NZ239994A - Immersion heater for spa pool with integral thermal fuse. - Google Patents

Description

,240701 239994 Patents Form # 5 NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION AFTER PROVISIONAL NOS : 239994 240701 DATED : 27 September 1991 22 November 1991 TITLE: Heater Control We, John Wickham Rossiter, a New Zealand citizen, of 141 Pah Road, Howick, Auckland, New Zealand, and Edwin Ernest Poynter, a New Zealand citizen, of 57 Ngapuhi Road, Remuera, Auckland, New Zealand, hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: xP Yo \$.l V E .JWP FEE CODE -1050 23 § 9 94 2407 FIELD OF THE INVENTION This invention relates to the control of heaters for use in swimming pools, spa pools, and other apparatus in which water or other liquids flow through a heating chamber.

BACKGROUND Spa pools and swimming pools typically have a heater controlled by a pump. The heater is usually in the form of an immersion heater situated within a heating chamber, with liquid flowing into the heating chamber and out to the spa pool or swimming pool. These heaters are typically of three kilowatts or more, and if they malfunction, they may cause the heating installation of the spa pool or swimming pool to catch fire. Although a thermostat may be provided in association with the contactor coil of the heater, to cut off power when a pre-set temperature limit has been surpassed, such apparatus can itself be the cause of fire. The reaction of the thermostat to overheating is often too slow, such that a fire has ignited before the thermostat registers excessive temperature and cuts off. It is believed that three or four fires occur every week in New Zealand through faulty heaters in spa pools or swimming pools.

It is an object of this invention to go at least partway towards providing a safer heater control, or one which will at least provide the public with a useful choice.

STATEMENT OF INVENTION An immersion heater to be fitted through an aperture in a fluid container, the heater including a wall to close the aperture, the wall having an interior face to contact the fluid in the container, and an exterior face, an electrical resistance heating element passing through said wall and sealed thereto, the element having a working length in which heat is generated when electric current is passed therethrough, said working length being positioned on that side of the wall having the interior face, and temperature sensing means located on that side of the wall having the interior face and in thermal contact with a part of said working length of the element.

OBJECT 23 9 9 9 4 ' 2407( Preferably the immersion heater includes a heat conductive housing extending through and sealed to said wall, the housing having a closed end positioned on the same side of the wall as the working length of the element, the housing being in thermal contact with said working length of the element and the temperature sensing means being located in said housing.

Conveniently, the temperature sensing means is arranged to stop the heating of said element upon actuation.

The sensing means may include a heat actuable circuit breaking switch, whereby an electrical circuit including the element can be disconnected by actuation of said switch, and wherein said disconnection is arranged to stop the heating of said element.

Preferably the housing and said one or more elements are formed at least in part from metal, and said housing is brazed or welded to said one or more elements.

Preferably the heater comprises two elements connected in parallel, with each element having a thermal fuse positioned electrically in series between the element and the mains supply to the heater.

In another aspect the invention provides water heating apparatus incorporating any of the above mentioned immersion heaters.

The invention is also concerned with a spa pool in configuration with such a heater.

As will become apparent from the following description of the preferred embodiment, it is preferred that the thermal fuses are separate from the thermostat controls and the contactor, as the contactor used in the control of heating elements for swimming pools and spa pools is sometimes the cause of the elements overheating.

These and other aspects of this invention, which should be considered in all its novel aspects, will become apparent from the following description which is given by way of example only, with reference to the accompanying drawings in which: 5269CSAM. 093/ms N.Z. PA7g?Ts" O'T-iCE 150GT1S93 | r:r a*;*'/" 23 9 994 2407 FIGURE 1 is a partial cross-section through a first preferred heating chamber for use with a spa pool or swimming pool; FIGURE 2 is a circuit diagram showing the placement of the thermal fuses in series with the heating elements; FIGURE 3 is a plan view of the heating chamber of Figure 1; FIGURE 4 is a plan view of a second preferred heating chamber; FIGURE 5 is a partial cross-section through a third preferred heating chamber. PREFERRED EMBODIMENT The preferred form of the present invention comprises a controller for an immersion heating element, for use in heating a spa pool or swimming pool, preferably comprising a pocket brazed or welded onto the heating elements, the pocket containing one or more thermal fuses, which are wired in series with the heating element(s) to prevent serious overheating of the heating elements and heating chamber, and thereby minimise the risk of fires. A twin element six kilowatt heater preferably has a pair of thermal fuses, each fuse being wired in series with each element, with each fuse having cut-out temperature Turning to Figure 1, a heating element for a spa pool or swimming pool is in the form of an immersion heater 10, in this case having two sets of elements 16 and 18 connected in parallel. Only one half of each set is shown in Figure 1, as the other half is positioned out of the plane of the paper, and for the purpose of this description has been omitted from the drawing, so that the fuse chamber can be better illustrated.

The heating chamber 10 has an inlet 11 and an outlet 12, inlet 11 being supplied with a flow of liquid from a pump or the like. The chamber conveniently has a screw thread, for reception of a boss 14 of the heating element, and may be provided with appropriate gaskets or seals 15 to provide a substantially watertight fit between the heating element and the chamber 10. The boss 14 is conveniently formed of brass, with the heating elements typically being formed of stainless steel, to minimise corrosion. However any of 60°C. 2c> 9 9 9 2407 suitable material may be used for the heating elements and the boss, depending upon the liquid, the temperature of the heating elements, and other design constraints.

Preferably the boss has a chamber 20 extending downwardly to the liquid within the heating chamber and brazed or welded to the heating elements. The chamber 20 is in the form of a socket or finger, preferably copper or brass, having a closed base 21 and being open at the top, for the reception of electrical leads 22,23 connected to thermal fuses 24, 25. These thermal fuses are designed to break the circuit if the temperature of the fuse exceeds a predetermined value. In this example, the thermal fuses have a temperature limit of 60°C, so that the circuit will be broken before the water in the chamber 10 boils.

Preferably the thermal fuses, and the wires 22 and 23 inserted into the pocket, are electrically insulated from one another and from the metal of the pocket 20, and the wires 22 and 23 are wired in series with the elements as shown in Figure 2.

A preferred thermal cut-off means is MICROTEMP (trade name) thermal fuse from Therm-O-Disc of the U.S.A. This type of thermal fuse is claimed by its manufacturer to provide an accurate, reliable solution to upper limit temperature protection. The MICROTEMP features include: Compact size facilitates mounting in confined spaces; High current interrupt capacity for applications requiring up to 25 amps; One shot operation cuts off electrical current permanently; Low resistance (<5 milliohms).

Variety of special configurations and mounting packages available.

The active trigger mechanism of the thermal cutoff is a MICROTEMP formulated, electrically non-conductive pellet. Under normal operating temperatures, the solid pellet holds spring loaded contacts closed. When a predetermined temperature is reached, the pellet melts, allowing the springs to relax, sliding a first contact away from the contact lead and the circuit is opened. Once the MICROTEMP thermal cutoff opens a circuit, the 5269CSAM.093/ms 23 9 9 9 circuit will remain open until the MICROTEMP thermal cutoff is replaced. This replacement helps ensure correction of the fault condition before the product can be operated again.

It will be appreciated that a wide variety of other fuses might equally be used, the above thermal fuse being an example only. Furthermore, other temperature actuated control elements such as thermostats might alternatively be mounted in the pocket 20.

As shown in Figure 3, it is preferred that the pocket 20 be welded or brazed onto the elements, so that heat is transmitted directly from the elements to the pocket 20. Under normal circumstances the flow of water through the chamber 10 will serve to cool the elements and the pocket, and both will remain at a predetermined, acceptable temperature. If for any reason water flow ceases, and/or either or both elements start to overheat, the change in temperature will be transmitted directly to the pocket 20, and thereby activate the thermal cutoff. This is considered to be preferable over indirect heat transmission from the elements, through water and/or steam in the chamber 10, to a separate pocket 20. Steam in particular is a relatively poor conductor of heat, and in cases of rapid overheating, the elements might become dangerously hot before the water or steam in the chamber 10 has transmitted sufficient heat to the pocket 20 to activate the thermal cutoff. Other methods of ensuring thermal contact between the pocket and the element(s), such as adhesives, straps or other fasteners might also be used to provide enhanced heat conduction.

It is preferred that the pocket 20 be positioned at the top of the chamber 10, rather than at the bottom of the chamber or projecting inwardly from one side. It will be appreciated that if for any reason the chamber 10 is partially emptied, the upper part of the elements, exposed to air, may overheat while lower parts are still covered and cooled by flowing water. The pocket 20 is however preferably brazed to the elements 16 and 18 below the uppermost entrant stem portion 19, which does not in practice actively heat.

The following dimensions relate to a prototype and are included for the purpose of illustration only, as the placement of the pocket and size and relative position of the pocket and elements may be varied to suit manufacturing or customer requirements. In this prototype, the elements 16 and 18 are 2 x 3000 watt elements, and are approximately 250mm long, and of 8mm diameter, looped back one on the other with a space in the 5269CSAM. 093/ma 7 2407 centre or to one side for reception of the pocket 20, which in this example is 80mm long and 12mm in diameter. The pocket is welded or brazed to at least one limb of each of the two elements, over a length of about 20mm, 60 to 80mm from the tops of the elements.

This arrangement is such that in use if the water in the chamber overheats because of a blockage, or malfunction of the pump, or malfunction of the heater, the increase in the temperature of the elements will be transmitted to the pocket 20, causing the thermal fuses to break the circuit when the temperature reaches substantially 60°C (or such other predetermined temperature depending upon the temperature chosen for each fuse). In this case two fuses are used, one for each element. As the elements are wired in parallel, it is preferred that the fuses have the same cut-out temperature. If the heating chamber is empty of liquid, and the heating element overheats, heat will again be conducted from the elements to the pocket, and the fuses will likewise cut-out when the temperature of the pocket and hence the fuses reaches the predetermined temperature.

Turning now to Figure 2, there is a schematic circuit diagram showing the arrangement of the fuses and the other controls for the heater.

A control box is connected through a high limit thermostat, an adjustable thermostat, and a pressure switch to a contactor coil, the contactor coil operating a relay which connects the mains to the heating elements. The purpose of the contactor coil is to electrically isolate the mains from the switches used to control the heaters. However, the contactor relay may malfunction, typically by sticking in the on position, resulting in overheating of the elements, and thus may cause a fire even though the temperature of the adjustable thermostat or the high limit thermostat is exceeded.

The adjustable thermostat is set by the user to control the temperature of the swimming pool or the spa pool, whilst the high limit thermostat is typically set at 45°C, and is designed to prevent injury to users, if the heater remains on, overheating the water in the spa pool. Despite the presence of these two thermostats, fires have occurred with spa pool heaters, as the thermostats are connected not to the heating elements but to the contactor coil, and thus if the contactor relay malfunctions, the thermostats cannot then switch off the mains supply to the heating element. By way of contrast, the presence of the thermal fuses between the mains supply and the heating elements means that if the heating elements overheat, the close proximity of the thermal fuses will break the circuit 23 9 9 9 4 to the heating elements, and will thus minimise the risk of a fire through the continued .

It is preferred that the thermal fuses are positioned in a pocket which is formed integrally with the heating element, so that in the event of a malfunction, it is a simple matter to remove the defective fuse and replace it with a new thermal fuse. This design also has the advantage that a new heating element with attached fuses can be retrofitted to existing spa pools. As shown in Figure 4 the pocket 20 need not be positioned centrally in the arrangement of heating elements, but rather might be positioned to one side, and welded or brazed to the limb of each element 16 and 18 on that side. It will be appreciated that the thermal fuses may alternatively be positioned on the inside of the boss, in thermal contact with the boss, and hence in thermal contact with the heating elements and to a lesser extent with the water in contact with the boss, or in another embodiment the thermal fuses may be in heat exchange with the liquid flowing into or out of the heating chamber, or may for example be positioned in a pocket or other chamber extending into the heating chamber so as to be in thermal contact with liquid in the heating chamber.

In other modifications, the heating elements are sometimes incorporated with the pump supplying liquid to the swimming pool or spa pool, and in such cases it is preferred that the thermal fuses are in thermal contact with the liquid in the pump/heating chamber.

It will be appreciated that the invention is concerned with the placement of thermal fuses in series with the heating elements, and in close proximity to the heating elements, so that overheating can be minimised as quickly as possible as excess heat will result in the thermal fuses breaking the circuit to the heating element.

It should be noted that the invention encompasses the use of a thermostat or other temperature actuated control element, positioned and arranged in the manner described above in respect of fuses. Alternatively as shown in Figure 5 the thermostat 26 may be regulated by a remote probe or thermometer 27 mounted in the housing 20, while the thermostat 26 itself is positioned elsewhere. This arrangement has the advantages of avoiding any restriction on the size or complexity of the thermostat used, and allowing the thermostat to be positioned in an easily accessible location. As shown, the thermostat may be provided in conjunction with one or more fuses 25 set to a higher temperature. application of power to the heating elements.

By this means moderate overheating may result in the heater being reversibly switched off by the thermostat, but if the thermostat fails or the heater continues to overheat for any other reason, the fuse can break a circuit to disconnect the elements.

In this example the actuation temperature of the thermal fuses has been chosen as 55° or 60°, with a view to preventing fires rather than substantial overheating of the water supplied to the spa pool or swimming pool. It will however be appreciated that by choosing a lower temperature closer to the temperature of the high limit thermostat, the thermal fuses (or additional thermal fuses) can be used to prevent dangerous overheating of the water supplied to the spa pool or swimming pool, instead of or in addition to the high limit thermostat.

We have found that the pocket should be in good thermal contact with the heater elements, and that this is best achieved by brazing a bronze pocket to each limb of the twin heater elements shown in the drawings. Other materials may be used for the pocket and other means of providing good thermal contact between the pocket and element(s) may be used. We believe it is preferable to use one pocket containing two thermal fuses but more than one pocket could be used.

It is possible for the pocket to physically or mechanically contact the element(s), though we have found that brazing or welding gives the best practical thermal connection between the pocket and elements. By "good thermal contact" we expect the heat actuable switch (whether it is a thermal fuse or thermostat) to cut-out within 40 seconds in a dry situation, ie, when the water supply has failed and before the elements start to glow red-hot.

If materials other than brass are used they should be assessed both in terms of thermal conductivity and resistance to corrosion. Stainless steel may be used as it is resistant to corrosion but it has a lower (though still acceptable) thermal conductivity than brass.

Finally, various other alterations or modifications may be made to the foregoing without departing from the scope of this invention, as characterised by the following claims:

Claims (15)

WHAT WE CLAIM IS:

1. An immersion heater to be fitted through an aperture in a fluid container, the heater including a wall to close the aperture, the wall having an interior face to contact the fluid in the container, and an exterior face, an electrical resistance heating element passing through said wall and sealed thereto, the element having a working length in which heat is generated when electric current is passed therethrough, said working length being positioned on that side of the wall having the interior face, temperature sensing means located on that side of the wall having the interior face and in thermal contact with a part of said working length of the element, and including a heat conductive housing extending through and sealed to said wall, the housing having a closed end positioned on the same side of the wall as the working length of the element, the housing being in thermal contact with said working length of the element and the temperature sensing means being located in said housing.

2. An immersion heater according to claim 1 in which the temperature sensing means is arranged to stop the heating of said element upon actuation.

3. An immersion heater according to claim 2 in which said sensing means includes a heat actuable circuit breaking switch, whereby an electrical circuit including the element can be disconnected by actuation of said switch, and wherein said disconnection is arranged to stop the heating of said element.

4. An immersion heater according to claim 3 in which said switch is connected in series with said element.

5. An immersion heater according to any of claims 1 to 4 in which said housing is formed at least in part from copper or brass.

6. An immersion heater according to claim 5 in which said housing is brazed to said one or more element.

7. An immersion heater according to claim 6 in which there are two of said elements, connected in parallel and the housing is brazed to both of said elements.. 5269C3AH.N93/mi 11 24 0

8. An immersion heater according to claim 3 or 4 in which the switch is not resetable.

9. An immersion heater according to claim 8 in which the switch is a replaceable fuse means.

10. An immersion heater substantially as described herein with reference to any one of the accompanying drawings.

11. Water heating apparatus, including a water container and an immersion heater according to any preceding claim, fitted through an aperture in the container.

12. Water heating apparatus according to claim 11, in which the temperature sensing means is located adjacent an uppermost heating part of said element, whereby upon a decrease in water level within said container said temperature sensing means is in a position adjacent a first part of the element to be exposed to air.

13. Water heating apparatus according to claim 11 or 12 in which the water container includes a water entry and a water exit, whereby water can be flowed continuously through the container.

14. Water heating apparatus substantially as described herein with reference to any one of the accompanying drawings.

15. A swimming pool, bath or spa pool installation including water heating apparatus according to any of claims 11 to 14. JOHN WICKHAM ROSSITER EDWIN ERNEST POYNTER By Their Attorneys JAMES W PIPER & CO S269CSAM>N93/aa