Plastic/compact heat exchangers for single-effect desalination systems

Use of plastic materials and compact heat exchangers remains to be found on a limited scale in the desalination industry. This work focuses on performance evaluation of plastic and compact heat exchangers in the single-effect mechanical vapor compression desalination system. The analysis considers a number of tubing materials that includes PTFE plastic, high steel alloys, Cu/Ni 90/10 and Cu/Ni 70/30, and titanium. The analysis includes determination of the specific power consumption and the evaporator and preheaters specific heat transfer area for various materials. Analysis is made as a function of variations in the condensate and the brine boiling temperatures. The specific cost is evaluated for various construction materials. Results show that the specific power consumption is independent of the construction material and depends only on the compression range. Also, the lowest specific heat transfer area is calculated for Cu/Ni 90/10. However, the lowest cost is obtained for the PTFE system because of the much lower cost of the material. Corrosion considerations and the associated reduction in the cost of corrosion inhibitors give an added edge for the use of PTFE plastic. Other factors that favor the use of plastic evaporators and preheaters include ease of construction and machining, lower installation and erection cost, ability to operate at higher top brine temperatures without fear of the effects of scale formation, use of acid cleaning, and virtual elimination of in-leakage problems. These merits of the plastic and compact heat exchangers and the analysis results should encourage the industry to take active steps in adopting these systems.