Numerous different types of dryers use gases (most commonly air) as the carrier. Mostly, they are categorized into a type of dryer (spray, flash or fluid bed, for example), mode of heating (direct or indirect), or mode of drying (batch or continuous). But, within these categories, there is a less commonly referred to dynamic: the flow. Flow refers to the direction of the gas relative to the direction of the feed travel. Flows are either cocurrent (or co-current, as it is sometimes written), counter-current, through-the-bed, cross, impingement or fountain. In this short series, I will describe these flows and touch on the factors that affect the selection of flow for a given application.
Cocurrent Flow. In cocurrent flow, the feed and the gas stream move in the same direction. Certain dryers such as a flash dryer intrinsically are cocurrent. Others -- rotary cascade or spray dryers, for example -- offer the cocurrent operating flow as an option.
Cocurrent flow enables the hottest air to interact with the feed in its wettest form. This allows the most energy to be transferred to the material with the lowest increase in product temperature. For many materials, this is beneficial. With cocurrent flow, the product and gas exit the dryer at similar temperatures; normally, the gas is a slightly higher temperature than the product. This setup maintains the gas at a temperature well above the gas's dewpoint, but it may necessitate a high-temperature pollution control system.
Counter-current flow presents the highest temperature gas to the driest product. This technology is suited for relatively robust products that have difficult-to-remove residual moisture, bound or hydrated moisture. With counter-current flow, the product frequently exits the dryer at a higher temperature than the gas. Depending on the type of gas circulation, the exhaust temperature may be close to the dewpoint, and if not properly accounted for, the saturated gas could clog fabric filters and cause other condensation-related problems.
Many through-the-bed systems may have multiple zones where the gas temperature is controlled to allow the most efficient drying condition. In single-zone systems, all of the material is exposed to the carrier at the same temperature. Through-the-bed systems are well suited to free-flowing feeds as well as feeds that require little physical movement or those that pose a problem for certain types of material-handling systems.