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 basic 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 designs. 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 led to the decline of forced circulation processes, such as MSF, especially in plant sizes less than 10,000 t/d. Plate heat transfer surfaces are the latest development in falling film technology. Various configurations of plates have been tried in the past. Among them have been dimpled plates and corrugated plates. The latest type of plates are pressed plate used in a falling film (PPFF). This new configuration has several advantages never before available to any thermal desalination process. The use of PPFF leads to higher heat transfer coefficients due to the combination of plate pattern and thickness which is based on 40 years of proprietary manufacturing technology. The PPFF system incorporates a patented distribution system, which provides greater control of fluid distribution and wetting of the surface as well as turbulent boundary layers promoted at low velocities due to the plate pattern. Conversely, all HTFF systems have a vapour space between the distribution system and the top row of tubes then successive gaps between each row. The new PPFF system removes these potential sources of scaling due to inadequate wetting found in HTFF. Perhaps the most practical new feature is the flexibility and access to the heat transfer surface. Normal cleaning procedures involve the typical cleaning in place (CIP) circulation of mild acid solution used by most desalination processes. However, the possibility of full access to the heating surfaces in the PPFF means that virtually no irreversible scaling will take place. Even the dreaded calcium sulphate scale, should it arise due to operational errors, can be readily and completely removed. The PPFF configuration can also be configured to allow future addition of the heat transfer surface. This makes it possible to plan for future plant capacity expansion with minimal investment today. While membrane systems may be able to all