Water tubes

WaterwaH furnaces were employed by the ancient Greeks and Romans for household services. A water boHet, found in Pompeii, was constmcted of cast bton2e and incorporated the water-tube principle (2). The earhest recorded instance of boHets performing mechanical work (130 Bc) was Hero s engine... 

Furnaces of this type, such as the steam locomotive furnace—boHet design, had the obvious disadvantage that pressure was limited to ca 1 MPa (150 psi). The development of seamless, thick-waH tubing for stationary power plants (ie, water-tube furnaces) and other engines for motive power, such as diesel—electric, has in many cases ecHpsed the fire-tube boHet. For appHcations calling for moderate amounts of lower pressure steam, however, the modern fire-tube boHet continues to be the indicated choice (5). 

A key development in water-tube furnace design was the Babcock and WHcox boHet of 1877 (Fig. 2) (3). This can be considered the direct evolutionary ancestor of the 1000 MW steam power plants a century later (see Steam). 

The Los Alamos water boiler served as a prototype for the first university training reactor, started in September 1953 at North Carolina State College. The cylindrical reactor core used uranyl sulfate [1314-64-3] UO2SO4, and cooling water tubes wound inside the stainless steel container. A thick graphite reflector surrounded the core. 

Seamless copper tubing is sold in water-tubing sizes (ASTM B88 and B306). These sizes are identified by a standard size designation dimensionally Vh in less than the nominal outside diameter. The tubing is also sold as outside-diameter copper tubing (ASTM B280). 

TABLE 10-31 Copper Water Tubing—Types K, L, M (ASTM B88) ... 

Extracted from ANSI B16.22—1973 with permission of the publisher, the American Society of Mechanical Engineers, New York. ( F — 32) 9 = C to convert inches to millimeters, multiply by 25.4 to convert pounds-force per square inch to megapascals, multiply by 0.006895. f Standard water-tubing sizes. 

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

How was it that water tubes reached such high temperatures We can give two probable reasons. The first is that "hard" feed water will - unless properly treated -... 

Fig 13 5 Temperature distribution across the water-tube wall. 

The steam for process heating is generated in either fire or water-tube boilers, using the most economical fuel available. The process temperatures required usually can be obtained with low pressure steam (tyq ically 25 psig), and steam is distributed at a relatively low pressure (typically 100 psig). Higher steam pressures are needed for high process temperatures. 

Cooling water required on site often is stored in towers storage tank problems or piping and valve malfunctions could cause loss of this component. If seawater is used, materials of construction must be more resistant to salt. Loss of steam purchased or generated in water tube boilers could result from boiler lube failure, turbine failure, or piping or valve malfunction. 

Waste-heat boilers. Waste-heat boilers can be designed to accept any grade of waste heat to produce steam or hot water. Designs can be based on water-tube boilers, shell and tube boilers, or a combination of the two. 

There is limited application for CO trim systems which are widely used on utility and other large water-tube boilers. The principle of operation is for an infrared beam to traverse the flue from emitter to sensor. The absorption of the infrared radiation is proportional to the CO content. 

Obrecht, M. F., Sastor, W. E. and Keyes, J. M., Integrated Design of Field Test Panel Pilot Unit for Investigating Pitting Corrosion of Copper Water Tube by Potable Water Supplies , Proc. 4th Int. Congr. Met. Corr., 1969, 576 (1972)... 

Table 17.7 Water tube boilers - examples of low-pressure boiler water standards...

In high heat flux (heat transfer rate per unit area) boilers, such as power water tube (WT) boilers, the continued and more rapid convection of a steam bubble-water mixture away from the source of heat (bubbly flow), results in a gradual thinning of the water film at the heat-transfer surface. A point is eventually reached at which most of the flow is principally steam (but still contains entrained water droplets) and surface evaporation occurs. Flow patterns include intermediate flow (churn flow), annular flow, and mist flow (droplet flow). These various steam flow patterns are forms of convective boiling. 

Typically, FT boilers tend to have lower rates of overall heat-flux and lower steam/water quality, and nucleate boiling predominates. Water tube (WT) boilers tend to have higher heat fluxes and higher steam/water quality under these conditions, annular flow convective boiling tends to dominate. 

Steam produced from a packaged boiler (fire tube or water tube) should always contain less than 5% entrained water. 

There are four fundamental types of boiler available today—electric boilers, fire tube (shell or FT) boilers, water tube (WT) boilers, and nuclear reactor boilers. Electric boilers apart, all other types are essentially developments from shell and tube heat-exchanger designs. 

Water tube boilers convert heat from burning fuel within a central, boxlike open furnace chamber to generate either hot water or steam (often at very high pressure, temperature, and output capacities). 

Water tube boiler design and construction provide for much greater capacity, pressure, and versatility than FT boilers because of the subdivision of pressure parts and the ability to rearrange boiler components into a wide variety of configurations. As a result, steam output may be from under 1,500 lb/hr to several million lb/hr. Designers have, over the years, developed WT boilers for many diverse industrial process applications. 

Water tube boilers are installed in all manner of commercial and institutional buildings, smaller industries, large industrial processors, and power generators. (In fact, the utility power industry is the single largest user of WT boiler capacity in the world.)... 

Boiler Types and Applications 2.3.1.3 Water Tube Exit Gas Section... 

Water Tube Steam Generators Steam generators for industrial applications may have internal boiler components such as drums, boiler bank, and membrane wall designed in one of several different arrangements. These arrangements are based on the position of the boiler drum and include the following ... 

Water tube boilers have a pressure gauge, vent cock, and drum safety valve on the top of the steam drum. Where superheaters are fitted, the steam takeoff line leads to the superheater, which is followed by a superheater safety valve, automatic nonreturn valve, and stop valve with a pressure-equalizing line and valve. 

NOTE Water tube boiler water-wall headers should not be blown down while the boiler is under load because it disturbs the natural circulation and may result in an overheated and bulging or ruptured tube. Usually header blowdown valves are locked closed and are only blown down when the boiler goes offline. 

Water tube boiler plants lacking a deaerator... 

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