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Round or Square Bins?



To many companies considering changing to a bin system, the question of round vs. square is a difficult one.



Round Bins, although not new on the market, have recently become more popular, due solely to the better flow characteristics. The funnel is symmetrical, as is the horizontal distance from the vertical center line. This is more logical then the square bin. The round bin will generally be taller then the square bin as the roundness of the container occupies less volume then a square geometry.


Square bins have been used for at least 40 years, and they can be seen today at almost every major pharmaceutical manufacturing plant. They work and quite well. However, they do have one flaw, which in some cases can cause them not to work: segregation during discharging. To avoid segregation, a 60 degree funnel (as measured from horizontal) combined with a properly sized discharge valve must be used. In cases where the products still segregate, the funnel can be made steeper, usually 70 degrees (as measured from horizontal). However, the steeper the funnel, the taller the bin will be. If ceiling height is limited, your bin capacity may also be smaller.
   
Typical Flow Problems:
Funnel Flow: This is the description of how funnel discharges when ratholing. The powder actually moves in a "last in, first out" manner. If you look into any typical bin (round or square) during discharging you will see an indentation in the powder forming. This indentation gradually leads to a vortex-looking shape which is called ratholing. The opposite of funnel flow is mass flow, whereby all the product moves towards the outlet in a somewhat equal manner without any ratholing.

Bridging: This is a very common problem, one that has made the rubber mallet a mainstay of many feeding systems. Simply put, the powder makes a bridge over the outlet blocking the flow. When designing a bin system, it is very important that the flow characteristics be taken into account. One of the biggest problems we see is when too-small of an outlet valve is used. There are many flow aids that can be employed. In general, vibration should be avoided if the bins have blended products in them (they may segregate).