Designing artifacts is a common although a vague task in a sense that neither intention of the designer nor his exploited there appears explicitly in the output of the design process. In order to make the process transparent, we propose a computer based system together with a knowledge base for designing chemical plants in terms of functional design. Taking design specifications as its input, the system generates a hierarchy of functions, each of which can be realized by either a unit or some subassembly. At the same time, it examines whether all conditions required for realizing each function, which are premised in the knowledge base, have been satisfied so that it can provide the designer with appropriate error messages.