###### Abstract  
In boiler houses, the biggest heat energy losses are caused by flue gas being released into the atmosphere. Installation of condensing heat exchangers allows reducing the temperature of the flue gas being released, condensation of water vapor, and, thus, efficient use of the waste heat. There are many investigations of average heat transfer in different types of condensing heat exchangers. They indicate also that the cooling water flow rate and its temperature are important parameters defining water vapor condensation efficiency. Investigations of local condensation heat transfer in condensing heat exchangers are very limited. Only recently experimental investigations of the flue gas temperature and Re number effect on local condensation heat transfer in the model of the condensing heat exchanger at a constant cooling water flow rate and its temperature have started being published. In this paper, for the first time, detailed experimental investigations of the cooling water flow rate and its temperature effect on local condensation heat transfer of the water vapor from the flue gas in the model of the condensing heat exchanger (long vertical tube) are presented. The results revealed that at higher flue gas Re\({}_{\text{in}}\), the effect of the cooling water flow rate and its temperature has a stronger impact on local heat transfer distribution along the test section. biofuel flue gas; condensation; vertical tube; cooling water flow rate; inlet temperature; local total heat transfer 202212412650 2022122412650 2022122412650 2022122 2022122 2022122 2022122 2022122 2022122 202122 202212 2022122 202212 202212 2022122 202212 202212 202212 202212  
usually takes place in the initial part of the test sections where the largest quantity of water vapor is condensed, and therefore high heat and mass transfer rates are obtained there. As for flue gas, the situation is the opposite since the gas typically contains more non-condensable gas and less water vapor. Unfortunately, studies devoted to the condensation process of water vapor from flue gas and especially the impact of the cooling water flow rate and its inlet temperature are rather limited. The analytical and experimental analyses of heat and mass transfer processes in a horizontal flue gas condensing heat exchanger, which was composed of a few sections, are presented in [12]. Rather good agreement was obtained between predicted and experimentally measured values. The authors also analyzed the influence of the cooling water inlet temperature on total condensation efficiency. The obtained results showed that the dependency of condensation efficiency on the cooling water inlet temperature is rather strong. An increase in the cooling water mass flow rate resulted in a gradual increase (up to \(\approx 10\%\)) in the condensation efficiency, which was, however, limited to a certain cooling water flow rate. After a certain cooling water mass flow rate was achieved, no further increase in it occurred because of the increased condensation efficiency. The results presented in [13] also show that among various inlet parameters, the cooling water flow rate and temperature have a pronounced effect on average heat transfer (HT) even in a short (x/d \(\approx 9\)) finned tube with a flue gas Re number in the range of 4200-12,800. Flue gas condensation experiments in a horizontal bundle-type heat exchanger showed that the average heat transfer increases sharply when the cooling water flow rate was increased from 3.3 L/min to about 10 L/min. However, when the cooling water flow rate exceeds 10 L/min, the increase in the total heat transfer slows down. Based on that, it was concluded that the excessive cooling water flow rate is not able to enhance HT. The influence of the cooling water flow rate on the condensate flow and average heat transfer was also determined.