The Effect of the Welding Technology on the Thermal Performance of Welded Finned Tubes Used in Heat Exchangers

Abstract

Due to the use of welded finned tubes in heat exchangers, gas blocks are characterized by very good electric energy production performance, and their reliability can reach 98%. Finned tubes used in heat exchangers are usually welded. Such tubes should be characterised by the following properties: high thermal performance, good resistance to high-temperature corrosion in a flue gas atmosphere, and appropriate joint strength and hardness. The most popular technology for welding finned tubes is MAG. MAG-welded joints are characterised by considerable spattering, weld discontinuities (up to 60%), and non-axial fin alignment. Such irregularities result in a considerably reduced heat flux due to the heat transfer resistance occurring in the welded joint. It has been shown that with weld discontinuities of 20%, the thermal performance of welded finned tubes reduces considerably. This paper proposes the possibility of welding finned tubes with a laser. The thermal properties of laser-welded finned tubes were compared with those of MAG-welded tubes. It was found that the thermal performance of welded finned tubes was three times higher than that of plain tubes. However, the thermal performance was not found to be affected by the welding technology. The correct heat flow at the level of 95% occurs in the tube/fin joint even with joint penetration of 0.01 mm, however, the absence of metallic continuity in a joint results in a drastically reduced thermal performance (by 50%) and the overheating of fins, which affects their durability.

1. Introduction

In modern gas boilers, such as water heaters, evaporators, and steam superheaters, welded finned tubes are used to increase the heat exchange efficiency (Figure 1) [1]. Due to the use of welded finned tubes in heat exchangers, gas blocks are characterized by very good electric energy production efficiency, and their reliability can reach 98% [2]. Welded finned tubes are usually applied where the heat transfer coefficients on the two sides of a barrier have significantly different values (e.g., flue gas and steam). The presence of fins on heat exchanger tubes increases the heat exchange surface even up to 30 times in comparison to non-finned tubes, leading to the heat exchange flux increasing by nearly 300% [1,3].

The basic finned tube manufacture technologies include wrapping a fin strip directly onto the tube, mechanical bending of the wound strip [4], mechanical working [5,6,7,8,9], high-frequency welding [10,11,12], and electric arc welding, most frequently using the MAG method [13]. Recently, there have been attempts at manufacturing laser-welded finned tubes [14,15].

Tubes with a wound or bent fin strip are not used in the energy industry due to their separable structure, which does not provide sufficient heat exchange conditions. Additionally, at time passes, the joint quality decreases due to the loss of bonding as a result of different thermal expansion coefficients of the materials and the vibrations involved in the boiler’s operation [16].

Finned tubes used in heat exchangers are usually welded. Such tubes should be characterised by the following properties: high thermal performance, good resistance to high-temperature corrosion in a flue gas atmosphere, and appropriate joint strength and hardness, as required by technical regulations, especially the requirements for quality level B according to ISO 5817 (for arc welding) and ISO 13919 (for laser welding) [17].

Significant structural factors determining the properties of welded finned tubes include weld continuity, weld shape, and weld microstructure. These factors determine the thermal performance of the pipe and thus the performance of the entire heat exchanger.

The most popular technology for welding finned tubes is MAG [18,19,20]. The structure and properties of welded finned tubes with fillet or butt welds beneath the fins are well-described in the literature [14,15,17]. The manufacture of finned tubes with fillet welds is characterized by low efficiency (welding rate of up to 2.5 m/min) and the welded joint can be affected by weld discontinuities (up to 20%) and irregularities in the geometry of the fillet welds [17]. The lack of full penetration and the non-symmetric weld increase the welded joint’s susceptibility to crevice corrosion. MAG-welded joints located beneath the fins, in turn, are characterised by considerable spattering, weld discontinuities (up to 60%), and non-axial fin alignment. Such irregularities result in a considerably reduced heat flux due to the heat transfer resistance occurring in the welded joint. In work [21], it was shown that with weld discontinuities of 20%, the thermal performance of welded finned tubes reduced considerably.

At present, lasers are not used for welding thick-walled boiler components for the power industry due to the limited beam power, difficulties with precise joint preparation, the possibility of hardening both in the narrow HAZ and in the weld, and the hot cracking susceptibility of such welds [20,22]. However, the advantages of disc and fiber lasers, in particular the stability of the welding process, higher joint preparation tolerance, high welding energy concentration, and considerably increased welding efficiency, have prompted attempts at the laser welding of finned tubes. The results of those attempts indicate that the quality of the welded joints depends on the welding parameters, in particular the welding rate and the beam power [23].

Issues related to laser-welded finned tubes have not been extensively investigated and described in the literature. For this reason, research was undertaken to determine the influence of laser welding parameters on the structure and properties of finned tubes used in heat recovery boilers.

In this paper, the thermal performance of a finned pipe is defined as the ratio of the heat flux through a finned pipe to the heat flux through a smooth pipe, i.e., without a rib. The effect of the degree of remelting of the ribbed pipe joint on the thermal performance of the entire pipe was also determined. The results of these studies are particularly relevant to design engineers who plan to use modern laser-welded finned tubes in heat exchangers. The research was carried out on a test stand developed at Energoinstal S.A., Poland.

2. Testing Methodology

The complex finned pipe model was simplified to a flat baffle, allowing the steady-state heat flow flux to be defined. In a stationary state, a heat flux ($\dot{Q}$) passing through a finned barrier (W) is defined by surfaces A₁ (the surface on the water or steam side) and A₂ (the surface on the flue gas side); thickness δ; and heat flow conditions T₁, α₁, T₂, and α₂ (Figure 2).

$$\dot{Q}={\dot{Q}}_W={\dot{Q}}_2=Q_1=A_1{\alpha }_1\left(T_1-T_{W1}\right)=A_{WM}\frac{{\lambda }_W}{{\delta }_W}\left(T_{W1}-T_{W2}\right)=A_{20}{\alpha }_2\left(T_{W2}-T_2\right)+A_{2\dot{\mathrm{Z}}}{\alpha }_2\left(T_{\dot{\mathrm{Z}}M}-T_2\right),$$

(1)

where:$\dot{Q}—\mathrm{heat} \mathrm{flux},$ ${\dot{Q}}_1$—heat flux flowing from medium 1 to the surface of barrier W, ${\dot{Q}}_W$—heat flux conducted through barrier W, ${\dot{Q}}_2$—heat flux penetrating the surface of barrier W from medium 2, $A_1$—barrier surface on the side of medium 1, m², $A_{WM}$—average (integral) barrier surface, m², $A_{20}$—surface between the ribs, m², $A_{2z}$—side fin surface, m², ${\alpha }_1$—coefficient of heat transfer from medium 1 to the barrier surface, W/(m² · °C), ${\alpha }_2$—coefficient of heat transfer from the barrier surface to medium 2, W/(m² · °C), T₁—temperature of medium 1, °C, T_W1—barrier temperature on the side of medium 1, °C, T_W2—barrier temperature on the side of medium 2, °C, T₂—temperature of medium 2, °C, T_ŻM—average fin temperature, °C, ${\lambda }_W$—coefficient of heat transfer through barrier W, and ${\delta }_W$—thickness of barrier W.

After applying the thermal performance for the fin:

$${\epsilon }_{\dot{\mathrm{Z}}}=\frac{T_{\dot{\mathrm{Z}}M}-T_2}{T_{W2}-T_2},$$

(2)

one obtains:

$${\dot{Q}}_2={\alpha }_2\left(T_{W2}-T_2\right)\cdot \left(A_{20}+A_{2\dot{\mathrm{Z}}}\cdot {\epsilon }_{\dot{\mathrm{Z}}}\right)$$

(3)

Following a transformation, the Peclet equation is obtained [4]:

$$\dot{Q}=k\cdot A_1\cdot \left(T_1-T_2\right),$$

(4)

where the following formula:

$$\frac{1}{k}=\frac{1}{{\alpha }_1}+\frac{{\delta }_W\cdot A_1}{{\lambda }_W\cdot A_{WM}}+\frac{A_1}{{\alpha }_2\cdot \left(A_{20}+A_{2\dot{\mathrm{Z}}}\cdot {\epsilon }_{\dot{\mathrm{Z}}}\right)}$$

(5)

defines the heat transfer coefficient k for the finned barrier in relation to the barrier surface on the plain (non-finned) side [24,25]. However, the equation above does not take into consideration the contact resistance between the base tube and the fin (at the welded joint) and may only be applied to heat calculations, e.g., rolled finned tubes produced by a deepening or thinning method.

In the case of welded tubes, the contact resistance must be taken into consideration, i.e., the resistance offered to heat transfer by the welded joint (RS) [25,26,27]. Thus, the heat flux on the finned side can be described by the following formula:

$${\dot{Q}}_2=A_{20}\cdot {\alpha }_2\left(T_{W2}-T_2\right)+{\dot{Q}}_{22},$$

(6)

where

$${\dot{Q}}_{22}=A_S\cdot \frac{1}{R_S}\left(T_{W2}-T_S\right)$$

(7)

and

$${\dot{Q}}_{22}=A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}\left(T_S-T_2\right)$$

(8)

By transforming the equations above, one obtains:

$$A_S\cdot \frac{1}{R_S}\left(T_{W2}-T_S\right)=A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}\left(T_S-T_2\right),$$

(9)

and

$$\left(T_{W2}-T_S\right)=\frac{{\dot{Q}}_{22}}{A_S\cdot \frac{1}{R_S}},$$

(10)

and

$$\left(T_S-T_2\right)=\frac{{\dot{Q}}_{22}}{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}$$

(11)

After summing up the temperature drops, one obtains:

$$\left(T_{W2}-T_2\right)={\dot{Q}}_{22}\cdot \frac{\left(A_S\cdot \frac{1}{R_S}\right)+\left(A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}\right)}{\left(A_S\cdot \frac{1}{R_S}\right)\cdot \left(A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}\right)},$$

(12)

and

$$\left(T_{W2}-T_2\right)={\dot{Q}}_{22}\cdot \frac{\left[1+\left(\frac{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S\cdot \frac{1}{R_S}}\right)\right]}{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}},$$

(13)

and

$$\left(T_{W2}-T_2\right)={\dot{Q}}_{22}\cdot \frac{\left[1+\frac{R_S\cdot A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S}\right]}{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}$$

(14)

The heat flux on the finned side is described by the following equations:

$${\dot{Q}}_2=A_{20}\cdot {\alpha }_2\cdot \left(T_{W2}-T_2\right)+\frac{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{\left[1+\frac{R_S\cdot A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S}\right]}\cdot \left(T_{W2}-T_2\right)$$

(15)

and

$${\dot{Q}}_2=\left\{A_{20}\cdot {\alpha }_2+\frac{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{\left[1+\frac{R_S\cdot A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S}\right]}\right\}\cdot \left(T_{W2}-T_2\right)$$

(16)

After summing up the temperature drops from Equation (6) transformed to (17):

$$\left(T_1-T_{W1}\right)=\frac{{\dot{Q}}_1}{A_1\cdot {\alpha }_1},$$

(17)

from Equation (7) transformed to (18):

$$\left(T_{W1}-T_{W2}\right)=\frac{{\dot{Q}}_W}{A_{WM}\cdot \frac{{\lambda }_W}{{\delta }_W}} ,$$

(18)

and from Equation (13) transformed to (19):

$$\left(T_{W2}-T_2\right)=\frac{{\dot{Q}}_2}{A_{20}\cdot {\alpha }_2+\frac{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{\left[1+\frac{R_S\cdot A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S}\right]}},$$

(19)

Equation (20) is obtained, which describes the heat transfer from medium 1 to medium 2 in the case of resistance at the fin/base tube joint (for welding):

$$\left(T_1-T_2\right)=\dot{Q}\cdot \left\{\frac{1}{A_1\cdot {\alpha }_1}+\frac{1}{A_{WM}\cdot \frac{{\lambda }_W}{{\delta }_W}}+\frac{1}{A_{20}\cdot {\alpha }_2+\frac{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{\left[1+\frac{R_S\cdot A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S}\right]}}\right\}$$

(20)

This is the Peclet Equation (4), where k is described by Equation (21):

$$\frac{1}{k}=\frac{1}{A_1\cdot {\alpha }_1}+\frac{1}{A_{WM}\cdot \frac{{\lambda }_W}{{\delta }_W}}+\frac{1}{A_{20}\cdot {\alpha }_2+\frac{A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{\left[1+\frac{R_S\cdot A_{2\dot{\mathrm{Z}}}\cdot {\alpha }_2\cdot {\epsilon }_{\dot{\mathrm{Z}}}}{A_S}\right]}}$$

(21)

The thermal performance tests of laser-welded and MAG-welded finned tubes were performed on a dedicated test stand, whose diagram is shown in Figure 3. The thermal performance of a finned tube was defined as the ratio of the heat flux passing through a finned tube to the heat flux passing through a plain tube.

The dedicated test stand enables the performance of tests in both natural and forced heat convection. A two-phase thermosyphon was used as the heat exchanger, with the condensate flowing to the evaporator counter current to the steam flowing to the condenser. Water was used as the working fluid, enabling measurements at temperatures of 100–220 °C at pressures of 1–25 bar.

In order to confirm the uniform temperature distribution across the finned tubes, the temperature distribution was monitored during the experiment using a thermographic camera (Figure 4). On this basis, it was found that the designed and built test stand provided uniform temperature distribution across an entire tube and enabled the simulation of heat exchange in finned tubes.

The thermal performance tests were performed for finned tubes laser-welded with various beam power and welding rate parameters (designated as 1–5) and for finned tubes welded using the MAG method, with a butt weld beneath the fin and a fillet weld (designated as 6 and 7, respectively) (

Table 1. Technological parameters of the laser welding of finned tubes and MAG-welded methods tubes.

Joint No.	Power of Laser Beam/Welding Arc, kW	Welding Rate, m/min	Rotational Speed, rpm	Number of Fins Per 1 Running Meter of Pipe	Gas Flow Rate, dm3/min	Comments
1	2.2	3.0	20	150	5	laser
2	3.2	5.2	50	150	5	laser
3	4.0	7.5	100	150	5	laser
4	6.0	15.0	150	150	5	laser
5	6.5	22.5	200	150	5	laser
6	2.2	4.5	40	150	14	MAG butt weld under the fin
7	3.6	1.8	15	150	14	MAG fillet weld

).

The test results were compared to a heat flux passing through a plain tube under the same conditions (temperature and pressure). The tests were performed for the temperatures of the medium inside the tube (steam) ranging from 100 °C to 170 °C.

3. Results

Figure 5 shows the measurement results for the heat flux transferred in the tested tube section (800 mm) from steam to air under stabilized thermal conditions of the test stand. Figure 6 shows the measured value of the heat flux transferred by a finned tube from steam to air, reduced by the heat flux transferred by a plain tube under the same thermal conditions.

Figure 7 shows the heat fluxes transferred by the finned tubes compared to a plain tube, whereas Figure 8 shows the heat fluxes passing through the laser-welded finned tubes compared to the MAG-welded finned tube with a fillet weld.

The analysis of the heat fluxes measured under the test conditions indicates that the use of fins to expand the heat exchange surface area increased the heat flux by three times, i.e., in the test conditions at 100 °C, from approx. 0.2 kW for a plain tube to approx. 0.6 kW for finned tubes. Additionally, at 170 °C, the heat flux increased more than twofold (from 0.5 kW for the plain tube to over 1 kW for the finned tubes) (Figure 5). The comparison of the heat fluxes transferred by the laser-welded and MAG-welded finned tubes did not reveal any significant differences. The heat fluxes transferred by the finned tubes, reduced by the heat flux passing through the plain tube, fell within the range of 0.3 kW to 0.4 kW at 100 °C and from 0.57 kW to 0.6 kW at 170 °C (Figure 6).

The comparison between the relative heat fluxes transferred by the finned tubes and the plain tube indicates that at 100 °C, the difference amounts to 60%, and as the temperature rises, the difference decreases to less than 20% at 170 °C (Figure 7). Such changes in the relative heat fluxes transferred by the finned tubes confirm information contained in the literature, to the effect that any type of welded joint provides correct heat exchange. The analysis of the heat fluxes transferred by the laser-welded finned tubes compared to the heat flux transferred by the MAG-welded tubes did not reveal any significant differences in thermal performance either (Figure 8). This indicates that laser-welded finned tubes are characterised by the same thermal performance as MAG-welded finned tubes.

The tests were complemented by an analysis of the temperature distribution field during heat transfer in the laser-welded and MAG-welded tubes. A test stand equipped with a forced heat convection system and a thermographic camera was built to assess the temperature field. A TERMEK heater (120 cm long, 1000 W) with a Kanthal wire was used for heating the outer surface. The heater was appropriately bent to ensure its direct contact with a layer of an aluminium sheet approx. 1 mm thick, which made it possible to even out the temperature distribution and to heat the fins uniformly (Figure 9). A thermostat was also installed, whose thermocouple was located between the aluminium layer and the fins, which allowed control over the temperature of the aluminium sheet. The regulation range of the thermostat was from 50 °C to 320 °C. The ends of the copper tubes were adapted for the purpose of connecting to the water supply system, to ensure efficient cooling. It had been determined experimentally that, with the thermostat set to 170 °C, cooling with air blown by a fan was sufficient.

In the designed experiment, heat transfer occurs from the heat source, i.e., the aluminium sheet, across the fins, the steel tube bonded to the fins by means of a laser or MAG weld, the tin layer, and the copper tube (Figure 9). Air heated by the aluminium sheet rises and heats the side fin surfaces by natural convection, which reflects the actual service conditions (Figure 10).

With much cooler air flowing through the copper tube, heat is absorbed through natural convection and its exchange is accelerated, which makes the temperature difference clearly observable on the examined surface. Figure 11 shows the test results for a laser-welded tube in the form of temperature distribution maps, whereas Figure 12 presents the results for the MAG-welded tube.

The result analysis revealed that the temperature at point SP01 stabilised at 97 °C for the laser-welded tube (Figure 11c). Based on the thermograms, it was found that, in the laser-welded tube, the heat transfer from the fin to the tube was uniform across the entire fin width. This stems from the complete penetration of the weld (Figure 11a,b). As for the MAG-welded tube, the temperature at point SP02 stabilised at 88 °C (Figure 12c).

It was found that the heat flow from the rib to the tube occurred through the MAG fillet weld to a much greater extent (Figure 12a,b). This was caused by the shape of the fillet weld, affected by incomplete penetration. Operation under such conditions for a long time may cause damage and cracks due to the non-uniform temperature in the material, the overheating of the material on the weld root side, and potential high-temperature corrosion in the area of lack of penetration.

4. Analysis of the Test Results

An analysis of the structural solutions applied in modern recovery boilers and gas boilers showed that finned tubes were usually manufactured based on tubes having diameters of from 31.8 to 63.5 mm and made of low-carbon steels (e.g., P235, P355) or low-alloy steels of the 15Mo3, 13CrMo4.4, and 10CrMo4.10 types, according to DIN 17175. Such steels are considered to have good weldability. On this basis, tubes made of steel grade P265GH, having a diameter of 51.0 mm and wall thickness of 3.6 mm, and a flat bar of steel grade DC01, measuring 19.5 mm × 1.0 mm, were selected for the purposes of the tests.

In order to evaluate the effect of weld continuity and shape on the thermal performance of the tubes, a model diagram of a tube/fin welded joint with incomplete penetration was developed (Figure 13). The contact resistance (Rs,[(m²·°C)/W]) of a finned tube is defined as the ratio of the gap width (G_s) to the heat transfer coefficient on the flue gas side.

The contact resistance for tube Rs with external transverse fins is a function of the gap width (G_s) between the tube and the fin, the gap length (δ—G_p) (Figure 13), and the heat transfer coefficient on the flue gas side (λ₂). The value of this resistance can vary from 0, in the case of complete penetration, to G_s/λ₂ [28,29].

Welding irregularities offer contact resistance to heat flow and affect the thermal performance of a tube/fin joint, understood as the ratio of the heat flux in a tube with welding irregularities to the heat flux in a tube of the same geometric dimensions, but unaffected by irregularities:

$${\eta }_s=⥂\frac{\dot{Q}}{{\dot{Q}}_X}\cdot 100\%$$

(22)

where: η_s—thermal performance of the joint, $\dot{Q}$—heat flux measured for a finned tube with welding irregularities, and ${\dot{Q}}_X$—heat flux measured for a finned tube without welding irregularities.

The following values were applied to the calculations of h thermal performance: fin heat transfer coefficient λ₁ = 52 W/m °C, air heat transfer coefficient λ₂ = 0.025 W/m °C, fin thickness s = 1.0 mm, and fin height h = 19 mm (Figure 14). Figure 14 shows the results of the calculations depending on the heat flux flowing through a tube/fin joint with incomplete penetration in relation to the heat flux flowing through a tube/fin joint with complete penetration, i.e., G_p/s = 100%.

The analysis of the heat flux flowing through the welded tubes, taking into account welding defects in the form of discontinuities or incomplete penetration, indicates that weld continuity is crucial to the correct heat flow through a finned tube. The analysis of the influence of the joint penetration level on the thermal performance of a finned tube revealed that even penetration covering approx. 1% of the fin width ensured an appropriate heat transfer of more than 95% (G_p = 0.01 mm). However, insufficient joint penetration reduces its durability due to the rate of corrosion in a flue gas atmosphere and thermal and fatigue deformations. A discontinuity in the joint (G_p/s = 0%) leads to immediate overheating of the fin and a 50% decrease in thermal efficiency, even if the gap is 0.01 mm wide (Figure 14).

The main reason for the use of welded finned tubes is their thermal performance, i.e., the ratio of the heat flux passing through a finned tube to the heat flux passing through a plain tube. The evaluation of the heat flux transferred from steam to air by the laser-welded finned tubes showed that it was similar to the case of the MAG-welded finned tubes (Figure 6). The thermal performance of the welded finned tubes is over three times higher than for the plain tube (Figure 5).

5. Conclusions

The welded finned tubes are among the main components of heat exchangers, which determine the thermal performance of the entire exchanger. A very important factor determining the ability of finned tubes to conduct heat is the quality of the welded tube–rib joint. Welding technologies for this type of pipe are numerous, both by arc methods and laser welding. Based on the tests performed, it was found that the heat efficiency of welded finned tubes was three times higher than that of plain tubes, which confirms the validity of using a rib along the entire length of the tube.

The welding technology used very often determines weld discontinuities. The number of discontinuities in the weld determines the degree of connection between the tube and the rib. However, the heat efficiency was not found to be affected by the welding technology if the joints are constructed correctly. The correct heat flow at the level of 95% occurs in the tube/fin joint, even with joint penetration of 0.01 mm; however, the absence of metallic continuity in a joint results in a drastically reduced heat efficiency (by 50%) and the overheating of fins, which affects their durability. These results confirm the need to connect a permanent rib to the pipe along its entire length so as to ensure metallic continuity in the joint.

However, a detailed analysis of the temperature distributions in the tests showed slight differences in MAG-welded pipes and laser-welded pipes. Due to the shape of the joint, a more homogeneous heat flux distribution was observed for laser-welded pipes, which is related to the symmetrical weld in relation to the fillet weld made by the MAG method. It should therefore be concluded that the tests have shown that laser-welded finned tubes have a more uniform heat flow compared to MAG-welded tubes with a fillet weld. This results in less stress and deformation of the fin.