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.
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| Impingement flow occurs when the
gas is controlled directly onto the feed at an angle perpendicular to
the direction of the feed. |
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Impingement Flow. Impingement flow occurs when the gas is controlled directly onto the feed at an angle perpendicular to the direction of the feed. The fundamental difference between impingement flow and through-the-bed flow is that with impingement systems, the feed is not permeable. Examples of impingement dryers include paint drying ovens, solid component overhead ovens and cabinet ovens.
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.
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| Fountain flow occurs when either the feed or the gas change direction inside the dryer. |
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Fountain Flow. Fountain flow is not all that common, but it is a recognized flow. It occurs when either the feed or the gas change direction inside the dryer. An example of a fountain flow dryer is a spray dryer. Depending on whether the feed or the air changes direction, the temperature of the gas to which the feed is exposed can be controlled. For spray dryers, the feed and the product may experience both the hottest and the lowest (exhaust) carrier temperature. In some dryer designs, the temperature can be controlled for each zone.
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.
This is the art of drying technology, an aspect that I will continually discuss. The palette includes the various equipment types and the flexibility of the flows. In Part 3, I'll explain flow flexibility.
