Experimental and theoretical investigation of a radiative heat pipe ceiling for uniform cooling and heat recovery in a ceramic roller hearth kiln

Abstract

The ceramics manufacturing sector is among the most energy intensive industry in Europe. The greenhouse gas generated by this industry represents a large amount of the greenhouse gas generated across the industrials sectors. During the tile manufacturing process, the slow cooling of the tile is among the most important step as the strength and lifespan of the tile will be determined by the quality of cooling. The main objective of this study is to investigate and apply a radiative heat pipe ceiling solution that will provide a uniform tile surface temperature across the kiln section to reduce the internal stresses of the tile and to recover heat from the slow cooling process. To provide a uniform tile temperature, a heat pipe with a near horizontal evaporator was proposed. In a first approach, a cylindrical single heat pipe with a near horizontal evaporator section and cooling water jacket was tested. The single heat pipe was exposed to a radiative ceramic heat source in a fully instrumented laboratory kiln. The system was studied at different filling ratios to assess the effect of the filling ratio on the heat transfer rate, the heat pipe thermal resistance, and the heat pipe temperatures. The geyser boiling occurring at low heat flux was discussed and analysed to avoid this phenomenon in the full-scale system. By using the experience gathered during the single heat pipe test, module heat pipes were proposed with different evaporator heat transfer area. The module heat pipes were composed of a near horizontal evaporator section with different number of parallel pipes connected to a bottom collector and a shell and tube header. The impact of the change in the evaporator surface area by changing the number of evaporator pipes for the same overall heat pipe dimensions on the heat transfer rate, the heat pipe thermal resistances and the heat pipe working temperatures were discussed. The impact of the filling ratio was assessed to verify the assumptions made in the single heat pipe study. The systems were tested at flow rates from 5 L/min to 20 L/min and heater temperatures from 200 °C and 500 °C. A theoretical modelling tool was developed using VBA to predict the performance of the single heat pipe and the module heat pipes. The modelling tool was built based on an electrical analogy approach, considering the different thermal resistance within the system. Each thermal resistance was based on correlations validated by the experimental data. The modelling tool was then used to predict the heat transfer rate for all the tests carried out in this study. The prediction obtained was within 10%. The modelling tool predicted the impact of the heat pipe ceiling on the tile temperature. The potential heat recovered by the system was predicted to be 1277 MWh per year.