Talk about compressed air metering
First, the nature of compressed air and pre-treatment compressed air comes from the atmosphere, while the atmosphere contains a certain amount of water vapor, which is a kind of humid air. Engineering thermodynamics believes that the water vapor pressure in wet air is usually very low, which can be regarded as a special case of ideal mixed gas, which is calculated as an ideal gas. However, when the humid air is compressed, the pressure and temperature will increase (even after passing through the aftercooler, it can only drop to about 40 °C), and some of the water vapor may cause phase change to precipitate water droplets, while the compressed air is still in the pipeline. The water will continue to precipitate due to the temperature drop, becoming a mixture of wet saturated air and water, that is, a two-component gas-liquid two-phase flow. In addition, compressed air often contains some solid particles and oil stains, and there are strong vibrations and pulsations at the outlet of the air compressor, which are unfavorable for subsequent production and measurement.
There are three indicators for measuring compressed air quality: dryness and humidity, dust content, and oil content. In order to meet production and metering requirements, the compressed air should be properly pretreated - dried and purified.
Second, compressed air metering, instrument selection and installation 1. Compressed air that can be regarded as an ideal gas can be regarded as an ideal gas for compressed air that has been well pretreated, so that it is simple in flow calculation and compensation, general flow Manufacturers will do it.
Compressed air metering is generally volumetric flow. Compressed air will change in pressure and temperature during transmission, so the instrument should choose a temperature-compensated intelligent flowmeter. Manufacturer selection is very important, one must consider quality, and the second should consider maintenance. In addition, the following points should be noted in the selection and installation of the instrument: understand the gas volume and pressure (use, maximum and minimum) of the pipeline, and distinguish whether it is standard flow or working flow, whether it is gauge pressure or absolute pressure. If the pipeline is large and the flow rate is small, the shrinkage treatment should be considered. The installation location should be considered comprehensively to meet the measurement requirements.
According to many years of experience, for the place without vibration or vibration, the spiral vortex intelligent flowmeter (DN矣200mm) can be selected. After continuous improvement and development, the reliability and stability of this kind of instrument are greatly improved, which is good for measuring compressed air. meter. Compared with the vortex street, the lower limit flow rate is nearly half, and the metering effect is better. Our company's compressed air primary and secondary metering use a large number of such instruments, easy to install and use, has basically replaced the throttling device. Its range ratio is up to 15:1, special treatment can be higher; the minimum straight pipe section requirements: straight pipe section 5D, rear straight pipe section 3D; power supply mode: 3.6V lithium battery or 24V DC power supply; liquid crystal display.
For large vibrations or large pipe diameters, a throttling device can be used. Such a watch is preferably integrated, that is, the transmitter is directly mounted on the throttling device to eliminate the error caused by the water in the positive and negative pressure chambers of the pressure transmitting tube and the differential pressure transmitter. Due to the narrow range of the throttling device, the differential pressure transmitter should have a wider range.
The quality of the instrument is closely related to the installation. The installation should be standardized. In particular, the instrument cannot be clamped for welding or cutting. If the flowmeter is installed outdoors, it should be covered with a meter box to prevent sun and rain and affect the life.
―The sampling frequency of some flowmeter manufacturers is set to 6s. For example, the gas time for large load gap is only two or three seconds. In order to make the measurement effect better, the sampling frequency can be shortened to the manufacturer.
Some workshops require high pressure and use less gas. This is not a good measure. Consider small flow compensation or estimation.
From the actual use, the dehumidification effect of the refrigerating dryer is not as good as that of the adsorption type. The compressed air may still precipitate water due to the temperature drop during the transmission. If the time is long, water will form in the low or no flow place, so pay attention to drainage.
2. Compressed air in two-component gas-liquid two-phase flow. Some air compressor stations only post-cool and filter the compressed air to remove impurities and moisture. At this time, the compressed air is saturated and the temperature is high. In the transmission, the water gradually precipitates due to the temperature drop, and becomes a two-component gas-liquid two-phase flow, which is a difficulty in the measurement of compressed air. Now we don't have a good instrument for measuring two-phase flow. We can only use existing instruments to measure, but we should understand the errors and problems that exist.
How much water is released during the transfer of compressed air, and its mass gas content can be estimated. The common pressure of compressed air is (0.4~0.8) MPa. If it is cooled and filtered, the compressed air exits as wet saturated air, the temperature is 45 °C, the temperature of the flowmeter is 10 °C, and the pressure dew point and atmospheric dew point are converted. Figure and the relationship between the moisture content of the dew point of the atmosphere can be obtained from Table 1. It can be seen from Table 1 that the temperature of the compressed air drops by 35C during the transfer, the amount of precipitated water does not exceed 15g/m3, and the mass gas content is as high as 99%. On the surface, 1m3 compressed air effluent Not much, but there are more than tens of thousands of cubic meters. When compressed air flows, most of the water will be carried away in the air, and in some places, water will form in some places. Therefore, the necessary drainage facilities should be provided at the low point and end of the pipe network for automatic and manual drainage.
It is very difficult to accurately describe the flow conditions of the gas-liquid two-phase flow.
When the mass gas content is high, from the actual situation, it is possible to use the homogeneous flow model to approximate the treatment. Compared with the experimental values, the error of the calculation model of the homogeneous model decreases with the increase of the pressure or mass gas content, especially when the mass gas content is greater than 0.98.
The homogeneous flow model assumes that the gas-liquid two-phase fluid is uniformly mixed and has the same flow rate. It can be regarded as a single-phase fluid with average fluid physical properties, and its homogeneous density is liquid phase density; X-mass gas content.
Because PL is much larger than PG, and X is not less than 0.9, the upper formula can be simplified to X. It is generally not necessary to install the online instrument for measurement. It may be substituted by manual test value or estimated value.
In terms of selection, for the place where there is no vibration or vibration, it is the first to push the vortex intelligent flowmeter. The vortex street can also consider DN300mm. This type of meter measures the volume flow, which is independent of the fluid density. The precession vortex flowmeter is completely different from the vortex generated by the Karman vortex flowmeter. The former is forced oscillation and the latter is natural oscillation. The former has a vortex generator that forces the fluid to rotate around the flow axis and move forward, which objectively promotes a more even mixing of air and water.
For the throttling device, both the volumetric flow rate and the mass flow rate are related to the density, and the influence of the density on the flow measurement accuracy is as important as the differential pressure. It can be considered to calculate the homogeneous density. There is a need to inform the flowmeter manufacturer first.
Third, compressed air quality and price Compressed air quality, high pressure, the inevitable cost. If there are several air compressor stations in a unit, the air pressure produced by each station will be different and the quality will be different. It is unreasonable to use a price charge. It is advisable to set different air prices for settlement. Otherwise, the air volume should be considered. make up. Even in a single air compressor station, the pressure requirements may be different. To stabilize the network pressure and energy saving, consider installing an intelligent flow controller in front of each workshop to automatically adjust the flow and pressure of each pipeline.
Table 1 List of temperature drop and precipitation water quantity, mass gas content rate Table pressure dry air density pressure dew point 45 pressure dew point 10 precipitation water volume dew point (C) water content (g/m3) atmospheric dew point (C) water content (g/m3 In the table: 1 the amount of water is the difference between the water content when the compressed air temperature drops from 45 ° C to 10 ° C; 2 the mass gas content is the fraction of the gas phase mass flow in the two-phase mass flow, expressed by x. The mass gas content is also referred to as dryness for a single component gas-liquid two-phase flow.
2014.9 China Measurement 111
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