The Heat Transfer Characteristics of Vacuum Insulated Tubing

Abstract

Thermal effects in vacuum insulated tubing (VIT) have been studied in previously reported experiments2. VIT is usually characterized by a single thermal conductivity ( k -value) based on a one-dimensional radial heat transfer analogy. Despite its simplicity, however, this physical model masks the underlying complexity of the two-dimensional axisymmetric heat transfer field.Recent theoretical insights have demonstrated that VIT behaves as a fin around the weld and coupling regions, where up to 90% of the overall heat loss can occur. As a result, VIT does not have a single k -value; instead it has a potentially wide range of k -values, depending on boundary conditions. Recent numerical and experimental investigations of vertically aligned VIT demonstrate the highly sensitive nature of thermal performance with respect to these boundary conditions1. With this insight, previous test data2 is reinterpreted in a more revealing light. It can be demonstrated that the 10-fold spread of k -values from these tests is attributable to explainable boundary phenomena.Another issue which requires clarification is the k -value definition. A standard definition is presented here which 1) is physically related to the effective thermal conductivity of the vacuum and 2) is relatively stable for different VIT configurations.Transient behavior, including startup, cool down during shut in, ground soak, and convection cell development will be explained, as well as the implications for long-term thermal stability.The discussion concludes with calculations of k -value for VIT under significantly varied boundary conditions: different annular fluids, different soils, with and without interior or exterior coupling insulators, and using high-temperature steam injection.