High Temperature Rotating Heat Pipe Setup

Abstract

The heat pipe assisted annealing project is a collaboration between TU Delft, Tata Steel and Drever International. It is aimed at developing a continuous annealing line where the implementation of rotating heat pipes leads to a 70% reduction in energy consumption by reusing heat recovered at the cooling step. Rotating heat pipes are closed cylindrical containers, containing merely a working medium operating at its vapor-liquid equilibrium. They are highly effective heat transfer devices, due to evaporation of the medium on one end of the pipe, and condensation on the other. Rotation about the longitudinal (horizontal) axis allows for return of the condensate to the evaporation side and maintaining of the cycle. For further development of the project, an experimental setup is to be designed that can operate at 423 to 873 K (150 to 600C°) without exceeding an internal pressure limit of 0.5MPa. The setup has to facilitate measurements that provide insight about heat and mass transfer phenomena inside the heat pipe while operating conditions are similar to those in a continuous annealing line. Working fluids capable of operating at the specified temperatures have been identified in view of their vapor pressure, chemical stability and safety aspects. Rotational velocity and dimensions of the heat pipe are based on resemblance of flow regimes expected in the continuous annealing line and comparison of experimental data with other studies. Heating and cooling loads provided by the setup are equal to those caused by steel strips in the annealing process. The equipment required to do so is selected with respect to the uniformity of the heating/cooling profile and ease of implementation in the setup. Essential measurements have been identified to provide temperature (internal and external) and pressure profiles, heat fluxes through the pipe shell and insight about internal flow regimes, velocities and phase change rates. Steady and safe operation is ensured by procedures concerning filling, safety and control and by selection of auxiliary equipment. To combine all the required equipment and instrumentation and arrange for manufacture of the setup, preliminary CAD drawings have been made that illustrate the intended design concepts. Biphenyl, phenanthrene and cesium, in combination with a 316 stainless steel shell, have been selected to cover the temperature range up to 607 K, 717 K and 873 K respectively, while complying with the specified criteria. Nevertheless, the TU Delft test setup is restricted to 717 K because cesium requires an inert external environment for safe operation. The working fluid study has also revealed how critical purity and cleanliness of the heat pipe is. Accordingly, each fluid will have its own heat pipe and thus the setup should support modularity. The dimensions are established at 500 mm length, 50 mm inner diameter and a 5 mm shell thickness. A maximum rotational velocity of 1000 rpm covers all possible internal flow patterns when the heat pipe is loaded up to 10% of its volume. A maximum heat input of 2.27 kW supplied at the evaporator ensures heat fluxes up to 30kW/m2 at the condenser. Trace heating and mist cooling can supply the necessary heat flows uniformly without compromising safety nor impeding the replacement of heat pipes. Six pairs of thermocouples in the heat pipe shell, distributed over the length, enable both heat flux and temperature profile measurements. Five thermocouples located inside the heat pipe, and distributed over the length as well, provide an inner temperature profile and indirect pressure measurements. Ultrasonic technology is selected for the measurement of the liquid layer thickness, but requires further investigation. Arrangements have been made for manufacture of the heat pipe embedded with the thermocouples. Additional equipment with the appropriate specifications and its suppliers have been identified. The design concepts will be finalized for manufacture and thus the high temperature rotating heat pipe setup will be realized.