The invention relates to a flue tube of a condensing heat exchanger to be used in central heating systems and tap water systems.

There are known heat exchangers that are equipped with means for turbulence stimulation in flue gas flowing therethrough, intensifying the heat exchange process.

European patent application EP2384837 describes a heat exchanger tube with a non-circular cross section, especially rectangular, formed of a tube of a circular cross section, and having a corrugated surface in the longitudinal and transverse direction, with a corrugation amplitude varying between 0.2 and 1.2 times the outer diameter of the circular tube.

European patent application EP1429085 describes a tube with a variable cross section along its length and the planar portion in the central area.

The aim of the invention is to develop a flue tube for a fired condensing heat exchanger, intensifying the heat exchange process while reducing flue gas flow resistance, and preserving the condensing nature of the phenomena occurring in the heat exchanger. In particular, the aim is to modify the circular shape of the tube surface, by generating major flow disturbances in flue gas, with little deformation to minimise stresses resulting from deformation, which directly affects the life cycle of the heat exchanger.

This aim is achieved by developing a new geometry of the flue tube.

A flue tube of a condensing heat exchanger, characterised in that it has embossed pits formed along the length of the tube, and said embossed pits are pointed towards the centre of the tube, and said pits are situated opposite each other, and two opposite embossed pits create a section, where the distance between the tips of the pits measured inside the tube in the section being 1.0 mm at most, and the ratio of the length of the tube to its cross-sectional circumference washed by flue gas is between 2.5 and 6.5, and the tube at the top and bottom has a cylindrical shape.

Preferably, the upper cylindrical part of the tube has a length of between 0.25 and 1.5 of its circumference of the cross section washed by flue gas. Preferably, the arrangement of adjacent sections of embossed pits are not collinear along the length of the tube, and in particular, they are arranged at an angle of 90° or 45° to one another.

Sections of the pits are preferably arranged uniformly along the length of the tube, or the distance between the sections of the pits decreases along the length of the tube.

Preferably, the pits have a circular, oval or drop-like shape.

Preferably, the cross section of the tube between adjacent sections has a circular shape or square shape with rounded vertices and sides curved towards the axis.

The developed proportions and arrangement of pits in accordance with the invention preserves the condensing nature of the phenomena occurring in the heat exchanger, while reducing the flow resistance of flue gas through the tube and increasing the flow turbulence.

Further benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings.

In the drawings:

Fig. 1 shows an isometric view of the tube, with drop-shaped pits evenly spaced along the length of the tube;

Figs. 2 and 3 - the tube as in Fig. 1 , in a side view;

Fig. 4 - the tube as in Fig. 1 , in a top view;

Figs. 5 and 6 - the tube as in Fig. 1 , in an axial section;

Fig. 7 - the tube as in Fig. 1 , in a cross section;

Fig. 8 - the tube in isometric view, with circular-shaped pits evenly spaced along the length of the tube;

Fig. 9 and Fig. 10 - the tube as in Fig. 8, in a side view;

Fig. 11 - the tube as in Fig. 8, in a top view;

Fig. 12 and Fig. 13 - the tube as in Fig. 8, in an axial section;

Fig. 14 - the tube as in Fig. 8, in a cross section;

Fig. 15 - the tube in an isometric view, with oval-shaped pits evenly spaced along the length of the tube;

Fig. 16 and Fig. 17 - the tube as in Fig. 15, in a side view;

Fig. 18 - the tube as in Fig. 15, in a top view;

Figs. 19 and 20 - the tube as in Fig. 15, in an axial sectional view;

Fig. 21 - the tube as in Fig. 15, in a cross section;

Fig. 22 - the tube in an isometric view, with drop-shaped pits evenly spaced along the length of the tube, in a different version;

Figs. 23 and 24, the tube as in Fig. 22, in a side view;

Fig. 25 - the tube as in Fig. 22, in a top view;

Fig. 26 and Fig. 27 - the tube as in Fig. 22, in an axial sectional view;

Fig. 28 - the tube as in Fig. 22, in a cross-sectional view;

Fig. 29 - the tube in an isometric view, with circular-shaped pits evenly spaced along the length of the tube, in a different version;

Figure 30 and Fig. 31 - the tube as in Fig. 29, in a side view;

Fig. 32 - the tube as in Fig. 29, in a top view;

Fig. 33 and Fig. 34 - the tube as in Fig. 29, in an axial sectional; Fig. 35 - the tube as in Fig. 29, in a cross-sectional view;

Fig. 36 - the tube in an isometric view, with oval-shaped pits evenly spaced along the length of the tube, in a different version;

Figs. 37 and 38 - the tube as in Fig. 36, in a side view;

Fig. 39 - the tube as in Fig. 36, in a top view;

Fig. 40 and Fig. 41 - the tube as in Fig. 36, in an axial sectional view;

Fig. 42 - the tube as in Fig. 36, in a cross-sectional view;

Fig. 43 - the tube in an isometric view, with drop-shaped pits evenly spaced along the length of the tube;

Fig, 44 and Fig. 45 - the tube as in Fig. 43, in a side view;

Fig. 46 - the tube as in Fig. 43, in a top view;

Fig. 47 and Fig. 48 - the tube as in Fig. 43, in an axial sectional view;

Fig. 49 - the tube as in Fig. 43, in a cross-sectional view;

Fig. 50 - the tube in an isometric view, with circular-shape pits unevenly spaced along the length of the tube;

Fig. 51 and Fig. 52 - the tube as in Fig. 50, in a side view;

Fig. 53 - the tube as in Fig. 50, in a top view;

Fig. 54 and Fig. 55 - the tube as in Fig. 50, in an axial sectional view;

Fig. 56 - the tube as in Fig. 50, in a cross-sectional view;

Fig. 57 - the tube in an isometric view, with oval-shaped pits unevenly spaced along the length of the tube;

Fig. 58 and Fig. 59 - the tube as in Fig. 57, in a side view;

Fig. 60 - the tube as in Fig. 57, in a top view;

Fig. 61 and Fig. 62 - the tube as in Fig. 57, in an axial sectional view;

Fig. 63 - the tube as in Fig. 57, in a cross-sectional view;

Fig. 64 - the tube in an isometric view, with drop-shaped pits evenly spaced along the length of the tube, in a different version;

Fig. 65 and Fig. 66 - the tube as in Fig. 64, in a side view; Fig. 67 - the tube as in Fig. 64, in a top view;

Fig. 68 and Fig- 69 - the tube as in Fig. 64, in an axial sectional view;

Fig. 70 - the tube as in Fig. 64, in a cross-sectional view;

Fig. 71 - the tube in an isometric view, with circular-shaped pits unevenly spaced along the length of the tube, in a different version;

Fig. 72 and Fig. 73 - the tube as in Fig. 71 , in a side view;

Fig. 74 - the tube as in Fig. 71, in a top view;

Fig. 75 and Fig. 76 - the tube as in Fig. 71 , in an axial sectional view;

Fig. 77 - the tube as in Fig. 71, in a cross-sectional view;

Fig. 78 - the tube in an isometric view, with oval-shaped pits unevenly spaced along the length of the tube, in a different version;

Fig. 79 and Fig. 80 - the tube as in Fig. 78, in a side view;

Fig. 81 - the tube as in Fig. 78, in a top view;

Fig. 82 and Fig. 83 - the tube as in Fig. 78, in an axial sectional view; Fig. 84 - the tube as in Fig. 78, in a cross-sectional view.

A flue tube of a condensing heat exchanger, in its example embodiment, has embossed pits directed towards the inside of the tube 1. At a predefined height, the flue tube 1 has two embossed pits 2 facing each other and forming together a separate section. The distance between the tips of the pits in the tube in the section is 0.5 mm, and the ratio of flue tube 1 length L to its cross-sectional circumference washed by flue gas is 3.5. Adjacent sections of pits 2 are located along the tube 1 at an angle of 90° to one another. The tube 1 at the top is cylindrical, with a length H which is 1.0 times its cross-sectional circumference washed by flue gas.

In the embodiments shown in Figs. 1 to 7 and Figs. 22 to 28, the pits 2 are drop-shaped; in other variants shown in Figs. 8 to 14 and Figs. 29 to 35, the pits 2 are circular in shape; and in another, shown in Figs. 15 In these embodiments of the invention, the sections of the pits 2 are evenly arranged along the length L of the tube 1.

In other embodiments of the invention, shown in Figs. 43 to 49 and Figs. 64 to 70 with drop-shaped pits, Figs. 50 to 56 and Figs. 71 to 77 with circular pits, and Figs. 57 to 63 and Figs. 78 to 84 with oval pits, the sections of the pits 2 are arranged unevenly along the length L of the tube in such a way that the distance S between the sections decrease along the length of the tube.

In the embodiments of the invention, described above and shown in Figs. 1 to 21 and Figs. 43 to 63, the cross section of the tube 1 between adjacent sections has a circular shape, and for those shown in Figs. 22 to 42 and Figs. 64 to 84, it forms a square with rounded vertices and sides curved towards the axis.

In other variants of the embodiment described above, the distance between the tips of the embossed pits 2 inside the flue tube 1 in the section cannot be greater than 1.0 mm, and the ratio of flue tube 1 length L to its cross-sectional circumference washed by flue gas is between 2.5 and 6.5, and the length L of the upper cylindrical part of the flue tube 1 is between 0.25 and 1.5 times the circumference of its cross section washed by flue gas.

It was found that for a flue gas temperature of 1 450°C to 1 550°C, at the inlet to the flue tube and with a flow rate of 0.83 - 0.89 kg h of flue gas in the tube (for natural gas, C0 ₂ = 9%) and an initial temperature of 30°C and dT = 20 K of heated liquid, with a counterflow of 22 - 26 1/h for the tube, the flue gas at a distance of 155 - 225 mm from the inlet to the tube reaches the dew point, and the pressure drop is not more than 375 Pa.