## 3 Results
### Effect of the Cooling Water Temperature  
To reveal the effect of cooling water temperature on the distribution of temperatures along the tube and on the local Nu\({}_{\rm t}\) number, an additional series of experiments was performed with the cooling water inlet temperature (t\({}_{\rm cw\mathchar 22\mathchar 22\mskip-2.0 mu \)in}\) of about 14-15 \({}^{\circ}\)C and the flow rate of 120 kg/h (\(\approx\)2 L/ min). There are no significant differences between the results obtained for higher and lower cooling water inlet temperatures (cf. Figures 2b and 8a): the temperature values are only slightly higher in Figure 8a. Moreover, the difference in the temperatures of the flue gas, the tube wall, and the water in the case of a higher cooling water inlet temperature is smaller than in the case of a lower cooling water inlet temperature. Smaller temperature differences resulted in the decreased influence of condensation heat transfer, and, therefore, from x/d = 0 until about x/d = 100, the Nu\({}_{\rm t}\) was smaller in comparison to a lower cooling water inlet temperature (Figure 9). From x/d \(\approx\) 100 until the tube end, the Nu\({}_{\rm t}\) was decreasing and was the same for both water inlet temperatures. Figure 8: Temperature distribution in the heat exchanger at \(\text{Re}_{\text{in}}\approx 9500\), at flue gas temperatures (**a**) \(\text{t}_{\text{in}}\approx 83\) \({}^{\circ}\)C, (**b**) \(\text{t}_{\text{in}}\approx 115\) \({}^{\circ}\)C, and at similar cooling water inlet temperatures: (1) center of the tube, (2) the inner wall of the tube, (3) cooling water, (4) the dew point temperature. Figure 7: Comparison of the local Nu\({}_{\text{t}}\) distribution in the heat exchanger at \(\text{Re}_{\text{in}}\approx 21,000\), different flue gas inlet temperatures and cooling water flow rates. Figure 9: Distribution of the local Nu\({}_{\text{t}}\) in the heat exchanger at \(\text{Re}_{\text{in}}\approx 9500\) and at different cooling water inlet temperatures. Condensation efficiencies for the cases with water inlet temperatures of \(\approx\)8 \({}^{\circ}\)C and \(\approx\)14 \({}^{\circ}\)C did not differ significantly and were about 60 and 50%, respectively, indicating that a more intense condensation was obtained at a lower cooling water inlet temperature. Temperature distribution results obtained for a higher flue gas inlet temperature presented in Figure 8b do not indicate any drastic changes in comparison with similar flue gas inlet temperatures, but lower cooling water temperature (Figure 2d). Here (Figure 8b) it should also be noted that the tube wall and water temperatures are by a few degrees higher than at a lower water inlet temperature (Figure 2d). Only negligible differences in the Nu\({}_{\rm t}\) obtained in the case of a lower and a higher cooling water inlet temperatures were determined from the beginning of the tube until x/d \(\approx\) 90-100 (Figure 10). After that, Nu\({}_{\rm t}\) was decreasing. However, when the cooling water inlet temperature was lower, the Nu\({}_{\rm t}\) stabilized and remained in the range of 78-88, while when the temperature was higher, the Nu\({}_{\rm t}\) decreased sharply until the end of the tube. Hence, the results show that only a certain length of the pipe is involved in higher condensation heat transfer. The condensation efficiencies in this case do not differ significantly and for t\({}_{\rm cw-in}\approx\) 8 \({}^{\circ}\)C, it is \(\approx\)58% and for t\({}_{\rm cw-in}\approx\) 15 \({}^{\circ}\)C it is \(\approx\)52%. The comparison of the Nu\({}_{\rm t}\) distribution in the condensing heat exchanger (Figure 11) shows that it remains almost constant until about x/d \(\approx\) 80-90, and starts to decrease farther on. Hence, only a certain length of the heat exchanger is efficiently used for condensation heat transfer. Figure 11: Comparison of the local Nu\({}_{\rm t}\) distribution in the heat exchanger at Re\({}_{\rm in}\approx\) 9500, at different flue gas inlet and cooling water temperatures.