Refrigerator compartment and method for accurately controlled temperature
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a separate compartment and method in a combination refrigerator-freezer having means for drawing cold air from the freezer and mixing the cold air with compartment air for circulation through the compartment. 
     2. Description of the Prior Art 
     A refrigerated compartment having a fan that draws air from the compartment and mixes that air with air drawn through a flue from the top of the freezer compartment is disclosed in U.S. Pat. No. 3,090,209. A thermostat is included, located in the refrigerated compartment, to control the operation of the fan. 
     U.S. Pat. No. 3,659,429 discloses a fast chill space in a refrigerator having an auxiliary timer controlled fan adjacent an inlet passage from the freezer, which passage includes a thermostatically controlled damper. The inlet passage draws air from the top of the freezer adjacent evaporator openings from which cool air is discharged during cooling cycles. 
     U.S. Pat. No. 3,747,361 discloses a refrigerator having a fast chill compartment similar to that disclosed in the &#39;429 patent which includes a switch for continuously running the fan. In an alternate position, the switch varies the fan operation dependent on the compartment thermostat state. 
     U.S. Pat. No. 3,733,841 discloses an air flow control which mixes refrigerator compartment air with freezer compartment air and utilizes a temperature sensor to control operation of the compressor. 
     U.S. Pat. No. 4,358,932 discloses a fast chill compartment which utilizes a microcomputer to control the fan operation time. 
     U.S. Pat. No. 3,122,005 discloses a refrigerator-freezer combination having an evaporator in the divider wall and a fan that draws air in through the side wall of one of the compartments. 
     U.S. Pat. No. 3,005,321 discloses a multiple compartment refrigerator-freezer which utilizes two fans to control the air flow and temperature. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides a device and method for supplying cool, relatively constant temperature freezer compartment air to a separate compartment to establish and maintain an accurate temperature within the compartment. The present invention also provides a device and method for maintaining a low temperature gradient throughout a long-term food storage compartment. 
     The present invention is embodied in a refrigerator-freezer having a separate compartment maintained at an accurate temperature, preferably near or just below the freezing point of water, to provide a storage location for long-term storage of perishable foods without freezing. The compartment, known as a &#34;super cool&#34; compartment, includes a cold air siphon to draw chilled air from near a mid-portion of the freezer compartment into a plenum chamber in the controlled temperature compartment. The freezer air is mixed in the plenum chamber with air from the super cool compartment and circulated by a high capacity fan throughout the compartment so that a low temperature gradient is maintained across the entirety of the compartment. 
     The low compartmental temperature gradient and the accurately controlled temperature of the present super cool compartment enable foods to be stored for extended times without risk of spoilage. The temperature can be set to and maintained at a very precise point so that freezing is not required. At lower temperatures, even longer storage can be provided for foods that are less susceptible to chilling injury. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a side-by-side refrigerator-freezer, including an accurately controlled temperature compartment according to the principles of the present invention; 
     FIG. 2 is a partial perspective view, in phantom, of the refrigerator-freezer of FIG. 1 showing an air flow duct arrangement and fan housing for the compartment of the present invention; 
     FIG. 3 is a vertical cross-section along lines III--III of FIG. 2 showing additional details of the present invention. 
     FIG. 4 is a cross-section taken along lines IV--IV of FIG. 3 showing the freezer air siphon and plenum chamber of the present invention; 
     FIG. 5 is a horizontal cross-section taken along lines V--V of FIG. 3 showing additional details of the plenum chamber; 
     FIG. 6 is a partial elevational view taken along the lines VI--VI of FIG. 4 and showing the relationship of the freezer air siphon and the fan housing outlet; and 
     FIG. 7 is a vertical cross-section of another embodiment of the present invention having a controlled temperature compartment below the refrigerator compartment in a side-by-side refrigerator-freezer. 
     FIG. 8 is a vertical cross-section of a further embodiment of the present controlled temperature compartment in a top-freezer type refrigerator-freezer. 
     FIG. 9 is a cross-section along lines IX--IX of FIG. 8 showing the arrangement of the freezer air siphon and return duct. 
     FIG. 10 is a diagram of an electrical control circuit as used in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a side-by-side refrigerator-freezer, generally at 10, including a cabinet 12 containing a refrigerator compartment 14 having an openable door 16 and a freezer compartment 18 having an openable door 20. A common dividing wall 22 separates the refrigerator compartment 14 from the freezer compartment 18. A plurality of shelves 24 are mounted within both the refrigerator and freezer compartments 14 and 18, as well as on the interior surfaces of the doors 16 and 20. In the embodiment shown, a chilled water and ice access panel 26 is provided at an exterior of the freezer door 20. A controlled temperature compartment 28 is provided above the refrigerator compartment 14 that includes an interior door 30 which is accessible upon opening of the refrigerator door 16. 
     In FIG. 2, the controlled temperature, or supercool, compartment 28 is shown above the refrigerator compartment 14 and beside the freezer compartment 18. A plenum housing 32 having a shaped fan housing portion 34 within which is rotatably mounted a fan 36 is mounted on an inside wall 38 of the super cool compartment 28. The plenum 32 includes air inlet slots 40 spaced from the fan housing 34. As the fan 36 operates, air is drawn from the temperature controlled compartment 28, through the inlet slots 40, and into a plenum chamber 42 in the interior of the plenum 32. 
     A freezer air siphon, or dip tube, 44 is mounted within the freezer compartment 18 and includes an upper end 46 in communication with the plenum chamber 42, as well as a lower end 48 extending to near a mid-portion of the freezer compartment 18. The lower end 48 has at least one freezer air inlet 50 formed therein so that freezer air is drawn from a mid-portion of the freezer compartment 18, along the siphon 44, and into the plenum chamber 42. 
     A return duct 52 extends through the divider wall 22 between the controlled temperature compartment 28 and the freezer compartment 18 through which air returns from the compartment 28 to the freezer 18. A temperature selection control 54 is provided on a front face portion 56 of the plenum 32 by which the temperature within the controlled temperature compartment 28 can be adjusted. 
     As shown by the arrows, the operation of the fan 36 creates an air flow from the fan housing 34 of the plenum 32 into and throughout the interior of the controlled temperature compartment 28. After circulation, a portion of the air returns into the plenum chamber 42 at the air inlets 40 and is carried therethrough for recirculation by the fan 36. The operation of the fan 36 creates a low pressure zone within the plenum chamber 42 which causes air to be drawn from the freezer compartment 18, into the siphon inlet 50, upward through the freezer air siphon 44 and into the plenum chamber 42, where it is mixed, or tempered, with the air from the controlled temperature compartment 28 prior to being exhausted by the fan 36. As air is removed from the mid-portion of the freezer 18 by the siphon, or dip tube, 44, air within the controlled temperature compartment 28 is returned to the freezer compartment 18 through the air return duct 52. The present invention, thereby, mixes cool freezer air with air circulating through the controlled temperature compartment 28 to provide temperature regulation of the air within the compartment 28. 
     Referring to FIG. 3, the controlled temperature, or super cool, compartment 28 is partially enclosed by insulated walls 58 of the cabinet 12, including a lower insulated wall 60 between the compartment 28 and the refrigerator 14. The door 30 completes the enclosure and is mounted to the lower wall 60 by a hinge 62. A gasket 64 may also be provided at the perimeter of the door 30 to ensure an effective seal between the generally warmer refrigerator compartment 14 and the super cool compartment 28. 
     The temperature selection control 54, in the embodiment shown, is a slide control projecting from the face portion 56. The slide control 54 adjusts control circuitry 66 contained within the plenum chamber 42. Included in the control circuitry 66 and mounted just within the plenum chamber 42 at the inlet openings 40 is a temperature sensor 68. The temperature sensor 68 detects temperature changes in the air flowing in through the inlet slots 40 and, through the control circuitry 66, switches into and out of operation a fan motor 70 which drives the fan 36. The temperature sensor 68 is preferably mounted close enough to the air slots 40 to sense the ambient temperature within the compartment 28 even when the fan 36 is not in operation. Inlet slots 40 are located in the front portion of compartment 28 while fan 36 is located at the rear portion of the compartment. This arrangement assures adequate air flow and hence low temperature gradients throughout the controlled temperature compartment. 
     The plenum housing 32 is mounted to the refrigerator-freezer dividing wall 22 by a plurality of screws 72 extending through sockets 74 in the plenum 32. Mounted within the plenum chamber 42, in some embodiments, is a heater 76. The heater 76 is controlled by the control circuitry 66 to warm the air that was drawn into the plenum chamber 42 by the fan 36 should the temperature within the temperature controlled compartment 28 fall below the predetermined set temperature set by the slide control 54. A small dead band is built into the control circuitry 66 so that during heating the air temperature is heated to the set temperature minus the dead band. It will be understood that the set temperature can be fixed in some embodiments. 
     The fan 36 is preferably of relatively large capacity compared to the size of the compartment 28 so that a high volume of air is circulated therethrough to maintain an extremely low temperature gradient within the compartment 28. 
     The upper end 46 of freezer air siphon 44 is generally at the same level as the air return duct 52. Otherwise, there may be a tendency for a thermally induced air flow between the freezer 18 and the super cool compartment 28. 
     FIG. 4 shows the freezer air siphon 44 extending into the freezer compartment 18 to draw freezer air from near a mid-portion thereof. Generally, cycling of the evaporator, or cooling mechanism, shown at 78, in the freezer compartment 18 causes temperature variations within the freezer 18. The temperature variations are particularly wide at the top of the freezer compartment 18 since cold air is forced to the top of the freezer compartment 18 when the cooling mechanism 78 is on and, when the cooling mechanism 78 is off, warmer air collects at the top of the freezer compartment 18. Air below the top of the freezer compartment 18 is less subject to temperature variations, and, in particular, air near the mid-portion of the freezer is relatively constant in temperature. 
     For purposes of the present invention, mid-portion, or horizontal mid-portion, refers to that portion of the freezer compartment spaced from the top and from the bottom of the freezer compartment in which a lower temperature variation occurs as the freezer cooling mechanism cycles. 
     In the embodiment shown, the air inlet 50 of the freezer air siphon 44 is disposed just above a shelf 24, below which is an ice maker 80. In this portion of the freezer compartment 18, the temperature variations are considerably reduced over that of the top of the freezer compartment 18 and, therefore, a more controlled temperature air is drawn into the plenum chamber 42 for mixing with the air within the controlled temperature compartment 28. 
     Also referring to FIG. 4, the slide control 54 includes a display 82 indicating the temperatures to which the air temperature within the controlled temperature compartment 28 may be set. The fan housing portion 34 can be seen projecting beyond the body of the plenum 32 to more effectively direct the air flow generated by the fan 36 throughout the compartment 28. 
     Referring now to FIG. 5, the freezer air siphon 44 is disposed at the rear of the freezer compartment 18 while the air return 52 is at the front of the freezer compartment 18. A transverse cool air duct 84 at the top 46 of the siphon 44 extends through the wall 22 spaced somewhat laterally and, as can be seen in FIGS. 3 and 6, above the fan 36. Such arrangement provides for even distribution of the controlled temperature air, as well as for some mixing of the freezer air with the compartment air within the plenum chamber 42 and for controlled return of the compartment air to the freezer 18. 
     In FIG. 6, the relative sizes of the fan housing 34 and the transverse duct 84 of the freezer air siphon 44 are shown. The fan housing 34 is considerably larger than the transverse duct 84 so that a relatively large quantity of the temperature controlled compartment air is mixed with a relatively small quantity of colder freezer air. The siphon 44 is formed to accommodate the cooling mechanism 78 and to extend to just above the ice maker 80. The siphon 44 additionally prevents air from the cooling mechanism from being forced into the transverse duct 84 when the fan 36 is not operating. 
     In FIG. 7 is shown a controlled temperature compartment 100 disposed beneath a refrigerator compartment 102, which draws cool air from a freezer compartment 104 through a freezer air siphon 106 extending upwardly from compartment 100 along the dividing wall 108 and then downwardly to near a mid-portion of the freezer compartment 104. An air return duct 110 is provided between the top of the controlled temperature compartment 100 and the top of the freezer compartment 104. The embodiment shown in FIG. 7 includes many of the features of the above-discussed embodiment, including drawing relatively constant temperature air from below the top of the freezer compartment 104. The freezer air siphon 106 and the return duct 110 both cross the adjoining wall 108 near the top thereof to prevent thermally induced air flow. The freezer air siphon 106 is shown adjacent the adjoining wall 108 while the return duct 110 extends alongside the siphon 106 and spaced from the wall 108 to prevent excessive warming of the freezer air as it passes downward to the controlled temperature chamber 100. 
     As shown in FIG. 8, a controlled temperature compartment 200 can also be provided in a refrigerator-freezer 202 which has a freezer compartment 204 above a refrigerator compartment 206. The refrigerator compartment 206 is separated from the freezer compartment 204 by a first dividing wall 208, while the controlled temperature compartment 200 is separated from the refrigerator compartment 206 by a second dividing wall 210. A main door 212 is hingedly mounted at the front of the refrigerator-freezer 202 for access to the refrigerator compartment 206. An interior freezer door 214 provides access to the freezer compartment 204, and an interior controlled temperature compartment door 216 provides access to the controlled temperature compartment 200. 
     Within the controlled temperature compartment 200 is a plenum housing 218 defining a plenum chamber 220, the plenum housing 218 being provided with a fan housing portion 222 projecting therefrom. The plenum housing 218 is mounted on an insulated rear wall 224 of the refrigerator-freezer 202. A freezer air siphon 226 is disposed in the insulated rear wall 224 and has a lower end 228 in communication with the plenum chamber 220. The freezer air siphon 226 extends upwardly along the insulated rear wall 224 and has an upper end 230 in communication with a horizontal mid-portion of the freezer compartment 204. 
     An evaporator fan 232 is mounted in a divider 234 which separates an evaporator 236 from a main portion of the freezer compartment 204. In the illustrated embodiment, the upper end 230 of the freezer air siphon 226 is in communication with a suction side of the evaporator fan 232. This prevent the evaporator fan 232 from forcing freezer air down the freezer air siphon 226 to the controlled temperature compartment 200, and thereby prevents inaccurate temperature sensing by a sensor (not shown) within the plenum chamber 220. 
     Another view of the top-freezer type refrigerator-freezer unit 202 is shown in FIG. 9. The freezer air siphon 226 extends to behind a fan 238 mounted within the fan housing portion 222 of the plenum housing 218. In communication with the controlled temperature compartment 200 and outside of the plenum housing 218 is a lower end 240 of a return duct 242. The return duct 242, like the freezer air siphon 226, extends vertically along the insulated rear wall 224 and has an upper end 244 in communication with a horizontal mid-portion of the freezer compartment 204. Like the upper end 230 of the freezer air siphon 226, the upper end 244 of the return duct 242 is on the suction side of the evaporator fan 232. The upper end 244 of the return duct 242 is at a slightly higher elevation than the upper end 230 of the freezer air siphon 226. This prevents a reverse thermal cycling of the freezer air which could cause erroneous temperature sensing and control within the controlled temperature compartment 200. The upper ends 244 and 230 could also be at the same horizontal level to achieve this effect. 
     The control circuit 66 is shown in FIG. 10 including the slide control 54, the temperature sensor 68, the fan motor 70, and the heater element 76. An operational amplifier 300 is provided in the control circuit 66 which functions as a comparator and includes a positive feedback loop 302 including a resistor 304. The provision of positive feedback in the comparator 300 causes the comparator to exhibit hysteresis. 
     A reference voltage is set at an inverting input 306 of the comparator 300 by the slide control 54 which includes a movable contact 308 for connection with one of a plurality of pre-set resistance connections 310-318, as well as an open circuit connection 320 and a short circuit connection 322 to ground. The open circuit connection 320 corresponds to a 20° F. voltage reference level at the inverting input 306 of the comparator 300. Each of the connection points 310-318 have connected thereto resistors of respectively decreasing resistance so as to provide voltage reference levels in 2° F. increments. The short circuit connection 322 corresponds to an off-position of the slide control 54. 
     A non-inverting input 324 of the comparator 300 is fed with an input voltage as determined by the temperature sensor 68. Upon the input voltage at the non-inverting input 324 reaching the reference voltage level at the inverting input 306, the comparator 300 abruptly changes the voltage at an output 326 which is fed to a first thyristor 328, shown as a triac. The triac 328 controls the supply of power to the fan motor 70. 
     A second operational amplifier 330 also functions as a comparator, however, without the provision of feedback. A non-inverting input 332 of the comparator 330 is supplied with the reference voltage level as established by the slide control 54, while an inverting input 334 is supplied with an input voltage as determined by the temperature sensor 68. A voltage dividing resistor 336 maintains a voltage difference between the input 324 of a first comparator 300 and the input 334 of the second comparator 330, so that the second comparator 330 is triggered at a lower temperature. The resistor 336, thus, defines the dead band. 
     An output 338 of the comparator 330 is connected to a gate 340 of a second thyristor 342, also shown as a triac. When the triac 342 is triggered by the comparator 330, power is supplied to the heater element 76. The second comparator 330 and second triac 342 can obviously be eliminated from embodiments of the circuit 66 not requiring a heater 76. 
     A zener diode 344 and capacitor 346 are provided in the circuit 66 for power regulation. 
     The amplitude of the hysteresis provided by the first comparator 300, in a preferred embodiment, is approximately equal to 1° F. For instance, when the slide control 54 is set at the 20 degree connection 320, the comparator 300 triggers when the temperature sensor 68 senses a temperature of 20° F., thereby starting the fan motor 70. The fan motor 70 continues to run until the comparator 300 is reset at approximately 19° F., thereby preventing extremely rapid cycling of the fan motor 70. 
     The present invention, thus, provides a means and method for maintaining a compartment at a set temperature. Cold freezer air is drawn from a central region of a freezer and is tempered by mixing with air already circulating within the compartment. 
     In tests of the present device, the fan runs nearly continuously, either at varying speeds or on a rapid duty cycle. The resulting air circulation prevents air stratification within the compartment. 
     The set temperature is maintained throughout the compartment to within a fraction of a degree Fahrenheit. Such accurate temperature control enables foods to be stored for long periods, often without freezing injury. For example, ground beef was stored at 23° F. in a frozen condition for 51 days without detectable flavor loss. Fresh shellfish was stored in an unfrozen state at between 29° and 32° F. for three to seven days. And cherries were stored in a non-frozen storage zone of 32°-35° F. for 10 to 14 days without spoilage or chilling injury. 
     Although the present invention is disclosed and described in a combination refrigerator-freezer, it is within the bounds of the present invention to provide a controlled temperature compartment in conjunction with a freezer unit. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.