By Scott Heffern
Director of Bio-Pharmaceutical Systems
Custom Powder Systems
Tip speed (angular velocity) is a constant that can be used to establish an initial blending speed for a given bin geometry. The blender speed may need to be adjusted upon further testing but is a good starting point. The tip radius is the distance perpendicular to the blender rotational center measured to the furthest top bin corner.
TS = 2100 in/min tip speed constant
TR = Tip Radius (in inches)
RPM = blender rotational speed (in revolutions per minute)
PI = 3.1415
RPM = TS/(PI x TR x 2)
These speed values are approximate based on average density materials 40#/ft3, good flow properties, and particle sizes within 40% of each other. This formula is not relevant for IBC capacities less than 200 liters.
Adjusting for less than ideal products
Product density influence on blend speed
- Increase blend speed for denser products
- Decrease blend speed for lighter products that are prone to aeration.
Product flow properties influence on blend speed
- Decrease blend speed for products with poor flow properties
- Increase blend speed for products with excellent flow properties
Particle size influence on blend speed
- Particle size variation over 40% - blend speed change has little effect. Draw blend samples much more frequently (at least every minute of blend time) during initial blend testing because batches with large particle size variation can de-blend if run too long.
- Generally speaking, blending speed that is lower than optimum does not affect blend uniformity. Blending too fast can degrade blend uniformity because the product becomes airborne due to centrifugal force. Airborne product allows heavier particles to segregate from lighter particles.
- A pronounced "thump" can be heard once per revolution if blend speed is too fast. This is the sound of airborne product impacting a bin sidewall.
SOURCE: Custom Powder Systems