Selection of Infrared Heating Tubes for Drying Hydrated Wet Materials: Short-wave or Medium-long Wave?

Capillary multi-porous colloidal materials, as previously mentioned, are one of the most common types of materials encountered in everyday life and production processes. Examples include wood, leather, and food. These materials are a major focus in the study of drying due to the relative ease with which water can be expelled from large capillaries, whereas the extraction of water from micro-capillaries or cell walls is considerably more challenging. Consequently, the migration process of internal moisture in these materials involves both large and micro-capillaries, including the expulsion of free water within cell cavities.

The energy consumed in the bonding of water within the material manifests not only in the expulsion of water from cell walls or at equilibrium moisture content but throughout the entire drainage process. Thus, the drying process should be viewed as a comprehensive transfer of energy and matter. Given the complex structure of materials, such as thermosensitive and biologically active materials (e.g., seeds), the mechanisms of heat and mass transfer processes are complex.

Water in materials can be chemically bound, physico-chemically bound, or mechanically bound. Chemically bound water, where water is bonded to solids by chemical forces (e.g., water of crystallization in copper sulfate pentahydrate, CuSO4·5H2O), is typically challenging to remove through heating and is not generally considered part of the drying process, although successful drying using carbon fiber infrared heating has been achieved with dolomite balls.

Physico-chemical binding occurs when water or solvents bind to materials through hydrogen bonds or van der Waals forces. The interaction between water molecules and the material happens at the molecular level, where the first layer of liquid molecules binds strongest to the material and subsequent layers bind weaker. Changes in surrounding media can easily disrupt these layers beyond the first.

Mechanical binding involves water forming surface tension within the capillaries of the material. The combined force of water with large capillaries is weak, similar to pure water, where the vapor pressure of surface moisture equals the saturated vapor pressure of pure water at any temperature, facilitating easy evaporation of water. In micro-capillaries, a concave meniscus forms strong bonds with the capillary walls, and its surface saturation vapor pressure is lower than the saturation vapor at the same temperature.

Materials such as wood, food, fruits, powders, fibers, paints, and coatings reflect, transmit, and absorb infrared radiation. Unlike liquids, colloids, capillary porous colloids, and amorphous solids, they not only exhibit vibrational spectra but also rotational spectra. The energy from the infrared spectra is absorbed by the material, converting it into thermal energy.

During radiative heating, materials only gain energy by absorbing radiation. Radiation that is transmitted or reflected does not contribute to heating, making absorption rates a critical parameter for how effectively radiative energy is utilized by the material. Analysis of the absorption spectra of materials like apples, dried apples, potatoes, dried potatoes, tea leaves, wood, and paint reveals that capillary porous colloids absorb the least in the short-wave range, with absorption rates increasing with wavelength and reaching maximum absorption peaks at the medium-long wave boundary.

Given these characteristics and the effects of water molecules within materials, such as wood and paint containing hydroxyl and alkyl groups, significant absorption bands are evident in the 3-6μm wavelength range. Water within materials significantly influences the absorption spectrum, with liquid water showing three absorption peaks between 5μm-17μm, making these the optimal absorption peaks for infrared radiation in hydrated wet materials.

Based on the experimental data, drying hydrated wet materials effectively requires medium-long wave infrared heating tubes.

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