Plate heat exchangers - the new trend in thermal desalination

In recent years thermal desalination using multiple effect processes, often augmented by thermo-compressors, has become increasingly popular. The basi c process has remained unchanged since the late 1800s when scientists such as Rilleaux developed the concepts we use today. At the turn of the century these multiple-effect processes almost exclusively used submerged tube des igns. Over the years several other equipment configurations have been used with horizontal tube falling film (HTFF) finding new popularity in the last decade. The low pumping power consumption of falling film processes has le d to the decline of forced circulation processes, such as MSF, especially i n plant sizes less than 10,000 t/d. Plate heat transfer surfaces are the la test development in falling film technology. Various configurations of plat es have been tried in the past. Among them have been dimpled plates and cor rugated plates. The latest type of plates are pressed plate used in a falli ng film (PPFF). This new configuration has several advantages never before available to any thermal desalination process. The use of PPFF leads to hig her heat transfer coefficients due to the combination of plate pattern and thickness which is based on 40 years of proprietary manufacturing technolog y. The PPFF system incorporates a patented distribution system, which provi des greater control of fluid distribution and wetting of the surface as wel l as turbulent boundary layers promoted at low velocities due to the plate pattern. Conversely, all HTFF systems have a vapour space between the distr ibution system and the top row of tubes then successive gaps between each r ow. The new PPFF system removes these potential sources of scaling due to i nadequate wetting found in HTFF. Perhaps the most practical new feature is the flexibility and access to the heat transfer surface. Normal cleaning pr ocedures involve the typical cleaning in place (CIP) circulation of mild ac id solution used by most desalination processes. However, the possibility o f full access to the heating surfaces in the PPFF means that virtually no i rreversible scaling will take place. Even the dreaded calcium sulphate scal e, should it arise due to operational errors, can be readily and completely removed. The PPFF configuration can also be configured to allow future add ition of the heat transfer surface. This makes it possible to plan for futu re plant capacity expansion with minimal investment today. While membrane s ystems may be able to allow for the addition of extra membrane for planned increases in capacity, this has not before been possible with any thermal d esalination process, thereby the new PPFF configuration offers a very high design flexibility.