Issue link: https://www.e-digitaleditions.com/i/1197147
66 / January 2020 powderbulk.com both the vapor space and material are essentially constant, what do we measure to regulate the dryer's heat input so we can achieve the desired final product moisture content without overdrying or underdrying the product? Once again, directly determining the material moisture content to con- trol drying may be the best solution. Direct moisture- sensing methods Technologies suitable for directly determining material moisture include near infrared, radio fre- quency, and microwave. Each of these so-called "wave technologies" is suitable for controlling the dry- ing of powders and granules. Near infrared. The near-infrared (NIR) sensing method determines material moisture content by com- paring reflected energy at two NIR wavelengths. The NIR device proj- directly measure the material mois- ture content? Before we consider methods for doing this, let's look at how the same control limitations in convection drying can also apply to conduction drying. Controlling continuous conduction drying In a continuous conduction dryer, in which a metal wall separates the heating fluid (steam, hot water, or hot oil) from the material to be dried, the material is heated up to its moisture component's boiling point. At this point, the moisture is vaporized into the vapor space above the material, and then sweep air or applied vacuum removes the moisture vapor from the dryer. It's not uncommon for the material temperature to remain constant at the moisture's boiling point throughout the entire drying oper- ation. But when the temperatures of process problems. Let's say that the dryer's final drying zone is used for product cooling rather than drying: If the moisture content of the mate- rial entering this cooling zone is too high, the final product discharging from the cooling zone won't have the desired moisture content. The same thing will happen if the final drying zone provides annealing (heat treating to improve product strength) at high temperature. If the moisture content of the material entering this zone is too high, the annealing operation won't produce the correct product quality, as is often seen when processing some polymer resins. Because factors other than material temperature and out- let air temperature can affect the final product's moisture content in diffusion-controlled drying, we need another sensing parameter for effective dryer control. Why not