Loss-in-Weight, Gain-in-Weight Feeders
by Joseph Marinelli

It seems appropriate at this time of year to discuss loss-in-weight and gain-in-weight approaches to feeding bulk solids. In previous articles, we have discussed volumetric feeding vs. gravimetric feeding. If you require feed accuracy in the ¼% to ½% range, a gravimetric system will provide it (depending on the material being handled).

A gravimetric feeder relies on weighing the material to achieve a required discharge rate or batch. This approach should be used when the above stated accuracy is required, when the material's bulk density varies, or when you simply need to record the weight of material used for a particular process. A disadvantage of a feeder that weighs material is that it is usually more expensive than a volumetric device.

Loss-in-weight (LIW) systems are the state of the art when feed accuracy is essential. In a loss-in-weight system, everything is weighed. Load cells are attached to the bin or hopper that are capable of weighing the entire bin, feeder, and contents. The load cells sense a change in weight that occurs while your product is discharging and sends a signal to a controller that speeds the feeder up or slows it down.

Figure 1.  Loss-in-Weight, Continuous	Figure 2. Loss-in-Weight, Batch

A LIW system does exactly what it states, measuring the loss in weight of material discharged from the system. As such, it can be used both in continuous and batching systems. They can even be used for liquids simply by replacing the feeder with a pump. A continuous LIW system is shown in Fig. 1. Fig. 2 shows a batch LIW system that incorporates a bulk mode (fast feed) and a trim mode (slow feed to top off the batch).

A disadvantage of LIW systems is that obviously they cannot weigh while they are being filled. Fill time is critical in that a typical LIW system will switch to volumetric feeding while the filling process continues. When the bin is filled, it switches back to gravimetric. Screws, belts, rotary valves and louvered or vibratory pan feeders can be used to control discharge.

Gain-in-weight (GIW) systems are used only for batching applications. The receiving container rests on a scale or on load cells and the system controls the discharge from the filling bin, which can use a volumetric feeder to control rate. In a gain-in-weight application, batch accuracy of 1/4% at two standard deviations is not unusual.

The disadvantages of this type of system are: headroom requirements, the fact that only one material can fill the GIW hopper at a time and the GIW hopper must empty completely with little or no buildup. A GIW system is shown in Fig.3. As with the LIW batch application, the GIW system incorporates a bulk mode (fast feed) and a trim mode (slow feed to top off the batch).

Help others by posting your comments, suggestions and experiences with bulk solids feeding or any other materials handling concerns you may have on our On-Line Help Forum.  For past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.