Experimental study on scale inhibition performance and fouling characteristics of spiral insert in heat transfer tube

Abstract

In order to study the anti-scaling and descaling performance in the process of cooling crystallization, the effects of flow velocity and reciprocating displacement on fouling resistance and overall heat transfer coefficient under two spiral insert schemes are experimentally studied. The system studied in the experiment is a saturated sodium sulfate solution, which forms crystalline fouling during the experiment. The micro structure of the fouling layer is investigated to explain the variation of macroscopic thermal parameters. The average fouling thickness curve is obtained by heat transfer theory analysis. The results show that the maximum decrease of the asymptotic value of the fouling resistance of the fixed spiral insert is 63 % with the increase of the flow velocity. When the reciprocating displacement of the spiral insert increases to one pitch, the asymptotic value of the fouling resistance decreases by 83 %, and the overall heat transfer coefficient decreases by only 3 %. The particle size and porosity of fouling at different observation positions gradually increased, while the shape of fouling crystals changed from small to regular, and the degree of bonding changed from tight to loose. Statistics on the particle size of fouling particles show that the fouling particle size increases from 2-8 μm to more than 30 μm. For the fixed spiral inserts, the porosity of the fouling layer is up to 42 %, but the porosity of the middle layer increases rapidly with the flow velocity. With the change of flow velocity, the flow field characteristics in the tube are the main reason for changing the structure of the fouling layer. With the increase of reciprocating displacement, the collision effect of the spiral on the fouling layer is more comprehensive, and the interlayer bonding of the fouling layer is completely destroyed. In the field of industrial cooling crystallization, it can greatly improve the heat transfer efficiency of the inner wall crystallization process.