Important.
The complete compressed air system consists of an air compressor unit, a post-processing unit, a conveying line, a pressure control, and a pneumatic component, that is, a compressed air pneumatic system. The working principle of the system is: the compressor compresses the air to the rated pressure, the post-processing equipment processes it, the pipe network is transported to the gas unit, and then the pressure is used to drive the pneumatic tools and components to work. The typical system is as shown.
Typical compressed air pneumatic systems indicate that with the emphasis on energy conservation, more and more people are beginning to study the aerodynamic efficiency of compressed air system output. Due to the complicated process, it is difficult to obtain accurate and clear results. Many people have only a rough, fuzzy concept about the efficiency of pneumatic systems, and lack accurate quantitative knowledge.
This paper aims to study the compression and aerodynamic processes through the thermodynamic analysis of PV maps, and accurately determine the output energy efficiency of the various process conditions of the system, and can be visually displayed through the diagram. The purpose is to make everyone clear the system's various forms, process efficiency, optimize the technical requirements and design parameters of the system, and rationally select high-efficiency pneumatic tools and components; also provide research on compressed air as power drive and energy storage.
2 Compressed air system energy analysis The compressed air energy in the compressed air system is composed of enthalpy, kinetic energy and potential energy. Compressed air delivery, use process, exercise energy and potential energy are relatively small to be negligible. The energy formula of flowing compressed air is: Cp - isobaric specific heat 0 air temperature For the convenience of analysis, this paper converts the energy in the compressed air system, simplifying that the compressor group works to compress the air, the energy is stored in the compressed air, and the compressed air The pneumatic tools and components release energy to output mechanical work to achieve power transmission. In this paper, the energy process of the analysis system is realized by analyzing the compression or expansion of the compressed air volume V and the pressure P change process.
Performance concept. The external function of compressed air is the part of compressed air that can be converted into mechanical energy. This part of energy is called the effective energy of compressed air. The effective energy is the functional force of compressed air under isothermal expansion in the relative environment, and it is the ideal function of compressed air.
The effective aerodynamic forces contained in the compressed air stream are: a compressed air absolute pressure - a compressed air volume flow - an air volume P. - atmospheric pressure type (2), (3) can be used to calculate the compressed air of each process in the system Effective energy.
The PV diagram of the system thermodynamic process is shown.
The basic process of system thermodynamics PV-like compressor (taking screw compressor as an example) thermodynamic working process: 1-2-3-4 direction, the work done on gas is: 1-2-3-4 -1 area 5. The aerodynamic effective energy thermodynamic working process is: 4-5-2-1 direction, the mechanical work done by the gas is: 4-5 -4 area, which is the isothermal expansion process, equal to the isothermal compression work, is an ideal process.
3 Typical Compressed Air System Pneumatic Process In order to gain an intuitive understanding of efficiency, the PV diagrams in this paper will all be scaled and can be displayed by comparison of the area shown. In order to facilitate the analysis of the diagram, the air volume of the pneumatic unit in the PV diagram is the same as that of the compression unit; the pneumatic components select the pneumatic cylinder and the air motor, and take the thermodynamic process as an example.
A typical compressed air pneumatic system is: a compressor unit, a post-processing device, a pressure regulating device, a pneumatic machine and a pipe, as shown. This paper mainly studies the energy efficiency of thermodynamic processes of compressed air aftertreatment, transportation, pressure regulation control and pneumatic principle. Compressor technology is very mature, and the efficiency is very clear. This article simplifies the use of compressor specific power. (Because the unit input specific power actually reflects the energy efficiency of the compressor, the specific power mentioned in this paper is all the unit input specific power).
At present, the power air compressor and the screw compressor occupy the majority. In this analysis, the screw compressor power diagram is taken as an example to analyze the compression process, the specific power value, and the compression process efficiency. Compressor, pneumatic process PV diagram, compressed air as the ideal gas, the process is ideal, no pressure loss. The various energy losses of the compressor are considered in the specific power (other types of compressors, including turbo compressors, you can get the efficiency of the compression process by comparing the specific power).
The system parameters are taken as general compressed air power system. The compressor inlet pressure is: 0.1MPa, the output pressure is: 0.8MPa (the pressure is absolute pressure in the figure and the figure), and the air compressibility index is: n = 1.1, PV shows The power diagram is as shown. The unit input specific power range of the compressor is 5.6~17.4kWm3/min), the specific power value of the micro compressor is large, and the specific power value of the large centrifugal compressor is small. In this paper, the energy efficiency value of 6.9kW/(m3/min) of a typical 55~90kW screw compressor can be used as the average power of the compressor for calculation efficiency.
-2-3-4-1 is the compression process of the compressor to the air. The compressor's work on the air Wm is 1 -2-3-4-1; the compressor has an efficiency of 55.9% for air. Analysis of the situation: (1) compressed air delivery pressure loss. Compressed air is subjected to post-treatment and pipeline transportation, and there is pressure loss; (2) gas pressure loss. When the gas unit is used for control adjustment and gas use, pressure loss will occur again.
3.1 Compressed air delivery pressure loss situation In actual operation, after the compressor outputs compressed air, there will be pressure loss after the post-processing equipment and pipeline. Usually, the pressure difference of 0.1MPa is normal. If the system design and operation management are unreasonable. , there will be a large pressure difference. The following analysis of the output energy efficiency of several differential pressure losses occurred.
After the compressed air is produced, it is cooled and then post-treated.
During the conveying process, the pipeline is relatively long and the compressed air will improve cooling. This paper assumes that the pipeline transportation process and the environment are fully heat exchanged and cooled to the same temperature as the environment, that is, the normal pneumatic cylinder and air motor of the =T, the compressed air work is close to the isobaric driving process. In this paper, pneumatic cylinders and gas motors are typical, and they are studied in the isostatic process.
(1) Assuming that there is no pressure loss in compressed air delivery, such as -2-5-4 is the ideal isothermal compression process, the effective energy of compressed air is equal to the isothermal compression work: the area of ​​1 -2-5-4-1 is also the formula ( 2) The effective energy indicated is the maximum work that compressed air can make externally, and it is also the maximum energy that pneumatics can obtain. The calculated effective energy efficiency of the compressed air system is calculated to be 50.6%. The compressed air aerodynamic work expansion is basically not utilized. 2 The thermodynamic process is 4-5-6-1; the system output aerodynamic energy is 4 -5-6- The area of ​​1-4, the grid area in the figure; the system output aerodynamic energy efficiency is 21.1% (2) when pressure loss occurs in compressed air delivery, 5, 6, 7, and 8 are shown. The compressed air works as a 5-6-7-1 process; the effective energy 压缩 of the compressed air is 5-6-7-1 -5, which is the grid area in the figure. Below is the energy efficiency of compressed air output to pneumatic components for several pressure losses.
=0.7MPa, see, system output pneumatic energy efficiency: 20.7%; when the pressure loss is 0.2MPa, namely: Pd-P! =0.2MPa, P3=Pi=0.6MPa, see, system output aerodynamic energy efficiency: 20.1% pipe network without pressure loss when the system PV diagram pipe network 0.2MPa pressure loss when the system PV diagram reasonable piping, joints, there should be no Excessive decompression, this process is often adiabatic expansion, or close to adiabatic endothermic expansion, due to the short process, compressed air can not get better heat. In order to facilitate the analysis, this paper studies the pressure loss of gas unit as the adiabatic expansion process.
The following process takes the delivery pressure loss to be 0. 7MPa), and when several pressure loss values ​​appear in the gas unit, the output efficiency of the compressed air to the pneumatic component is obtained. For example, ~12,6-7 is the adiabatic expansion line, and the compressed air works as the process of 9 -7-8-1; the effective energy 压缩 of the compressed air is the area of ​​9-7-8-1-9, that is, the grid area in the figure. .
Volume efficiency: 19.3%; = 0.4MPa, see, system output aerodynamic energy efficiency: 18.1%; volume efficiency: 16.1%. 3.2 gas unit pressure loss situation in the compressed air system, when the gas unit is close to the pneumatic components, will Installation of pressure regulating control components, but also a large number of not for the convenience of analysis, this article will not be analyzed as an ideal state of the process, the actual process is very complex, there are many factors affecting the aerodynamic efficiency of the compressed air system, the system has many Energy loss, such as the larger ones are: (1) usually 10%~30% leakage loss 6'7; (2) compressor unit gas volume loss, efficiency loss 5%~20%; (3) compression If the operating parameters of the machine are unreasonable, there will be 3%~10% efficiency loss. For example: 8MPa pressure air compressor, the operating pressure will increase by 6% for every 0.1MPa increase. Therefore, it can be concluded that in the actual operation of the system, the output aerodynamic energy efficiency will be about 20% lower than the efficiency described in the previous section, or even more.
Due to the different efficiency of the compressor, the difference in the efficiency of the compressor to the air is also very large, the efficiency of the large compressor is relatively high, and the efficiency of the small compressor is much lower. If the system uses a small compressor, the efficiency is even only about 1/2 of the calculation in this paper.
The actual compression air pneumatic system, the mechanical loss of pneumatic tools and components, and the loss of residual air in the piping have a great impact on system efficiency. According to 8, the efficiency of the pneumatic cylinder drive system is only 6%. Now there are various energy-saving type research booms, and many people are conducting research on compressed air energy storage. In order to balance the use of electricity, etc., when the electricity is low, the electric energy is converted into compressed air energy storage energy, and when the electricity peak is used, the compressed air is released to be converted into electric energy and other energy; and the other is to convert the electric energy into compression. Air energy storage for driving cars, ships, etc. Due to the high pressure storage, the lower efficiency of the high pressure compressor and the decompression loss before the release of the stored energy will reduce the efficiency of the system.
This paper analyzes the aerodynamic thermodynamics process of compressed air system by TV graphic method, and provides a convenient and intuitive way to understand the output energy efficiency of compressed air system and affect the efficiency factor. By analyzing the typical parameter system as an example, the aerodynamic efficiency of compressed air system is obtained. The situation is clearly understood.
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