为了寻找抽汽背压式汽轮机（back pressure extraction steam turbine，BEST）最佳的级数方案，以配置BEST的二次再热机组为设计基础，保持总回热级数不变，BEST级数从4级增加到8级，对比不同BEST级数下机组的可行性、变工况特性及热经济性。结果表明：从BEST功率相对给水泵功率的裕量来看，当BEST级数等于4级，机组负荷在100%~75%THA工况时，BEST功率无法满足给水泵功率的需求，可行的方案选取BEST级数从5级增加到8级，BEST每增加1级，BEST功率相对给水泵功率的平均裕量增加5~7 MW；对于抽汽压力，当负荷降低时BEST所在的除氧器前的抽汽压力随着BEST级数的增加而减小，BEST所在的除氧器后的抽汽压力随着BEST级数增加而增大；对于抽汽流量，当负荷降低时第1级抽汽流量随着BEST级数增加而增加，BEST所在的除氧器前的抽汽流量随着BEST级数的增加而减小，BEST所在的除氧器后的抽汽流量随着BEST级数的增加而增加；从热经济性来看，随着BEST级数增加机组的热耗率也随之增加，5级BEST方案为机组热经济性最佳的方案。

In order to find the optimal stages scheme of back pressure extraction steam turbine (BEST), based on the design of the secondary reheat unit equipped with BEST, keeping the total number of regenerative stages unchanged, the number of BEST stages was increased from 4 to 8, and the feasibility, variable working condition characteristics and thermal economy of the unit under different BEST stages were compared. The results show that from the perspective of the margin of the BEST power relative to the feedwater pump power, when the number of BEST stages is equal to 4 and under the condition of the unit load of 100% to 75% THA, the BEST power cannot meet the power demand of the feedwater pump.A feasible solution selects the number of BEST stages to increase from 5 to 8. For every additional stage of BEST, the average margin of BEST power relative to feedwater pump power will increase by 5 to 7 MW; For extraction pressure, when the load decreases, the extraction pressure in front of the deaerator where BEST is located decreases with the increase of the number of BEST stages, and the extraction pressure behind the deaerator where BEST is located increases with the increase of the number of BEST stages;For the extraction steam flow, when the load decreases, the first stage extraction steam flow increases with the increase of the number of BEST stages, the extraction steam flow in front of the deaerator where BEST is located decreases with the increase of the number of BEST stages, the extraction steam flow behind the deaerator increases with the increase of the number of BEST stages; From the perspective of thermal economy, as the number of BEST stages increases, the heat consumption rate of the unit also increases. The 5stage BEST solution is the best solution for the thermal economy of the unit.

TM621