Determination of heat transfer coefficients in stirred tank systems

Abstract

Heat transfer coefficients for helically coiled and continuously stirred tanks were determined using flat paddle turbine impellers. Heat transfer experiments have been carried out by varying the inlet temperature, flowrate of hot stream and agitator speed in the two flat bottom cylindrical stirred tanks in series. Experiments were also repeated for a single tank. The investigation was conducted under both steady-state and unsteady-state conditions with water being used, as the liquid to be mixed. The individual film heat transfer coefficients were calculated from the experimental overall heat transfer coefficients by using the Modified-Wilson Graphical Method. Two computer programs were also developed to check the validity of the analytical model used, and to compute film heat transfer coefficients and temperatures. A Reynolds number range of 19439 to 76727 was used for the agitated side, that there has been very little investigation in this region. In the coil side, the Reynolds number range was varied from 1100 to 14000. The convective heat transfer coefficients for unsteadystate conditions were found to have the same value as for steady-state conditions. The values of the individual film heat transfer coefficients found were in agreement with the previously published values for similar cases and the analytical model for heat transfer in two consecutive stirred tanks gave same results in agreement with the experimental results. Agitator speed was changed between 200 and 500 rpm. Higher values of heat transfer coefficients were observed with increasing agitator speeds.