“…Some of these studies, where simplified models were obtained, are: Chien et al [18]; Speedy-Goodwin [19], McDonald-Kwantny [20]; Herget-Park [27] andÅström-Eklund [5]. Other works for coal fired units are Kwan and Anderson [26] and Armor et al [28]; the first one applies the distributed properties along the waterwall tubes in a linearized model solution and the second presents a module modeled system but no equation was shown. Maffezzoni [22] proposes that the two-phase flow in the evaporator of a steam generator may be treated as a single homogeneous phase with equivalent average fluid properties.…”