KR102158294B1 - Gas flowmeter capable of measuring at sonic speed and subsonic speed - Google Patents

Abstract

The present invention relates to a gas flowmeter for measurement at sonic and subsonic speeds. The gas flowmeter of the present invention comprises: a pipe body connected to a pipe; a sonic nozzle formed inside the pipe body and having a converging unit and a diffusion unit; a first pressure sensor installed at a front end of the sonic nozzle to measure a pressure; a temperature sensor installed at the front end of the sonic nozzle to measure the temperature; a second pressure sensor installed in the diffusion unit adjacent to the converging unit in the sonic nozzle to measure a pressure; an operation unit calculating a flow rate of gas in accordance with measured values of the first pressure sensor, the temperature sensor, and the second pressure sensor; a display unit for displaying the flow rate of the gas; and a control unit for controlling the display unit. When the flow speed of the gas flowing through the sonic nozzle is sonic, the flow rate is calculated from the measured values of the first pressure sensor and the temperature sensor. Also, when the flow speed of the gas flowing through the sonic nozzle is subsonic, the flow rate is calculated from measured values of the first and second pressure sensors. Accordingly, the flow rate can be measured in all flow rate ranges such that a measurement range is wide and the flow rate can be accurately measured.

Description

Gas flowmeter capable of measuring at sonic speed and subsonic speed}

The present invention relates to a gas flow meter capable of measuring at the speed of sound and subsonic.

Flowmeters are devices that measure the amount of fluid such as gas or liquid flows in a unit time, and are classified into impeller flowmeters, differential pressure flowmeters, area flowmeters, volumetric flowmeters, ultrasonic flowmeters, and the like according to the measurement method or principle.

The differential pressure flow meter installs a throttling mechanism in the pipeline and measures the flow rate by using the pressure difference before and after the throttling mechanism. Differential pressure flowmeters are widely used for industrial purposes due to their relatively simple method and wide range of applications, and venturi tube flowmeters and sonic nozzles are mainly used.

Venturi tube flowmeter is a flow meter that measures the flow rate by generating differential pressure by inserting a venturi tube with a reduced cross-sectional area in the fluid passage. When the flow velocity at the neck of the venturi nozzle is subsonic, the pressure upstream of the venturi nozzle and the pressure at the neck The flow rate is calculated by measuring the difference in (upstream pressure-pressure in the neck).

Such a venturi nozzle flow meter has a simple structure and low price, but it cannot measure the flow rate when the speed of sound is in the neck, so the flow rate measurement range is narrow, and the measurement accuracy is low in places where the flow rate in the pipe is small.

In order to compensate for the disadvantages of the Venturi tube flowmeter, a flowmeter using a sonic nozzle composed of a converging section and a diversing section is sometimes used.

The sonic nozzle can calculate the flow rate by measuring the temperature and pressure upstream of the nozzle when the gas flow rate at the nozzle neck is sound speed. When the pressure at the rear end of the nozzle and the pressure at the front end (rear end pressure/shear pressure) falls below the critical pressure ratio (usually 0.8), the flow velocity at the nozzle neck becomes the speed of sound.

At this time, since the flow rate does not change even if the pressure on the downstream side of the nozzle is lowered, that is, the flow rate of the gas is not affected by the pressure on the downstream side, the flow rate can be calculated by measuring only the pressure and temperature at the front end of the nozzle.

These sonic nozzles can measure more accurately than other flow meters, so they are used in calibration institutions and standard institutions. There are drawbacks.

To solve this problem, when the flow velocity of gas at the nozzle neck is sonic, the flow rate is measured in the same way as the sonic nozzle, and in the case of subsonic velocity, the difference between the upstream pressure and the downstream pressure of the nozzle (upstream pressure-downstream pressure There was a method of measuring the flow rate using ). The downstream pressure of the nozzle measures the pressure at the tip of the nozzle diffusion.

However, there is a problem in that the flow rate cannot be accurately measured because the difference between the upstream pressure and the downstream pressure of the nozzle varies depending on the shape of the nozzle, surface roughness, and the like. In addition, the difference between the upstream pressure and the downstream pressure of the nozzle is small, so that the flow measurement accuracy is lowered.

Accordingly, there is a need for a flow meter that can accurately measure the flow rate without limiting the measurement range by measuring the flow rate in all flow velocity ranges (sonic velocity, subsonic velocity).

Korean Patent Registration No. 10-1195491

Accordingly, the present invention was invented in view of the above circumstances, and it is an object to provide a gas flow meter capable of measuring the flow rate at sound and subsonic speeds capable of accurately measuring the flow rate while having a wide measurement range since the flow rate can be measured in all flow rates There is this.

The gas flow meter according to the present invention for realizing the object as described above includes a pipe body connected to the pipe; A sonic nozzle formed inside the tube and having a converging portion and a diffusion portion; A first pressure sensor installed at a front end of the sonic nozzle to measure pressure; A temperature sensor installed at the front end of the sonic nozzle to measure temperature; A second pressure sensor installed in the diffusion unit adjacent to the converging unit in the sonic nozzle to measure pressure; A calculation unit for calculating a flow rate of gas according to measured values of the first pressure sensor, the temperature sensor, and the second pressure sensor; A display unit displaying the flow rate of gas; A control unit for controlling the display unit; Including, when the flow velocity of the gas flowing through the sonic nozzle is a sound velocity, the flow rate is calculated from the measured values of the first pressure sensor and the temperature sensor, and the flow velocity of the gas flowing through the sonic nozzle is subsonic velocity. In this case, the flow rate is calculated from the measured values of the first pressure sensor and the second pressure sensor.

In addition, an opening is formed through the tube body so as to contact a neck portion that is a boundary between the converging portion and the diffusion portion, and the second pressure sensor is installed through the opening.

In addition, the calculation unit distinguishes between a sound speed and a subsonic speed from a ratio of the measured values of the first pressure sensor and the second pressure sensor.

A method of measuring a flow rate in a gas flow meter in which a sonic nozzle having a converging part and a diffusion part is formed inside a tube according to another embodiment of the present invention is to measure an upstream pressure by a first pressure sensor installed at the front end of the sonic nozzle. The step of doing; Measuring a temperature by a temperature sensor installed at a front end of the sonic nozzle; Measuring neck pressure by a second pressure sensor installed in the diffusion unit adjacent to the converging unit in the sonic nozzle; Including, when the flow velocity of the gas flowing through the sonic nozzle is a sound velocity, the flow rate is calculated from the upstream pressure and the temperature, and when the flow velocity of the gas flowing through the sonic nozzle is subsonic, the upstream pressure and Measure the flow rate in a gas flow meter that calculates the flow rate from the neck pressure.

According to the present invention, it is possible to provide a gas flowmeter capable of measuring a flow rate in all flow rate ranges, thus enabling measurement at sound and subsonic speeds capable of accurately measuring the flow rate while having a wide measurement range.

1 is a diagram showing a gas flow meter according to an embodiment of the present invention.
2 is a cross-sectional view showing the configuration of the sonic nozzle of FIG. 1 in detail.
3 is a graph showing the pressure ratio according to the distance of the nozzle.
4 is a block diagram illustrating a method of measuring a flow rate in a gas flow meter according to an embodiment of the present invention.

Hereinafter, the configuration and operation of a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, in adding reference numerals to elements of each drawing, it should be noted that only the same elements are marked with the same numerals as possible, even if they are indicated on different drawings.

1 is a diagram showing a gas flow meter according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing in detail the configuration of the sonic nozzle of FIG. 1. 3 is a graph showing the pressure ratio according to the distance of the nozzle. 4 is a block diagram illustrating a method of measuring a flow rate in a gas flow meter according to an embodiment of the present invention.

1, the gas flow meter 100 of the present invention includes a tube body 110, a sonic nozzle 120, a first pressure sensor 130, a second pressure sensor 140, a temperature sensor 150, and an operation unit ( 160), a control unit 170, and a display unit 180.

The pipe body 110 may have a circular pipe shape and is connected to a pipe.

The sonic nozzle 120 is formed in the tube body 110 and has a converging portion 121 in which a cross-sectional area through which a fluid flows is reduced, and a diffusion portion 122 in which a cross-sectional area is increased. Here, the fluid moves from the converging portion 121 to the diffusion portion 122.

The boundary point between the converging portion 121 and the diffusion portion 122 is generally referred to as the neck 123 of the sonic nozzle 120. The inner diameter of the neck 123 is the smallest, and the cross-sectional area through which the fluid flows is the narrowest.

An opening 124 is formed in the diffusion part 122 of the sonic nozzle 120 through the tube body 110 in a portion adjacent to the neck 123. If the opening 124 is formed so as to overlap the neck 123 of the sonic nozzle 120, the neck 123 is damaged so that it cannot play the role of the sonic nozzle 120, so the opening 124 closes the neck 123 It is formed to be in contact with the neck 123 without damaging it.

The first pressure sensor 130 is installed at the front end of the sonic nozzle 120 to measure the upstream pressure (P ₁ ). Here, the upstream refers to a section before the fluid flows into the sonic nozzle 120.

The temperature sensor 150 is installed at the front end of the sonic nozzle 120 to measure the upstream temperature T ₁ .

The second pressure sensor 140 is installed to be inserted into the opening 124 formed in the diffusion part 122 of the sonic nozzle 120 and measures the pressure P ₂ of the neck 123.

The calculation unit 160 calculates the flow rate of the gas according to the measured values of the first pressure sensor 130, the second pressure sensor 140, and the temperature sensor 150.

The display unit 180 may be formed in the form of a display, and may display, for example, an upstream temperature T ₁ , an upstream pressure P ₁ , a neck pressure P ₂ , and a flow rate of gas.

The controller 170 collects the upstream temperature (T ₁ ), the upstream pressure (P ₁ ), and the neck pressure (P ₂ ) from the sensors, and displays the flow rate of the gas calculated by the calculation unit 160 on the display unit 180. The entire system of the gas flow meter 100 can be controlled.

Hereinafter, a method of measuring the flow rate of a gas using the gas flow meter 100 of the present invention, regardless of whether the velocity of the gas is sound or subsonic, will be described in detail.

First, the upstream pressure P ₁ is measured by the first pressure sensor 140 installed at the front end of the sonic nozzle 120 (S110).

Next, the upstream temperature T ₁ is measured by the temperature sensor 150 installed at the front end of the sonic nozzle 120 (S120).

Next, the neck pressure P ₂ is measured by the second pressure sensor 140 inserted and installed in the opening 124 formed in the diffusion part 122 of the sonic nozzle 120 (S130).

The calculation unit 160 calculates the ratio of the upstream pressure (P ₁ ) and the neck pressure (P ₂ ) of the sonic nozzle 120 from these measured values.

The velocity of the gas passing through the sonic nozzle 120 is gradually accelerated as it passes through the converging portion 121 and becomes the maximum at the neck 123. The ratio of the rear end pressure and the shear pressure of the sonic nozzle 120 When the pressure) is equal to or less than the critical pressure ratio (generally 0.8), the sound velocity is maintained at the neck 123 of the sonic nozzle 120.

In this case, since the flow rate does not change even if the pressure on the downstream side of the sonic nozzle 120 is lowered, that is, the flow rate of the gas is not affected by the downstream pressure, only the pressure and temperature at the front end of the sonic nozzle 120 are measured. Flow can be calculated.

)는 공기인 경우 0.5283으로 일정한 값이 된다. 공기 외의 가스인 경우, 상류압력(P₁)과 목 압력(P₂)의 압력비(

)는 아래의 수학식 1로 계산할 수 있다.When the flow velocity is sonic, the pressure ratio between the upstream pressure (P ₁ ) and the neck pressure (P ₂ ) (

) Is a constant value of 0.5283 for air. For gases other than air, the pressure ratio between the upstream pressure (P ₁ ) and the neck pressure (P ₂ ) (

) Can be calculated by Equation 1 below.

[Equation 1]

Here, Κ is the ratio of the static specific heat and the static pressure specific heat, and is a value determined according to the type of gas.

)로부터 가스의 유속이 음속인 경우를 판단할 수 있다. In this way, the calculating unit 160 is the pressure ratio (

), it is possible to determine the case where the flow velocity of the gas is sound velocity.

)가 0.5283이면, 연산부(160)는 소닉노즐(120)을 통해 유동하는 가스의 유속이 음속인 경우로 판단하고, 제1 압력센서(130)에 의해 측정한 상류압력(P₁)과 온도센서(150)에 의해 측정한 온도(T₁)로부터 아래의 수학식 2를 이용하여 유량을 계산한다(S140). The flowing gas is air and the pressure ratio (

) Is 0.5283, the calculation unit 160 determines that the flow velocity of the gas flowing through the sonic nozzle 120 is the speed of sound, and the upstream pressure P ₁ measured by the first pressure sensor 130 and the temperature sensor The flow rate is calculated from the temperature (T ₁ ) measured by 150 using Equation 2 below (S140).

[Equation 2]

(kg/s)

(kg/s)

Here, Q _m is the flow rate (mass flow rate), C _d is the runoff coefficient (value obtained by calibrating the flow meter), C ^* is the critical flow function (physical properties of the gas), and A is the cross-sectional area of the neck 123 (㎡ ), P ₀ is the upstream pressure (Pa), R is the gas constant (8312 m·N/kg·mol·k), M is the molecular weight of the gas (kg·mol), and T _o is the upstream temperature (K )to be.

In this way, when the flow velocity of the gas is the sound velocity, the flow rate can be calculated by substituting the upstream pressure (P ₁ ) and the temperature (T ₁ ) in Equation 2.

)가 0.5283보다 커질 때, 즉 P₂가 0.5283P₁보다 커질 때, 연산부(160)는 소닉노즐(120)을 통해 유동하는 가스의 유속이 아음속인 경우로 판단한다.Pressure ratio(

When) is greater than 0.5283, that is, when P ₂ is greater than 0.5283P ₁ , the calculation unit 160 determines that the flow velocity of the gas flowing through the sonic nozzle 120 is subsonic.

When the flow rate of the gas is subsonic, using Equation 3 below from the upstream pressure P ₁ measured by the first pressure sensor 130 and the neck pressure P ₂ measured by the second pressure sensor 140 The flow rate is calculated (S150).

[Equation 3]

(kg/s)

(kg/s)

Where Q _m is the flow rate (mass flow rate), C _d is the outflow coefficient (a value obtained by calibrating the flow meter), β is the tightening ratio (nozzle neck diameter/tube diameter), ε is the expansion coefficient, and A is the neck (123) is the cross-sectional area (m2), ㅿP is the pressure difference (upstream pressure-neck pressure) (Pa), and ρ is the upstream gas density (kg/m3).

In order to calculate the upstream gas density, the upstream pressure (P ₁ ) and temperature (T ₁ ) must be measured, and the pressure difference can be obtained by measuring the upstream pressure (P ₁ ) and neck pressure (P ₂ ), and the remaining values are You know, so you can calculate the flow rate.

)이다. 곡선 (ⅰ),(ⅱ),(ⅲ),(ⅳ)는 노즐 후단의 압력을 다르게 할 때 노즐 거리에 따른 압력비를 나타내고, 압력의 세기는 (ⅰ) > (ⅱ) > (ⅲ) > (ⅳ) 순이다.In the graph of FIG. 3, the horizontal axis is the distance along the nozzle, and the vertical axis is the upstream pressure (P ₁ ) and the pressure ratio according to the nozzle distance (

)to be. Curves (i), (ii), (iii), (iv) represent the pressure ratio according to the nozzle distance when the pressure at the rear end of the nozzle is different, and the strength of the pressure is (i)>(ii)>(iii)> ( Iv) in order.

In the graph, curves (i) and (ii) represent the subsonic speed, and (iii) and (iv) represent the speed of sound.

Referring to FIG. 3, it can be seen that the pressure at the neck portion of the sonic nozzle is lower than the downstream pressure P _B at the rear end of the neck.

In particular, in the case of subsonic speed, if the flow rate is calculated using the difference between the downstream pressure (P _B ) and the upstream pressure (P ₁ ), it can be seen that the difference in pressure is small and it is difficult to accurately calculate the flow rate.

When the flow rate of the gas is subsonic, the flow rate can be more accurately calculated by calculating the flow rate from the pressure difference between the upstream pressure (P ₁ ) and the neck pressure (P ₂ ) as in the present invention.

In addition, since the downstream pressure (P _B ) at the rear end of the neck varies depending on the shape of the nozzle or surface roughness, it is difficult to accurately measure the flow rate. In the present invention, since the flow rate is calculated using the neck pressure (P ₂ ) in the case of subsonic speed, the flow rate can be more accurately measured because it is not affected by the shape or surface roughness of the nozzle.

As described above, in the gas flow meter 100 of the present invention, the opening in the diffusion portion 122 adjacent to the converging portion 121 of the sonic nozzle 120 so as not to damage the neck 123 portion of the sonic nozzle 120 A 124 is formed, and a second pressure sensor 140 is installed through the opening 124 to measure the neck pressure P ₂ .

The operation unit 160 determines whether the flow velocity is a sound velocity or a subsonic velocity from the pressure difference between the upstream pressure (P ₁ ) and the neck pressure (P ₂ ), and calculates the flow rate by different equations applied in the case of sound speed and the case of subsonic speed. .

Accordingly, in the gas flow meter 100 of the present invention, it is possible to measure the flow rate at both the sound velocity and the subsonic velocity, and it is not affected by the shape of the nozzle or the surface roughness, and thus the flow rate can be more accurately measured. .

It is obvious to those of ordinary skill in the art that the present invention is not limited to the above embodiments and can be implemented with various modifications or modifications within the scope not departing from the technical gist of the present invention. will be.

100: gas flow meter
110: tube
120: Sonic Nozzle
121: convergence unit
122: diffusion
123: neck
124: opening
130: first pressure sensor
140: second pressure sensor
150: temperature sensor
160: operation unit
170: control unit
180: display

Claims (4)

In the gas flow meter,
A pipe body connected to the pipe;
A sonic nozzle formed inside the tube and having a converging portion and a diffusion portion;
A first pressure sensor installed at a front end of the sonic nozzle to measure pressure;
A temperature sensor installed at the front end of the sonic nozzle to measure temperature;
A second pressure sensor installed in the diffusion unit adjacent to the converging unit in the sonic nozzle to measure pressure;
A calculation unit for calculating a flow rate of gas according to measured values of the first pressure sensor, the temperature sensor, and the second pressure sensor;
A display unit displaying the flow rate of gas;
A control unit for controlling the display unit;
Including,
An opening is formed through the tube body so as to contact a neck portion that is a boundary between the converging portion and the diffusion portion, and the second pressure sensor is installed through the opening,
The calculation unit separates the sound speed and the subsonic speed from the ratio of the measured values of the first pressure sensor and the second pressure sensor,
When the flow velocity of the gas flowing through the sonic nozzle is the speed of sound, the flow rate is calculated from the measured values of the first pressure sensor and the temperature sensor,
When the flow rate of the gas flowing through the sonic nozzle is subsonic, the gas flow meter calculates a flow rate from measured values of the first pressure sensor and the second pressure sensor. delete delete delete

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