A General Purpose Network Solver?

I often get frustrated if I want to solve a simple fluid flow or thermal problem that is in the form of a network with branches, parallel paths etc. These tend to be a bit to difficult for a "back of the envelope" solution, which is always my preferred approach, so you end up writing code to iterate to find solution.

The easy way would be to use a commercial package like Aspen Hysis, SINDA or ESATAN, but these are very expensive for the once a year or thereabouts that I need to solve something like this. It would be nice to have a simple, low cost solver available.

I thought there would be plenty of open source ones available, but I have not been able to find a simple network solver, so I am going to resort to writing it myself. I did do one once before, but I seem to have lost the source code....It never got finished properly anyway.

I have started the project at http://code.google.com/p/openproc. There is no code worth talking of there yet, but the Wiki describes the sort of design I am thinking of. Any offers of help greatly appreciated!