The design and the follow-up of a once-through circulation boiler differs from the design and the follow-up of a conventional boiler. A specific thermodynamic model has to suit very high pressure, sub- and supercritical steam properties. Mathematical models have to be adapted to account for the disappearance of the conventional economiser, boiler and superheater. Empirical equations corresponding to each part of the traditional boiler are no more possible. In a once-through heat recovery boiler the location of the boiling point is no more fixed. General equations have to be used for each tube of the boiler. The mathematical complexity as well as the number of equations is increased. This paper presents one subcritical 180 bar once-through heat recovery boiler model. Comparison with usual boilers in terms of mathematical results are presented, including the description of a specific mathematical model, especially developed, using the Belsim-VALI software, to represent a once-through boiler.
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