Improvement in the circulating efficiency by radial flow impellers in evaporating crystallizers operated in horizontal cascade crystallizers

For decades the impellers in evaporating crystallizers (vacuum pans) have been installed within the central draft tube of the calandria. The axial flow impellers - so called "Kaplan type" - are nowadays the standard impellers in evaporating crystallizers. In comparison with this standard type the performance of radial flow impellers installed below the heating chambers of two evaporating crystallizers at the Goslawice sugar factory was investigated during the 2004 campaign. The two evaporating crystallizers were operated continuously as the 3rd as well as the 2nd stage of B (raw sugar) and C (after product sugar) cascade crystallizers. The investigations included running each impeller at three different values of the rotational speed and measuring the power consumption of the agitator drive while determining the heat transfer coefficient in the respective evaporating crystallizer. In a similar manner, the performance of standard Kaplan impellers installed in the central tubes of the heating chambers of both evaporating crystallizers was investigated. In relation to Kaplan impellers, the radial flow impellers made it possible to attain markedly higher heat transfer coefficients at lower values of the rotational speed. By comparing their power consumption at similar levels of the heat transfer intensity, a comparison between the two impeller types with respect to the circulating efficiency was also made. In B3 evaporating crystallizer, the circulating efficiency of the radial flow impeller was higher than that of the Kaplan impeller by a factor of 2.8. In C2 evaporating crystallizer, the corresponding factor was 2.6.