## 2 Materials and Methods  
The investigations of the flue gas water vapor condensation at different inlet cooling water flow rates and temperatures were done using the experimental setup shown in Figure 1. Wood pellets were incinated in the automatic boiler (50 kW). The incineration-generated flue gas flew from the boiler to the experimental section. To obtain different flue gas inlet temperatures at the exit from the boiler, the economizer of the boiler was used. The test section was composed of two long vertical stainless steel tubes. The internal calorimetric tube was about 5.8 m long, 0.034 m wide (x/d \(\approx\) 170), with its wall thickness of 2 mm. The internal tube (in which the condensation took place) was surrounded by an outer tube, which was about 5.9 m long and 0.108 m wide. The flue gas flow rate entering the internal vertical calorimetric tube was adjusted by valves. After passing the calorimetric tube, the flue gas was discharged to the atmosphere through the chimney. The chimney and the test section were insulated with Rockwood insulation (50 mm thickness). The cooling of the test section during the first series of experiments was ensured with water from the municipal network. The water was supplied into the gap between the two tubes, and was of a relatively low temperature (8-9 \({}^{\circ}\)C). The passed water was discharged into the sewage system. In order to have a higher inlet cooling water temperature (14-15 \({}^{\circ}\)C), the second series of experiments was performed. During the second series of experiments, the water from the storage tank was supplied by the water pump. It circulated through the coil-type air-cooled heat exchanger in a closed circuit (Figure 1). The water flow rate was measured at its discharge by weighting. The required flow rate was regulated by a corresponding valve. To have uniform cooling water inlet (when entering test section) and outlet (when leaving test section) temperatures, the water was mixed in the mixers (see Figure 1). The water condensed during the experiments was collected in a collection vessel. More details of the experimental setup are presented in [19]. Experiments were performed at two Reynolds numbers (Re \(\approx\) 9500 and Re \(\approx\) 21,000) at the inlet into the calorimetric tube, at two different cooling water flow rates (about 60 and 120 kg/h), at two different cooling water inlet temperatures (about 8-9 \({}^{\circ}\)C and 14-15 \({}^{\circ}\)C), and at different flue gas inlet temperatures. The water vapor mass fraction in the flue gas was between 15 and 16%. Such a fraction was achieved by water spraying into the furnace of the boiler. About 20 calibrated chromel-copel thermocouples were mounted in the center of the calorimetric tube. These thermocouples measured the flue gas temperature along the tube. Furthermore, 20 thermocouples were also mounted along the inner wall of the calorimetric tube. Ten thermocouples were mounted in the middle between the tubes for cooling water temperature measurement. Each water mixer was installed with three thermocouples. Keithley automatic data acquisition system was used for the data collection from the thermocouples. Measurements of inlet relative humidity (RHin, %) and inlet flue gas temperature (tin, \({}^{\circ}\)C) were recorded with a KIMO C310 sensor. To measure the flue gas flow rate at the inlet, a bellmouth with Pitot-Prandtl's tubes connected to the micromanometer was used. The cooling water flow rate was determined by weighting. The properties of flue gas (mixture of non-condensable gas and water vapor) in the equations below were obtained using formulas for mixture properties calculation [22]. Local properties were determined using the flow temperature measured in the center of the inner tube.