Self-regulating Electric Fluid-sump Heater

Abstract

A self-regulating temperature-controlling semiconductive element having a so-called anomalous positive temperature coefficient (PTC) of resistivity is sandwiched between electrical contacts attached thereto and encapsulated within electrically insulating but thermally conductive material to form an assembly to be placed in heat-exchange relationship with a fluid the temperature of which is to be controlled. In particular, the assembly is immersed in oil within an oil sump of a compressor, internal-combustion engine or the like; or attached to the outside of the casing of such a sump. The anomalous PTC characteristic of the heating element results in maintaining a desired stable and safe fluid temperature. Thus oil may be safely maintained at a temperature to prevent piston clogging mixtures of cold oil and refrigerant such as "Freon" in the case of a compressor, or to thin cold engine oil for easier starting.

Description

BACKGROUND OF INVENTION

In refrigerator compressors refrigerant such as "Freon" may as a liquid migrate from the condenser into the compressor oil sump if the compressor is relatively cold. Here it is damaging to the operation of the compressor. Therefore it is desirable to employ a sump heater to maintain the compressor at a temperature above that of the condenser, so as to prevent such migration.

Formerly, constant-resistance heaters were used for heating of fluids such as oil in compressor and engine sumps. These were not self-regulating. This was both uneconomical and sometimes dangerous due to overheating.

The advantage of the heating element used according to the invention resides in the fact that due to the anomalous PTC resistance characteristic, the temperature of the heating element will not exceed a safe value. This is true 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 will remain relatively constant as will the heater and fluid temperature. 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.

In contradistinction, in prior-art devices employing resistance heaters, the amount of power is determined by the voltage applied. Due to the v.sup.2 /R expression, a voltage increase results in a power and temperature increase, and this temperature rise is added to the existing ambient temperature, whatever it is. Therefore, additional heat is added to a compressor which employs a constant resistance heater during operation even when it is not needed, which is detrimental to the life and safety of the insulation and the oil.

Initially, upon 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 the well known I.sup.2 /R expression 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 raise the fluid temperature. During low ambient temperature conditions, the heating element resistance remains low (as at R in FIG. 8), even though the heating element is at the anomaly because of the heat sink effect of the cold oil 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. By means of the invention, this optimum utilization of power without wastage or excess generation, avoids the disadvantages of the prior art mentioned above.

Referring to the drawings:

FIG. 1 is a top view of a fragmentary section of an oil sump illustrating an enclosed immersion-type heater made according to the invention;

FIG. 2 is a longitudinal vertical section as indicated by line 2-2 of FIG. 3;

FIG. 3 is a horizontal cross section, as indicated by line 3-3 of FIG. 2, parts being shown in elevation;

FIG. 4 is a side view showing application of a second form of the invention;

FIG. 5 is a vertical section taken on line 5-5 of FIG. 4;

FIG. 6 is a vertical section taken on line 6-6 of FIG. 5;

FIG. 7 is an enlarged isometric view of another form of the invention; and

FIG. 8 is a chart illustrating various temperature functions of a heating element having anomalous PTC characteristics.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Referring to FIGS. 1--3, there is illustrated at numeral 1 an oil sump of a compressor, internal combustion engine or the like. This carries oil 3 which is circulated by suitable means (not shown) to lubricate the machine parts. As known, such oil under certain low ambient temperature conditions becomes thick and sluggish, making starting of the apparatus difficult while causing other difficulties such as above pointed out in connection with compressors employing "Freon" or the like as a refrigerant.

At numeral 5 is illustrated a metal heat-conductive housing in the form of a flat lancelike steel wall, terminated by a supporting plate 7 in which is a slot 8. The housing 5 may be introduced through an opening 9 in the sump 1. The plate is welded, brazed or otherwise suitably attached and sealed to the sump 1. An appropriate size of the housing 5 is 1 .times. 0.25 .times. 3.5 inches and a wall thickness of 60 mils. It freely transmits heat. If desired, fins (not shown) may be attached to housing 5 to improve heat transfer between the housing and the oil.

As illustrated in FIGS. 2 and 3, the housing 5 is hollow and contains a heater assembly. This assembly comprises a heater element of semiconductor material in slab form 13, this material being of the type having a PTC anomaly such as illustrated in FIG. 8. The slab 13 is diagrammatically shown. Its dimensions may for example be 1 .times. 0.75 .times. 0.125 inches, but any convenient size may be used. 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 resistance change. As the temperature rises, a temperature T with concomitant resistance R is reached (FIG. 8) beyond which the resistance rapidly increases with only a small increase in temperature (see resistance R' at temperature T'). This temperature may be for example, but without limitation, 250.degree. F.

Appropriate materials which have the desired anomalous 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 (BaPbTiO.sub.3), carbon-black-filled polyethylene or polypropylene polymers, or the like. Soldered or otherwise appropriately conductively attached to the opposite sides of the slab 13 are contact strips 15 which may be composed of "Kovar" or other contact means compatible with heater materials employed. Attached to each contact strip is one wire terminal 17 of a pair of leads 19 which have suitable terminations for attachment to a 240 volt 60 cycle power circuit (for example). The terminations and power circuit are not shown, being conventional.

A suitable electrically insulating but thermally conductive material 21 is infilled between the assembly 13, 15, 17 and the walls of the housing 5. This may be epoxy resin, silicone rubber or the like. Or it may be formed by a rubber or heat-shrunk "Mylar" covering containing the assembly. In the case of the epoxy filler, the containment of the assembly 13, 15, 17 in the housing 5 is permanent. In the other two cases, the assembly may be removed and replaced through the slot 8 in the plate 7.

In FIGS. 4--6 is shown another form of the invention in which the oil sump for the oil 3 is numbered 23. In this case the sump has no opening for the heater. Bolted exteriorly to the sump is a plastic housing 25 for containing the heater assembly. In this case the semiconductive slab is numbered 27, being sandwiched between conductive terminals 29 electrically attached thereto and having connections at 31 with connecting wires 19. The assembly 27, 29, 31 is encapsulated in an epoxy layer 33 for electrical insulation and heat conduction. It is pressed against a wall of the sump 23 by a spring consisting of a flat plate 35 from which spring-leaves 37 are struck. The spring 35, 37 presses the assembly 27, 29, 31, 33 against the outside of the sump 23. Appropriate dimensions of the heating member 27 for a 240 volt circuit are for example approximately 1 .times. 0.75 .times. 0.125 inches. The plastic housing 25 traps an air layer that resists efficient heat flow from member 27 to the ambient air outside of the housing 25.

In FIG. 7 is shown another form of the invention in which a round sleeve 39 of the semiconductor material having the PTC anomaly is electrically connected to a wire conductor 41. The sleeve 39 has a conductive strip 43 conductively connected to the outside circumference and extending in the direction of the wire 41. The connected assembly 39, 41, 43 is held in a metal cup 45 with flange 47 for attachment to a sump with the cup 45 extending through an opening provided therein and into the oil carried within the sump. Flange 47 will be suitably attached, such as by welding, to the sump casing, or by inserting into a mating recessed portion formed in the casing. Appropriate dimensions for the round heater are approximately but not restricted to 0.75 .times. 0.330 OD .times. 0.080 ID inches for a 240 volt circuit. Epoxy potting material is shown at 51.

In view of the above, it will be seen that the heater assembly is carried in a container extending through an opening in a sump and into the contained oil (FIGS. 1--3 and 7) or in a container which is attached to the sump wall (FIGS. 4--6). In both cases the heat-exchange relationship with the oil is close. This relationship may be made closer, if desired, by simply suspending the heater assembly in the oil but some type of encasement such as described is preferred.

In operation, when the heater is first excited and ambient temperature is low, the heater initially draws substantial power (watts) during initial heating of the cold oil. While ambient temperature is low, the temperature of the heater will not rise above T due to the large heat sink and since the power consumed will be relatively high. As the ambient temperature increases, the heater temperature will increase to T' because there is less dissipation of heat from the heater. The temperature increase from T to T' corresponds to a resistance increase of R to R' and a power decrease of Q to Q' (FIG. 8). Thus the device is especially economical due to the low power consumed when the temperature of the ambient increases so that the heater operates under conditions of continuous temperature regulation in the environs of T'. On the other hand, a conventional resistance heater draws power according to line voltage. Since power varies with the square of line voltage, large variations in power (and heating) will result from normal line voltage variations. This causes wide temperature variations. According to the invention under line voltage changes of as much as 10 percent, the controlled temperature does not vary substantially.

Among the advantages of the invention are the following:

1. The oil is not unnecessarily heated and any resulting breakdown or dangerous flashing is avoided;

2. Electrical insulation requirements for the heater are minimized since it is not required continuously to carry large currents. The heater will operate at temperatures not very much above 250.degree. F;

3. large energy drain from the power circuit is avoided when substantial heating is not required such as under higher ambient temperature conditions. This decreases operating costs;

4. Temperature is controlled without moving parts;

5. Increased safety is obtained. For example, when prior ordinary resistance heaters become detached from the sump while energized, they have been known to burn themselves up;

6. The heater operates to maintain substantially constant temperature conditions regardless of ambient temperature or expected over-voltage changes as high as 10 percent.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing 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.

Claims

I claim:

1. A fluid-sump heater comprising a hollow, flat housing having an apertured wall, flange means for attaching the housing to the side of a fluid sump, with said wall adjacent the outside of said sump a flat heater assembly therein, said heater assembly comprising a flat mass of resistance material having an anomalous positive temperature coefficient of resistance, electrical contacts conductively attached thereto on opposite sides, line terminals connected with said contacts, a rigid flat block of thermally conductive electrical insulation encapsulating said assembly of the material, contacts and line terminals adapted to engage an outside surface of the sump in close heat-exchange relationship, and spring means within the housing for pressing the flat encapsulated assembly through said wall aperture against the outside of the sump.

2. A heater according to claim 1 wherein the flat mass of material is approximately centrally located in said insulation.

3. A heater according to claim 1 wherein said flat mass material is selected from the group consisting of carbon-black-filled polyethylene and polypropylene polymers, doped barium titanate, doped barium strontium titanate and doped barium lead titanate.