Measurement of Negative Pressure Air Flow

Measurement of negative pressure air flow There are not many measurement objects for negative pressure air flow, and it is often used in production processes that require negative pressure air, such as in the production process of cigarettes. Negative pressure air is also an energetic working medium, and the consumption of negative pressure air is measured for energy consumption assessment. (!) The characteristics of negative pressure air flow measurement ① does not allow flow measurement to introduce significant pressure loss. The negative pressure of the negative pressure air comes from the vacuum pump. The negative pressure generated by many powerful vacuum pumps is only tens of kilopascals negative. For example, the absolute pressure at the inlet is 301^?&For the vacuum pump, the negative pressure converted from strong power is only 1701^3. If the flow meter is installed on the negative pressure pipeline and the resistance is increased, resulting in a large pressure loss, the power loss will be greatly increased, which is related to energy saving. It runs counter to the purpose. ② The flow density is small, which brings difficulties to the selection of the instrument. ③ After the flowmeter is installed on the negative pressure pipeline, if there is leakage, it is difficult to detect, and power is wasted unconsciously. Flowmeter selection Due to the constraints of the first feature above, orifice flowmeters, turbine flowmeters, positive displacement flowmeters, etc. are negated. Due to the constraints of the second feature above, the choice of vortex flowmeter is also negated. Because in the pipeline where the flowmeter is installed, the fluid with an absolute pressure of 3(^1^, its density is only 1/3 of the air density under normal pressure conditions, and the thrust of the fluid vortex on the sensor is correspondingly smaller, so it cannot be measured. Ultrasonic As far as the first constraint condition is concerned, the flow meter is an ideal choice, but it needs to be checked by the acoustic impedance. Due to the existence of the second characteristic, the acoustic impedance of the specific measurement point becomes very small, so that it is difficult to match the impedance The so-called acoustic impedance (^(^'; 01 mouth 6!!') refers to the damping and resistance of the medium to the sound wave transmission, which is equal to the ratio of the sound pressure to the volumetric displacement velocity of the medium. In ultrasonic flow measurement, Acoustic impedance is proportional to sound velocity and fluid density, so the lower the absolute pressure of the measured medium, the smaller the acoustic impedance.

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