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 cross, impingement and fountain flows and factors affecting dryer selection.
Cross Flow. Cross flow systems direct the flow of gas across the material, parallel to the surface of the material, perpendicular or parallel to the direction of material flow. Examples of cross flow dryers include batch or continuous ovens, tray dryers and certain conveyor or band dryers.
Cross flow frequently is associated with low cost systems because it can be achieved with little more than a tray or pan. This type of technology is cost effective and versatile although not the most efficient method of process drying. Carrier temperature can be controlled and ramped in batch pro-cesses to present the most effective combination to the operation.
Like other dryers, impingement systems can have controlled temperatures and zones and frequently are modulating to provide a profiled or ramp-and-soak drying cycle.
Regardless of the flow type selected, to achieve efficient drying in any system, many factors must be considered. Among others, these factors include:
- Presenting the feed to the carrier (heat source) in the most efficient manner.
- Maximizing the exposed surface area.
- Constantly exposing new surface areas.
- Removing excess vapor from the system to prevent moisture block.