JP3267943B2 - Thermal flow meter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a thermal flow meter.

[0002]

2. Description of the Related Art A conventional thermal flow meter is shown in FIG.
As shown in (2), two U-shaped thin metal tubes 32, 33 having the same thermal conductivity are arranged in a flow tube 31 of a fluid to be measured so as to be perpendicular to the flow direction of the fluid, Temperature sensor 3
Some are provided with 4, 35, and are mainly used for gas flow rate measurement. In the figure, reference numeral 36 denotes a metal thin tube support.

In the above-mentioned conventional thermal type flow meter, an electric current is applied to one of the thin metal tubes 32 to heat it, and the other thin tube 33 is not heated. The amount of heat carried by the fluid is measured from the difference, and the flow rate is calculated. Specifically, the amount of current flowing through one of the thin metal tubes 32 is adjusted so as to keep the difference between the temperatures detected by the two temperature sensors constant, the amount of heat carried by the fluid is measured from the amount of current, and the flow rate is determined from the amount of heat. Seeking.

A conventional thermal flow meter requires two metal thin tubes, a heated thin metal tube 32 and a non-heated thin metal tube 33, which are not energized, and have a complicated structure, many seals, and a reduced size of the flow meter. It is difficult to reduce the price.

[0005] In the thermal type flow meter shown in FIG.
The temperature sensors 34 and 35 are provided one by one in each of the thin metal tubes 32 and 33, and are of a central measurement type in which only the central portion of the flow path tube is a measurement point. The total flow must be calculated from the flow velocity distribution.

Therefore, if there is a disturbance in the flow velocity distribution in the flow path pipe, a measurement error occurs. In addition, if the temperature in the flow path pipe is not constant and there is a temperature distribution, the amount of heat taken by the fluid is accurately measured. In this case, a measurement error also occurs.

By the way, if a large number of temperature sensors are arranged in each of the thin metal tubes 32 and 33 to provide a multi-point measurement type, the average temperature in the flow path tube can be measured, and the above-described measurement error can be reduced. However, in the conventional thermal flow meter, if a large number of temperature sensors were to be provided, the structure and electrical connection would be more complicated, and could not be put to practical use.

[0008]

[Purpose] The purpose of the present invention is to use only one thin metal tube, so that the structure is simple and the size can be reduced, and a large number of temperature sensors can be easily arranged. Accordingly, it is an object of the present invention to provide a thermal type flow meter which can realize a multi-point measurement type flow meter, is hardly influenced by a flow velocity distribution and a fluid temperature distribution in a flow path pipe, and has a small measurement error.

[0009]

In order to achieve the above object, a thermal flow meter according to a first aspect of the present invention provides a thin metal tube through which an electric current for heating flows in an axial direction in a flow pipe for a fluid to be measured. Is provided at right angles to the flow direction of the fluid, and in the middle of the metal thin tube, the both ends of the metal current bypass line having a sufficiently smaller electric resistance value than the metal thin tube are appropriately spaced in the axial direction of the metal thin tube. A non-heating portion through which the heating current hardly flows was provided in the metal thin tube by connection, and a temperature sensor was disposed in each of the non-heating portion and the metal thin tube in the heating portion of the metal thin tube through which the heating current flows. There are things.

[0010] A thermal flow meter according to a second aspect of the present invention comprises:
A thin metal tube through which a heating current flows in the axial direction is provided at right angles to the flow direction of the fluid in the flow path tube of the fluid to be measured, and the electrical resistance value of the thin metal tube is smaller than that of the thin metal tube. By connecting both ends of the metal current bypass line at an appropriate distance in the axial direction of the thin metal tube, a non-heating portion in which the heating current hardly flows in the thin metal tube is provided, and the non-heating portion, A temperature sensor is provided in each of the metal thin tubes in the heating portion of the metal thin tube through which the heating current flows, and a radiator plate is provided at least around the metal thin tube between the heating portion and the non-heating portion.

A thermal flow meter according to a third aspect of the present invention comprises:
A thin metal tube through which a heating current flows in the axial direction is provided at right angles to the flow direction of the fluid in the flow path tube of the fluid to be measured. At least two heat sinks made of conductive metal
By connecting both ends of a metal current bypass line having an electric resistance value sufficiently smaller than that of the metal thin tube to the heat sink, the heating current hardly flows through the metal thin tube. A heating section is provided, and a temperature sensor is provided in each of the non-heating section and the metal tube in the heating section of the metal tube through which a heating current flows.

Further, in the first to third inventions,
The heating section and the non-heating section are provided alternately in the axial direction of the thin metal tube.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a thermal flow meter according to the present invention will be described below in detail with reference to specific examples shown in the accompanying drawings. FIG. 1 shows a first embodiment which is a basic structure of a thermal type flow meter according to the present invention. In a flow path tube 1 for a fluid to be measured, a single thin metal tube 2 is formed at right angles to the flow direction of the fluid. The upper and lower ends of the thin metal tube 2 penetrate the upper and lower supports 3 and 3 and protrude out of the tube.

The thin metal tube 2 is made of a conductive metal having a small thickness and an appropriate electric resistance, and generates heat when an electric current flows.

A plurality of (three in FIG. 1) heat radiating plates 4a, 4b, 4c are provided outside the thin metal tubes 2 in the flow channel tube 1.
The heat sink has a large heat transfer area as compared to the thin metal tube, and has a large heat capacity.For example, as a disk shape having a hole at the center for allowing the thin metal tube to penetrate. is there.

Electric wires 6a, 6b from a power source 5 outside the flow path tube 1 are connected to upper and lower heat radiation plates 4a, 4c closest to the flow path tube wall, respectively. Are connected to both ends of a metal current bypass line 7 having a sufficiently smaller electric resistance than a thin metal tube, and a central and lower heat radiating plate 4b, 4c. Are set so that the potentials are substantially equal.

Therefore, all the heating current from the power supply 5 flows through the thin metal tube between the upper heat radiating plate 4a and the central heat radiating plate 4b, so that the upper and central heat radiating plates 4a and 4b of the thin metal tube are heated.
a, a heating portion A is provided between the heat radiating plate 4b and the heat radiating plate 4c between the central portion and the lower heat radiating plate 4c.
A non-heating portion B is located between b and 4c. The reason why almost no current flows in the non-heating portion B of the thin metal tube 2 will be described later in detail.

Temperature sensors 8a and 8b are disposed in the thin metal tubes 2 in the heating section A and the non-heating section B, respectively, and outputs from the temperature sensors 8a and 8b are transmitted through signal lines 9a and 9b. The signal is sent to the temperature signal output terminal 10.

Next, the operation of the thermal flow meter having the above-described configuration will be described. The heating current from the power source 5 flows from the upper radiator plate 4a to the central radiator plate 4b via the thin metal tube 2 by the electric wire 6a, and the upper radiator plate 4a and the central radiator plate 4
The heating section A of the thin metal tube 2 generates heat between b. The heating current passing through the heating section A flows from the central heat radiating plate 4b to the lower heat radiating plate 4c through the current bypass line 7, and returns to the power supply by the electric wire 6b.

At this time, the heating current slightly flows through the non-heating portion B of the thin metal tube. The heating current amount is I, the electric resistance value of the non-heating portion is R ₁ , and the current amount flowing through the non-heating portion is I ₁ ,
Assuming that the electric resistance value of the current bypass line 7 is R ₂ , according to Ohm's law, I ₁ = I · R ₂ / (R ₁ + R ₂ ) (1)

Here, the electric resistance value R ₂ of the current bypass line
Is sufficiently smaller than the electric resistance value R ₁ in the non-heated portion B of the thin metal tube, so that R ₂ / (R ₁ + R ₂ ) is almost equal to 0. Therefore, from the equation (1), I ₁ is It is almost equal to zero.

Further, the heating value of the non-heated part B due to the Joule _{heat, πdh (T-T a)} = R 1 · I 1 2 can be expressed as (2), the I ₁ = 0 (2 ), T = _Ta , and the temperature of the non-heated portion is equal to the temperature of the fluid. That is, the temperature of the metal thin tube detected by the temperature sensor 8b in the metal thin tube in the non-heated portion B is equal to the temperature of the fluid. Can be considered. Where d is the diameter of the thin metal tube, h
Is the heat transfer coefficient, T is the temperature of the metal tube, and _Ta is the temperature of the fluid.

The heat from the heating section A is transmitted to the non-heating section B side and the upper flow path pipe wall side by heat conduction of the thin metal tube, but the heating section A is transmitted to the upper and central heat radiation plates 4a and 4b. Since the heat radiating plates are sandwiched between the heat radiating plates 4a, 4b,
The heat transmitted to the fluid is radiated to the fluid, and almost all of the heat from the heating portion A is taken away by the fluid without being transmitted to the non-heating portion B or the flow path tube wall.

Therefore, the temperature detected by the temperature sensor 8b in the heating section A and the temperature sensor 8a in the non-heating section B
If the heating current is controlled so as to keep the temperature difference of the detected temperature constant, the heating current is proportional to the amount of heat generated in the heating unit A,
In addition, since the calorific value is a function of the flow rate of the fluid, the flow rate of the fluid is obtained from the heating current. In the case of the flow meter according to the first embodiment, it is necessary to make appropriate corrections according to the flow velocity distribution and the temperature distribution of the fluid in the flow path pipe.

In the first embodiment described above, the heating unit A
And the non-heating portion B are provided in the axial direction of the thin metal tube 2 at one position each in the axial direction, and the thermal flow meter having the basic structure is described. The thermal flow meter of the present invention is described in the first embodiment. As shown in FIG. 2, a multi-point measuring type thermal flow meter is used without complicating the structure, as in the second embodiment shown in FIG. be able to.

More specifically, a large number of heat radiating plates 11, 11 are arranged in the axial direction of the thin metal tube 1, and a large number of heating portions A and non-heating portions B are provided alternately. Are provided with temperature sensors 12 and 13, respectively.
, And the temperatures of the non-heating portions B, B, the average temperature of the heating portion and the average temperature of the non-heating portion, that is, the average temperature of the fluid, can be obtained.

Therefore, there is a great merit that the measurement error due to the flow velocity distribution and the temperature distribution of the fluid in the flow path tube can be reduced. In the second embodiment as well, the heating current is controlled so as to keep the difference between the average temperatures of the heating sections A, A and the non-heating sections B, B constant, and the flow rate of the fluid is determined from the heating current. .

In the thermal type flow meters of the first and second embodiments described above, the metal thin tube 2 is a straight tube and is provided in the diameter direction of the flow path tube. What is necessary is just to provide on one surface which becomes a right angle with respect to,
In some cases, the metal tube is formed in a U-shape, loop shape, or wavy shape, and the structure and electrical connection do not become complicated even if the shape of the metal tube is changed.

Although the electric wires from the power source are connected to the heat sinks at the upper and lower ends, the electric wires from the power source may be connected to the upper and lower ends of the thin metal tube protruding outside the flow path tube. Also, the current bypass line may not be connected to the heat sink but may be directly connected in the middle of the thin metal tube.

When an electric wire or a current bypass line is directly connected to a thin metal tube as described above, the heat radiating plate may not be made of a conductive metal, but may be made of a material having excellent heat radiating properties (thermal conductivity). In this case, for example, an insulating material such as a ceramic material may be used.

Further, although one temperature sensor is provided for each of the heating section A and the non-heating section B, the number of temperature sensors provided for each heating section and the non-heating section may be plural. ,
For example, one temperature sensor is provided in the non-heating section B to detect the temperature of the fluid at one point, and this detected temperature is used as the representative fluid temperature. In some cases, the flow rate is measured by detecting the heating section temperature and comparing the heating section temperature with the representative fluid temperature.

[0032]

The thermal flow meter according to the present invention is provided with a heating section and a non-heating section in one metal thin tube. Therefore, two metal thin tubes are arranged as in a conventional thermal flow meter. Thus, there is no need to perform complicated wiring over both metal thin tubes, a simple structure and a low-cost thermal flow meter can be realized, and the flow meter can be downsized.

Further, since a heat radiating plate having a larger heat transfer area than the thin metal tube is arranged at least between the heating part and the non-heating part, the heat from the heating part is prevented from being transmitted to the non-heating part. Therefore, it is possible to eliminate a measurement error due to heat transfer from the heating unit to the non-heating unit.

In addition, since the structure is simple, a large number of temperature sensors can be arranged in a thin metal tube to realize a multi-point measurement type thermal flow meter. And the measurement error due to temperature distribution can be suppressed to an extremely low level.

[Brief description of the drawings]

FIG. 1 is a partially cutaway front view showing a first embodiment of a thermal flow meter according to the present invention.

FIG. 2 is a partially cutaway front view showing a second embodiment of the thermal flow meter according to the present invention.

FIG. 3 is a partially cutaway perspective view showing an example of a conventional thermal flow meter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Flow path pipe 2 Metal thin tube 3 Support 4a, 4b, 4c Heat sink 5 Power supply 6a, 6b Electric wire 7 Current bypass line 8a, 8b Temperature sensor 9a, 9b Signal line 10 Temperature signal output end 11 Heat sink 12, 13 Temperature Sensor

Claims (4)

(57) [Claims] A thin metal tube through which a heating current flows in an axial direction is provided at right angles to a flow direction of a fluid in a flow path tube of a fluid to be measured, and is provided in the middle of the thin metal tube more sufficiently than the thin metal tube. By connecting both ends of a metal current bypass line having a small electric resistance at appropriate intervals in the axial direction of the metal thin tube, a non-heating portion through which the heating current hardly flows is provided in the metal thin tube. A thermal flowmeter comprising a temperature sensor disposed in each of a non-heating portion and a metal thin tube in a heating portion of a thin metal tube through which a heating current flows. 2. A thin metal tube through which a heating current flows in an axial direction is provided at right angles to a flow direction of a fluid in a flow channel tube of a fluid to be measured, and is provided in the middle of the thin metal tube more sufficiently than the thin metal tube. By connecting both ends of a metal current bypass line having a small electric resistance at appropriate intervals in the axial direction of the metal thin tube, a non-heating portion through which the heating current hardly flows is provided in the metal thin tube. A non-heating portion and a temperature sensor are respectively arranged in the metal thin tubes in the heating portion of the metal thin tube through which the heating current flows, and a radiator plate is provided at least around the metal thin tube between the heating portion and the non-heating portion. Thermal flow meter. 3. A thin metal tube through which a heating current flows in the axial direction at right angles to the flow direction of the fluid, and a radiating plate made of a conductive metal in the middle of the thin metal tube. At least two or more are disposed at appropriate intervals, and both ends of a metal current bypass line having a sufficiently smaller electric resistance value than the metal thin tube are connected to the heat radiating plate, respectively. A thermal flowmeter comprising a non-heating portion through which a heating current hardly flows, and a temperature sensor disposed in each of the non-heating portion and the metal thin tube in the heating portion of the metal thin tube through which the heating current flows. 4. The thermal flow meter according to claim 1, wherein said heating section and said non-heating section are provided alternately in the axial direction of said thin metal tube.