*3.2. Heat Recovery Hot Water Boiler in an Existing Plant*

A heat recovery hot water boiler with nominal thermal power of 53.3 MW in an existing combined heat and power (CHP) plant was selected as the second test case. The boiler contained two counter-flow tube bundles which were mounted in the vertical portion of the flue gas duct (i.e., the tubes are horizontal, see Figure 9). Both bundles consisted of several passes, and each pass was composed of four staggered tube rows with 48 tubes per row. In the bottom bundle, the first pass was unfinned, the second pass used plain round fins, and serrated fins were utilized in the third pass. The top bundle contained solely tubes enhanced with serrated fins. The built-up area of the heated portion of each bundle was ca. 7.6 × 4.0 m. All stream-related data presented in this article were obtained by the operator of the boiler in the course of a guarantee test.

The boiler was driven by flue gas exiting from a gas turbine. Because the temperature field (see the measurement array shown in Figure 9) was almost uniform, the corresponding boundary condition was specified in both the models discussed further as a constant. The outlet temperature of flue gas was estimated by the operator because the respective quantity had not been measured during the guarantee test. All the necessary data are summarized in Table 3.


**Table 3.** Data provided by the operator of the heat recovery hot water boiler.

<sup>1</sup> Calculated from time-dependent data obtained using the measurement array shown in Figure 9; <sup>2</sup> Estimated (not measured during the guarantee test).

**Figure 9.** Schematic of the investigated boiler. Top bundle collector was connected to the bottom bundle distributor via an external duct as indicated using the dashed lines. The temperature field measurement array, consisting of 12 (4 × 3) sensors, was only installed in the duct during the guarantee test.

The simulation carried out using the developed computer code mentioned in Section 2.3 included both water distribution in the tube side and heat transfer between the flue gas and water. To assess the accuracy of the predicted temperatures, the boiler was also analyzed using an industry-standard tool, namely, HTRI Xchanger Suite. Please note that, with respect to the requests of the manufacturer of the boiler and the operator of the CHP plant, no other data regarding the apparatus can be explicitly specified in this article. For the same reason, neither the HTRI Xchanger Suite case files nor the simplified 3-D CFD model of the flue gas duct discussed in the following sections can be made available.
