Abstract: Taking 1 000 MW direct air cooling units as an example, by using the method of effective energy-heat transfer units, a mathematical model of direct air cooling units under variable operating conditions has been elaborated. By considering the influence of pressure drop of the exhaust pipe and environment heat dissipation loss on the exhaust steam pressure of the last stage in the steam turbine in direct air cooling units, the curves of condenser pressure change with environment temperature, face velocity and exhaust steam flow were obtained. The linear relationship between the exhaust steam pressure of the last stage and its influence factors was analyzed, which can provide a reference for choosing the suitable exhaust steam pressure and for improving economical efficiency under off-design operation conditions in 1 000 MW air-cooling units.

Key words: condenser pressure, direct air-cooling, off-design, characteristic curves