(671c) The Effect of Impeller Type on Heating Times in a Jacketed Agitated Vessel

A great many industries have a genuine need to control the temperature of a working media within an agitated vessel. This can be accomplished by installing an external jacket on the vessel sides, and / or the vessel bottom. Pumping a heat transfer fluid through a vessel jacket will add or remove heat from the working media as it mixes. The agitation system that circulates the working media promotes not only mixing, but also temperature uniformity throughout the media and more effective heat transfer between it and the vessel wall. Mixing literature (e.g., J.R. Nunhez, Chapter 14, Advances in Industrial Mixing, pp 491-532, John Wiley & Sons, New York, NY, 2016) contains numerous empirical correlations to calculate process-side heat transfer coefficients under desired operating conditions, as well as heating / cooling times of the working media given certain physical properties of it and the heat transfer fluid. These correlations indicate that process-side heat transfer coefficients are dependent upon impeller type, or perhaps more accurately, upon flow patterns created by the different impellers.

This experimental study compares the performance of three common impeller types while heating up a batch of water in a jacketed vessel. A total of 42 different heat transfer tests were completed at nine different sets of operating conditions in an 18.1inch (46cm) ID stainless steel vessel with dished bottom and a dimple jacket. Impeller type and dimple jacket surface area (i.e., vessel wall vs. vessel bottom vs. both) were varied during these tests. All tests mixed a constant water volume (21gal; 79.4L) using one of three different impeller types including a narrow hydrofoil (A310), a Rushton turbine (R100), or a 4-45-PBT (A200). Diameters of the A200 and R100 impellers are 7inches (17.8cm), while that of the cast A310 is 6.8inches (17.3cm), the nearest-to-the-standard diameter that is available in the cast A310. Each test monitored temperature in the agitated water using two vertically oriented thermocouples mounted 1inch (2.5cm) from the vessel wall, and either 2inches (5.1cm) below the liquid surface or 0.5inch (1.3cm) above the vessel bottom. A dedicated, PLC-controlled oil heater supplied heat transfer fluid to the vessel jacket at a temperature of 95-105deg F (35-41deg C). Each test simultaneously monitored increasing water temperature, decreasing oil temperature, oil flow rate, and total run time. A test lasted only as long as needed to raise the temperature of the agitated water to approximately 30deg F (17deg C) above the initial water source temperature (~55-60F; 13-15.8deg C). Each impeller was rotated at the requisite mixing speed to impart a fixed power level of 1.08 hp/kgal. Calculations suggest that heat up due to ongoing agitation never exceeded 2deg F (1deg C).

Test results show that differences between the two water-monitoring thermocouples were always within one degree of each other. Trends in heat up times with the different impellers generally match the trends predicted by correlations in the literature (e.g., see reference above). These trends also compare as expected when using either the vessel’s dimple-jacket bottom or dimple-jacket wall but displayed some unexpected behavior when simultaneously using both. The results also showed a strong dependence on the behavior of the heating oil supply. For example, an accurate measure of oil flow rate was necessary to close the system energy balance, and fluctuations in oil flow rate and pressure drop produced unanticipated challenges in analyzing the water heat up trends.