How does the expander works?
The expander is installed right after the existing short-term conditioner and is suitable for any pellet mill brand. Conditioning of the feed takes place before the expander in the short-term conditioner with the addition of saturated steam and eventually liquids (molasses, fats), in order to ensure the success of the desired treatment.
Like a single-screw extruder, the expander consists of a thick-walled mixing tube, which is divided into individual spiral segments and is equipped with an over-mounted worm shaft to which mixing and kneading elements are attached. The pipe is equipped with stop pins and steam nozzles and the shaft transports the feed into the compression zone. This shearing and kneading force load, leads to a further heating up of the product, besides the addition of steam.
The inlet mash temperature must be about 70 °C. The addition of saturated steam provides the required increase in the moisture content of the material to over 17% and produces a decrease in the mechanical power requirement, by the production of hydrothermal energy, whilst maintaining the desired effects for changing the materials.
At the outlet, the pressure suddenly falls to the ambient pressure, whereby the majority of the added water spontaneously evaporates (flash evaporation). Due to this phenomenon, the temperature of the product sinks suddenly (below 90 °C).
The structure of the expanded material, which directly leaves the expander discharge, can be crumbly or lumpy, depending upon the recipe and the set temperature of the product.
After eventual cooling and granulating, the product can be brought into market as the final product. However, in most cases, a pellet press is connected downstream to the expander. In this last case, the expander provides a distinct improvement in the properties of the pellets of the feed mixture.
The frontal die is mobile and, depending on the desired conditioning result, the pressure is adjusted by means of a hydraulic unit at the piston during operation, allowing determining the intensity of kneading work, product temperature and energy consumption continuously without delay. By turning the die it’s possible to increase or reduce the space where the product leaves the machine, changing the pressure and the extrusion molding of the final product. The frontal cutter can rotate at variable speed, determining the granulated size.
Through the length of the expander many special injectors add steam under high pressure. The maximum pressure comes up to 90 bar, with temperature of 100-120 °C, although only for a short time, because the entire process takes only a few seconds. This is the reason why the expander is called also STHT (Short Time, High Temperature).
The product quality of expanded material can be affected by the following process parameters:
- hydrothermal energy production, conditioning
- mechanical energy production, shaft revolutions
- grade of milling of the raw material
- selection of the additives