CA1152550A - Heater with distributed heating element - Google Patents

Abstract

ABSTRACT

A heater for heating liquids such as water in an aquarium comprises a water-impermeable housing including a heating section for immersion in the water. Within the heating section of the housing is a flexible, distributed planar heating element. The housing is resistant to breakage from mechanical and operating abuse and has a high heat transfer coefficient and high heat distortion temperature.
To prevent the housing from melting when a control element of the heater malfunctions, preferably the aquarium heater includes temperature limiting means such as a circuit protection element comprising a material having a positive temperature coefficient of resistance. Preferably, the heating section of the housing is fabricated with large surface area walls to accommodate the planar heating element, and is provided with coupling means for maintainig the heating element thermally coupled to the walls of the heating section. The coupling means can be a ribbed support for holding the heating element against the heating section walls.

Description

BACKGROUND OF THE INVI~NTIO

This invention relates to heaters for liguids, and particularly aquarium heaters.

Aquarium heaters conventionally comprise wire-wound S heatins elements such as resistance heating wires wound about -a hollow ceramic core. The heating element is in series with a bi-metal thermostat which is set by means of a knob to open when the temperature of the air around the thermostat exceeds a selected temperature in the range of typically 25 to 45C.
The heating element is housed in a cylindric~l glass container, such as a test tube.

- Although these aquarium heaters are the standard of the industry, they often malfunction in use. In particular, the glass housing is relatively fragile and can develop cracks.
Mechanical abuse can cause cracks or fractures in the housing.
A cracked housing can also result from operational abuse, uch as ope~ating the heater with a low water level in the aguarium, and then filling the aquarium with cold water, thereby thermally shocking the glass. Because water can then leak into the housing, current leaka~e can occur, which can result in electro-cution of fish in the aquarium, and sometimes even a shock to an ichthyologist.

Another problem with conventional aquarium heaters is that the bi-metal thermosta~ can malfunction, usually in the closed mode or can by human error be set in the fully closed mode. If this occurs, the fish can be killed.

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Another form of operational abuse that occurs is ~peration of the aquar ium heat2r when it i~ not immer~ed in water, such as when the heater is removed from the aquarium for cleaning of the aquarium. The heater can then be a source of fire because the heater can get much hotter in air than in water.
In addition, if the hot heater is immersed in water, breakage of the glass housing can occur, with excessive leakage current - resulting.

In response to these problems with electric~heaters for aquariums, Underwriters Laboratories, Inc. recently promulgated UL 1018 standard regarding electric aquarium equipment. This standard, which took effect on ~anuary 1, 1979, reguires for UL
approval that in normal operation, aquarium heaters have a leakage of current which does not exceed 0.5 milliamperes. Under abnormal use, a heater is required to: ~1) be able to be ~dropped three times from a height of three feet off a hardwood surface; (2), be suspended from a three foot length of flexible cord and released so that it swings by the cord against a vertical wall of hardwood; and (3~ be able to take a crushing force using a 12-inch square flat steel plate with a crushing load of 100 lbs. force. In spite of such abuse, the heater is required to remain intact with a leakage current of no more than O.S milliamperes. Aquarium heaters with a glass housing cannot pass these mechanical abuse tests.

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There are other UL tests that heaters with glass enclosures cannot satify. ~or example, an immersible heater with its control set for maximum heating is required to be able to operate in free air until it is well heated and then dipped suddenly into water at room temperature. The aquarium heater is req~ired to undergo this test five times, and still exhibit no dielectric brea~down, demonstrate no adverse mechanical or operational e~fects, and bave no increased likelihood of fire, electric shock or injury to a person.
Another ~L test ~hat conventional heaters cannot pass is the fire test. In this test, the heater i5 placed on a softwood board. The heater is covered with cbeesecl~th, set to maximum heat, and operated for 7 hours. No glowing ~r fl~ming of the cheesecloth or tissue paper i8 permitted.

It is evident that there i~ a need for ~n immersible beater suitable for use in aquariums, where the heater performs satisfactorily, not only in normal oper~tion, but al60 when subjected to aissuse ~nd abuse ~n ordinary and e~traordinary operation.

:' SUMMARY

The present invention is directed to an immersible ~eater ~uitable for liquids, and particularly ~ater, ~ith these features. A particular feature of the heater ~ that its housing is not formed of glass, but rather comprises a strong, highly heat conductive, heat resistant ~aterial having a 55~

notched Izod impact strength of ~t least about 0.5 ft-lbs~in, a heat transfer coefficient of at least about 2 BT~-inches per hour-square foot-F, and a heat distortion temperature at 264 PSI of at least about 350F. Preferably, the housing is al~o electrically insulating. The preferred ~aterial is filled polyphenylene sulfide. Polyphenylene sulfide is ~uffi~iently ~trong that a housing made of it can suffer a great amount of mechanical abuse, ~nd still maintain its structural and electrical integrity. Bowever, replacing the giass hou5ing o a conve~tional aquarium heater ~ith a polyphenylene fiulfide ~ousing does not by itself yield a satisfactory aquarium heater. This is because a housing made of polyphenylene fiulfide can be melted by the hot resistance ~eating wire of the heating element, particularly if the bi-metal thermostat of the aquarium heater malfunctions in a closed mode.

Therefore, the heater of the present invention has as its heating element a distributed area heating element capable of providing a maximum of about 10 watts per square inch of surface area, and preferably at least about 1 watt per sguare inch of surface are~. Because of this low energy density, it is not possible to ~elt the housing during nor~al operation when the housing is immersed in water. To prevent melting of the housing if the bi-metal thermostat malfunctions or by error is set in a fully closed position ~nd the housing is not 2s immersed in water, preferably the heater ~ncludes temperature S5~
limiting means such as a circuit protection element comprising at least two electrodes and a PTC element ~Positive Temperature Coefficient of resistance element; described below) composed of a PTC composition. The circuit protection element is selected and positioned so that it is capable of preventing the heating element from heating the housing to a temperature at which the housing melts. Instead of or in addition to using a circuit protection element, the heating element can comprise a PTC composition that serves as the temperature limiting means.
Because of the low energy density of the heating element, preferably the housing provides a large heat transfer area and preferably the heating element is thermally coupled to the walls of the housing. A preferred large surface area housing comprises a hollow, thin, elongated heating section, the heating section being oval in cross section. Preferred means for thermally coupling the heating element to the housing comprises a support for maintaining the heating element against the internal surface of both of the wider side walls.
In accordance with ~he present invention, there is provided an electrical heater for heating a liquid comprising:
a. a water impermeable, electrically insulating housing comprising a hollow heating sec-tion for immersion in the liquid, the walls of the heating section being from 0.05 to 0.2 inches thick and being composed of a material having a notched Izod impact strength of at least 0.5 foot-pounds per inch, a heat distortion temperature at 264 psi of at least 350F, and a thermal conductivity of at least 2 BTU-inches per hour-square foot- F;
b. a distributed heating element within the heating section of the housing, the heating element, when the heater is connected to a 120 volt AC
power supply, having an energy output of 1 to 10 watts per square inch of surface area of the heating element, at least a part of said heating element ~",,.

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being in intimate engagement with the interior surface of the heating section;
c. an adjustable thermostat which prevents the heating element from heating the housing to a temperature above the heat distortion temperature thereof; and d. a circuit protection device which comprises a PTC element composed of a PTC composition, and which in the event of failure of said adjustable thermostat, prevents the heating element from heating the housing to a temperature above the heat distortion temperature thereof.

DRAWINGS
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings where:
Figure 1 is a circuit diagram of an aquarium heater according to the present invention;

- 6a 55~) Fig. 2 is a longitudinal cross-sectional view of an aquarium heater according to the present invention and having a circuit as shown in Fig. l;

Fig. 3 is a front elevation ~iew of the aquarium heater of Fig. 2;

Fig. 4 is a cross-sectional view of the heater of Fig. ~ taken on line 4-4 in Fig. 3; ~nd Fig. 5 is a cross-sectional view ~imilar to that of Fig. 4 of another heater according to the present invention.

DESCRIPTION

The prese~t invention is directed to immersible heaters for liquids, where the heater can resist mechanical and operational Abuse. The heaters described herein and shown in the figures are suitable for beating water in aquariums.
The heaters are also useful for heating liquids other than aquarium water, such as photographic developing solutions.

~ ig. 1 ~hows a circuit diagr~m f~r an aqu~rium heater 10 of the present invention and Fig. 2 shows ~ cross-sectional view of an aquarium heater of the present invention.
Referring to these figures, te~per~ture limiting means such as a circuit protection device 12 is connected in series with a heating element 14 and means for regulating the heating element for controlling the aquarium water temperature.
The regulating means can be a bi-metal ther~ostat 16 which is set by means of a knurled knob 1~ to open ~hen the temperature of the ~ir arouna it exceed5 a temperature ~n the ss~

range of 25 to 45C. A capacitor 18 (not shown in Fig. 2~
is connected in parallel with the therasostat 16. A plug 20 enables the heater 10 to be connected 1:o a 120 volt AC power supply (not shown). A neon lamp 22 and a current limiting resistor 24 (not shown in Fig. 2) are connected in parallel with the heating element 14 and the circuit protection device 12, so that the lamp 22 is lit when AC power i~
supplied via the plug 20 and the thermostat 16 is closed. A
second neon lamp 26 and a resistor 28 (not sh~wn in Fi~. 2) are connected in parallel with the circ~it protection device 12 so ~hat the lamp 26 i5 lit ~hen the device i5 activated, but not when the aq~arium heater 10 is in a normal operating mode. These comp~nents ~re secured to a molded plastic cap 30 which is attached to a bousing 32. A gasket 34 provides a water tight seal between the cap 30 and the housing 32.

In a preferred circuit of the present invention, tbe capacitor 18 is 0.01 microfarad, the resistors 24 and 28 are 220 kiliohms, and the neon l~mps 22 and 26 are ~odel numbers NE-2RR20 ~vail~ble from General Elect~ic. The heating element 14 is typically ~0, 75, or lOO watts. Both neon lamps ~re ~ounted under w~ter tight, translucent covers (no~ ~hown). ~he ~E-2RR2~ light ~vailable fro~ ~ener~l Electric includes in one component both the neon light and the 220 kilohms resistor.

The circuit shown in Pig. 1 ~ comparable to tbe ~`4 ~ f ~ O ~

circuit shown in Fig. 8 of co-assigned ~k~. Patent Applicntion -5C) Serial No. 340,998 flled on November 30, 1978 by ~ddleman, et al. The heater 10 of the present invention differs from the aquarium heater described in the '998 application in the type of heating element 14 used, and the sha~e, size, and operational characteristics of the housing 32. The heating element of the '998 application is a conventional, high tem~erature wire-wound heating element comprising resistanoe heating wires wound about a hollow ceramic core, while the heating element 14 of the heater 10 of the present invention is a low energy density, distributed heater, as described in detail below. m e housing of the heater described in the '998 application is a conventional small surface area, test tube shaped housing, while the housing 32 of the heating element 14 has a large surface area configuration.
Another im~ortant difference between the heater 10 of the present invention and that descriked in the '998 application is that the heating element 14 of the present invention is thermally coupled to the walls of the housing. A
large surface area housing and thermal coupling are needed because of the low energy density and relatively low temperatures of the heating element 14. By "thermal coupling", there is meant that the resistance to heat transfer frQm the heating element 14 to the housing 32 is minimized and is less than the resis-tance provided by the air gap between the glass housing and the wire-wound heat-ing element of cQnventional aquarium heaters. Such thermal ~255~

coupling can be provided by coupling means such as a heat ~ransfer fluid such as silicone oil between the housing and the - heating element, or preferably by maintaining the heating element in intimate engagemen. with the housing. As shown in Fig. 2, the preferred coupling means i~ a 6upport core member 36 which serves to main'ain the heating element 14 against the internal surface of the housing 32.

The housing and c~p preferably are fabricated of the same material for ease in securing them together and for convenience in molding. It is a sine qua non of the present invention that the housing is able to rem~in intact in spite of substantial mechanical abuse, including the abuse specified in the above-described tests of Underwriters Laboratories. The housing must be fabricated of a material having a notched Izod impact strength of at least about 0.5 ft-lbs/in. As used herein, the notched Izod impact strength of a material is determined according to ASTM test method D256. It has been determined that materials with such a high impact strength are satisfactory for aquarium heater housings. It should be noted that glass does not meet this impact strength requirement.

It is also desirable that the housing recist melting at elevated temperatures which can result from operatio~al abuse, i.e., operating the heater 10 when it is not immersed in water or only partially immersed in water. Therefore, it is preferred that he material used to form the housing has a i2~5~

heat deflection temperature at 264 psi of ~t least about 350F, and preferably at least about ~OODF. As used herein, heat deflection temperature is that temperature measured according to ASTM testing method D648.

It is desirable that the housing be able to rapidly transfer heat from the heating element to the surrounding medium, both for efficient utilization of electricity ~nd to avoid excessive heating of the housing. For rapid and efficient heat transfer to the surrounding medium, it i~
preferred that the housing be formed of a material having a thermal conductivity of at least about 1.5 BTU-inches per hour-square foot~ nd more preferably at least about 2 ~U-inches per hour-square foot-F.

Preferably, the housing is made of a material that is electrically insulating, i.e., substantially electrically non-c~nductive, to avoid current leakage into the water of an ~guarium. In addition, preferably the housing is formed of material that is not chemically degraded or dissolved by ~ solvents, and of course, not adversely affected by water. For ease in fabrication of the housing, preferably the housing is made of a material 1:hat is capable ~f being injection molded.

There is no available material which is perfect, especially when co~t is considered. Materials considered suitable for the housing include polypropylene, phenylene oxides, phenylene 6ulfides, polysulfones, polyether ketones, ~;255~

and polyether sulfones, all with and without fillersO Talc is not a satisfactory filler because it absorbs water. Corrosion resistant metallic housings can, in some instances, be suitable, although metallic housings are not ellectrically insulating.
~pon consideration of molding charactleristics, cost, mechanical properties, electrical propertieE, and thermal properties, the preferred material for the housing is glass filled phenylene ~ulfides such as those marketed by Phillips Chemical C~mpany ~ ,~fks under the trade ~s Ryton R-B and Ryton R lOo These materials can have a notched Izod impact strength of at least 0.5 ft-lbs/
in., a heat deflection temperature at 264 psi in excess of 500F, and heat transfer coefficient in excess of 2 BTU-inches per hour-square foot-F. Another advantage of polyphenylene sulfide is that it is relatively inflammable, having a UL 94 flammability rating of V-O. Both Ryton R-8 and R-10 are poly-phenylene sulfide resins filled with mineral and glass materials.

~ owever, even a housing 32 composed of polyphenylene sulfide can melt if subjected to operational abuse such as operation of the heater 10 in air with the thermostat 16 ~et at an elevated temperature, or if the thermostat malfunctions in a closed mode. Such melting, of course, would not occur with a glass housing. To avoid this melting, the heater 10 includes one or more temperature limiting means. The temperature limiting means is different from the temperature regulating means. The temperature regulating means is the primary 55~
control devlce that can be manually operated by the owner of the heater to main-tain the aquarium water at a desired temperature. The temperature limiting means is a back up safety element that usually is not manually operated by the owner and that prevents the heating element from beccmlng hotter than a desired temperature. The temperature limiting means can be no more than a wax pellet which melts at selected te~perature below the melting point of the housing, or it can be a back up bi-metal thermostat. Alternatively, the heating element can ccmprise a PTC ccmposition with a sufficiently low switch temperature Ts (i.e., the temperature at which the resistance of the PTC composition ccmmenoes to in-crease sharply) that the heating element cannot get so hot that it can cause thehousing to melt or soften. In this version of the present invention, there need not be a separate temperature limiting means, but rather the temperature limit-ing means is built right into the heating element. Suitable heaters having a built-in temperature limiting means are described in co-assigned U.S. Patent 4,177,376, December 4, 1979.
The preferred temperature limiting means is the circuit protection ele-ment or device described in the afore-mentioned application Serial No. 340,998.
This device is preferred because it is infinitely reversable, i.e., even if the devioe is tripped by excessively high temperatures, on oe the heater cools dcwn, the device switches back to an inactive m~de and allows the heater to continue to heat a~uarium water. Such a circuit protection devioe comprises at least two electrndes and a PTC element ca~posed of a PTC camposition. As described in application Serial No. 340,998, this type of circuit protection device generally comprises an electrically insulating jacket which surrounds the PTC element and the electrodes and through which pass the leads to the electrodes.
me most preferred circuit protection device is a PolySwitch* thermal limit device model No. TLD-01-AA150W sold by Raychem Corporation of Menlo Park, *Trade Mark -13-ss~
California. Such a device demonstrates a sharp increase of dR (change in resis-tance with change of temperature) at about 75&. The resistance of a TID-01-AP150W device at 50C is about 0.2 ohm, at 75C, it is about 1 ohm, at 100C it is about 15 ohms, and at 125 & it is about 200,000 ohms. A TLD-01-AP150W devi oe has a maxImlm pass current at 75F inactivated of 2 amps, a maximNm allowance inrush current at 120V of 10 a~ps, a residual current when activated (at 120v, in still air) of 0.02 amp, and a temperature after activation of 260F.

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~ ven if the heater 10 is provided with a circui~
protection device, a housing 32 could still fail by ~elting if a conventional heating element comprising resistance heating wires ~ound about a hollow ceramic core is used becau e the ~ire has ~ small ~urface ~rea and gets very hot to ~eliver the necessary BTU's. ~o overcome this problem, accordi~g to the present invention, the heating element 14 is a distributed heater that provides no ~ore than about 10 watts per square inch of surface area. As used herein, when ~ wattage value per sguare inch is given for a heating element, the basis is the surf~ce area of the heating element, i.e., the beating element 14 produces no ~ore than 10 watts per square inch surface area of the beating element 14. With such a low energy density, there i5 no danger of the bousing 32 failing by melting when the heater is in water, particularly wben it i5 composed of polyphenylene ~ulfide. Because of the temperature limiting means and use of the distributed he~ting elements, even ~f the heater is operated in ~ir, the housing cannot melt or ~often.

There is ~notber reason that it i5 important that the heating element produce no ~ore than ~bout 10 watts per ~quare inch. If the heating element produces more than 10 watts per sguare inch, there can be insufficient time for tbe circuit protection device 12 to respond to overheating. Thi~
problem can be overcome by using a bighly sensitive circui~

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protection device. However, this would create the problem of nuisanoe tripping.
To avoid an uneconcmically large and inconveniently bulky heater lO, the heating element 14 has an energy output of at least about 1 watt per square inch~ preferably an energy output fram about 3 to about 7 watts per square inch, and most preferably, an energy output of about 5 watts per square inch. Prefer-ably the heating ele~ent is sufficiently flexible so that it can conform to the configuration of the interior surface of the hollow housing 32 so that it can be thermally coupled to the housing.
Exemplary of heating elements having a sufficiently low energy density are conductive polymeric heaters, including temperature self-limiting conductive polymeric heaters; mesh metal heaters with a busbar at parallel edges; and thin film evaporative heaters formed by evaporating metal on a plastic substrate~ A
preferred heating element is one which ccmprises an electrically conductive film sandwiched between two sheets of electrically insulating material, where the electrically conductive film comprises a conductive material dispersed in a non-cQnductive colloidal silica binder, with a pair of spaced electrodes or busbars secured to the film. Such heating elements are described in U.S. Patent No.
3,179,544.

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An advantage of these preferred heaeing elements is that they can be ~ubjected to high temperatures ~ithout degradation. This is because the electrically insulating material can be ~sbestos, ~nd becau~e colloidal silica binders can withstand high temper~tures. In fact, a~ noted in Patent No. 3,197,5~4, during the manufacture of these preferred heating elements, they are heated to 350F.

Another advantage of using a beating element as described in Patent No. 3,179,5~4 is that it is fle~ible, and thus can easily be ~haped to conform to the configuration of the housing. A further advantage is that even if water leaks into the housing and the heating element becomes wet, the - resistance of this type of heating element does not decrease, but instead increases by no more than about 20~. Therefore, the heating element does not ~hort out. When the heater 10 is activated, the heating element can revert to normal operation by driving off the ~ater and drying out.

A further advantage of u~iny the heating element~ of Patent ~o. 3,179,544 is that the current flows between the electrodes in ~ path perpendicular to the longitudinal &xi~ of the electrodes. Therefore, there i~ an infinite number of parallel flow paths, so that even if a portion of the heating element becomes damaged, the heating element can continue to operate.

S5~) ~ eating elements described in Patent No. 3,179,544 are commercially available from Raychem Corporation under the trademark CelloTherm~. CelloTher~ heating elements compri~e qraphite as the conductive ~aterial and asbestos as the electrical insulating material. They can easily be connected to the thermostat 16 by stapling each busbar of the heater to one of the metal strips of the thermostat or to connecting leads.

As noted earlier, the h~using needs to provide a large heat transfer surface area because o~ the low energy density of the distribut~d heating element. Shapes that are suitable for the housing include cylindrical, spherical, cubical, oval in cross section, and rectangular in cross ~ection. A preferred shape is rectangular in cross section because it maximizes the surface area to volume ratio of the housing. Another advantage of a rectangular shape is that is matches the generally rectangular shape of the aquarium. A
preferred housing is shown in Fig. 2. Another preferred housing is oval in cross-section, as ~hvwn in Fig. 5. ~ousings oval in cross section e~hibit less stress cracking than housings rectangular in cross-section, which are subject to such cracking at the corners.

As shown in Fig. 2, the ~ater-impermeable housing ~? 7~e~; na~s rl5 - 32 concl~c_ an upper, open portion 40 to whlch the cap 3~
is secured and a hollow, thin, elongated, wedge-shaped heating section 42 for immersion in water in an aguarium.

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The upper portion of the housing includes a depending flange 44 supporting a mounting screw 46 that i~ used to fasten the heater to an aquarium wall. The ~,crew 46 presses against the outside ~all of an aquarium while ribs 48 hold the heating section of the housing away from the inside wall of the aquarium.

As shown in Fig. 4, the heating section 42 comprises four substantially planar side walls, two opposing side walls ~0 being substantially wider than the other two side walls 51.
This shape provides a large surface area for ~eat transfer from the heating element to t~e ~edium to be heated, and has a high surface area to volume ratio. Due to the low energy densi~ies of the distributed heating element 14, such a large surface area is required. For e~ample, for a heating element having an energy output of S watts per square inch, 20 ~quare inches of effective surface area are required to have a standard size 100 watt aquarium heater. ~or n conventional wire-wound heating element, the effective surface area typically is only about 6 inches or less.

For good thermal coupling between the heating element 14 and the housing, the heating element is maintained ag~inst the walls of the housing. Thi~ can be effected by many techniques, including (1) bonding the heating element to the housing by ~n adhesive such as an epoxy adhesive; ~2) molding the heating element into the wall of the housing; t3) softening the housing by heat or solvents ~nd embedding the heating element into tSe wall of the housing; (4) relying on the inherent ~sprin~iness~

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of a CelloTherm~ heating element by r~lling the heatinq element into the shape of a cylinder and placing it inside a tubul~r ` h~using; or (~) using ~ support member for holding the heating element in place.

The support member can be a beat expandabie member ~uch as a beat expandable core of memory plastic or ~emory metal that upon heating irre~ersibly expands to press the heating element aqain~t the ~alls of the hou~ing. The heat for expansion can be provided by the heating element it~elf. The support member can also be a spring member. Preferably, the support member is the rigid support core 36 a5 shown in Fig. 2.

As shown in ~ig. 2, the heating element 14 is bent into a narrow ~U~ so that it can be held by the support core 36 against the internal surface of each of the wider side walls 50. The bottom of the ~ i5 cut out, e~cept for the conductors.
If this were not done with CelloTherm~ henters, then undesirable cracking could occur at ~he bend of the ~a The support core 36 is wedge-shaped so tbat it conforms to the ~hape ~f the ~eating ~ection of the housing.
~o 8y makinq both the support 36 ~nd the heating section 42 wedge-shaped, the ~upport 36 can act to press the heating element 14 against the internal 6urface of the heating ~ection for optimum heat t~ansfer to the liquid medium surrounding the housing. The support 36 has a plurality of projections such a~
parallel, ~paced apart r~bs 54. The ribs ~re ~ubstanti~lly ~;255~) parallel to the longitudinal a~is of the heating section of the housing. Each of the ~ider ~urfaces of the core has these ribs 5~. The heating element is wrapped around the core, i.e., the support core 36 sits ~ithin the ~U~O

The core 36 can be made of the same material that the housing 3~ is made, such as polyphenylene sulfide. ~ow-ever, to minimize cost, the core can be made of less expensive material, such as polypropylene. Although polypropylene does not have the high melt temperature possessed by polyphenylene sulfide, softening, or even local melting, of the core does not result in a dangerous situation as does failure of the housing.
Therefore, the core can be made of ~ less e~pensive, lower performance material than polyphenylene sulfide.

As shown in Fig. 4, there is a gap 58 in the ribs for mounting of the circuit protection device 12. The location at which the circuit protection device is mounted in relationship to the ~ater level and the heating element is important to the proper operation of the heater 10. The circuit protection device needs to be located in a position ~here it can sense the temperature of the he~ting element. As ~hown in Fig. 2, it is desirable th~t the circuit protection device be proximate to both the top of the heating element snd the normal water level. At this location, the cir~uit protection device is capable of tripping before the housing melts, but the circuit protection device ~experiences minimal nuisance tripping.

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It is desirable that the wall~ of the heating section of the housing be as thin ~s possible for fast and efficient transfer of heat to aquarium water. Therefore, preferably the walls are no ~ore than about 0.2 inch ~hick. ~oweYer, if the walls are made too thin, pin holes can result during the molding operation, providing a ~ater leakage path through the housing, and the h~using may not have adequate strength to pass the UL crushing test. ~herefore, preferably the ~alls are ~t least about 0.05 inch thick. ~ptimally, tbe walls are sbout 0.1 inch thick.
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The heater described herein has many substantial advantages compared to prior ~rt heaters. Not only does it satisfactorily heat water as does ~ conventional ~test tube" aquarium heater, but it is ~lso resistant to ~echanical and operational abuse. The housing resists cracking in situations where glass would normally crack. In addition, the housing, even when hot, does not crack when immersed in ccld water.
Furthermore, because of the distributed heating element 14 and the temperature limiting means, the housing is protected from melting. Because the heating element is pressed firmly against the housing by a ribbed support core 36, efficient heat transfer to water can be obtained. An aquarium heater according to the present invention ~atisfies the requirements of Underwriter Laboratories standard lOlB, something no known commercially ~vailable heater can ~ccomplish.

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Although the present inventlon has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. ~or esample, instead ~f one heating element 1~, two heating elements in par~llel or series can be used. Further~ore~ t~o or more temperature limiting means can be used. Therefore, the spirit and scope of the appended claims ~hould not necessarily be limited to the description of the preferred versions contained herein.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrical heater for heating a liquid comprising:
a. a water impermeable, electrically insulating housing comprising a hollow heating section for immersion in the liquid, the walls of the heating section being from 0.05 to 0.2 inches thick and being composed of a material having a notched Izod impact strength of at least 0.5 foot-pounds per inch, a heat distortion temperature at 264 psi of at least 350°F, and a thermal conductivity of at least 2 BTU-inches per hour-square foot-°F;
b. a distributed heating element within the heating section of the housing, the heating element, when the heater is connected to a 120 volt AC power supply, having an energy output of 1 to 10 watts per square inch of surface area of the heating element, at least a part of said heating element being in intimate engagement with the interior surface of the heating section;
c. an adjustable thermostat which prevents the heating element from heating the housing to a temperature above the heat distortion temperature thereof; and d. a circuit protection device which comprises a PTC
element composed of a PTC composition, and which in the event of failure of said adjustable thermostat, prevents the heating element from heating the housing to a temperature above the heat distortion temperature thereof.

2. A heater according to Claim 1 in which the heating element comprises a conductive polymer.

3. A heater according to Claim 2 in which the conductive polymer has a positive temperature coefficient of resistance.

4. A heater according to Claim 1,2 or 3 in which the housing is composed of polyphenylene sulfide.

5. A heater according to Claim 1,2 or 3 which further comprises a thermal coupling means for maintaining the heating element thermally coupled with the housing.

6. A heater according to Claim 1,2 or 3 which further comprises a thermal coupling means comprising a support core member which biases the heating element against a plurality of the walls of the heating section.

7. A heater according to Claim 1,2 or 3 in which the hollow heating section is oval in cross-section.

8. A heater according to Claim 1,2 or 3 in which the hollow heating section is wedge-shaped.

9. A heater according to Claim 1,2 or 3 wherein the heating element has an energy output of 3 to 7 watts per square inch.