A new arrangement of blades in scraped surface heat exchangers for food pastes
The present study analyses the performance of a scraped surface heat exchanger (SSHE) equipped with a new arrangement of blades to achieve higher thermal efficiencies than the conventional SSHE. This new design of exchanger, named here as scraped surface heat exchanger with alternate blades, A-SSHE, may be particularly suited to treat high viscous fluids, like food pastes. Experimental and numerical tests were carried out on an industrial scale A-SSHE used to heat hazelnut paste, an intermediate product widely used in Italian confectionery industry. Experimental tests include physical, chemical and rheological characterization of the hazelnut paste and the evaluation of the overall heat transfer coefficient as a function of rotational speeds and mass flow rates. Three-dimensional axial-symmetric CFD simulations of the A-SSHE were performed by using the software Fluent 6.2. For comparison, the same numerical tests were carried out for an equivalent SSHE with a conventional blades design (C-SSHE). Our studies show that the A-SSHE gives heat transfer coefficient values almost twice that of an equivalent C-SSHE, and that the numerical results are consistent with the experimental observations. The analysis of the fluid dynamics and of the thermal profiles suggests that the higher heat transfer efficiency of A-SSHE may be attributed to the occurrence of back mixing phenomena.