Paddle Dryer Modern paddle dryers are real all-rounders

Editor: Dominik Stephan

Sometimes, what it takes to get the job done is highly specialised equipment. Nevertheless, quite often universal all-rounder units with their high flexibility have a lead: By combining process steps like sterilizing, crystallizing, calcining, coating or reacting substances, modern paddle dryers can be used in a variety of applications.

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Paddle dryers have multiple applications – these include drying materials or substances from 50% moisture to ppm values, improving a substance's morphology, or reducing amounts of waste. (Picture: Nara)
Paddle dryers have multiple applications – these include drying materials or substances from 50% moisture to ppm values, improving a substance's morphology, or reducing amounts of waste. (Picture: Nara)

Pellets, pastes and powders — paddle dryers are versatile devices. They can be used to dry, heat up or cool down any medium, from animal feed to plastics and polymers providing a thorough mixing and gentle heat transfer.

Nara has developed a paddle dryer that provides an indirect heat transfer, so that the product has no direct contact with the transfer medium. The hollow shaft, paddles, and the double-walled trough are heated or cooled with water, steam or oil, providing a large contact surface with the product.

The gas flow through the product during drying removes vapours and prevents condensation. This minimizes the emission of gas and the correlating loss of heat.

From a commercial point of view, a paddle dryer has several advantages: Electric dryers can only be used for short-time drying processes, since they do not allow the configuration of a residence time. Fluid bed dryers have greater air and gas consumption and require larger-sized components such as fans and air vents. Further, more hot gas is released to the environment. Thus, the energy loss of fluid bed units is significantly higher.

Drying Modified Starch

Paddle dryers can be used for a variety of applications — one is drying modified starch. In contrast to native starch, modified starch must be kept at a certain temperature during drying to obtain the desired characteristics.

An application for wheat is similar to this procedure. In this case, the paddle dryer was also used as a reactor in order to weaken the wheat’s gas holding capacity. In a second process which is also reactive, wheat powder was roasted, enabling it to then be used as a component in curry powder.

Sterilizing Animal Feed

Another challenge is the partial sterilization of animal feed, or additives for forage. When animal feed is being processed, the target is to reduce the amount of microorganisms below a critical limit. This is achieved by raising the product’s temperature from room temperature to 75–80 °C by heating.

Steam is also fed into the process in order to accelerate the desired reduction in bacteria. The paddle geometry is adjusted accordingly to ensure optimal residence time and particularly even transport of the product through the machine. Good results have already been achieved when sterilizing fish meal and wheat starch.

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