Steam Treated Stems (STS) is a technique for expanding cut tobacco stems.In the STS unit (Figure 2 .7) the stems pass through a Rotary Airlock into an Expansion Chamber, where they are subject to the intensive effect of a group of high velocity steam nozzles. A high rate of heat and mass transfer induces an increase in both product moisture and temperature.
As the particles accelerate away from the Expansion Chamber into the Post Expansion Chamber Tapered or Parallel Tube, this is followed by flash evaporation of the water present in the tobacco cellsThe moisture gain in the STS process is typically 3%, and the stems are discharged directly into a High Expansion Dryer (HXD) or EVA Dryer or into the infeed of a Fluid Bed or Rotary Dryer via a Vibratory Conveyor. The dryer may be a Rotary Dryer, EVA Dryer, Fluid Bed Dryer or HXD, good results have been obtained with the combination of STS and each of these drying methods.If a degree of expansion below the maximum obtainable is required (e.g., for cigarette design reasons), the STS unit is supplied with the facility to control the product expansion.This is achieved by proportioning the injected steam flow with dilution air at the same pressure. The degree of expansion can be varied continuously between the minimum and maximum for the product.