Once-through steam generator design subject to minimum heat capacity constraint

Various types of once-through superheating steam generators have found application in central powe stations. The once-through steam generator makes possible a higher thermal efficiency but can be less stable in certain transients than a recirculating boiler. Because of the limited liquid inventory, questions have been raised about the desirability of sacrificing heat capacity in order to obtain increased thermal efficiency by superheating the vapor. In an effort to configure a once-through boiler subject to a liquid inventory constraint, a once-through steam generator design has been optimized in the sense that heat transfer area is minimized as a function of certain free variables, using Powell's conjugate direction technique. A suitable configuration is then compared to a two-pass superheating steam generator, which has two tubing arrays--hence an extra degree of freedom making satisfaction of the minimum heat capacity constraint easier. It is shown that in the two-pass design, the liquid inventory can be substantially increased--hence making the thermal response more sluggish--with only a small penalty in heat transfer surface area. However, the time to boil dry in the event of a loss of fee water without auxiliary feed is about twenty minutes, compared to the better part of an hour for amore » recirculating boiler.« less