One of the food industry’s constant goals is to reduce manufacturing costs whilst retaining control over food quality. The process optimisation of heat transfer is therefore a particular priority for the many food products that are made by heating or cooling a raw material or mixture of materials. A number of distinct types of heat exchangers are routinely used in the food sector. The fluid flow and heat transfer in classical plate and tubular heat exchangers is normally relatively straightforward and easy to understand and optimise. Matters are a great deal more complicated in scraped-surface heat exchangers (SSHEs) where both operational and modelling difficulties are often present. SSHEs are often used to process products that require more sophisticated heat-transfer mechanisms;
In devices such as in SSHE, food is pumped axially through a long cylindrical annulus. The outer cylinder, sometimes called the stator, is heated or cooled and the inner “rotor” cylinder (which may be assumed to be thermally insulated) rotates at a prescribed velocity. Blades are attached to the rotor and continuously scrape foodstuffs from the stator, the heat-exchange surface. A typical SSHE cross-sectional configuration is shown in Fig.
The materials that are processed in an SSHE encompass a broad spectrum of rheological behaviour. Some possess a yield stress and exhibit viscoelastic or pseudoplastic properties and many have a highly temperature-dependent viscosity. Some processed foods also involve multiphase flow and/or particle suspensions. Crystallisation, freezing and other phase changes may also take place. Generally, however, the main aim in SSHE operation is to ensure that the heat transfer from the stator to the food is both (a) maximised and (b) distributed as evenly as possible within the food. In order to achieve these twin goals, process engineers may alter many aspects of a particular machine. Blade configuration, annular gap width, axial length, rotation speed and pump pressure may all be changed not only for optimisation purposes, but also to avoid the occurrence of flow regimes that are regarded as undesirable. One such flow regime, which is normally to be avoided at all costs, involves “channelling”. Under certain conditions large regions of the material being processed may pass through the heat exchanger in an essentially thermally unaltered fashion. The existence of such flow “channels” can render the final product completely useless.
How can channelling be avoided? Currently, the theoretical understanding of SSHEs is limited. Food manufacturers therefore usually rely on empirical knowledge, and may expend significant resources adapting operating parameters to new materials or products. The aim of this study is to try to produce a simplified model of flow and heat transfer in an SSHE and thereby allow the phenomenon of channeling to be approached from a theoretical point of view.