film boiling

Film boiling is a form of evaporation of liquid at very high heat flux densities. Is formed on the heating surface, a continuous vapor film, in contrast to the well-mixed liquid at boiling has a high heat insulating effect. The heat transfer coefficient is considerably lower than the film boiling in nucleate boiling because the heat radiation is dominated by the film boiling of the heat transport. This has the consequence that for the same heat flux, the wall temperature rises significantly and can result in a system imposed heat flux to the destruction of the heating surfaces.

In technical applications must therefore be taken to ensure that the heat flux densities are well below the transition point from the bladder to the film boiling. The critical heat flux is dependent on the saturation pressure of the steam and the surface structure of the heating surface. It is at atmospheric pressure for the media pairing steam / steel at> 1000 kW / m². Steam boilers are therefore designed so that a heat flux of 300 kW / m is not exceeded, in order to avoid above all in a compact boiler design with dense boiling tube arrangement, the onset of film boiling. The transition to film boiling is reduced by oils or high salt content in the boiler water.

Clearly the film boiling is characterized by the behavior of a water droplet on a hot stove comprehensible (see Leidenfrost effect). Drops below a vapor film on which the droplet moves through the plate forms. The drop of water evaporates very slowly due to the poor heat transfer.

• Thermal Engineering
• Technical Chemistry
• Steam boiler technology