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.
Special continuous flow dryer types, which are used as first stage dryers in two-stage drying systems, are the rotary drum dryers in the Philippines and the fluidized bed dryers which were successfully commercialized in Thailand in the nineties. Both types use extremely high temperatures (up to 110-120°C) for rapid removal of the surface moisture and can only dry down to 18% MC without damaging the grains (see also "Two stage drying"). While the rotary drum dryers were mainly disseminated through government programs the fluidized bed dryers in Thailand were accepted by the private sector and are well integrated in combination drying systems that include either large scale in-store drying facilities with several hundred tons capacity or mixed flow heated air dryers for second stage drying to storage MC.
The fluidized bed dryer consists of a drying chamber with an air speed of around 2.3 m/s, a bed thickness of 10 cm in which the grain is exposed to the drying air for 10-15 minutes. Capacities of commercial units range from 1-10 t/h. A diesel burner or a rice hull furnace is used as heat source and a system for re-cycling 50-70% of the drying air is provided to improve energy efficiency. Typically head rice yields are reported to be up to 5% reduced compared to samples dried at ambient air while the effect on whiteness is minimal.