## 3 Results  
### Effect of the Cooling Water Flow Rate  
Figure 2 presents variations of temperatures along the experimental model at \(\text{Re}_{\text{in}}\approx 9500\)), different inlet flue gas temperatures and in cases when the cooling water flow rate was 60 kg/h and 120 kg/h. The dew point temperature (\(\text{t}_{\text{dp-in}}\)) was calculated as indicated in [24]. The obtained \(\text{t}_{\text{dp-in}}\) was in the range between \(\sim\)59 and 63 \({}^{\circ}\)C. The inlet cooling water temperature (\(\text{t}_{\text{cw-in}}\)) measured in the inlet water mixer was about 8-9 \({}^{\circ}\)C.  
From the results shown in Figure 2, it is clear that the dew point temperature (curve 4) is well above tube wall temperature (curve 2), and hence water vapor condensation should start on the internal wall of the tube already from the beginning of the tube. This was confirmed by the heat transfer results (\(\text{Nu}_{\text{t}}\), see Figure 3). As Figure 2a shows, the flue gas temperature (curve 1) rapidly decreased by approx. 20 \({}^{\circ}\)C from x/d = 0 to x/d \(\approx\) 25, and then only a small temperature decrease between x/d \(\approx\) 25-40 was obtained. After that, the temperature gradually decreased until the end of the tube. Then, as some amount of vapor had condensed and the temperature difference between the tube wall and flue gas had decreased (Figure 2a, curves 1 and 2), the heat transfer also started to decrease. Nonetheless, it was still high and at x/d \(\approx\) 170 of the tube, the \(\text{Nu}_{\text{t}}\approx\) 110 (Figure 3a). Curve 3 (see Figure 2a) shows that the cooling water temperature was increasing from \(\mathrm{x}/\mathrm{d}\approx 170\) (inlet) until \(\mathrm{x}/\mathrm{d}=0\) (the outlet). The change in temperature until \(\mathrm{x}/\mathrm{d}\approx 85\) was  
Figure 3: Variation of the \(\mathrm{Nu_{t}}\) along the heat exchanger at \(\mathrm{Re_{in}}\approx 9500\), different inlet temperatures (smaller (**a**), higher (**b**)) as well as cooling water flow rates. Figure 2: Temperature variation in the heat exchanger at \(\mathrm{Re_{in}}\approx 9500\), different inlet flue gas temperatures and cooling water flow rates of 60 kg/h (**a**,**c**) and 120 kg/h (**b**,**d**): (1) flue gas in the tube center, (2) the inner wall of the tube, (3) cooling water, (4) the dew point temperature. not significant; however, it became more expressed from \(\rm x/d\approx 85\). This means that the heat gained by the water in this part of the condensing heat exchanger was rather high due to the prevailing condensation process and high heat transfer (Figure 3a; Nu\({}_{\rm t}\) varied in the range of 110-150). In this case, the water temperature increase through all the test section was about 21 \({}^{\circ}\)C, i.e., from \(\approx 9\)\({}^{\circ}\)C to \(\approx 30\)\({}^{\circ}\)C.  
The results of temperature distributions for almost the same flue gas inlet parameters, but at a higher cooling water flow rate are presented in Figure 2b. For the flue gas temperature (Figure 2b, curve 1), two clear zones of the linear temperature change could be distinguished: one is from \(\rm x/d=0\) until \(\rm x/d\approx 70\), where a steep temperature decrease is obtained, and the other from \(\rm x/d\approx 70\) until \(\rm x/d\approx 170\), where much less of a decrease is obtained. Figure 2b (see curves 2 and 3) shows that the temperatures of the cooling water and tube wall have similar characters. The water temperature increase in this case (Figure 2b, curve 3) is not as clearly expressed as it was with a lower cooling water flow rate (Figure 2a, curve 3). The cooling water temperature increases from the beginning of the section, i.e., from \(\rm x/d\approx 170\), and until \(\rm x/d\approx 70\) the increase is only by 3-4 \({}^{\circ}\)C (Figure 2a, curve 3).