GB2298479A

GB2298479A - Heat exchanger

Info

Publication number: GB2298479A
Application number: GB9504120A
Authority: GB
Inventors: Robert Meadows
Original assignee: Kohler Mira Ltd
Current assignee: Kohler Mira Ltd
Priority date: 1995-03-01
Filing date: 1995-03-01
Publication date: 1996-09-04
Anticipated expiration: 2015-03-01
Also published as: IE960168A1; GB9504120D0; GB2298479B

Abstract

A heat exchanger comprising an inlet 6 and an outlet 8 at opposite ends of a tank 4 containing at least one heating element 12 for heating water flowing through the tank 4 wherein a chamber 26 in the tank 4 forms a pocket of air which is compressed to prevent the tank 4 rupturing if water remaining in the tank 4 freezes when the heat exchanger is not in use. As an alternative to a pocket of air, a resiliently deformable member, to accommodate the increased volume as ice is formed, may be used to prevent the tank rupturing. The tank may include heating element spacer means in the form of baffle 20, suitably apertured with hole 24 to seat chamber 26. The tank may include flow guide 22 defining a helical flow path around each heating element 12.

Description

Heat Exchanaer This invention relates to heat exchangers for instantaneous water heaters of the type in which water is heated as it flows through a heat exchanger containing one or more electric heating elements.

In the known instantaneous water heaters, problems can arise if water freezes in the heat exchanger tank. Ice forming can rupture the tank which increases the risk of an electric short or, if the heating elements are switched on, steam could form in the tank and the whole unit explode.

The tank can be provided with a pressure relief valve that operates to release excess pressure developing in the tank due to the formation of steam but such valve does not prevent the tank being ruptured if the water freezes in the tank.

The present invention has been made from a consideration of this problem.

According to the present invention a heat exchanger for an instantaneous water heater comprises a tank having an inlet and an outlet, at least one heating element for heating water flowing through the tank, and anti-burst means to accommodate the increased volume of ice formed if the water freezes in the tank.

By this feature of the anti-burst means, the development of excessive pressure which can cause the tank to rupture if the water freezes is prevented.

Advantageously, the anti-burst means comprises an air pocket positioned in the tank so as to be compressed by an increase in volume of the ice formed as the water freezes.

Preferably, the air pocket is formed and/or, maintained by collecting air released from the water as it is heated when the heat exchanger is in use.

Advantageously, a baffle plate is provided in the tank to distribute the incoming water flow and air bubbles in or released from the water as it is heated are trapped by the baffle plate for collection in the air pocket.

Preferably, the air pocket is formed in a tube having an open lower end seated on the baffle plate and aligned with a hole in the baffle plate through which the air bubbles can pass into the tube.

In a preferred construction, the tank is of elongate tubular form, preferably cylindrical, with a plurality of elongate heating elements extending lengthwise of the tank and spaced apart in the circumferential direction to form a central space or gap in which the tube is located.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein: FIGURE 1 is a side view of a heat exchanger according to the invention; FIGURE 2 is a vertical section of the heat exchanger tank showing the heating elements and flow guide; and FIGURE 3 is a vertical section of the tank, heating elements and flow guide showing the air reservoir.

The heat exchanger 2 shown in the drawings comprises a cylindrical tank 4 with an inlet 6 at the lower end and an outlet 8 at the upper end in which a temperature sensor 10 is located. The tank 4 is made of suitable plastics material but this is not essential and other materials including metal could be used.

The tank 4 contains four U-shaped electric heating elements 12 carried by an end cap 14 screwed onto the upper end of the tank 4 and sealed by an O-ring 16 located between the tank 4 and a neck 18 of the end cap 14 received within the upper end of the tank 4.

The heating elements 12 may be of the same or different power rating with suitable switching means (not shown) being provided to enable different combinations of heating elements 12 to be selected to vary the power input as desired.

Each heating element 12 has a pair of spaced parallel legs 12a extending lengthwise of the tank 4 and connected by a U-bend 12b at the lower end of the tank 4. The heating elements 12 are spaced apart in the circumferential and vertical directions to provide a compact nested arrangement in which each heating element 12 is free to expand/contract in response to temperature change independently of the other heating elements 12.

The legs 12a of the heating elements 12 are supported at the lower end of the tank 4 by a perforated sheet metal baffle plate 20 which acts as a spacer for the heating elements and causes the incoming water flow to be evenly distributed.

Above the baffle plate 20, the legs 12a of the heating elements 12 are surrounded by a helical flow guide 22 which extends from the baffle plate 20 to the upper end of the tank 4 and confines the water flow to a helical flow path around the heating elements 12. The flow guide 22 is a separate component located in the tank 4 but this is not essential and the flow guide 22 could be formed integrally with the tank 4.

The baffle plate 20 has a hole 24 in an upset centre portion aligned with the open lower end of a tube 26 seated on the baffle plate 20 in the space between the legs 12a of the heating elements 12. The tube 26 is closed at the upper end of the tank 4 by the end cap 14 and provides an air reservoir to trap and maintain an air bubble of pre-selected volume within the tank 4.

In use, water flowing through the tank 4 is heated by the heating elements 12 to a pre-determined temperature according to the selected power input and flow rate of the water. The selected power input may correspond to switching on 1, 2, 3 or 4 of the heating elements 12.

The water is swirled around the legs 12a of all the heating elements 12 by the helical flow guide 22 so that the water is thoroughly mixed. In this way, separate streams of heated and unheated water are prevented from developing, especially when one or more of the heating elements 12 is switched off. As a result, the temperature of the water detected by the sensor 10 at the outlet 8 is much more uniform and consistent.

If the water in the tank 4 freezes when the water heater is not in use, the air bubble trapped in the tube 26 is compressed so as to prevent excessive pressure developing in the tank 4 which may cause the tank 4 to rupture causing a leak and risk of electrical short when the water thaws.

In use, air bubbles released by initial heating of the incoming water flow by contact with the U-bends 12b of the heating elements are guided by the baffle plate 20 towards the upset centre portion from where they pass through the central hole 24 and are trapped in the tube 26 to maintain the required volume of the air within the tank 4.

It will be understood that the invention is not limited to the embodiment above-described and that the principle of the anti-burst means has application to any heat exchanger in which there is at least one heating element in a tank for heating a continuous flow of water through the tank.

Although the invention has been described with particular reference to the use of an air bubble to prevent the tank rupturing if the water freezes in the tank, it will be appreciated that the same result may be achieved by other means. For example the anti-burst means may comprise a resiliently deformable member or material arranged to accommodate the increased volume as ice is formed in the tank.

The air reservoir for trapping the air to form an air bubble may be of any suitable size and/or shape and may be located at any suitable position.

The flow guide may be separate from or integral with other parts of the heat exchanger, for example the end cap.

The heating elements may be of U-shape or any other suitable shape with the number and arrangement of the heating elements being chosen according to the required power input(s).

The heating elements may be of any size and/or shape with the number and arrangement of the heating elements being chosen according to the power requirements.

The tank may be fitted with a suitable pressure relief valve to release excess pressure which may be produced in use as a result of steam forming in the tank for any reason.

The provision of the helical flow guide to avoid random fluctuations in the outlet water temperature forms the subject matter of our co-pending UK Patent Application of even date.

Claims

Claims:

1. A heat exchanger for an instantaneous water heater comprising a tank having an inlet and an outlet, at least one heating element for heating water flowing through the tank, and anti-burst means for accommodating formation of ice caused by water freezing in the tank.

2. A heat exchanger according to Claim 1 wherein the anti-burst means is compressible for accommodating increased volume of ice formed by water freezing in the tank.

3. A heat exchanger according to Claim 1 or Claim 2 wherein the anti-burst means is provided by a pocket of air in the tank.

4. A heat exchanger according to Claim 3 wherein the air pocket is formed by air trapped in the tank.

5. A heat exchanger according to Claim 4 wherein air released from water heated in the tank is trapped in the air pocket.

6. A heat exchanger according to Claim 4 or Claim 5 wherein trapped air forming the air pocket is contained in a hollow body within the tank.

7. A heat exchanger according to Claim 6 wherein the or each heating element is of U-shape having spaced parallel legs extending lengthwise of the tank between which the hollow body is located.

8. A heat exchanger according to Claim 7 comprising a plurality of U-shaped heating elements.

9. A heat exchanger according to Claim 7 or Claim 8 comprising spacer means arranged to locate the legs of the or each heating element in the circumferential direction.

10. A heat exchanger according to claim 9 wherein the hollow body comprises a tube which is open at one end and closed at the other end to trap and retain air forming the air pocket in the tank.

11. A heat exchanger according to Claim 10 wherein the spacer means has a hole aligned with the open end of the tube and is arranged to guide bubbles of air from water in the tank towards the hole for collection in the tube.

12. A heat exchanger according to any one of the preceding claims including a flow guide defining a helical flow path around the or each heating element.

13. A heat exchanger according to any one of the preceding Claims including a temperature sensor for monitoring the outlet water temperature.

14. A heat exchanger according to any one of the preceding Claims including a pressure relief valve for releasing excess pressure caused by formation of steam in the tank.

15. A heat exchanger comprising a tank containing at least one heating element for heating water flowing through the tank and means for preventing tank rupture if water freezes in the tank when the heat exchanger is not in use.

16. A heat exchanger substantially as hereinbefore described with reference to the accompanying drawings.

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