13.4.2 Steam-to-steam reheaters - part 1
Vertical reheaters have been developed in recent years by NEI-Parsons for Canadian machines and by Deutsche Babcock and Wilcox for machines in Europe, particularly in Germany.
A typical NEI-Parsons steam-to-steam reheater is shown in Fig 2.105. The shell consists of a large vertical cylinder with dished ends top and bottom. The structure is supported on two integral tubular supports which penetrate the shell and provide support for the tube elements and integral framework. The tube elements are therefore allowed to expand vertically from this tubular support near the base inside a sliding linkage arrangement. The tube elements are also positioned centrally within a support frame which locates the elements inside the shell and ensures that the shell-side steam does not by-pass the tube elements. A full diameter bolted joint is provided near the top of the shell to allow the upper dished end to be removed for access to the tube elements.
Each identical tube element contains a number of straight externally-finned tubes welded into similar upper and lower headers. Welding is done with an automatic TIG welding system, using a two-pass technique followed by full NOT. The headers are rectangular in section and trough-like in shape, the side opposite the tube sheet being in the form of a coverplate or lid. The covers are bolted to the headers and a soft metal gasket is incorporated between the cover and the trough section to ensure a leak-tight seal.
The tubes in each element are arranged in a narrow rectangular pattern with the long span being perpendicular to the direction of the shell-side steam flow. This results in the minimum shell diameter, and facilitates element interchangeability and ease of manufacture, erection, leak detection and replacement of elements, if necessary.
Steam condenses as it passes down the tubes and the vertical tube arrangement obviates problems of slug-water flow that can occur with horizontal configurations. Complete drainage during shut down or element isolation is also ensured by the vertical arrangement.