Thermal Properties Of Pre-Insulated Pipeline With Vacuum Layer And Thermal Analysis Of Buried Pipeline

Centralized heating system is the important basic establishment to saving the primary energy, reducing the pollution and improving heating quality for a modern city. The heating pipelines is one of the most essential parts in the heating network to feeding the thermal mediums such as the hot water and steam for the centralized heating system. It is important in improving the heat transfer efficiency of the heating network and hence the energy efficiency of the heating system to reducing the heat loss of the heating pipeline. It is a new technique that the heating pipeline employs vacuum insulation to improve the insulations and keep the thermal characteres of the heat mediums, reduce the corrosion and monitor the leak for the heating pipeline in the heating network. However, to the best of the author's knowledge, no data have been reported in the open literatures on the thermal properties for the vacuum insulation in the heating pipeline tested in vacuum as a function of the pressure and the temperature, especially as the vacuum insulation is in low or medium vacuum. In China, there has no been found in open literatures on it. Even now, the reasonable vacuum pressure and the thermal properties of the insulating material layer and vacuum layer in the vacuum insulation, the thermal performance of the pipeline, the thermal influence of the thermal bridges on the protective layer for the pipeline and the method to weaken the thermal bridges have not been dicussed yet. Also, the empirical formulas derived by the Former Russian scholars are employed to analyse the heat tranfer of the buried pipe and the duct, conventionally. They have the deficiencies on calculate heat transfer on these conditions, which the multi-laid adjacent directly buried heating pipes are applied, the multi-layer insulation are applied on the buried pipe and the centeres are not horizontal from each other. They also have the deficiency on the generalization for the consequential calculations and analysis.Firstly, Experiments were performed to measure the combined heat transfer and equivalent thermal conductivities of the insulating material in vacuum and rarefied air employed in the insulation of pre-insualted heating pipeline with vacumm layer. Then, how does these characters impact the thermal properties of the insulation are discussed for the pipeline, which are the different the vacuum pressures and the thermal media temperatures, the different diameters of the inner steel pipe&steel jacket and the thicknesses of vacuum layer. Equivalent thermal conductivities of vacuum layer and insulating material layer are compared. Also, the ways to improve this insulation of this type of heating pipeline are presented. In addition, the methods to calculate the heat transfers are developed based on experimental data and theory of aerothermodynamic in rarefied gas. Results show that heat transfer of this pipeline is a strongly degressive function of vacuum pressure of insulation, especially when this insulation is below 5kPa. Vacuum pressure of this pipeline should be controlled below 2kPa to achieve desirable insulating effect. Equivalent thermal conductivity of insulating material layer is lower and drops sharper with vacuum pressure than that of vacuum layer distinctively, but decrease of vacuum pressure improves much more insulating effect of vacuum layer than that of insulating material layer. Also, the every increment of the thickness of the insulating material layer results in the reduction of the heat loss of the pipeline in all the vacuum pressure.Secondly, the three-dimension temperaure field and the distribution of heat flux for the thermal bridges in the pre-insulated heating pipeline with vacuum layer was simulated based on the finite element method and CFD software. Then, how does the different impact characters influent the thermal impact region and termerature field in the thermal bridge of the pipeline are discussed. Those impact characters include the vacuum pressures and the thermal media temperatures, the diameters of the inner steel pipe and steel jacket and the thicknesses of vacuum layer etc. Then, the recommended two ways to weaken the thermal bridges are derived, which are to also controll the vacuum pressure of this pipeline below 2kPa or to controll the vacuum pressure about 5kPa or at least or below 5kPa after the proper addition of insulating material on the proper areas of the pipeline.Thirdly, a new analytical method is derived for the thermal analysis of multi-laid adjacent heating pipeline and the pipeline with multi-layer insulations.The method employs the complex function method and conformal mapping, Multi-polar coordinate, Residue Theorem and Fourier series expansion technology. Then, the impact of the different characters on the calculative results of the heat losses of the examples are discussed, by employing the presented analytical method and the Russian empirical formulas for the pipeline. Those impact characters include the buried depth and the buried position, the termperature of the heat medium and the diameters of the pipes and their insulations. The results derived by employing the Russian empirical formulas all shows good agreement with the predicted results by the presented analytical method for the relative deviations are below 0.6% with the variations of all the impact characters.Finally, a new semi-analytical method is obtained for the thermal analysis of buried heating duct or non-circle pipe. The method employs the complex function method and conformal mapping, point collocation method for nonorthogonal boundary discretization and Fourier series expansion technology. Then, the impact of the different structure or thermal characters on the calculative results of the heat losses of the examples are discussed, by employing the presented semi-analytical method and the Russian empirical formulas, for the heating duct. The relative deviations of the results by employing the presented semi-analytical method and the Russian empirical formulas drops as the buried depth increase accordingly. The relative deviation of the results of the two method is a strong function of the height and the width of the heating duct, also the ratio of the height to buried depth and the ratio of the width and the height of the heating duct. The relative deviations have no concern with the temperature of the inner-surface of the duct's envelope, the temperature of the ground and their temperature difference. Also, the increment of the ratio of the height to buried depth or the ratio of the width and the height of the heating duct all results in the increment of the the relative deviations. Moreover, the bigger the ratio of the height to buried depth of the heating duct is, it causes the more relative deviations as the increments of the ratio of the width and the height of the heating duct. All these may be helpful to validate the valid range and the deficiencies of the Russian empirical formulas and to improve it reasonablly for the thermal analysis of the buried heating pipeline or duct.