Optimized silo capacity needs to consider the following

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  • What is the capacity of my bulk raw material delivery vehicle?

You must be able to fit at least 1 truck load of raw material into your silo each time you order a new load.

  • What is the output of my plant in tonnes/hr and what is the worst case usage of this raw material?

So if your recipe uses 50% of this raw material per batch and you manufacturer at 6 tonnes per hour, you require 3 tonnes per hour of this material.

  • When will your plant manufacture?

Consider here if you run 24/7 or 8 hour/day for 5 days.

  • When I call, how long will the supplier take to deliver the raw material.

First, you should only call the raw material supplier when you can fit at least 1 load in your silo. You need the full load in your silo.

Second, the volume of product left in your silo needs to support production until your next delivery. e.g. You can calculate your raw material requirement based on 24 hour response by your supplier. A 10 hour production schedule using 3 tonne/hour of this material and 24 hours to supply, will need 10 x 3 = 30 tonnes of material.

  • Your silo must hold 1 truck load plus production requirement until next delivery.

Assuming each truck delivery is 32 tonne, from our example, your silo must hold 32 + 30 = 62 tonnes.

  • What is the average bulk density of this raw material?

The silo capacity must be converted to a volume (from a weight). To calculate the volume of the silo we multiply the weight by the raw material bulk density. Assuming our raw material is sand, we can determine at a bulk density of 1.6 (see table) that the volume needed to store 62 tonnes is 1.6 x 62 = 99.2 cubic metres. The bulk density may be calculated by weighing a sample of your raw material and determining geometrically what volume it occupied. We can assist with this calculation.

  • What is the angle of repose for your raw material?

This is the angle or slope formed when the product starts to flow. It is also the angle that we need on the cone of the silo so that all products will flow out of the silo. Many products have an angle of repose less than 45 degrees and by default our silos use 30 degrees for infeed and outfeed calculations. We can calculate the approximate angle of repose using the Tilting Box Method that places the material within a box with a transparent side. It should initially be level and parallel to the base of the box. The box is slowly tilted at a rate of approximately 1/2 degree/second. Tilting is stopped when the material begins to slide in bulk, and the angle of the tilt is measured.

  • Are there any other constraints?

Perhaps the discharge outlet needs to be above your down-stream process to allow gravity feed, or there is a dust collector and walkway on the top of the silo. Certainly the height of your factory roof is a constraint, and our most economical tanks have transport constraints of 3.5 metres diameter.. Try to identify any conditions on the silo design that will limit its height or diameter.

To demonstrate the typical silo sizing process we will work through the calculations for a simplified cylindrical silo with conical bottom.

  • The silo capacity is 100 m3
  • The angle of repose is assumed to be 45°
  • The ceiling height is 20 metres
  • The clearance under the discharge is 0.5 metre
  • The infeed and dust collector needs 2m with a service factor
  • The diameter is 3 metres (suggested)

The discharge angle of repose – the CONE is based on the angle of repose of your product or in this case 45 degrees. Cone Volume=1/3πr2 X h1 where r is the silo radius and h1 is the height of the cone. H1 = r = 1.5m for angle of repose =45° The capacity of the cone is 3.5 m3

The filling angle of repose – the MOUND Is also based on the angle of repose of your product or in this case 45 degrees fed from centre. Mound volume=1/3πr2 X h3 where r is the silo radius and h3 is the height of the mound. H3 = r=1.5m for angle of repose =45° The capacity of the mound is 3.5 m3

The straight walled section – the BODY The body of the silo is a regular cylinder where the product will fill the volume. (does not include the filling mound) Body Volume =πr2 X h2 If r is the silo radius and h2 is the height of the straight sided cylinder. Also, Body capacity = Silo capacity- mound capacity-cone capacity Body capacity is 100-3.5-3.5 = 93m3 Body height(h2) =body volume / πr2 If r is the silo radius and h2 is the height of the straight sided cylinder. Body height is 13 metres

The final geometry. This silo has the following vital statistics

Capacity (m3)100
Silo diameter(m)3
Height Under Cone (m)0.5
Height Of Cone (m)1.5
Height Above Cone (m)13
Total Silo Height (m)15
Height Above Silo (m)2
Height Under Roof (m)20

Let us design your next silo or tank In practice your silo may be any shape and capacity and our engineers will model the silo to suit your workplace, flowpath and material properties.