Analysis of Entropy Production and Analysis of Small Refrigeration System
1 This paper mainly introduces the development of micro-refrigeration system and entropy production model, and analyzes the change of entropy yield of various components in the system as the size of refrigeration system decreases. 2 Entropy production model of micro refrigeration system 2. 1 Introduction to Micro Refrigeration System The micro-refrigeration system is a typical example of the application of advanced micro-energy systems in engineering. The US military has applied this advanced technology to the Iraq war and has attracted widespread international attention. At present, there are only a few research institutions in the United States that are conducting research on micro-refrigeration systems. There are no relevant research reports in Europe and Japan. The research in China is still blank. The author conducted systematic and in-depth research on the micro-refrigeration system and developed a prototype of a micro-vapor compression refrigeration system. The micro-refrigeration system consists of a micro-motor, a micro-triangular rotor compressor, a parallel flow condenser, a capillary tube and a micro-spiral evaporator. As shown, the 12V high-energy lithium-ion polymer battery is used as the power source for continuous operation for 2 hours. System size: 265 mm × 250 mm × 120 mm, total weight of about 2. 85 kg, can produce about 300W of cooling capacity at 40 ° C ambient temperature. The efficiency of the compressor is the key to the entire refrigeration system. The primary goal of the micro-refrigeration system is light weight, and the triangular rotor compressor has the advantages of simple structure, high efficiency, long life, low vibration, low noise, small volume and high speed. The weight is only 0. 4kg. The weight is only 0. 4kg. The weight is only 0. 4kg. The compressor is semi-closed and connected to the brushless DC motor via an elastic coupling for variable speed operation. The cooling method of the compressor is air-cooled, the cylinder and the end cap are cooled by air, and the triangular rotor is cooled by lubricating oil. The advection condenser is developed by a tube-and-belt condenser. It is also composed of a flat tube and a heat sink. It absorbs various new technologies of tube-and-belt type, with advanced structure, high heat transfer coefficient, low material consumption, and shape. Small in size, it is one of the most promising forms of condensers at present, and it is a technically mature product. We designed and developed a miniature multi-parallel flow condenser for the micro-refrigeration system, which is based on the parallel flow structure and is provided with a variable-transition structure and optimized design. The micro-refrigeration system uses a micro-spiral evaporator, which weighs only 0.16 kg, and enhances heat transfer through roughening of the flowing surface and internal twisting. Because the cooling capacity of the system is small, and the demand for cooling capacity does not change much, the capillary is used as the throttling device of the refrigeration system. 2. 2 Entropy production model of micro refrigeration system In conventional refrigeration systems, in addition to cryogenic refrigerators, heat leakage inside the system is negligible. The micro-refrigeration system is small in size, and the heat leakage from the high-temperature component (condenser) to the low-temperature component (evaporator) cannot be ignored. In the actual refrigeration system, when the system has heat exchange with the outside, the entropy production can be divided into two parts, internal entropy production and external entropy production. In the micro-refrigeration system, internal entropy production accounts for the majority. Therefore, we mainly discuss the entropy production inside the micro refrigeration system. 2. 2. 1 compressor One of the reasons for the inconsistency between the volumetric efficiency and the isentropic efficiency of the compressor is the heat transfer between the internal high temperature chamber and the low temperature chamber, due to the internal heat transfer, the entropy is produced in the S gen) H. TDDD compressor due to heat transfer. Entropy production, J / KQ WD heat transfer, J hD heat transfer coefficient, W / (m 2 K) A WD heat transfer area, m 2 ΔTD heat transfer temperature difference, flow friction of refrigerant in K compressor is caused by entropy production Another factor. Many scholars have studied the entropy production caused by viscous friction in the flow channel. The formula S gen) ΔPDDD compressor entropy production caused by viscous friction, J / K m Mass flow rate of DDD refrigerant, density of kg/sÏDDD refrigerant, kg/m 3 fDDD friction factor DDDD runner diameter, m ADDD runner cross section, m 2 TDDD refrigerant temperature, K 2.2.2 condenser and evaporation Device The entropy production caused by the temperature difference heat transfer and the flow friction in the heat transfer process of the heat exchanger can be calculated by the following formula. The ideal expansion process in the refrigeration system is an isentropic process, and the cooling effect is optimal at this time. In an actual vapor compression refrigeration system, the expansion process is actually an isobaric process, inevitably entropy production. The entropy production in this process is equal to the entropy change. 2.2.4 Internal heat leakage in the system Because the temperature of the evaporator is lower than in a low temperature environment, the external space or high temperature components of the system will continue to transfer heat to it. In a micro-refrigeration system, the harm of such heat transfer is not large, and the following analysis only considers the case of irreversible heat conduction. The internal heat leakage entropy of the system is the sum of the total entropy production in the J/K refrigeration system. 3 Entropy production analysis of micro refrigeration system Compared with the traditional device, the micro-refrigeration system developed by the author is not only the overall size, but also has an internal structure and a flow channel size that are reduced by one to two orders of magnitude. Therefore, the entropy production changes caused by the miniaturization of the system need further discussion. To this end, we select several different sizes of refrigeration systems to compare the entropy yield under the same working conditions. Model B is the cooling system we have developed. The other structural parameters are the same as model B. The method is designed. Structural parameters of some components in each system. According to the formula, the calculation formula of the entropy yield in the condenser and the evaporator is the same, and the condenser is taken as an example to calculate only the structural parameters of the condenser. It is clear from the formula that the size of the entropy yield in the capillary is independent of the size of the capillary, and no further calculation is performed here. Since the processing methods of the micro-refrigeration system prototype are basically the traditional processing technology, the surface processing precision of the system components of various sizes is considered to be the same in the analysis. 3. 1 Entropy yield calculation in the compressor According to the formula, the entropy yield of the internal mass flow rate of the compressor can be obtained. The Js/(kgK) can be seen. The entropy production in the compressor consists of two parts: the entropy production caused by the irreversibility of the viscosity of the refrigerant and the compression. Entropy production caused by irreversible heat transfer between the high temperature chamber and the low temperature chamber. Because the compressors have the same operating conditions and similar structural dimensions, we can use the corresponding structural dimensions to calculate the equation. D is calculated by R - e, L is calculated by R + e, A is calculated by Rb, assuming that the heat transfer coefficient is the same between heat and pressure in each compressor, and the heat transfer coefficient can be approximated by the empirical value of gas-gas heat transfer. Calculation. It is possible to derive the entropy yield of compressors of various specifications, and the curve of entropy yield in the compressor as a function of size (creation radius). As can be seen from the figure, as the size of the system decreases, the entropy yield in the compressor increases. 3.2 Entropy yield calculation in the condenser For the sake of simplicity, it is assumed that the Nu number in the laminar flow region is constant, the friction factor is inversely proportional to the size, and the heat exchange amount of the condenser or the evaporator is proportional to the flow rate of the refrigerant. The entropy yield per unit mass flow caused by the pressure drop per unit mass flow caused by the temperature difference, and the parameters in the Js/(kgK) formula can be calculated according to the empirical formula. The curve of entropy yield in the condenser as a function of size (runner diameter). The entropy yield caused by the temperature difference decreases as the size of the heat exchanger decreases, and the heat transfer caused by friction increases as the size of the heat exchanger decreases. The total entropy yield in the heat exchanger does not change significantly with the size reduction. 3.3 Entropy yield calculation caused by irreversible heat conduction in the system According to the formula, the entropy yield caused by irreversible heat conduction in the system assumes the same thermal conductivity k of irreversible heat conduction in each system, and defines a dimensionless entropy yield. In addition, assuming that the connecting line between the high and low temperature components of the system is proportional to the length of the heat exchanger, A can be calculated from the cross-sectional area of ​​the heat exchanger, and L is calculated from the length of the heat exchanger. The change in N s varies with system size, and the entropy yield caused by irreversible heat conduction in the system increases as the size decreases. The curve of the entropy yield caused by irreversible heat conduction in the system is the statistical result of the thermodynamic perfection of different specifications of the refrigerator. The curve is fitted by the least squares method. The thermal perfectity of the refrigerator is η= COP Real / COP ideal, it can be seen that the size effect of the cooling capacity of the system is very obvious, which decreases as the cooling capacity of the system decreases. 4 Conclusion The prototype of the micro-refrigeration system developed by the author is introduced. The entropy production model of the vapor compression refrigeration system is established based on the prototype. The variation of entropy yield of each component in the system is analyzed with the reduction of the size of the refrigeration system. After the size of the refrigeration system is reduced, the entropy yield in the compressor and the entropy yield caused by irreversible heat conduction in the system increase, while the entropy yield caused by the temperature difference in the heat exchanger decreases, and the entropy yield caused by friction increases. The total entropy yield is basically unchanged. For micro-refrigeration systems, improving the processing accuracy of micro-compressors and reducing internal heat leakage are the key to the successful operation of the system.
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