Copper industry knowledge: Performance comparison of copper tubes and stainless steel heat exchange tubes
Stainless steel heat exchange tube: a device made of stainless steel, one of the components of the heat exchanger, placed inside the cylinder, used for heat exchange between two media.
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Stainless steel heat exchange tubes have the following advantages:
1. The heat exchange tube uses 0.5-0.8mm thin-walled tubes to improve the overall heat exchange performance. Under the same heat exchange area, the overall heat transfer coefficient is 2.121-8.408% higher than that of copper tubes.
2. Because the material is SUS304 high-quality stainless alloy steel, it has a higher hardness and the steel degree of the tube is significantly improved. Therefore, it has strong impact resistance and vibration resistance.
3. Because the inner wall of the tube is smooth, the thickness of the boundary layer flow bottom layer is thinned, which not only strengthens the heat exchange, but also improves the anti-scaling performance. In order to eliminate welding stress, heat treatment is carried out at a high temperature of 1050℃ in a protective gas. ??? Steel pipes are leak-checked by differential pressure, and the air pressure test is up to 10MPa, with no pressure drop for 5 minutes.
Copper tube: also known as red copper tube, a kind of non-ferrous metal tube, is a pressed and drawn seamless tube. Copper tube has the characteristics of being strong and corrosion-resistant, and has become the first choice for modern contractors to install water pipes, heating and cooling pipes in all residential commercial houses. Copper tube is the best water supply pipe.
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Advantages of copper tube:
1. Light weight, good thermal conductivity, and high low-temperature strength. Commonly used in the manufacture of heat exchange equipment (such as condensers, etc.). Also used to assemble low-temperature pipelines in oxygen production equipment. Copper tubes with small diameters are often used to transport pressurized liquids (such as lubrication systems, oil pressure systems, etc.) and as pressure measuring tubes for instruments.
2. Copper tubes have the characteristics of being strong and corrosion-resistant, and have become the first choice for modern contractors to install water pipes, heating and cooling pipes in all residential commercial houses.
3. Copper pipes have many advantages: they are strong and have the high strength of general metals; at the same time, they are easier to bend, twist, crack, and break than general metals, and have certain anti-frost heave and impact resistance. Therefore, once copper water pipes are installed in the water supply system of buildings, they are safe and reliable to use, and even do not require maintenance and maintenance.
Disadvantages of copper pipes: As far as copper pipes are concerned, the high price is its biggest disadvantage. It is currently the most high-end water pipe. It is usually installed by welding technology and will not leak for life. The connection at the interface of copper pipes mainly depends on the construction process level, and the construction quality requirements are high.
The following will explain the differences between copper pipes and stainless steel heat exchange tubes from the following aspects
The performance comparison of copper pipes and stainless steel heat exchange tubes is as follows:
I. Performance comparison of copper pipes and stainless steel heat exchange tubes: thermal conductivity
Since the thermal conductivity of copper pipes is 100W/m℃ and the thermal conductivity of stainless steel pipes is 13W/m℃, this will of course affect the overall heat transfer coefficient. However, the wall thickness of stainless steel pipes can be reduced to 0.5~0.8mm, while the wall thickness of copper pipes cannot be less than 1.2mm due to strength and erosion wear.
According to the formula: Rc= (1) Where: Rc - thermal resistance, m2k/w. λ - thermal conductivity, W/(m.k).
δ - pipe wall thickness, m
When the pipe material is constant and λ is unchanged, according to formula (1), the smaller δ, the smaller Rc, and the greater the heat transfer coefficient. This can narrow the gap between the overall heat transfer coefficients of stainless steel pipes and copper pipes.
Since the inner and outer walls of copper pipes are rougher than stainless steel, they are prone to scaling, which increases the thermal resistance of copper pipes, which in turn narrows the gap between the overall heat transfer coefficients of copper pipes and stainless steel pipes.
II. Performance comparison of copper and stainless steel heat exchange tubes: convection heat release
When using stainless steel pipes or copper pipes, the flow rate in the pipe is turbulent. The biggest factor affecting convective heat release is the thickness of the laminar bottom layer, because heat transfer in the laminar bottom layer is heat conduction, and the thermal conductivity of water is very low. Under the same flow state, the thickness of the laminar bottom layer depends on the roughness of the inner wall of the tube. The inner surface of the copper tube has oxides, and its roughness is much greater than that of the stainless steel tube. The thickness of the laminar bottom layer of the copper tube is greater than that of the stainless steel tube. This makes the convective heat release coefficient of the stainless steel tube greater than that of the copper tube.
Rw=(2)
Where: Rw--Convective heat release thermal resistance, m2k/w. αw--Convective heat release coefficient, w/m2.k. ?According to formula (2), the larger αw is, the smaller Rw is.
III. Performance comparison of copper tube and stainless steel heat exchange tube: condensation heat release coefficient
There are two types of condensation heat release coefficients: film condensation and bead condensation. The bead condensation heat release coefficient is much larger than the film condensation heat release coefficient. However, it is still unclear whether the outer wall of stainless steel tube or copper tube has more bead condensation, but it can be said that most of the outer walls of the two tubes are film condensation. The heat release coefficient of film condensation is closely related to the thickness of the film, because the heat is conducted inside the film, the thermal conductivity of the water film is particularly low, and the thickness of the film depends on the roughness of the outer wall of the tube. The outer wall of the copper tube is much rougher than that of the stainless steel tube due to the oxide layer. Therefore, the condensation heat release coefficient of the outer wall of the stainless steel tube is greater than that of the outer wall of the copper tube.
Rm=(3)
Where: Rm--condensation heat release thermal resistance of the outer wall of the tube, m2k/wαm--condensation heat release coefficient of the outer wall of the tube, w/m2.k. According to formula (3), the larger αm is, the smaller Rm is.
IV. Overall heat transfer coefficient of performance comparison of copper tube and stainless steel heat exchange tube
K=(4)
Where: R--total thermal resistance, m2k/w. K--overall heat transfer coefficient, w/m2.k.
From (4), we can know that: if the convection resistance, conduction resistance and condensation heat release resistance are all reduced, the total heat resistance will be reduced: if the total heat resistance is reduced, the overall heat transfer coefficient will increase.
Under the same wall thickness, the overall heat transfer coefficient of the stainless steel pipe is 6% lower than that of the copper pipe. Due to the use of stainless steel pipes that are thinner than copper pipes, the overall heat transfer coefficient and condensation heat release coefficient of the stainless steel pipe are larger than those of the copper pipe, which improves the overall heat transfer coefficient of the stainless steel pipe.
Heat transfer performance comparison table of copper pipes and stainless steel heat exchanger pipes
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V. Long-term economic performance comparison of copper pipes and stainless steel heat exchanger pipes
With the increase of operating time, the oxide layer of the copper pipe will become thicker and thicker, and the heat transfer effect will become worse and worse. However, stainless steel will basically not oxidize, or the oxidation speed is very slow. Therefore, if the stainless steel pipe heat exchanger and the copper pipe heat exchanger are put into operation at the same time, the longer the operating time, the better the economy of the stainless steel pipe heat exchanger will be than that of the copper pipe heat exchanger. At the same time, the copper tube has a much stronger adsorption capacity for debris in the cooling water than the stainless steel tube, which greatly reduces the economic efficiency of the equipment.
Sixth, the performance comparison of copper tube and stainless steel heat exchange tube safety performance comparison
Engineering characteristics table of copper tube and stainless steel heat exchange tube
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It can be seen from the above table: the yield strength and tensile strength of stainless steel tube are higher than those of copper tube, the service life of stainless steel tube must be longer than that of copper tube, the thermal expansion coefficient is lower than that of copper tube, and it is closer to the tube sheet. Therefore, it is not easy to damage the tube or affect the expansion due to thermal expansion and contraction.
Corrosion resistance table of copper tubes and stainless steel heat exchange tubes
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From the above table, we can see that stainless steel tubes have the following advantages over copper tubes: good erosion resistance, can resist the impact corrosion of steam with water droplets at high speed; good ammonia corrosion resistance: resistant to water side impact corrosion, realize copper ion-free system, and the pH value can be increased to reduce the corrosion rate, and the cooling water flow rate can be increased to 2.3m/s, up to 3.5m/s, which can not only improve the overall heat transfer coefficient, but also reduce the deposition of impurities in the tube.
