Configuration of Evaporators for Once-Through Heat Recovery Steam Generators of Combined-Cycle Units

Abstract

Various designs are examined of evaporators for horizontal or vertical once-through heat recovery steam generators (HRSG) of combined-cycle units. Special attention is given to the Benson boilers manufactured by Siemens, which are most widely used now. Vertical HRSG with evaporators having horizontal heating surface sections are also analyzed. Their advantages and disadvantages are outlined. It is demonstrated that HRSGs with vertical heating coils provided with equalizing headers are more advantageous. The equalizing headers are connected with stubs to bottom U-bends of the coils. All heating surfaces are drained by gravity. The temperature conditions of this evaporator were investigated in a GTU load range of 25–100%. It has been found that partial load operation results in poorer heat transfer due to the boiling crisis. However, the maximum temperature jump in the post dryout region will be approximately 25–30°С, which is quite acceptable for reliable operation of low-alloy steel tubes. Assessment of the flow stability at the precrisis conditions demonstrates that the two-phase flow in the evaporator is stable over the applicable load range of HRSG. The calculations were performed for the evaporator without an orifice at the coil inlets. On the whole, the thermal-hydraulic operating conditions of these boilers are much better than those of Benson boilers. From the construction standpoint, a coil evaporator with equalizing headers is simpler and less metal intensive. A quite high liquid velocity is kept at all loads. Dynamic properties are also advantageous, since the material and the geometry of metal intensive elements of the evaporator, such as separators and water tanks, have been selected to improve performance of the overall HRSG.