Sentinel boiler

The Sentinel boiler was a design of vertical boiler, fitted to the numerous steam waggons built by the Sentinel Waggon Works.

The boiler was carefully designed for use in a steam wagon: it was compact, easy to handle whilst driving, and its maintenance features recognised the problems of poor feedwater quality and the need for it to be maintained by a small operator, rather than a major locomotive works.

Although this design was used in most of Sentinel's products, they also produced larger boilers of quite different types for their railway locomotives.

Sentinel boilers are vertical, as was common for many designs of steam wagon, so as to reduce the effects of tilting due to hill climbing or uneven roads disturbing the water level. It also provides a compact boiler that leaves adequate space in the cab for the crew, controls and coal bunker, whilst leaving as much as possible of the wagon's overall length available for the useful load.

The boiler is a watertube boiler, with these tubes contained within a vertical, cylindrical outer drum. This drum is double-walled and forms a water jacket around the boiler, with a large vertical flue within. The inner flue has a complex cross section. It is stepped in three diameters, tapering towards the top. The central region is square in section, rather than round.

The main heating surface is provided by water-tubes in this squared section. These tubes are short, straight and pass between the flat faces of the squared section in a grid pattern. There are eight layers of tubes, four banks of six each way.

A space is left in the centre of the water-tube banks for the firing chute. The firebox is top-fired (and lit) through this chute and there is no side firedoor. The lower part of the water-jacketed barrel surrounds the firebox. The narrow waterspace here encourages rapid steam raising. Firing is simple, with a thick fire relative to its area and fuel simply poured down. Beneath the grate is a water-filled ashpan, to prevent hot embers falling onto the road. Draught is controlled by a damper in the side of the ashpan, between the grate and the water tray.

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