Conversion of Existing Concrete Tower Silos to Dry Grain Storage: Part B
Table of ContentsIntroductionThis Factsheet is the companion to one entitled, Conversion of Existing Concrete Tower Silos to Dry Grain Storage, Part A (Order No. 88-069), which deals with topics such as silo condition, structural strength, horizontal and vertical pressures, extra hooping requirements, moisture control, roofs, walls and floors. AerationBasic information on the aeration of commonly used grain storages is dealt with in the OMAFRA Factsheet, Grain Aeration, Agdex 110/717. Figure 1. Aeration system - required to maintain uniform temperature and moisture within the grain mass. A tall column of grain presents relatively high resistance to airflow (static pressure) as indicated in Table 1. Using this table and the storage capacities shown in Table 2, it is possible to select a fan for most situations. Example:
As shown in the example, considerable pressure is required to move air through an appreciable depth of grain. Therefore, every effort must be made to prevent air from short-circuiting through walls or around silo doors. Aeration to control temperature change and moisture movement within the grain mass can be accomplished using properly designed ducts, either built into the floor, or placed on top. Fully perforated floors may be used, but are not necessary since converted silos should not be used for drying grain. Similarly, the use of a silo for "in-bin" cooling is not recommended because of the associated higher airflow requirements and static pressure limits. For example, cooling grain which comes hot from a dryer normally requires five (5) times the air flow of a conventional aeration system. Thus a 50' column of corn would create a static pressure exceeding 12" - far beyond the capacity of most aeration fans.
Grain HandlingLoadingOne of the problems facing the owner of a newly converted silo is how to put the grain into storage - keeping in mind the height of the structure, the tendency to damage the grain during this operation, and often the need to achieve fairly high inflow rates.
An inclined auger will be acceptable only for very short structures - 40' or less. The majority of farmers will use either a bucket elevator or pneumatic conveyor as their silo loading equipment. Employing a forage blower to elevate dry grain is not recommended due to substantial break-up of the material. Fines cause great difficulty in properly aerating the storage; they also serve as an ideal host for mould development. In addition, as the percentage of fines increases, the grade of the grain drops rapidly. There is currently available a specialized piece of equipment to be used with a forage blower to elevate dry grain without causing severe breakage. It is simply a short length of hydraulically-driven auger which introduces grain into the filler pipe immediately above the fan. Here, the blower is working solely as an air mover- a function it performs with low efficiency. As a result, it is only effective for silo heights up to 35' to 40', since its capacity drops considerably for taller silos. A bucket elevator is an efficient, though costly type of conveyor. Such an investment may be justified if multiple uses are possible. This will be the case if it is part of a grain drying facility or a feed processing centre (Figure 2). Where a bucket elevator will be used only for loading and unloading a tower silo, one should look to the lighter-duty, lower capacity units which are currently available. Figure 2. Converted silo with bucket elevator as part of a total grain storage and handling centre. Pneumatic conveyors for handling grain have become popular within the last few years. Low maintenance and high flexibility are their obvious strong points. Even where a silo stands several hundred feet away from an existing grain centre a low volume pneumatic conveyor can be used. Using this conveyor to load other storage bins will help to justify its cost. Higher capacity, P.T.O.-driven, trailer-type units may be available on a rental basis, and will often prove to be the most attractive means of solving the loading problem. Vertical augers offer an alternate, though less popular option for filling a silo. While they may cost only a fraction of the price of a bucket elevator, they do have high maintenance requirements related to bearing and gearbox replacement. Their capacity is only about half that of a similar size inclined auger, while power requirements are roughly five (5) times those of a bucket elevator for similar capacities. Loading vs. QualityGrain damage (cracks, breakage, etc.) is a concern when filling a tower silo due to the long fall the grain must undergo, particularly during the nearly empty stage. The situation is even worse if the material enters the silo from a long, or even moderate, length of down-spouting. Since the grain is already moving rapidly and is in a concentrated stream, it is not restricted to the same terminal velocity which a dispersed flow of grain experiences. Every effort should be made to momentarily stop the descent of the grain, and have gravity reaccelerate it in a dispersed state. Flow retarders, cushion boxes, and cyclones are all methods of accomplishing this objective. Another way to minimize quality problems related to grain impact is to handle a product which initially has few stress cracks. Refer to OMAFRA Factsheet, Harvesting and Storing Quality Grain Corn, Agdex 111/736, for a discussion of drying strategies which minimize stress cracking. UnloadingAll converted silos should be unloaded from the centre. Opening a hole in the sidewall at the bottom, or up some distance from the bottom to provide an overhead load-out is not recommended, since unbalanced sidewall loads will result. Any circular storage structure is designed on the basis that pressure will be exerted relatively uniformly at all points around the wall circumference. However, if one unloads from the side, the top surface will take on a slope, resulting in unbalanced side-wall pressures (Figure 3). This creates a tendency for the structure to go out-of-round. Thus, inserting a load-out auger through a silo door at the bottom of the silo can well lead to structural problems. Figure 3. Silo tapped for overhead load-out or bottom side load-out has unbalanced forces. Another phenomenon often associated with off-centre unloading is termed "rat-holing". An accumulation of fines or some mouldy grain may disrupt the normal grain flow pattern. If grain flows only from directly above the load-out point along the sidewall, then unbalanced forces are again present (Figure 4). Obviously, the only way to maintain balanced wall pressures is by employing bottom centre unloading. Figure 4. Rat-Holing leads to unbalanced sidewall forces. Constructing a new concrete floor provides the opportunity to place a centre hopper and tube to receive an under-floor unloading auger. (If a fully perforated floor is installed, the auger need not be placed in the concrete.) As with any other circular storage with a flat floor, a sweep auger can be used to complete the unloading once gravity flow has stopped. EconomicsThere are so many variables involved in the cost of silo conversion that it is impossible to suggest a standard dollar figure. However, the checklist on the following page may assist in estimating the overall cost for a particular silo.
Total Cost: ______ Cost Per Bushel: ______ Items 1 through 6 are related to the existing silo conditions and thus are highly variable. However, the final five items are required for every silo and can be readily estimated. The costs and advantages/disadvantages of alternative grain storage should be carefully assessed before proceeding. The per bushel costs should be especially considered in order to make valid comparisons. In some situations, it may be wise to leave that silo standing empty, while in other cases it may make sense to put an existing silo to work as dry grain storage. Related Links
For more information: Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
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