Investigation of Flashing Flow in a Siphon to Extract Condensate in Paper Dryer Application

Abstract

The paper industry uses steam to dry paper web through cylinder dryers. As steam condenses inside the dryer, the condensate is removed by means of either a stationary or a rotary siphon. However, during the siphoning process for transporting the condensate, flashing of the condensate occurs, which could cause backflow or discontinuity in the siphoning process. To resolve this flashing issue, two approaches have been employed: (a) increasing the amount of steam supplied to the cylinder to “blow-through” the stalled condensate–steam mixture and (b) reducing the back pressure by inducing “suction” through a thermocompressor from downstream. The objective of this study is to investigate and improve understanding of the flashing phenomena during condensate transport through the siphon and piping system in order to develop means to reduce the excessive steam consumption during the paper drying process. A computational fluid dynamics (CFD) simulation is performed that uses the Eulerian–Eulerian multiphase method. The steady-state case is first solved to obtain the flow field without flashing. Then, the transient method is initiated by employing evaporation and condensation model. The results show that reduction of local pressure triggers flashing; however, flashing in turn reduces local temperature and subsequently induces condensation, resulting in an alternating flashing and condensation behavior. To maintain continuity of the siphon flow, the inlet pressure fluctuates corresponding to the variation of total vapor volume ratio inside the siphon. The results will be used to modify the current siphon system design and operating practices to reduce the steam consumption.