Analysis of heat flow and “channelling” in a scraped-surface heat exchanger

Abstract

Scraped-surface heat exchangers (SSHEs) are widely used in industries that manufacture and thermally process fluids; in particular, the food industry makes great use of such devices. Current understanding of the heat flow and fluid dynamics in SSHEs is predominantly based on empirical evidence. In this study a theoretical approach (based on asymptotic analysis) is presented for analysing both the flow and heat transfer in an idealised SSHE (a cylindrical annulus) for Newtonian fluids. The theory allows the effects of scraping-blade configuration, pumping rates, annular shear velocity and material properties all to be accounted for. The analysis relies on asymptotic simplifications that result from the large Peclet numbers and small geometrical aspect ratios that are commonly encountered in industrial SSHEs. The resulting models greatly reduce the computational effort required to simulate the steady-state behaviour of SSHEs and give results that compare favourably with full numerical simulations. The analysis also leads to what appears to be the first theoretical study on the undesirable phenomenon of “channelling”, where fluid passes through the device in an essentially unheated or uncooled state. A parametric study is also undertaken to investigate the general circumstances under which channelling may occur.