17 - What are the key characteristics of a hot-air dryer?
Hot-air dryers are suited to preheating and drying non-hygroscopic or slightly hygroscopic plastics. These attractively-priced systems use ambient air to dry plastics by carrying the surface moisture away, but without the use of desiccants. Use of ambient air means that drying results will vary depending on prevailing weather conditions.
Hot-air dryers operate within an open cycle. Ambient air is drawn through a filter, heated, and then passed through the resin granulate in the drying hopper before being vented to the factory floor. Some dryers feature a changeover valve to allow partial recirculation of the humid air. Drying temperature and air flow rate can be controlled via the heater/blower unit. Hot-air dryers transfer heat via convection.
The current relative humidity of the ambient air is a key factor when using hot-air dryers, as it has a direct impact on results. As ambient air is heated, its relative humidity sinks, increasing its moisture holding capacity. At a given drying temperature, the climate-dependent moisture content of the ambient air and its temperature give rise to different relative humidities and thus to different equilibrium states.
This means that the performance of hot-air dryers varies between summer and winter. For example, drying a nylon resin with hot air in the summer can cause the material’s moisture to increase rather than decrease.
It is not possible to achieve very low residual moisture with hot-air dryers. It is theoretically possible to increase drying temperature, especially for materials with high initial moisture content; however, there is a risk of thermal damage to the material.
Hot-air dryers are also suitable for preheating plastics to keep the thermal feed conditions constant. Often the hot-air dryer is mounted directly on the feed-throat of the processing machine. Preheated resin increases the energy efficiency of the system, improving productivity and reducing power consumption.
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