Pneumatic systems using compressed air as a medium have been widely used. People have noticed that the compressed air system consumes about 10% of the national electricity consumption per year. For the current pneumatic application system, it can save 10%~30% of the air consumption. This is very valuable for a power-poor country like China.
To save energy in a pneumatic system, first of all, how much energy is used in a pneumatic system or a unit's pneumatic application system, how is this energy used, and then further analysis of which energy can be taken to save it? . This first encountered two problems, one is how to measure the amount of energy of the flowing and non-flowing compressed air; the second is how to determine the size of the energy.
1 Energy Analysis of Compressed Air In pneumatic systems, the flow of compressed air is quite complex. As shown, the pneumatic system that pushes the cylinder movement, when the reversing valve 1 is reset, the cylinder 2 has an atmospheric pressure pa in the rodless chamber, and the rod chamber has a supply pressure ps, and the entire pneumatic system is in a non-flowing state. However, the static compressed air in the rodless chamber already has a certain amount of energy. When the valve 1 is reversing, the compressed air is inflated into the rodless cavity of the cylinder, and at the same time, the compressed air without the rod cavity passes through the collection period: the teaching and research work of the control. The reversing valve is vented outward. At the beginning of the inflation, the pressure ratio Pa at both ends is also generally smaller than the critical pressure ratio b of the pneumatic inflation circuit, so it is inflated at the speed of sound. Until the ratio of the pressure P to the ps in the cylinder is greater than the value of b, the cylinder is inflated to the subsonic inflation. As P/Ps increases until 1, 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, gradually becomes 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 (ie, energy loss, mechanical energy is converted into heat energy). And on any section of the flow, flow parameters (such as speed, pressure, etc.) are not uniform. During the operation of the cylinder, there is also a phenomenon of external work and heat exchange with the outside world. It can be seen from the above analysis that 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 non-uniform, incompressible and compressible flow, a non-ideal fluid (with pressure loss), and an unsteady flow (flow and exhaust flow). It is difficult to analyze various energies (such as thermodynamic energy, kinetic energy, pressure energy, heat work exchange, etc.) and their mutual transformation for such complex flows, but we analyze the energy and flow compression of static compressed air. The total energy consumed by air for a certain period of time can be achieved. This makes it possible to carry out many tasks such as the cost of compressed air, energy saving analysis, and the like.
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 I2, the external heat is transferred to the gas in the cylinder, and at the same time, the gas in the cylinder acts on the outside as W, according to the conservation of energy and The conversion law has Q=(I2-)+W(1)0; it releases heat to the outside, Q<0. The gas in the cylinder works externally, W>0; the outside works on the gas in the cylinder, W<0. The thermodynamic energy changes, process work and process heat of a particular state change process are shown in Table 1.
In Table 1, the pressure, temperature and mass volume of the gas are respectively. Ai and -h represent the change in thermodynamic energy per unit mass of gas and the change in enthalpy, respectively. Cp is the mass constant pressure heat capacity, Cv is the mass constant volume heat capacity, let =01/(is the heat capacity ratio, also called the isentropic index. w and q are the process work and process heat per unit mass of gas. C is the specific heat capacity .
It can be seen from Table 1 that the specific performance of energy conservation and conversion is 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 represented by 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 the enthalpy of a stationary gas is the total energy of the gas (including thermodynamic energy and mechanical energy).ç„“h is the energy of a unit mass of gas that has a viscous microscopic kinetic energy when flowing. When 1 kg of 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 latter (also called pressure energy, which is the driving work generated by the pressure p) pV Also brought into the system, so in the process of thermal changes, although the thermodynamic energy 1 and the propelling power Pv can be converted into each other, only the converted driving power can be used as mechanical energy, that is, the thermodynamic energy that is not converted into the propelling work cannot be used as mechanical energy. of.
Therefore, for a gas of mass m and volume V, the total mechanical energy Ep can be derived from the following equation using the gas state equation pV=RT.
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 p in the formula (3) should be in gauge pressure. Because the movement of the cylinder must overcome the reaction force of the existing atmospheric pressure, the actual total mechanical energy must be deducted from the atmospheric pressure. Several specific thermal processes are specified. The thermal process is equal to the volume process v=C isobaric process p= C isotherm process T=C adiabatic process q=0 state equation thermodynamic energy change ç„“ change process work w=! Pdv finds p2=0.3925MPa (absolute pressure) under the action of p2, the sound velocity can be blown in the nozzle. Let S2 be the critical section (M=1). According to the gas dynamics function table, from 0.275, find the Mach number in the S1 pipeline å£=0.16, so £=1.018, the following formula can be calculated through the nozzle. Flow rate qa to determine the maximum total flow power of blowing (767-657) x5 = 5505 Measured reference of compressed air energy From the above analysis, the following formula for calculating 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. From equation (8), the total flow power over a section of a slowly varying flow in an incompressible flow can be calculated. From equation (13), the total flow power over a section of a slowly varying flow in the compressible flow can be calculated. 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 from 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 pressure of P and a volume of V can be expressed by the work of isothermal expansion of the compressed air into atmospheric pressure. The reason is that, in the isothermal expansion process, the compressed air is the largest in terms of external work compared to other processes (such as adiabatic processes). Therefore, the effective energy of compressed air is considered to be and the effective power when the compressed air flows is compressed air with a pressure of p and a volume of V. During the process of pressure isothermal expansion from p to pressure Pa, heat is absorbed from the outside, and the heat is again All are used for external work, and the size is expressed by the formula (15), but the formula (15) is not the effective energy of the compressed air itself having a pressure of p and a volume of V. 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 calculate the effective power of the flowing compressed air by using equation (16).
For example 1, according to formula (3), the total mechanical energy of the compressed air is 21×105. If calculated according to formula (15), the effective energy of the compressed air can reach 49.9×105. The difference is too large.
6 Determination of compressed air energy The total mechanical energy of static compressed air can be calculated by equation (3), so that the gauge pressure p of compressed air having a volume of V can be measured.
To determine the total flow power of the flowing compressed air, in addition to measuring the volume flow rate qa in the standard state, the total pressure p and the static pressure p on the slowly varying flow section are also measured. Since the inner diameter of the connecting pipe in the pneumatic circuit is relatively small, it is difficult to measure the total pressure and the static pressure. As an engineering problem, not as a research topic, you can not care about the measurement of 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 energy saving measures and operation can be further studied. The cost is calculated.
Measuring the maximum total flow power, from equations (9) and (14), it is only necessary to measure the air source pressure ps and the air source temperature Ts used by the pneumatic circuit or device, and then measure the standard through the pneumatic circuit or device. The volume flow rate qa in the state can be.
The PFA series digital flow switch is used to measure the volume flow rate qv under pressure or the volume flow rate qa under standard conditions. Since qv is the volume flow in the state where the pressure in the pipeline is p, the p should be measured simultaneously. Knowing p and qv, equation (17) can be used to calculate qa. As shown in the charge and exhaust circuit, the maximum total flow power Nmx is a function of time, if the change in Nmx and time t is measured as shown. Then, for a certain period of time T, the total mechanical energy consumed by the pneumatic circuit is the area enclosed between the curve and the time axis, which is expressed by the formula.
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