An experiment with a single-effect submerged vertical tube evaporator in multi-effect desalination

Abstract

There is a shortage of fresh water in many countries of the world. Singapore imports a large fraction of its fresh water requirements from a neighbouring country to meet its domestic and industrial water demands. Desalination is an option and Singapore intends to pursue it further to obtain about 10% of requirements within the next two years. It will also select an appropriate method for the production of fresh water from the sea. The multi-effect distillation (MED) process is one of the reliable methods of desalination. Among the limitations, which prevented wider applications of MED, scaling is the most common. These result in higher fresh water production cost and operational difficulties. A submerged vertical tube evaporator (SVTE) has been investigated in this study making use of waste heat. This can be as part of a cogeneration, power and water, or on board ships where waste heat is available. Inside the evaporator, 175 single fluted aluminum tubes have been used with an inner diameter of 13 mm and a length of 0.5 m. In order to obtain the thermal performance of the evaporator, a series of experiments have been conducted under different operating conditions. Experimental and numerical results have been obtained under process conditions that are realistic for commercial small-scale desalination unit. The thermal behaviours of the SVTE have been evaluated for different temperatures and flow rates of heating medium and feed water, and the chamber pressure. For the conditions considered in this study, a vapour production rate of 3.3 tons/day and an average overall heat transfer coefficient of about 1000 W/m2K have been found.