Thermal and hydraulic aspects of falling film evaporation
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Some thermal and hydraulic aspects of falling film evaporation were investigated. Experiments were made on two single effect evaporators. The first with a tube, 15 m long and 48 mm in diameter and the second which had the choice of one of three tubes, all 10 m long, with diameters (23, 40 & 48 mm). The apparatus was designed to enable the measurement of overall heat transfer coefficients. The effects of process variables, on the product-side and on the steam-side heat transfer coefficients, were determined by fixing conditions on the other side of the tube. Experiments were conducted on water, sucrose solution and skim milk. Aspects evaluated were temperature driving force, boiling temperature, tube liquid loading, feed temperature, non-condensable gas concentration in steam, de-aeration rates, product concentration (sucrose solution, milk) and feed thermal history (milk). The results obtained have been compared with theory and empirical data from the literature. A numerical integration procedure for the boiling side Nusselt number was developed to account for the change in boiling-side fluid properties over the length of the tube. The effect of boiling film Reynolds number on the overall heat transfer coefficient proved complicated with a local maximum being found in the transition flow region. A model description was developed to mimic the steam flow and condensation behaviour in the steam-side annulus of a single tube falling film evaporator. The numerical method initially chosen to solve the model proved inapplicable. A range of methods for measuring the amount of non-condensable gases present in vacuum steam were considered. The in-line measurement of pressure and temperature and the comparison of the pressure of a collected sample before and after freezing out the component were evaluated.