Pressure vessel

A cuboid pressure vessel, comprising:

• a square-shaped top wall;
• a square-shaped bottom wall which is opposed to the top wall;
• and
• an external wall which connects the periphery of the top wall to the periphery of the bottom wall, wherein
• the external wall is provided with:
• an external-wall main body which includes a first side wall having a first opening that allows a heat exchanger to be inserted to an area surrounded by the external wall, and a second side wall having a second opening that allows the heat exchanger to be inserted to the area surrounded by the external wall;
• a first lid which is configured to cover the first opening and which is secured to the first side wall so as to occlude the first opening;
• and a second lid which is configured to cover the second opening and which is secured to the second wall so as to occlude the second opening,
• the bottom wall is provided with:
• a bottom-wall main body which is connected to the lower end of the external wall;
• and a bottom-wall reinforcement rib which is provided in an upright manner on the outer surface of the bottom-wall main body, and
• the bottom-wall reinforcement rib is provided in an upright manner on the outer surface of an area, within the bottom-wall main body, which contains a central section of the bottom-wall main body and which excludes four corner sections, a region adjacent to the first lid, and a region adjacent to the second lid of the bottom-wall main body.