This paper is about a concept that began with the need to increase the evaporator capacity at Mourilyan Mill by a small increment, sufficient only to meet immediate needs. Current practice is to plan for future expansion when considering today's requirements. This could have meant installation of a vessel having 4000 m2 of heating surface, when the present requirement may only be for a vessel of 1000 m2 heating surface. The intention at Mourilyan was to outlay expenditure for a vessel to suit present needs, but designed so that modules could be added to it at any time to increase its size, there being no limit to ultimate size as long as space for expansion was available.