The horizontal-return tubular boiler

In the horizontal-return tubular (HRT) fire-tube boiler the boiler shell is a horizontal cylinder closed at each end by a flat tube sheet or head. The fire tubes, which are usually 3 to 4 in. in diameter, extend through the boiler from one tube sheet to the other and are rolled or expanded into the tube sheets at each end, thus serving not only as flues through which the hot combustion products flow but also as tie rods to hold the flat tube sheets in place against the steam pressure in the boiler The flat surfaces of the heads above the tubes are braced to the boiler shell by diagonal braces.

The boiler supported by a steel frame, is provided with a brick setting which encloses the furnace, and is fired by a single-retort underfeed stoker. The gaseous products of combustion from the stoker pass over a bridge wall at the rear of the stoker which is intended to promote turbulence, then through the brick furnace under the boiler shell to the rear of the boiler. They then flow through the boiler tubes to the front of the boiler after which they pass a damper and are discharged to a chimney.

A water level is maintained a short distance above the top tubes so as to provide adequate surface for the separation of the steam from the water and, at the same time, to keep water in contact with all surfaces across which hot gases are

flowing. The water level in the boiler is indicated by a water column which is connected to the boiler by two pipes, one above and one below the water level. The water in the water column is thus maintained at the same level as in the boiler, and this level is indicated by a glass tube attached to the water column.

A blow-off line is connected to the bottom of the drum at the rear. Valves in this line are opened periodically and some of the boiler water is blown to a sewer, thus carrying out of the system the impurities that are coming into the boiler in the feedwater. It is common practice in these small boilers to add chemicals to the feedwater. These chemicals are intended to prevent the scale-forming impurities in the feed- water from precipitating on the heating surfaces as an adherent scale. If the boiler produces dry steam, all these impurities remain in the boiler. They must be removed by periodic blowdown in order to maintain the concentration in the boiler water below a level that will cause scale formation.

The boiler shell is provided with suitable opening for the attachement of spring loaded safety valves, feed-water inlet, a steam outlet nozzle, and manholes or cleanouts,

Since this boiler is provided with a brick furnace which is external to the boiler itself, it is known as an externally fired boiler.