Mass flow, in a very many cases, is required to ensure uninterrupted discharge from your bin. In mass flow, all the material in the bin moves while any is being withdrawn. This means that the walls of the hopper are steep and smooth enough to overcome wall friction and allow material to slide.

There are three important design considerations to evaluate when incorporating a mass flow bin into your system:

• The opening size required to prevent arching
• The wall angle required to ensure flow along the walls
• The discharge rate required to satisfy your process

Keep in mind, as I have stated many times before, the feeder design is as important as the bin design; however, I will focus on the bin design in this series.

The first consideration is the opening size required to prevent arching in a mass flow bin. Just because material slides at the walls does not necessarily mean it will flow without arching. But, just how large does the hopper opening need to be to prevent arching? There are two types of arch that occur with bulk solids: particle interlocking and cohesive.

Particle interlocking occurs as particles lock together to form a mechanical bond. The outlet size required to prevent this is directly related to the size of the particles. Particle interlocking usually occurs with particles that are two inches in size and greater. The rule of thumb to eliminate particle interlocking is to size the hopper opening as follows:

• Cones:  6 to 8 times the largest particle size.
• Wedges (slotted):  3 to 4 times the largest particle size.

Consider yourself as just one particle of many thousands at a baseball game when it ends. All of a sudden there is a rush to the exits that only allow so many people through at a time. This is more of a flow rate limitation but, for just a moment though, an interlocking arch occurs as two, three or four people try to get through the same doorway at once. Much the same occurs in a hopper as several large particles converge on the opening (although, particles don't care who won the ball game).

My next article will focus on cohesive arching, which effects more bulk solids than interlocking arching.