Figures 3.19a and 3.19b Show an inverted loop superheater commonly used in packaged boilers, and Fig. 3.19c Shows a horizontal tube design with vertical headers. Superheaters operate at high tube wall temperatures; hence their design should be carefully evaluated. The convective superheater design located behind several rows of screen section operates at lower tube wall temperatures than the radiant design, though the steam temperatures may be the same. Figure 3.20 shows the results from a computer program for a superheater located very close to the furnace section and beyond several rows of screen tubes.

Option a shows the results for a packaged boiler generating 150,000 lb/h of steam at 650psig when a 14-row screen section is used. The gas temperature entering the superheater is 1628°F. For the steam temperature of 758°F, the superheater tube wall temperature is 856°F. The surface area used is 1833 ft2.

In option b, a nine-row screen section is used. The gas temperature entering the superheater is 1801°F. The superheater tube wall temperature is 882°F. However, owing to the higher log-mean temperature difference, the surface area required is smaller, namely 1466 ft2. It can be shown, as discussed under life estimation below, that the difference in the life of the superheater for a 26°F difference for alloy steel tubes such as T11 can be several years. By the same token, one may wonder about the life of the radiant design with a gas inlet temperature of 2187°F. Tube sizes are typically 1.5-2in. OD, and materials used range from T11, T22, and T91 to stainless steels, depending upon steam and tube wall temperatures. Generally, bare tubes are used; however, I have designed a few packaged boilers, which are in operation in gas-fired boilers, using finned superheaters to make the design compact.

Steam velocity inside the tubes ranges from about 50 ft/s at high steam pressure (say 1000-1500psig) to about 150 ft/s at low pressure (150-200psig). The turndown conditions and maximum tube wall temperatures determine the number of streams used and hence the steam pressure drop. In inverted loop superheaters, the headers are inside the gas path and are therefore protected by refractory. A few evaporator tubes are provided in the superheater region to ensure that steam blanketing does not occur at the mud drum and that steam bubbles can escape from the mud drum to the steam drum.

Flow distribution through tubes is another concern with superheater design. If long headers are used, multiple inlets can reduce the nonuniformity in steam flow distribution through the tubes as shown in Fig. 3.21. Inlet and exit connections from the ends of headers should be avoided because they can

Result in flow distribution problems. In arrangement 1, the inlet and exit connections are on opposite ends, causing the greatest difference in static pressure at the ends of the headers, and should be avoided. Arrangement 2 is better than 1 because the flow distribution is more uniform. However, arrange­ment 3 is preferred, because the central inlet and exit reduce the differential static pressure values by one-fourth, so the flow maldistribution is minimal.

Two temperatures are of significance in the design of superheater tubes. One is the tube midwall temperature, which is used to evaluate the tube thickness per ASME code. (The published ASME stress values have increased during the last few years and therefore the latest information on stress values should be used in calculating the tube thickness.) The outer wall temperature determines the maximum allowable operating temperature, sometimes known as the oxidation limit. Table 3.6 gives typical maximum allowable temperatures for a few materials.

One can vary the tube thickness to handle the design pressure, but if the outermost tube temperature gets close to the oxidation limit, we have to review

The design. In large superheaters, different materials and tubes of different sizes may be used at different sections, depending on the tube midwall and outer wall temperatures. In all these calculations one has to consider the nonuniformity in gas flow, gas temperature across the cross section, and steam flow distribution through the tubes. Because of their shorter lengths, a few tubes could have higher flow and starve the longer tubes.