This section applies to steam generators heated by process fluids with temperatures less than 1000°F.…
Very High Temperature Waste Heat Steam Generators
High temperature waste heat steam generators are services where the heating gas is over 1000°F and can cause rapid failure of pressure containing parts that are not continuously water wetted. High temperature in-shell steam generation is discussed below for horizontal units, and in Section 390 for vertical units. This manual does not address tube side steam generation. Figure 300-8 is a schematic of a high temperature horizontal shell side steam generator.
This type of unit is used in hydrogen, ammonia and sulfur plants. Inlet gas temperatures range from about 1500°F to 3000°F. The bypass pipe shown in Figure 300-8 is required in hydrogen and ammonia plants to control downstream temperature. Bypass is not used in sulfur plants.
Shell side circulation is driven by the density difference between the fluid in the annular gap (mostly liquid) and the lighter two-phase mixture in the bundle (internal circulation). For best circulation, 90 degree square tube layout is used. The annular gap should be about 10% of the bundle diameter.
Operation is limited by “dryout” just downstream of inlet tube ferrules near the center of the bundle. When dryout occurs, the tube temperature jumps to within a few hundred degrees of process inlet temperature, and the tube may fail within minutes in very high temperature units. In lower temperature units, local dryout causes local deposition of solids, restricted water circulation, more extensive dryout, and eventual tube failure a few months after initial dryout.
Dryout heat flux in submerged bundles increases with process gas temperature and pressure, and with the ratio of heat transfer surface area to peripheral flow/outflow area. Appendix E provides methods to evaluate thermally induced dryout, and calculate maximum recommended heat flux.
Water concentration is maximum at the same location where dryout is most likely to occur (central tubes near ends of ferrules). Blowdown should be located close to this region.
Higher quality water is required in horizontal shell side steam generators with internal circulation than with external circulation because the maximum percentage of vapor is higher. Within the recommended operating range (without dryout), water is about 25% more concentrated than in systems designed for a maximum of 5% vaporization.
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