First, the properties of compressed air and pretreated compressed air are from the atmosphere, and the atmosphere contains 1 quantitative water vapor, which is a kind of wet air. Engineering thermodynamics believes that water vapor pressure in wet air is usually very low, and can be regarded as a special case of ideal mixture of gas, which is calculated as an ideal gas. But the wet air is compressed, the pressure and temperature will rise (even after cooler also can only be reduced to about 40 DEG C), part of which water vapor may produce the precipitation of water droplets, while the compressed air in the pipeline will continue to decline in transmission due to the temperature of the water, a mixture of wet saturated air and water. That is a two-component two-phase flow. In addition, compressed air often contains some solid particles and oil contamination. There are strong vibration and pulsation at the outlet of air compressor, which are unfavorable for subsequent production and metering.
There are 3 indexes to measure the quality of compressed air: the degree of dry and wet, the amount of dust and the amount of oil. In order to meet the production and measurement requirements, the compressed air should be properly pretreated - drying and purifying.
Two, compressed air metering, instrument selection and installation 1. can be regarded as compressed air of ideal gas, which can be regarded as an ideal gas for compressed air which is well pretreated, which is simpler in terms of flow calculation and compensation. Generally, flowmeter manufacturers will do it.
The measurement of compressed air is the volume flow. The pressure and temperature of compressed air will change in transmission, so the instrument should choose the intelligent flow meter with temperature and pressure compensation. The choice of the manufacturer is very important, one should consider the quality, and two should consider the maintenance. In addition the instrument selection and installation should pay attention to the following points: understand the pipeline volume and pressure size (usually, maximum and minimum), is the standard flow or distinguish the flow conditions, is still the absolute pressure gauge. If the pipe is large and the flow is small, the shrinkage tube should be considered. The installation position should be considered comprehensively to meet the measurement requirements as far as possible.
According to many years of experience, the vortex swirling intelligent flowmeter (DN 200mm) can be selected for places with little vibration or vibration. After continuous improvement and development, the reliability and stability of the instrument are greatly improved, which is a good instrument for measuring compressed air. Compared with the vortex street, the lower limit flow rate is nearly half, and the measurement effect is better. This kind of instrument is widely used in the one or two grade measurement of compressed air in our company. It is convenient to install and use, and it has basically replaced the throttle device. Its range is up to 15:1, and the special treatment is higher. The minimum straight pipe requirements are straight pipe 5D, rear straight section 3D, power supply mode: 3.6V lithium battery or 24V DC power supply; LCD display.
In case of large vibration or large pipe diameter, a throttle device can be used. It is best to choose the integrated form of the meter, that is, the transmitter is directly installed on the throttling device to eliminate the error caused by the water in the positive pressure chamber and the negative pressure chamber of the differential pressure transmitter. Because the range of the throttle device is narrow, the differential pressure transmitter should choose the wider range.
The quality of the instrument is closely related to the installation, and the installation should be standardized, especially if the instrument can not be clamped for welding or cutting. If the flowmeter is installed outside, we should add an instrument box to prevent the sun and rain, and affect the life of the flowmeter.
The sampling frequency of some flowmeter manufacturers is set to 6S, for example, the gas time of large load gap is only two or three seconds. In order to make the metering effect better, the sampling frequency can be shortened to the manufacturer.
Some studio pressure high required by gas less, this situation is not a good measure, can be considered small flow compensation or estimation.
From the actual use, the dehumidification effect of the refrigerating dryer is not as effective as that of the adsorption type. The compressed air may still get water out of the water because of the temperature drop during the transmission, and the time will be long. At low or non flowing place, the water will form, so we should pay attention to drainage.
The 2. is a two-component two-phase flow of compressed air or compressed air station for compressed air after cooling and filtering, removing impurities and moisture therein, the compressed air is saturated with high temperature, in the transmission due to temperature decrease precipitation water, become a kind of two-component two-phase flow this is a difficulty, compressed air metering. At present, we have no special instrument for measuring the two phase flow. We can only use the existing instruments to measure it, but we need to know the errors and the existing problems.
How much water is precipitated during the transmission of compressed air and how much gas content of its mass can be estimated. The compressed air used for pressure (0.4~0.8) MPa, if after cooling and filtering, the compressed air station for wet saturated air, the temperature is 45 ~ C, the temperature meter at 10 DEG C, the pressure dew point and atmospheric moisture content and atmospheric dew point dew point conversion chart table can be obtained from table 1. table 1 shows. The compressed air in the transmission temperature drop 35C, precipitation water does not exceed 15g/m3, the rate of up to 99%. on the surface of 1m3 compressed air water not much gas quality, but many tens of thousands of cubic meters. When the compressed air flows, most of the water is taken away in the air, and water is formed in some places when the flow is not flowing. Therefore, there should be necessary drainage facilities in the low-lying area and end of the pipe network for automatic and manual drainage.
It is difficult to accurately describe the flow status of the gas-liquid two phase flow.
When the mass gas content is high, the homogeneous flow model can be used to approximate the process from practice. Compared with the experimental value, the error of the homogeneous model formula decreases with the increase of pressure or mass fraction of gas content, especially when the mass gas content rate is greater than 0.98, the error is smaller.
The homogeneous flow model assumes that the gas-liquid two phase fluid is uniformly mixed and the flow velocity is the same, which can be regarded as an average flow.