Determining the appropriate fin spacing for high performance and low footprint of direct contact ambient air-based vaporization systems

Abstract

The continuous growth of frost over the tubes during the operation of ambient air vaporizers (AAV), which are used to re-gasify the liquefied permanent gases, eventually creates ice bridges. This phenomenon not only increases the thermal resistance but also restricts the natural convective airflow. In the present paper, the effect of fin spacing on the performance of the natural and forced draft AAV is analysed under different weather conditions. Appropriate fin spacing at different operating conditions is calculated by the three-dimensional model solved in ANSYS Fluent. A dynamic numerical model is developed to predict the frost properties in a forced draft vaporizer. A higher fin spacing not only increases the footprint but also impairs the rate of heat transfer in a forced draft AAV. At 298 K air temperature and 0.7 RH, the rate of heat transfer through a forced draft AAV can be enhanced by 8% with the appropriate fin spacing. The forced draft vaporizers are less sensitive to weather conditions compared to the natural draft vaporizers. For the identical fin specification, the number of tubes needed for vaporizing 240 Nm3hr−1 of LN2 is 16 for the forced draft, while it is 36 for a natural draft AAV. With reduced fin spacing, the footprint of the vaporizer block can be reduced by as much as 65% in a forced draft vaporizer for the identical vaporization capacity and ambient conditions.