Drying and Cooling of Bakery Grist at Its Finest with Carousel Dryer-Cooler (Published in dei-Germany).
This article is published in dei June 2022
Thermal treatment of baking grist - Carousel drying and cooling
Many materials require gentle mechanical and thermal treatment during drying. Baking grist is an example of such material. Baked grist is a pre-processed, dough-like mass with a moisture content of approx. 16% made from large-milled wheat or rye, intended for baking wholemeal bread. On entering the dryer it is quite pasty and sticky. The drying temperature must not exceed 50 °C to preserve the nutritional value. Baked grist can be dried with a combined dryer/chiller, reducing the moisture content from 16% to 10% at the outlet. Carousel technology, a proven drying technology that is used in many industries, is very well suited for this. The Taiwanese manufacturer Idah adapts these drying systems exactly to the respective customer needs.
Built-ins loosen clumps
To simplify the packaging process, the grist should be free-flowing after drying/cooling. Therefore, constant agitation and mixing are required during the drying process. In a specific application, however, the product stuck together on the upper drying deck of a carousel dryer to such an extent that a large lump with a diameter of around 3 m was formed. The Idah designers were able to quickly solve this problem by retrofitting what is known as a paddle worm. The paddles continuously loosen the product on the top level of the carousel dryer. In a bunker dryer, this problem would probably have been very difficult or even impossible to solve as the product can form a layer up to one meter thick. In addition, the material's weight can significantly increase the adhesive effect. The multi-level design of the carousel dryer has an extremely positive effect here. The carousel dryer is combined with a cooling system to avoid permanent exposure to heat on the material to be dried, which can cause physical and chemical changes in grain.
TK-300 series carousel dryer for drying/cooling baking grits
Moisture and temperature changes of the grist with a combined dryer-chiller with a drying temperature of 75 °C (A) and 87 °C (B)
Combined drying and cooling
The Idah Carousel Dryer Cooler consists of multiple decks with a tilting floor in each deck to offload the material onto the lower deck. The discharge helps to mix the material and expose it to the drying and cooling air, allowing for effective heat transfer. On the decks, the material rotates in a constant flow of air and thus experiences an even drying and cooling process. The drying and cooling air are guided past the incoming product in a cross-countercurrent flow to achieve maximum heat transfer. A rotary valve in the inlet prevents the uncontrolled intake of ambient air. At the same time, the material is continuously fed into the dryer. With the help of a special mechanical device, dough-like material can be broken up and loosened. This ensures an optimal flow of drying air and effective heat transfer. For cooling, the cooler exhaust air is routed through the dryer section. This offers the simplest possible way to use the waste heat from the cooler without additional equipment.
Process data for baking grist
The graphs show the changes in temperature and humidity of the baking grist in a TK-300 series carousel dryer. The dryer has a diameter of approx. 3 m, the throughput of baking grist is 4.5 t/h, the residence time for drying is 23 minutes, and for subsequent cooling 15 minutes. The inlet temperature of the baking grist is around 87 °C. The product leaves the dryer at a temperature of approx. 13 °C. The direct effect of the special mechanical device installed between Deck 2 and Deck 3 can be seen in the graph on the left. Before installing the device, the temperature dropped to 35 °C. The additional device helped the product flow freely and the product temperature rises from about 43 to about 58 °C just before the grist falls into the cooling zone. The graphic on the right shows the influence of the higher drying temperature under otherwise identical conditions. A similar temperature to Deck 2 can be achieved if the drying temperature is increased to 87°C. This condition results in a high temperature before the grist enters the cooler, which can lower the nutritional value. In addition, the final moisture content is around 1% lower than with a drying temperature of 75 °C. Comparing the combined dryer/cooler with the pure cooler shows that only an air duct with additional heat exchangers between two floors has to be inserted and the main shaft has to be slightly lengthened.
Concept of the carousel cooler (left) and the combined carousel dryer/cooler (right)