Several Methods of Fluid Flow Measurement, Work is Easier with RenhotecIC Tools

 

Whether it is a screw blower or a screw vacuum pump, the flow parameters are the main performance indicators. At the same time, in the application of enterprises, in order to ensure the actual flow rate, many times the manufacturer's products will be re-measured. Today, screw fan manufacturers will talk about several methods of fluid flow measurement.

 

The methods of fluid flow measurement mainly include differential pressure type, velocity type, volumetric type and mass type. There are several types of these methods, each of which has different fluid adaptation characteristics and errors.

 

Differential pressure fluid flow measurement includes:

 

Throttling variable pressure drop type, the principle is that for a resistance member of a certain shape and size, a certain pressure measurement position and a straight pipe section before and after, under certain fluid parameters, the front and rear differential pressure of the resistance member and the fluid volume flow have a certain relationship. Functional relationship. This measurement is suitable for measuring the flow rate of single-phase, homogeneous fluid in a circular cross-section pipe, and the fluid must be fully turbulent when passing through the throttling piece, and the flow velocity is subsonic. In addition, there are other differential pressure types, including dynamic pressure tube type and throttling variable cross section.

 

Velocity fluid flow measurement includes turbine type, Karman vortex type, electromagnetic type and ultrasonic type:

 

Among them, the turbine type uses the proportional relationship between the angular velocity of the turbine placed in the fluid and the fluid flow rate, and measures the volume flow rate of the fluid through the turbine speed. It has high precision, large range, and fast response, but requires high fluid cleanliness and no inclusions. Solid particles are not suitable for the flow of liquid with too high viscosity.

 

The Karman vortex street type uses a symmetrical bluff body (vortex generator) placed in the fluid, and two rows of regular vortices (Karman vortex street) will be alternately generated from both sides of the downstream of the bluff body. Within the range, the frequency of vortex separation is proportional to the average flow velocity of the fluid in the pipe, and inversely proportional to the width of the bluff body. The vortex generates pressure pulsation on both sides of the rear part of the bluff body, and the pressure measuring element in the wake behind converts the pulsation frequency signal of the pressure into a flow signal output. This kind of fluid flow measurement accuracy is also high, the measurement range is wide, and there are no moving parts. It is suitable for the measurement of gas, liquid, vapor and some mixed-phase fluids, but it has higher requirements for straight pipe sections.

 

The electromagnetic type is to use the conductive fluid medium to flow in the vertical direction in the magnetic field to cut the magnetic force line, it will generate an induced electromotive force on the electrodes on both sides of the pipeline, and then convert the fluid flow signal and output it. Electromagnetic fluid flow measurement is sensitive and suitable for dirty, corrosive media and liquid-solid two-phase flow liquid with solid particles, but the measured fluid must be conductive, and it is not resistant to high temperature media and is susceptible to electromagnetic interference.

 

Ultrasonic methods mainly include propagation time method and Doppler method:

 

The former uses different fluid velocity and ultrasonic propagation velocity to measure the fluid velocity and flow; the Doppler law uses the Doppler frequency shift generated by the scattering of ultrasonic waves by solid particles or bubbles in the fluid to determine the fluid velocity. This form can be installed on the outer wall of the pipeline, and the cost will not change with the diameter of the pipeline. It is suitable for various clean liquids and gases, but for liquids with suspended particles or bubbles exceeding a certain range, it will affect the sound transmission, and the accuracy is not high. Temperature changes can easily cause errors.

 

Volumetric fluid flow measurement includes oval gear type, waist wheel type, scraper type, etc. The principle is to fill a certain volume of space with the measured fluid, and then discharge the fluid from the outlet, and then according to the volume flow rate of the fluid discharged per unit time, The volumetric flow rate of the measured fluid can be determined directly.

 

Mass fluid flow measurement is divided into direct and indirect:

 

Direct type is to directly use heat, differential pressure or momentum to detect mass flow, such as angular momentum type, calorimetric type and Coriolis force type, etc.; indirect type mass flowmeter is to measure the volume flow rate at the same time, and then measure the measured fluid The density or pressure, temperature and other parameters can be used to calculate the mass of the fluid.

 

The accuracy of mass fluid flow measurement is not affected by the temperature, pressure and density of the fluid.