Thermal analysis of vacuum layer in steel jacketed vacuum insulated directly buried steam pipelines

Based on the theory of aerothermodynamics for rarefied gas, discusses the principle of compound heat transfer in the vacuum layer. Presents a method for calculating the equivalent thermal conductivities for this layer. Analyses the effect of thermal characters on heat transfer in vacuum layer, such as vacuum pressure and temperature of steam, and discusses the percentages of conduction, convection and radiation in vacuum layer and their variability. Presents how to optimize the thermal performance of the pipelines. Results prove to be satisfactory for the maximum of deviation rate between calculated data and experimental data is 79%. The pressure in vacuum layer should be controlled under 2 kPa to achieve desirable insulating effects. The dominating way of heat transfer in vacuum layer is the convection when the vacuum pressure is above 1013 kPa, and it is the radiation when the vacuum pressure is under 1 kPa. The convection in vacuum layer should not be neglected.