GB2164731A - Electric water heater - Google Patents

Abstract

An instantaneous electric water heater 1 has elongate heating elements 4 detachably mounted in a tubular casing 2 for heating water flowing through the casing 2 to provide a continuous supply of hot water on demand. Each heating element 4 extends lengthwise of the casing 2 and has opposed end portions 8a,8b which project outwards through aligned through bores 6,6' of a respective end fitting 3,3' of the casing 2. Thermal expansion/contraction of the heating elements 4 relative to the casing 2 is accommodated by slidably sealing the opposed end portions 8a,8b in the bores 6,6' by means of respective O-rings 10 retained in the bores by releasable closure caps 12,12' attached to the outer face of each end fitting 3,3' respectively. Each heating element 4 also has an external annular collar 11 on one end portion 8a which is located and retained in the bore 6 by the closure cap 12 whereby each heating element 4 is releasably secured in the casing 2 and may be assembled/disassembled independently of the others by removing the closure caps 12,12' and sliding the heating element 4 into/out of the casing 2. <IMAGE>

Description

SPECIFICATION Electric water heater This invention relates to electric water heaters and in particular, though not exclusively, to so-called instantaneous electric water heaters.

Instantaneous electric water heaters provide a source of hot water on demand by heating a supply of coid water flowing continuously through the heater. Such heaters are commonly provided in ablutionary installations, especially for showering, either when a supply of hot water from a conventional storage cylinder is not readily available or when the demand for hot water is irregular making it uneconomical to heat and maintain a relatively large volume of water contained in a storage cylinder at a raised temperature until required.

The known instantaneous electric water heaters rely on a heating element of considerable heating capacity, generally about 7 kW, located in the fluid flow path to heat rapidly the water to the required temperature. In order to obtain the necessary heat transfer to ensure rapid heating of the continuous flow of water, the heating element is usually mounted in a hollow casing of relatively small volume and the heating element is of convoluted profile, e.g. coiled, to provide a large surface area for heat exchange with the water flowing through the casing.

A disadvantage of such known water heat ers is that the heating coil is of complex shape and is a close fit in the casing. As a result, manufacture of the heating element is complicated and requires a high degree of accuracy to meet the close tolerances necessary for assembly. Furthermore, the heating element is usually secured in position by one or more soldering operations which can result in heat damage to the sealing of the sheathed heating coil, leading to moisture absorption which in turn destroys the insulation resistance of the powder separating the heating coil from the sheath causing failure of the heating element.

It is an object of the present invention to provide an electric water heater which mitigates at least some of the problems and disadvantages aforementioned.

According to the present invention there is provided an electric water heater comprising an elongate hollow casing having a respective end fitting at each opposed end, an inlet for supply of water to the casing and an outlet for delivery of heated water from the casing, an elongate heating element extending lengthwise of the casing and having opposed end portions, each end portion projecting outwards through one respective end fitting of the casing for connection to an electrical power source for heating water flowing through the casing and, a respective seal between each end portion of the heating element and the associated end fitting, at least one of said seals being constructed and arranged to allow relative axial movement between the heating element and the casing.

By providing seals between the opposed end portions of the heating element and the casing, the water heater according to this invention may be assembled without the soldering operations previously required to secure the heating elements. In this way possible heat damage to the heating element is eliminated.

Additionally, by constructing at least one seal to allow relative axial movement between the heating element and the casing, thermal expansion/contraction of the heating element relative to the casing is accomodated in a simple and effective manner.

Preferably both seals are constructed and arranged to allow relative axial movement between the heating element and the casing and the heating element is detachably mounted in the casing. For example, the heating element may have the opposed end portions slidably located in aligned axially extending through bores in the end fittings and the seals may each comprise an O-ring positioned in a respective one of the bores.

Advantageously, the bores are each of stepped profile having a wider bore portion opening towards the outer face of the associated end fitting and the O-rings are received in the wider bore portions. Preferably each end fitting has a respective apertured closure cap which locates over the projecting end portion of the heating element and is releasably secured to the outer face of the end fitting to close the wider bore portion and retain the 0ring therein.

By this arrangement each O-ring is axially movable within the associated bore portion thereby reducing wear on the O-ring caused by the thermal expansion/contraction of the heating element in service. Additionally, inspection of the O-rings for wear and/or replacement is facilitated, requiring only the removal of the adjacent closure cap.

Alternatively, each O-ring may be located in a groove formed in either the bore portion or the projecting end of the heating element.

Preferably the heating element is linear and has an external annular collar, provided for example by an olive, on one end portion which is received in the wider bore portion of the associated end fitting to locate and retain the heating element in the casing.

By this feature the heating element may be inserted axially into the casing from one end to locate in the aligned bores, the O-rings fitted over the projecting end portions after which the closure caps are attached to retain the O-rings in the bores and secure the heating element in the assembled position. In this way, not only is assembly of the water heater achieved in a simple and efficient manner but removal of the heating element for repair/replacement is also facilitated by the detachable and slidable mounting of the heating element.

Preferably the casing comprises a cylindrical tube of predetermined length and diameter closed at each end by the end fittings, and the heating element comprises a cylindrical rod containing a wire heating coil connected to a respective terminal at each end of the rod for connection to the power supply. Conveniently, the inlet to the casing is provided by a tubular spigot on one end fitting and the outlet from the casing is provided by a tubular spigot on the other end fitting. In this way casings of any required capacity to suit a particular application can be produced by cutting the tube to length from stock of the appropriate diameter and attaching the end fittings.

With the construction of water heater according to the present invention, the velocity of the water flowing through the casing is maintained laminar thereby maximising the rate of heat exchange to ensure rapid heating of the water. Additionally, by appropriate selection of the dimensions of the casing and heating element the volume of water in the casing can be kept small to give a fast response time when the user adjusts the required temperature of the hot water.

Furthermore, the heat transfer is gradual along the length of the casing and is consistent from one casing to another so that the delay in sensing a change in the temperature of the hot water from that selected can be reduced by sensing a proportion of the change at a fixed point along the length of the casing. As a result, a closed loop control system can be used for monitoring the temperature of the hot water from the outlet to give a better degree of control in maintaining constant the selected temperature.

Yet a further advantage of the elongate casing is that the water heater can be kept relatively slim thereby providing a greater range and freedom of choice of mounting positions.

This is of particular benefit in shower installations where the space available for mounting the water heater in a position accessible to the user to allow control of the temperature whilst showering may be limited.

According to a further feature of the invention the water heater includes a plurality of spaced apart heating elements each constructed and arranged as aforedescribed.

With this arrangement, each heating element is separate from the others and is assembled and can be removed independently. As a result, maintenance of the water heater is greatly facilitated whilst the provision of more than one heating element permits a wider operating range and greater control of the selected temperature by appropriate selection of the heating capacity of each heating element and the selective operation of some or all of the heating elements.

In known manner the water heater will include suitable controls for regulating the water flow through the casing, selecting the temperature of the hot water produced and monitoring and maintaining the selected temperature.

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawing wherein: FIGURE 1 is a plan view of an electric water heater according to the present invention; and FIGURE 2 is a longitudinal section on the line A-A of Figure 1.

Referring to the drawing, there is shown an instantaneous electric water heater 1 comprising an elongate hollow casing 2 having a respective end fitting 3,3' at each opposed end and four elongate heating elements 4 extending lengthwise of the casing 2 for heating water circulated continuously through the casing 2.

Additionally, the water heater includes suitable controls (not shown) for regulating the flow of water through the casing 2, selecting the temperature of the heated water produced and monitoring and maintaining the selected temperature. Such controls are well known to those skilled in the art and will not be further described herein as they form no part of the present invention.

The casing 2 comprises a cylindrical tube 5 of predetermined length and diameter to which the end fittings 3,3' are attached, for example by welding. The tube 5 is conveniently cut to length from an extrusion of metal or plastics having the predetermined diameter. In this way casings of any required volume may be produced by selecting and cutting the appropriate length of tube and attaching the end fittings. The end fittings 3,3' are formed as pressings from metal or plastics and both the tube 5 and end fittings 3,3' may have a suitable surface finish applied thereto as desired.

Each end fitting 3,3' is formed with four axially extending through bores 6,6' respectively and a laterally projecting tubular spigot 7,7' respectively. The bores 6,6' of each end fitting are uniformly spaced apart in the circumferential direction and the spigots 7,7' provide an outlet and an inlet respectively for water circulating through the casing 2.

Each through bore 6,6' is of stepped construction having a wider bore portion 6a,6a' opening towards the outer end face of the associated end fitting and each through bore 6 of the end fitting 3 is aligned with a respective through bore 6' of the opposed end fitting 3.

Each heating element 4 comprises a respective cylindrical rod 8 of a suitable heat conducting and electrically insulating material and containing a wire heating coil (not shown) of suitable heating capacity connected to a respective terminal 9,9' at each opposed end of the rod.

Each rod 8 has a length greater than the length of the tube 5 and attached end fittings 3,3' and the opposed end portions 8a,8b of each rod project outwards through a respective pair of aligned through bores 6,6' and are sealed in the wider bore portions 6a,6a' by a respective O-ring 10.

Each rod 8 has a respective external annular collar provided by an olive 11 on one end portion 8a which is received in the associated wider bore portion 6a, and each end fitting 3,3' has a respective apertured closure cap 12,12' releasably secured to the outer face by a respective screw 13,13' to close the wider bore portions 6a,6a' to retain the O-rings 10 therein and secure the rods 8 in the assembled position.

As will now be appreciated from the foregoing description, each rod 8 is slidably sealed at the opposed end portions by the 0rings 10. As a result, on assembly of the water heater, the rods 8 are mounted without any soldering operations and, in use of the water heater, thermal expansion/contraction of each rod 8 relative to the casing 2 is accommmodated in a simple and efficient manner by relative axial movement between the rod 8 and casing 2 through the sliding seals.

Consequently, heat damage to the heating elements is avoided thereby ensuring the electrical integrity of the heating elements 4 is maintained.

Furthermore, each heating element 4 is detachably mounted separately from the other heating elements, permitting each heating element to be removed independently for repair or replacement by removing the closure caps 12,12' and sliding the element 4 out of the casing 2. As a result, not only is maintenance of the water heater facilitated but, by selective operation of some or all of the heating elements, the heating capacity of the water heater may be varied as desired. Additionally, by exchanging one or more heating elements with heating elements of different heating capacity, the same water heater may be adapted for a range of applications having different heating requirements. For example, in the above-described water heater, a heating capacity in the range 6-10.8 kW may be obtained by the appropriate selection and control of heating elements of 2, 2.5 and 2.7 kW.

It will be understood that the invention is not limited to the embodiment above-described which may be modified in a number of ways, for example any number of heating elements may be provided and the dimensions of the casing and heating elements may be varied as desired.

Finally, although in the above-described embodiment, there is a sliding seal arrangement at each opposed end portion of each heating element, it will be appreciated that this is not essential and that thermal expansion/contraction of each heating element relative to the casing could be accomodated by the provision of a sliding seal arrangement at one end portion only and a seal arrangement at the other end portion by which the other end portion is fixed or otherwise located against axial movement. However the above-described arrangement is preferred as providing a detachable and slidable mounting which facilitates simplified assembly and removal of the heating elements.

Claims (12)

1. An electric water heater comprising an elongate hollow casing having a respective end fitting at each opposed end, an inlet for supply of water to the casing and an outlet for delivery of heated water from the casing, an elongate heating element extending lengthwise of the casing and having opposed end portions, each end portion projecting outwards through one respective end fitting of the casing for connection to an electrical power source for heating water flowing through the casing and, a respective seal between each end portion of the heating element and the associated end fitting, at least one of said seals being constructed and arranged to allow relative axial movement between the heating element and the casing.

2. An electric water heater according to claim 1 wherein both seals are constructed and arranged to allow relative axial movement between the heating element and the casing.

3. An electric water heater according to claim 1 or claim 2 wherein the heating element is detachably mounted in the casing.

4. An electric water heater according to claim 3 wherein the opposed end portions of the heating element are slidably located in aligned axially extending through bores in the end fittings and the seals each comprise an 0ring positioned in a respective one of the bores.

5. An electric water heater according to claim 4 wherein the aligned bores are each of stepped profile having a wider bore portion opening towards the outer face of the associated end fitting and the O-rings are received in the wider bore portions.

6. An electric water heater according to claim 5 wherein each end fitting has a respective apertured closure cap which locates over the projecting end portion of the heating element and is releasably secured to the outer face of the end fitting to close the wider bore portion.

7. An electric water heater according to claim 6 wherein the heating element has an external collar on one end portion which is received in the wider bore portion of the associated end fitting to secure the heating element in the casing.

8. An electric water heater according to any one of the preceding claims wherein the casing comprises a cylindrical tube of predetermined length and diameter and the heating element comprises a cylindrical rod containing a wire heating coil connected to a respective terminal at the opposed ends of the rod.

9. An electric water heater according to any one of the preceding claims wherein the inlet is provided in one end fitting and the outlet is provided in the other end fitting.

10. An electric water heater according to any one of the preceding claims having a plurality of spaced apart elongate heating elements, each heating element having opposed end portions projecting outwards through one respective end fitting.

11. An electric water heater according to claim 10 wherein each heating element is detachably mounted separately from the other heating elements.

12. An electric water heater substantially as hereinbefore described with reference to the accompanying drawing.