Dear Mr VK Aggarwal,

We understand your proeject and have done similar projects involving weighing-batching.

The feeding speed of hopper (which is on load cell) controls the delivery speed of the screw conveyor, subject to it's maximum speed. We cannot control the feeding speed of the hopper because some times it may be full, sometimes empty and some times partially full, hence the feeding speed cannot be controlled. The direct effect of this is that the material in the screw converyor will not be the same always, thus you are not sure whether the exact quantity weighed is transfered or not.

Alternatively you should place an intermediate dosing tank which should be placed on load cells after screw conveyor, and above the reactor or where the weighed material is to be supplied. This will guarantee the accuracy desired.

We can provide such system or give you proper consultancy for this project.

Nitin Jain - Indore

nitin999jain@yahoo.com ■

The capacity of the hopper is not the criteria. The amount / level to which it is filled is what controls the rate of flow. If we delay the switching off of the screw conveyor for some time, you will reasonable accuracy but not perfection, because the conveyor does not discharges total material in inclined position and there will be a fixed amount of material always lying in the conveyor as dead stock. This fixed amount will vary in certain range which will give you some variation, hence perfection wcannot be achieved.

Where is your site located?

Nitin Jain ■

It is not good practice to use a steep elevator as a feeding device because of the degree of fallback that takes place when the screw is stopped. In this instance the roatary valve controls the amount taken from the hopper. The real questionis whther there is a flexible connection to isolate the screw elevator from the hopper weight. Presumably this is so with a 7 meter long elevator, as otherwise the loss in weight would reflect the amount delivered into the tank.

Assuming that a suitable flexible allows the amount taken from the tank to be accurately measured the query boils down to: - will the elevator deliver the same amount out that is put in after the first batch has primed the system. The answer is generally yes, provided that the elevator is run on for a consistent time after the bulk of the material has discharged. The reason is that an elevator at this angle is not completely self clearing and the delivery tends to tail off progressively when the infeed stops.

In detail there is likely to be some weight transfer with a tubular type flexible connection. The most efficient way to isolate weight in these circumstances is to employ a diaphragm type flexible, but these are not commonly used.

Another point is that it would seem that the hopper has a verical outlet to the rotary valve and is therefore probably of conical construction. The outflow rate and infeed conditions to the elevator will not in these circumstances be dependent pon the depth of bed in the supply hopper until the hopper is almost empty. In any case, the load cells will record how much goes out. The best way to secure the most accurate result is to run the elevator as fast as practical to attain rapid terminal effects. Unless the elevator has been well designed it will probably rattle and not be well suited for this duty. ■