Fabrication of polymer film heat transfer elements for energy efficient multi-effect distillation

Abstract

Conventional multi-effect and multi-stage flash distillation use aluminium brass, cupro-nickel and/or titanium heat transfer surfaces. Polyolefins such as high density polyethylene (HDPE) and polypropylene (PP) have better corrosion resistance than these, which permits thinner walls. For identical internal & external convection coefficients, 20–50 μm thick HDPE and PP film heat transfer elements have from 71–105% of the U value of 1 mm cupro-nickel tubes. Experience has shown them to last as well as — and in some high-scaling water re-use applications better than — titanium elements. But per unit area they cost far less. This low cost — especially when combined with durable low cost vacuum vessels — permits the installation of more thermal conductance (UA) than with metal heat transfer surfaces. This leads to a lower temperature difference Δ T 1 between the condensing and evaporating sides, and a lower specific energy consumption. We describe the impulse welding of thin HDPE and PP films on specially developed apparatuses to produce tube-like heat transfer elements. Some of these were pressure tested up to several bars at room temperature to determine the strength of the weld lines. We make brief remarks about the design, construction and testing of a simple falling saline film vapour compression desalinator — with vertically oriented air mattress-like multi-tube polyolefin film heat transfer elements. Designed for operation with a small Δ T 1. From the frictional pressure drop Δ p f of the condensing vapour for laminar flow inside a film tube, we relate the film tube diameter, length, U value, temperature and the ratio R T = Δ T 1/Δ T f of temperature difference Δ T 1 to frictional temperature drop Δ T f.