Nusselt’s correlations in agitated tanks using the spiral coil with Rushton turbine and PBT 45° impeller. Comparison with tanks containing vertical tube baffles

Abstract

One of the main reasons for employing coils to heat and cool liquids in tanks is its low purchase cost. The technique is used in a great variety of industrial processes such as: the production of paint, food additives, pharmaceutical products, polymers and fuel. A project to design a spiral coil, which is also called “pancake coil” for a given impeller and tank size involves the determination of the overall heat transfer coefficient, which is a function of the Nusselt (Nu), Reynolds (Re) and Prandtl (Pr) numbers, besides the ratio between the bulk temperature viscosities and the wall temperature (Vi). The current article aims mainly at determining two calculation expressions for the Nusselt number, one for the pitched blade turbine (PBT) and another for the Rushton turbine (RT). The secondary objective is to compare the efficiency of heat transfer in spiral coils to vertical tube baffles. The models obtained for the PBT and RT impeller were:Nu=0.81Re0.64Pr0.33Vi0.14-PBTNu=0.10Re0.83Pr0.33Vi0.14-RTThe models had an adjusted determination coefficient (R2-sq) of 0.97 and 0.98 for PBT and RT, respectively. The work found that in spiral coil heating for Reynolds between 2000 and 60,000 the PBT is recommended and for Reynolds from 60,000 to 500,000 the RT is indicated. When comparing heating efficiency, if the process is carried out using the PBT, the spiral coil should be used. Whereas, when a RT is employed, the most adequate heat transfer surface is the vertical tube baffles.