Measurements of Heat Transfer Coefficients to Cylinders in Shallow Bubble Columns

High heat transfer coefficients and large interfacial areas make bubble columns ideal for dehumidification. However, the effect of geometry on the heat transfer coefficients outside cooling coils in shallow bubble columns, such as those used in multi-stage bubble column dehumidifiers, is poorly understood. The generallyoverlooked entry and coalescing regions become important in shallow bubble columns, and there is disagreement on the effects of the coil and column diameters. In this paper, a method is presented for measuring the heat transfer coefficient between coil and liquid in a shallow bubble column. Horizontal cylindrical probes are used to measure the heat transfer coefficient over a range of gas velocities. The liquid depth and the diameter, height, and horizontal position of the cylinder are also varied. Existing correlations for tall columns tend to underpredict the heat transfer coefficient and do not account for all effects of geometry. The highest heat transfer coefficients (above 8000 W/m2-K) are recorded on cylinders placed 4 cm high. No significant effect of cylinder diameter is observed. Based on the results, recommendations are made regarding bubble column dehumidifier design.