SU855401A1 - Thermal flow meter - Google Patents

Description

(54) HEAT FLOW METER

one

The invention relates to instrument engineering and is intended to measure the velocity and flow rates of liquids and gases, and can also be used to control flows with time-varying thermophysical properties of the measured and the environment.

Devices for measuring the flow rate, a measuring tube with a heater and temperature-sensitive elements located on it, a heat meter system consisting of two heat meter thermometers, an auxiliary heater and an electrically insulating tape, the meter heater and heat sensitive elements are included in the measuring circuit, and the heat meter thermometers are known. and an auxiliary heater - to the zero-temperature stabilization scheme for the heat metering system 1.

However, the presence in the known device between the heat metering system, the flow of heat insulation and the wall of the pipe through which gas or liquid flows, leads to the fact that maintaining a zero temperature differential on the heat insulation has increased inertia, and this reduces the accuracy

mass flow measurements at rapidly changing ambient and flow temperatures.

Closest to the present invention is a heat flow meter that contains a measuring section of a pipeline with a heater located on it, two measuring thermosensitive elements installed on both sides of the heater, a compensating thermo sensitive element installed in front of the first measuring thermosensitive element in the direction of flow, bridge circuits, the first of which includes the first in the direction of flow of the measuring and compensating those The sensing element, 5 body element, the second includes two measuring temperature-sensitive elements and output unit 2.

In this device, it is not possible to fully compensate for the inconsistencies from a change in the ambient temperature, the flow temperature at the inlet to the measuring section, which affects the accuracy of the measurement.

The purpose of the invention is to improve the measurement accuracy.

This is achieved by the fact that a heat flow meter containing a measuring section of a pipeline with a heater located on it, two measuring thermosensitive elements installed on both sides of the measuring heater, a compensating thermo sensitive element installed in front of the first measuring thermosensitive element in the direction of flow, the measurement circuit in the form of two bridge circuits, the first of which includes the first in the direction of flow of the measuring and compensation ter the sensing elements, the second includes two measuring temperature-sensitive elements and an output unit, equipped with three heat-measuring elements, each of which is made in the form of a temperature-sensitive element and a heater combined with it, located in three sections of the measuring pipeline between the compensation and measuring thermal receivers and the heater, and in the diagram measurements additionally introduced two bridge circuits, two comparison circuits, three control circuits, two keys, schemes - determining the unbalance sign, a subtraction circuit, a comparison circuit with a reference signal, wherein the third bridge circuit includes thermal receivers of heat meters, respectively, located between the compensating temperature sensitive element and the first measuring heat sensitive element and between the first measuring heat sensitive element and the measuring heater, the fourth thermal bridge includes heat receivers of the heat meter elements, respectively located between the compensatory temperature-sensitive element and the first measurement an effective temperature-sensitive element and between the measuring heater and the second measuring temperature-sensitive element, the outputs of the first and third bridge circuits are connected through the first comparison circuit to the input of the first control circuit, the output of which is connected to the inputs of switches connected to the heaters of the heat meter elements located between the compensation and the first measuring thermosensitive element and between the first measuring thermosensitive element and the measuring heater, at the same time to the second odes determination via keys unbalance sign circuit connected to the output of the first bridge circuit, the outputs of the second and fourth bridge circuit via the second comparing circuit and a second control circuit connected to the heater teplomernogo element arranged between the measuring heater and the second probe thermosensitive

the element, the outputs of the first and fourth bridge circuits are connected to the third comparison circuit, the outputs of which are connected to the measuring heater and to the output unit, through the subtraction circuit and the circuit of comparison with the reference signal.

FIG. 1 shows a diagram of the primary transducer of a heat flow meter; in fig. 2 is a measurement block diagram; in fig. 3 - curves of temperature variation of a gas or liquid stream and heat metering elements at a stream temperature lower than the ambient temperature; in fig. 4 - curves of temperature variation of the controlled flow and heat metering elements at the flow temperature higher than the ambient temperature.

The primary transducer for a heat flow meter includes a housing consisting of two parts 1 and 2, inside which a measuring tube 3 is placed, which is a measuring section of a pipeline, connected to part 2 of a housing through an electrically insulating sleeve 4. Part 1 of a housing and measuring tube 3 are made of a magnet a soft material, which allows to protect the internal part of the primary converter from the action of external electromagnetic and electrostatic fields. The measuring tube 3 contains measuring heater 5 and temperature-sensitive elements 6 and 7, made in the form of nets of platinum wire, wound on heat and electrical insulating frames, compensatory temperature-sensitive element 8. In three sections of the measuring tube between the compensation and measuring temperature-sensitive elements and the heater heat measuring elements 9–11 are located, each of which is made in the form of a temperature sensitive element 12–14 wound from wire bifilically with auxiliary and heaters 15-17 and heat-insulated from each other with frameworks of measuring thermosensitive elements and heater, and from the measuring tube with insulating cups 18. All elements are fixed in measuring tube 3 with a nut 19. For output of wires Hermetic connector 20.

The measuring block diagram includes a bridge circuit 21, which includes the first in the direction of flow measurement and compensation temperature-sensitive elements 6 and 8, a second bridge circuit 22, which includes two measuring temperature-sensitive elements 6 and 7, a third bridge circuit 23, which includes

the thermal receivers of the heat metering elements 12 and 13, located respectively between the compensation thermosensitive element, the first measuring thermosensitive element and the measuring

a heater, a fourth bridge circuit 24, which includes thermal sensors of heat meters 12 and 14, two comparison circuits 25 and 26, three control circuits 27-29, a comparison circuit 30 with a reference signal, two keys 31 and 32, an unbalance sign determination circuit 33, heaters 15-17 of heat metering elements, subtraction circuit 34, output unit 35. FIG. 3 and 4 curve 36 depicts the change in the temperature of a stream of gas or liquid along the length of the measuring section, curve 37 depicts the change in temperature of the three heat-measuring elements.

Heat flow meter works as follows.

From the graphs it can be seen that AtH At- (At2 + At) (Fig. 4),

 At4- (Atz + Ats) (Fig. 3),

HO At2 is Aft, and At3 is Atj therefore in the general case

AtH - At-At2 + Att) from where

 7

t I - temperature flow in place

Where

installation of a compensation thermal receiver 10; t i is the flow temperature at the installation site of the first measuring thermal receiver 6;

t is the flow temperature in front of the measuring heater 5;

tj is the flow temperature at the installation site of the second measuring thermal receiver 7;

t4, tj, t e is the temperature of heat-measuring thermal receivers 12-14; AtH- -fci-increment of the flow temperature with mass flow rate M due to the introduction of thermal power W released on the measuring heater 5, here Cf is the specific isobaric heat capacity of the flow.

At, t2-t;

At, t5- (tH + AtH);

At, t, -ta; At3 (tH-f Atn) -tj;

Atz tM-t2; At ts-tt

 ;

Since At g and Ats cannot be measured, since a temperature-sensitive element cannot be placed at the location of the measuring heater, At a and At a must be equal and equal to the difference At i, which can be measured using a compensation 8 and the first measuring 6 temperature-sensitive element. In this case, At 2

 At 3 At, and M g- 2b-THG-,.,

To achieve the equality Atz At3 At t, it is necessary that the lengths of the sections located between the compensation.

the measuring thermosensitive elements and the measuring heater were the same (L in figs. 3 and 4) and the laws of variation of the flow temperature due to heat exchange with the environment on each

of these plots were the same. For this, taking into account the proportional dependence of the intensity of heat exchange with the environment on the temperature difference between the flow and the channel wall, an equal temperature difference is created between the channel wall in each of these areas and the flow at the entrance to this section t4-tt t5-t2 t6- (tH + At) (Fig. 3), t, -t 4 ti-ts (tH + At) -16) (Fig. 4). The role of the channel wall in each of the sections

5 performs a heat meter element including a heat meter thermopile and an auxiliary heater.

Maintaining the equality t4-tf t5-tz is as follows. From this equality we have

ts-t4 ti-ti OR tg-14. Ati, the value of At t is measured by the first bridge circuit 21, the value of tj-t4 by the second bridge circuit 22, and their equality is maintained by auxiliary heating of the heat meter elements 15 and 16.

If At, 0 (Fig. 3) and At, ts-t, then the unbalance voltage of the first 21 and second bridge circuit 22 is fed to the input of the comparison circuit 26, the output of which causes an increase in the output voltage of the control circuit 27 supplied to the keys 31 and 32. At the same time, a signal is output from the output of the unbalance sign circuit 33 to open only the key 32, which turns on the heater 16 of the heat element. The temperature of the heater 16 and the heat meter thermal receiver 13 tg increases until the equality Ati t5-t is established. The output signal of the comparison circuit 26 becomes zero and the output voltage of the circuit

 Control 27 is stabilized on. achieved level. The key 31 is closed. And the heater of the heat meter element 15 is turned off. When (FIG. 3) the Ati t5-t 4 signal from the output of the comparison circuit 26 causes

a decrease in the voltage at the output of the control circuit 27, which leads to a decrease in the temperature tg until the equality Ati t5-14 is established. The key 31 is closed, and the auxiliary heater 15 is disconnected.

Claims (2)

When At, 0 (fig. 4) and / Ati / ts-t. unbalance voltage of the first and second bridge circuits 21 and 22 is fed to the input of the comparison circuit 26, the signal from the output of which causes an increase in the output voltage of the control circuit 27 supplied to the keys 31 and 32. At the same time, the signal from the output of the unbalance detection circuit 33 a key 31, which turns on the heater 15. The temperature of the heater 15 and the heat meter thermal detector 12 t increases until the equality / uti / ts-t is established. In this case, the signal from the output of the comparison circuit 26 becomes equal to zero, and the voltage at the output of the control circuit 27 is stabilized at the achieved level. The key 32 is closed and the heater 16 is turned off. If D1.0 (Fig. 4) and Mty t5-tA, then the signal from the output of the comparison circuit 26 causes a decrease in the voltage at the output of the control circuit 27, which leads to a decrease in the temperature, i, until the equality / Ati / t6-1l. When this key is closed, and the heater 16 is turned off. Maintaining the equality ts-ti t6- (tn + Atfl) is as follows. From equality we have te-1 / At (Fig. 3 and 4). The value of te-t is measured by the third bridge circuit 23, the value of At is the fourth bridge circuit 24, and their equality is maintained by the heater of the heat meter element 17. The values of At and te-t / are only positive, since At with the minimum thermal insulation measuring tube 3 is greater than At (-Atj, due to the heat exchange of the flow with the environment. If At te-t ;, then the unbalance voltage of the third 23 and fourth 24 bridge circuit enters the input of the reference circuit 25, the output of which causes an increase in voltage at the output of circuit 29 control The temperature supplied to the heater 17. Its temperature and, accordingly, the temperature of the heat meter thermal detector 14 tg increases until the equality At t6-t is established. At the same time, the signal from the output of the comparison circuit 25 becomes equal to zero and the voltage at the output of the circuit 29 If A; t6-14, then the signal from the output of comparison circuit 25 causes a decrease in voltage at the output of control circuit 29, which leads to a decrease in temperature 1 until the equality At t6-t4 is established. Thus, the law of change in the flow temperature due to heat exchange with the environment is maintained the same in all parts of the measuring tube, so that. In these cases, the power W consumed by the measuring heater 5 is a measure of the mass flow rate of gas or liquid M, if the value of At-2Ati is constant and is achieved as follows. The value of Ati is measured by the first bridge circuit 21, the unbalance voltage of which is applied to one input of the subtracting circuit 34, and the voltage of the unbalance of the fourth bridge circuit 24 measuring the value of At. The output voltage of the subtraction circuit 34 corresponds to the value of M-2 At i and is fed to the input of the comparison circuit 30 with the reference signal V reference, which corresponds to the selected and predetermined value At-2 At 1. If the output voltage of the subtraction circuit 34 is less than the reference voltage Coporn, then the signal from the comparison circuit 30 with the reference signal causes an increase in the output voltage of the control circuit 28 supplied to the measuring heater 5. Its temperature and, consequently, the flow temperature increases as long as the corresponding rank 2 At-At t vyodnoe voltage from the subtractor circuit 34 e ref is a reference voltage Vopor Noe. At the same time, the signal from the output of the AU comparison circuit with the reference signal becomes equal to zero, and the output voltage from the control circuit 28 stabilizes at the achieved level. If the output voltage from the subtraction circuit 34 is larger than the reference voltage, then the signal from the comparison circuit 30 with the reference signal causes a decrease in the output voltage of the control circuit 28, which leads to a decrease in the flow temperature tn until the output voltage from the subtraction circuit 34 does not compare with the reference voltage. Thus, the constant value of At-2At is maintained, and the mass flow rate of gas or liquid M is proportional to the power W consumed by the measuring heater 5, which is measured by the output unit 35, which is, for example, a wattmeter calibrated in units of mass flow. Since in all three sections of the measuring tube located between the compensation, measuring thermosensitive elements and the measuring heater, the same law of temperature variation of the flow due to heat exchange with the environment is maintained regardless of the temperature difference between the flow and the environment, the accuracy of measuring the mass flow at ambient temperature more flow temperature. Thus, the measurement of the temperature difference between the flow and the channel wall and keeping it constant at the entrance to each of the three sections of the measuring tube is carried out by thermal meters located on the inner surface of the measuring tube and playing the role of the channel wall, the inertia of the variable mass increases. flow rates at rapidly changing flow and ambient temperatures. The invention The thermal flow meter containing a measuring section of a pipeline with a heater located on it, two measuring temperature-sensitive elements installed on both sides of the heater, a compensation temperature-sensitive element established by the first measuring temperature-sensitive element in the direction of flow, the measurement circuit in the form of two bridge circuits, the first of which includes the first in the direction of flow of the measuring and compensating thermosensitive The second one includes two measuring temperature-sensitive elements and an output unit, characterized in that, in order to improve the measurement accuracy, it is equipped with three heat-measuring elements, each of which is designed as a temperature-sensitive element and a heater combined with it, located in three sections of the measuring pipeline between compensatory and measuring thermal receivers and a heater, and two bridge circuits, two comparison circuits, and three control circuits are additionally introduced into the measurement circuit. Here, the unbalance sign detection circuit is a subtraction circuit, a comparison circuit with a reference signal, while the third bridge circuit includes thermal receivers of heat meter elements located respectively between the compensation temperature sensitive element and the first measuring temperature sensitive element and between the first measuring temperature sensitive element and the measuring heater, fourth bridge circuit included thermo3. 1 S 13 1B - 15 12. 8 ..... illLl Ll..l  I and I-J I ° 1 h. OI / 7i  //////// 777 The output of the receivers of heat metering elements located respectively between the compensation temperature sensitive element to the first measuring temperature sensitive element and between the measuring heater and the second measuring temperature sensitive element, the outputs of the first and third bridge circuits are connected through the first comparison circuit to the input of the first control circuit, the output of which is connected to the key inputs, connected to the heaters of heat meters, located between the compensation and the first measuring the sensing element and between the first measuring sensing element and the measuring heater, while the second inputs of the keys are connected via the first circuit circuit to the second key inputs, the second and fourth bridge circuits are connected to the heater of the heat meter element located through the second comparison circuit and the second control circuit between the measuring heater and the second measuring temperature-sensitive element, the outputs of the first and fourth bridge circuits through the circuit you Itani and a circuit comparing a reference signal connected to the third comparing circuit, the outputs of which are connected to the measuring to the heater and the output unit. Sources of information taken into account in the examination 1. P.Korotkov. Thermal flowmeters. L., “Mechanical Engineering, 1969, p. 92 2.Korotkov P. A. Thermal flowmeters. L., “Mechanical Engineering, 1969, p. 150 (prototype).