The steam humidifier without steam

Using the flash-boiling effect

Water, and therefore moisture, is a natural constituent of air. For our physical well-being, a relative humidity of 40-55% at a room temperature of 20-23°C is ideal. To maintain these humidity conditions all year round, air humidification is an indispensable process step within modern air handling systems.

Similarly, maintaining precise humidity conditions is of elementary importance in a wide range of industrial manufacturing processes. In addition to air washers and high- and medium-pressure humidifiers, steam humidifiers are used in smaller air handling systems in particular. Water is evaporated in a pressure vessel and injected into the air via steam lances. A large proportion of the water vapor condenses out and forms a fine mist of droplets that gradually evaporates as the air flows on.

The process developed by SUPART, on the other hand, does not involve complete evaporation of the water. Instead, an aerosol is generated from the injected water and the air. From a physical point of view, this is a multi-component system in which the water (disperse phase) is distributed within the air (dispersant). Characteristic of disperse systems are the particle size of the disperse phase and the aggregate states of the dispersed substance and dispersant.

According to these distinguishing characteristics, a classic steam humidifier ideally has a molecularly disperse, homogeneous water distribution. In contrast, a conventional high-pressure or medium-pressure humidifier, also known as a spray humidifier due to the droplet spectra formed (dT > 20μm), is a heterogeneous system. The water is present in the form of macroscopically detectable droplets in the air and is thus coarsely dispersed. Aerosols take an intermediate position. Here, the individual molecules of water are aggregated into such extended droplets that they are separated from the gas phase by a phase boundary. However, the droplets are so small (dT< 1μm) that their behavior largely The water droplets are “quasi-homogeneously distributed” in the air, so to speak.

Since the air temperature under normal conditions is below the saturation temperature of the steam, such a “quasi-homogeneous aerosol” is also formed immediately in a steam humidifier by condensation of the water vapor.

Such an aerosol is formed either by droplet condensation from the vapor phase or directly by atomization of the liquid. In this process, work must be done against the force of surface tension and against the viscous forces of the liquid. The mechanical energy to overcome these forces comes from the energy content of the injection jet. Depending on the origin of the atomization energy, a distinction is made between kinetic and thermal fragmentation. Conventional high- and medium-pressure humidifiers use only the kinetic energy of the injection jet; in the approach developed here, the thermal energy is also used.

Before injection begins, the water is raised above the boiling temperature at ambient pressure for this purpose. Due to the rapid pressure drop at the nozzle outlet, the injected liquid is in a state of superheating. If the temperature inside the liquid can be equalized below the boiling temperature level by heat transfer to the environment, the liquid evaporates only at the surface of the injection jet. If, on the other hand, the temperature in the core of the injection jet is not lowered fast enough by the heat transport to the outside, boiling nuclei are activated. The boiling nuclei give rise to vapor bubbles which, as they grow, pierce the liquid phase and thus break up the injection jet. This process of dissipating the thermal energy of the injection jet is known as flash boiling. This process achieves a fine droplet distribution similar to that of the steam humidifier. However, since the water is not completely evaporated, the energy required to generate the aerosol is only a fraction of that required by a steam humidifier. Also advantageous is thermal sterilization of the water by placing the heating element directly in front of the nozzle.