Trommels are best used as a scalping device to remove large/oversize pieces from a stream, when these particles are a small percentage of the stream. They are not very efficient as compared to regular screens, but they also beak up clumps of material,

As standalone devices they work well on friable feed, discharging boulders and breaking up lumps.

On grinding mill discharges they keep the pebbles and other debris out of the product stream.

That is if you refer to Mill trommels ,

If you refer to trommel screens or drum screens then there are quite a few options ,
The Rota spiral screen is a rotary drum screen with build in spiral to remove oversize fast , can cut as low as 80 - 100 micron and could scalp Cyclone overflow ,
or remove the undersize from mill feed if installed on Cyclone underflow ,and so reducing
gold recycling back to the mill and gold lock up .

Rota spirals are relatively cheap and simple screening machines.

What is your application?

If the efficiency is concerned the design (diameter, length, single or double decks) must be correct depending on the application and suitable to the material. The screen opening size is also very important and water addition if required. Care must be taken to control the wear to avoid the running at very large screen opening size than recommended.

The trommel screen is like a micron screen, it remove large objects (oversize, bushes, plants, rocks) from feed from the mine to avoid choked feed pipes and or plant instability, and the (undersize/micron size feed) moves through the plant for processing.

Basically its starts with your trammel feed from the mine enters the trommel and then separation take place, finer minerals from courser minerals... the finer feed minerals go to plant processing throughput and courser to the extendable/tailings/waste. The trommel has to do with width, microns, diameter, length, size.