Bin Volume

Have you ever tried to fit 15 ton of feed into a 15 ton bin and it didn’t fit? My cousin called the other day after mixing up a batch of hog feed and it wouldn’t fit into his bin. He measured the ingredients weight with the scales on this grinder mixer and made the normal batch size but what he didn’t think about was switching the main ingredient from corn (48 lbs/cu.ft.) to barley (38 lbs/cu.ft.). The joke was on him, the lighter feed ingredient took up more volume and didn’t fit in his bin. Lesson learned, when you’re in the process of researching your next bin, consider the following points as you compare and make you’re buying decisions.
  • How large of a bin do I need? It depends on how much material you want to load into the bin at any given time including what’s already in the bin. Most commodities are sold by weight so its easiest when shopping for a bin to compare volume tonnage of the material bulk density you plan to put in the bin. Be sure when comparing tonnages from different bin manufactures that you are comparing tonnage based on the same bulk density. A 975 cu. ft. bin will hold 24 tons of 52 lbs./ft3 material compared to 19.5 tons of 40 lbs./ft3 material. Square bin manufactures may rate their bin using heavier materials to make them look like they hold more. If the price looks too good to be true, you might want to check the facts. FCS specifies the material bulk density used to determine bin tonnage capacity so you know exactly what you’re getting. Look for bulk density information on product web pages or bin drawings.
  • Air Volume or True Usable Volume? It is easy to calculate the capacity of a bin based on its geometric volume. For most bins this is a fairly easy calculation combining the volume equations of several basic shapes; cylinders and cones for round hopper bottom bins and pyramids and cubes for square or rectangular bins. But is this really the most accurate way of calculating what the bin will actually hold? It is not. Most dry bulk materials will not completely fill up the air volume of the bin. For example, a square bin with pyramid top will not fill to the corners. The material piles up under the opening in a conical shape (see figure to the right). The most accurate way to calculate the capacity of a bin is to calculate the true usable or live volume based on the filling angle of repose of the material you plan to store. The filling angle of repose is a material property based on it flow characteristics. It is the maximum angle in which a material can be piled without slumping. This angle is measured from the horizontal plane up to the steepest angle of descent. Not all materials have the same filling angle of repose. Grains like wheat, corn and soybeans have a fairly low angle of repose, approximately 16 degrees and tend to spread out when piled. Oats are next at 18 degrees followed by grain sorghum and rice at 20 degrees. Pelleted feeds have a wide range of angle of repose, 25 to 38 degrees, depending on size and shape of the pellet. DDGS and Corn Gluten Meal have a very high filling angle of repose of 44 degrees. Its important to understand the characteristics of your bulk material. FCS maintains a library of common dry bulk materials and does in-house material flow testing. Let us know how we can assist you with your next project.
  • The shape of the bin and roof angles are also factors. As material piles up in a bin, it forms an inverted cone shape. Round bins will fill evenly out to the eve of the bin but square bins (filled from the center) will fill to the eve at the midpoint of the side wall but may not fill out to the corners depending on the filling angle of repose and roof angle. FCS designs standard round bins with a 35 degree roof angle and square bins with a 40 degree roof angle to maximize fill capacity. When the angle of repose is less than the roof angle, the material will flow out to the corners of a square bin. If the angle of repose is steeper than the roof angle, the bin will fill to the opening and not fill out into the corners. Flat roofs have an even greater discrepancy between air volume and usable capacity. While flat roofs are great working surfaces and walkways, they are very inefficient when it comes to usable volume. As you can see in the figure to the right, this bin has a large unusable air space due to the 35° angle of repose. FCS Manufacturing takes these factors into consideration when calculating the capacity of a bin to ensure the amount of commodity you desire will actually fit.
  • Inlet location is another factor that impacts the usable volume of a bin. Center fill locations are the most efficient because material can pile symmetrically within the walls of the bin. When constraints require an offset or eccentric fill location, the amount of usable volume in the bin decreases. As is seen in the figure to the left, the right offset inlet location causes material to pile on the right side of the bin reducing the live volume.
  • At FCS, we utilize 3D modeling software combined with angle of repose of the bulk material plus the inlet location to create the actual usable volume of the bin. Without 3D modeling, it is nearly impossible to calculate usable volume of offset fill or special compartment shapes by hand. We strive to provide the most accurate capacity ratings so your bin holds what you need to make your project a success.
  • So when you’re shopping for your next bin, be sure and check whether the manufacturer is advertising air volume or usable volume and compare the bulk density of your material with the bulk density used to calculate tonnage capacity of the bin.