Heat load capability matching principle and its applications to anti-freezing of air-cooled condenser

•It is of use to propose anti-freezing measures for air-cooled condenser.•Heat load capacity matching principle for the anti-freezing is suggested.•Anti-freezing steam flow rate increases with decreasing ambient temperature.•Anti-freezing fan flow rate increases with increasing exhaust steam flow rate.•Back pressure can be much more reduced at secure steam flow rates.

Air-cooled condenser in power plants takes a risk of freezing in extremely cold days, so it is of benefit to the safe and economical operation of direct dry cooling system to propose the anti-freezing principles and take measures. On the basis of the heat load balance between the exhaust steam and cooling air, the heat load capacity matching principle for the anti-freezing of air-cooled condenser is proposed with reference to the freezing point of water. By applying heat exchanger model to the finned tube bundles of air-cooled condenser, the thermo-aerodynamic behavior of cooling air, the condensation of exhaust steam and the sensible heat rejection of condensate in a representative air-cooled condenser cell are synchronously modeled and resolved. The correlations among the ambient temperature, flow rate of cooling air, exhaust steam flow rate and quality, and back pressure of turbine that prevent the air-cooled condenser from freezing are discussed, and the anti-freezing flow rate of exhaust steam, back pressure of turbine and flow rate of axial flow fan are obtained. The results show that the anti-freezing flow rate of exhaust steam and back pressure both increase with decreasing the ambient temperature and increasing the flow rate of axial flow fan, from which derive the secure steam flow rate that can reduce the back pressure as much as possible to improve thermal efficiency. The anti-freezing fan flow rate increases with increasing the exhaust steam flow rate and ambient temperature, but varies little with back pressure. The increased steam quality will result in a higher heat load at the steam side, which allows a higher rotational speed of fan to be free of freezing for air-cooled condenser. The application of heat load capacity matching principle to the anti-freezing of air-cooled condenser contributes to the secure and optimal operation of dry cooling system in power plants.