In process engineering systems, each plant item is associated with a set of process variables.
For hydraulic : Pressure-mass flow,volume flow.
For thermal: température, specific enthalpy _entropy flow, enthalpy flow, thermal flow, quality of steam....
For chemical : chemical potential, chemical affinity, molar flow...
Use pseudo bon dgraphs allows to manipulate more intuitive variables and easily measurable (concentration, enthaly flow, ...) therefore easy to simulate.
Entropy is not conserved.
Mass m stored by any accumulator,
Total enthalpy (or internal energy) U stored by any heated tank,
Number of moles N accumulated in a reactor.
Thermal energy Q stored by any metallic body.