The time dependent processes are studied on a numerical basis, using an unsteady process simulation software. Such software tools solve the one-dimensional un­steady conservation equations for mass, energy and momentum [2]:

• TOC o "1-5" h z mass balance: dA^+dApv = Q (1)

dt d z

• momentum balance: dApv + dApv +dAp = Sm (2)

dt d z d z

• energy balance: dAph + dApvh = se (3)

dt d z e

On the basis of a detailed software evaluation [2,3,4], reviewing more than 15 simula­tion tools, the unsteady simulation software APROS from the finish supplier VTT was selected. The Advanced PROcess Simulation environment APROS is one of the very few software tools, that provides a 6-equation model for calculating the conservation equations for water and steam separately. This is of interest for the calculation of so called zero flow conditions appearing during start up and shut down when the flow velocities decrease to zero. Most of the other tools provide 5-equation models only, neglecting the momentum differences between the liquid and gaseous phases. Fur­ther advantages are the various interfaces providing several alternatives for connect­ing external software to the simulation programme like the IAPWS-IF97 for the thermodynamic properties of water and steam [6]. A long list of reference users working with APROS especially in Europe is another advantage. The number of available power plant components is very large and nearly every plant type from fluidized bed boilers to even coal gasification combined cycles including automation and electrical systems can be modelled.