By Todd D. Messmer
How can I improve my screw feeder’s gravimetric performance?
As Applications Engineers we get asked this question quite often.
In answering, the first thing we need to understand is the material we are feeding. Gravimetric feeder performance is most always affected by how well the material feeds volumetrically. The closer we can fill the flights of the feed screw volumetrically to 100%, the better the feeder will perform gravimetrically. In order to get the material to feed better volumetrically, understanding the material characteristics of what we are trying to feed must be analyzed first.
So let’s look at some of the most common material characteristics with respect to volumetric feeder performance. Remember, we must first improve how the feeder feeds volumetrically before we can look at improving gravimetric performance.
Free Flowing: Plastic pellets are great free flowing materials. They feed under gravity without the need for special design considerations or flow enhancements.
Adhesive: These materials like to stick to everything. Pigments are notorious for adhering to all types of contact surfaces. Often we need to clean feed screws and tubes just to keep the material from building up on them. Avoiding feeders with internal agitation systems is recommended. We may need to look at different coatings such as Teflon or more polished contact services. Systems to self-clean the inside of the feed tube should be taken into consideration.
Cohesive: These materials like to pack like a snow ball and are typically associated with a high angle of repose. They need flow aids such as internal agitation, air sweeps, air pads to create movement or external vibration to break up the clumps. Adding cross wires on the end of the feed tube to get the material to “pack” better into the flights of the feed screw can help.
Cohesive materials like to pack like a snowball and need flow aids such as internal agitation, air sweeps, or air pads to create movement or external vibration to break up the clumps.
Aeratable / Floodable: Materials typically associated with a low angle of repose. They behave like a fluid when aerated and will easily flush out of a feed screw if it is not designed properly. A feed screw with a center rod vs. an open flight is often needed with these materials. I would recommend doing smaller refills more frequently with these types of materials versus a larger refill which can often aerate the material in the feeder causing it to flood out.
Hygroscopic: These materials retain moisture very easily. Often times we hear customers say that they left material in the feeder and when they came back the next morning it had solidified because it had collected moisture from the environment. Blanketing the material with clean dry air or Nitrogen can help in keeping moisture out of the feeder.
Pressure Sensitive: These materials are prone to packing if used on large volume hopper extensions. Again frequent smaller refills may help keep the material from packing. For those feeders that utilize external paddle agitation with flexible walled hoppers require close attention to the level of frequency the paddles are agitating the hopper walls. Higher frequency agitation or vibration can often pack these materials.
Pressure-sensitive materials are prone to pack if used in large-volume hopper extensions. Sometimes external paddles, as shown above, are used to agitate flexible-walled hoppers to mitigate this problem. In some cases, though, higher-frequency agitation or vibration will cause these materials to pack.
Low Melt Temp: These materials tend to break down, melt, or caramelize when excess friction/energy is used on them. My recommendation would be to use a larger diameter feed screw spinning at a lower rpm than a smaller diameter feed screw spinning at a higher rpm with these types of materials.
If all else fails inquire about the testing capabilities of your material handling supplier. Often times they have had experience feeding the material and can suggest ways of improving performance. Material testing is often free of charge and can be witnessed firsthand.
Now, after taking the steps to improve material feeding volumetrically let’s take a look at several factors that will affect the gravimetric performance of the feeder.
Vibration: Vibration is detrimental to the operation of the gravimetric system because of the sensitivity of the scale, and special provisions must be taken to eliminate any vibration to the scale. Some suggestions to minimize vibration are as follows: isolate the decking that the weighing system rests on; reinforce the decking around the weighing equipment so the decking flexes a minimal amount; mount the weighing equipment on a high mass pedestal (i.e. concrete block table); mount the weighing equipment on vibration isolators; mount the weighing equipment on structural members, and not on the decking itself.
Heating/Air Conditioning and Ventilation Ducts: Heating, air conditioning and ventilation ducts cause air disturbances, which could translate to false scale movements and changing scale weights. These ducts must be noted, and may need to be re-routed away from the gravimetric system especially for those systems with small load cell capacities required for very accurate measurements.
Open Windows and Doors: Like the ventilation ducts, open windows and doors can create air disturbances for the gravimetric system. Doors and windows especially to the outside, should be noted, and special precautions may need to be taken to make sure the doors and windows remain closed.
Ambient Temperature: The ambient temperature where the gravimetric system - the scale, the controller and the feeder - is going to be placed must not exceed the temperature specified in the scale systems specifications as load cells are temperature compensated.
Hazardous Areas: Provisions for the Class, Division and Group of hazardous areas must be taken into consideration. These areas typically require the need for intrinsic barriers which will degrade the raw signal of the load cell due to the voltage drop across the barrier.
Electrical Power: The controller of the gravimetric system requires "clean power" much as a computer requires a clean line. This line should be free from any large inductive or capacitive loads. If uncertain about the condition of supply power, an isolation transformer or a UPS (Uninterruptible Power Supply) is recommended.
Large Inductive and/or Capacitive Equipment: The scale and the scale cables (excitation and signal) must be separated from large inductive and/or capacitive loads, such as arc welders, large motors, etc.
AC Voltage Power Wiring: All cables associated with the gravimetric system should be run in separate conduit from all high voltage AC signals.
Radio Frequency Equipment: The scale and the scale cables must be isolated from radio frequency-generating equipment.
Support Systems: The floor, balcony, mezzanine, etc., that the gravimetric system is mounted to must have a rigid construction for providing a solid platform as mentioned earlier.
The Distance from the Feeder and Scale to the Controller: For distances greater than 25 feet (eight meters), contact the manufacturer for cabling recommendations.
Electrical Ground: A solid electrical ground must be available for both the feeder and the electrical controller.
Preventative-maintenance programs are offered by some material handling equipment suppliers. These should include a feeder audit and recommendations by a field service engineer for improving feeder performance.
Scale/Feeder Mounting: The mounting table or mounting base for the scale must be solid and preferably afford some vibration isolation between the scale and the floor.
Outdoor Installations: Provisions must be taken for any gravimetric equipment that is going to be installed outdoors. Extreme temperature variations should be noted and avoided if at all possible. Cabinet heaters may be required to keep the controller and the load cells at a nominal temperature.
Flexible Connections: Any electrical, plumbing, etc. connections to the gravimetric feeder must be made with flexible conduit/piping/tubing to create a minimal effect on the movement of the gravimetric scale. Use factory recommended flexible connectors for feeder inlet and discharge ends if the scale system is part of a gravimetric feeder.
Maintenance Access: Consideration should be given to allow maintenance personnel access in order to maintain the scale, gravimetric feeder and the gravimetric controller.
Corrosive Atmospheres: Any corrosive vapors, dust, etc., should be noted and recommendation should be given on how to prevent corrosion; that is to say, what kind of material will resist the corrosive effects of the material in the atmosphere.
Refill Mechanism: The mechanism that is used to automatically refill gravimetric feeders must be tight- closing so the material cannot enter the feeder’s extension hopper other than during the refill time. In addition, the refill device must be sized to refill the required amount of material so the feeder does not starve out.
Refill Venting: The rapid introduction of the dry material into a feeder hopper extension during a refill causes a pressure to build up inside the hopper extension equivalent to the volume of air displaced by the volume of dry material. This pressure must be relieved, either by leaving the refill gate open so the displaced air can move into the refill hopper or by providing a vent in the feeder hopper extension.
Vacuum Systems: Vacuum or pressure systems, either at the infeed or discharge of a feeder may affect the gravimetric system by causing adverse suction or pressure on the system. These ancillary systems must be properly vented to prevent these conditions.
Contact your material handling supplier to see if they offer a preventative maintenance program that includes a feeder audit and recommendations by a Field Service Engineer for improving feeder performance. There typically is a small fee associated with this service, but when weighed against the alternatives of poor accuracy and frequent down time, the feeder evaluation is worth the expense.
Todd D. Messmer is the Applications Engineering Manager for Schenck Process located in Whitewater Wisconsin.