Several applications come to mind where a bulk solids flow meter may be a great fit for a certain application.
Inaccurately loading a container, truck, or rail car can have undesirable repercussions. Overloading with too much material is not only a loss of revenue but could also result in stiff fines when shipping. There is also the additional time lost for unloading and reloading the container. A flow meter can be used for a more accurately fill.
A flow meter can be used to obtain a master rate that different feeders can be ratioed off of. This is a common application in large plastic and food extrusion processes. The flow meter feeds in the base material acquiring a master rate to which the additive feeder(s) can be ratioed.
A flow meter is a great tool for totalizing both raw material in and finished material out of a process for calculating inventory and process losses.
A flow meter can be used to meter a material into a process. Here a pre-feed device such as a rotary airlock or screw conveyor is used in conjunction with the flow meter. Typically, the flow meter controller will send a control magnitude signal to the VFD supplying power to the pre-feeder motor. A set point is entered in the controller and the prefeeder speeds up or slows down based on the rate of material flowing though the flow meter.
There are four basic flow meter types:
These flow meters calculate a rate via material striking an internal impact plate that is attached to a load cell either directly or through a linkage. The controller then calculates the rate by the amount of force exerted on the plate/load cell.
Material slides on a chute located inside of the flow meter. As with the impact plate the chute is attached to a load cell either directly or through a linkage. The weight of the material on the chute is measured by the load cell and converted into a rate by the controller.
There are flow meters on the market that are a combination of both impact and deflection flow meters.
This type of flow meter typically has a rotating wheel with vanes or slates inside of it. A motor attached to a load cell rotates the wheel. As material enters the flow meter it passes through an opening in the wheel striking the vanes as it is discharged out of the flow meter. The resulting Coriolis effect of the material hitting the vanes is transferred up to the motor connected to the load cell. The controller calculates the rate based off the amount of force exerted on the load cell by the motor. From a cost perspective the impact flow meters are less expensive than the Coriolis and combination-type styles.
Accuracies also vary with the Coriolis flow meters being more accurate than impact flow meters, which is why there is a premium on the cost for a Coriolis flow meter. Accuracies for Coriolis units fall in the range of 0.5%, while impact flow meters are typically 2%.
As with any feeding device flow meters are not without their limitations. Feed rate and bulk density minimums must be taken into consideration. Flow meters do not work well on lower rates. They are typically designed for measuring tn/hr vs. lb/hr. Lower bulk densities will result in a decrease in accuracy as the load cells cannot sense the material weight.
There are limitations on the minimum and maximum particle size of the material. Larger particles can cause plugging, whereas smaller particles i.e. dust will not be measured at all.
Adhesive materials will build up on the sensing plates in a flow meter. Not only does this cause housekeeping issues it ultimately throws off the calibration of the unit making it less accurate.
Want to know if a flow meter is the right choice for your application? Consult with an applications engineer from a material handling company. They can help determine if there is a fit for a flow meter in your process.
Todd D. Messmer is the applications engineering manager for Schenck Process, Whitewater, WI.