Heat transfer with heating and cooling granular materials in a scraped-surface homogeniser

Abstract

The study focused on the transfer of heat from the scraped-surface homogeniser heat-exchange wall into granular foodstuff materials. The object of the study was the dependence of the heat transfer coefficient under conditions of heating and cooling of dried egg white, wheat starch, whole mustard seeds, wheat semolina and peeled rice. The heat transfer values are required for specifications of exchangers and homogenisers used for heat processing of granular materials, e.g. inactivation of micro-organisms in dried egg white. The tested materials were placed into the homogeniser and subjected to heating and cooling. Temperatures were read for the granular materials near and directly upon the heat transfer surface. The data obtained were used to calculate the values of the surface heat transfer coefficient (SHTC) for separately heating and cooling under varying rotational speed of the scrapers. The results indicate that the rotational speed had no significant influence over the surface heat transfer coefficient. The acquired data were transformed into a relation among dimensionless variables of the general form Nu=c1·PéC2. Relationships thus obtained can be used for calculations of the surface heat transfer coefficient in heat-exchangers with scraped-surface heat-exchange walls.