The invention concerns a flue tube of a condensing heat exchangers designated for central heating and utility water installation.

Known are heat exchanger tubes fitted with devices to turbulise the combustion gases flowing inside the tubes, which intensify the heat exchange process.

Known from published European patent application EP2384837 is a heat exchanger tube of other-than-circular shape in cross-section, rectangular in particular, made of a tube circular in cross-section and having undulated surface with the undulation spreading both sideways and lengthways and the amplitude of the undulation ranging from 0.2 to 1.2 of the inner diameter of the tube circular in cross-section.

Known from the published European patent application EP 1429085 is a tube the shape of which in cross section changes along its length and with a flat section in the centre.

The purpose of the invention is to develop a flue tube for a fired condensing heat exchanger which would intensify the heat exchange process while reducing the flow resistance of the combustion gas and retaining the condensing nature of the processes which occur in the exchanger.

The purpose was achieved by developing a new geometry of the flue tube.

A flue tube of a condensing heat exchanger according to the invention is characterised in that it features dents formed along the length of the tube and said dents are pointed towards the centre of the tube, and said dents are positioned opposite each other, and two opposite dents mark out a section, where the distance between the lowest points in the dents measured inside the tube in the section is 1.0 mm or less, and the ratio of the length of the tube to the tube cross section circumference around which the combustion gases flow falls within the range from 2.5 to 6.5.

Preferably, the tube is cylindrical in shape in its top and/or bottom section(s).

Preferably, the length of the cylindrical top section of the tube ranges from 0.25 to 1.5 of the tube's cross section circumference around which the combustion gases flow.

Preferably, the adjacent dent sections are arranged otherwise than collinearly to each other along the tube length, in particular at the angle of 90°.

Preferably, the dent sections are arranged at even distances along the tube length, or the distances between the dent sections decrease along the tube length.

Preferably, the dents are circular, oval, or drop-like in shape.

Preferably, the cross section of the tube in the fragment between the adjacent sections is circular, oval, or square in shape, with rounded apexes and sides indented towards the axis.

The designed proportions between the tube dimensions and the shape and arrangement of the dents according to the invention ensures retaining the condensing nature of the processes which occur in the exchanger while reducing the flow resistance of the combustion gases inside the tubes and increasing the flow turbulences.

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.l shows the tube in the isometric projection with drop-shaped dents arranged at even distances along the tube length;

Fig.2 and Fig.3 show the tube as in Fig. 1 in side view;

Fig.4 depicts the tube as in Fig. 1 in bird's eye view;

Fig.5 and Fig.6 illustrate the tube as in Fig. 1 in axial section;

Fig.7 shows the tube as in Fig. l in cross section;

Fig.8 shows the tube in the isometric projection with circular dents arranged at even distances along the tube length;

Fig.9 and Fig.10 show the tube as in Fig. 8 in side view;

Fig.l 1- depicts the tube as in Fig. 8 in bird's eye view;

Fig.12 and Fig.13 illustrate the tube as in Fig. 8 in axial section;

Fig.14 shows the tube as in Fig.8 in cross section;

Fig.15 shows the tube in the isometric projection with oval dents arranged at even distances along the tube length;

Fig.16 and Fig.17 show the tube as in Fig. 15 in side view;

fig.18 depicts the tube as in Fig. 15 in bird's eye view;

Fig.19 and Fig.20 illustrate the tube as in Fig. 15 in axial section;

Fig.21 shows the tube as in Fig.15 in cross section;

Fig.22 shows the tube in the isometric projection with drop-shaped dents arranged at even distances along the tube length in another variant;

Fig.23 and Fig.24 show the tube as in Fig. 22 in side view;

Fig.25 depicts the tube as in Fig. 22 in bird's eye view;

Fig.26 and Fig.27 illustrate the tube as in Fig. 22 in axial section;

Fig.28 shows the tube as in Fig.22 in cross section;

Fig.29 - shows the tube in the isometric projection with circular dents arranged at even distances along the tube length in another variant;

Fig.30 and Fig.31 show the tube as in Fig. 29 in side view;

Fig.32 depicts the tube as in Fig. 29 in bird's eye view; Fig.33 and Fig.34 illustrate the tube as in Fig. 29 in axial section;

Fig.35 - shows the tube as in Fig.29 in cross section;

Fig.36 shows the tube in the isometric projection with oval dents arranged at even distances along the tube length in another variant;

Fig.37 and Fig.38 show the tube as in Fig. 36 in side view;

Fig.39 depicts the tube as in Fig. 36 in bird's eye view;

Fig.40 and Fig.41 illustrate the tube as in Fig. 36 in axial section;

Fig.42 shows the tube as in Fig.36 in cross section.

Fig.43 shows the tube in the isometric projection with drop-shaped dents arranged at uneven distances along the tube length;

Fig.44 and Fig.45 show the tube as in Fig. 43 in side view;

Fig.46 depicts the tube as in Fig. 43 in bird's eye view;

Fig.47 and Fig.48 illustrate the tube as in Fig. 43 in axial section;

Fig.49 shows the tube as in Fig.43 in cross section;

Fig.50 shows the tube in the isometric projection with circular dents arranged at uneven distances along the tube length;

Fig.51 and Fig.52 show the tube as in Fig. 50 in side view;

Fig.53 depicts the tube as in Fig. 50 in bird's eye view;

Fig.54 and Fig.55 illustrate the tube as in Fig. 50 in axial section;

Fig.56 shows the tube as in Fig.50 in cross section;

Fig.57 shows the tube in the isometric projection with oval dents arranged at uneven distances along the tube length;

Fig.58 and Fig.59 show the tube as in Fig. 57 in side view;

Fig.60 depicts the tube as in Fig. 57 in bird's eye view;

Fig.61 and Fig.62 illustrate the tube as in Fig. 57 in axial section;

Fig.63 shows the tube as in Fig.57 in cross section.

Fig.64 shows the tube in the isometric projection with drop-shaped dents arranged at uneven distances along the tube length in another variant; Fig,65 and Fig, 66 show the tube as in Fig. 64 in side view;

Fig.67 depicts the tube as in Fig. 64 in bird's eye view;

Fig.68 and Fig.69 illustrate the tube as in Fig. 64 in axial section;

Fig.70 shows the tube as in Fig.64 in cross section;

Fig.71 shows the tube in the isometric projection with circular dents arranged at uneven distances along the tube length in another variant;

Fig.72 and Fig.73 show the tube as in Fig. 71 in side view;

Fig.74 depicts the tube as in Fig. 71 in bird's eye view;

Fig.75 and Fig.76 illustrate the tube as in Fig. 71 in axial section;

Fig.77 shows the tube as in Fig.71 in cross section;

Fig.78 shows the tube in the isometric projection with oval dents arranged at uneven distances along the tube length in another variant;

Fig.79 and Fig.80 show the tube as in Fig. 78 in side view;

Fig.81 depicts the tube as in Fig. 78 in bird's eye view;

Fig.82 and Fig.83 illustrate the tube as in Fig. 78 in axial section;

Fig.84 shows the tube as in Fig.78 in cross section.

In the exemplary embodiment, a flue tube for a condensing heat exchanger has a dents 2 formed along the length of the tube 1 , and the dents are pointed towards the centre of the tube 1. At the predetermined point there are two dents 2 made in the tube 1 opposite each other, which mark out a section. The distance between the lowest points in the dents 2 measured inside the tube in the section is 0.5 mm, and the ratio of the length L of the tube 1 to the tube cross section circumference around which the combustion gases flow is 3.5. The adjacent sections of the dents 2 are mutually arranged at the angle of 90° along the length of the tube 1. In the top section, the tube 1 is cylindrical in shape and the length H of the top section amounts to 1.0 of the tube cross section circumference around which the combustion gases flow. In the embodiment variants shown on Figs. 1 to 7 and Figs. 22 to 28, the dents 2 are drop-like in shape; in other variants which are shown on Figs. 8 to 14 and Figs. 22 to 35 the dents 2 are circular in shape; and in further variants shown on Figs. 15 to 21 and Figs. 36 to 42 the shape of the dents 2 is oval.

In the invention embodiment variants referred to above the sections of the dents 2 are arranged in even distances along the length L of the tube 1.

In the other invention embodiment variants shown on Figs. 43 to 49 and Figs. 64 to 70 (drop-shaped dents 2); Figs. 50 to 56 and Figs. 71 to 77 (circular dents 2); and Figs. 57 to 63 and Figs. 78 to 84 (oval dents), the sections of the dents 2 are arranged at uneven distances along the length L of the tube in such a way that the distance S between the sections reduces along the tube length.

In the invention embodiment variants described above and shown on Figs. 1 to 21 , and Figs. 43 to 63, the tube 1 in the f agment between the adjacent sections is circular in cross section, and in the variants illustrated on Figs. 22 to 42, and Figs. 64 to 84 the shape of the tube cross section is square with rounded apexes and sides indented towards the axis.

In yet other embodiment variants the tube between the adjacent sections can be oval in cross section.

In other embodiment variants described above the distance between the lowest points in the dents 2 measured inside the flue tube 1 in the section cannot be larger than 1.0 mm and the ratio of the length L of the flue tube 1 to the tube cross section circumference around which the combustion gases flow falls within the range from 2.5 to 6.5, and the length of the cylindrical flue tube 1 in its top section ranges from 0.25 to 1.5 of the tube cross section circumference around which the combustion gases flow.

It has been found that with the combustion gas temperature at the flue tube inlet ranging from 1450 to 1550 °C, the flow of the combustion gas in the flue tube between 0.83 and 0.89 kg/h (for natural gas, CO ₂ = 9%), and the initial temperature of 30 °C and dT=20K of the heated liquid in counterflow of 22-26 1/h per tube, the combustion gas reaches the dew point at the distance of 155-225 mm from the tube inlet, and the pressure drop does not exceed 375Pa.