Issue link: https://www.e-digitaleditions.com/i/1197147
January 2020 / 65 diffusion-controlled drying zone, these measurements can be mislead- ing for determining the final prod- uct moisture content. The problem is, where do you take the measure- ment? Multiple factors — the mate- rial temperature, the material resi- dence time at a given temperature, and even the drying air's humid- ity — can significantly influence the final product moisture content. The material temperature may also change along the zone's length (that is, vary with residence time). Mea- suring the material temperature also doesn't account for the influence of the drying air's humidity. Relying only on temperature measurements in diffusion-con- trolled drying zones can result in rather than the material tempera- ture, to adjust the inlet air tempera- ture in this zone. The subsequent drying zones then provide the residence time and additional heating required for diffusion-controlled drying in the falling-rate regime. Mate- rial moisture content in these diffusion-controlled zones depends strongly on the material tempera- tures, material residence time, residence time distribution 1 , and, in some cases, the humidity of the drying air in contact with the material. Although measuring the material temperature or the zone's outlet air temperature is typically used to control the heated inlet air temperature or the airflow rate to a on the maximum heated inlet air temperature to prevent heat dam- age to dried material deposits on the dryer wall or to prevent a fire or dust explosion in a dryer handling an organic material. The dryer's allowable inlet and outlet air temperatures for achiev- ing the required final product moisture content without thermal degradation are defined in pilot- plant tests during the dryer's design phase. Since the material residence times in the flash dryer and spray dryer are less than 30 seconds, dry- ing in this equipment is normally in the constant-rate drying zone, where heat transfer, not moisture diffusion, is the controlling step. From the drying curve shown in Figure 1, we can see that the mate- rial temperature is constant during constant-rate drying, corresponding to the drying airstream's wet bulb temperature. This means that the material temperature has little effect on the final product moisture level, so the control system can simply measure the outlet air temperature. Cross flow. In a continuous, cross-flow convection dryer, such as a belt conveyor dryer or flu- id-bed dryer, the heated air flows through the material perpendicu- lar to the material's flow direction. Since constant- and falling-rate drying can occur in this type of dryer, it's well-suited to being divided into drying zones. In each zone, material is separated from the material in the preceding and following zones so the temperature of the heated inlet air to each zone can be independently controlled. The first zone is typically designed for constant-rate drying, where the wet bulb temperature's evaporative cooling effect on the material can allow the use of a higher inlet air temperature. As with the cocurrent flow convection dryer, the control system can sense this zone's outlet air temperature, FIGURE 1 Drying rate and material temperature Drying time (minutes) Material temperature (°F) Constant-rate drying zone (heat-transfer-controlled) Critical moisture level Key: Drying rate Material temperature 160 300 140 250 120 100 200 80 150 60 100 40 20 50 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 Falling-rate drying zone (diffusion-rate-limited) Material moisture (percent dry basis)