Less energy; how to measure the amount of energy.
1 Energy Analysis of Compressed Air In pneumatic systems, the flow of compressed air is quite complex.
The pneumatic system that pushes the cylinder movement. When the reversing valve 1 is reset, the cylinder 2 has no atmospheric pressure port in the rod cavity and the supply pressure in the rod cavity. 8. The entire pneumatic system is in a non-flowing state, but there is no rod cavity. The static air inside is already fixed. When the valve 1 is reversed, the compressed air is inflated into the rodless cavity of the cylinder. At the same time, the compressed air without the rod cavity is passed through the teaching and research work of the draft control.
The reversing valve is vented outward. At the beginning of the inflation, the pressure ratio at both ends is also smaller than the critical pressure of the pneumatic inflation circuit, so that it is inflated at the speed of sound. Until the ratio of the pressure in the cylinder to the ps is greater than the value of b, the cylinder is inflated to the subsonic inflation. Then, take the increase until the inflation is changed from subsonic inflation to low speed inflation until the inflation is stopped. The cylinder-free cavity is the sound velocity exhaust from the beginning, and gradually becomes the subsonic exhaust low-speed exhaust until the exhaust stops. In the process of charging and discharging, in the flow direction, there is also a pressure loss due to the flow, that is, energy loss, and mechanical energy is converted into heat energy. And on any cross section of the flow, flow parameters such as velocity pressure and the like are not uniform. During the operation of the cylinder, there is also a phenomenon of external work and heat exchange with the outside world. From the above analysis, the cylinder itself has the thermodynamic energy of the internal gas, the heat exchange with the outside world and the energy conservation and conversion of the external work. The gas flow in a pneumatic system is a kind of non-incompressible and compressible flow. There are non-ideal fluids. There is pressure loss. Unsteady flow is the charge and exhaust flow. It is very difficult to analyze various energies such as thermodynamic kinetic energy, pressure energy, heat exchange, etc. and their mutual transformation, so we analyze the energy of static compressed air and the flow of compressed air in a fixed time. The total energy consumption is that you can do more work.
2 Energy analysis of gas without obvious macroscopic motion When the gas in the pneumatic component or system has no obvious macroscopic motion, that is, the gas flow velocity is much smaller than the microscopic molecular motion velocity, such as the gas movement in the cylinder during the operation of the cylinder. During the change of the state of the cylinder gas from the state of thermodynamic energy to the state of thermodynamic energy, the outside sends heat to the gas in the cylinder. At the same time, the gas in the cylinder acts on the outside as energy conservation and conversion law, and there is Q. =IrIi+W1 The gas in the cylinder works externally, 0; the outside works on the gas in the cylinder, and the thermodynamic energy changes in several specific states change process power and process heat, etc.
product. The difference between the thermodynamic energy and the enthalpy change of the unit mass gas. It is the mass constant pressure heat capacity, which is the mass constant volume heat capacity, let =! It is the heat capacity ratio, also known as the isentropic index. And 9 are the process work and process heat per unit mass of gas. Is the heat capacity.
The specificity of energy conservation and conversion is now related to the thermal process. For example, in the isovolumic process, the heat absorbed from the outside is used to increase the thermodynamic energy of the gas in the cylinder, which is now a rise in temperature. In the isothermal process, the thermodynamic energy does not change. When isothermal expansion, the heat absorbed from the outside is used for external work; while isothermal compression, the external work on the gas is all converted into heat released by the gas to the outside.
Talking about the gas energy without obvious macroscopic motion usually means that the gas is in a certain state, that is, the beginning and end of the thermal process, and it is not necessary to analyze the energy in the state change process. We know that a stationary gas is an energy that includes a microscopic kinetic energy when a unit mass of gas flows. When 1 gas flows into the system through the outside, not only the mass potential mechanical energy of the gas is brought into the system, but also the propulsive work obtained from the back can also be called pressure energy. Because of the pressure, the generated driving force is also brought into the system. The system, so in the process of thermal changes, although the thermodynamic energy 1 and the propulsion work can be transformed into each other, only the transformed work can be used as mechanical energy, that is, the thermodynamic energy that is not converted into the work can not be used as mechanical energy.
Therefore, for a gas of mass and volume, using the gas state equation, = and D, the total mechanical energy 1 can be derived from the following formula.
This total mechanical energy is the ability of the compressed gas to have mechanical work. The total mechanical energy is the product of the pressure P of the compressed air and the volume V.
It should be noted that the pressure in Equation 3 should be on a pressure gauge. Because pushing the cylinder movement must overcome the reaction force of the atmospheric pressure existing in the outside world, the actual total mechanical energy must be deducted from the atmospheric pressure. The specific thermal process is equal volume process, =0 isobaric process isotherm process Ding adiabatic process mouth =, state equation The change process of thermodynamics of 辜 辜 çƒ çƒ åŠŸ 功 功 = = = = = = = åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ åŠ
It should also be noted that the total mechanical energy of the compressed air calculated according to Equation 3 is not the total energy of the compressed air. Quality is! The total energy of the compressed air of 1 is from the formula 2. It should be that the total energy is the total energy calculated based on the complete stopping of the gas molecules. It is of course impossible to obtain this energy because the gas molecules cannot stop moving. We only emphasize here that the total mechanical energy of the compressed air calculated according to Equation 3 is not the compressed air. It should also be noted that the air at atmospheric pressure also has mechanical energy, because the atmospheric pressure gas still has a function relative to the vacuum state. The ability of mechanical work.
Example l It is known that a 3m3 gas tank stores air with a pressure of 0.7MPa and a temperature of 293, asking how much mechanical energy the air in the gas tank has, and answering the formula 3, the total mechanical energy of the air in the gas tank is 0.73 gas incompressible. Energy Analysis During Flow The energy conservation equation for a unit volume of gas established between the constant incompressible and two slowly varying flow sections 12 is the inter-pressure loss head.
The sum is equal to the static pressure on the 2 cross section, and the pressure port at 10 places between the 2 dynamic pressure +1 and the 1 to 2 cross section. It is called the total pressure and can be written in the above formula. The total pressure should not contain the pressure loss term. Since the pressure loss has been converted into heat energy, it can no longer be reversibly converted into mechanical energy, so the mechanical energy only contains pressure energy and kinetic energy. And still in the above formula, should be based on pressure gauge.
And both are based on absolute pressure, there is flow power N.
The volume flow that will be converted to the standard state is = this. For the incompressible flow, it can be approximated that the temperature 1 in the pressurized state is equal to the standard state, and the total flow power on the slowly varying flow section in the pneumatic circuit can be calculated from Equation 7 or Equation 8. The flow is provided by the source pressure Ps. The maximum total flow power provided by the gas source is obtained. Example 2 It is known that the pressure of a pneumatic circuit is 0.8 屯. The connection of the 40 series pressure reducing valve is 艮12. The pressure of the valve with the inner diameter of the valve is 16 PCT, and the pressure after the valve is stabilized at 0.32. The flow rate through the valve is 3000LminANR. The gas temperature before and after the valve is 293. The total flow of the gas before and after the valve. What is the power, if the valve is stable, how much energy should be supplied by the gas source, how much energy is lost through the pressure reducing valve, and the first section is 1 section, and the valve is 2 section, then 1 = total valve The pressure line + the morale + the sect of the father's circuit, the flow of gas should provide the energy 6 =, the loss of input energy. 1.
4 When the gas is compressible, the energy is analyzed. When the gas is in a compressible flow, the temperature changes, and the thermodynamic energy is involved in the conservation and conversion of energy. Therefore, under the adiabatic condition of the element, the energy conservation equation of the unit mass gas is the pressure energy 1 and The sum of kinetic energy remains the same and can be transformed into each other. However, the thermodynamic energy of P 2 must be converted into mechanical energy and kinetic energy before it can be used as mechanical work. Therefore, the total flow power of gas in compressible flow should be the product of the sum of pressure energy and kinetic energy per unit mass of gas and mass flow. There is a pressure at 10, called total pressure, the density is called total density, the temperature is called total temperature, and if the mass flow rate is converted to the volume flow rate 4 under standard conditions, then the absolute pressure in the above formula is already. If it is known to measure the volume flow through the loop in the standard state and measure or calculate the total pressure on a slowly varying flow section. And total temperature Ding. , the total flow power N on the section can be calculated by Equation 10.
When in the formula 13, and Ding. When the air source pressure ps and the temperature Ts of the pneumatic circuit are 5, the N calculated by the equation l3 is the maximum total flow power of the circuit.
Example 32 blowing circuit, I know the source pressure 01! 7. The gas source temperature 1 is the set pressure of the 293 pressure reducing valve. The effective cross-sectional area between the outlet of the pressure reducing valve and the nozzle inlet is 4, 2. The effective sectional area of ​​the nozzle is 2, and the total blowing is required. What is the flow power, what is the maximum total flow power of the blow, the blow 53, how much energy is lost in the loop, and the absolute pressure. Under the action of the following formula, the sound velocity can be blown in the nozzle. Let 32 ​​be the critical section = according to the gas dynamics function 5 from 275, find the Mach number in the S1 pipeline M1 = 0.165, so i = 1.0185 130.39953 absolute pressure. According to Equation 2, the maximum total flow power of the blown air can be calculated by the flow rate of the nozzle. 5 The metric of the compressed air energy is discussed. From the above analysis, the following formula for calculating the compressed air energy is proposed.
The total mechanical energy is used to measure the energy of the static compressed air. The total mechanical energy can be calculated from Equation 3.
The total flow power is used to measure the energy of the flowing compressed air. Equation 8 calculates the total flow power over the flow cross section of a slowly varying flow section in an incompressible flow. The energy loss between the two slowly varying flow sections can be calculated from the difference between the total flow powers of the adjacent two slowly varying flow sections.
The maximum total flow power is used to measure the amount of energy that has been used by a pneumatic circuit or pneumatic device. The maximum total flow power of the incompressible flow can be calculated by Equation 9. The maximum total flow power of the compressible flow can be calculated from Equation 14.
There is a view that the effective energy of compressed air with a volume of pressure can be expanded by isothermal expansion of the compressed air into atmospheric pressure. Compared with other processes such as adiabatic processes, the theoretical work is the largest. Therefore, it is considered that the effective energy of the compressed air is that the effective power when the compressed air flows is when the pressure is, the volume is compressed air, the pressure is expanded from the isothermal to the pressure team, and the heat is absorbed from the outside, and the heat is used again. To work externally, the size is up to the formula 15, but the formula 15 is not the pressure, the volume, the effective energy of the compressed air itself. Like the isometric process, although the external work is zero, it does not mean that the gas of the process has no effective energy. Therefore, it is also wrong to use Equation 16 to calculate the effective power of the flowing compressed air.
For example, according to Equation 3, the total mechanical energy of the compressed air is 21刈05. If calculated according to Equation 15, the effective energy of the compressed air can be up to 6. The measurement of the compressed air energy The total mechanical energy of the static compressed air can be calculated by Equation 3. Therefore, as long as the volume of compressed air is measured to be 0.
In order to determine the total flow power of the flowing compressed air, in addition to measuring the volume flow rate in the standard state, it is also necessary to measure the relatively small cross-flow section, and it is difficult to measure the total pressure and pressure. As a project, and not as a research class, you can not care about the measurement of the total flow power. As long as the maximum total flow power is measured, it is known how much energy is used in the pneumatic circuit or pneumatic device, so that the energy saving measures and progress can be improved. Run costing.
Measuring the maximum total flow power, from Equations 9 and 14, it is only necessary to measure the pressure of the air source used in the pneumatic circuit or device, and the temperature of the air source to measure the volume flow through the standard state of the pneumatic circuit or device. .
Volume flow rate under pressure or volume flow under standard conditions.
Because the volume flow rate in the state of the pressure in the pipeline, it should be measured at the same time 0. Known, and can be calculated using Equation 17.
For a charge and exhaust circuit like the one, the maximum total flow power is a function of time, and if the change of wind and time 1 is measured. Then, in a fixed time period, the total mechanical energy consumed by the pneumatic circuit is the area enclosed by the 4 curve and the time axis. The formula is Wu Yushi. University Physics 1. Xi'an Xi'an Jiaotong University Press, 1990.
810 China Ltd. The second edition of modern practical pneumatic technology. Beijing Machinery Industry Press, 2004.
Xianglichun air pressure is a province of labor. Annualized Jフフレ, パ, Xu Wencan. Blowing energy saving habits. Hydraulic Pneumatics and Sealing, 2005, 5.
Abramovich Practical Gas Dynamics 1. Beijing Higher Education Press, 1955.
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