Continuing my discussion of flow types and the factors that affect flow type selection for a given application, in this issue I will look at flow flexibility.
Although each type of flow -- cocurrent, counter-current, through-the-bed, cross, impingement and fountain -- has been described singularly in the past two columns, it is more common for a combination of these flows to occur within the drying process. For example, in a rotary cascade dryer, the primary process is through-the-bed. That is, as the material falls from the top of the drum to the bottom and creates a curtain, the gas will flow through-the-bed. However, depending on whether the exhaust is extracted from the feed or discharge end, the secondary process may be counter-current or cocurrent. As the process air passes above the product (laying on the bottom of the drum as it moves along the length of the dryer), the air will be in a cross flow direction above the product bed. Similarly, for a rotary louver dryer, the air flows through the product in a through-the-bed fashion. Once the air has passed through the product, it will pass over the material in a co- or counter-current direction and cross flow over the bed of material.
Worth specific mention is the physical motion of the feed. Although the flow itself does not indicate whether the material is stationary or moving, the principles remain. In a batch oven, for example, the feed is still -- the tray does not move, nor does the product on the tray. In a conveyor dryer, the feed remains relatively still on the belt, but the belt is moving; hence, the bed is moving. In a rotary dryer, the drum is rotating and the feed is moving (falling) relative to the drum. In a fluid bed, spray or flash dryer, the feed is well disbursed in a dilute state and moving while the physical dryer remains stationary. Therefore, flow is a relative condition.
Factors Affecting the SelectionOther than the intrinsic nature of the dryer dictating the flow characteristic, several other important factors should be examined in selecting the best flow condition.
Some products display a skinning or crusting when exposed to high temperatures. This phenomenon seals the surface of the feed and reduces drying effectiveness, frequently without the potential of recovery. In these instances, careful temperature control is necessary, and cocurrent drying systems are not recommended.
Other products have a significant encapsulated moisture contained in the inner interstices of the feed that will cause the product to explode if heated to quickly. If attrition and particle size is important, this condition should be avoided. Again, co-current systems may not be suitable.
Some products have a physical temperature limit above which they degrade, change color, change state or change molecular structure (calcine). For these, a counter-current system potentially would be a problem and would not be recommended.
When selecting the flow characteristic, keep product-specific criteria in mind.