles the machine's capacity. Going back to the example, the double ASC will yield 8 t per hour, which applies also to the standardizer (grader) for soybean seeds.
When assessing the efficiency of ASC it is advisable to consider the screen area as an index of capacity, or the volume of seed processed per screen m2 per hour; the area considered will be that of the two screens performing the same task, the first and third or the second and fourth. For soybean, the appropriate index is of around 800-1000Kg.h-1.m2 screen, whereas for rice is around half those values; 400-500Kg.h-1.m2 screen.
In general, all machines that comprise the equipment for seed processing operate more efficiently under a continuous flow of seeds and since not all machines at the SPU have the same capacity it may be important to attach to all of them a seed load regulator device, which will ensure that each machine will operate at the desired rate. This device can be a container or bin with a flow regulation scheme with a capacity equivalent to that needed for a 1 hour operation by the machine it feeds.
The first machine to operate whenever a seed lot enters the SPU is the pre-cleaning machine (e.g. ASC bearing one or two screens), located just outside the seed reception site. After going through this machine the seed can be dried, stored in bins or continue straight ahead to the cleaning and grading machines (Figures 1 and 2). When seeds enter the line of cleaning and grading, the first to be accessed is the ASC, with four or more screens and two air separations (basic SPU machine).
How can we know the exact number of machines that need to be acquired to mount the SPU? Following the latter example for soybean seeds, the first thing to do is to calculate the SPU capacity as a whole: 2,500 t will be volume entering the SPU. From harvest until the time the seed needs to be commercially available, the time elapsed is of approximately four months (May to August), which means there are 856 hours to process the 2,250 t of seed (more or less 96 full days x 8h per day shifts), which will render a SPU capacity of 3.0 t per hour. With these basic data, a processing line can be chosen with one ASC yielding 4 t per hour, a line of 20 double coils (200kg.h-1.pair-1), a standardizer at 4t.h-1 and a gravity table also at 4t.h-1.
If there's need to reduce the processing time or operate with the machines of lower yield, one alternative can be to carry processing and drying operations simultaneously or establish two 8 hour shifts per day.
The spiral separator often operates at a high level of noise, caused by the velocity of impact from dry soybean seeds against the metallic wall of the outer spiral, from where the seed is unloaded. The manufacturers of spoiral separators should build them completely closed, that is without the outer coil, which will be substituted by a closed box. This will not only reduce the noise of the operation but will soften the impact of seed against the walls by giving the rubber sheet at the bottom of the box the proper inclination.
Regarding the gravity table, if the separation of three seed sizes is desirable, individual tables for each size can be mounted or seeds from two different sizes can be stored in a relief bin after exiting the standardizer, so that they can go through the gravity table on at a time.
Bulk and bag storage
In most SPU, the volume of received seeds (tons per hour) largely exceeds the capacity to clean and grade, so that storage bins to keep the batch until due processing should be considered. Following the example in which the equipment to process soybean seeds has a capacity of 3t.h-1, the expected entry volume of seed is of 2.5t, which will allow a new entry after a 40 day period, or a daily reception of 62.5t.
Assuming that the machines operate effectively 12h per day (subtracting the time needed for cleaning, maintenance and other factors contributing to their paralyzation), for a two 8-hour shifts, the equipment would be able to process 1440t in a 40 day period (3x12x40=1440), thus remaining 1,06t to be processed at a later time (Figure 3).
Seed storage after processing requires that care is taken on handling, placement and environmental conditions within the SPU, so that the risks of losing quality are minimized. The space allocated to store the clean batch should be large enough to allow for handling and bagging, which normally occurs at the same site.
The seeds that are stored at this stage must be piled in stocks which can eventually hold more than one lot, as long as it belongs to the same variety and they are duly identified. The stacks should have enough space between them (minimum 0,8m) to allow inspection, sampling, and pest control and aisles at least 3m wide so that transportation vehicles and lift trucks can operate properly.
The space needed for the seeds already bagged and ready for trade (2000t) is calculated as follows: in a warehouse with a 6m right foot there should be a minimum of 1.5m from the top of the stack to the roof. Thus, the height of the seed stack will be of 4.5m. Considering 15 bags per cubic meter (15 x 40kg per bag), we have 600kg or 0.6t.m3 or 2.7t.m2 so that to store 2000t, 741m2 will be necessary.
To calculate the free space and aisles (30%), proceed as follows: 741m2 are equivalent to 70% of the warehouse area so that total area = 1060m2, which equals to a warehouse of approximately 50m long, 20m wide and 6m high.
Summing up: using the simple example of a SPU for soybean seeds, the main steps on how to set it up were discussed, so that seed producer can have a general view on how to plan and make the appropriate decisions in relation to the equipment necessary.
The investment and technical knowledge necessary to plan and mount a SPU are, without doubt, considerably higher than those for a grain processing unit. Differences arise from features of the drying unit, operational specialty, machinery involved and the infrastructure available for the storage of bagged seed.
This will inevitably impact on the final seed prize; however, the superior quality of the lot will render it worthy.
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