Class 1 Sedimentation

In Class 1 sedimentation, the particles are dilute enough to have little interaction. Each particle may be assumed to fall at a constant rate. However, there is almost always a wide range of particles, and some may be more dense than others. In the following sketches, only three different particle sizes are considered, but there may be hundreds of different particles present.

The sketch on the left is for time zero with the suspension well mixed. The sketch on the right is 4 time units later. Note that some element near the center of the vessel would have the same concentration at time=0 and time=4 because as many particles are entering as leaving. If the element were near the top of the tank, there would be a difference if all of the large,blue particles had passed.

The left sketch is for t=8 and the right is for t=12. An element selected near the bottom at time = 8 would still have the same concentration as for the earlier times, but most horizontal sections would show differences. All of the large blue particles are gone by time = 12. Remember that there are actually many more different sizes of particles than shown in the sketches, so a real element about midway in the tank would be exhibiting differences in particle concentrations.

The sketch at the left is for t=16 and the right is for t=20. The large, blue particles that have the greatest settling velocity are gone. Of course, they have collected on the bottom of the vessel.

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