Calculating Conveying Capacities

When designing a plant, it is important to consider the flow of material during a given time. This is the “mass flow rate” and is important when specifying equipment capacities for conveying and bins. Under sizing equipment will cause bottlenecks in production while over sizing equipment could result in overpaying for equipment much larger than required. The following can be used to calculate capacities for conveyors and bucket elevators. 

Most manufacturers will publish some specifications about their conveyors which will help with calculations. One such rate is cubic feet or cubic meters per hour. This term states how much volume can be conveyed per hour and is the most important when trying to calculate your conveying needs. This will help size the proper conveyor, but we need one more piece of information first.

Density

The above calculations will help determine how much volume of product moves per hour. Now we have to consider the product that we will be transporting. There are many resources online to help estimate bulk density. A simple search will yield several results. Some specialty items, such as those your plant produces, may need to be determined by a simple test.

Calculating Density

You can calculate density of a product very simply. You must construct or purchase a box of known dimensions, fill and weigh the box and then divide the results by the volume of the box. As an example, you could construct a box 30.48 cm long x 30.48 cm wide x 30.48 cm high (28,317 cubit cm, approximately 1 cubic foot). Next, zero or tare the scale with the box on it so the whole unit reads zero. Then, fill the box completely level to the top (not under it and not overflowing) and gather the results. Calculate the volume of the box (length x width x height) and take your product weight divided by this answer. The result will be density, which may need to be converted to more reasonable units such as pounds/cubic feet or kg/cubic meter.

Putting It All Together

Now that we have capacity and density, we can calculate a rate per hour. As an example, let’s assume we need to transport 1 ton of soybeans per hour with a screw conveyor. Is this achievable with a 6 inch conveyor? Our manufacturer says we can transport 180 cubic feet / hour. Our soybeans are whole and have a density of approximately 47 pounds/cubic foot. Simply multiply the density by the capacity gives us 8460 pounds/hour (180*47). This is a little over 4 tons per hour, which is more than enough for our 1 ton per hour requirement.

Other Items

You could also use this methodology to calculate bin sizes as well. Simply determine your bin size (volume) and they multiply by the product density. Then, if you have an approximate capacity, you could divide to see how much time it will take to empty the bin. For intermediate process bins, you may want up 30 minute holding capacity while larger day bin and storage bins may require 2-4 hours. Hopefully these ideas will help correctly size your conveying and storage needs and give you more insight on how to correctly calculate what your plant needs.