The design of cost optimal heat exchanger networks is a difficult optimisation problem due
both to the nonlinear models required and also the combinatorial size of the search space.
When stream splitting is considered, the combinatorial aspects make the problem even harder.
This paper describes the implementation of a two level evolutionary algorithm based on a
string rewriting grammar for the evolution of the heat exchanger network structure. A biological analogue of genotypes and phenotypes is used to describe structures and specific solutions respectively. The top level algorithm evolves structures while the lower level optimises specific
structures. The result is a hybrid optimisation procedure which can identify the best structures including stream splitting. Case studies from the literature are presented to demonstrate the capabilities of the novel procedure
