Steam generation in boiler tubes is based on nucleate boiling. This means that a multiple of small steam bubbles are generated on the inside wall of the tube which remains "wet". If steam generation in any portion of the tube becomes excessive, film boiling results, which increases resistance to heat transfer and elevates the tube temperature. Steam blanketing can occur at low heat fluxes, in the region of nucleate boiling, if the steam bubbles formed are not continuously removed. At heat flues of 400,000 Btu/hr/ft2 and above, nucleate boiling changes to film boiling. At this point, even the most vigorous circulation cannot prevent the formation of an insulating steam film on the heating surface. To provide a reasonable factor of safety, HRSG's should be designed for a maximum heat flux of 100,000 Btu/hr/ft2.
The operating pressure of the evaporator also has a direct relation to the required circulation to achieve nucleate boiling. Since at a low pressure, the specific volume of the vapor is greater than at higher pressures, the circulation ratio must be greater at the lower pressure than at the higher. So, where a 10 :
ratio may be satisfactory for a system operating at 600 psia, you may need a 25 : 1 ratio for a 100 psia system.For more specific information on circulation ratios and design of different configurations, see the Forced Circulation and Thermosyphon Circulation sections.
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