GB2138948A - Measuring fluid flow thermo-electrically - Google Patents

Abstract

A flow-meter 3 for measuring the flow-rate of a fluid 2 at a predetermined temperature in a conduit 1 comprises a thermally conducting probe 9 having a first end 8 in direct thermal contact with the fluid 2, whilst being thermally insulated at 10 from said conduit 1, a second end 12 of said probe 9 extending away from the fluid 2 and arranged e.g. by heater 13 to be maintained at a substantial temperature differential from said first end 8. First and second electrical temperature sensors 14,15 are disposed spaced apart from each other in the probe 9 for monitoring the temperature gradient along the probe. A third electrical temperature sensor 16 is disposed in direct thermal contact with the fluid 2 inside the conduit 1. Sensors 14-16 are connected to means 19 for processing temperature-dependent electrical outputs received from said sensors 14-16 so as to produce, in accordance with an empirical formula, an electrical output related to the flow rate of said fluid 2 which can be displayed. By providing a further temperature sensor in a return conduit, a measure of heat supplied can be obtained. <IMAGE>

Description

SPECIFICATION Flow meter The present invention relates to flow-meters suitable for measuring the flow-rate of fluid in a conduit.

Conventional flow-meters generally rely on some mechanical interaction with the fluid flow and thus suffer from long-term reliability problems and relatively limited accuracy. Other recently proposed methods are relatively complex and expensive and thus unsuitable for widespread use such as for example in energy consumption meters in individual customer supply points in a district heating scheme, or in water consumption meters.

It is an object of the present invention to avoid or minimize one or more of the above disadvantages.

The present invention provides a flow-meter for use in measuring the flow-rate of a fluid at a predetermined temperature in a conduit, said flow meter comprising a thermally conducting probe having a first end formed and arranged so as to be disposable, in use, in substantially direct thermal contact with the fluid inside the conduit, whilst being substantially thermally insulated from said conduit, with a second end of said probe extending away from the fluid and formed and arranged so as to be maintained in use, at a substantial temperature differential from said first end, first and second electrical temperature sensors spaced apart from each other in the direction from said first end to said second end for monitoring the temperature gradient between said first and second ends, a third electrical temperature sensor disposable in use, in substantially direct thermal contact with the fluid inside the conduit, each of said sensors being connected to signal processing means formed and arranged for processing temperature dependent electrical outputs received from said sensors so as to produce, in use, an electrical output related to the flow rate of said fluid, and a display means in communication with said processing means for displaying a measurement corresponding to said processing means electrical output.

Preferably the probe is provided with an electrical heating means, most preferably with a substantially constant thermal output, at said second end.

The present invention also extends to a method of measuring the flow-rate of a fluid in a conduit comprising the steps of disposing a probe of a flow-meter of the invention with its first end in direct contact with the interior space of said conduit with said probe thermally insulated from said conduit, disposing the third temperature sensor in substantially direct thermal contact with the interior space of the conduit remote from said probe, monitoring the electrical outputs of said sensors and processing them in the processing means of said flow-meter, and reading the display means thereof.

Naturally the invention also extends to a flow-meter of the invention when connected to a conduit for conveying fluid. With a flow-meter of the present invention it is possible to obtain relatively accurate measurements of fluid flow rate in a simple and reliable manner with little if any perturbation of the fluid flow. The measurements so obtained may moreover either be read directly or combined with other measurements prior to display.

Thus for example the flow-rate measurement may be combined with a measurement of the fluid temperature differential between the supply and return points in an individual supply installation of a district heating scheme, so as to provide an energy consumption measurement.

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example-of a preferred embodiment illustrated with reference to the accompanying drawing in which: Figure 1 is a generally-schematic sectional view of a flow-meter of the invention; and Figure 2 is a schematic circuit diagram thereof.

Figure 1 shows a length of conduit 1 for carrying a fluid comprising hot water 2, said conduit 1 having inserted along said length a flow meter 3.

In more detail the flow-meter 3 comprises a generally tubular support 4 having enlarged inner-diameter end portions 5 in which are fixedly received respective ends 6 of the conduit 1 and at a longitudinally intermediate point in one side an aperture 7 in which is mounted one end 8 of a probe 9 with its thermal insulation sleeve 10 of a suitable water resistant insulating material e.g. a plastics laminate such as Tufnol.

The one end is disposed so as to be substantially flush with the inner wall 11 of the tubular support 4 which in turn is continuous with that of the conduit 1 in order to minimize perturbation of the water flow 2 in the conduit. The water flow path should desirably be substantially straight at this point for a length corresponding to at least 10 times the internal bore diameter.

The other end 12 of the probe extends radially outwardly of the conduit 1 and is provided with a low power electrical resistance heating element 13. In between the heating element 13 and the one end 8 of the probe 9 along the longitudinal axis of the latter are disposed in spaced apart relation first and second temperature sensors in the form of thermistors 14,15 respectively. A third thermistor or temperature sensor 16 is attached to the outer wall 17 of the conduit 1 the latter being of metal e.g. copper in this case so that said outer wall 17 inside the flow-meter housing 18 (which desirably includes low density thermal insulation around the probe and other parts) upstream of the probe 9 is substantially at the same temperature as the water flow 2 therein.

Alternatively the third sensor could be disposed within the flow 2 itself.

As shown schematically in the drawings the thermistor temperature sensors are connected to a signal processing means 19 formed and arranged for monitoring variations. The electrical heating element 13 is provided with a suitable electrical power supply P.S.20 which could be in the form of a battery. The probe 9 is of a high thermal conductivity metal such as aluminium.

In more detail the signal processing means 19 includes for each channel corresponding to one of the thermistors 14-16, an analogue-to-digital converter means 21 for converting the magnitude of a thermally influenced electrical signal e.g. voltage drop across the thermistor. The corresponding digitized signals are then linearized with a linearizing means 22 which is formed and arranged to correct the slightly non-linear temperature response of the thermistor. The linearized signals which are directly proportional to the temperature at the thermistor locations are then conducted to an algebraic processor means 23 for processing the temperature related signals in accordance with the formulae given hereinbelow, the output of the processor means 23 corresponding to flow-rate being displayed on any suitable visual display means such as a liquid crystal display.In the case of a domestic energy meter there will not normally be any requirement for this information separately and accordingly a fourth thermistor 25 is employed to monitor the return temperature of the return water flow from the consumer unit thereby providing in turn by comparison with the third sensor reading a measure of the temperature drop at that consumer unit. The processor means 23 in this case includes means for combining continuously the flow-rate and the temperature drop of the fluid flow in the consumer unit to provide an output of the total heat energy extracted from the water flow which information is then displayed on the display means 24.Naturally in order to facilitate meter readings the display means can be located remotely from the probe either elsewhere on the consumer's premises or in a centralised location being connected to the processor means as required e.g. via the telephone system. In practice the meter would also normally include an integrator means for integrating the flow rate or energy consumption rate with respect to time so as to produce a cumulative total flow passed or energy consumed which would then be displayed.

As explained above the flow meter is formed and arranged so that there is an appreciable temperature differential e.g. 20"C between the two ends of the probe so that there is an appreciable heat flow along the probe. This heat flow may be in either direction but more conveniently is from the probe to the liquid but in any event is minimal in relation to the thermal energy content of the liquid.

From the temperature measurements of the first and second sensors, T1 and T2, respectively, and the relative separations thereof from the first end of the probe, a and a+b respectively, the temperature Ts at the interface between the probe and the liquid is, having regard to the linear temperature gradienttherealong, given by the equation Ts=Ti -b-(T2-T1) The heat flow along the probe per unit cross-sectional area is given by the expression K1 (T2-T1) b (K' = thermal conductivity of probe) which is of course the same as the heat transfer to the fluid at the interface between the first probe end and the liquid which is given by the expression h (Ts-To) where h is the heat transfer coefficient and To the liquid temperature given by the third temperature sensor.

Thus K1 (T2-T1) = h (T1 -a (T2-T1) -To) b whence K1=b(T1-T0) -1 ha a (T2-T1) Thus since K1 and a are constant (b T1 -T, -1 hoc(aT2-T1 -1 If the other temperature dependent fluid properties affecting the heat transfer coefficient namely kinematic viscosity, thermal conductivity and Prandtl number are approximately constant i.e. To is approximately constant say at 70 "C (which would normally be the case in a district heating scheme), then h Q08 (where Q is the volume flow rate).

Thence if b=a (T1-T0 )- 1.25 Q a. -1) T1 -1 > (1) ('2 Thus once the meter has been calibrated to determine the appropriate empirical constant to be applied to the temperature measurements as processed by the processor means in accordance with the above algebraic relation, readings of the flow rate can be obtained directly.

It will be appreciated that the flow-meter of the present invention can be employed for measuring the flow-rates of various fluids the resulting measurements being read either directly or being combined with other measurements e.g. temperature drop, simultaneously carried out on the fluid. If desired flow-rate may also be measured for fluids whose temperature is not constant by simply applying suitable corrections in accordance with the known relationships between the relevant temperature dependent fluid properties mentioned above and the fluid temperature (as measured by the third temperature sensor).

Example Measurements were carried out on water at 600C (To) flowing along a conduit of 20 mm internal bore (d).

At this temperature viscosity n = 0.48.10-6 m2/s, thermal conductivity of the water k = 0.65 W/m"C, and the Prandtl number Pr = 3.0. Using an aluminium (thermal conductivity K1 = 170 W/m C) probe having a diameter of 10 mm corresponding to a cross-sectional area A of 25 rr mm2, and wherein the thermistors are mounted 15 mm (a) and 30 mm (a +b) from the first end, then with a heat flow of about 6W (supplied by the heating element) the following results are obtained using known flow rates to 'calibrate' the meter.

flow rate Temperature Ql/min To C T1"C T20C 1. 15.5 60.1 75.0 80.7 2. 13.0 59.5 75.7 81.5 3. 8.1 59.4 79.3 84.9 From the above results the empirical constant for formula I above was found to be 27.1 I/min.

This was then used to determine unknown flow rates from the following temperature readings, the unknown flow rates being subsequently determined by direct measurement of the volume passed.

Temperature Flow rate limit To C T1"C T20C calculated actual 1. 60.3 75.8 81.6 14.25 14.5 2. 59.9 77.0 82.9 12.2 11.7 3. 59.6 81.7 87.2 6.8 6.5 As may be seen from the above the actual values corresponded to the calculated ones within + 5%.

With reference to the relation between the heat transfer coefficient and the flow rate given in the formula h Q0.8 and the formula I derived therefrom it should be noted that the value of the index given as 0.8 or 1.25 is in fact an empirical constant which is dependent to some extent on the geometry of the conduit and the probe and its disposition in the conduit. Thus whilst the index 1.25 given provides an approximate indication of the algebraic relation between the flow rate and the experimentally determined quantities in formula I, the accuracy of the meter may be further improved if required by determining the value of the index giving the 'best fit' for a graph of a large number of different measurements over the working range of the meter for a given meter probe configuration and arrangement.

Claims (17)

1. A flow-meter for use in measuring the flow-rate of a fluid at a predetermined temperature in a conduit, said flow-meter comprising a thermally conducting probe having a first end formed and arranged so as to be disposable, in use, in substantially direct thermal contact with the fluid inside the conduit, whilst being substantially thermally insulated from said conduit, with a second end of said probe extending away from the fluid and formed and arranged so as to be maintained, in use, at a substantial temperature differential from said first end, first and second electrical temperature sensors spaced apart from each other in the direction from said first end to said second end for monitoring the temperature gradient between said first and second ends, a third electrical temperature sensor disposable in use, in substanitally direct thermal contact with the fluid inside the conduit, each of said sensors being connected to signal processing means formed and arranged for processing temperature dependent electrical output received from said sensors so as to produce, in use, an electrical output related to the flow rate of said fluid, and a display means in communication with said processing means for displaying a measurement corresponding to said processing means electrical output.

2. Aflow-meter as claimed in claim 1 wherein the probe is provided with an electrical heating means.

3. A flow-meter as claimed in claim 2 wherein said heating means has a substantially constant thermal output, at its second end.

4. A flow-meter as claimed in any one of claims 1 to 3 wherein said first end of the probe is provided with fin means for increasing the surface area thereof in contact with the fluid.

5. A flow-meter as claimed in any one of claims 1 to 4 wherein said processing means is formed and arranged to process the electrical outputs of said sensors in accordance with the formula (T1-T0 1 ) Q=C ( -1 ) T2-T1 ) where Q is the flow rate, C is an empirical constant, and X is an empirically determined index, and T2, T1, and To are temperature values corresponding to said electrical outputs of said sensors.

6. A flow-meter as claimed in claim 5 wherein X -1.25.

7. A flow-meter as claimed in any one of claims 1 to 6 wherein said processing means includes integrating means formed and arranged for integrating the flow measurement or a product thereof with respect to time so as to provide a cumulative measurement.

8. A flow-meter as claimed in any one of claims 1 to 7 wherein is provided a fourth temperature sensor formed and arranged so as to be disposable in substantially direct thermal contact with the fluid inside the conduit downstream of a thermal energy extraction means between said fourth sensor and said flow-meter, said fourth sensor being connected to said processing means and said processing means being formed and arranged so as to determine the temperature differential between the third and fourth sensors and calculate the product of said differential and the flow-rate so as to provide a measurement of the thermal energy extraction rate therebetween.

9. A flow-meter as claimed in any one of claims 1 to 8 wherein said probe is mounted in a tubular body having first and second ends formed and arranged for connection, in use, to respective conduit sections, so as to provide a substantially rectilinear flow pathway through and between said conduit sections.

10. A flow-meter as claimed in claim 9 wherein said first end of the probe is mounted generally flush with the inner wall of said tubular body.

11. A flow-meter as claimed in claim 9 or claim 10 wherein is provided an insulating collar between said first end of the probe and said tubular body.

12. A flow-meter as claimed in any one of claims 9 to 11 when connected in series with the conduit.

13. A flow-meter as claimed in any one of claims 1 to 8 wherein said first end of said probe is mounted in an aperture in the wall of said conduit.

14. Aflow-meter as claimed in claim 12 or claim 13 when connected to a conduit of high thermal conductivity, wherein said third sensor and any said fourth sensor is mounted in substantially direct thermal contact with the outside wall of said conduit.

15. A method of measuring the flow-rate of a fluid in a conduit comprising the steps of disposing a probe of a flow-meter according to claim 1 with its first end in direct contact with the interior space of said conduit with said probe thermally insulated from said conduit, disposing the third temperature sensor in substantially direct thermal contact with the interior space of the conduit remote from said probe, monitoring the electrical outputs of said sensors and processing them in the processing means of said flow-meter, and reading the display means thereof.

16. A flow-meter substantially as described hereinbefore with particular reference to Figures 1 and 2 of the accompanying drawings.

17. A method of measuring the flow-rate of a fluid in a conduit substantially as described hereinbefore with particular reference to Figures 1 and 2 of the accompanying drawings.