Flow instabilities in a vertical tube reboiler

摘要

The dynamic thermohydraulic behaviour of a vertical tube reboiler is investigated by experiments and by simulation. Natural circulation reboilers are widely used in chemical industries, especially as column evaporators. Their main components are an evaporator and a separator which are connected in a loop. The subcooled product fluid coming from the separator is heated, partially evaporated and fed back into the separator. Entering the separator the two-phase flow is separated into liquid and vapour phase due to gravity. As a result of the partial evaporation the mean density of the fluid decreases. Caused by the density difference between evaporator and separator contents, a natural circulation occurs. Under certain conditions flow instabilities will occur, triggered by different causes as for example feedback between vapour generation, flow velocity and pressure loss. To investigate these instabilities, to distinguish between them as to their type and to enable a forecast for these instabilities a technical pilot plant scale test facility is used. The main components are a vertical tube evaporator, a separator and a condenser. The evaporator is heated by steam. It is designed as a straight tube heat exchanger with seven tubes in parallel of 4 m length each. By varying various operating parameters such as product vapour pressure, heat load and submergence level, stable and unstable conditions can be observed and analysed. As a working fluid deionized water is used. Based on the experimental results and criteria found in literature a stability diagram is in development. Besides the ongoing experiments a computer model based on the differential volume method is in development to simulate the dynamic thermohydraulic behaviour of the system. The obtained experimental data is used to validate the model.