Heating Apparatus

Abstract

A heater assembly is disclosed comprising a flat mass of positive temperature coefficient of resistivity material attached to a first heat sink plate which extends beyond the flat mass in length and width. The flat mass has a flame sprayed layer of aluminum on opposite sides, a second flame sprayed layer of copper on at least one opposite side to permit attachment of a terminal, and the other opposite side being attached to the heat sink plate by electrically and thermally conductive solder or epoxy material. The flat mass and plate in one embodiment is received in a shrinkable electrically insulative sleeve and in another embodiment in a thermally conductive can, both the sleeve and can being sealed so that the assembly can be immersed in fluid for heating thereof. The heaters of both embodiments are received in a pocket formed in a second heat sink formed of elongated flexible foil having pressure sensitive adhesive on one surface to facilitate attachment of the assembly to a surface to be heated.

Description

BACKGROUND OF INVENTION

This invention relates to heaters and more particularly to self-regulating heaters. There are many applications in which it is desired to provide an inexpensive heater which dissipates large quantities of heat when a demand for heat is indicated and one which can be used either in moisture laden ambient or actually submerged in liquid.

For instance, in air conditioner apparatus there is a need for maintaining the temperature of the compressor above that of the condenser to prevent migration of the refrigerant, such as "Freon" in a liquid form, from the condenser into the compressor. Constant resistance heaters have been used for heating of fluids such as oil in compressor and engine sumps; however, these are not sufficiently economical since further temperature controls are required to limit the heat output of the heater. Further, since such control normally entails thermal overshoot, that is cycling within a range from a minimum to a maximum regulation temperature, there is a danger of either over-heating if the range is chosen so that maximum heat production can be achieved or sluggishness and having a slow response if a safety factor is used, that is if the range is chosen well under the maximum allowable or safe temperature.

In coassigned U.S. Pat. No. 3,564,199 a self-regulating heater particularly useful for heating oil in a compressor sump or the like is described and claimed which overcomes the above-mentioned limitations. Essentially that heater has a positive temperature coefficient (PTC) of resistivity so that the temperature of the heating element will not exceed a safe value even with normal changes in ambient temperature and voltage. Only the power dissipated determines the amount of power that will be consumed by the heating element. An increase in voltage drives the resistance to a higher value and due to the P = V.sup.2 /R relationship, the power remains relatively constant as do the heater and fluid temperatures. An increase in ambient temperature also causes the resistance to increase, and due to the P = V.sup.2 /R relationship, this increase serves to reduce the amount of power thereby preventing the fluid temperature from exceeding safe limits. In other words, the heat supplied by the heater varies with changes in ambient temperature. In both these conditions, where an ambient temperature and voltage change occurs, the temperature of the heating element remains relatively constant due to its anomalous PTC characteristic.

Upon initial energization, the PTC heater resistance R is low so that it draws a comparatively large current I and generates a comparatively large amount of power due to I.sup.2 /R and causes the PTC to heat. When the heating element reaches its anomalous temperature, it self-regulates to produce an amount of heat sufficient to balance heat dissipation. During low ambient temperature conditions, the heating element resistance is lowered even though the heating element is near the anomaly because of the heat sink effect of the cold oil in which the heater is either immersed or in heat transfer relationship therewith, which increases the heat dissipation of the heating element and due to the V.sup.2 /R relation, a large amount of heat is generated. At high ambient temperatures the PTC heating element dramatically increases its resistance concomitantly decreasing the power generated. Thus the PTC heater optimizes utilization of power without wastage or excess generation. However, employment of PTC heaters as described does have certain limitations. One such limitation is that to be effective, the size of the PTC elements must be comparatively small. It becomes proportionately more expensive and more difficult to produce a PTC element as its size is increased. Another related limitation is a phenomenon called "banding" in large PTC elements. This phenomenon is associated with the inherent poor thermal conductivity of ceramic PTC material and evidences itself by a small banded portion reaching temperatures above the anomaly at the same time the temperature of the remainder of the element remaining below the anomaly and thus diminishes the total amount of heat generated and deleteriously affects the self-regulating characteristics of the PTC element. The thicker the PTC element the more likely this phenomenon can occur. Thus there is a finite limit to the size of the PTC element which can be provided to meet applications in which not only a high rate of heat generation but also a large quantity of heat is desired.

It is therefore an object of the present invention to provide a heater which is inexpensive to produce, reliable in operation and one in which the possibility of banding phenomenon occuring is minimized. Another object of the invention is the provision of a PTC heater construction which is particularly suited for generating large quantities of heat and one which can be operated in a moisture ladened atmosphere and even immersed in fluid as well as in normal atmospheric conditions.

Briefly, the heater assembly of the present invention comprises a first heat sink, larger in width and length than the PTC element, in close thermally conductive relationship with the PTC element, the heat sink and PTC element received in a sealed housing which in turn is received in a pocket formed in a second flexible, elongated foil heat sink. Heat generated in the PTC element is efficiently dissipated from the element to the body to be heated by the combination of heat sinks. The foil is provided with an adhesive backing to facilitate attachment to any convenient surface.

Referring to the drawings:

FIG. 1 is a top plan view of a PTC element mounted on a heat sink plate;

FIG. 2 is a cross sectional view taken on lines 2--2 of FIG. 1;

FIG. 3 is a top plan view of a sealed housing containing the FIG. 1 PTC element and heat sink plate;

FIG. 4 is a cross sectional view taken on lines 4--4 of FIG. 3;

FIG. 5 is a top plan view showing the FIG. 3 housing received in a pocket formed in a flexible, elongated foil;

FIG. 6 is a top plan view of a second form of sealed housing, partly broken away, containing the FIG. 1 PTC element and heat sink plate;

FIG. 7 is a cross sectional view taken on lines 7--7 of FIG. 6;

FIG. 8 is a top plan view showing the FIG. 6 housing received in a pocket formed in a flexible, elongated foil; and

FIG. 9 shows a view similar to FIG. 7 but of another form of the invention in which a plurality of PTC elements are employed.

Similar reference characters indicate corresponding parts throughout the several views of the drawings. Dimensions of certain of the parts as shown in the drawings may have been modified or exaggerated for the purpose of clarity of illustration.

Referring to FIGS. 1 and 2 there is illustrated a heater unit 10 comprising an element 12 mounted on plate 14 formed of a good thermally and electrically conductive material, such as copper. Element 12 may be in the form of a flat mass of ceramic like material having a PTC characteristic at temperatures above an anomaly. While any convenient size element can be used, keeping in mind that the thicker the pill the greater the chance of thermal banding occuring, one which may be used for example is 1 .times. 0.50 .times. 0.150 inches. Examples of material which have the desired PTC characteristics are, for example, lanthanum-doped barium titanate (Ba.sub..997 La.sub..003 TiO.sub.3), doped barium strontium titanate (BaSrTiO.sub.3), doped barium lead titanate (BaPbIiO.sub.3, or the like. When such material is placed in a power circuit, it initially draws a substantial amount of current which rapidly raises its temperature to a certain value without substantial change in resistance. As the temperature continues to rise an anomaly temperature is reached beyond which the resistance rapidly increases with only a small increase in temperature.

Element 12 has electrically conductive layers 16, 18 on spaced opposite flat surfaces forming an ohmic contact. It is preferred to apply these layers by flame spraying aluminum as set forth in copending application, Ser. No. 340, filed Jan. 2, 1970, assigned to the assignee of the instant invention in order to maximize bond strength with low contact resistance. One layer of aluminum is then coated with a solderable layer 20, such as copper. While copper layer 20 can be applied by flame spraying in a manner taught in the aforementioned application, Ser. No. 340, any convenient method may be employed since there is no substantial difficulty in achieving a good electrical and mechanical bond between the aluminum and copper layers. In order to improve heat transfer from element 12 to the first heat sink plate 14 it is preferred to attach the element by using a thin layer 22 of electrically and thermally conductive epoxy such as C-409 of Amicon Corporation, Lexington, Massachusetts, an epoxy having approximately 60 to 70 percent of silver by weight. A thin layer of epoxy is placed between plate 14 and element 12 and then cured as by baking for a half hour at 300.degree.F.

For convenience of manufacturing as well as lower cost, element 12 may alternatively be attached to heat sink plate 14 by solder such as a tin-lead solder. The solder forms a thermally and electrically conductive layer between the element and the heat sink. When solder is used in lieu of the epoxy, both opposite surfaces of the element 12 are coated with copper.

In order to decrease the total thermal resistance of the assembly, plate 14 also may serve as a terminal member and is provided with ears 24 which clampingly engage lead L1. Lead L2 is attached to copper layer 20 as by soldering at 26.

As seen in FIGS. 3 and 4, the basic unit 10 is placed in housing 30 which is made of any thermally conductive material such as aluminum. In order to electrically isolate unit 10 from housing 30, a thin sheet 32 of electrically insulative and thermally conductive material is placed therebetween. Sheet 32 may, for example, be a polyester film with acrylic or silicon pressure sensitive adhesive on opposite sides thereof to fixedly mount unit 10 in housing 30. It will be realized in the event that housing 30 is formed of an electrically insulative and thermally conductive material that sheet 32 need not be employed and that unit 10 can be attached directly to housing 30. Once unit 10 is mounted in the housing, the housing is infilled with appropriate thermally conductive, electrically insulative potting compound 34 to effectively seal the housing 30 from penetration by any moisture or fluid to which it may later be exposed.

Lastly, housing 30 is received in a pocket 36 formed in an elongated flexible foil 38 of aluminum or similar thermally conductive material. Foil 38, which may be 0.010 inches thick may be formed of a top and bottom sheet of foil each having one surface provided with pressure sensitive adhesive with pocket 36 formed therebetween so that housing 30 is maintained in close heat transfer relationship with the foil and the top and bottom foil layers in turn are maintained in close heat transfer relationship with each other as well as providing a convenient means for mounting the finished heater assembly to any desired surface. As seen in FIG. 5 the foil extends from the pocket in two opposite directions forming flexible wings. Thus foil 38 forms a second heat sink and greatly increases dissipation of heat from element 12 to the medium which is to be heated. Heat is conducted by the foil from both the top and the bottom of housing 30, due particularly to the large surface area of the foil, and efficiently transferred by conduction to the medium which is to be heated, e.g. oil in the crankcase.

The combination of the first and second heat sinks results in a heater with improved heat dissipation characteristics which in turn results in a heater which generates larger quantities of heat than comparable heater elements of the same size and thus minimizes the risk of thermal banding occurring in satisfying the particular heat requirements. Essentially this is made possible by the improved thermal path. As mentioned supra, when PTC material is initially energized in the cold state a relatively large quantity of heat is generated due to the low resistance of the PTC material (P = V.sup.2 /R). This high rate of heat generation is continued until the anomaly temperature is reached beyond which the resistance rapidly rises with concomitant decrease in heat generation. However, the improved heat path between PTC element 12 and that which is to be heated (the oil) by means of epoxy or solder bond of element 12 to first heat sink plate 14 and thence through housing 30 to elongated foil 38 effectively keeps the resistance of element 12 slightly below the anomaly until the temperature of the oil approaches the anomaly temperature due to the increased heat dissipation. In effect, as rapidly as heat is generated by element 12 it is transferred away, keeping the resistance of the PTC element low which results in maintaining a high level of heat generation for a longer period of time, that is until the temperature of the oil or whatever is being heated, approaches the anomaly. This combination of dissipating heat sinks therefore enables a small PTC element to generate more heat and further tends to minimize banding problems not only because a smaller PTC element can be used for a given heat demand, but also becuase of the improved means for transferring heat away from the ceramic body.

FIG. 5 shows an alternative housing 40 of heat shrinkable electrically insulative material such as irradiated polyolefin into which unit 10 is inserted. Housing 40 may be in the form of a sleeve which is heat sealed on one end 42 by inserting the end between two heated members which are then brought together to cause the material to soften and coalesce. End 44 may be closed with a clamp 46 and sealed with infilled potting compound 48 such as a silicon rubber sealant to produce a liquid seal. As seen in FIG. 8, housing 40 is then inserted in foil in the same manner as housing 30 shown in FIG. 5. In applications where the sealing requirements are less stringent or where a portective sheath is not required, the FIG. 8 assembly is particularly responsive to changes in heat demand since it is less massive than the FIGS. 3, 4 housing.

In certain applications where even more heat is required than can be obtained from the embodiments described above, we have found that mounting a plurality of PTC elements 12 on an enlarged heat sink plate 14', as seen in FIG. 9, and encased in housing 40' effectively avoids the problem of thermal banding. The PTC elements are connected in parallel by conductors 50 joining copper layers 20 of elements 12. Housing 40' is then placed in a pocket formed in foil heat sink in the same manner as described in the other embodiments.

As many changes could be made in the above constructions without departure from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

We claim:

1. A self-regulating heating device comprising:

an element composed of ceramic type material having a steeply sloped positive temperature coefficient of resistance, the element having first and second surfaces spaced from each other;

the first surface having a layer of aluminum bonded thereto and forming an ohmic contact therewith and a solderable layer of copper bonded to the aluminum layer;

the second surface having a layer of aluminum bonded thereto and forming an ohmic contact therewith;

a first heat sink comprising a plate of thermally conductive material;

a first terminal means attached to the heat sink;

a second terminal means soldered to the copper layer; and

electrically and thermally conductive epoxy bonding the aluminum layer on the second surface to the heat sink plate.

2. A self-regulating heating device as set forth in claim 1 in which the first heat sink plate extends beyond the element in length and width.

3. A self-regulating heating device as set forth in claim 2 including at least one additional element composed of ceramic type material having a steeply sloped positive temperature coefficient of resistance, the additional element havine first and second surfaces spaced from each other;

the first surface of the additional element having a layer of aluminum bonded thereto and forming an ohmic contact therewith an a solderable layer of copper bonded to the aluminum layer;

the second surface of the additional element having a layer of aluminum bonded thereto and forming an ohmic contact therewith;

electrically and thermally conductive epoxy bonding the aluminum layer on the second surface of the additional element to the heat sink plate; and

an electrical conductor soldered to respective copper layers to connect the elements in parallel circuit relationship.

4. A heater assembly comprising:

electrical resistor means including at least one element composed of ceramic type material;

first heat sink means including a thermally conductive plate, the electrical resistor means attached to the plate;

second heat sink means comprising an elongated flexible foil member formed with a pocket therein, the foil member extending substantially beyond the pocket, the electrical resistor means and the first heat sink means received within the pocket; and

means to electrically separate the electrical resistor means and first heat sink means from the second heat sink means.

5. A heater assembly according to claim 4 including a layer of pressure sensitive adhesive on a surface of the foil to facilitate attachment of the heating assembly to a surface to be heated.

6. A heater assembly as set forth in claim 4 in which the electrical resistor means has a steeply sloped positive temperature coefficient of resistivity whereby the heater assembly is self-regulating.

7. A heater assembly as set forth in claim 6 in which the electrical resistor means includes an element composed of a doped barium titanate.

8. A heater assembly as set forth in claim 7 in which the doped barium titanate is in the form of a flat mass having first and second surfaces, a flame sprayed layer of aluminum on the first and second surfaces of the flat mass, a flame sprayed layer of copper on the aluminum layer on the first surface, and means electrically and thermally attaching the second surface to the first heat sink plate.

9. A heat assembly as set forth in claim 8 including a first terminal wire attached to the first heat sink plate, a second terminal wire attached to the copper layer on the element, a shrinkable electrically insulative sleeve telescopically receiving and shrunk about the heater assembly therein, the sleeve having an end which is closed by heat sealing and a second end through which the terminal wires extend, the second end sealed by electrically insulative potting material whereby the assembly can be inserted into fluid material for heating thereof.

10. A heater assembly as set forth in claim 8 including a first terminal wire attached to the first heat sink plate, a second terminal wire attached to the copper layer on the element, a can of thermally conductive material having a closed end, depending walls and an open end, a sheet of electrically insulative and thermally conductive material with pressure sensitive adhesive on opposite sides thereof placed in the can on a depending wall portion, the heater assembly received in the can, the first heat sink plate having a surface opposite to that on which the element is mounted in contact with the sheet and electrically insulative potting compound sealingly filling the can.

11. A heater assembly according to claim 4 in which the electrical resistor means includes at least two resistor elements connected in parallel circuit relationship.

12. A heat assembly as set forth in claim 4 including a first terminal wire attached to the first heat sink plate, a second terminal wire attached to the resistor means, a shrinkable electrically insulative sleeve telescopically receiving and shrunk about the heater assembly therein, the sleeve having an end which is closed by heat sealing and a second end through which the terminal wires extend, the second end sealed by electrically insulative potting material to prevent entry of liquids and moisture into the assembly.

13. A heater assembly as set forth in claim 4 including a first terminal wire attached to the first heat sink plate, a second terminal wire attached to the resistor means, a can of thermally conductive material having a closed end, depending walls and an open end, a sheet of thermally conductive material with pressure sensitive adhesive on opposite sides thereof placed in the can on a depending wall portion, the heater assembly received in the can, the first heat sink plate having a surface opposite to that on which the element is mounted in contact with the sheet and electrically insulative potting compound sealingly filling the can.

14. A heater assembly according to claim 4 in which the foil member includes a top and bottom sheet, a layer of adhesive is provided on the same side of each sheet whereby the sheets are fastened together and the assembly can be fastened to an object to be heated.

15. A heater assembly according to claim 4 in which the foil member extends beyond the pocket in at least two directions forming flexible wings extending from the first heat sink means to facilitate attachment in close heat transfer relationship to surfaces of various configurations.