This paper develops and analyses a model of a falling film evaporator with mechanical vapour recompression for the purpose of examining the coupling and functional controllability of the three core control loops for effect temperature, product dry mass fraction and product flowrate. The Relative Gain Array is used to assess coupling, and the functional controllability analysis uses the methodology of Skogestad and Postlethwaite(1). This determines if adequate levels of disturbance rejection can be achieved with allowable levels of manipulation. The aim is to understand the fundamental properties of the process which limit the ability of the control system to reject disturbances. A first principles model of appropriate complexity which captures the essential mechanisms is developed and used. Initially a single pass evaporator is studied, the applicability of the results to multi pass systems is considered.