In a vapour generator, Fig. 1, comprising pressure vessel 10 containing water space 30 through which a large number of U-shaped thinwalled tubes 38 carry heating fluid from header 44 to header 46, a tube support system 70, 72, 74, 86 prevents lateral vibration of the tubes while allowing longitudinal thermal movement. The tubes are spaced apart in closely spaced layers by the support plates 70, 72, 74 which lie parallel to and between the layers in rows perpendicular to the layers of tubes. A baffle structure 28, 26, 24 forms an annular passage 32 down which cold water from inlets 58 flows to enter space 30 at the bottom. The vapour generated in space 30 passes through separators 62 in baffle 28 and driers 64 to outlet 22. The horizontal sections 68 of the tubes 38 are each positioned between a pair of tabs 901, 9011 on plates 70a, Fig. 7, formed by bending out at right-angles the tabs formed by cutting two rows of E-shaped holes. The length of the tabs is made equal to the tube diameter so that the tubes are constrained by the opposite faces of adjacent plates in a row. The lower imperforate extensions 82a of the plates 70a used in the central row have locating holes 92 through which passes connector rod 94. The lower end of the extensions (82b) of plates (70b) (Fig. 9, not shown) used in the other rows have two vertically extending locating lugs (102) which fit between legs 66 of pipes 38. The upper ends 84 of plates 70 are welded between pairs of retainer plates 104 to the lower arched edge 110 of a structural member 86. Members 86 have upper flanges 106 to increase rigidity and are slidably mounted at 112 on baffle 24. The vertical legs 66 of tubes 38 are positioned by tabs 90, 90b of plates 72, Fig. 14, in a manner similar to that above except that the plates 72 extend horizontally and are fixed at their outer ends to baffle 24 and at their inner ends to a connecting rod 128 via plates (127). A rectangular recess 160 in the outer end of each plate 72 fits over annular support 124 plug welded to baffle 24. Adjacent plates are bolted to opposite legs of U-shaped pieces 138 acting to constrain the outer tubes 38 in some cases. In region 40 the legs 66 of the tubes are positioned between tabs 154 formed along both edges of plates 74, Fig. 21, by making oblique cuts into the plate edges and bending out the triangular tabs so formed. The outer ends of plates 74 are rigidly mounted on baffle 24 (as plates 72) except for the uppermost rows which are slidably mounted. The inner ends of the plates 74 in the uppermost rows are fixed to rod 94 but the other plates 74 are merely rested on top of the uppermost of plates 72.
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