The existing power and desalination capacity installed in the Middle East represent a huge asset which yearly requires considerable investment. In particular desalination plants reliability is essential as the entire economy depend on water produced by desalination plants and the interval allowed for shut down and rehabilitation works is limited to few weeks per year. Several desalination plants constructed in the 70s are still successfully in operation and their life can be further extended for other 10 years or more. The real problem is that the design methods adopted at the construction times of these plants became obsolete and do not comply with modern criteria of water and power production. Upgrading and up-rating of existing brown fields will therefore more and more play a key role in the framework of an intelligent rehabilitation work ensuring comparatively fast investment recovery times and immediate benefits in plant output and management. In June 2004 Deutsche Babcock was awarded a contract for the rehabilitation of Ras Abu Fontas units 3,4,5,6 in Qatar. The rehabilitation project is aiming at further extending the service life of the 5 MIGD MSF units, which were installed between 1975 and 1978 by Weir Westgarth. Ras Abu Fontas units 3,4,5,6 have like several existing MSF desalination plants installed in the same period present an available heat transfer surface which could be theoretically used to increase the distillate water capacity of the plant. Taking advantage of this feature, Deutsche Babcock proposed, along with the rehabilitation work, a 3% augmentation of distillate production by partial redesign of evaporator auxiliary systems. This project followed the same philosophy adopted for the up-rating carried out along with the rehabilitation of SEWA Layyah unit 4-5 [1] The present paper aims to describe the design approach to this achievement.