Considerations on the design principles for a binary mixture heat recovery boiler

The use of a binary mixture as a working fluid in bottoming cycles has in recent years been recognised as a means of improving combined cycle efficiency. There is, however, quite a small number of studies dealing with components of plants which employ fluids other than water, and particularly binary mixtures. Due to different specific volume, viscosity, thermal conductivity and Prandtl number heat recovery boilers designed to work with water require certain modifications before they can be used with binary mixtures. Since a binary mixture is able to recover more heat from the exhaust fumes than water, the temperature difference between the hot and the cold fluids is generally lower over the whole recovery boiler; this necessitates greater care in sizing the tube bundles in order to avoid an excessive heat transfer surface per unit of thermal power exchanged. The aim of this paper is to provide some general criteria for the design of a heat recovery boiler for a binary mixture, by showing the influence of various dimensional parameters on the heat surface and pressure drop both in the cold and the hot side. Heat transfer coefficients and pressure drops in the hot side were computed by means of correlations found in the literature. A particular application was studied for the ammonia-water mixture, used in the Kalina cycles, which represents one of the most interesting binary cycles proposed so far.