GB2281379A

GB2281379A - Hot water supply system

Info

Publication number: GB2281379A
Application number: GB9317483A
Authority: GB
Inventors: James Melville Buick
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-08-23
Filing date: 1993-08-23
Publication date: 1995-03-01
Also published as: GB9317483D0

Abstract

A hot water supply system is based on a two stage contra-flow heat exchanger 1 using a hot primary fluid to heat secondary water. A measurement of the temperature attained by the secondary water at the junction 10 of the heat exchanger stages is used to predict the secondary water outlet temperature and to control the flow of primary fluid in order to stabilise the secondary water outlet temperature. The primary fluid flow is controlled electrically or electronically by actuation or speed control of a pump 13 or by a valve in response to a thermocouple, thermistor or bimetallic sensor. Alternatively, the control means may be actuated mechanically by a wax, bimetallic or shape memory alloy sensor. The primary fluid may be steam instead of hot water. A remote temperature adjuster 21, e.g. for a shower, may be provided. The second stage of the heat exchanger may be detachable for de-scaling. Single and plural heat exchange constructions are described. <IMAGE>

Description

HEAT EXCHANGER WATER HEATER A heat exchanger system for heating water to a controlled temperature for domestic or other uses.

Stored primary systems for the supply of hot water by heating mains secondary water are well known. These comprise a tank containing hot water which is heated by one or more of a variety of heat sources with a heat exchanger either inside the primary store where the secondary flow simply absorbs heat from its surroundings, or externally, where the primary side flow is usually pumped.

The internal heat exchanger systems require large heat exchangers because the slow relative movement of the primary water gives poor heat transfer per unit area. The systems with external heat exchangers su++er an initial delay while the pumped primary water reaches the heat exchanger and transfers heat to the secondary flow water. Temperature control of the hot water supplied by these systems is usually by means of a thermostatic mixing valve which adds cold water to the secondary water after it emerges fr from the heat exchanger. In hard water supply areas these systems can suffer from scaling because the heat exchanger is operated at thermal store temperature (about SOC).Systems designed to overcome this scaling problem involve considerable additional complexity and therefore cost, or accept a lower performance by having a lower temperature store.

Th current invention provides an improved performance at lower cost than existing stored primary systems. It relates to a heat exchanger including a temperature sensing means and a primary flow control means for heating water to a controlled temperature for domestic or other purposes using heat fro primary hot source fluid which may be from a thermal store in the known systems noted above.It provides a reverse flow heat exchanger based water heater arranged to reduce lags in both the initial output of hot water and the subsequent control of its temperature by having a suitable physical layout and control means. It comprises two heat exchanger stages in series in which the secondary water temperature rise in a first stage is used by a control means to predict the temperature rise which will occur in its passage through a second stage.

The system lead so produced compensates for lags arising elsewhere in the water temperature control system, enabling the control means t regulate the +low 3f primary heating +luid such that the secondary water outlet temperature is well stabilised.

As the cold mains temperature is reasonably constant and the required output temperature is set, the necessary temperature rise is known. When a supply of hot primary source fluid is fed to one path through a reverse flow heat exchanger and cold supply water is fed to the opposing path, heat will transfer from the hot primary fluid to the secondary water resulting in a rise in the secondary water's temperature as it progresses towards its outlet. The temperature of the secondary water measured at some point along its path through the heat exchanger can be used to predict at what temperature it will emerge. So when this measured temperature varies from that required for the emerging water to be at the required hot water temperature, the variation can be used by a control means to apply a correction by varying the flow of primary fluid.

Because the secondary water is gradually approaching the required temperature as it progresses through the heat exchanger, provided that the control lags are taken into account and that the system response is sufficiently rapid, even a simple on/off control of the primary flow can achieve adequate control of outlet temperature. This use of on/off control of the primary heating fluid flow into the heat exchager, depending on whether the secondary water temperature at the point of measurement, between the two heat exchanger stages, is below or above a preset temperature, which has been selected to result in the required outlet temperature is a useful but not essential feature of the invention.

It will be appreciated that total lag compensation required for a given secondary flow rate depends on the response of the temperature sensor plus the lag of the control system in changing the rate of flow of primary heating fluid into the heat exchanger plus the lag as heat flows from the hot primary to the secondary. It will also be appreciated that the lag compensation raquired will vary with secondary flow rate, being greater at higher flow rates. To reduce the lag compensation error, it is therefore necessary either to make the control system lag small so that the error is never important or, to provide the control system with tha ability to vary the lag compensation with secondary flow rate. For simplicity, the former is prefered providing a very simple and effective hot water supply system.

The control of outlet water temperature by control of flow from the pump circulating the primary water through the heat exchanger results in the mean temperature of even the secondary outlet end of the heat exchanger usually being well below the heat store temperature. It will be appreciated however that the peaks of temperature may lead to some scaling within the heat exchanger but this is not likely to be a problem except at the hot end and will be less severe than in the known systems referred to above.

A first embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which : Figure 1 shows a top view of a first embodiment of the invention wherein the coiled heat exchanger is mounted on top of a primary storage tank with the associated sensor, control means and circulator pump indicated. The outer heat exchanger tube is shown cut away at both ends so that the construction can be indicated.

Referring to Figure 1. In this embodiment of the invention the heat exchanger 1 is made of four small copper tubes 2 in parallel carrying primary water from the hot end 3 to the cold end 4 inside a larger diameter copper tube 5 carrying secondary water in the opposite direction that is from the cold inlet 6 to the hot water outlet 7. The bundle of small tubes has been given a slow twist and has a copper wire lacing which holds the tubes in a bundle, separates them from each other and the outer tube wall and provides turbulence to improve heat transfer.

The whole heat exchanger which is over four metres long, is coiled and is mounted on top of a primary heat store cylinder 8 within the store's insulation 9, but insulated from the store such that the coil tends to an intermediate temperature between the store and the surroundings but less than the required hot water outlet temperature. This position for the heat exchanger also minimises the distance from the top tapping 22 of the store to the primary input to the heat exchanger helping to reduce lag.

A thermocouple 10 using the outer copper tube 1 as one arm is attached to the heat exchanger at an intermediate point along its length but towards the secondary outlet end 7 and connected to the control means by cable 18. The thermocouple position forms the division between the two stages of the heat exchanger. The control means 11 amplifies the thermocouple voltage, compares the result with a reference voltage set with the adjuster 12 and powers a standard heating circulator pump 13 via a solid state relay, included in the control means and cable 19, whenever the reference voltage is the greater. It will be appreciated that the voltages represent the measured temperature and the set, intermediate point temperature respectively. This means of temperature sensing introduces very little lag as there is no delay between the tube and the thermocouple. The control means is left permanently powered but only operates the pump when the heat exchanger sensor falls below its set temperature. In trials, during periods of no demand for hot water the pump is powered for one or two seconds three to five times an hour. This can be reduced by adjusting the insulation. Power consumption of the control circuit is less than one tenth watt. The heat loss from the heat exchanger adds little or nothing to the heat store loss. No flow sensor is required. The whole control means from the temperature sensor to the pump control relay cost less than half the cost of the standard domestic heating circulator pump.

The system lag reducing features incorporated allow the second stage of the heat exchanger to be relatively short so that a wide range of flow rates can be accomodated while maintaining an adequately stable outlet temperature. In trials, the embodiment described has provided hot outlet water controlled to within a temperature range of 3 degrees C for a variety of secondary flow rates and primary +luid temperatures. The cycle time and duty cycle of the control system both vary with store temperature and secondary flow rate but are typically five seconds and ten to sixty percent respectively. It has been found that the second eat exchange stage can be one fifth or less of the size o+ the first stage.An air bleed 14 and an isolation valve 15 are +itted between the primary heat store outlet 9 and the hot end 3 of the heat exchanger. . To prevent gravity circulation overheating the eat exchanger a null flow valve 1A is fitted in the pipe taking primary water from the heat exchanger cold end 4 to the pump 13 for return to the store via a low level tapping 17. The small secondary water volume of the heat exchanger allows the use of a short vertical stub pipe 20 to serve as an expansion vessel.

The invention could be employed in many alternative embodiments in which details of the heat exchanger, temperature sensor, control means and flow control means may vary but in which they are combined in a way which still retains the control principle of having a predictive element in the control loop. Some of these alternative embodiments could have:- A heat exchanger system as herein described where the control means provides modulation of the pump speed instead of on/off control.

A heat exchanger system using hot water stored ready for use eg. in a domestic hot water cylinder as the heat source in order that a mains or other water supply can be heated to a controlled temperature for use eg. in a shower where mains pressure and elimination of temperature fluctuations resulting from other draw-offs are very desirable.

A heat exchanger system using a primary fluid other than water from a thermal store as the heat input source eg. steam or hot water from a distribution main, in which case its flow may be controlled by a valve instead of a pump as appropriate.

Alternative means, other than a pump, of controlling the flow of primary heating fluid through the heat exchanger by a valve or diverter operated by the control means to control the primary flow.

Alternative means of sensing the temperature eg. a thermistor, standard type of thermocouple, an immersed bi-metallic sensor operating a micro-switch or reed-switch controlling power to the flow control means either directly or for example, through a relay.

A control means comprising a wax-stat, bi-metallic actuator, memory-metal actuator or other suitable means capable of sensing the secondary temperature at an intermediate point between a first ant a second heat exchanger stage and directly, mechanically operating a means to control the primary flow A different means of controlling the supply of electric power to the flow control means, eg. a triac or electro-mechanical device rather than a solid state relay.

Other types of heat exchanger for example, plate, folded tube, multi-tube or concentric with the inner tube finned.

The second stage heat exchanger of a different type +rom the first stage and easily detachable for de-scaling or replacement.

A second stage heat exchanger in which the incoming primary flow mixes with primary fluid already partly cooled by transfer or heat to the secondary water within this stage, before it impinges on the secondary water containing elements, thus reducing the tendency to scaling.

A heat exchanger as herein described, wherein a temperature adjuster included within the control means 12 allows for setting of the maximum secondary outlet temperature and a remote temperature adjuster 21 allows the outlet temperature to be set up to this limit. This arrangement would be particularly beneficial for a shower where the control could be accessible by the user. It would also simplify the plumbing as the shower would require only one supply pipe.

4 plurality of heat exchangers with their individual control means and circulator pumps, each set to provide water at different temperatures for example 65C for kitchen and 45C +or children's or elderly person's washing. One or more of these could be a dedicated supply to a shower as described in the previous paragraph.

A heat exchanger using hot water stored ready for use in a domestic hot water cylinder as the heat source in order that a mains or other supply can be heated to a controlled temperature for use in a shower as a means of adding a mains pressure shower to an existing domestic hot water system.

A control means in which the lag compensation is electronically adjusted by the use of a delay circuit arranged to reduce the control lead from a maximum value, for which the sensor position has been set, to a value appropriate for the current lower rate of flow. This may be appropriate for non-domestic situations where the range of secondary flow rates experienced is wide.

Claims

1 A not water supply reverse flow heat exchanger system including temperature sensing and primary flow control means wherein a primary heat supply fluid supplies heat @@ the secondary water and in which the secondary outlet water temperature is stabilised by a control means which adjusts the flow rate of the primary heating fluid through the heat exchanger in response to a measurement of the temperature of the secondary water at a point part @ay along the she exchanger.

2 A two stage heat exchanger system as in Claim 1 wherein the secondary flow temperature is measured at the junction of the stages and the division between the stages is sited such that, for typical secondary flow rates, the control lead s@ introduced reasonably matches the total control lags arising from temperature measurement and processing, operation of t@@ @ imary fluid flow control means, primary fluid movement and heat transfer to the secondary fluid within the latter stage of the heat exchanger.

3 A heat exchanger system as in Claim 2 in which the control means operates the primary flow control means to be on o@ off depending on whether the temperature measured at the selected point on the heat exchanger is respectively below or above that needed at this point for the secondary water to emerge at the required temperature.

4 A heat exchanger system as in Claim 2 in which the control means operates the primary flow control means to modulate th@ flow depending on whether the temperature measured at the selected point on the heat exchanger is below or above that @eeded at this point for the secondary water to emerge at the required temperature: a lower temperature resulting in 3 increased flow and a higher temperature resulting in a decreased flow.

5 A heat exchanger system as in Claims 3 or 4 wherein the source of primary fluid is a primary storage tank Or water kept hot for use only as a source of stored heat.

6 A heat exchanger system as in Claims 3 or 4 wherein the source of primary fluid is a secondary storage tank of water kept hot ready for use.

7 A heat exchanger system as in Claims 3 or 4 wherein the source of primary fluid is a heating main marrying steam or hot water.

8 A heat exchanger system as in Claims 5, 6 or 7 wherein the flow control means is a pump.

9 A heat exchanger system as in Claims 5, 6 or 7 wherein the flow control means is a valve or diverter operated by the control means to control the primary flow.

10 A heat exchanger system as in Claims 8 or 9 wherein the temperature sensor is a thermistor or standard thermocouple or a thermocouple using the copper tube containing the secondary water as one arm as in the first embodiment herein described.

11 A heat exchanger system as in Claims 8 or 9 wherein the temperature sensor is of some other type eg. a bimetallic sensor operating a micro-switch or reed switch, controlling the flow control means either directly, through a relay or through an electric or electronic controller.

12 A heat exchanger system as described in Claims 8 or 9 wherein the control means comprises a wax-stat or other suitable means of sensing the secondary temperature at an intermediate point between a first and a second heat exchanger stage and directly, mechanically operating a means to control the primary flow.

13 A heat exchanger system as in Claims 10 or 11 wherein the control means uses a solid state relay, triac or electro-mechanical device to operate the flow control means.

14 A heat exchanger as in Claims 12 or 13 wherein the type of construction is as described herein for the first embodiment.

15 A heat exchanger as in Claims 12 or 13 wherein a different type of construction is employed for example, plate, folded tube, multi-tube or concentric with the inner tube finned.

16 A heat exchanger as in Claims 12 or 13 wherein the second stage heat exchanger is of a different type of construction from the first stage.

17 A heat exchanger as in Claims 14, 15 or 16 wherein the second stage heat exchanger is easily detachable for de-scaling or replacement.

18 A heat exchanger as in Claims 15, 16 or 17 wherein the second stage incoming primary flow mixes with primary fluid already partly cooled by transfer of heat to the secondary water within this stage, before it impinges on the secondary water containing elements.

19 A heat exchanger system as in any of Claims 14 to 18 wherein a temperature adjuster included within the control means allows +or setting of the secondary- outlet temperature

20 A heat exchanger system as in any of Claims 14 to 18 wherein a temper ature adjuster included within the control means allows for the setting of the maximum secondary outlet temperature and a remote temperature adJuster allows the outlet temperature t be adjusted up to this limit

21 A plurality of heat exchanger systems as in Claims 19 and/or 20, each with their individual control means and circulator pumps, adjustable to provide water at different temperatures

22 A heat exchanger system as in Claims 19 and/or 20 using hot water stored in a domestic hot water cylinder, ready for use, as the heat source, in order that a mains or other supply can be heated to enable the addition of a a shower to an existing domestic hot water system

23 A heat exchanger system as in any of the previous claims in which the system lag compensation is electronically adjusted.

24 A heat exchanger as in any of Claims 19 to 23 which is mounted within the same overall insulation as the thermal store from which the primary fluid is drawn 25 4 eat exchanger based water heater system substantially similar in principle of operation to that herein described.