THE HORIZONTAL-RETURN TUBULAR BOILER

The horizontal straight tube boiler covers a range of capacity and pressure between that of the fire-tube boiler and the large central steam generator. (1) The horizontal straight tube boiler is limited to an hourly production of about 10,000 lb steam per ft of boiler width. It is simple in operation and has low draft loss.

The straight tube boiler is made up of banks of tubes that are usually staggered, the tubes are inclined at an angle (5 to 15 deg) to promote circulaiion and expanded at the ends into headers.

The header (either a box header or a sectional header) provides flat surfaces for tube connections. (2) The drum may be either longitudinal (long) or across (cross) with reference to the axis of the boiler tubes. Some boilers have a portable firebox with wrapper and furnace sheets instead of a drum. The high end is usually the firing end. The area of the heating surface (and the capacity) is varied by changing the tube length and the number of tube row in both height and width. The tubes, 3 to 4 in. in diameter, are spaced 7 to 8 in. on centers horizontally and 6 in. on centers vertically (except slag screen tubes, which are on about 12 in. centers). The tubes are all of the same diameter and length, never over 18 to 20 ft.

As the pressure increases, the header design changes. Greater tube spacing is required, and the tubes must be smaller in diameter.

Internal fireside baffles may be horizontal (parallel with and between the tubes) or vertical (across the tubes). (3) In the headers opposite the tube end, there is a handhole of sufficient size to permit removal or renewal of the tubes and the inspection of tubing and cleaning of the tube interior. Handholes are elliptical in shape, machined to form a smooth gasket seat and fitted with forged steel handhole plates.

Superheaters with a maximum temperature rise of about 100° F may be installed. They are termed overdeck and interdeck depending upon their location in the boiler.

Circulation. (4) The water then circulates through the downcomers to the rear header and finally to the tubes to complete the cycle. In the long drum boiler, the water is diverted by a baffle plate back through the steam drum. In the cross drum boiler, steam separators (drum internals), are often used to eliminate entrained moisture and precipitates, thereby purifying the steam. (5)

Fuels and fuel firing. The horizontal straight tube boiler is suitable for operation with oil, gas, coal, bagasse, or wood.

Burning methods include oil and gas burners with hand or stoker firing. Pulverized coal firing is rarely used. (6)

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.

(7) 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. (8)

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. (9)

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. (10) 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 attachment 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.