The annular gap expander is a high temperature, short time conditioner primarily used for pre-treatment of feed prior to pelleting [1, 2]. The device resembles a single-screw extruder, but differs by discharging material over an annular gap outlet design instead of forcing it through a fixed die. The expander is capable of greater throughput with lower energy costs. Expanders were used first in Northern Europe in the late 1980s to improve pellet quality while using high levels of liquids and by-product ingredients without compromising throughput requirements. Laws passed in Northern Europe requiring feed meet rigid thermal processing standards for biosecurity reasons, accelerated acceptance of the technology. North America first embraced the technology in 1993. Evidence indicates expanded feed offers several benefits. While not a complete list, documented benefits include animal performance improvements, lower feed moisture, greater formulation flexibility, improved feed hygiene, increased mill throughput, manipulation of feed bulk density and particle size, ability to crumble without pelleting, and longer pellet die life. While representing an alternative to traditional conditioning technologies, expanders must be operated correctly for optimal results. Pellet mill energy costs are lower when using expanders; however, total manufacturing energy costs increase. Users must determine, under their own conditions, the cost:benefit relationship.
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Fancher et al. (1996) studied this question.