What are the gas flow meters_ Types of gas flow meters

Principle of gas flowmeter

Thermal gas mass flowmeter adopts the principle of thermal diffusion, which is a technology with excellent performance and high reliability under adverse conditions. Typical sensing elements include two thermal resistors (Platinum RTDS), one is a speed sensor, and the other is a temperature sensor that automatically compensates for gas temperature changes. When the two RTDS are placed in the medium, the speed sensor is heated to a constant temperature higher than the ambient temperature.

Gas waist wheel flowmeter

Another temperature sensor is used to sense the temperature of the medium. The gas mass flow through the speed sensor is calculated from the heat transfer of the sensor element. With the increase of gas flow, the heat carried by the medium increases. And the sensor temperature decreases accordingly. In order to keep the temperature constant, it is necessary to increase the working current through the sensor, and the increased part of the current is proportional to the flow rate of the medium.

Type of gas flow meter.

There are many kinds of flow meters, some can be used to measure liquid medium, some can be used to measure gas, some can be used to measure steam, etc. Only when we understand the working principle and characteristics of these flowmeters, we will not be at a loss when selecting them. So, what are the flow meters for measuring gas? Next, we will briefly understand the types of gas flow meters.

1. velocity flowmeter.

Gas turbine flowmeter

Velocity flowmeter is a kind of flowmeter based on the principle of directly measuring the velocity of full pipe in closed pipe. Industrial applications mainly include:

① Turbine flowmeter: when the fluid flows through the turbine flow sensor, the turbine is forced to rotate under the thrust of the fluid, and its speed is proportional to the average flow rate of the pipeline. The rotation of the turbine periodically changes the magnetoresistance of the magnetoelectric converter, and the magnetic flux in the detected coil periodically changes, generating a periodic electric pulse signal. Within a certain flow rate (Reynolds number), the electric pulse signal is positively proportional to the volume flow of fluid flowing through the turbine flow sensor

② Vortex street flowmeter: a streamline vortex generator is placed in the fluid, and the fluid is alternately separated on both sides of the vortex generator, releasing two rows of regularly alternating vortex streets. Within a certain flow rate (Reynolds number), the vortex separation frequency is positively proportional to the fluid volume flow through the vortex street flow sensor

③ Vortex precession flowmeter: after the fluid passes through the rotator composed of spiral guide vanes, it is forced to strongly rotate around the center line to form a vortex wheel. When passing through the enlarged tube, the vortex center moves along the conical spiral shape. Within a certain flow rate (Reynolds number), the precession frequency of the eddy current is positively proportional to the fluid volume flow through the precession eddy current sensor

④ Time difference ultrasonic flowmeter: when ultrasonic wave passes through flowing fluid, the propagation speed along downstream direction and countercurrent direction is different within the same propagation distance. In a wide range of flow (Reynolds number), the time difference is positive with the volume flow (average velocity) of the measured fluid in the pipe

Velocity gas flowmeter is generally composed of flow sensor and display instrument. For temperature and pressure changes, pressure gauge (sensor or transmitter), thermometer (sensor or transmitter), flow totalizer (temperature pressure compensation) or flow computer (temperature pressure and compression factor compensation) shall be configured. For occasions with high accuracy requirements (such as trade natural gas), an on-line chromatograph is added to continuously analyze the composition or physical properties of the mixed gas and calculate the compression factor, density and calorific value.

2. positive displacement flowmeter.

Roots flowmeter (gas waist wheel flowmeter)

There is a fixed large space and a group of rotating bodies inside the volume flowmeter, such as waist wheel, diaphragm, drum, scraper, elliptical gear, piston and screw, which divide the space into several small spaces with known volume. The rotating body rotates continuously under the action of fluid pressure difference, and continuously discharges fluid from a small space of known volume. According to the rotating speed of the rotor in a certain period of time, the fluid volume can be calculated.

In the standard state, the volume flow calculation formula of the volume flowmeter is the same as that of the velocity flowmeter. Volume flowmeter is a kind of mechanical instrument, which is generally composed of measuring body and integrator. When temperature and pressure change, pressure gauge (sensor or transmitter), thermometer (sensor or transmitter), flow integrator (temperature and pressure compensation) or flow computer (temperature and pressure and compression coefficient compensation) shall be provided.

3. differential pressure flowmeter.

Orifice flowmeter (differential pressure flowmeter)

Differential pressure flowmeter is based on Bernoulli equation and fluid continuity equation. According to the throttling principle, when the fluid flows through throttling parts (such as standard orifice, standard nozzle, large diameter nozzle, classical venturi nozzle, venturi nozzle, etc.), The pressure difference between front and back is proportional to the square of flow. In the differential pressure flowmeter, the standard orifice differential pressure flowmeter is widely used because of its simple structure, low manufacturing cost and sufficient research.