In this study

a mathematical model for comprehensive heat balance calculation under variable conditions is established based on the actual operation data of a 300 MW extraction heating unit. Combining with the theoretical calculations and actual operation data

the economy of extraction heating unit with rotating board is calculated. And then the economies of three different heating steam sources under variable operating conditions are compared. The calculation results show that the operation of the rotating board has a negative effect on the medium pressure cylinder efficiency. The lower the load

the greater the decline of the medium pressure cylinder efficiency

resulting in lower heating gains. At high loads

the three-stage extraction heating unit using rotating board has the largest energy savings. However

under the heating conditions of 260 MW and below

because of the large throttling loss of the rotating board

the three-stage extraction heating unit has poor economy. The economy of heating unit with reheating cold-side is the best.