The Net Pressure Driving Force Due To Boiling in Calandria Tubes

摘要

Vapour formation due to boiling in calandria tubes provides the driving force for natural circulation in vacuum pans. The net pressure difference generated across a calandria tube is determined by the average density deficit in the tube relative to the downcomer (i.e. the amount of vapour in the tube) and the pressure loss due to friction and acceleration. This paper presents a mathematical model of the two-phase flow of molasses and vapour in a calandria tube assuming equilibrium thermodynamics and steady state conditions. The model can predict the net pressure driving force and the evaporation rate produced by a but as a function of parameters such as the heat input, mass flow rate, liquid height above the calandria and boiling point elevation. Additionally, the model yields detailed profiles of temperature, absolute pressure, volume fraction and other variables as a function of distance along the calandria tube. Results are presented in the form of characteristic curves representing the net pressure difference available to drive natural circulation as a function of applied heat and mass flow rate. The circulation rate and evaporation rate in a natural circulation loop may be determined by matching the appropriate characteristic curve to the system resistance curve for the loop. An example is presented to illustrate the application of the method.