As with any separation it depends on the assemblage of the feed material and what you are wanting to achieve, but generally speaking. The TBS (density separators, up-current classifiers, etc) can accept a feed solids ranging from 35-75% solids as long as the unit is designed to account for the excess water at the lower end of the feed solids range. 

In typical mineral sands applications they are normally used in conjunction with spiral separators either on the front end of the plant for desliming and/or light-heavy gangue mineral rejection. This can also help to smooth out any minor feed fluctuations going into plant. Or they are used on the back end of the plant for final cleaning of the concentrate to remove fine silica or very fine HM. 

I have also seen them used to make cut sizes in the silica sand industry. The typical feed range is 1mm down to 53um with more area required for processing the finer the material you are treating. I hope this helps. 

Interesting discussion. Is this equipment (TBS) similar to the old hydrosizers used for size separation in the tungsten and tin industry. The throughput was quit low but otherwise worked extremely well.

You are correct. Hydrosizer is another common name used in the industry. Yes, the throughput can be low, especially if you are trying to make a separation in the finer size range. But at the same time you can really load them up as the particle size of the material increases. 

I've found that they generally produce the same D50 cut size over a considerable range of feed size distributions, the D50 size can be steered over a wide range by varying the teeter bed water, and that they are very critical on water quality. You especially want to avoid or strain out leaves, wood fiber, minnows, and other organic matter because it plugs the water nozzles. You also have to be careful that solids don't enter the water pipes and plug them up when the machine is shut down.

The high % solids in the underflow can be a plus depending on the downstream process, but the overflow tends to be very dilute.

Their capacity per square foot or meter of floor space is low.

Competing equipment performing a similar function include hydro-separators (highly loaded thickeners without teeter water), cyclones, other classifiers like screw classifiers, and fine screens like Derricks.

Like a cyclone, particle density is significant. A larger low density particle will report to the overflow with smaller high density particles.

We've used them in iron ore and silica sand. with D50 splits ranging from 200 to 50 microns. We've never had occasion to try them at coarser or finer sizes.

Excellent points, especially on the teeter water quality.

We normally use teeter bed for separating a mineral assemblage into heavies (mostly of value) and lights to go to next operation . So we have to manipulate the design variables of the equipment and operating variables( such as feed rate )to get the required bed density to get the best separation.

Another up-current classifier to consider is the Reflux Classifier which is finding traction in the beneficiation of fine coal in place or in conjunction with spiral classifiers. These can seemingly be used for a top size as high as 2mm and as low as 250/150 microns. It is sensitive to water quality just as with the TBS.
The hydrosizer can separate different fractions, also the TBS?, for instance to feed different sizers to the shaking tables?

If we go back to the original point raised in the first instance, teeter bed is the phenomena created by manipulating design and operating variables to get a bed density to separate heavies from lights. Many machines like hydrosizers use this phenomena. There is no centrifugal force affect in this unit.

Spirals operate on centrifugal forces also and so separation characteristics differ.

Which one to use depends on the characteristics of the particulate assemblage such as liberation/either significant differences are there between wanted and unwanted minerals etc,

Look at the ore and decide the machine. 

Thanks to you all. i want to use this machine for coal for size range of 0.3 mm up to 3 mm. you mentioned good points.

I found useful http://www.saimm.co.za/Journal/v102n07p385.pdf