Thermal management apparatus for a sealed enclosure

[0002] Not applicable.

[0003] Not applicable.

[0004] This invention relates to sealed enclosures in which electronic components are housed, and more particularly, to apparatus for controlling the temperature within the enclosure to protect the components regardless of the temperature extremes to which the enclosure is subjected.

[0005] Certain electronic equipment, for example, traffic monitoring equipment, includes sensors and associated electronic processing circuitry housed in a sealed enclosure. To perform the traffic monitoring function, the enclosure is typically installed outdoors atop a pole or similar support. As a result, the enclosure and the equipment housed in it may be required to operate at temperatures ranging between −40° C. to +50° C. (−40° F. to +122° F.). At very high temperatures, ambient heat must be extracted from the enclosure; while in extremely cold conditions, heat must be supplied to the enclosure.

[0006] A traditional approach to thermal management in electrical and electronic equipment is use of heat sinks with components which cannot be allowed to get too hot. Heat sinks are used because their surface area is larger than that of the electronic components mounted on a heat sink and this helps to readily dissipate heat generated by the components. Heat transfer is often enhanced by forcing air flow over the heat sink, typically by use of a fan. The warm air drawn away from the components is then vented outside the enclosure in which the components are housed, typically through openings in the enclosure.

[0007] Another approach to thermal management involves use of liquids and thermoelectric cooling for heat extraction. Liquid cooling requires a working fluid, typically chilled water, and a pump to move the fluid from a source through a piping arrangement for heat produced by the components to be drawn to the fluid as it flows past the components. Thermoelectric cooling requires an electrical supply to operate. Again, forced air flow over the components may be used to increase heat extraction, and this requires openings in the enclosure so the heated air can be discharged to the outside environment. With heat sinks, liquid cooling, and thermoelectric cooling, the heat transfer is not 100% efficient, so a certain amount of heat remains inside the enclosure.

[0008] When electronic equipment is housed in a sealed enclosure, the above heat extraction approaches are not effective because air within the enclosure is not exchanged with the outside environment. Resulting heat build-up produces a temperature rise within the enclosure which is detrimental to the electronics. A means is therefore required to transfer heat from the equipment to the outside of the enclosure. In certain electronic applications in which components are housed in a sealed enclosure, a printed circuit board is used which has a copper inner-layer. Electronic components requiring heat extraction contact this copper layer. Edges of the board contact sides of the enclosure. While effective, this approach is very expensive. Further, this approach does not have the capability of disabling the heat extraction mechanism when the equipment is operating in a cold environment and heat needs to be retained within the enclosure to maintain the equipment within a desired band of operating temperatures.

[0009] Among the several objects of the present invention may be noted the provision of apparatus for extracting heat from a sealed enclosure, the heat being generated by electronic components housed within the enclosure;

[0010] the provision of such apparatus in which the enclosure is an outdoors installation and the temperature of the surroundings about the enclosure can vary widely from very hot conditions to extremely cold conditions;

[0011] the provision of such apparatus to function effectively to remove heat from within the sealed enclosure to the atmosphere when the outside temperature is warm or hot and the heat generated by the components can cause their operating temperatures to be excessively high and damage the components;

[0012] the provision of such apparatus to further have the capability of generating and retaining heat within the enclosure when the outside temperature is cold and the heat generated by the components is needed to maintain their operating temperature within a normal range of operating temperatures;

[0013] the provision of such apparatus utilizing heat pipes and heat sinks to accomplish heat transfer out of the sealed enclosure;

[0014] the provision of such apparatus to further utilize strip heaters to heat the components within the sealed enclosure when the outside temperature falls below a predetermined temperature;

[0015] the provision of such apparatus in which the heat pipes have a heat transfer capacity equal to or greater than the amount of heat produced by the equipment (including electronic components) housed within the enclosure but lower than the total amount of heat produced by both the equipment and the heaters;

[0016] the provision of such apparatus in which the heat sinks employed limit the maximum temperature reached within the enclosure at a predetermined maximum temperature outside the enclosure;

[0017] the provision of such apparatus to efficiently and effectively maintain the temperature within the sealed enclosure to a range of temperatures which allow all of the equipment to function properly and with no equipment failures occurring because of temperature conditions within the enclosure;

[0018] the provision of such apparatus which can be configured for the type of equipment housed within the enclosure, as for example, stacked modular electronic components; and,

[0019] the provision of such apparatus which provides a relatively low cost solution for maintaining the temperature within the sealed enclosure to a safe operating temperature.

[0020] In accordance with the invention, generally stated, apparatus is provided for controlling the temperature within a sealed enclosure in which are housed electronic components. Operation of these components produces heat, and operation of the components preferably occurs within a desired temperature range. A heat sink extends through a sidewall of the enclosure to conduct heat from within the enclosure to the atmosphere. Additional heat sinks are mounted within the enclosure in a thermal transfer relationship with the components. Heat generated by operation of the components is transferred to these heat sinks thus drawing the heat away from the components. A heat pipe which extends between the respective heat sinks now conducts the heat to the first said heat sink for the heat to be expelled to the atmosphere. A heat generator within the enclosure generates heat to heat the components. A thermostat senses when the temperature within the enclosure falls below a predetermined temperature to energize the heat generator. As a result, the temperature in the enclosure is maintained within the desired range by appropriately removing heat from or generating heat within the enclosure and controlling the heat transfer path. Other objects and features will be in part apparent and in part pointed out hereinafter.

[0021] In the drawings, FIG. 1 is a representation of a sealed enclosure installed outdoors on a support structure;

[0022] FIGS. 2A-2C are respective top, side, and rear views of the enclosure with apparatus of the present invention being shown installed in the enclosure;

[0023] FIG. 3 is a representation of one installation of strip heaters within the enclosure;

[0024] FIG. 4 is a similar representation of a second installation of the strip heaters;

[0025] FIG. 5 is a graph illustrating the total heat within the enclosure as a function of ambient temperature;

[0026] FIG. 6 is a simplified representation of thermal sources and thermal sinks within the sealed enclosure;

[0027] FIG. 7 illustrates one method of collection of heat generated within the enclosure by a heat pipe;

[0028] FIG. 8 illustrates a second method of heat collection;

[0029] FIG. 9 is a simplified representation of a heat pipe used with the apparatus of the present invention;

[0030] FIGS. 10A-10C respectively illustrate an evaporator, wick, and condenser portion of a heat pipe; and,

[0031] FIG. 11 is an electrical schematic for thermostat controlled strip heater portion of the apparatus.

[0032] Corresponding reference characters indicate corresponding parts throughout the drawings.

[0033] Referring to the drawings, FIG. 1 shows an enclosure indicated generally 10 in which electronic components are housed. In the application shown, enclosure 10 is mounted on a support S so to be above a roadway R over which pass automotive vehicles V. An imaging system and associated processing electronics are housed within enclosure 10 for observing traffic flow over the roadway and providing traffic information to a control site at which traffic flow over different portions of a road network is monitored. Operation of the imaging system and processor is described in co-pending U.S. patent application Ser. No. 08/965,942, titled Compact Video Processing System for Remote Sensing Applications. One of the features of enclosure 10 is that it is a sealed enclosure. For this and similar applications of the system described in the co-pending application, it is important that the components not be exposed to moisture, dust, dirt, and debris, and that insects and birds not be able to gain access to the enclosure. Heretofore, while the sealed enclosure provides these advantages, there has not been an efficient way to manage the heat load and heat requirements for the equipment housed within the enclosure.

[0034] Referring to FIGS. 2A-2C, enclosure 10 is shown to include a number (three) of circuit boards 12 which are mounted in a horizontal, stacked configuration within the enclosure. Electronic components E are mounted on the various circuit boards to perform the functions required of the imaging system. As described in the above referenced co-pending application a rear wall 14 of the enclosure is removable so the stacked circuit boards can be removed from within the enclosure. However, when this rear section of the enclosure is in place, the enclosure is completely sealed; that is, there are no other openings in any of the sidewalls 16, 18, front wall 20, or top and bottom walls 22, 24 respectively forming enclosure 10.

[0035] When installed, enclosure 10 is subjected to a wide range of temperatures. A typical range to which the enclosure is subjected is −40° C. to +50° C. (−40° F. to +122° F.), not taking into account any added effects of wind and humidity. As in any electronic circuitry, operation of the electronic components generates heat, and this affects the temperature within the enclosure. Since optimal performance occurs if the electronic components operate within a desired temperature range, it is important to control the internal enclosure temperature. Apparatus 30 of the present invention accomplishes this function.

[0036] The apparatus first includes a heat sink 32 which, as shown in FIGS. 2A-2C extends through rear wall 14 of enclosure 10. Heat within the enclosure transferred to heat sink 32 is conducted through the rear wall of the enclosure to the atmosphere. The portion of the heat sink on the outside of the enclosure has a large surface area so to allow a substantial heat transfer to the atmosphere. For this purpose, the heat sink includes a base 34 which extends substantially across the rear wall of the enclosure. The height of heat sink 32 is also substantial with respect to the overall height of the enclosure. A number of fins 36, twelve of which are shown in FIGS. 2A and 2C, extend rearwardly of the enclosure from base 34.

[0037] A second heat sink 40 has a thermal interface with the electronic components E for heat generated by the electronic components to be transferred to the heat sink. This draws the heat away from the electronic components. Heat sink 40 can comprise more than one heat sink depending upon the number of circuit boards mounted within enclosure 10. For the three circuit boards 12 shown in FIGS. 2A and 2B, electronic components E are mounted on both sides of each circuit board. In FIGS. 7 and 8, it is shown that heat generated by the components is both radiated into the interior of the enclosure, as well as conducted from the components to a second heat sink through a thermal interface 42 with the heat sink. The second heat sink 40 first includes a fin 44 which physically contacts the thermal interface elements 42. As indicated by the arrows in FIG. 8, heat from a component is conducted from the component through the thermal interface to the fin 44 associated with the side of circuit board 12 on which the component is mounted. In addition, and as shown in FIG. 7, heat radiated by the component is absorbed by one of plurality of fins 46 which are mounted in the enclosure in spaced relation with respect to the components and the circuit boards.

[0038] Apparatus 30 next includes a heat pipe 50 which conducts the heat absorbed by fins 44 or 46 to heat sink 32 so to remove the heat from within enclosure 10. As shown in FIGS. 10A-10C, heat pipe 50 contains a working fluid. As indicated in FIG. 10A, at an evaporator end 52 of the heat pipe, heat enters the heat pipe and vaporizes the working fluid within the pipe. The vaporized fluid now produces a pressure gradient within the pipe which forces the vaporized fluid toward a condenser end 54 of the pipe. At the condenser end of the pipe, the working fluid condenses and the heat drawn from the working fluid is transferred to heat sink 32 so it can be discharged to the atmosphere. An inner surface of the heat pipe forms a wick by which the working fluid flows back to the evaporator end of the heat pipe by a capillary action. Heat pipe 50 is an L-shaped pipe having a longitudinal reach extending generally the length of enclosure 10, the heat pipe extending immediately beneath upper wall 22 of the enclosure. At the rear end of the enclosure, the heat pipe has a vertical reach which extends down to the level of an inner end of heat sink 32, this end of the heat sink being within the enclosure. The condenser end of the heat pipe is attached to heat sink 32 by a clamp 56 so the heat pipe can thermally transfer heat to the heat sink.

[0039] While the foregoing portions of apparatus 30 are used to remove heat from the enclosure, it is sometimes desirable to generate heat within the enclosure. For this purpose, apparatus 30 includes at least one, and preferably two, heaters 60, 62. These heaters are, for example, each 50w. strip heaters whose length is at least that of the length within enclosure 10 through which the circuit boards and their connected electronic components extend. This is as shown in FIG. 2B. As shown in FIGS. 3 and 4, the strip heaters can either be mounted so to extend along opposite sides of the enclosure (FIG. 3), or along the bottom of the enclosure (FIG. 4). In either embodiment, the strip heaters are attached to respective brackets 64, 66 so to be positioned with respect to the circuit boards. If one of the heaters fails, the amount of heat produced by the other heater, when it is energized, is sufficient to raise the enclosure ambient temperature to the range in which the electronic components are designed to properly function.

[0040] A control circuit for the strip heaters is shown in FIG. 11 to include a power source external to the enclosure. Power from this source is routed to the strip heaters through a connector (not shown) in rear wall 14 of the enclosure. Each heater 60, 62, is on a separate circuit. A sensor 70 including respective thermostats 72, 74 controls application of power to the heaters. Each thermostat is normally open, but will close when the temperature within the sealed enclosure falls below a predetermined temperature. This temperature is, for example, +4° C. (+39° F.). By energizing one or both the heaters at the appropriate time, the temperature in enclosure 10 is maintained within the desired range at which the electronic components properly function.

[0041] Referring now to FIG. 6, operation of the apparatus of the present invention can be understood with respect to the diagram which illustrates the heat sources and heat sinks within sealed enclosure 10. The electronic components E and the heaters 60, 62 supply heat to the enclosure, while heat pipe 50 and heat sinks 32 and 40 transfer heat to the atmosphere. When the unit is operating at a nominal ambient temperature Ta which is, for example +20° C. (+68° F.), the strip heaters are deactivated and the only heat source is the electronic components. These generate, for example, 30w. of heat (P_Ein FIG. 6). This heat is removed from within the enclosure by the heat sinks and heat pipe (P_HSin the Fig.). This operating condition exists for an ambient temperature range of −1C to +50C (+30° F. to +122° F.). The lower limit is based upon a thermostat transition from heaters “off” to “on”, at +4° C. (+39° F.), and a temperature differential ΔT of 5° C. (9° F.) between the inside and outside of enclosure 10. When the temperature falls below −1° C., the heaters are activated and supply an additional 100w. of heat within the sealed enclosure (P_Hin FIG. 6). This makes the total heat input into the enclosure 130w. The capacity of heat pipe 50 is 30w. Since the heat load is now 100w. more than the pipe's capacity, the heat pipe stops working. This is beneficial because if the heat pipe kept working, it would nullify the effects of having the heaters turned “on”. Again, failure of one of the heaters would not effect the overall functioning of apparatus 30 to maintain the appropriate ambient temperature within enclosure 10, since even with 80w. of heat within the enclosure, this is still adequate to dry out the heat pipe.

[0042] The heaters are kept “on” for an ambient temperature range of −40° C. to +3° C. (−40° F. to +37° F.). The upper limit is based upon the thermostats turning the heaters 60, 62 “off” at +13° C. (+55° F.), and a temperature differential ΔT of 10° C. (18° F.) between the inside and exterior of enclosure 10 when heat pipe 50 is not operational.

[0043] Finally, FIG. 5 represents an ideal curve of the net heat input to enclosure 10 as a function of temperature. It will be appreciated that the actual transitions depend upon location of the thermostats within the enclosure. As shown in the Fig., at the lower end of the ambient temperature range, there is a net heat input of 130 w. within the enclosure. At the upper end of the range, all of the heat is removed from the enclosure so the net heat input is 0 w.

[0044] What has been described is apparatus providing a low cost solution for maintaining the temperature within the sealed enclosure at a safe operating temperature. The apparatus controls the heat requirements within a sealed enclosure including both heat extraction and heating depending the temperature within the enclosure. The apparatus employs heat pipes and heat sinks to transfer heat out of the enclosure, and strip heaters for heating components within the enclosure, as appropriate. Transfer capacity of the heat pipes is greater than the amount of heat produced by components housed within the enclosure, and the heat sinks limit the maximum temperature reached within the enclosure. When the strip heaters are operational, the heat they produce, when combined with that of the electronic components is sufficient to dry out the heat pipe so it does not remove heat from the enclosure which could cause the ambient enclosure temperature to fall. The apparatus efficiently and effectively maintains the temperature within the enclosure to a range of temperatures within which the components function properly and no failures occur due the temperature within the enclosure. The apparatus is configured for the type of equipment housed within the enclosure.

[0045] In view of the foregoing, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained.

[0046] 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.