A mechanism for heat transfer in a Votator-type scraped-surface heat exchanger

Abstract

The mechanism for heat transfer in a SSHE consists of three steps, viz.: • Penetration of heat by conduction in a thin layer close to the heat transfer surface in the time between two scrapings. • Partial temperature equalization in the boundary layer that builds up on the scraper blade. • Convective radial transport from the cooled layer to the bulk of the liquid. Taylor vortices contribute to this radial transport. Heat transfer measurements were conducted under various conditions. The heat transfer coefficient can be described by means of an equation resulting from penetration theory, modified by an empirically determined correction factor ϕ. In ϕ are included: • The incompleteness of the temperature equalization in the boundary layer. • The effect of radial dispersion. • The decrease in driving force for heat transfer due to axial dispersion. When the influence of the axial dispersion on heat transfer is absent (high mass flow rates), ϕ is a function of Pr. If Taylor vortices are present, the value of ϕ is independent of Re R . Below the critical Reynolds number ϕ decreases due to insufficient radial mixing. At low mass flow rates the influence of the axial dispersion on the driving force for heat transfer is important. The apparent decrease in heat transfer coefficient due to this effect can be calculated. To do so, the plug flow with dispersion model is applied in which the axial dispersion coefficient is calculated from the standard deviation of the residence time distribution curve.