In order to improve the energy efficiency ratio of air conditioning and realize energy saving, the most direct technical approach is to improve the heat transfer performance of air conditioning heat exchanger (evaporator and condenser) . The most effective measure to control the cost of air conditioning products is to study and use heat transfer enhancement technology to improve the heat transfer capacity per unit area of heat exchanger, control its volume and quality, save materials and improve heat transfer coefficient.

1. Firstly, the heat transfer efficiency of the heat exchanger and the air-conditioning cost-performance ratio are closely related to the proper selection of the diameter and tooth shape parameters of the rifled tube. The initial diameter of the copper tube is Φ9. As the distance between the Tube and the tube is reduced, the fin efficiency is increased, the effective area of heat transfer is increased, the flow resistance of air passing through is reduced, and the heat transfer is enhanced. After 1995, the diameters of some household air conditioners were further reduced to φ6 mm, even φ5 mm, and the heat transfer efficiency was further improved.

Analysis and research on tooth profile parameters of rifled Tube 1. At the beginning of 1980, the saw-tooth shape (or mountain-tooth shape or triangle shape) was the first one to be used in our country, and the trapezoid-grooved inner-grooved tube with higher heat transfer efficiency was used in 1984 In 1994, a deep grooved tube with a Trapezoidal grooved inner grooved tube with a thin bottom wall appeared; in 1997, a grooved tube with a herringbone (also known as a w-shaped grooved inner grooved tube was developed; and in 2002, a cross-grooved inner grooved tube with a secondary grooved top was developed, that is, micro-secondary groove inside the thread Tube, can be simply referred to as the cross-tooth inside the thread tube (as shown in the following figure) .

2. “Ordinary tooth”and “Non-ordinary tooth”internal thread pipe common tooth mainly refers to: single-spiral trapezoidal groove inside thread pipe, zigzag shape (or mountain tooth shape, triangle) inside thread pipe. Non-ordinary teeth mainly refers to: herringbone slots, Cross slots inside the threaded pipe. The difference between the two applications. In general, the heat transfer efficiency of the grooved and cross-grooved tubes is higher than that of the ordinary single-spiral trapezoidal grooved tubes, especially when the refrigerant is a mixed refrigerant, the most obvious advantage of the inner grooved tube is to adopt herringbone slots and cross slots. 2. Herringbone grooved tube is the best refrigerant for R 407 °C. This is because: R 407 °C is a mixture of three non azeotropic refrigerants, which can be easily stratified when flowing in a spiral tube (especially a smooth tube) in a single spiral groove, and turbulent in two directions when flowing in a zigzag groove (w-shaped groove) , gas-liquid conversion is carried out at the same time, effectively avoiding the reduction of heat transfer efficiency caused by delamination. Although R407 C can be fully mixed in the cross-cogged rifled tube, it is better to use the R407 C refrigerant product in the cross-cogged rifled tube because of the high pressure drop. 3. The internal grooved tube with crossed slots is the most suitable R410A refrigerant. R410A refrigerant product can overcome the negative effects of increased pressure drop in cross-recessed grooved tubes due to high system pressure at high flow rates, the R410A refrigerant can be used in the cross-cogged rifled tube because the internal surface area of the cross-cogged rifled tube is larger than that of the herringbone rifled tube.

4. The limiting factors of the application of non-common tooth internal threaded pipe. Although herringbone grooved and Cross grooved have the advantages mentioned above, the herringbone grooved and cross grooved inner grooved tubes require high materials and have higher production costs, at present its application does not have the ordinary single-spiral trapezoid groove inside the thread tube popularization. For each type of rifled tube, there are different tooth shape parameters, and there are many combinations. The different tooth shape and geometric parameters have great influence on the heat transfer performance of the rifled tube. 5.3 factors to be considered in the design of rifled pipe. According to the research of the engineer, the combination and optimum design of the tooth shape and geometric parameters of the rifled tube should consider three factors, which are geometric size, heat transfer efficiency and processing technology, only in this way can the heat transfer performance of the internally threaded Tube be fully improved. In addition to diameter (i. e. outside diameter) D, inside diameter D and tooth shape, there are bottom wall thickness t w (m m M) , tooth height H F (m m m) , tooth number (I. E. Thread Number) N, spiral angle β, tooth top angle α and groove bottom width W, etc. (as shown in the following figure)

3.8 design factors of internal thread tube tooth shape 1. The bottom wall thickness of internal thread tube has been decreasing for nearly 20 years. At present, the bottom wall thickness of internal thread tube is generally in the range of 0.25 ~ 0.3 m. the thinner the bottom wall thickness is, the better the heat transfer effect is, but the thin bottom wall will weaken the strength of the tube and the stability of the tooth, which is not only bad for the quality of U-BEND and welding, but also bad for the stability of the tooth. 2. The tooth height is an important factor which affects the heat transfer. Increasing the tooth height will increase the heat transfer area of the inner surface and the ability of piercing the liquid film. At present, the tooth height of internal thread pipe is generally in the range of 0.1 ~ 0.25 m. 3. The existence of Helix Angle Helix angle is to make the fluid rotate, make the fluid in the pipeline produce the secondary flow which is different from the radial flow, increase the intensity of the turbulent flow, so that the convective heat transfer is strengthened and the heat transfer coefficient is increased, therefore, the increase of spiral angle can enhance the heat transfer coefficient, but with the increase of spiral angle, the pressure loss also increases, so the spiral angle is not the bigger the better, but there is a reasonable range. At present, the spiral angle of the inside thread pipe is usually in the range of 10 ° ~ 25 ° .

4. The small tooth-top Angle of tooth-top angle is beneficial to increase the heat transfer area of inner surface, decrease the thickness of liquid film for condensation heat transfer and increase the vaporization core for evaporation heat transfer, but if the tooth-top angle is too small, the anti-expansion pipe strength of internal spiral tube tooth is too small, the heat transfer efficiency will decrease with the reduction of tooth height after tube expansion and the increase of tooth deformation. Therefore, the tooth apex angle of the internal thread tube should be as small as possible under the premise of ensuring the anti-expansion strength of the tooth, which is generally between 40 ° and 60 ° at present. 5. Increasing the number of teeth, I. E. The number of threads, can increase the number of vaporized cores, which is beneficial to the measure of boiling heat transfer and increase the heat transfer area of the inner surface. But if the number of teeth increases too much, the space between the teeth will be too small, which in turn will weaken the stirring intensity of the fluid in the tube, increase the thickness of the liquid film between the teeth, increase the thermal resistance, and reduce the heat transfer ability, so that the heat transfer efficiency of the threaded tube approaches to that of the smooth tube, therefore, the number of teeth should be controlled within a certain range is appropriate. At present, the number of teeth of internally threaded tubes is generally between 50 and 70, among which, when internally threaded tubes are used to make evaporators, the number of teeth is between 50 and 60, and when internally threaded tubes are used to make condensers, the number of teeth is between 60 and 70. In addition, when the pipe diameter is small, take a small value; when the pipe diameter is large, take a large value.

6. The width of the groove bottom and the width of the groove bottom are favorable for heat transfer, but the width of the groove bottom is too large, and the degree of the tooth height being pressed down after the tube expansion and the deformation of the tooth shape are increased, so the heat transfer efficiency will be reduced, it’s better to have a wider bottom. 7. Increasing the circumference of evaporation can increase the number of vaporization cores and improve the efficiency of evaporation heat transfer. Therefore, for the evaporator pipe, the greater the cross-section of the circumference of the better. The increase of the moistening perimeter can be achieved by increasing the tooth height and decreasing the tooth vertex angle. 8) for the condensing tube, the effect of condensation heat transfer can be increased by increasing the area of liquid storage. At present, the area of liquid storage is increasing, which can be realized by increasing the tooth height. Among the above parameters, diameter, tooth shape, bottom wall thickness, tooth height, tooth number, helix angle and tooth top angle are the main tooth shape parameters. When the main tooth parameters are determined, the parameters such as groove bottom width, moistening perimeter and liquid storage area are also determined.

4. On the basis of analyzing the different effects of different tooth shape parameters on the heat transfer performance of air-conditioning heat exchanger, engineers have developed a high energy efficiency and low cost air-conditioning product, the experiment of optimizing and adjusting some tooth profile parameters of the original screw tube was carried out. On the basis of the original inner-threaded tube a with the outer diameter φ9 mm, he designed and fabricated five kinds of inner-threaded tubes (B, C, D, e, F) with the same outer diameter φ9 mm and different tooth shape parameters, to replace the heat exchangers made of type A internal threaded tubes on the R F 28 W air conditioners. Under the same test conditions, the performance of the whole machine is tested one by one. The tooth profile parameters and test results of each internal threaded pipe are shown in the table below.

The experimental results are as follows: 1. According to the analysis, the energy efficiency ratio of the air conditioner has been improved from 2.77 to 2.91, which is increased by 5.1% . It is shown that although the number of teeth is favorable to increase the heat transfer area of the inner surface and the number of vaporized cores, which is favorable to the boiling heat transfer, if the number of teeth is too large, the space between teeth and the width of the bottom of the groove will be too small, which in turn will reduce the intensity of the fluid being stirred in the tube, the thickness of liquid film between teeth is increased, the thermal resistance is increased, and the heat transfer ability is decreased. 2. The spiral angle of the c-type tube was increased from 18 ° to 24 ° on the basis of the b-type tube, but it was found that the energy efficiency ratio was not improved, but decreased by 6.9% on the basis of the b-type tube. This is because the larger the Helix Angle is, the better the heat transfer coefficient, but as the Helix Angle continues to increase, the resistance and pressure loss will also increase, so the larger the Helix Angle is, the better it is, but there should be a reasonable value, too Large will reduce the performance of heat transfer. 3. Based on the B type pipe, the d type pipe reduces the tooth top angle from 56 ° to 50 ° . The results show that the energy efficiency of the air conditioner is 3.8% higher than that of the B type pipe. This is because reducing the tooth apex angle can increase the heat transfer area of the inner surface, decrease the thickness of the condensation liquid film, and increase the vaporization core and stability of the evaporation heat transfer. Of course, the tooth top angle can not be too small, otherwise, the internal thread of the pipe teeth of the anti-expansion pipe strength is too small, will cause the screw teeth of the degree of increase.

4. The e Type Tube is based on the D type tube, the tooth height is increased from 0.18 mm to 0.22 mm. However, the installed test found that the increase of tooth height, the corresponding air-conditioning energy efficiency ratio did not improve, but decreased by-4.3% . This is because the stability and anti-back-teeth ability of the internal rifled tube decrease after increasing the tooth height, and the internal rifled Tube deforms seriously after the tube is expanded. 5. On the basis of the e type pipe, the f type pipe reduces the tooth height from 0.22 mm to 0.14 mm. The test results show that the energy efficiency of the corresponding air conditioner is 10.8% higher than that of the original a type pipe (1.7% higher than that of d type pipe) . Furthermore, the mass per unit length of the rifled tube decreased from 88 g/m to 81 G/M, which reduced the material cost of the rifled tube by nearly 8% . This is because when the tooth height is lowered properly, the stability of the tooth is improved, and the degree of the inverted tooth is decreased. 6. It should be noted that although theoretically reducing the thickness of the bottom wall can improve the heat transfer effect and reduce the material cost of the rifled tube, considering that the thickness of the wall outside the bending part of the rifled tube will be thinned after u bending, the amount of thinning is influenced by many factors.

For example, if the tensile strength of the inner-threaded copper pipe is too high and the copper pipe is too hard and the elongation is too low, then the reduction will increase, and cracks will easily appear on the outside of the bending part of the u-shaped copper pipe, the stability of internal thread tube will decrease greatly and the degree of tooth inversion will increase. Therefore, in order to ensure the reliability and durability of the air conditioner, I. E. The quality of the product, the thinning test of the bottom wall has not been done.

5. Conclusion 1. Through the experiment, we think that the heat transfer Coefficient will not be increased indefinitely by increasing the heat transfer area, and the inner surface area of the threaded copper tube will be increased to a certain extent, the improvement of heat transfer coefficient should be realized by the influence of other parameters. 2. It is more significant to optimize the parameters of tooth shape than to increase the area of heat transfer. 3. Because of the difference of heat transfer mechanism between evaporation and condensation, the influence and effect of several parameters on heat transfer coefficient are mutually restricted, so it is necessary to start from strengthening heat transfer mechanism, through the guidance of scientific theory and detailed experimental verification, the optimal design of tube-type parameters was carried out, from which the tube-type parameters with good performance of evaporation and condensation heat transfer were found. 4. By optimizing the tooth shape parameters of the internal spiral tube, not only can the heat transfer coefficient of the refrigerant and the heat transfer performance in the heat transfer tube be greatly increased, but also the performance and energy efficiency ratio of the air conditioner be improved and the energy consumption be reduced, and can greatly reduce the quality of the internal thread tube unit length, reduce the production cost of air conditioning products. 

Source: Refrigeration Air Conditioning Heat Exchanger

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