Weighing screw conveyor with automatic calibration.
Weighing screw conveyor perform a dual function – they transport the bulk material from A to B and measure the throughput (t / h) and the absolute quantity delivered (t) simultaneously.
Especially for fine and dusty products, such as white lime, hydrated lime, cement and starch a weighing screw conveyor provides the advantage of dust-free conveying. Due to the dual function (conveying and weighing) this results in an economical solution compared to conventional weight feeders and solids flow – measuring instruments.
Requirements for the use of weighing screw conveyors:
1. To ensure optimum transport of the bulk material in conjunction with a precise weighing, the inlet- and the outlet connection must be fully decoupled. The input feed should be controlled via a rotary valve or a second screw conveyor.
2. The screw should only be filled to maximum 40% of the conveying capacity -otherwise a part of the conveyed product is thrown over and double-weighted.
3. Evt. Caking in the conveyor tube or on the screw flights should be zeroed periodically.
Especially the third point can often not be realized in practice. Screw conveyor often dose additives (eg. hydrated lime in the flue gas desulfurization) and are in operation 24 hours arround the clock.
In order to still be able to use the serious advantages of weighing screw conveyor, Hense Wägetechnik has developed a concept for automatic “self-calibration” in continuous operation.
Functionality:
The weighing screw is feed via a small hopper with a rotary valve in the discharge. The hopper is mounted on 4 precise “Clevermount” – weighing modules. The static hopper weighing system and the “weighted screw conveyor are connected to the same evaluation electronic.
The “self calibration cycle” can be started periodically (over a programmable time) or externally from the PLS system. In the first step the hopper is filled to an adjustable weight. This value can be preselected in the evaluation electronic. The weighing electronics transmits a signal “start calibration cycle” to the parent PLS system. The filling of the hopper is stopped during the calibration. After a short settling time, the electronics stores the container weight (kg) and the state of the totalizer (kg). The electronic system now “waits” until the weight of the product in the hopper is reduced by an adjustable value (eg. 100 kg). Now the electronics compares the reference quantity (eg. 100 kg) withdrawn from the hopper to the counter of the weighing screw – on exceeding a defined fault tolerance, the system performs an automatic correction calibration. Furthermore, all data is logged in a log table and optionally set a tolerance alarm.