In past Industry Insight columns, I have discussed the importance of the feeder in handling your powders. I have discussed feeders such as screws, belts, rotary valves, etc. It is vitally important that the bin and feeder work together to ensure uninterrupted flow. In this column, I will discuss feeder do’s and don’ts in the form of frequently asked questions:
Am I restricted to steep conical hoppers to ensure mass flow? (Hang in there I will get to feeders!)
Not necessarily. Wedge-shaped hoppers are a great alternative to conical hoppers. Wedges, such as chisel or transition type, are more forgiving from a flow standpoint. Keep in mind that in a conical hopper, your powder must converge 360° around the hopper, while a wedge-type configuration converges in only one direction (along the sloping sidewalls of the wedge). Wedges use slotted openings that require smaller widths to prevent arching and shallower hopper slopes to ensure mass flow. Because of the long slot, wedges will also provide a higher discharge rate. Here is where the feeder comes into play. To discharge your material uniformly from slot opening, a properly designed feeder is required to maintain flow over the length of the slot.
* Screw feeders with a conical shafted section and half-pitch flights that attach to an increasing pitch section are a practical and reliable approach to ensuring mass flow.
* Belts feeders use a properly designed interface to deposit your powder uniformly onto the belt; but, more importantly maintain the mass flow pattern in the bin above.
My feeder design is not critical, so if I attach any type of feeder to my mass flow bin, my powder should still flow in mass flow, right?
Wrong! The feeder design is as important as the bin design. An improperly designed feeder may draw your product preferentially from the bin, converting it from mass flow to funnel flow. If this happens, you have wasted money modifying your bin to achieve mass flow. The feeder must be designed to discharge product over the entire cross-sectional area of your outlet. If you take advantage of the benefits of a wedge-shaped hopper configuration, you will likely use a screw or belt to control discharge rate. This feeder must increase in capacity in the discharge direction, thereby pulling from the entire outlet cross-section.
My powder is easy flowing so that I can use a conical hopper with a reasonable size opening and proper hopper slope to ensure mass flow. Can I simply use a rotary valve feeder to control discharge rate?
A rotary valve will likely work well. However, if you mount the rotary valve to the cone outlet, problems may arise as follows:
* Rotary valves mounted directly to cone outlets tend to pull material preferentially from the bin and not over the entire outlet cross-section. This causes the powder to flow in funnel flow, creating preferential flow channels that could jeopardize the structural integrity of the bin.
* When your powder discharges from the pockets or veins of the valve, it is replaced by air that tends to flow up against the discharging powder potentially holding it up.
Each of these issues is resolved by:
* Inserting a vertical spool piece (a piece of pipe) that is 1½ to 2 diameters tall in between to cone outlet and valve. For example, if the cone opening is 12”, the spool piece should be 18 to 24 in. tall. The preferential flow channel will expand within the vertical spool piece allowing the powder to flow over the entire outlet cross-section of the cone outlet, maintaining mass flow.
* Installing a vent line from the rotary valve housing up to the top of the bin or to a dust collector will prevent the counter current air flow from holding up your powder. Keep in mind that this vent line should be as vertical as possible to prevent dust buildup.
These are just some of the questions I receive regarding feeder design. Remember the all-important statement that the feeder design is as important as the bin design and hopefully you will experience a problem-free startup.
Joseph Marinelli is president, Solids Handling Technologies. For more information, visit solidshandlingtech.com.