There are three indexes to measure compressed air quality: dry and wet degree, dust content and oil content.In order to meet the requirements of production and measurement, the compressed air should be properly pretreated -- dried and purified.
Compressed air can be regarded as ideal gas to the compressed air after a good pretreatment, can be regarded as ideal gas, so in the flow calculation and compensation aspects are relatively simple, general flow meter manufacturers will do.
The general measure of compressed air is volume flow.Compressed air in the transmission of pressure and temperature will change, so the instrument should choose temperature and pressure compensation intelligent flowmeter.Manufacturer choice is very important, one must consider the quality, two must consider the maintenance.In addition, the following points should be paid attention to the selection and installation of the instrument: understand the amount of gas used in the pipeline and the size of pressure (common, maximum and minimum), distinguish whether it is the standard flow rate or the operating flow rate, whether it is the gauge pressure or absolute pressure.If the pipeline large, small flow rate to consider the shrinkage treatment.Installation location should be considered comprehensively to meet the measurement requirements.
According to many years of experience, the intelligent flowmeter (DN 200mm) can be selected for the places with no vibration or small vibration. After continuous improvement and development, the reliability and stability of this kind of instrument are greatly improved, which is a good instrument for measuring compressed air.Compared with vortex street, the lower limit flow is nearly half smaller, and the measurement effect is better.This kind of instrument is widely used in the first and second level measurement of compressed air in our company. It is easy to install and use and has basically replaced the throttling device.It has a range ratio of 15:1, higher for special treatment;Minimum straight pipe section requirements: straight pipe section 5D, back straight pipe section 3D;Power supply: 3.6v lithium battery or 24V dc power supply;Liquid crystal display.
Throttling device can be used in case of large vibration or pipe diameter.This kind of table had better choose the integration, that is, the transmitter is installed directly on the throttle device, to eliminate the error caused by the water in the positive and negative pressure chamber of the pressure tube and the differential pressure transmitter.Because the throttling device has a narrow range, the differential pressure transmitter should choose a wide range.
The quality of the instrument is closely related to the installation, the installation should be standardized, especially the instrument cannot be clamped up for welding or cutting.If the flowmeter is installed outdoors, it should be covered by an instrument box to prevent the sun and rain from affecting the service life.
Some flowmeter manufacturers set the sampling frequency to 6s, such as large load gap gas use time is only two or three seconds, in order to better measurement effect, the manufacturers can propose to shorten the sampling frequency.
The pressure of some workshops is high and use gas little, this kind of circumstance is bad to measure, can consider small flow compensation or estimate.
From the practical use, the dehumidification effect of the refrigerated dryer is not as good as the adsorption type, compressed air in the transmission may still be due to temperature drop precipitation water, a long time in low-lying or not flowing place will form water, so pay attention to drainage.
2. A two-component gas liquid two phase flow of compressed air or air compressor station only after the compressed air cooling and filter, remove the impurities and moisture, then the compressed air is saturated and the temperature is higher, in transit will reduce temperature precipitation water gradually, become a kind of two-component gas liquid two phase flow, this is a key problem for compressed air metering.At present, we don't have a good instrument for measuring two-phase flow, so we can only use the existing instrument to measure, but we should have some understanding of the errors and problems caused by it.
Compressed air in the transmission process of how much water precipitation, its mass gas content, how much can be estimated.Compressed air pressure is commonly used as (0.4 ~ 0.8 MPa), if after cooling, filtering, outbound compressed air for wet saturated air and temperature of 45 ° C, temperature in 10 ° C, the flowmeter by dew point pressure and atmospheric dew point conversion diagram and atmospheric dew point moisture content relational tables available table 1. From table 1, the compressed air temperature drop in the transmission of 35 C, precipitation of water is not more than 15 g/m3, quality gas rate is as high as 99% on the surface of 1 m3 compressed air out of the water is not much, but more than tens of thousands of cubic meters.When compressed air flows, most of the water is carried away by mixing with the air. When it does not flow, water accumulates in some places.Therefore, necessary drainage facilities should be provided in low-lying areas and ends of the pipe network for automatic and manual drainage.
It is difficult to describe the flow state of gas-liquid two-phase flow accurately.
When the gas content of mass is high, the homogeneous flow model can be used to approximate the process.Compared with the test value, the error of the calculation formula of homogeneous model decreases with the increase of pressure or mass gas holdup, especially when the mass gas holdup is larger than 0.98.
The homogeneous flow model assumes that the gas-liquid two-phase fluid is uniformly mixed and the velocity is the same. It can be regarded as a single-phase fluid with average fluid physical characteristics, and its homogeneous phase density is liquid density.X -- mass gas holdup.
And because PL is far more than PG, and X - generally not less than 0.9, when estimating the formula can also be simplified for X how to determine, generally no need to install online instrument to measure, may as well with artificial test value or estimate value into.
In terms of type selection, the intelligent flowmeter of first push into vortex should be adopted for places with no vibration or small vibration. DN300mm can also be considered for vortex street, which measures volume flow and has nothing to do with fluid density.The vortex generated by precession vortex flowmeter is completely different from that produced by kaman vortex flowmeter. The former is forced oscillation while the latter is natural oscillation.The former has a vortex generator that forces the fluid to rotate around the axis of the flow and to move forward, objectively mixing air and water more evenly.
For throttling devices, both volume flow and mass flow measured are related to density, and density has the same important influence on flow measurement accuracy as differential pressure, so homogeneous phase density can be considered for calculation.The flowmeter manufacturer should be notified of this requirement.
Compressed air quality and price compressed air quality, high pressure, high inevitable cost.If a unit has several air pressure stations, the air pressure produced by each station is different, and the quality is not the same, such as the use of a price charge is not reasonable, appropriate to establish a different air price for settlement, otherwise we should consider the amount of air compensation.In order to stabilize the pressure of pipe network and save energy, intelligent flow controller can be installed in front of each workshop to automatically adjust the flow and pressure of each pipeline.
Water content (g/m3) water content (g/m3) water content (g/m3)(2) mass gas content is the share of the mass flow rate of the gas phase in the mass flow rate of the two phases, expressed by x.The mass gas content is also called dryness for one component gas liquid two phase flow.