Good Dryer Design For High Quality Aquafeed

Design requirements

What are the design requirements to achieve a dryer with ± 0.5% moisture tolerance?

• The product layer height need to be lower (normally less than 20cm). A high layer will cause un-even drying in which the bottom product gets more drying as air enters than the product on the top where air leaves.
• A round shape design in which optimize airflow distribution. Since it has no dead spots inside the dryer, also improves air distribution and hygiene as well.
• A counter-flow air stream, giving an optimized heat transfer goes through the perforated circular trays and product. The product is moving in circles, in a cross-counter airflow.
• The product in the dryer is rotating and discharging into a lower deck and tumble product for mixing. This will be done in each deck before the product is leaving the dryer with very uniform moisture, it also implies all the product gets the same treatment bypassing different air volume and temperatures.
• The product flow is First-in, First-out (FIFO). Every pellet is getting the same retention time that is similar to continuous production, the operator can easily identify the batch of product quality and correct it immediately if need be.

Energy efficiency

Another indicator for the dryer performance is energy efficiency, drying is a very energy-intensive operation. When extrudate discharge from the extruder, the moisture content can be 20-30%, the dryer energy is significant and easily more than 50% of the plant’s energy consumption. Thus, it is a very challenging task to optimize the drying process efficiency without compromising the quality. The ultimate goal is energy-saving and product quality improvement. Several studies in the literature show how the drying inlet/outlet parameters impact energy efficiency. In conclusion, the inlet, outlet process air temperature, and air volume are three major factors to determine the amount of water evaporation.
Basically, the value of air volume equal to drying surfaces multiplies air velocity. The drying surface is fixed by a specific design, the velocity is limited by the size and density of pellets, therefore only the delta T is the flexible parameter for energy efficiency. The rule of thumb for delta T is as following,

• The airflow temperature that leaves the dryer need to be lower
  The lower the outlet airflow temperature means the more air energy is used for water evaporation.
• The airflow temperature that enters the heater needs to be higher
  The higher inlet air temperature meaning the airflow needs less heater energy to approach the setpoint temperature.

Good air system

Therefore, a good air system like a carousel dryer has the following characteristics and as shown in Figure 2.

• Using the recirculation air system
  Using the cooler air or dryer exhaust air as the make-up air.
• Independent heating zone
  The desire air volume and delta T could be set in each specific zone.
• Counter-flow design
  The definition of counter-flow in the dryer design is the material drop from up to down and the airflow motion is bottom-up, as some literature showed, the counter-flow design will be more than 10% better efficiency than a cross-flow dryer like a belt or horizontal dryer.
• Easy control
  Since the delta temperature, air volume and retention time all of the parameters will impact the dryer performance, this is necessary to design a simple rule that allows the operator easy to follow as shown in Figure 3. In the carousel dryer design, the operator only needs to verify the product outlet moisture, then adjust the inlet airflow temperature until approaching the target moisture. In practice, one percent moisture drop required 3℃ temperature raise.