GB2427460A

GB2427460A - Heat exchanger having intertwined helically coiled heating elements.

Info

Publication number: GB2427460A
Application number: GB0612240A
Authority: GB
Inventors: Colin Martyn Harwood
Original assignee: Kohler Mira Ltd
Current assignee: Kohler Mira Ltd
Priority date: 2005-06-21
Filing date: 2006-06-21
Publication date: 2006-12-27
Anticipated expiration: 2026-06-21
Also published as: IE20060462A1; GB0512590D0; GB0612240D0; GB2427460B

Abstract

A heat exchanger 101 having at least two heating elements 103. 104 being arranged so that helically coiled portions of the elements are intertwined. The heating elements 103, 104 are electric heating elements used to heat water and are arranged in a double helix and may be connected to power supply to operate one or both of the elements 103, 104. Heat exchanger 101 comprises a tank 102 having a chamber 102a housing the elements 103, 104. Water enters the tank 102 via a tangential water inlet 106, passes up over the elements 103, 104 into an outlet 107 of an outlet pipe 108 that extends from the top of the chamber 102a to the bottom. A sensor may be located on an end cap 105 opposite outlet 107 to monitor the water temperature and a thermal switch controls the power supply to the elements 103, 104. Chamber 102a may be flushed with cold water when the power to the elements 103, 104 is turned off. The double helix arrangement and reversing the polarities of the heating elements 103,104 reduces limescale. The heat exchanger 101 is for an instantaneous water heater suitable for providing temperature controlled water to an ablutionary appliance such as a shower or hand washer.

Description

IMPROVEMENTS IN OR RELATING TO HEAT EXCHANGERS This invention concerns improvements in or relating to heat exchangers. The invention has particular application to heat exchangers for continuous flow heaters of the instantaneous type where fluid is heated as it passes through the heat exchanger. Instantaneous electric water heaters that provide a supply of hot water on demand are commonly used in ablutionary installations for showering, hand washing and the like. The known instantaneous electric water heaters typically allow the user to select the power input and/or adjust the water flow rate to obtain the desired outlet water temperature. To make temperature selection easy, it is necessary for the shower unit to respond quickly to adjustments made by the user. The time taken for the shower unit to respond to an adjustment is directly proportional to the amount of water being processed in the heat exchanger. Hence, it is desirable for the internal volume of the heat exchanger to be low. Commonly used heat exchangers consist of a chamber with an inlet and an outlet for water in which one or more electric heating elements are arranged to heat the water passing through the chamber. The heating elements have a helical coil of resistance wire inside a protective sheath. The sheath is usually a metal tube with the resistance wire packed in electrically insulating compressed powder in the centre of the tube. The tube is generally made of copper as this has the appropriate corrosion resistance in water and is ductile so that the complete element can be bent to a compact shape that enables the chamber volume to be kept small. The power output of the resistance wire inside the heating element sheath together with the requirement for a reasonable working life dictates the total length of the heating element to be contained within the chamber. Nowadays, there is a trend to increase the power output of electric showers for rapid heating of the water with the result that the length of the heating elements to be accommodated within the chamber has to be increased. For example, a power output of 10.8kW may require the heating elements to have a total length of over 1.8 metres. The most cost effective and compact shape to achieve this length is to form the heating element into a helix. Typically two coiled heating elements are provided either one above the other or side-by-side within the heat exchanger chamber with a switch for the user to energise either one or both heating elements to vary the power input to the heat exchanger. The high power output of the heat exchanger combined with the continuous flow of water through the chamber during operation of the heat exchanger gives rise to conditions in which lime scale can rapidly form in the heat exchanger. In hard water areas an electric shower can fail due to lime scale formation in an unacceptably short time. Examination of shower heat exchangers that have failed show that there is scale adhered to the coils of the elements and a lot of flakes of loose scale that can fill at least half of the volume of the heat exchanger chamber. In many cases the scale forms between adjacent coils of the element and distorts the element by spreading the coils apart. The failure mode, as lime scale forms, may be due to the scale having an insulating effect so that the element operates at a much higher temperature and the resistance wire can burn out. Alternatively, the distortion may be sufficient to cause mechanical failure of the element and/or the element fixing in the heat exchanger. Another potential mode of failure, as loose scale collects in the chamber, may be due to the scale affecting the water circulation so that a temperature-limit switch responsive to the outlet water temperature operates erratically. We have found that the initiation of lime scale formation occurs where the temperatures are highest. This is typically between adjacent turns of the coils of the element and in areas of the heat exchanger where the water flow is poor leading to localized hot spots where the temperature becomes higher than most of the rest of the heat exchanger. Once scale starts to form the flow through the heat exchanger becomes further disrupted causing more hot spots where scale forms. The present invention has been made from a consideration of the foregoing problems and disadvantages of the known heat exchangers. According to one aspect of the present invention there is provided a heat exchanger having at least two heating elements arranged so that helically coiled portions of the elements are intertwined. Preferably, two heating elements are provided with the helically coiled portions forming a double helix within the heat exchanger chamber. This arrangement allows the pitch of the helically coiled portions to be greater as there is no discontinuity between the pair of elements as in the case of two heating elements arranged in tandem, one above the other. The increased spacing improves the flow of water between the turns and reduces the tendency for hot spots to develop between adjacent turns. As a result, the rate of formation of limescale within the chamber is reduced and the working life of the heat exchanger increased. Preferably, the end portions of the heating elements remote from the upper end of the double helix are bent up to the upper end so that both end portions of each heating element can be attached to the power supply at one end of the chamber. By only having the lower end portions of the two heating elements bent up along the whole length of the double helix, the outer chamber can have a more uniform cross section that avoids discontinuities where water may be trapped. In this way, an even flow pattern in the heat exchanger is promoted which contributes to a reduction of localised hot spots. Preferably, the heat exchanger chamber has a water inlet at the lower end and a water outlet at the upper end with the inlet arranged substantially tangential to the chamber so that the water flows around the chamber as it flows upwards through the chamber to the outlet. In this way, air is expelled from the chamber so that the heating elements are immersed in water and heat transfer between the turns of the coiled portions and the water is improved. Preferably, the tangential inlet causes the water to flow in a roughly helical manner with the opposite hand to the turns of the coiled portions of the heating elements so that the water crosses the turns transversely. This enhances heat transfer and also reduces hot spots generating between the turns. Preferably, the helically coiled portions of the heating elements have the same number of turns so that successive turns of one element are separated by a turn of the other element. In this way, each heating element extends between the upper and lower ends of the chamber and heat transfer between the water and each heating element occurs over substantially the whole length of the chamber. Preferably, the helically coiled portions have the same pitch and diameter and are axially aligned. This assists in maintaining a uniform water flow pattern and uniform heat transfer along the length of the chamber between the inlet and outlet. User operable means may be provided to allow the user to control the outlet water temperature. The user operable means may allow the user to adjust the power input and/or flow rate to control the outlet water temperature. The heating elements may have the same or different power outputs and the user may be able to select different power settings by energising either one or both heating elements. Alternatively or additionally, the user may be able to select different flow rates by adjusting a flow control valve. Preferably, the outlet opens to an outlet pipe that extends from the upper end of the chamber to the lower end and the coiled portions of the heating elements extend around the outlet pipe. In this way, the heated water can exit from the bottom of the heat exchanger remote from the electrical connections to the power supply at the top of the heat exchanger. In addition, the pipe reduces the volume of the water that is being heated within the chamber so the speed of response to any change in the operating conditions is enhanced. Preferably, a temperature sensor is provided opposite the outlet for monitoring the outlet water temperature and a thermal switch responsive to the temperature sensor is provided to interrupt the power supply to the heating elements when a pre-determined outlet water temperature higher than the normal operating temperatures is detected. In this way, the risk of the user being scalded by very hot water is reduced. Preferably, a phased shut down is provided to flush the chamber with cold water when the heat exchanger is turned off to cool the heating elements and reduce the risk of "hot shots" occurring when the heat exchanger is re-started. Preferably, the heating elements are wired so that the magnetic fields associated with the heating elements can further assist with the minimisation of lime scale build up and thus improve working life of the heat exchanger. For example, where two heating elements are employed these may be wired with reversed polarities. According to another aspect of the present invention there is provided a heat exchanger having a chamber through which water can flow from an inlet to an outlet, and at least two electrical heating elements arranged within the chamber for heating water flowing through the chamber, each element being in the form of a helical coil wherein at least one turn of one coil is disposed between a pair of turns of the other coil. According to yet another aspect of the present invention there is provided a heat exchanger having a pair of electrical heating elements, each heating element having a helically coiled portion comprising a plurality of turns wherein the heating elements are arranged with turns of one element alternating with turns of the other element. According to a still further aspect of the present invention there is provided a heat exchanger having a pair of electrical heating elements, each heating element having a helically coiled portion wherein the heating elements are arranged with the helically coiled portions forming a double helix. The heating elements of the double helix may comprise two congruent helices with the same axis and offset along the axis. According to yet another aspect of the present invention there is provided an instantaneous water heater comprising a heat exchanger according to the preceding aspects of the invention. According to a further aspect of the present invention, there is provided an ablutionary installation comprising an instantaneous water heater according to the preceding aspect of the invention. The instantaneous water heater may be arranged to deliver water to one or more outlets of the ablutionary installation. For example a spray fitting for hand or body washing. The spray fitting may be fixed, for example wall mounted, or movable, for example incorporated in a handset. The spray pattern may be adjustable. According to a still further aspect of the present invention, there is provided a method of reducing limescale formation in a heat exchanger having at least two electric heating elements comprising the steps of providing each heating element with a helically coiled portion and arranging the helically coiled portions so that turns of the helically coiled portion of one element are intertwined with turns of the helically coiled portion of the other element. The invention will now be described in more detail by way of example only with reference to the accompanying drawings wherein:- Figure 1 is a perspective view of a known heat exchanger; Figure 2 is a perspective view of the heating elements of the heat exchanger shown in Figure 1; Figure 3 is a perspective view of a heat exchanger according to the present invention; Figure 4 is a perspective view sectioned through the inlet of the heat exchanger shown in Figure 3; Figure 5 is a perspective view sectioned through the heater tank of the heat exchanger shown in Figure 3; and Figure 6 is a perspective view of the heating elements of the heat exchanger shown in Figure 3.

prior art heat exchanger 1 for an instantaneous electric water heater for a Referring first to Figures 1 and 2 and the drawings, there is shown a shower or other ablutionary appliance. The heat exchanger 1 comprises a tank 2 housing two heating elements 3,4 having helically coiled portions 3a,4a respectively arranged in tandem one above the other and straight end portions 3b,3c, 4b,4c that extend through an end cap 5 at the upper end of the tank 2. The end portions 3b,3c,4b,4c are connected to terminals for connection to an electrical power supply via a switching mechanism (not shown) for operating either one or both of the heating elements 3,4 to provide different power settings according to user selection. The tank 2 has a radial inlet 6 in the sidewall at the lower end for connection to a source of cold water and an outlet 7 at the upper end close to the end cap 5 that opens to an outlet pipe 8. The outlet pipe 8 extends axially within the tank 2 from the upper end and passes through the bottom wall of the tank 2 for connection to an ablutionary appliance such as a shower handset via a flexible hose (not shown). The helically coiled portions 3a,4a of the heating elements 3,4 extend around the pipe 8 within the tank 2. In use, cold water enters the tank 2 through the inlet 6 and flows up the tank 2 over the heating elements 3,4 to the outlet 7 at the upper end of the outlet pipe 8. The water flows from the outlet 7 down the outlet pipe 8 for delivery to the ablutionary appliance. The water is heated by heat exchange with the heating elements 3,4 as it flows through the tank 2 and the temperature of the outlet water leaving the tank 2 is dependent on the inlet water temperature, the selected power input to the tank 2, and the rate of flow of the water through the tank 2. The user can adjust one or both of the power setting and flow rate to achieve the desired outlet water temperature and/or to accommodate seasonal variations in the inlet water temperature. As shown, the tandem arrangement of the heating elements 3,4 one above the other requires an enlarged space 9 between the lower end of the upper heating element 3 and the upper end of the lower heating element 4 to accommodate the connections to the straight end portions 3c,4b. In addition, the tank 2 has steps 10,11 in the internal surface to conform to the shape of the heating elements 3,4 in order to reduce the volume of the heat exchanger for fast response. The enlarged space 9 and steps 10,11 create pockets where water flow through the tank 2 can circulate producing dead spots where the local temperature becomes higher than the rest of the tank 2 leading to increased formation of limescale which further disrupts flow through the heat exchanger causing more hot spots where limescale forms. Referring now to Figures 3 to 6 of the drawings, there is shown a heat exchanger 101 according to the invention. For convenience, like reference numerals in the series 100 are used to indicate parts corresponding to Figures 1 and 2. The heat exchanger 101 comprises a tank 102 providing a heat exchanger chamber 102a housing two heating elements 103,104 having helically coiled portions 103a,104a arranged in a double helix with the turns of one coiled portion 103a intertwined with the turns of the other coiled portion 104a. As shown, both heating elements 103,104 extend the length of the tank 102 and the coiled portions 103a,104a form congruent helices with the same axis and having the same number of turns with the same pitch and diameter so that successive turns of one coiled portion are separated by a turn of the other coiled portion. In this way, the spacing of the turns of the double helix is substantially uniform along the length of the coiled portions 103a,104a without any increased spacing to accommodate bent end connections such as arises with the tandem arrangement of the heating elements 3,4 shown in Figures 1 and 2. Straight end portions 103b,103c,104b,104c of the heating elements extend through an end cap 105 at the upper end of the tank 102 for connection to an electrical power supply (not shown) for operating one or both of the heating elements 103,104 to provide different power settings according to user selection. The operation of the heat exchanger 101 is similar to that of Figures 1 and 2 with the power setting and/or flow rate being adjustable to control the rate of heating of the water flowing through the heat exchanger to achieve the desired outlet water temperature. A sensor (not shown) located on the end cap 105 opposite the outlet 107 monitors the outlet water temperature and a thermal cut-out switch is operable to interrupt the power supply to the heating elements 103,104 if a pre-determined outlet water temperature higher than the normal operating temperatures is detected to reduce the risk of scalding. A phased shut-down may also be provided to flush the tank 102 with cold water when the heat exchanger is turned off to remove any residual heat in the heating elements 103,104 that could otherwise heat the water remaining in the tank 102 to an elevated temperature causing a shot of very hot water being discharged when the heat exchanger is turned on again shortly after being turned off. The above-described arrangement of the heating elements 103,104 in a double helix results in each heating element 103,104 having a short end portion 103b,104b connected to the upper end of the coiled portion 103a,104a and a long end portion 103c,104c connected to the lower end of the coiled portion 103a,104a. In this way, the internal cross-section of the tank 102 is substantially uniform and presents a smooth (continuous) internal surface along the length of the tank 102 without any steps such as arise with the tandem arrangement of the heating elements shown in Figures 1 and 2. As a result, both the heating elements 103,104 and the tank 102 are constructed so that water flow through the tank 102 is not affected by discontinuities in either the coiled portions of the heating elements 103,104 or the internal section of the tank 102. In this way, the formation of pockets where water can be trapped creating localised hot spots that can increase the rate of formation of lime scale on the elements 103,104 and/or over-heating of the elements 103,104 leading to premature failure of the heat exchanger 101 is reduced or eliminated. Furthermore, the double helix allows the helix angle of the coiled portions 103a,104a of the heating elements 103,104 to be increased leading to a larger spacing between the coils. More specifically, employing heating elements constructed from copper tube of 6.35mm diameter, a pitch spacing of the turns of the coiled portions 103a,104a of 9.5mm can be achieved producing a gap of 3.15mm between adjacent turns with the double helix arrangement of the coiled portions according to the present invention. In contrast, to achieve the same power output with the prior art tandem arrangement of the coiled portions shown in Figures 1 and 2, the pitch spacing of the coils is 8mm producing a gap of only 1.65mm. Thus, the double helix arrangement of the present invention allows the gap between adjacent turns of the coiled portions to be almost doubled in size compared to the equivalent tandem arrangement of the prior art. In this way, water flow through and around the turns of the coiled portions 103a,104a is improved so that heat transfer is enhanced which results in improved cooling of the heating elements 103,104 and further reduces hot spots. As a result, the surface of the heating elements operates at a lower temperature and the rate of limescale formation is reduced. A further advantage of the increased helix angle is that the curvature of heating element is reduced during manufacture which helps to avoid damage to the powder packing and the resistance wire of the heating elements 103,104 and in turn helps to prevent premature failure of the heating elements 103,104 caused by overheating. As best shown in Figure 5, the tank 102 has a tangential inlet 106 at the lower end that is arranged to input a similar motion to the water on entry to the tank 102 that is in the opposite direction to the helices of the heating elements 103,104. As a result, the water flows across the heating elements 103,104 as it passes from the lower end to the upper end. In this way, heat transfer from the heating elements 103,104 to the water is improved, which also enhances efficiency and reduces or prevents overheating of the elements 103,104. The above features of the heat exchanger according to the present invention have been found to result in an improved working life. Thus, in tests, heat exchangers according to the invention with the double helix arrangement of the heating elements have shown an improvement in working life of up to 60% compared with an equivalent prior art tandem arrangement of the heating elements. Moreover, for a given power rating, this improved life can be achieved without any significant increase in the overall size of the heat exchanger and may even allow the size of the heat exchanger to be reduced. As a result, the heat exchanger can be employed in place of comparable heat exchangers in existing water heaters without change to the styling of the water heater. Furthermore, it may be possible to provide more compact heat exchangers that improve the options for the design and styling of new water heaters. We may wire the heating elements with reverse polarities although this is not essential. By reversing the polarities, the magnetic fields associated with the heating elements can further assist with the minimisation of lime scale build up and thus improve working life of the heat exchanger. It will be understood that the invention is not limited to the embodiment above-described. For example, we may provide heating elements with intertwined coiled portions of the same or different length according to the required total power input and power settings to be provided. We may provide more than two heating elements with intertwined coiled portions. Other changes that can be made will be apparent to those skilled in the art. While the invention has been described with reference to a heat exchanger for a shower installation, it will be understood the invented heat exchanger may be used in other installations, for example hand washers.

Claims

1. A heat exchanger having at least two heating elements arranged so that helically coiled portions of the elements are intertwined.

2. A heat exchanger according to claim 1, wherein two heating elements are provided with the helically coiled portions forming a double helix within a heat exchanger chamber.

3. A heat exchanger according to claim 2, wherein both end portions of each heating element are attached to the power supply at one end of the heat exchanger chamber.

4. A heat exchanger according to claim 2 or claim 3, wherein the heat exchanger chamber has a water inlet arranged substantially tangential to the chamber so that the water flows around the chamber as it flows through the chamber to a water outlet.

5. A heat exchanger according to claim 4, wherein the water inlet and water outlet are arranged so that water flows upwards through the heat exchanger chamber 6. A heat exchanger according to claim 4 or claim 5, wherein the water inlet and water outlet are arranged so that air is expelled from the chamber.

7. A heat exchanger according to any of claims 4 to 6, wherein the tangential inlet causes the water to flow in a roughly helical manner.

8. A heat exchanger according to claim 7, wherein, turns of the coiled portions of the heating elements are of opposite hand to the helical flow of the water so that the water crosses the turns transversely.

9. A heat exchanger according to any of claims 2 to 8, wherein the helically coiled portions of the heating elements have the same number of turns so that successive turns of one element are separated by a turn of the other element.

10. A heat exchanger according to any of claims 2 to 9, wherein the helically coiled portions of the heating elements have the same pitch and diameter. 11. A heat exchanger according to any of claims 2 to 10, wherein the helically coiled portions of the heating elements have the same axis.

12. A heat exchanger according to any of claims 2 to 11, wherein each heating element extends between upper and lower ends of the heat exchanger chamber and heat transfer between the water and each heating element occurs over substantially the whole length of the chamber.

13. A heat exchanger according to any of claims 2 to 12, wherein user operable means is provided for adjusting power input and/or flow rate for selectively controlling outlet water temperature.

14. A heat exchanger according to claim 13 wherein the heating elements have the same or different power outputs and the user operable means is arranged so that either one or both heating elements can be energised to provide different power inputs. 15. A heat exchanger according to any of claims 2 to 14, wherein a flow control valve is provided and the user operable means is arranged so that the valve can be adjusted to provide different flow rates.

16. A heat exchanger according to any of claims 4 to 7, wherein the water outlet opens to an outlet pipe that extends from an upper end of the heat exchanger chamber to a lower end of the chamber, and the coiled portions of the heating elements extend around the outlet pipe. 17, A heat exchanger according to any of claims 4 to 7, wherein a temperature sensor is provided opposite the outlet for monitoring the outlet water temperature and a thermal switch responsive to the temperature sensor is provided to interrupt the power supply to the heating elements when a pre-determined outlet water temperature higher than the normal operating temperatures is detected. 18. A heat exchanger according to any of claims 2 to 17, wherein a phased shut down is provided to flush the heat exchanger chamber with cold water when the heat exchanger is turned off to cool the heating elements and reduce the risk of "hot shots" occurring when the heat exchanger is re-started.

19. A heat exchanger according to any of claims 2 to 18 wherein the heating elements are wired with opposite polarity.

20. A heat exchanger having a chamber through which water can flow from an inlet to an outlet, and at least two electrical heating elements arranged within the chamber for heating water flowing through the chamber, each element being in the form of a helical coil wherein at least one turn of one coil is disposed between a pair of turns of the other coil. 21. A heat exchanger having a pair of electrical heating elements, each heating element having a helically coiled portion wherein the heating elements are arranged with the helically coiled portions forming a double helix.

22. A heat exchanger having a pair of electrical heating elements, each heating element having a helically coiled portion comprising a plurality of turns wherein the heating elements are arranged with turns of one element alternating with turns of the other element.

23. A heat exchanger according to any of claims 20 to 22, wherein the heating elements form a double helix consisting of two congruent helices with the same axis and offset along the axis.

24. An instantaneous water heater comprising a heat exchanger according to any preceding claim.

25. An ablutionary installation comprising an instantaneous water heater according to claim 24. 26. A method of reducing limescale formation in a heat exchanger having at least two electric heating elements comprising the steps of providing each heating element with a helically coiled portion and arranging the helically coiled portions so that turns of the helically coiled portion of one element are intertwined with turns of the helically coiled portion of the other element.

27. A method according to claim 26, wherein the heating elements are wired with opposite polarity.

28. A method according to claim 26 or claim 27, wherein the helically coiled portions of the heating elements have the same axis with the same pitch and diameter.

29. A heat exchanger substantially as hereinbefore described with reference to Figures 3 to 6 of the accompanying drawings. 30. A method of reducing limescale formation in a heat exchanger substantially as hereinbefore described with reference to Figures 3 to 6 of the accompanying drawings.