IE930637A1

IE930637A1 - Heat exchanger

Info

Publication number: IE930637A1
Application number: IE930637A
Authority: IE
Inventors: Nicholas John Beck; Robert Meadows
Original assignee: Caradon Mira Ltd
Priority date: 1992-09-07
Filing date: 1993-08-30
Publication date: 1994-03-09
Also published as: GB9218927D0; GB2270370A; GB2270370B

Abstract

A heat exchanger for an instantaneous electric water heater has one or more U-shaped heating elements 2 extending lengthwise of a casing 1 provided with an internal baffle 7 adjacent to a water inlet 8 for promoting non-turbulent substantially laminar flow along the or each heating element 2. The bends 4 of the elements 2 may be vertically spaced apart and the legs 3 circumferentially spaced apart, and supported by the baffle 7. The heat exchanger is provided with a temperature sensor 13 in a water outlet 11, a pressure relief valve, a flow sensor, and a flow stabiliser. The casing 1 is of circular or oval cross-section, and made of metal or plastics. Baffle 7 is a perforated sheet or mesh or wire gauge.

Description

This invention relates to heat exchangers, particularly heat exchangers for use in instantaneous electric water heaters for ablutionary appliances such as showers.

In an instantaneous electric water heater, water is heated as it flows 5 through a heat exchanger containing one or more heating elements and the user selects the power input to the heat exchanger and adjusts the water flow rate to obtain the desired outlet water temperature. B For application to ablutionary appliances such as showers, it is 2½ desirable for the heat exchanger to have a low volume relative to the hB 10 surface area of the heating elements for efficient heat transfer and fast v\ L> response to adjustments to the flow rate. I Γ It is already known to provide a large surface area by the use of i -CZ coiled heating elements. Such elements, however, are expensive to manufacture and, in use, can give rise to fluctuations in the outlet water o temperature due to turbulence within the water flowing through the heat exchanger as it crosses each coil of the element.

These disadvantages are reduced by the use of straight heating elements which are easier and less costly to manufacture and, in use, avoid turbulence within the water which tends to flow along the elements. As a result of such laminar flow along the heating elements, the water is progressively heated as it flows through the heat exchanger giving rise to a steadier outlet water temperature and faster response to flow adjustments as compared with heat exchangers employing coiled heating elements.

A further advantage of using straight heating elements as compared with coiled heating elements is that the residual heat of the heating elements when the heat exchanger is switched off is more evenly distributed through the water remaining in the heat exchanger. As a result, the accumulation of very hot water within the heat exchanger that is discharged first as a hot shot which the user does not expect when the heat exchanger is switched on again is reduced.

Typically, heat exchangers employing straight heating elements are of elongate, slim profile with the heating elements projecting from opposite ends for connection to the power supply. With this arrangement premature failure of the heating elements can arise due to the repeated expansion and contraction that occurs as the power is switched on and off, especially where -3feL<30637 the heating elements are secured at opposite ends of the heat exchanger by soldering or the like.

A solution to the problem of expansion and contraction of straight heating elements is disclosed in our UK Patent No. 2164731 and consists in the provision of sliding seals at each end of the heating element to accommodate the change in length of the heating element. Such seals, however, add to manufacturing costs and are also susceptible to failure.

The present invention has been made from a consideration of the disadvantages above discussed.

It is a preferred object of the present invention to provide a heat exchanger in which the advantages of substantially laminar water flow along the heating elements can be obtained without the provision of sliding seals to accommodate thermal expansion and contraction of the heating elements.

According to the present invention we provide a heat exchanger for an instantaneous water heater having a water inlet and a water outlet and at least one heating element of generally U-shape.

The main advantage of the invention is that the effects of expansion and contraction of the heating element are substantially equal in each leg and can be accommodated without the provision of sliding seals for the ends of the heating element projecting from the heat exchanger for connection to the power supply.

A further advantage of the invention is that the water flow along each leg of the heating element is substantially laminar enabling steady outlet water temperature, fast response to flow adjustments and reduced hot shot to be obtained with a reduction in the overall length of the heat exchanger.

Preferably, the heat exchanger includes an internal baffle for damping out turbulence caused by water entering the heat exchanger and/or flowing over the bend portion connecting the legs of the heating element. By this feature of the internal baffle, non-turbulent substantially laminar water flow along each leg of the heating element is promoted.

In a preferred construction, each leg of the heating element extends lengthwise of the heat exchanger substantially parallel to the direction of water flow through the heat exchanger, and the baffle provides a support for the heating element adjacent to the bend portion.

Where more than one heating element is provided, the heating elements are preferably of different lengths with the bend portions -4ii 9s connecting the legs vertically spaced apart and the legs circumferentially spaced apart. In this way, the heating elements can be closely packed without touching so that the volume of the heat exchanger can be kept small.

Other features, benefits and advantages of the invention will be apparent from the following description of an exemplary embodiment with reference to the accompanying drawings wherein:FIGURE 1 is a longitudinal section through a heat exchanger embodying the invention; and FIGURE 2 is a section on the line 2-2 of Figure 1.

The heat exchanger shown in the drawings comprises a cylindrical casing 1 in which four U-shaped heating elements 2 are disposed. The casing 1 is cut to length from a copper tube of the appropriate diameter.

Each heating element 2 has a pair of parallel legs 3 connected by a smoothly curved bend 4 at one end and consists of a wire coil surrounded by an outer cover of electrically insulating material.

The free ends of the legs 3 project from the top of the casing 1 through an end fitting 5 secured to the casing 1 and are provided with terminals 6 for connection to an electrical power source (not shown).

The heating elements 2 are of slightly different lengths with the curved bends 4 vertically spaced apart and the legs 3 circumferentially spaced apart and supported by a transverse baffle 7 comprising a perforated sheet or mesh or wire gauze.

By this arrangement, the heating elements 2 are closely packed without touching each other enabling the volume of the casing to be kept small.

An inlet 8 for connection to a water supply, for example mains cold water, is provided at the bottom of the casing 1 by an end fitting 9 secured to the casing 1 and sealed by an O-ring 10. An outlet 11 for connection to an ablutionary appliance, for example a shower head, is provided adjacent to the top of the casing 1 by an elbow fitting 12 secured to the casing 1 and mounting a temperature sensor 13 for monitoring the water outlet temperature.

The heat exchanger is suitable for use in an instantaneous water heater having user operable controls (not shown) for selectively energising one or more of the heating elements 2 to vary the power input and for -5» ®3 0 8X7 adjusting the water flow rate to heat the water flowing through the heat exchanger to a selected temperature.

The effects of thermal expansion and contraction on the legs 3 of the heating elements 2 are equal thereby avoiding the need for any sliding seals or similar means to accommodate the change in length of the heating elements 2 and the elements 2 can be secured to the end fitting 5 by soldering or any other suitable means providing a watertight seal. In addition, the bends 4 are not subjected to forces sufficient to cause failure of the heating elements 2 by the change in length of the legs 3.

The legs 3 of the heating elements 2 extend lengthwise of the heat exchanger substantially parallel to the direction of water flow and the baffle 7 has the effect of damping any turbulence caused by the water flowing into the heat exchanger over the bends 4. In this way, non-turbulent substantially laminar flow along the legs 3 of the heating elements 2 is promoted so that the water is progressively heated as it flows through the heat exchanger enabling a steady outlet water temperature and fast response to flow adjustments to be obtained. In addition, when the heat exchanger is switched off, all motion of the water ceases and the residual heat remaining in the heating elements is dissipated to all the water remaining in the heat exchanger so that the accumulation of very hot water which is discharged first as a hot-shot when the heat exchanger is switched on again is reduced and can be avoided by allowing a short delay between switching off the power to the heating elements 2 and stopping the water flow.

The temperature sensor 13 is operable to switch-off the heating element(s) 2 at a pre-determined outlet water temperature higher than that for comfortable showering to reduce the risk of the user being scalded.

The heat exchanger may also be provided with a pressure relief valve (not shown) arranged to relieve excess pressure within the casing 1 for example if the heating elements 2 are switched on with the outlet blocked or with insufficient water flow.

Yet another safety feature may be provided by a flow sensor (not shown) arranged to control the power input to the heat exchanger so that the heating elements 2 are switched on only when there is sufficient water flow through the heat exchanger. -6Μ 930637 An additional safety feature may be provided by a flow stabiliser (not shown) arranged to maintain constant a selected flow rate so that the outlet water temperature is unaffected by fluctuations in the water supply pressure.

It will be appreciated that the invention is not limited to the 5 embodiment above-described. For example, the number of heating elements may be chosen to suit any desired power input requirements and, for some applications, a single heating element may be sufficient.

The casing may be of circular cross-section or any other suitable cross-section as desired, for example oval, and may be made from metal or plastics as desired.

Claims

1. A heat exchanger for an instantaneous water heater comprises a water inlet and a water outlet and at least one heating element of generally U-shape.

2. A heat exchanger according to Claim 1 wherein the inlet and outlet are arranged at or near opposite ends of the heat exchanger.

3. A heat exchanger according to Claim 1 or Claim 2 wherein the heating element extends lengthwise of the heat exchanger substantially parallel to the direction of water flow through the heat exchanger.

4. A heat exchanger according to any one of the preceding Claims comprising a plurality of heating elements of generally U-shape spaced apart circumferentially and axially.

5. A heat exchanger according to any one of the preceding Claims further comprising an internal baffle for promoting non-turbulent substantially laminar water flow along the or each heating element.

6. A heat exchanger according to Claim 5 wherein the baffle provides a support for the or each heating element adjacent to the inlet.

7. A heat exchanger according to Claim 5 or Claim 6 wherein the baffle comprises a perforated sheet or mesh or gauze.

8. A heat exchanger according to any one of the preceding Claims further comprising a temperature sensor for monitoring the outlet water temperature.

9. A heat exchanger according to any one of the preceding Claims further comprising a pressure relief valve arranged to relieve excess pressure.

10. A heat exchanger according to any one of the preceding Claims further comprising a flow sensor arranged to control the power input to the heat exchanger.

11. A heat exchanger according to any one of the preceding Claims further comprising a flow stabiliser arranged to maintain constant a selected flow rate.

12. A heat exchanger for an instantaneous water heater substantially as hereinbefore described with reference to the accompanying drawing.

13. An instantaneous water heater incorporating a heat exchanger according to any one of the preceding Claims.