Designing a batching and mixing system

by Fred Fairchild
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Many factors affect designing a modern batching and mixing system for a feed mill. This includes the required production capacity, the number of ingredients needed, the size of the mixer required and the mix cycle time.

The batching/mixing system capacity is usually the key element of the whole production process. Let us assume that we want to average making 30 tons per hour (tph) of mash feed out of the mixer. In order to average this capacity, we need to include an efficiency factor to make up for time lost in production due to such things as availability of all the ingredients, the amount and rate of inclusion of each ingredient in a particular formula, amount of hand adds, and required sequencing between formulas. I normally use an efficiency factor of 80%. This would make the design capacity for the batching and mixing system 37.5 tons per hour. We determine that design requirement by dividing the 30 tph/.80 = 37.5 tph.

Let us assume we want to normally make the batches in 3-ton sizes, and that most of the formulations have a density of 40 pounds per cubic foot. The cubic size for a 3-ton batch is determined by converting 3 tons into pounds and dividing by the density. 3 tons x 2000 pounds per ton/40 pounds per cubic foot = 150 cubic feet. This tells us that we will need a mixer of at least 150-cubic-foot capacity.

The batching/mixing system capacity is usually the key element of the whole production process. 
With this information, we can now determine the cycle time available for each batch to be mixed and discharged. Batches per hour is design capacity per hour divided by tons per batch = 37.5 tph/3 tons per batch = 12.5 batches per hour. Cycle time = 60 minutes per hour/12.5 batches per hour = 4.8 minutes per batch. Converting to seconds per cycle = 4.8 x 60 = 288 seconds per mixer cycle. Assume that it takes 15 seconds to dump a batch from the scale into the mixer and 15 seconds to discharge a batch from the mixer into the surge below it. Deducting these times from the mixer cycle time leaves a scale cycle time of 258 seconds or 4.3 minutes. The addition of all ingredients put in the scale hopper plus any hand adds must be done in 4.3 minutes. The feeders must have a combined capacity to provide that all the ingredients are put into the scale in 4.3 minutes or less. A typical batching scale and mixing system is shown in Fig. 1.