For new power unit, HEI standard can be used to confirm the leakage of dry air in condenser. And the capacity of the vacuum system can be confirmed according to the temperature of circulation water and the back pressure of condenser. Meanwhile, the capacity of the vacuum pump also should be confirmed by the demand of hogging time. For the situation that planning to replace of existing vacuum pumps or ejectors, the design capacity of vacuum pump can be enlarged properly according to experiential coefficient.