Natural-circulation boiler

Table A.2 [1] shows Hitachi s recommendations on high pressure natural circulation boilers and once-though type boilers.

Application of Low pH Coordinated Phosphate Treatment for Natural Circulation Boilers... 

Hitachi recommends applying low pH coordinated phosphate treatment for natural circulation boilers as Hitachi s standard for the following reasons. Hitachi has experienced water wall tube explosions that originated in hard zinc scale adhesion. It was thought that zinc dissociated from condensation tubes of copper alloy and deposited on water wall tubes. 

Application 150-200 bar natural circulating boiler Hitachi standard Once through super critical boiler Hitachi standard Once through super critical boiler Hitachi standard... 

Certain boilers employ forced circulation, whereby a pump helps impart the circulation through the downcomer lines to the waterwaH header, particularly to improve or control circulation at low loads. Forced-circulation pumps are also required in high pressure and supercritical pressure boilers, because once the pressure within a boiler approaches the critical pressure, 22.1 MPa (3208 psia), the densities of the water and steam become similar, limiting or eliminating the potential for natural circulation. 

Steam-Generator Circulation System Circulation systems for utility application are generally classified as natural circulation and forced or pump-assisted circulation in drum-type boilers, and as once-through now in subcritical- and supercriticaf-pressure boilers. The... 

Natural-circulation and convection boiler banks are the basic design features on which a line of standard industrial boilers has been developed to accommodate the diverse steam, water, and fuel requirements of the industrial market. 

Guerrieri, S. A. and R. D. Talty, A Study of Heat Transfer to Organic Liquids in Single-Tube Natural-Circulation Vertical Tube Boilers, presented at AIChE Heat Transfer Sym., Louisville, KY, Mar. 20 (1955). 

There are two basic types of watertube boilers assisted and natural circulation. Assisted circulation might apply where heat is from convection rather than a radiation source such as a waste heat application. Natural circulation is more suited where radiant heat and high gas temperatures are present. 

Heat transfer coefficients In modem, natural circulation, fossil fuel WT boilers, overall heat transfer coefficients vary between 5,000 and 20,000 Btu/hr/ft2/°F (28.4-113.6 kW/m2/°K). 

Boiler water (and two-phase mixtures) circulation rates Natural circulation rates vary according to design but are typically 1 to 2 m/sec (3.3-6.6 ft/sec). 

At the same time, the natural circulation that results from the combustion process provides effective cooling of all tubes, headers, drums, and other boiler surfaces. 

NOTE The downcomers, although part of the boiler steam-water circulation system, are located outside the boiler so that theFW-BW mixture flowing within the downcomers remains cool relative to the steam-BW. Thus, density differences remain and natural circulation continues. 

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. 

Figure 3.13. Reformed gas waste-heat boiler, principal features of typical natural circulation fire-tube boilers (Reprinted by permission of the Council of the Institution of Mechanical Engineers from the Proceedings of the Conference on Energy Recovery in the Process Industries, London, 1975.)...

Guemeri, S. A, and R. D. Tatty, 1956, A Study of Heat Transfer to Organic Liquids in Single Tube Natural Circulation Vertical Tube Boilers, AIChE Chem. Eng. Prog. Symp. Ser. 52(18) 69—77. (4) Gungor, K. E., and R. H. S. Winterton, 1986, A General Correlation for Flow Boiling in Tubes and Annuli, Int. J. Heat Mass Transfer 29(3) 351—358. (4)... 

The boiler shown in Fig. 15.2 relies on natural circulation. The density difference between the water plus steam in the downflow pipes and the riser pipes causes the water to circulate through the pipes. A typi-... 

Treatment of Boiler Water for Natural-Circulation Units... 

There arc four methods of internal treatment in common use on natural-circulation drum-type boilers ... 

In contrast, fire-tube boilers are much better suited for natural circulation and the steam drum can sit in piggyback-fashion, right on top of the boiler. This makes it possible to provide each boiler with its own separate steam drum, which allows greater flexibility in the plot plan. But it took some time before this boiler type was accepted in ammonia plants as the stress pattern is more complex and less predictable than in water-tube boilers. 

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