The steam, which leaves a large part of its heat to the system, returns to this tank by returning as condensate water. After the condensate tank, the hot water is sent back to the boiler.
Since the system is closed, there is no steam leakage to the outside. The feed water to be sent to the boiler stays in the condensate tank without losing its heat, so the amount of heat required to heat the water in the boiler is reduced.