###### Abstract
The cooling water temperature increase from 10 \({}^{\circ}\)C to 16 \({}^{\circ}\)C resulted in the condensate flow rate decreasing by approximately 12%, but the average heat transfer coefficient increased by about 15% [14]. The authors did not explain the reason for the heat transfer coefficient increase. The condensation of air and vapor mixture, simulating flue gas, in a rather short vertical pipe (x/d \(\approx 70\)) was investigated in [15]. The volumetric water vapor fraction in the experiments was \(\approx\)17%. It was obtained that condensation efficiency in such a case was only about 30%, but the determined total average HT appeared to be twice as high as the heat transfer in dry air experiments (i.e., with no condensation). Simulated flue gas condensation results in a vertical tube of x/d \(\approx 32\) with a water vapor mass fraction of about 5-12% and the inlet Reynolds number of 4600-14,000 were presented in [16]. It was obtained that the average condensation heat transfer coefficient for a lower inlet Re number as well as for the water vapor mass fraction of about 5% was very similar to the sensible heat transfer coefficient. When the mass fraction was increased to 12%, the obtained average HT coefficient became five times higher than the sensible HT coefficient. Experimental and theoretical results allowed concluding that condensation heat transfer starts dominating if there is an increase in vapor mass fraction and the flue gas Re number. Similar conclusions were obtained in [17; 18], etc. Results obtained in the experimental set-up (a vertical tube as a mock-up of a condensing heat exchanger) in [19] allowed to conclude that the temperature and the humidity of inlet flue gas significantly affect local heat transfer. Experiments showed that there were regimes, when convection was prevailing in the initial part of the heat exchanger, and, hence, the heat transfer was not efficient there. The total local HT started increasing once the flue gas temperature reached the dew point temperature. The injection of water before the experimental set-up additionally moistened the flue gas, and, therefore, much better conditions were created for condensation. The water injection into the flue gas flow significantly changed the variation of temperatures in the heat exchanger [20]. The injected water caused the local total heat transfer increase by at least a few times in comparison to the case without injection. The impact from different temperatures of the injected water on the local total heat transfer was recorder only until the middle of the test section. In these studies, however, no investigations of effect of the temperature and flow rate of the cooling water on HT were performed. The results of theoretical and experimental investigations of water vapor condensation in a vertical tube (x/d \(\approx\) 80) from the flue gas obtained by incinerating natural gas are presented in [21]. The experiments were done at rather small Reynolds numbers, i.e., the Re number was varied from about 2500 to about 5000. The results showed that the average Nusselt number for various cooling water temperatures and flue gas Re numbers was between 35 and 70. It was also emphasized that the wall temperature was an important factor as it had a significant influence on the condensation rate and heat transfer. Thus, the literature review shows that the investigations are mainly oriented to the estimation of the average heat transfer in relatively short flue gas condensers. But these investigations also indicate that the cooling water flow rate and its temperature are important parameters defining water vapor condensation efficiency. The condensing heat exchangers used in the industry are rather long (usually x/d \(>\) 100) and local total HT investigations in such condensing heat exchangers are very limited.