An essential component of any dosing system is the load cell. The load cell will measure the force applied and translate it to a weight that is readable by the user and can be used for running the process.
This section is touching 2 main ways to dose found in the industry : Gain-in-Weight dosing and Loss-in-Weight dosing. The key difference in between those 2 dosing approaches is the position of the load cells.
The dosing Gain-in-Weight is done by measuring weight of a hopper located after the dosing instruments. It is somehow similar to what many cooks do in the kitchen when they weigh on a scale the quantity of product required for their recipe.
Several ingredients from several hoppers can be dosed to the hopper on load cells. A restriction is however that the dosing must be sequential. Since the weight is measured by the hopper downstream, if all ingredients are dosed at the same time, the dosing cannot be tracked anymore. On the other side, such sequential dosing can be done in advance while the rest of the process after the dosing hopper is occupied at performing other tasks. It is called dosing in hidden time.
Figure 1 : Dosing principle for a Gain in Weight system
Contrary to GIW dosing, the hopper on load cells driving the dosing is located above the dosing instrument. It means that, when the dosing instrument runs and deliver the product, the weight of the hopper from which the product is taken is decreasing, it is losin weight.
It means that different dosing to the same equipment (a mixer for example) can be performed independently which can represent a gain of time. On the other hand, the dosing cannot be performed in hidden time since the equipment receiving the powder needs to be free. It could still be possible to combine the advantage of both technics (LIW and GIW) by dosing LIW to a hopper. All dosing can be done at the same time. while it can be performed in hidden time regarding the rest of the process. However, this solution should be reserved to very specific cases since it requires more equipment, height and pose the problem of making sure that all the ingredients dosed to the hopper can be discharged properly.
Figure 2 : Dosing principle for a Loss in Weight system
By its nature, Gain-In-Weight dosing is a batch operation and therefore cannot be applied for continuous dosing. Wherever continuous dosing is required (feed of a mixer for example), it will have to be done by Loss-In-Weight feeders. Several feeders can be used at the same time to feed the process downstream. Such application requires however complex automation for the LIWF. The control of the LIWF is actually mainly aiming to filter the perturbations that may occur on the LIWF load cells during the dosing.
Table 1 : List of common dosing equipment
|Screw feeder||Screw conveyors are one of the most common and simple way to perform a dosing. It can be used both in LIW and GIW, can have different sizes. The design of the powder inlet as well as the screw flight pitch is of importance to manage to perform a quick and precise dosing.|
|Vibrating tubes||Vibrating tubes have the advantage to be gentle with the product, easy to clean and maintain and can reach high throughput. The inertia experience at the end of dosing make it however more difficult to control to get a right precision. It should therefore be reserved for major ingredients.|
|Rotary valve||Rotary valves can be used for dosing but do not constitute a preferred solution. It is difficult to reach a good accuracy and those equipment are expesive when buying and complex of maintenance. It must be reserved to very specific applications (in case there is an ATEX concern for example)|
|Vibrating valve||Some suppliers can actually propose a butterfly equipped with a special actuator that controls the flux of product by vibrating. This kind of dosing offers a compact solution that can be used for major / small ingredients.|
|Loss In Weight Feeders||Loss in Weight Feeders refer to special machines built under the LIW models but that come stand alone with their own controller. They are used mainly for ingredients requiring a very high degree of accuracy and can have a specific design to adapt to the characteristics of each ingredients (twin screw, agitated hopper...). Such systems are also used in continuous dosing ; for this particular application, the quality of the software allowing to filter the load cells signal and drive the dosing equipment is of prime importance for the quality of the dosing.|
The key in achieving a good dosing is mainly in the care applied to properly isolate the load cells from any disturbance of the environement. Of course, the basics is to have load cells correctly calibrated and giving weights within the limits defined by the supplier. However, if a disturbance happens, the load cells will read a weight inacurrate but will anyway give a value : this is where it can be trick for the plant operator, can the value be trusted or not ?
To trust the value, it is necessary to ensure that no force, except those due to the weight measured, apply to the load cells. It is particularly important to make sure the following.
Table 2 : Key parameters to achieve an accurate dosing
|Flexible connections||The flexible connections are neither too tensed nor too compressed. This particularly important to check this point after a service (maintenance, cleaning...)|
|Filters||Discharging a powder is inducing some air movement. If the air is not properly vented (downstream hopper) or admitted (upstream hopper), the air can push on the hoppers and disturb the load cells reading.|
|External disturbances||No vibration is transmitted to the hopper during the dosing, no one is pushing a hopper, no strong air draft|
|Other connections||No connection is preventing the hopper to move freely|
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