A new algorithm for variable condition was put forward and applied on a NZK600-24.2/566/566 unit to establish the minimum unit heat consumption objective function using main steam initial pressure and load as the constrained condition.When the unit high pressure regulating valve worked with different opening degrees

the optimal initial steam pressure was determined by optimizing.The result showed that

this off-design calculation method can meet the precision requirements of engineering calculation.The calculated optimal initial steam pressure under variable condition was higher than that of the design and test value.The calculated unit heat consumption was lower than the design and test value.Fixed pressure operation should be adopted when the unit load was above about 82% of rated load.On the premise of satisfying the load demands

the unit economical performance under sliding pressure operation condition with regulating valve fully opened was not necessarily higher than that under regulating valve throttling condition.In a certain range

the unit economy of both schemes that with three valves fully opened and one valve throttling and with three valves throttling were better than the one with four valves fully opened.