Table 1.17. Performance of three commercial backward-curved centrifugal fans

(in cubic meters per minute)

FanFower Rating (kw)	250-Pa Pressure	750-Pa Pressure	1250-Pa Pressure
7.5	359	314	266
11.2	475	423	368
15.0	543	505	453

Figure 1.16. Typical centrifugal drying fan

Figure 1.17. Operating conditions for an in-bin wheat-drying system and a maize-drying system.

System Operating Condition. If the system curve and the fan curve of a grain-drying system are plotted on the same graph, the operating condition of the fan/drying system is located at the intersection of the two curves. This is illustrated in Fig. 1.17 for an in-bin system in which both maize and wheat are dried. The axial-fan output for maize is 283 m³/min, but for wheat it is only 170 m³/min because of the larger static pressure of wheat.

On large grain silos, two fans frequently are placed in parallel in order to increase the airflow rate. (Only identical fans should be chosen for parallel installation.) The performance of a parallel-airflow system is shown in Fig. 1.18; the airflow characteristics of the system are found by adding the flowrate of the fans at a particular static pressure.

Figure 1.18. Performance of identical fans in parallel.

Relative drying rates of selected grains

Table 1.18.

	22% w.b. Moisture Content	18% w.b. Moisture Content	14% w.b. Moisture Content
Peas	95	35	15
Maize	135	50	15
Wheat	210	100a	35
Canola	—	1150	500

Drying Rate. The drying rate of grain in a grain-drying system depends on the drying rate of the individual grain kernels. In general, small kernels lose moisture more readily than large kernels, and naked seeds dry faster than covered seeds. This is illustrated in Fig. 1.19, in which the drying curves of maize, rice, and wheat at 49°C are drawn. Maize contains the largest kernels and dries the slowest; rice and wheat seeds are of comparable size but rice kernels are covered and therefore dry slower than wheat kernels.

Grain kernels dry at a falling rate. This is shown in Fig. 1.20, in which the drying and the drying-rate curves are drawn for a typical grain species. In Table 1.18 data are tabulated for the relative drying rates of several grains. As the kernels increase in size, and decrease in moisture content, the drying rate decreases.

Typical equations for the drying rate of the major grains follow.

Figure 1.19. Comparison of drying rates of dent maize, medium rice, and soft wheat at 49^◦C.

Figure 1.20. Grain drying during falling-rate period.