Heated air drying

postharvest-lowcost-dryerCompared with traditional sun drying, heated air drying or mechanical drying allows for suitable drying air conditions to be set. In this method, drying can be carried out any time of the day or night.

Using mechanical dryers may also reduce the labor costs, especially if some form of mechanical turning or stirring of grain is practiced, as in the case of re-circulating dryers. Grain re-circulation allows to uniformly dry grains, and the automatic drying air temperature control to maximize the drying rate and at the same time reduce over-heating or over-drying. The following are options for heated air drying:

Batch dryer


Fixed-bed batch dryers usually have rectangular bins with plenum chamber underneath (flat bed dryer, box dryer, inclined bed dryer) or circular bins with central duct (Vietnamese low-cost dryer). 

heated-air-drying-2Bed configuration of fixed bed batch dryers

The most common fixed bed dryers are flat bed dryers which have a very simple design. Grain is laid out on a perforated screen, and dried by forcing air from below. The air fan that provides the drying air is usually a simple axial flow fan that is powered by a diesel engine or by an electric motor. A kerosene burner or a biomass stove provides drying heat. The capacity of the dryer varies from one to ten tons.

Generally the drying floor is flat although dryers with reclining sections (to facilitate unloading) or vibrating sections (to facilitate stirring) exist as well. The height of the layer is usually 40 cm. The most common smaller dryers have a capacity of one to three tons per day with drying times of six to twelve hours. For drying of paddy in tropical areas, an air temperature of 40−45ºC is normally used with a heater capable of raising the air temperature 10−15ºC above ambient. An air velocity 0.15−0.25 m/s is required and typical fan power requirements are 1.5−2.5 kW /ton of paddy. The efficiency of these dryers as well as the head rice recovery is improved by stirring the grain during drying.

Other fixed bed dryers have a cylindrical duct made out of porous materials with a central duct for drying air delivery. These models save floor area and small scale units can be made out of very cheap materials such as woven bamboo mats thus keeping the dryer affordable for small farmers. However, an inherent problem of this dryer type lies in its circular design because the inner layers of the grain bulk contain less grain than the outer layers. Air velocities and thus drying potential are therefore larger close to the center of the dryer where the drying air enters the grain bulk and the air velocity decreases on its radial path through the grain. At the outlet, the drying rate which is already lower because of adsorbed water is further reduced by the lower specific air volume. This leads to even higher moisture gradients compared to flat bed dryers. Circular bin dryers made out of locally available materials, however, offer very affordable solutions to farm level drying, especially when they are used for ambient air drying with low temperatures where the moisture gradient is minimized.

To reduce the moisture gradient that develops during drying and to eliminate the need for mixing, some manufacturers have introduced devices for reversing the airflow in some fixed bed dryer models. This reduces the moisture gradient and thus improves the quality of the dried paddy but it adds to costs. Compared to the more complicated re-circulating batch dryers this is still a feasible solution where simple design is needed and operator skills are low. Principle (left) and example of a mobile reversible-flow batch dryer (right)

heated-air-drying-3Principle behind reversible-flow batch dryerheated-air-drying-4Mobile reversible-flow batch dryer

In general, mechanically dried grains will produce better quality rice compared to traditional sun drying. It will lead to more uniform drying of grain and higher milling yield and head rice recovery. 

Since rice quality is becoming more important to rice consumers, medium-sized grain dryers have become a common sight throughout Asia. For production of premium quality rice or seed, mechanical drying with heated air dryers is highly recommended.

Table: Overview of commercially available dryers