GB2368896A - Heat exchange system, temperature sensor arrangement and operation - Google Patents

Abstract

The system comprises a pumped circuit for supplying heated primary fluid to a heat exchanger (33), through which a secondary fluid to be heated passes. The system further includes primary and secondary water temperature sensors (47, 42), so connected that if the secondary sensor (42) fails or malfunctions the primary sensor (47) takes over its control function. This arrangement prevents the stopping of the pump (34) in the event of failure or malfunction of the secondary sensor (42), as it is overridden by the primary sensor (47) keeping the pump (34) running, thereby maintaining the supply of heated water from the secondary side of the heat exchanger (33). The second sensor may be included in the system specifically for this purpose, or it may already have its own control function when the system is operating normally. The override operation may take place automatically or by manual operation of a switch. The primary sensor (47) may control the speed of the pump (34) to control the temperature of the secondary water leaving the heat exchanger (33). The primary and the secondary sensors (47, 42) may be arranged to sense the rate of change of temperature of the primary and secondary water.

Description

Improvements Relating to Heating Apparatus This invention relates to heating apparatus for the heating of a fluid (herein the secondary fluid) by means of another, hot fluid (the primary fluid) by heat exchange, and is a heating apparatus of the general kind set forth in our European Patent Application No. 0 989 372 A3, to which reference is made for details.

The problems with existing heating apparatus are set forth in said European Application, and the object of the invention the subject of the European Application was to provide a controlled heating system in which the flow switch of the existing apparatus could be eliminated.

In accordance with the invention the subject of the European Application (the prior invention) there is provided a heat exchange system wherein the primary fluid is circulated by means of a pump through a heat exchanger through which secondary fluid is also circulated, and including sensing means for controlling the rate of heat supplied to the heat exchanger via the primary fluid, characterised in that the sensing means comprises a temperature sensor means which is arranged to sense the rate of change of temperature of the primary and/or secondary fluid issuing from the heat exchanger.

The temperature sensor means is preferably electronic in nature.

Preferably, the temperature sensor means is arranged to sense at least the rate of change of temperature of the secondary fluid issuing from the heat exchanger.

The sensing of the rate of change of the temperature of the secondary and/or the primary fluid may be by measuring the temperature of the fluid at intervals, say half a second for example, and comparing each reading with the previous one or preferably two, in order to avoid the detecting of spurious temperature spikes There is an advantage in sensing the rate of change, as opposed to the absolute temperature, in that the system can distinguish the condition which represents the flow of secondary and/or primary fluid, from other general operational conditions of the system, but it is to be noted at this time that the present invention, set forth hereinafter, although preferably providing an apparatus which detects rate of change of temperature, need not be limited to detecting rate of change, but could operate on detection of absolute temperature.

The arrangement of the prior invention enables the requirement of the prior art to provide a flow switch to be removed, by carrying out all the functions of the flow switch.

In the prior invention, preferably, the temperature sensing means is adapted to sense the rate of change of temperature of both the secondary and primary fluid.

When the sensing system is set to detect the rate of change of temperature, then the temperature condition of the primary and secondary water can be accurately distinguished from the other conditions affecting the absolute temperature of the secondary and/or primary fluid.

As stated the primary fluid and the secondary fluid are preferably the primary and secondary water of a domestic thermal storage water heating system. The temperature sensor means of the secondary hot water may be arranged to detect the temperature of the hot secondary water issuing from the heat exchanger. The sensing control system provides that the temperature of the secondary water is being sensed at a frequency even if there is no water flow. As long as there is no rate of change of temperature of the secondary water, the pump is off. When a tap is opened and the secondary water starts to flow, its temperature will start to fall, and its rate of fall is detected. If that rate of fall is within operating limits, the pump is switched on and heat is supplied via the resulting flow of the primary water through the heat exchanger. The pump is controlled (regulated) in the preferred arrangement, by the rate of change of the temperature of the primary water, with the objective of keeping the issuing hot secondary water at a pre set temperature, for example 55OC.

There is constant regulation. This regulation may comprise increasing or decreasing the speed of and/or stopping and starting the pump.

It is arranged in the prior invention that if, for any reason, the sensor in the secondary side fails and gives no output readings, or malfunctions by giving erroneous readings outside normal operating ranges, such that no or clearly wrong signals of temperature are fed back to the control system, the pump is stopped, showing a fault.

It has been found in practice that this fault is occurring at a rather higher frequency than had been anticipated, and although the system functions as designed, the stopping of the pump means no hot water until the fault is repaired, which is unacceptable to customers.

This is the problem which the present invention addresses, and in accordance with the invention, there is provided a heat exchange system wherein hot primary fluid is circulated by means of a pump through a heat exchanger through which secondary fluid to be heated by the primary fluid is also circulated, and including first and second water temperature sensing means connected in the system for controlling the rate of heat supplied to the heat exchanger via the primary fluid, characterised in that the first and second sensing means are so connected that the second takes over or can take over the control function of the first (rather than stopping the pump which is what happens in the prior art), either automatically or by the operation of a manual control, in the event that the first fails or malfunctions.

The first sensing means is preferably a sensor for sensing the temperature of the secondary water (secondary sensor) and the second sensing means is preferably for sensing the temperature of the primary water (primary sensor).

Where the primary sensor is used to control the temperature of the secondary water, it is used either by functioning in a new mode or in a mode as in the prior invention, in that the temperature of the secondary water will be a function of the temperature of the primary water issuing from the heat exchanger. Although the secondary and primary water sensors in the prior invention sense the rate of change of temperature, the present invention could be used where the sensors sense or one of them senses absolute temperature. In a modified arrangement of the invention, if the secondary sensor fails or malfunctions, and the pump stops, there may be a manual device which can be operated to put the sensor for the primary control into the mode in which it controls the temperature of the issuing secondary water.

The control system may also have a high temperature fail safe control, which is that if the temperature of the secondary water reaches a pre set high, for example 72OC, the pump is again switched off.

The temperature sensor means in the primary side preferably repeatedly senses the temperature of the primary water issuing from the heat exchanger, at the same or at a different frequency as that used for the secondary sensor. One function of the primary sensor is to regulate the pump, as described above, at a first rate of change of the temperature of the issuing primary water, but a second function of the primary sensor according to the invention, is the control of the temperature of the issuing secondary water, as indicated above.

The primary sensor also serves to switch off the pump when the primary water temperature starts to rise at a certain, different rate, showing that the flow of secondary water has been terminated, for example by the closing of the tap. This sensor in normal operation will switch off the pump before the secondary sensor does so due to a high temperature in the secondary side.

The primary sensor is also arranged to switch off the pump should the primary water issuing from the heat exchanger reach a pre set maximum, for example 45OC, to avoid overheating for any cause. Additionally, the control system may be such that if the primary temperature sensor senses a low threshold temperature, for example 35OC, the primary pump is switched on regardless of what information is being supplied from the secondary sensor.

The two sensors therefore work in tandem in a first mode, and in sequence in a second mode in which the secondary sensor has either failed or is detected as giving spurious readings, providing excellent control and enabling the elimination of the conventional flow switch.

The invention in one main embodiment of thermal storage (and a modification thereof), and an embodiment where there is no thermal storage, will now be described, by way of example, with reference to the accompanying drawings, wherein: Fig. 1 is a circuit diagram of a thermal storage water heating apparatus according to a known configuration; Fig. 2 is a circuit diagram of a thermal storage heating apparatus according to an embodiment of the invention; Fig. 2A is a diagram similar to Fig. 2, but in a modified form, showing a modified form of the invention ; Fig. 3 is a graph illustrating the characteristics of operation of the apparatus of Fig. 2. and Fig. 4 is a circuit diagram of a water heating apparatus of non thermal storage type, to which the invention can also be applied.

Referring to Fig. 1, reference 10 indicates a thermal storage tank containing a body of primary water which is heated by means of a boiler 12. To effect this heating, the water is circulated from the boiler through pipe 14 by means of the boiler pump 16. The pump draws the water from the store 10 and delivers it to boiler 12. From the boiler 12, the heated water is returned to the store via the pipe 18. Heating of the water in the store is carried out under the control of a pair of store thermostats 20 and 22 of which 20 is a limiting thermostat and controls the maximum store temperature, whilst the thermostat 22 serves to control the temperature of the store accurately to 82OC, plus or minus 3 C. It will be seen that there is a manual reset and overheat cut out device 24 in line 18, which operates to cut out the boiler 12 should it overheat to the extent that the water in line 18 reaches a temperature in the region of 10SoC.

The device 24 can be used manually to reset the apparatus after the cut out.

The device 24 is linked to the thermostats 20 and 22 and also to the pump 16, so that the device 24 or the thermostat 20 or 22 by detecting an appropriate temperature can cause the pump 16 to stop.

Connected to the store 10 is a twenty litre expansion vessel 26 which serves to accommodate expansion of the water in the tank 10.

The apparatus illustrated is an integrated thermal storage system in that the water in the tank 10 serves to heat the dwelling in which the apparatus is located, and to supply the heat for the secondary water which is connected at the dwelling taps and showers.

For the heating circuit, the water in the tank 10 is circulated through pipe 28 via a central heating pump 30. The pipe is connected to the appropriate number of radiators 32.

For the heating of the secondary water, a plate heat exchanger 33 is used. A circulating pump 34 serves to pass water from the tank 10 through line 35 and to return it to the tank 10 via line 36. The secondary water is supplied from the mains via line 38 and it passes through the heat exchanger in contra flow to the primary water from the tank 10. The heated secondary water emerges on line 40, which contains a sensing thermister 42, and is delivered to the dwelling consumption points. The line 38 contains a flow switch 44 and electrical control lines 46 and 48 connect the sensor 42 and the flow switch 44 to a speed controller 49 which in turn controls the speed of the pump 34 via electrical control line 51.

Finally, the circuit may also include an integral or external clock and room thermostat; these items are indicated generally by the reference 50.

The basic operation of the circuit illustrated will now be described.

The boiler (or in an alternative arrangement electric immersion heaters), under thermostatic control, supplies heat to the store by circulating water from the boiler through the tank 10, in known manner. The heat is supplied depending upon the condition of the store 10, and not upon the instantaneous demand for space heating or hot water. The room thermostat and/or clock 50 dictates when the space heating circuit is operational, and the opening of a hot water dispensing point i. e. the opening of a tap, dictates when the heat exchanger 33 is operational.

The arrangement provides that there is control of the pump 34 when there is a demand for hot water at say a tap. This will be detected by the actuation of the flow switch 44, which causes starting of the pump 34 to circulate hot water through the heat exchanger 33. The pump speed control 49 controls the speed of the pump depending upon how much the temperature sensed by sensor 42 deviates from a predetermined level.

As indicated herein, the flow switch is a problem as regards effective functioning, and cost, and the prior invention provides a means whereby, by the sensing of temperature rate of change, the flow switch need not be used and need not be provided..

The embodiment of the invention shown in Fig. 2 is an arrangement similar to Fig. 1, except that the flow switch has been eliminated. Similar reference numerals have been used to designate similar parts.

In the embodiment, the supply line 38 passes directly to the heat exchanger 33, and there is no flow switch as in the apparatus of Fig. 1. The sensor 42 is connected to controlling logic in the form of a microprocessor 43, and the sensor 42 is sensed at regular intervals, for example half a second whereby the rate of change of temperature of the secondary water issuing from the heat exchanger on line 40 is constantly monitored. As explained herein, when the temperature at sensor 42 falls at a pre set rate, indicating the flow of secondary water, the pump 34 is started, for example by virtue of a signal from the microprocessor on line 45.

The control of the apparatus is by the microprocessor 43 in the manner explained herein, in that when the pump is running, its speed is controlled by a primary sensor 47 in the line 36, which is sensed at a frequency to detect the rate of change of the primary water issuing from the heat exchanger 33. Depending upon that rate of change, so the speed of the pump 34 is controlled by the processor 43 so as to maintain a pre set temperature of the water issuing from the taps, again as explained herein.

An operational condition of the apparatus is indicated by the graph of Fig. 3. In the graph of Fig. 3 temperature detected by sensor 42 against time is indicated. In period 1 it is assumed that there is no demand for hot water, but that the water in pipe 40 at the sensor is still hot (the system is set up to ensure that the temperature of the water at the primary at sensor 47 is at a pre set minimum). With the passage of time, the temperature starts to drop with natural heat loss, and the temperature drifts downwards at a slow rate. Detection of this slow rate of change will not cause the pump 34 to start.

Period 2 however indicates that a tap has been turned on, and secondary water starts to flow out of line 40. The temperature of the water sensed by the sensor 42 starts to drop at a higher rate, and this is detected by the sensor 42, and/or the control logic, which in turn causes the pump to switch on to add heat to the secondary water, whose temperature will start to rise or its rate of fall will decrease. The control of the pump speed to maintain a pre set temperature of the water issuing from the tap is under the detection of the rate of change of the temperature of the primary water by sensor 47.

The prior invention provides the advantage that the flowing secondary water provides a temperature rate of change, and so by detecting this characteristic, the flow switch can be eliminated. The primary sensor provides the control of the pump to ensure that the temperature of the secondary water, when flowing, will stay within pre-set limits.

The present invention provides the additional advantage in that the primary sensor 47 is also used to control the temperature of the secondary water issuing from the line 40 in the event that the sensor 42 fails or starts to read spuriously (give readings outside the ranges it is set to detect). In such case, the microprocessor 43 detects the failure or malfunctioning of the sensor 42, and instead of stopping the pump 34 and indicating a fault, as in the prior invention, the sensor 47 is instructed or set to take over control of the temperature of the secondary water, and the flow of hot water is maintained. At the same time there may also be an indication that there is a fault with the secondary sensor, so that at the consumer's convenience, the sensor 42. can be replaced or repaired. What is unacceptable to today's consumer is that the hot water flow is stopped for even the shortest period, unless the fault is a dangerous one.

In controlling the temperature of the secondary water using the primary sensor, the primary sensor may operate in the same mode as when the secondary sensor is functioning properly or in a different mode having regard to the fact that the sensor 42 has failed or is malfunctioning.

The temperature of the secondary water issuing from line 40 is a function of various parameters, including primary inlet water temperature, primary water outlet temperature, cold water inlet temperature, hot water outlet temperature, hot water flow rate, primary water flow rate, and the thermal characteristics of the heat exchanger, but within the operating limits of the apparatus the temperature of the primary outlet water is more or less directly representative of the hot water outlet temperature, and therefore can be used at acceptable accuracy standards, as the means to control the temperature of the secondary outlet water, if the sensor 42 fails or malfunctions.

When the sensor 42 fails, no signal is fed back therefrom. This also happens should the cables to the sensor break.

The sensors 42 and 47 in conjunction with the microprocessor also provide the additional control functions mentioned herein.

Fig. 2A basically illustrates the arrangement of Fig. 2 in a more practical manner and the components of Fig. 2 already described have the same reference numerals. In the Fig. 2A arrangement, the microprocessor 43 is shown as embodying the device 24, and the pump speed control 49. The dotted lines show that the various items (including the clock and room thermostat (s)) which are to be controlled or give control signals are connected to the single processor 43, which is the practical way that the arrangement of Fig. 3 would be set up. The operation of Fig. 2A is the same as that of Fig. 2.

In the arrangement of Fig. 4, which is not a thermal storage system, the boiler 100 supplies the heat to the primary water, which is circulated by the pump 101. The primary water is heated on demand, and is circulated by pump 101 either directly to the central heating radiators 102, or to the heat exchanger 104, when there is a demand at the tap (line 106). Normally the demand at the tap has priority, and normally a flow switch 108 detects when the demand is made. With the invention, the flow switch can be eliminated in the same manner as it is eliminated in the thermal storage embodiment, with the same effect. The same control system can be used.

It can be seen that by virtue of the invention the problem of providing a flow switch can be overcome and various methods can be adopted, whilst there is no sacrificing of the obtaining of effective control of the temperature of the secondary water to keep it at or within close limits of a preset required value in that if there is failure or malfunctioning of the secondary sensor, the control is automatically or manually switched to the primary sensor so that hot water will not be lost simply because of the failure or malfunction of one sensor.

In another modification, it may be possible to use two secondary sensors, one of which is dormant, but is put into use in the event of failure or malfunction of the first, again either automatically or manually, again to ensure that there is no loss of hot water unnecessarily.

Also, there may be two primary sensors, one of which is dormant, but becomes the means of controlling the temperature of the secondary water in the event of secondary sensor failure or malfunctioning.

Claims (4)

CLAIMS.

1. A heat exchange system wherein hot primary fluid is circulated by means of a pump through a heat exchanger through which secondary fluid to be heated by the primary fluid is also circulated, and including first and second water temperature sensing means connected in the system for controlling the rate of heat supplied to the heat exchanger via the primary fluid, characterised in that the first and second sensing means are so connected that the second takes over or can take over the control function of the first (rather than stopping the pump which is what happens in the prior art), either automatically or by the operation of a manual control, in the event that the first fails or malfunctions.

2. A heat exchange system according to claim 1, wherein first sensing means is used to sense the temperature of the secondary water, and the second sensing means is used to sense the temperature of the primary water, and in normal operation the second sensing means controls the speed of the pump to control the temperature of the secondary water issuing form the heat exchanger.

3. A heat exchange system according to claim 2, wherein the first and second sensing means are arranged to sense the rate of change of temperature of the secondary and primary water.

4. A heat exchange system substantially as hereinbefore described with reference to Figs. 2,2A, 3 and 4 of the accompanying drawings.