Abstract:
A food warming cabinet keeps food warm and on display in serving trays using infrared heaters instead of hot water. The infrared heaters are located below the trays and direct IR at the trays. In an alternate embodiment, the IR heaters can also conduct heat into the trays. Using IR instead of water saves energy because it shortens warm-up time. Using IR also eliminates contaminated water and enables separate and individual temperature control of each tray. Tray temperature is maintained under computer control using contact or optical/IR temperature sensors.

Description:
BACKGROUND 
       [0001]    Prior art food warming cabinets or “steam tables” are well known. They are commonly found in restaurants and/or food service institutions that display foods for consumers to select from in that they hold food in open trays from which the food can be served. 
         [0002]    Prior art steam tables are typically comprised of a cabinet having a some form of flat or planar countertop having one or more openings into which one or more open-top food serving trays are placed such that the tray bottoms are embedded within or just above a body of hot water held in an open tank or tub within the cabinet. The food temperature inside the tray is, therefore, determined by the temperature of the water held in the open tank, as well as ambient temperature as there is always a heat loss from the trays and water into the air above the trays. The water can be kept heated in some embodiments by an electrically-resistive heating element but maintaining the foods&#39; temperature at a sufficiently high temperature to prevent spoilage is problematic. 
         [0003]    Water has a relatively high specific heat. Prior art steam tables are therefore able to maintain relatively stable tray temperatures but keeping the water in the tank hot is inherently problematic because food in the trays should be kept at temperatures sufficiently high to prevent spoilage. Maintaining food tray temperatures inherently requires the water in the tank to be hot when the food-containing trays are first installed in the steam table and to thereafter be kept hot. Prior art steam tables, therefore, require a significant amount of starting energy to be input to the water, just to make it usable. Keeping the water sufficiently hot requires a heater within the steam table, which should operable in a closed room such as a restaurant serving area, i.e., electric, but which is also safe to use in an inherently wet environment. 
         [0004]    Another problem with prior art steam tables, which use water in an open tank, is that the water itself eventually become contaminated with food products making them subject to contamination from food products that inevitably find their way in the tank. The tanks should be thoroughly cleaned on a regular basis. 
         [0005]    Cleaning the water tanks in a steam table is problematic. The table must of course be taken out of service and the cleaning process requires the water to be drained, the tank sanitized and then re-filled with clean water. 
         [0006]    Yet another problem with prior art steam tables is that the specific heat of water precludes the ability to quickly change the temperature in the food holding trays or to keep side-by-side trays at different temperatures. Prior art steam tables maintain foods at a single temperature, i.e. the temperature that the water can be heated to, which can never exceed 212° Fahrenheit or 100° Centigrade. 
         [0007]    A food warming table or cabinet which overcomes the problems found in prior art steam tables would be an improvement over the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a perspective view of a food warming cabinet; 
           [0009]      FIG. 2  is a cross-sectional diagram of the food warming cabinet shown in  FIG. 1 ; 
           [0010]      FIG. 3  is a perspective view of an electrically-powered infrared energy source; 
           [0011]      FIG. 4  is a plan view of one embodiment of a planar infrared energy source; and 
           [0012]      FIG. 5  is a plan view of an alternate embodiment of a planar infrared energy source. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  is a perspective view of a food warming cabinet  10 , comprised of a base  14  mounted on wheels  12 , which enable the food warming cabinet  10  to be moved about. A top surface of the base  14  is provided with a countertop  16  into which rectangular openings are cut or formed to allow two stainless-steel serving trays  24 A and  24 B to inserted or held substantially within the base  14  of the cabinet  10 . Alternate embodiments of the cabinet  10  include trays that have other shapes, including for instance, square, round, elliptical or triangular. For the sake of brevity, however, trays or pans of any shape are considered to be encompassed by the term, tray. 
         [0014]    The trays are formed with a lip that rests on the top of the countertop  16 , but which is not visible in  FIG. 1 . The trays  24 A and  24 B thus rest on the countertop  16  but are nevertheless substantially inside within the base  14 . 
         [0015]    Each tray has a substantially planar bottom. Since the trays are rectangular, they have four sidewalls that can be considered to extend upwardly from the planar bottom of each tray, substantially orthogonal thereto. The tray sidewalls define both a peripheral edge as well as periphery of the tray. The open tops enable food to be placed into the tray and removed from the tray. A sanitary hood  18  is supported above the trays  24 A and  24 B and the countertop  16  by two vertical posts  20 . 
         [0016]      FIG. 2  is a cross-sectional view of the food warming cabinet shown in  FIG. 1 .  FIG. 2  also depicts a functional block diagram of other apparatus within the cabinet  14 . 
         [0017]    Both food holding trays  24 A and  24 B have an electrically-powered infrared energy source or “IR source”  34 A and  34 B located directly below the trays&#39; planar bottoms  25 A and  25 B respectively. The first tray bottom  25 A is separated from the first IR source  34 A by an air gap  44 . The second tray bottom  25 B is in direct physical contact with the second IR source  34 B. 
         [0018]    Infrared energy emitted from the electrically-powered infrared energy sources  34 A and  34 B is directed upwardly toward the food tray bottoms  25 A and  24 B. The IR energy is absorbed by the tray material, causing the temperature of the trays to rise. Stated another way, the IR heats the bottoms  25 A and  25 B of the trays and as a result, food items in the trays  24 A and  24 B. A distinct advantage of having two separate IR sources  34 A and  34 B to heat corresponding trays  24 A and  24 B is that the temperature of the corresponding trays can be different from each other and individually controlled. 
         [0019]    By way of example, a first tray  24 A can be maintained at or near a first temperature, within a first temperature range between one-hundred eighty and two-hundred degrees Farenheit. The second tray  24 B can be maintained at a second temperature, greater than the first temperature, within a second temperature range between one-hundred ninety and two-hundred degrees. 
         [0020]    Tray temperatures can be individually controlled and kept within different or the same temperature ranges using tray temperature information obtained from one or more temperature sensors  48  thermally coupled to different trays  24 A and/or  24 B. The sensors  48  are preferably embodied as thermistors and/or semiconductors in direct contact with either the bottom or sides of the trays. Such devices are well-known in the electronic art and provide a resistance or voltage proportional to temperature. 
         [0021]    In  FIG. 2 , a single temperature sensor  48  is depicted as being in direct thermal contact with the bottom  25 A of the first tray  24 A using an articulated leaf spring  52 , one end of which is attached to the underside of the countertop  16 . The spring  52  is sized, shaped and arranged to extend downward from the underside of the countertop  16 , bend and extend underneath the tray  24 A. The spring  52  is thus configured to hold the temperature sensor  48  against the bottom  25 A of the tray  24 A. 
         [0022]    The spring  52  is sized, shaped and arranged to deflect upwardly and downwardly as needed, according to the depth of the food tray  24 A but to hold the sensor  48  against the bottom  25 A of the tray  24 A whenever the tray  24 A is in the cabinet  10 . Electrical connections to the spring-mounted sensor  48  are provided via conductors that are carried over the leaf spring  52  from a connector block  54 . 
         [0023]    Wires extend from the connector block  54  to a central processing unit or CPU  40 , which is programmed to read the signals from the temperature sensor  48 . The CPU  40  thereafter adjusts electric power provided to the planar infrared energy source  34 A that provides heat energy to the tray  24 A by opening and closing a software-controlled solenoid  38 A. The solenoid  38 A is electrically coupled to an electrical energy source  30 , typically embodied as ordinary line voltage. 
         [0024]    While the left-side tray  24 A is separated from the IR sources  34 A by an air gap  44 , the right-hand side food holding tray  24 B is depicted as having its bottom surface  25 B in direct contact with a second, substantially planar infrared energy source  34 B. In such an embodiment, the tray bottom  25 B receives thermal energy directly from the energy source  34 B. Since the tray  24 B is metal and therefore thermally conductive, heat provided into the tray bottom  25 B is readily conducted through-out the tray and into food stuffs inside the tray. 
         [0025]    Sensing the temperature of the tray  24 B and/or food within the tray  24 B can be difficult if a direct sensor is to be used. In at least one alternate embodiment, tray temperature and the tray&#39;s contents temperature is measured using an optical temperature sensor  62  positioned inside the base  14  and directed to an exterior surface of a tray. In another alternate embodiment, an optical infrared energy sensor  64  is mounted to the underside of the hood  18  and positioned above one or more of the food holding trays  24  in order to measure the tray temperature by the amount of IR radiated from the tray contents or the tray bottom if the tray is empty or nearly empty. In embodiments that use optical/IR sensors, the sensors detect infrared emitted from the trays, the tray contents or tray surfaces, such as the sensor  62  inside the cabinet base  14 . The sensors  62  and  64  are configured to send a corresponding temperature-indicative signal to the CPU  40 . The CPU  40  thereafter modulates the current provided to a corresponding planar IR heating source  34 A and/or  34 B responsive to the measured temperature of a tray. The tray temperature is thus controlled by monitoring emitted IR. 
         [0026]    In yet a third alternate embodiment, tray temperature can be inferred from the temperature of the planar IR heat sources  34 A or  34 B. In  FIG. 2 , a temperature sensor  48  can also be attached to the underside or top side of a planar infrared energy source  34 B as shown. 
         [0027]      FIG. 3  is an exploded view of a planar infrared heating element  70  used in the food warming cabinet  10  shown in  FIG. 1  and  FIG. 2 . The planar heater  70  is comprised of an electrically-resistive heating element wire (wire)  74 , preferably bonded to a thermally and electrically insulating substrate  76 . The wire  74  is laid down on the substrate  76  with a predetermined pattern in order to optimally heat the trays  24  and their contents. 
         [0028]    The insulating substrate  76 , which carries the heating wire  74 , laid on top of a rigid metal support substrate  80 . The support substrate  80  maintains the planarity of the substrate  76 , which prevents the heater wire  74  from fracturing. 
         [0029]    An infrared-transmissive front layer  84  is attached to the top side of the substrate  76  using an appropriate adhesive placed around the perimeter of the substrate  76 . The IR transmissive front layer  84  protects the heating element wire  74  from mechanical damage and reduces the likelihood of an electrical short circuit due to a liquid coming into contact with the wire. An optional second infrared transmissive layer  88  can be used and provided with an ultraviolet filter to screen or shield the transmission of ultraviolet light. The second infrared-transmissive glass which optionally includes a UV filter layer provides a cleanable surface. 
         [0030]    As mentioned above, a bottom  25  of a tray  24  can be separated from a planar infrared energy source  70  by an infrared-transmissive material or layer, such as air, quartz or glass. As was also mentioned, a tray bottom can be in direct contact with the infrared energy source  70 . In one alternate embodiment, a thermally conductive material, e.g., metal, is located between the energy source  70  and the tray bottom  25  to provide heat to the tray using conduction instead of radiation. In another alternate embodiment, an IR partially transmissive material is placed in the air gap  44  such that heat is transferred into a tray  25  by radiation and conduction. 
         [0031]    The type of material between the energy source  70  and tray bottom  25  can be determined in part by the gap  44  or spacing between the tray bottoms  25  and the infrared energy source  70 . The gap  44  is effectuated by the dimensions of a metal frame  90  attached to the underside of the countertop  16  by fasteners  92 . As the vertical dimension of the metal frame  90  is increased, the infrared energy source  70  will be farther from the underside of the trays  25 . 
         [0032]      FIG. 4  shows one embodiment of a wire layout on a substrate  76 . In  FIG. 4 , the infrared energy source wire  74  is arranged in several rows, each of which is comprised of several boustrophedonic rows or patterns  104 ,  106  and  108 . The rows of boustrophedons identified by the letters A, B, C, C′, B′ and A′ identify boustrophedons of three different widths or pitch and which are themselves identified by reference numerals  104 ,  106  and  108 . 
         [0033]    As can be seen in the figure, the boustrophedons of rows A and A′ and which are closest to the outside lateral edges  102 , are spaced more closely than are the boustrophedons or loops of rows B and B′. Similarly, the boustrophedons of rows B and B′ are spaced more closely than are the boustrophedons or loops of rows C and C′ 
         [0034]    Winding the electrically-resistive material as shown in  FIG. 4  imbues the planar heating element  100  with an infrared emission pattern wherein the concentration of emitted IR is greater around the perimeter and along the edges  102  than is the IR emitted in the middle or central areas of the heating element  100 . By weighting the emitted infrared energy more heavily at the edges of the heating element  100 , a greater amount of infrared energy is emitted toward the corresponding peripheral edges of the food holding trays than is emitted toward interior areas of the food holding trays. Stated another way, the closer and more-numerous windings of resistive material  74  near the edges  102  emit more IR than do the more widely-spaced and less numerous windings near the interior areas. The greater IR emitted from around the periphery of the heating element  100  transfers a correspondingly greater amount of heat energy into the periphery of the trays. 
         [0035]    Referring now to  FIG. 5 , there is shown an alternate embodiment of a planar infrared heating element  110 . In this figure, the electrically-resistive heating element material  74 , which is also applied to a thermally and electrically-resistive substrate  76  is laid down in crenellate patterns. The crenellations of the rows AA and AA′ and which are adjacent to the lateral edges  102 , are more numerous and closer to each other than are the crenellations for rows BB and BB′. The more-closely spaced crenellations around the lateral edges  102  thus imbue the heating element  110  with the substantially the same characteristic described above for the heating element  100  shown in  FIG. 4 , namely the ability to concentrate more emitted infrared energy at the lateral edges of the trays  25  than would otherwise be possible. 
         [0036]    Those of ordinary skill in the art will recognize that the structure described above and shown in the figures lends itself to a method of heating food in a tray held in a food warming cabinet  10 . That method includes simply upward toward the bottom of the tray such that the amount of infrared energy per unit area that is directed along the peripheral sides or edges of the tray is greater than the infrared energy directed to the interior of the tray. By directing the infrared energy as such, thermal losses, which occur more at the tray periphery than in the center can be compensated for the amount of heat energy being input. 
         [0037]    The infrared energy concentration at the periphery of the tray edges can be effectuated using either boustrophedonic or crenellated rows of electrically-resistive material through which an electric current I is passed. 
         [0038]    The foregoing description is for purposes of illustrations only. The true scope of the invention is set forth in the appurtenant claims.