Patent Number: 
Section: claims

1. A steam generator comprising:a heat transfer tube group that is formed of a plurality of heat transfer tubes each having a U-shaped end opposite to ends that are secured to a tube plate, and each having a heat medium flow from one secured end toward another secured end;an annular channel that is formed so as to cover the circumference of the heat transfer tube group and that has an opening, at a lower portion thereof, that communicates with the heat transfer tube group;a water supply unit having a semi-ring shape that is disposed at an upper portion of the annular channel and that supplies water to a descending-side portion serving as an area where the heat medium in the heat transfer tubes descends toward the another secured end; anda steam/water separator that is disposed above the heat transfer tube group and that separates water heated while passing from the annular channel along the circumference of the heat transfer tubes into steam and hot water,wherein an air bubble removing member for removing air bubbles is provided in a rising-side portion that includes the annular channel,wherein the air bubble removing member is a water supply member having a semi-ring shape that is located on the rising-side portion to supply water to the rising-side portion for condensing the air bubbles,wherein the water supply member having the semi-ring shape is branched from the water supply unit to supply water in an amount that is sufficient to condense the air bubbles, andwherein a pipe diameter of the semi-ring shaped water supply member being smaller than a pipe diameter of the semi-ring shaped water supply unit. 2. A steam generator according to claim 1, wherein, in a tube supporting plate that supports the heat transfer tubes, the opening area in the rising-side portion is larger than the opening area in the descending-side portion. 3. A steam generator according to claim 1, wherein a heat-transfer-tube partitioning plate that partitions the rising-side portion and the descending-side portion is provided at a lower portion of the heat transfer tube group, and a channel in the descending-side portion partitioned by the heat-transfer-tube partitioning plate is a serpentine channel. 4. A steam generator according to claim 1, wherein anti-vibration bars inserted between the U-shaped ends of the heat transfer tubes adjacent to each other are arranged such that the arrangement density in the rising-side portion is lower than the arrangement density in the descending-side portion. 5. A steam generator according to claim 1, wherein a plurality of steam/water separators are provided, of which a steam/water separator that is positioned in the descending-side portion has an orifice channel resistance provided at the sides of its inlet port which restricts the ease with which the supply of water flows as compared to a steam/water separator that is positioned in the rising-side portion not having the orifice channel resistance at the sides of its inlet port to restrict the flow of supplied water thereby allowing the water supplied to flow more easily.