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The mass flowmeter
Published on:2015-1-6 14:35:04 Hits:

Section Overview

Flowmeter now widely used, whether it is the differential pressure, target type, turbine, and other types of electromagnetic or volume from the principle point of view are sufficient to measure the volume of traffic. Due to the size of the volume of the fluid affected by temperature, pressure and other parameters, when the temperature of fluid to be measured, Zuohua pressure, volume flow should be measured in terms of the corresponding value of the standard state, or some agreed state. But in fact, when frequent changes in temperature, pressure, timely conversion is very difficult, and sometimes impossible. It is therefore desirable to measure the mass flow mass flow. In addition, the actual production, due to the product quality control of the production process, the mixing ratio of various materials were measured, cost accounting and automatic adjustment of the production process, but also must be aware of the mass flow. With the improvement of the level of development and automation of industrial production technology, such as large generators to achieve full 程自启 stop, for nuclear power plant gas-liquid two-phase flow requirements, as well as thermal power plants more accurate economic evaluation have made mass flow measurement technology is increasingly important:

Volume flow rate Q and the relationship between the mass flow M is

M = Q (10-1)

Or M = A (10-2)

---- Density where the measured fluid, kg / m 3;

A ---- fluid flow cross-section (typically duct flow area), m 2;

  ---- Average flow velocity at the flow cross-section A, m / s.

Indirect mass flowmeter points [] and direct derivation categories. According to equation (10-1) measuring the mass flow meter, you must first measure the volume flow of the fluid and then by density measured by a densitometer and multipliers, this instrument called a mass flow meter or indirectly derived mass flowmeter . Recently, due to confinement structure and density meter element characteristics, at a high temperature, high pressure still can not use. Only use a fixed value multiplied by the volume of traffic density. As we all know, the medium density with pressure, temperature changes and differences in the changes in operating conditions using a fixed density value will bring greater mass flow measurement error, it must be parameterized compensated accordingly developed a temperature and pressure compensation flowmeter. Detects the temperature of the measured fluid pressure, then at a certain mathematical model automatic conversion of the corresponding density values obtained with the product of the density value of the volume flow mass flow measurement can be achieved, it is known as temperature, pressure compensated mass flow meter. Temperature, pressure compensated mass flow meter is a special form of the current widely used in industry A derivation of mass flowmeter.

Directly detect the amount and quality of traffic related to reflect the size of the mass flow meter is called direct mass flow meter.

Development of direct mass flowmeter, the pressure last purpose is to make an output signal representative of mass flow rate and the measured medium, independent of temperature and other parameters, in order to solve when the medium parameters vary widely, the relationship between the density and the temperature, the pressure between the not as linear, while the use of temperature, pressure compensation automatic mode very difficult and complicated problems. This is also the temperature and pressure automatic compensation mass meteor count has been widely used, we must also carry out the grounds of direct mass flow meter research.

Because of the demand for direct mass flow meter urgency was more intense in recent years, so it is in a stage of rapid development, though there are already many types, but due to restricted principle, structure, maintenance, and price and other aspects of life, in order to use the industry has not been widely used. This chapter focuses on indirect mass flowmeter, direct mass flow meter for general description only.

Section direct mass flow meter

Direct mass flow juice, is a direct reflection of the sensing element mass flow meter, Pat currently use different principles to develop a variety of types, such as the type of momentum and moment of momentum, inertia force type, the Coriolis force type, differential pressure, vibration, thermal, etc. Each type there are a variety of structures, such as differential pressure are: Magnus ugljan quality meteor, vibration pitot tube mass flowmeter, the powder quality meteor meter bridge, fluid gushed shaped mass flowmeter. Vibrating are: cantilever vibration and rotational vibration type mass flowmeter, surface-wave range of mass flow meters and other types of narrative difficult one. Far more common applications are now the type carried out briefly, to have a representative structure for the highlights.

Current common direct mass flow meter with dual turbo mass flowmeter, mass flowmeter moment of momentum, inertia force type quality King meter, Coriolis mass flowmeter and thermal mass flowmeter.

Structural principle of double turbine mass flow meter is of two turbines connected by a spring, by the flow of fluid energy impact itself is rotated by the turbine blade after the two helical inclination caused by the different torque difference, the difference is balanced by the torque spring is connected, and forming a twist angle between the two turbines, twist angle is proportional to the size and mass flow measurement signal time difference caused due to torsion angle, mass flow rate can be obtained. The advantage of this structure is the use of energy within the sensing element to work without external energy, simple structure, but the high performance requirements of the elastic element, and the need to consider in the design flow by eliminating the disturbance after the first turbine of the second Effect of the turbine, and the twist vibration disturbance in the fluid may occur between the lower two turbine.

Moment of momentum and inertia force type mass flowmeter is based on the principles of Newton's second law of production, from the mechanical point of view, the quality is a measure of the inertia of objects. Objects by external force, the state of motion changes, the size and quality of the amount of change in motion related to measuring the rate of change of time;. Mass flow can be measured, whereby you can create a variety of structures located flowmeter qualitative moment of momentum. mass flow juice is fluid moment of momentum changes reflect mass flow. the typical structure of a memory in the meter housing, and a driven capstan London, respectively mounted on the minor axis, the motor driven at a constant angular speed W capstan Let equivalent radius of rotation of the fluid is l W, W is the average flow velocity of the fluid. If the mass of the fluid is m, then the moment of momentum J = m W = Q. Due to the driven wheel is spring restrictions, can not rotate, so the measured braking force of the spring to reflect short moment of momentum. This rate of change of momentum moment of time

1.gifValue because the T series, it can reflect the quality of flow measurement A M = A. The inertial mass flowmeters generally use manner by the fluid flowing through the transfer function to the time constant of the movable measuring tube, to give an additional acceleration, so that the movable pipe wall by the fluid acceleration in the opposite direction to the inertial force, this inertial force is proportional to the mass flow rate by measuring the moment of inertia of the inertial forces or mass flow can be measured. Compared with the twin-turbo mass flowmeter, moment of momentum and inertia force mass flowmeters require external energy to work. Up to a meter class and broader application of the current rapid development is what is called the Coriolis mass flowmeter, it is by measuring the Coriolis force changes to reflect the size of the mass flow. Refers to the so-called Coriolis force, in uniform angular velocity of the moving object in the frame of reference of the observer in the rotating frame of reference seems to be attached to the body in addition to the centrifugal force of inertia, but also playing another additional inertia the role of force in order to take advantage of Newton's second law to describe the state of motion of objects, this force is the Coriolis force, called the Coriolis force. For example, a disk transfer function reference, if the disk around the central axis of rotation, the angular velocity is D, set up an object from the center of rotation along the radius of the disk at a rate of S with respect to the disc as a uniform linear motion, in addition to the object by the centrifugal force of inertia, but also by the effect of the Coriolis force, the magnitude of the Coriolis force depends on the angular velocity of the disk radial velocity g S and objects. provided to the Coriolis force fc bearing table, then the expression for

1.gif(10-3)

Wherein mass m-- moving object;

v - the speed of the rotary motion of objects in the frame of reference;

w - angular velocity of the rotating frame of reference.

As described above, the presence of the Coriolis force is a radial velocity v and the rotation angular velocity w exist as a prerequisite, either zero velocity, will not produce a Coriolis force.

By formula (10-3) can be seen, when the rotational angular velocity is constant v, the Coriolis force is proportional to the product of fc mw quality and speed of the object, which is the use of Coriolis mass flow measuring force most original theory in accordance with. In flow measurement, so that the fluid is flowing with a velocity w q angular velocity of the movable tube, in order to achieve a condition A and s are simultaneously present, this movable tube known that the flow measurement tube. Measuring tube can be rotated mode or modes of vibration cycles to achieve the desired value w. When the fluid flows through the measuring tube, equivalent to flow through a predetermined angular velocity direction of the rotary cycle of the measuring tube, the same will generate Coriolis effect, and is relatively simple in structure.

To find the Coriolis force and mass flow relation to a single vibrating U-tube structure, for example, shown in Figure 10-1. Measuring tube under the electromagnetic drive system to the natural frequency of vibration power down periodically. When the fluid flows through the vibrating tube, the fluid is forced to accept the vertical momentum of the tube. To exercise our powers of vibrating tube up half cycle, for example, set its angular velocity is a, then the U-shaped pipe into the side of the fire is published by the Coriolis

1.gif(10-4)

Style towel m-- measuring quality of fluid in the pipe, kg;

m - is training fluid Shen Di, m / q

v - the angular velocity of the measuring tube to the top of the movement, rad / s.

1.gif

Figure 10-1 vibrating single u-shaped pipe flow measurement

Mass flow is defined as a unit of time flowing through the fluid mass flow section. That

M = (10--5)

Wherein m-- flows at time t measured mass of fluid in the pipe, kg;

t-- fluid flow through the measuring tube time.

Welcome for uniform fluid: (10-6)

Where l-- measuring tube length, m;

The formula (10--6) into equation (10-5), and then on behalf of the people of the formula (10-4) was

f c = 2 (10-7

By formula (10-7) to give

M = c (10-8)

Since the length l and turn the power of the angular velocity of the measuring tube are constant, so constant, set k = 1, then

M = kf c (10-9)

Wherein k ---- measuring tube length l and a constant related to the angular velocity;

Remaining symbols with the former.

By formula (10-9) shows that the mass flow rate is proportional to M and the Coriolis force fc. When the measuring tube structure and vibration-driven system to determine, k was known constants, measuring the Coriolis force fc can be obtained mass flow M, the same token, if the analysis of the measuring tube vibration half cycle of downward movement in or out when the fluid inside the tube side, will get the same conclusion.

Using different methods for measuring the Coriolis force fc, and choose not to ask the structure and form of the measuring tube in different ways so that the measuring tube to obtain the desired rotation angular velocity a, may be made of various types of Coriolis mass flowmeters. As long as all the measured fluid flow through the measuring tube, the mass flow rate of the fluid can be directly measured,

Single U-shaped vibrating tube, often make use of variable-shaped U-shaped measuring tube to reflect the size of the Coriolis force fc. Because the fluid in the U-shaped pipe into the flow direction of the opposite side and the outflow side, so both sides of the u-shaped pipe by pipe size relative to ask, ask the opposite side of the Coriolis force. Coriolis effect caused by the deformation of the measuring tube. Variable size and shape of the Coriolis force is proportional to the mass flow rate that is proportional. General Instrument detection method, the measurement of vibration at these two points on the tube located on the flow speed through the measuring tube on both sides of the electromagnetic inductor, and the time difference between the signals due to the deformation of the pipe caused by these two speeds, and then this signal is sent to the converter, the converter will be processed and converted into a signal directly proportional to the mass flow rate of the electrical signal output.

The use of two U-shaped tubes for flow measurement tube vibration, vibration and deformation phase two pipes 180 °, with the deformation of their synthesis to determine the mass flow rate, this can increase the instrument's sensitivity.

Coriolis mass flowmeter in addition to the use of U-shaped tubular structure, the existing products as well as straight-tube mass flowmeter, Li-Lee mass flow meter, mass flow meter spinning top, vibrating gyro mass flow meter, rotary vibration function type and cantilevered mass flowmeter.

Thermal mass flowmeter is currently developing a rapid direct mass flow meter, its basic principle is the use of external heat source for heating the fluid to be measured, the measurement of fluid flow due to changes in temperature field to reflect the quality caused significant traffic. Changes in the temperature field with the temperature difference between the front and rear side of the heater to represent. Fluid to be measured before and after the mass flow M with the heater discretion k is the temperature difference between the ends

1.gif(10--10)

Where P-- heater power;

J ----- equivalent of heat;

C p ------ measured fluid specific heat;

d - the temperature difference between the front and rear ends of the heater.

From the above equation, the use of a constant power method, the temperature difference is inversely proportional to the mass flow rate M, the measured temperature difference can be obtained using a constant M if the temperature difference method, the input power P and the mass flow rate is proportional to the heater, the heater input measured power P can be calculated M value. In use, a constant temperature difference method, in terms of the relationship, or a means to measure the characteristics of view are more constant power method is simple, the P value is read out from the power meter to get the value of M, which are widely used.

Thermal mass flowmeter contact and non-contact can be divided in two, depending on heat and temperature measurement methods.

1. Contact thermal mass flowmeter

Heating elements and temperature measurement devices such mass flowmeters are placed in the test fluids, direct contact with the fluid, often referred to as Thomas flowmeters suitable for measuring large mass flow of gases. The structure shown in principle Figure 10-2. Since the heating element and in direct contact with the measuring fluid to be measured, thus rendering the element susceptible to fluid corrosion and wear, affecting the instrument measurement sensitivity and service life. Measurement of high velocity, should not use corrosive fluid when it is contact shortcomings.

2. Non-contact thermal mass flowmeter

This meter heating and temperature measuring element are disposed outside the fluid conduit, not in direct contact with the fluid to be measured, to overcome the shortcomings of the contact type. Hot micro flow line (the typical structure of a non-contact type mass flowmeter) shown in Figure 10-3. Instrument measuring catheter, for thin-walled tubes of small diameter nickel, nickel tube external sides of the platinum resistance wire wound coil of 3,5 as the temperature, and as no amount of arms bridge R1, R2. Two intermediate temperature manganese copper coil wrapped around the heating coil 4, as the instrument heater. When the fluid is stationary, due to the temperature measuring coil mounted symmetrically on both sides of the heater and the equal value (about 100 each), thus measuring bridge is in balance. But when the fluid in the fluid in the pipe by the temperature resistance of nickel when it destroyed the temperature field of the heater, two temperature at different temperatures inside the coil, thus causing the resistance to change. Two coil resistance temperature range, destroyed the balance of the bridge. According to the bridge balance principle, namely, measured by the galvanometer 8 resistance value changes, the mass flow rate can be obtained M.

1.gif

Figure 10--2-contact thermal mass flowmeter structure principle

l, 3- thermocouple; 2 a heater; 4 a power meter

1.gif

Figure 10-3 non-contact thermal mass flowmeter

1- measuring catheter; 2- isothermal shell; 3- temperature coil; 4- heating coil

7- zero resistance; 8- galvanometer

Hot micro flow meter for measuring liquid and gas mass flow tiny. Measurable 0--100cm 3 / h of liquid flow and micro l0L / h left some small gas flow.

In order to simplify the structure, and some products removed heater, only measuring resistance with two, both as a heating element and as a temperature measurement devices. This design, due to the thermal inertia of reasons, reflecting the speed meter is relatively small, the sensitivity is low; the measured fluid temperature variations instrumented accuracy.

In order to improve the non-contact thermal mass flow meter measuring range, designed a mass flow boundary layer, which uses the measurement of fluid near the boundary wall to reflect the size of the heat conduction flow. In this way the flow measurement is generally controlled by the heat pipe wall heater given to maintain a constant temperature difference between inside and outside the boundary layer, and reflect the mass flow measurement based on thermal member.

Thermal mass flowmeter current rapid development are: hotline mass flowmeters, mass flow boundary layer, shunt and thermal mass flowmeters capillary IC substrate technology with thermal mass flowmeter.


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