The side walls (4) have a rigid metal envelope (42) containing metal beam(s) (43) which define a free volume at least 50% of the envelope volume. The free volume contains water or an aqueous solution which changes phase at less than 150 degrees C. The envelope has inlet(s) (63) for the water or solution, outlet(s) (42bis) for the steam and a water outlet in case the envelope leaks : A rigid enclosure with one or more side walls (4) and a roof (3) define an internal chamber. One or more bases (5) support of the side wall(s). The envelope has a thickness or size between 5 and 50cm, preferably between 10 and 50cm. The envelope is made of steel, preferably stainless, and has a free volume which can contain at least 0.5m3 of water or aqueous solution. The enclosure has several separate envelopes connected together by passages for the water or aqueous solution and by mechanical joints which can dilate when heated. The water inlet is connected to a level control maintaining the level of water in the envelope(s). The envelope(s) are also provided with an overflow (64) and are connected to means of avoiding the formation of gel. Each side wall is connected to at least one buttress (10) extending into the chamber (CI) and pressing on a base, to oppose an external load on the wall. The side walls have an insulating layer on the outside. The side walls define a circle or a polygonal shape with at least 5 sides, preferably at least 8. An independent claim is included for a gas or inflammable product treatment installation with at least part of the installation in an enclosure as described above.
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