Visual modelling is a graphical technique to represent and simplify process models. The approach is based on the idea of representing the process as a interlinked set of control volumes, whereby the links are the exchange of extensive quantities across the interface pieces of neighbouring control volumes. The control volumes, in a first stage have capacities representing the nodes in a graph and the transport of extensive quantity occurs across the common boundary pieces are depicted as the arcs connecting the nodes.
The graph can be structured to reflect more or less of the physical reality, in particular the geometry. In general though the method looses quickly this component and focuses on the capacity / transport behaviour of the represented plant.