Copper tubes are widely used in heat exchangers due to their excellent thermal conductivity, corrosion resistance, and malleability. As a key component affecting heat transfer efficiency, energy consumption, and operational stability, the size of copper tubes (mainly reflected in outer diameter, inner diameter, and wall thickness) shows significant performance differences in different types of heat exchangers. This article explores the performance characteristics of copper tubes with different sizes in air conditioning heat exchangers, large water-cooled chiller heat exchangers, and micro heat exchangers, and analyzes the intrinsic reasons for these differences, providing a reference for the rational selection of copper tube sizes in engineering applications.

In air conditioning heat exchangers, which are the most common application scenarios, copper tubes of different diameters show obvious differences in heat transfer efficiency, material cost, and refrigerant charge. The market generally uses copper tubes with outer diameters of 5mm, 7mm, 7.94mm, and 9.52mm, among which inner threaded copper tubes are more widely used than smooth copper tubes due to their enhanced heat transfer performance^{(1)}. Tests show that under the same mass flow rate of refrigerant, the heat transfer coefficient of 5mm inner threaded copper tubes is higher than that of 7mm ones, and the heat transfer coefficient of evaporators is generally higher than that of condensers^{(1)}. Specifically, the heat transfer coefficient of 5mm copper tubes can reach 1.43 to 1.86 times that of 7mm copper tubes as the dryness increases, while the air-side heat transfer coefficient of 5mm heat exchangers is 17% higher than that of 7mm ones^{(1)}.

However, the miniaturization of copper tube diameter also brings obvious pressure drop problems. The refrigerant-side pressure drop of heat exchangers with small-diameter copper tubes increases significantly; the friction pressure drop, acceleration pressure drop, and local pressure drop of 5mm copper tube heat exchangers are three times those of 7mm ones, which can lead to a 1.1℃ decrease in evaporation temperature^{(1)}. In terms of economic benefits, small-diameter copper tubes have obvious advantages: the material cost of 5mm inner threaded copper tubes is at least 55% lower than that of 9.52mm ones of the same length, and the refrigerant charge is only 26.6% of the latter^{(1)}. The copper consumption of 5mm copper tube heat exchangers is 26.74% lower than that of 7mm ones and 46.43% lower than that of 9.53mm ones, while their cooling capacity and energy efficiency ratio (EER) are slightly improved^{(4)}. Currently, 5mm small-diameter inner threaded copper tubes account for only 2.98% of the total usage in the air conditioning heat exchanger market, mainly due to the challenges brought by high pressure drop^{(1)}.

In large water-cooled chiller units, the selection of copper tube sizes is more closely related to the heat transfer capacity and system stability. Common copper tube diameters for condensers are 6mm, 10mm, 12mm, 14mm, and 16mm, while those for evaporators are 7.94mm, 9.52mm, 12.7mm, and 15.88mm^{(2)}. Generally, larger-diameter copper tubes have stronger heat transfer capacity, but excessively large diameters will increase hydraulic resistance, so the selection needs to balance heat transfer and resistance^{(2)}. In addition, the wall thickness of copper tubes also affects their performance: thicker walls enhance strength and corrosion resistance but increase costs, so factors such as the unit's operating environment and budget need to be considered^{(2)}. Copper tube materials such as C12200 and C12000 are commonly used; C12200 has good thermal conductivity and machinability, suitable for heat transfer media, while C12000 has excellent corrosion resistance and weldability, suitable for headers and condensers^{(2)}.

In micro heat exchangers, which are widely used in precision electronic equipment and small refrigeration systems, capillary copper tubes with an outer diameter of 1.5mm to 5mm are mainly used^{(3)}. These small-diameter copper tubes have a small inner diameter, which can enhance the disturbance of the fluid, improve the heat transfer coefficient, and meet the requirements of compact structure and high heat transfer efficiency of micro heat exchangers^{(5)}. However, due to the small diameter, the flow resistance of capillary copper tubes is large, and the wall thickness (0.3mm to 0.8mm) needs to be strictly controlled to ensure pressure-bearing capacity while reducing heat loss^{(3)}. Internally enhanced small-diameter copper tubes (such as microgroove tubes) can further improve heat transfer efficiency, making the heat exchanger smaller and more energy-efficient, and better adapting to the use of environmentally friendly refrigerants^{(5)}.

The performance differences of copper tubes with different sizes are essentially determined by the combined effect of heat transfer area, fluid flow state, and pressure drop. According to the heat transfer formula Q = k×A×Δt, the heat transfer capacity (Q) is related to the heat transfer coefficient (k), effective heat transfer area (A), and temperature difference (Δt)^{(3)}. Larger-diameter copper tubes have a larger heat transfer area per unit length, which helps to improve heat transfer capacity, but the fluid flow velocity inside the tube decreases, leading to a lower heat transfer coefficient^{(6)}. Small-diameter copper tubes have a smaller heat transfer area per unit length, but the fluid flow velocity increases, enhancing fluid disturbance and improving the heat transfer coefficient, while the pressure drop also increases significantly^{(1)}.

In conclusion, there is no absolute advantage or disadvantage in the size of copper tubes; their performance adaptability varies with different types of heat exchangers and application scenarios. In air conditioning systems, small-diameter copper tubes (5mm, 7mm) are suitable for occasions pursuing cost savings and compact structure, while larger-diameter copper tubes (9.52mm, 12.7mm) are more suitable for systems requiring stable operation and low pressure drop. In large water-cooled chillers, medium and large-diameter copper tubes are preferred to balance heat transfer capacity and hydraulic resistance. In micro heat exchangers, capillary small-diameter copper tubes are the core choice to meet the requirements of miniaturization and high efficiency. Rational selection of copper tube size based on the actual working conditions of heat exchangers is crucial to improving system performance, reducing energy consumption, and controlling costs.