Patent Application
20110171355
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.
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' temperature at a sufficiently high temperature to prevent spoilage is problematic.
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.
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.
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.
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.
A food warming table or cabinet which overcomes the problems found in prior art steam tables would be an improvement over the prior art.
The trays are formed with a lip that rests on the top of the countertop 16, but which is not visible in
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 24A and 24B and the countertop 16 by two vertical posts 20.
Both food holding trays 24A and 24B have an electrically-powered infrared energy source or “IR source” 34A and 34B located directly below the trays' planar bottoms 25A and 25B respectively. The first tray bottom 25A is separated from the first IR source 34A by an air gap 44. The second tray bottom 25B is in direct physical contact with the second IR source 34B.
Infrared energy emitted from the electrically-powered infrared energy sources 34A and 34B is directed upwardly toward the food tray bottoms 25A and 24B. 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 25A and 25B of the trays and as a result, food items in the trays 24A and 24B. A distinct advantage of having two separate IR sources 34A and 34B to heat corresponding trays 24A and 24B is that the temperature of the corresponding trays can be different from each other and individually controlled.
By way of example, a first tray 24A 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 24B 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.
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 24A and/or 24B. 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.
In
The spring 52 is sized, shaped and arranged to deflect upwardly and downwardly as needed, according to the depth of the food tray 24A but to hold the sensor 48 against the bottom 25A of the tray 24A whenever the tray 24A 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.
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 34A that provides heat energy to the tray 24A by opening and closing a software-controlled solenoid 38A. The solenoid 38A is electrically coupled to an electrical energy source 30, typically embodied as ordinary line voltage.
While the left-side tray 24A is separated from the IR sources 34A by an air gap 44, the right-hand side food holding tray 24B is depicted as having its bottom surface 25B in direct contact with a second, substantially planar infrared energy source 34B. In such an embodiment, the tray bottom 25B receives thermal energy directly from the energy source 34B. Since the tray 24B is metal and therefore thermally conductive, heat provided into the tray bottom 25B is readily conducted through-out the tray and into food stuffs inside the tray.
Sensing the temperature of the tray 24B and/or food within the tray 24B can be difficult if a direct sensor is to be used. In at least one alternate embodiment, tray temperature and the tray'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 34A and/or 34B responsive to the measured temperature of a tray. The tray temperature is thus controlled by monitoring emitted IR.
In yet a third alternate embodiment, tray temperature can be inferred from the temperature of the planar IR heat sources 34A or 34B. In
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.
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.
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.
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.
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′
Winding the electrically-resistive material as shown in
Referring now to
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.
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.
The foregoing description is for purposes of illustrations only. The true scope of the invention is set forth in the appurtenant claims.
