Convective drying is the most common way of thermal drying. Gases from combustion or air heaters circulate through or over the product and evaporate the solvent. Convective drying requires large flows of hot drying gas, which is the energy carrier. Its treatment requires special attention.
Broadly speaking, there are three types of convective drying processes:
Parallel flow: the hot drying gases and the humid product circulate in the same direction inside the dryer. This means that the hot gases lose temperature while the product increases it. Thus, the parallel flow is used for thermo-sensitive or flammable products. The advantage of parallel flow is that very high initial drying temperatures can be used without heating the product in excess which reduces chimney losses.
Cross flow: the flow of hot gases crosses the product layer. This process is easily regulated and controlled as well as being energetically efficient because the gases exhaust temperature can be maintained lower than the product discharge temperature.
Counter flow: the drying gases flow in the opposite direction of the product. This is the most favorable and efficient process for single-zone dryers, but requires that the treated products are thermo-resistant. Parallel flow is specially recommended in order to achieve very low final moistures. It also allows for a precise control of the operating conditions.