Comparison of the performance between stainless steel heat exchange tubes and copper tubes

1. The stainless steel heat exchange tubes have the following advantages:

The heat exchange tubes are made of thin-walled pipe materials with a thickness of 0.5 - 0.8mm, which improves the overall heat exchange performance. Under the same heat exchange area, the overall heat transfer coefficient of the stainless steel tubes is 2.121 - 8.408% higher than that of copper tubes.

Due to the use of SUS304 high-quality stainless alloy steel as the material, it has a higher hardness, and the stiffness of the tubes has also significantly increased. Therefore, it has strong impact resistance and vibration resistance.

Due to the smooth inner wall of the tubes, the thickness of the boundary layer flow layer is reduced, which not only strengthens heat exchange but also improves the anti-scaling performance.

To eliminate welding stress, heat treatment is carried out at a temperature of 1050℃ in a protective gas.

The pipes are inspected for leakage by pressure difference, and the gas pressure test is up to 10MPa, with no pressure drop within 5 minutes.

2. Detailed comparison:

I. Heat Conduction

The thermal conductivity of copper tubes is 100W/m℃, while that of stainless steel tubes is 13W/m℃. This naturally affects the overall heat transfer coefficient. However, the wall thickness of stainless steel tubes can be reduced to 0.5 - 0.8mm, while the wall thickness of copper tubes cannot be lower than 1.2mm due to strength and erosion wear.

According to the formula: Rc = (1)

Where: Rc - heat transfer resistance, m2k/w.

λ - heat transfer coefficient, W/(m.k).

δ - tube wall thickness, m.

When the pipe material is fixed and λ remains unchanged, according to formula (1), the smaller δ is, the smaller Rc is, and the heat transfer coefficient is larger. This can narrow the gap in the overall heat transfer coefficient between stainless steel tubes and copper tubes.

Because the inner and outer walls of copper tubes are rougher than those of stainless steel tubes, they are prone to scaling, increasing the thermal resistance of copper tubes. This further reduces the gap in the overall heat transfer coefficient between copper tubes and stainless steel tubes.

II. Convective Heat Release

Whether using stainless steel tubes or copper tubes, the flow velocity inside the tubes is turbulent. The largest factor affecting convective heat release is the thickness of the laminar flow layer, because the heat transfer in the laminar flow layer is by conduction, and the thermal conductivity of water is very low. Under the same flow state, the thickness of the laminar flow layer depends on the roughness of the inner wall of the tube. The inner surface of copper tubes has oxides, and its roughness is much greater than that of stainless steel tubes. The thickness of the laminar flow layer of copper tubes is larger than that of stainless steel tubes. This makes the convective heat release coefficient of stainless steel tubes larger than that of copper tubes. Rw=    (2)

Herein: Rw - convective heat rejection resistance, m2k/w.

αw - convective heat rejection coefficient, w/m2.k.

According to formula (2), the larger αw is, the smaller Rw will be.

III. Condensation Heat Rejection Coefficient

There are two types of condensation heat rejection: film condensation and droplet condensation. The condensation heat rejection coefficient of droplet condensation is much larger than that of film condensation. However, it is still unclear which type of pipe material (stainless steel pipe outer wall or copper pipe outer wall) has more droplet condensation. But it can be said that most of the outer walls of both pipe materials are film condensation. The size of the heat rejection coefficient of film condensation is greatly related to the thickness of the film, because the heat conduction inside the film is conducted through thermal conduction, and the thermal conductivity coefficient of the water film is particularly low, and the thickness of the film depends on the roughness of the outer wall of the pipe. Due to the oxide layer on the copper pipe outer wall, it is much rougher than the stainless steel pipe. Therefore, the condensation heat rejection coefficient of the stainless steel pipe outer wall is larger than that of the copper pipe outer wall.

Rm = (3)

Where: Rm - condensation heat rejection resistance of the pipe outer wall, m2k/w.

αm - condensation heat rejection coefficient of the pipe outer wall, w/m2.k.

According to formula (3), the larger αm is, the smaller Rm will be.

IV. Overall Heat Transfer Coefficient

K = (4)

Where: R - total heat resistance, m2k/w.

K - overall heat transfer coefficient, w/m2.k.

From (4), if the convective heat resistance, thermal conduction resistance and condensation heat rejection resistance all decrease, then the total heat resistance decreases: when the total heat resistance decreases, the overall heat transfer coefficient increases.

When the wall thickness is the same, the overall heat transfer coefficient of the stainless steel pipe is 6% lower than that of the copper pipe. Due to using a thinner stainless steel pipe than the copper pipe, the overall heat transfer coefficient and condensation heat rejection coefficient of the stainless steel pipe are both larger than those of the copper pipe, which improves the overall heat transfer coefficient of the stainless steel pipe.