Continuous flow dryer

Conventional Continuous Flow Dryer

Although not very common in Southeast Asia continuous flow dryers are used by some larger milling enterprises that handle large volumes of wet paddy. Conventional continuous flow dryers usually consist of either mixing or non-mixing columnar dryers with different systems of airflow with respect to the grain (see figure). 

• Cross flow dryers are of simple design. In the drying zone the grain moves downwards between two perforated metal sheets while the air moves horizontally through the grains. Since the grain is not mixed moisture gradients develop across the bed. They are also less susceptible to clogging than mixed flow dryers.
• In concurrent flow dryers the air moves in the same direction as the grain. This has the advantage that the air with the highest drying potential is in contact with the wettest grains. Higher air temperatures can be used for a fast drying process. Drying is rapid in the upper layers and slower in the lower layers, which suits the drying characteristics of paddy.
• In counter flow dryers the air moves upwards against the movement of the grain. This system is very energy efficient since the drying air continues to adsorb moisture on its way through the increasingly wet grain until the air outlet.
• Mixed flow dryers produce the best quality grains because of the continuous mixing effect. The inlet and outlet ducts can be placed in alternating pattern so that both concurrent flow and counter flow of the air can be achieved in one dryer.

A continuous flow dryer cannot be used as a stand alone machine but needs to be integrated in a larger system consisting of the dryer, several tempering bins and conveying equipment (Figure 12). It is not possible to dry the paddy in a continuous flow dryer from typical MC content down to levels for safe storage in one single pass. Typical drying MC reduction rates per pass are around 2%. One pass lasts 15-30 minutes at around 70ºC drying air temperature. Higher rates could be achieved by increasing either the drying air temperature or the retention time but both would negatively affect grain quality because of increased cracking. Continuous flow drying systems are therefore operated as multi-pass systems where the grain is moved to tempering bins for around 24 hours after each pass until the desired MC is reached (see also Section 3.3.4 Tempering). Sometimes the tempering bins are equipped with aeration facilities to cool down the grain with some additional low-temperature drying effect. Actual residence time in the continuous flow dryer in a multi pass system is 2-3 hours for a 10% reduction of moisture and is thus below that of a re-circulating batch dryer.

Continuous flow dryer operation needs to be carefully planned and requires good management in order to fully utilize the expensive equipment. In addition it requires continuous input of wet grains at a steady rate. The small scale farms, multitude of varieties, low labor and management skills and high capital investment needs are some of the reasons why continuous flow dryers are for the time being not feasible in most Asian countries.

Flash Dryer