WO2001098738A2

WO2001098738A2 - Fluid flow measurements

Info

Publication number: WO2001098738A2
Application number: PCT/GB2001/002808
Authority: WO
Inventors: Michael Bowling
Original assignee: Michael Bowling
Priority date: 2000-06-23
Filing date: 2001-06-22
Publication date: 2001-12-27
Also published as: AU2001274335A1; GB2363852A; WO2001098738A3; GB0015356D0

Abstract

The present invention provides a method for detecting the time taken for a fluid to flow through a predetermined section of conduit, which method comprises the steps of: (i) inducing a change in the temperature of the fluid; (ii) detecting the passage of the temperature-changed fluid passed each of a first temperature sensor at a location upstream on the conduit and a second temperature sensor at a location downstream on the conduit; and (iii) determining the time interval between detection of the temperature changed fluid at the first sensor and detection at the second sensor. The invention also provides a heat exchanger apparatus having a holding tube with associated temperature sensors.

Description

FLUID FLOW MEASUREMENTS

Field of the Invention

The present invention concerns improvements in and relating to fluid flow measurements and encompasses a method for determining the time taken for a fluid to flow through a predetermined section of conduit and to an apparatus incorporating such a conduit and adapted to enable the method to be carried out.

The invention particularly, but not exclusively, concerns determination of the dwell or holding time of a fluid such as milk in a dairy pasteuriser.

Background to the Invention The technique of pasteurisation of milk and similar consumable products originated with experimentation carried out by Louis Pasteur as early as 1864 when he established that micro-organisms were responsible for the fermentation of wines, vinegars, beers and milk. Pasteur established that boiling milk to over 100°C destroys substantially all pathogenic microbes but harms the nutritional value of the milk by denaturing the proteins and vitamins.

Pasteur's experiments lead to the development of a pasteurising processing in which the milk is heated to 65°C but no higher and held at this temperature for a period of approximately thirty minutes before again lowering the temperature to 4°C.

This had the effect of destroying any of the common pathogenic microbes present in milk while not harming the milk's constituents. Accordingly, Pasteur's work realised the first complete controlled thermal treatment of liquid foodstuffs to neutralise the harmful bacteria without spoiling the heat sensitive vitamins and proteins.

Over more recent years the pasteurisation process conditions have been refined to arrive at such processes as the modern HTST treatment in which milk is heated rapidly for short times to high temperatures and then cooled. For example, the international pasteurisation standard of 72°C for 15 seconds followed by cooling. The basic principle of the process remains the same in that the milk must be held at an elevated temperature for a predetermined period of time.

The importance of ensuring that the milk is properly pasteurised should not be underestimated. In recent years there have been a number of outbreaks of serious illness arising from food poisoning through lack of pasteurisation.

The traditional methods for ensuing that the milk not only reaches the required temperature but importantly is retained in the temperature holding tubes downstream of the heat exchanger for the required predetermined period comprise what are known as the conductivity method and the dye method. In the conductivity method the product (eg milk) is first removed and then replaced with water. Measurement is then made of the time taken for the water to flow from one point in the system to another by first adding salt to the water and then using electrical conductivity testers to monitor the time taken for the water with added salt to travel from the upstream point to the downstream point. The test is unrepresentative not least because specific heats, viscosities and densities of the water and milk or other product fluid will differ substantially. Moreover, the plant breakdown needed to exchange fluids may introduce contaminants and may vary the flow characteristics of the plant, so that this disturbance could invalidate the test results.

The dye method also requires the product to be replaced with water. In this case a dye is injected into the system and is monitored as it passes through transparent tubes that are respectfully inserted into the system at an upstream point and a downstream point. An observer times the passage of the dye with a stopwatch. For the same reasons as for the conductivity method, this method is unrepresentative, being vulnerable to invalidation by the disturbance and furthermore is very vulnerable to human error. There is a need for a far more efficient method and system for establishing the holding time for the food products in the holding tubes of pasteurisers and similar heat exchanger systems which addresses the severe drawbacks of the existing technology. Summary of the Invention

According to a first aspect of the present invention there is provided a method for detecting the time taken for a fluid to flow through a predetermined section of conduit, which method comprises the steps of: inducing a change in the temperature of the fluid; detecting the passage of the temperature-changed fluid passed each of a first temperature sensor at a location upstream on the conduit and a second temperature sensor at a location downstream on the conduit; and determining the time interval between detection of the temperature changed fluid at the first sensor and detection at the second sensor.

Preferably each temperature sensor is attached to the inner or outer surface of the conduit. Particularly preferably, the sensors are attached to the outer surface of the conduit, being mountable as a retrofit to existing pasteuriser apparatus or other such systems.

In the preferred embodiment the conduit is at the outflow of an inline fluid processor which is suitably a heat exchanger and particularly suitably a pasteuriser heat exchanger. The fluid is suitably a liquid and in the preferred embodiment is a consumable liquid food such as milk, the in line processor being a dairy pasteuriser heat exchanger and with the predetermined section of conduit being the holding tube at the outflow of the heat exchanger.

Preferably each temperature sensor comprises a thermocouple or thermistor being sensitive and capable of fast response. For optimal efficiency, the thermal sensors are suitably each linked to a microprocessor to record the time at which the temperature changed fluid passes each respective sensor or, better still, to calculate and record the time interval between the passage of the temperature changed fluid past the first detector and then past the second detector.

A respective timing means may be provided at each respective temperature sensor but preferably is provided at the microprocessor/sensor information processing means.

According to a second aspect of the present invention there is provided a heat exchanger for heating or cooling of fluid and having a section of conduit at the outflow of the heat exchanger through which section of conduit the fluid heated or cooled by the heat exchanger flows, in use, the section of conduit having thereon or therein a first temperature sensor positioned at or nearer a proximal end of the conduit adjacent the heat exchanger outflow to monitor the passage of the heated or cooled fluid past the first temperature sensor, the conduit further having thereon or therein a second temperature sensor positioned downstream of the first temperature sensor to monitor the passage of the heated or cooled fluid therepast.

Preferably the first and second temperature sensors are linked to a timing means and recording means. Particularly preferably they are linked to a common timing and recording means which suitably comprises a microprocessor and which is adapted to determine the time interval between sensing of passage of the heated or cooled fluid at the first sensor and at the second sensor.

By positioning the first sensor at or very close to the outflow of the heat exchanger as it enters the holding tube/section of conduit and positioning the second sensor at or near to the end of the section of conduit/holding tube the interval of time between sensing by the respective sensors is closely representative of the dwell time or holding time within the holding tube.

Preferably the temperature sensors are each provided with means for attaching them to the external surface of the conduit. Although thermocouples are particularly preferred, alternative preferred temperature sensors include thermistors or similar fast response sensors for detecting temperature.

Brief Description of the Drawings

A preferred embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, wherein:

Figure 1 is a schematic diagram of a milk pasteurisation plant embodying the second aspect of the present invention;

Figure 2 is a schematic enlarged view of the section of conduit of the apparatus to which the temperature sensors are attached; and

Figure 3 is a graph of sensed temperature against time for each of the two temperature sensors for a typical example temperature elevated fluid.

Description of the Preferred Embodiment

Referring firstly to Figure 1, this illustrates a milk pasteurising system comprising a heat exchanger 1 having an outlet 2 for the heated milk which is coupled to a holding tube 3.

As will be seen, the holding tube 3, in common with most pasteuriser holding tubes, is convoluted, having a zig-zag or spiral form to maximise the length of travel through the tube while constraining the tube within a compact volume. As for all holding tubes 3, the tube 3 suitably is lagged for efficient heat insulation, for safety and to maintain the temperature of the temperature elevated milk at, or at least as close as possible to, the desired temperature throughout the transit of the milk through the holding tube 3.

The start of the holding tube 3 is indicated at point A and the end of the holding tube at point B. The objective is for the holding tube 3 to maintain the temperature of the milk at the desired level as it travels from point A to point B. If the temperature of the milk does drop then it will be diverted back again to the heat exchanger 1 to complete the cycle again.

For milk pasteurisation the time of holding within the holding tube 3 is nowadays typically fifteen seconds at the required temperature of 72°C. For other liquid food products to be pasteurised such as, for example, egg mix, the holding time may need to be substantially longer, e.g. of the order of 2 minutes.

A first temperature sensor 4 is attached to the external surface of the conduit comprised by the holding tube 3 at a location which is suitably as close as reasonably possible to the start point A of the conduit. A second temperature sensor 5 is attached to the external surface of an extension of the holding tube 3 conduit suitably close to but downstream of the exit point B from the holding tube 3 conduit. As illustrated, the sensor 5 is actually slightly downstream of the exit point B from the holding tube 3. As a generality, the length of the holding tube(s) 3 is sufficient to enable accurate measurement of the time interval for flow between the temperature sensors 4, 5.

In order to carry out the method of the invention effectively it is necessary to induce a change in the temperature of the fluid flowing into and through the holding tube 3 in order that a temperature change front/leading edge be created that may be observed by the respective temperature sensors. To this end, the heat exchanger 1 may be adjusted for the purposes of the method to at least temporarily raise the temperature of the milk by a finite extent, say, 5°C while monitoring for the temperature change at the respective temperature sensors 4, 5.

The elevated temperature front/leading edge 7 (see Figure 2) first passes the first temperature sensor 4 and then passes the downstream second temperature sensors and the interval between the two is indicative of the duration/dwell time of the liquid within the conduit of the holding tube 3.

The temperature sensors 4, 5 are suitably each a fast response thermocouple which consists of two dissimilar metals joined together at one end (a junction) that produces a small thermoelectric voltage when the junction is heated. The change in thermoelectric voltage is interpreted by thermocouple thermometers as a change in temperature. Exposed junction thermocouples are particularly suitably for use in the present context since they are fast responding. The smaller the probe sheet diameter of the thermocouple the faster is the temperature response. The sensors 4, 5 are attached to the external wall of the holding tube 3 conduit and are sufficiently sensitive to enable detection of small changes in temperature. They may be externally lagged to insulate them from any environmental thermal anomalies including, for example, reflection of heat from other components. The sensors 4, 5 are linked to a microprocessor device 8 where the temperature signals from the respective sensors 4, 5 are, following conversion by a signal conditioner/analogue to digital converter, recorded and compared to determine the interval between sensing of passage of the leading edge 7 firstly past the first sensor 4 and then past the second sensor 5. Figure 3 is a graphical plot of the temperature readings from each of the two sensors 4, 5 over a period of time that encompasses two successive elevations of the fluid temperature by a few degrees.

The time base along the X axis in seconds spans a period of 500 seconds and the Y axis indicates variations in sensed temperature over a range between 22°C and 27°C within that time period and in response to raising the temperature of the milk from the heat exchanger in two successive steps.

The first plot 9 from the first sensor shows the initial temperature to be approximately 22°C and which starts to increase, following an initial induced temperature elevation, at approximately 120 seconds. This rises rapidly to 24°C and then remains at that level until approximately 440 seconds at which point the temperature rises again by a further approximately 3°C. The sensed temperature plot 10 of the second sensor shows that this closely parallels the sensed temperature for the first sensor but with a brief marked time lag between the times at which the sensed temperatures rise. Both for the first rise and the second rise the time lag is twenty seconds.

This twenty seconds delay is the time interval between the sensing of passage of the leading edge of the temperature elevated milk past the first sensor and then past the second sensor and is a good indication of the holding time of the milk within the holding tube(s) 3. This interval is suitably displayed by the microprocessor 8 on a monitor screen or other display means (not shown) to provide a reliable indication of the dynamic status of the system and enable the operator to judge whether the fluid has been processed satisfactorily.

The temperature sensors 4, 5 not only provide a guide to the holding time but also inherently provide a guide as to the temperature of the fluid within the holding tube(s) 3 so that the operator may, using these sensors alone, know both of the key parameters for proper pasteurisation. Although the temperature sensed by the sensors 4, 5 may not be exactly the temperature of the fluid within the holding tube 3 a calibration factor may be determined to enable the actual temperature of the fluid within the holding tube 3 to be determined from the temperature sensed by the sensors 4, 5.

In addition to enabling determination of temperature of products and the holding time, the information obtained by the microprocessor dynamically from the temperature sensors may be further used to calculate rate of flow from knowledge of the length, and optionally also diameter, of the relevant section of conduit. The information need not simply be provided passively by the system, however, it may be used actively to control diverter valves for cycling of the milk back through the heat exchanger to ensure proper pasteurisation. A diverter valve 6 is illustrated on Figure 1 downstream of the exit point B of the holding tube and downstream of the second sensor. Control signals from the microprocessor in response to the sensed temperatures from the temperature sensors may not only be used to control operation of the flow diverter valve 6 but also operation of the pump (not shown) that propels the fluid through the system to thereby further adjust flow rate appropriately. For example, the flow rate could be simply increased or decreased by altering the pump speed if the holding time is too long or too short, respectively.

Although a heat exchanger in the form of a pasteuriser has been described hereinabove in detail, the invention may be applied to, for example, a cooler for liquids or gases.

Claims

1. A method for detecting the time taken for a fluid to flow through a predetermined section of conduit, which method comprises the steps of: (i) inducing a change in the temperature of the fluid; (ii) detecting the passage of the temperature-changed fluid passed each of a first temperature sensor at a location upstream on the conduit and a second temperature sensor at a location downstream on the conduit; and (iii) determining the time interval between detection of the temperature changed fluid at the first sensor and detection at the second sensor.

2. A method as claimed in claim 1 , wherein each temperature sensor is attached to the inner or outer surface of the conduit.

3. A method as claimed in claim 2 wherein the sensors are attached to the outer surface of the conduit, being mounted as a retrofit to existing apparatus.

4. A method as claimed in claim 1, 2 or 3 wherein the conduit is at the outflow of an in line fluid processor such as, for example, a heat exchanger.

5. A method as claimed in claim 4 wherein the in-line fluid processor is a pasteuriser and the predetermined section of conduit is comprised, at least in part, by the holding tube at the outflow of the heat exchanger.

6. A method as claimed in any preceding claim, wherein each temperature sensor comprises a thermocouple or thermistor, being sensitive and capable of fast response.

7. A method as claimed in any preceding claim, wherein the thermal sensors are each linked to a microprocessor to record the time at which the temperature changed fluid passes each respective sensor, and/ or to calculate and record the time interval between the passage of the temperature changed fluid past the first detector and then past the second detector.

8. A heat exchanger for heating or cooling of fluid and having a section of conduit at the outflow of the heat exchanger through which section of conduit the fluid heated or cooled by the heat exchanger flows, in use, the section of conduit having thereon or therein a first temperature sensor positioned at or nearer a proximal end of the conduit adjacent the heat exchanger outflow to monitor the passage of the heated or cooled fluid past the first temperature sensor, the conduit further having thereon or therein a second temperature sensor positioned downstream of the first temperature sensor to monitor the passage of the heated or cooled fluid therepast.

9. A iitjat exchanger as claimed in claim 8, wherein the first and second temperature sensors are linked to a timing means and recording means.

10. A heat exchanger as claimed in claim 9 wherein the first and second temperature sensors are linked to a common timing and recording means which suitably comprises a microprocessor and which is adapted to determine the time interval between sensing of passage of the heated or cooled fluid at the first sensor and at the second sensor.

11. A heat exchanger as claimed in claim 8, 9 or 10 wherein the temperature sensors are each provided with means for attaching them to the external surface of the conduit.

12. A heat exchanger as hereinbefore described with reference to any suitable combination of the accompanying drawings.

13. A method for detecting the time taken for a fluid to flow through a predetermined section of conduit as hereinbefore described with reference to any suitable combination of the accompanying drawings.