The horizontal straight tube boiler

The horizontal srtaight tube boiler covers a range of capacity and pressure between that of the fire-tube boiler and the large central steam generator. It is used in industrial applications primarily for process steam, occasionally for heating, and sometimes for power generation. 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 circulation and expanded at the ends into headers.

The header (either a box header or a sectional header) provides flat surfaces for tube connections. It may be connected to the drum by means of circulation tubes (downcomers or downtakes for supplying water to the tubes, uptakes or risers for discharging water and steam from the tubes) or by sheet steel saddles. 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). The baffling is arranged for two or three gas passes across the tubes. 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 1000 F may be installed. They are termed overdeck and interdeck depending upon their location in

the boiler.

Circulation. The steam and water rises along the inclined tubes to the front headers, then through the headers and circulation tubes to the drum. 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. If the tubes discharge to the steam drum at or above the waterline, the boiler is known as an exposed-tube boiler, otherwise it is a submerged-tube boiler.

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. The firing is usually manually controlled.