When water is boiled the result is saturated steam, also referred to as "wet steam." Saturated steam, while mostly consisting of water vapor, carries some unevaporated water in the form of droplets. As the resulting "dry steam" is much hotter than needed to stay in the vaporous state it will not contain any significant unevaporated water. The fluid at that point may be a mix of steam and liquid droplets as it passes into the condenser. In contrast to a "subcritical boiler", a supercritical steam generator operates at such a high pressure (over 3,200 psi or 22 MPa) that the physical turbulence that characterizes boiling ceases to occur; the fluid is neither liquid nor gas but a super-critical fluid. This is because unavoidable temperature and/or pressure loss that occurs as steam travels from the boiler to the machinery will cause some condensation, resulting in liquid water being carried into the machinery. Superheated steam boilers evaporate the water and then further heat the steam in a superheater, causing the discharged steam temperature to be substantially above the boiling temperature at the boiler's operating pressure. The steam piping is directed through the flue gas path in the boiler furnace, an area in which the temperature is typically between 1,300 and 1,600 degrees Celsius (2,372 and 2,912 degrees Fahrenheit).
In the event of a major rupture of the system, an ever-present hazard in a warship during combat, the enormous energy release of escaping superheated steam, expanding to more than 1600 times its confined volume, would be equivalent to a cataclysmic explosion, whose effects would be exacerbated by the steam release occurring in a confined space, such as a ship's engine room. As a superheated boiler is usually operated at high pressure, the corresponding feedwater pressure must be even higher, demanding a more robust pump design. For instance, the ASME Boiler and Pressure Vessel Code is a standard providing a wide range of rules and directives to ensure compliance of the boilers and other pressure vessels with safety, security and design standards. All programs tend to mix messages of general injury prevention, safety, fire prevention, and escape in case of fire. They mix audio messages and visual cues with hands-on interaction. Also, small leaks that are not visible at the point of leakage could be lethal if an individual were to step into the escaping steam's path. Also, higher steam pressure will be possible than with saturated steam, enabling the steam to carry more energy.
The steam jet oriented in the direction of flue gas flow induces flue gases into the stack and allows for a greater flue gas velocity increasing the overall draught in the furnace. The pump used to charge the boiler must be able to overcome the boiler's operating pressure, else water will not flow. Collapsed or dislodged boiler tubes can also spray scalding-hot steam and smoke out of the air intake and firing chute, injuring the firemen who load the coal into the fire chamber. Often the water tubes connect large drums, the lower ones containing water and the upper ones steam and water; in other cases, such as a mono-tube boiler, water is circulated by a pump through a succession of coils. Extremely large boilers providing hundreds of horsepower to operate factories can potentially demolish entire buildings. Fire-tube boilers usually have a comparatively low rate of steam production, but high steam storage capacity. Thin and brittle metal shells can rupture, while poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized steam. Special methods of coupling steam pipes together are used to prevent leaks, with very high pressure systems employing welded joints to avoided leakage problems with threaded or gasketed connections.
If feed water is then sent into the empty boiler, the small cascade of incoming water instantly boils on contact with the superheated metal shell and leads to a violent explosion that cannot be controlled even by safety steam valves. Water tube boilers can be designed to exploit any heat source and are generally preferred in high-pressure applications since the high-pressure water/steam is contained within small diameter pipes which can withstand the pressure with a thinner wall. Historically, boilers were a source of many serious injuries and property destruction due to poorly understood engineering principles. The design of any superheated steam plant presents several engineering challenges due to the high working temperatures and pressures. For the professionals involved in a project, BIM enables a virtual information model to be shared by the design team (architects, landscape architects, surveyors, civil, structural and building services engineers, etc.), the main contractor and subcontractors, and the owner/operator. In addition to this, fire safety plans can also provide specialized information that, in the case of a hospital fire, can provide information about the location of things like the nuclear medicine ward. In case of a locomotive-type boiler, a boiler barrel extends from the firebox and the hot gases pass through a bundle of fire tubes inside the barrel which greatly increases the heating surface compared to a single tube and further improves heat transfer.
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