THE BENT-TUBE BOILER

A typical small two-drum water-tube boiler is fired by a spreader stoker equipped with a dump grate. By means of baffles, the gases are forced to follow a path from the furnace to the boiler exit. This arrangement of gas flow is known as a "three-pass" design. A water level is maintained slightly below the midpoint in the steam drum. Water circulates front the steam drum to the lower or mud drum through the six rows of tubes in the rear of the boiler-tube bank where the comparatively low gas temperature results in a low heat-transfer rate. Circulation is from the mud drum to the steam drum through the front boiler tubes and the side-wall furnace tubes. The side-wall furnace tubes are supplied with water from the mud drum by means of circulators connected to rectan­gular water boxes located in the side walls at the level of the grate. Water for the front-wall tubes is supplied to a round front-wall header by downcomer tubes connected to the steam drum and insulated from the furnaces by a row of insulating brick. Most of the steam is generated in the furnace-wall tubes and in the first and second rows of boiler tubes which can "see" the flame in the furnace and absorb energy by radiation.

Boilers of this type have been standardized in a range of sizes capable of generating 8,000 to 50,000 lb of steam per hr.

The position of the drums and the shape of the tubes result in a compact unit having a well-shaped and economically constructed furnace. By simple changes in the arrangement of furnace-wall tubes, the design can be adopted to almost any kind of firing equipment and fuel.

The bent-tube boiler offers many advantages over the straight-tube boiler, including the following: 1) greater economies in fabrication and operation because of the use of welding, improved steels, waterway construction, and new manufacturing techniques; 2) greater accessibility for inspection, cleaning and maintenance; 3) ability to operate at higher steaming rates and deliver drier steam.

The main elements of the bent-tube water-tube boiler are essentially drums (or drums and headers) connected by bent tubes. With a water-cooled furnace, bent tubes are arranged to form the furnace enclosure, making it integral with the boiler.

The early bent-tube boilers were of the four-drum type. Although many operators still prefer it, there is a decided trend to use two drums or three drums. In modern bent-tube units, the capacity is held to less than 20,000 lb steam per hr per ft of width. The smaller bent-tube boiler has been fairly well standardized into a relatively small number of types. The boiler is either of refractory wall or waterwall construction, sometimes with a steel casing designed for nonpressure operation. Popular boilers are the two-drum low head, the three-drum low head, two-drum inclined as well as various package boiler designs. The standardized design used in industrial plants is available in capacities to 100,000 lb steam per hr. Design pressure varies from 160 to 825 psi with temperatures up to 850° F.

Integral furnace. In its early development stages, the bent-tube boiler was set over a brick or refractory furnace and all heat-absorbing surfaces were inside the boiler itself. As furnace size and temperatures increased, refractory maintenance became excessive, particularly when firing with pulverized coal.

The higher gas temperatures caused increased slagging or fouling of the boiler, surfaces. The furnaces were first partly then later completely, water-cooled to overcome these difficulties. Besides decreasing maintenance and boiler slagging, the waterwalls also generated steam, provided ex­cellent circulation, and aided in obtaining higher capacities.

Furnace waterwalls were first applied to existing boilers, the water circulation being more or less independent of the boiler circulation. Later the furnace water-cooled surface and the boiler surface were integrated.

With the advent of pulverized coal, it became necessary to prevent the ash from slagging at the bottom of the furnace. This was accomplished by installation of a waterscreen, consisting of a crisscross of water tubes protecting the furnace floor (and the ash) from radiation. As furnace input increased, the entire floor was water-cooled and designed for continuous or intermittent discharge of molten slag. Still later a redesign of the floor resulted in the dry-ash furnace bottom.

Design. With the exception of models of obsolete design or of very recent development, such as the positive circulation boiler, the bent-tube boiler is inherently a multidrum boiler.

There may be two, three or four drums-one lower drum with the remainder at the top of the boiler. The lower drum is the mud drum, which has a blowdown valve for removal of sludge and concentrations of salts. The upper drums are steam and water drums.

Although they are called steam drums, actually some of them may be water filled. Steam separators (drum internals) eliminate entrained moisture and precipitates, purifying the steam.

The tubes are either inclined or arranged in vertical banks within the combustion space, or they may comprise water-walls backed with refractories. The bent tube allows great flexibility in design, particularly with regard to drum arrangement, as it may enter the drum radially.

Boiler and furnace wall tubes are usually supported by the drums or headers to which they are connected. Some boilers are bottom supported, and others are suspended from the upper drums.

The gas baffles are arranged in many different patterns, with the gas flowing across and along the tubes in one or more passes. The tendency toward slag adherence is decreased if tubes are vertical or nearly vertical. To avoid particularly abrasive coal ash or unsatisfactory fusion characteristics, the boiler design must consider both these possibilities.

The bent-tube boiler is suitable for operation with oil, gas, coal, bagasse or wood. Burning methods include oil or gas burners and stoker firing. For sizes over 100,000 lb steam per hr pulverized coal or crushed coal (cyclone furnace) firing is used. The firing is usually manually or semiaiito-matically controlled.