Experimental Investigation on Thermal Properties of a Steel-jacketed Steam Heating Pipeline with Vacuum Insulation

Abstract

The steel-jacketed steam heating pipeline employs vacuum insulation to improve the insulating effect and reduce the corrosion, and hence increases the heat transfer efficiency of the heating network and building energy efficiency. It is important in improving the thermal insulation to investigate the impact of factors that insulate the effects and thermal properties of the pipeline. The thermal insulation of this pipeline comprises the vacuum layer and the insulating material layer. Experiments were performed to measure the combined heat transfer and equivalent thermal conductivities of the insulating material in the vacuum and rarefied air employed in the pipeline's insulation. The thermal properties of this type of insulation at vacuum pressures of 0.5~1013mbar, employing thermal media temperatures of 343~573K and with different thicknesses of vacuum layer, are discussed for this pipeline, for which diameters of inner steel pipe/steel jacket are DN50/DN250, DN100/DN300, DN200/DN500 and DN500/DN850, respectively. The results show that reduction in vacuum pressure reduces the heat loss in the pipeline. The equivalent thermal conductivity of the insulating material layer is distinctively lower than the vacuum layer, but decreasing the vacuum pressure improves the insulating effect of vacuum layer substantially more than insulating the material layer. As the vacuum pressure decreases from 1013mbar (atmospheric pressure) to 10mbar at the thermal media temperature of 523K e.g., the reduction of equivalent thermal conductivities of vacuum layer is approximately three times greater than that of insulating material layer. The equivalent thermal conductivities of the vacuum layer are lower and decease faster as the vacuum pressure is lower than 100mbar, but the equivalent thermal conductivities of insulating material layer are lower and decease faster as the vacuum pressure is lower than 50mbar. The pressure in vacuum insulation should be controlled lower than 20mbar to achieve desirable insulating effects. Every 10mm addition of thickness of insulating material layer (every 10mm reduction of thickness of vacuum layer) decreases the heat loss of approximately 6.8 percent at the vacuum pressure of 0.5mbar.