2. Purpose of drying

Grain is often harvested at a moisture content that is too high for safe storage. Drying is the most common post-harvest process performed for the long-term preservation of grain. Microflora and insects typically will not grow at equilibrium relative humidities below 65%, so high moisture content grain is artificially dried to a moisture content that will result in an equilibrium relative humidity within the stored grain mass lower than 65%. Although, the exact equilibrium relative humidity required for safe storage is a function of grain temperature, kernel damage, risk level of the stored grain manager, and numerous other factors. Table 6.1 sum­marizes the equilibrium moisture content at three levels of equilibrium relative humidity and a temperature of 25°C. Shelled corn in sound condition should be dried to a moisture content below 14% for safe storage. Soybeans require a storage moisture content less than 12.5% for safe storage.

3. Classification of dryer types

Grain dryers can be subdivided into numerous categories: 1) on-farm or off-farm; 2) high temperature or low temperature; 3) continuous flow or batch.

Most off-farm dryers are high capacity (between 700 and 5,000bu/hr when drying corn from 25 to 15% moisture content) continuous flow systems that utilize high drying air temperatures (between 140 and 220°F) and high airflow rates (between 50 and 100ft³/min per bushel). They are further categorized based on the grain flow and air flow patterns through the dryer (Figure 6.1). Most high temperature, high capacity grain drying systems in the United States are based on the crossflow design (Figure 6.1a), although, mixed flow dryers are more common outside of the United States (Figure 6.1c). Typically the grain is cooled at high rates within the dryer prior to transfer to storage.

Figure 6.1 Types of continuous flow grain dryers.

Grain quality is significantly affected by the drying process and type of dryer. Two of the most significant variables that have a deleterious affect on grain quality are maximum kernel temperature and drying rate. For example, the head yield from rough rice is sig­nificantly affected by the drying rate and kernel temperature. However, corn and wheat are less sensitive to high kernel temperatures and the maximum temperature allowed is a function of end use. Maximum kernel temperatures will occur with crossflow driers due to the extended exposure of the kernels on the hot, dry air inlet side of the dryer. Very little drying occurs where the air is exhausted from the dryer. Concurrent flow dryers are not common. However, they result in excellent grain quality since the hot, dry air is intro­duced with the cold, wet grain. A high rate of evaporative cooling occurs near the air and grain inlet that minimizes the kernel temperature. Mixed flow dryers result in kernel tem­peratures, and hence grain quality, between crossflow and concurrent flow dryer designs. High temperature, continuous flow dryers are also common on U.S. farms, although with lower drying capacities than typically found at commercial facilities. On-farm high temperature dryers sometimes utilize delayed cooling systems (dryeration) where the grain is dried to within a couple of percentage points of the final desired moisture content, tem­pered, and cooled within storage or tempering bins. This results in higher dryer capacity and higher quality grain.

In-bin drying systems with ambient or heated air (temperature rise between 5 and 50°F) are often found on farm. There are numerous strategies and types of processes that can be accomplished with in-bin drying. Simple natural air drying systems employ fans with airflow rates between 0.5 and 3.0ft³/min per bushel and result in the highest grain quality, lowest specific energy consumption, and lowest drying capacities. In-bin stirrers can be used to increase moisture content uniformity, drying temperature, and drying rate. Some in-bin dryers continually move the corn from the bottom of the bin simulating the per­formance of a counterflow dryer.

Figure 1.15. Typical axial-flow drying fan.

Fans. The movement of air in a grain-drying system is caused by the operation of one or more fans. The two major fan types are the axial-flow type and the backward-curved centrifugal type.

In an axial-flow fan the air moves parallel to the fan axis and at a right angle to the field of rotation of the blades. An axial-flow fan is illustrated in Fig. 1.15.

In a centrifugal fan the air enters the housing parallel to the axis and is discharged perpendicular to the direction in which it enters the fan. A backward-curved centrifugal fan is shown in Fig. 1.16.

The characteristics of a fan can be expressed in a tabular form (see Table 1.17), by a fan curve (see Fig. 1.17), or by a fan equation (see Eq. [1.3]). Atypical empirical fan equation that relates the airflow rate (Q) to the static pressure (AP) is:

Q = a + bAP + cAP², (1.3)

where a, b, and c are fan-specific constants.

Axial-flow fans usually deliver higher airflow rates than centrifugal fans of equal power at static pressures below 1000 Pa. If a grain system operates at static pressures above 1200 Pa, a centrifugal fan delivers the higher airflow rate. An axial-flow fan is noisier but less expensive than an equivalent centrifugal fan.