CA1061845A

CA1061845A - Food serving system

Info

Publication number: CA1061845A
Application number: CA275,125A
Authority: CA
Inventors: George K. Shumrak; Anthony Mack
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-05-09
Filing date: 1977-03-30
Publication date: 1979-09-04
Also published as: BR7502872A; SE421744B; US3924100A; FR2269901A1; SE7502165L; DE2444442B2; IT1022097B; JPS50147048A; CA1030204A; LU72281A1; DE2444442A1; US4068115A; BE819960A; FR2269901B1; DK671374A; FI342474A; NO744271L; ES430521A1; AU7439974A; DE2444442C3

Abstract

ABSTRACT OF THE DISCLOSURE

A food serving tray comprising a horizontal support comprising a thermally and electrical insulating material and an upturned edge portion. An opening is provided in the support generally corresponding in size to the bottom surface of a food container. A circuit assembly base is secured to the bottom of the support and aligned with the opening. A disc on the tray is made of good heat conductive material and generally conforms in shape to the opening and aligned with the opening. The disc and base form a closed cavity in the tray with the disc forming a portion of a top surface of the tray. An electric heating ele-ment heat transfer device is secured to the tray and in contact with the bottom of the disc and with its bottom spaced from the base and lying in the cavity. An electrical contact is provided in the tray and is electrically connected to the heat transfer device by conductors in the cavity for carrying energy to the device to energize it.

Description

T~lis invention relates to the delivery of prepared meals to locations remote from the point of preparation and more par-ticularly comprises a new concept in the feeding of patients in hospitals, nursing homes and other institutions, the feeding of hotel guests in their rooms, etc. In the following description the invention is described in terms of its use in a hospital but it is to be borne in mind that the invention has wider applica-tions.
This application is a division of application Serial No. 208,918 filed September 6, lg74.
Hospitals use a variety of different systems for ser-ving meals to their patients. These systems generally fall into two categories, frequently identified as centralized and decen-tralized systems. And the systems in each category include a variety of different techniques intended to bring food to the individual patients at the desired temperature.
In the centralized systems, the food is prepared in a main kitchen where the entire meal is set up on individual pa-j tient's trays and the trays are transported directly to the patients. The heated pellet system, hot and cold cart system,and . insulated nestable tray-thermal column system all are categorized in the centralized system of food distribution.
~.., The decentralized systems generally fall into two cate-gories, namely, conventional and microwave systems. In conven-tional decentralized systems, the food is prepared in central kitchens and transported in bulk to floor pantries on the dif-ferent hospital floors where the patients' trays are assembled.
In the microwave systems, the prepared food is either ~cept in a freezer or refrigerator, and just prior to its serving, the food is allowed to thaw and is then heated in a microwave oven in the ` floor pantry. Thereafter,it is promptly delivered to the patient.

All the various centralized and decentralized systems have disadvantages. For example, the pellet systems employ a metal disc preheated to 2~0 - 450F. as the heat source in the tray to maintain hot food at desired temperature, and the pellets constitute a hazard to the patient and kitchen personnel. If the pellets are overheated, they cause the foods to overcook or dry out. The effective temperature retention time is limited to approximately 45 minutes. And the pellets are heavy and add substantial weight to trays. In hot and cold cart systems, wherein the carts for the trays have separate hot and cold sec-tions, there is a tendency to overcook and dry out the foods in the hot section. Utility outlets are needed in both the kitchen and on the patients' floors, and the carts normally are on current for 7 or 8 hours daily. And they do not generate heat in transit. The carts are difficult to clean and require substantial maintenance, and the carts are costly, heavy and have limited tray capacity. In insulated tray-thermal column systems wherein the trays nest on top of one another so that all the hot foods stack on top of each other and similarly allthe ~` coid foods stack on top of one another to form thermal columns, the trays are bulky, heat is conducted in the tray skins from the hot to the cold columns, and the systems have a limited ~`~ temperature retention time. Furthermore, without lids on the separate trays, the tray bottoms become soiled from the food in the next lower trays.
; In the decentralized systems, labor costs are relative-ly high as more people are required to operate the systems, food costs are higher because of waste and unauthorized consumption, food odors are created on separate floors because of the floor pantries, and dishwashing noise is created on the floors when china is used.
One important object of this invention is to upgrade the quality of food service. This object is accomplished first ~- by providing a system wherein the food is served at the intended

- 2 -temperature even thouyh the food may have left the point of preparation an extended period of time before it is actually delivered to the patients. Second, the eating experience is enhanced by providing a system wherein the patient is served his food conventionally in separate dishes and bowls rather than in large partitioned unattractive and essentially impersonal trays. In essence the object of this invention is to make the eating experience of patients in institutions as enjoyable as possible. This is in part the result of the wider range of foods available to the hospital (they need not avoid quick cooling food) and the elimination of overheating which destroys both the taste and nutri-tional value of the food.
To accomplish these and other objects, this invention includes among its features a self-contained mobile cart, se-parate removable food serving trays for each patient, and in-` dividual dishes, plates, bowls, etc. to be carried on the trays and which may be like those used in the home. The cart carries its own rechargeable low voltage power pack, which is designed ` ~ to supply the necessary energy to each ~y to maintain selected portions of the trays that in turn support the dishes containing foods at other than ambient temperature at the desired tempera-ture. The trays include isolated heater transfer devices which comprise a relatively small percentage of the total tray area, and those portions are sized so as to correspond to the size of the dishes containing the foods to be heated or cooled by them. The heat transfer devices do not affect the tempera-ture of the remainder of the tray, and consequently foods not actually on the transfer devices remain at their initial tem-peratures.
Each tray carries its own contacts that engage termin-als on the cart so that the trays and particularly their heater transfer devices may automatically be energized when the trays are mounted on the racks. Preferably, the separate dishes, plat~s, ~nd bowls are disposable, and they have the abilit~
to conduct heat between the heat transfer devi,ces and the foods contained in them. When the transfer devices are heaters, the containers must have sufficient stability so as to be capable of being subjected for extended periods to the heat of the heaters without burning or distortiny. In order to maximize the efficien-cy of the heaters, those plates and bowls which are intended to ,carry hot foods are provided with covers to reduce the heat loss.
In accordance with the present invention, there is pro-vided a food serving tray comprising a horizontal support com-prising a thermally and electrically insulating material and an upturned edge portion. An opening is provided in the support gen-erally corresponding in size to the bottom surface of a food con-tainer. A circuit assembly base is secured to the bottom of the support and aligned with the opening. A disc on the tray is made of good heat conductive material and generally conforms in shape to the opening and aligned with the opening. The disc and base form a closed cavity in the tray with the disc forming a portion of a top surface of the tray. An electric heating element heat transfer device is secured to the t~ray and in contact with the bottom of the disc and with its bottom spaced from the base and lying in the cavity. An electrical contact is provided in the tray and is electrically connected to the heat transfer device by conductors in the cavity for carrying energy to the device to energize it.

These and other objects and features of this invention will be better understood and appreciated from the following detailed description of several embodiments of the food system of this invention, read in connection with the accompanying .. :~
drawings.
Figure 1 is a perspective view of a cart and tray of this invention and showing one of a number of food trays that are carried by the cart, partially inserted on its rack.
Figure 2 is a fragmentary perspective view of the cart operating controls.
Figure 3 is a top plan view of a tray constructed in accordance with this invention.
Figure 4 is a bottom plan view of the tray of Fig-ure 3 with parts broken away to reveal details of the tray construction.
Figure 5 is a side elevation of the tray of Fig-ures 3 and 4.
Figures 6 and 7 are cross sectional views of the ~ tray taken along section lines 6-6 and 7-7 - of Figures 3 and 4 respectively.
Figure 8 is a fragmentary cross-sectional view showing how a tray makes electrical con-tact with the cart and with the tray fully inserted in the cart and taken along section line 8-8 of Figure 1.
Figure 9 is a fragmentary perspective view of the inside of the cart showing the electrical contacts of one rack.
Figure 10 is a simplified circuit diagram of the - electrical components of the cart and tray.
In Figure 1, a single cart 10 is shown, which is de- ,, ~
signed to move about the floor of the facility employing the feeding system of this invention. The cart illustrated has a pair of fixed axle wheels 12 and a pair of pivotally supported wheels 14 to enable the cart to move freely in any direction.
A pair of handles 16 are provided at the top of end panel 18 of the cart for the convenience of the operator.

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The cart illustrated is shown to have three separate tray compartments 20, 22 and 24, each subdivided by angles 26 - that define separate tiers for the trays to be carried in them.
The separate compartments are collectively defined by the bottom :` i A
wall-~, end panels 18 and 30, partitions 32, 34 and 36, top wall 38, rear panel 40 and sliding doors 42.
A power pack and control circuit section ~44 forms part of the cart and is mounted between partitions 32 and 34.
The power pack and control circuit contained in the section is designed first to supply the energy needed to energize the spot heat transfer devices on the several trays which may be carried in the tiers within the compartments 20, 22 and 24, and it may !' serve the additional function of supplying energy to literally-drive the cart about the floor. And,such an arrangement would of course, include means for recharging the power pack from an AC power source. The details of the power pack and control cir-cuit are not part of the present invention.
In Figure 2 the controls for driving the cart are '.'-~ :
~ suggested. The console 31 between handles 16 has a key operated . . .
- 20 switch 33 for connecting the power pack to the motor 42 (see ; Figure 10) for driving wheels 12. Two button operated switches 35 and 37 on console 31 respectively represent forward and re-- verse button controls for the cart. The operator need only turn the key 33 "on", press the forward or reverse button as desired , and squeeze the trigger handles 16 to move the cart about the hospital floor. (Handles 16 have switches 40 (see Figure 10) built into them which,when squeezed complete the motor circuit.) Steering, of course, is made possible by the pivotally mounted ~ wheels 14. Indicator light 39 glows when the battery is being -~ 30 charged.
A heater control logic panel 46 is shown on the front of section 44 and has three columns of push button switches. Each ~' ':
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horizontal row of tllese switches represent the controls for a single tray. The first vertical column of switch but-tons 4i3 are the "off" switches for each of the tray stations, and columns of switch buttons 50 and 52 represent the "on"swi-tches for the two separate heat transfer devices which are independently operated and which are provided in each tray. Of course a different switch arrangement could be used.
In Figures 3 to 8, one tray to be used with the cart of Figure 1 is shown. It will be appreciated that the cart may have any desired capacity (the cart illustrated holds about 18 trays), and the number is not a limitation of this invention.
Normally all of the trays used in the cart are identical, and may be used interchangeably on any of the racks in the separate compartments of the cart.
The tray 100 illustrated in the drawing is rectangular in shape and typically may be approximately 14 x 18 inches. Pre-ferably, the dimensions of the tray are the same as conventional food serving trays now in use and they are designed so that they may be used as conventional food carrying trays without the - 20 activation of their heat transfer devices in cafeterias and other ,, central food serving locations in the facility utilized by am-bulatory patients, guest, staff, etc.
The tray has an upturned rim 102 which extends about its periphery, and the main body of the tray is made of a high heat distortion, self extinguishing materials such as acrylo-nitrile-butadiene-styrene plastic (ABS) or a copolymer of poly-styrene and polyphenylene oxide. The entire exposed upper sur-face of the tray is made of such a material with the exception of the spot heaters that comprise the heat transfer devices which are incorporated into it, as described in detail below.
` In the embodiment shown, two heaters 104 and 106 are provided in the tray, and their total areas together constitute 1~ 4~

a small peLcentage of the total tray area. For example, the i diameter of heater 104 may be approximately 7 inches and the diameter of heater 106 may be approximately 4 inches. Of course different members and sizes of heaters may be used in accordance with this invention. In the illustrated configuration the heater 106 would normally be used to heat soup in a bowl placed on the heater, and heater 104 would be used to direct heat to food in 4 the main dinner plate. While the diameters of the heaters 104 and 106 differ from one another, their construction is quite similar as will be evident from the following description.
In the cross sectional view of Figure 7, the main tray body 108 which forms the horizontal support member of the :;
tray is provided with an opening 110 whose diameter defines the size of the local or spot heater 104. A bead 112 is molded in the support 108 immediately about the opening llO, which bead defines a r'ng or guide for positioning a plate on heater 104 and inhibits the plate from shifting or sliding off the heater.
A similar but smaller opening 109 defines the area of the smaller , . .
second heater 106, and it also is surrounded by a bead 113 for positioning a soup bowl on it. A premolded circuit assembly base 114 made of the same or a material similar to the body is secured to the bottom of support 108 and closes the openings 109 and -: ,~,. ~
110. The periphery 11~ of the assembly base extends radially beyond the edges of the openings, and it is essential that the base form a proper seal with the body as described in greater detail below so as to ma~e the tray fully immersible. Water must of course be prevented from entering the interior of the heaters.
The assembly base shown is provided with a number of shallow channels il5 for receiving the electrical wiring for the two heaters. The conductors 116 are connected to electrical con-tact rivets 117 that are exposed on the bottom of the base so that they make contact with suitable terminals in the cart which . .
' . , are described in detail below.
;s As shown in Figure 6, the contact rivets 117 are con-nected to the conductors 116 by suitable termina] rings 118 which are crimped or otherwise electrically joined to the rivets. And, the tops of the rivets are peaned over the rings to ensure con-tinued contact therewith. The upper ends of the rivets 117 are not exposed on the upper surface 120 of the tray, and if holes are provided through the body they are filled with silicone 121 or some equivalent material to seal the upper surface, as shown in Figure 6.
The assembly base 114 together with an aluminum disc 124 defines a cavity 126 which houses the operative elements of the heater assembly. The aluminum disc is of sufficient diameter so as to extend under bead 112. A silicone or other sealant un-der bead 112 is used to preserve the necessary immersible char-, - , acter of the tray. The edge of the base 114 is similarly sealed to the main body of the tray.
The heater 104 includes three positive temperature co-efficient (PTC~ pellets 128 of the type manufactured by Texas Instrument Company, which are disposed in the cavity 126 and are equally spaced about the bottom of the aluminum disc to which they are bonded. The pellets may be approximately 5/8 inch in diameter and are connected in series to conductors 116' and 116" as shown in Figure 4. The bottoms of the pellets are spaced from the assembly base 114 so as to form an air gap to prevent the base from becoming uncomfortably hot when the heaters are ; activated. A number of ribs 130 are provided on base 114 to maintain the gap and prevent sagging or other distortion of alum-inum disc 124.

; 30 The smaller heater 106 differs from the larger heater 104 only in the number of pellets 128 used. As shown, a single `
pellet is sufficient to provide the heat necessary for the smal-_ g _ L~ ~ ~

ler heater. Like pellets 128 in heater 104, the sin~le pellet of smaller heater 106 is bondea to the bottom of its aluminum disc 132 and ribs serve to space that disc from the base 114 to pre-serve the gap.
In Figure 1 a set up tray is shown wherein a soup bowl B is on heater 106, a dinner plate P is on heater 104, and the other normally used side dishes (dessert dish DD and bread and butter dish BD), utensils and napkin, and coffee cup CC are shown.
The bowl B and plate P disposed on heaters 106 and 104 respective-10 ly may typically be those shown in design patent No. D-229,812 dated January 8, 1974 owned by Sweetheart Plastics, Inc. The bowl and plate may be made of thermoformed sheet material, and ` each preferably has a bottom wall whose contours conform to that of the aluminum discs to provide a wide face to face contact area with the aluminum discs forming the upper surface of each of the heaters. In the preferred form of this invention the bowl and plate are held in place on the aluminum discs by the ~; beads 112 and 113 which surround them. Customarily trays made up as shown in Figure l with the food, dishes, cutlery, etc. in place are assembled in the cart 10 on each of the racks in the compartments 20, 22 and 24. The angles 26 are disposed in the cart approximately 4 1/2 inches apart so as to provide sufficient space on each angle for a set up tray. It is contemplated that when the food is dished onto the main dinnner plate P and the soup is poured in bowl B, they are at the desired temperature for eating, and it is the function of the cart and tray to main-tain the food at that temperature for so long as is required to deliver the food to the various patients in the institution.
When the set up trays are placed on the angles which comprise the racXs in the cart, the electrical connection is made between the rivet contacts 117 on the tray and the mating terminals for each rack in the cart. As shown in Fi~ure 9, brackets 134 are mounted in the rear of the compartmerlts adjacent each tier as defined by the angles 26. The brackets carry four spring-type terminals 136 which equal in nurnber the rivet contacts ~ , and the terminals are positioned automatically to make electrical contact with the rivet contacts on the trays by stops 26' on ' angles 26 and guide 26" when the respective trays are positioned properly on their angle tracks. The ribs 141 on the sides of the tray bottom ensure proper positioning of the tray on the angles in alignment with terminals. The ribs also provide a steady base for the tray by extending below the bottoms of contacts 117. The ,~ control board switches 48, 50 and 52 for each rack are connected in circuit with the terminals 136 and power supply 137 and switch 48 turns off both heaters 104 and 106, switch 50 when "on" causes - ~ heater 104 to become energized and the other switch 52 when "on"
causes heater 106 to become energized. When a tray is placed on the rack the appropriate switches for that rack are actuated so as to supply energy to either or both of the heaters as desired.
The loaded cart is then wheeled about the floor to the locations of the various patients where the trays are pulled from the - 20 rack individually and given to the patients. For so long as the trays remain in the cart, heat is supplied to the soup and/or ; entree to maintain the food at the desired temperature. Thus, unlike the prior art feeding systems, there is a continuous source of energy provided to continuously heat the food.
The PTC pellets are basically multi-modal resistance heaters. Below their critical temperature which may range from 120F. to 400F. depending upon composition, they have a low electrical resistance, while above it the resistance is very high. Near the critical temperature, the resistance varies be-` 30 tween extremes. Therefore, depending upon temperature a PTC
- heater will deliver different wattages. If the plate or bowl is colder than intended to cool the PTC heater below its critical tempera~ure, the lleater will operate a~ a high wattaye which will in turn quickly heat the bowl or pLate on it and oE course the PTC material will become hot. Ultimately, the wattage will decrease and the system will come into equilibrium. At equili-brium the heater will operate at just enough wattage to keep the food warm without overcooking it.
This characteristic of PTC heaters may be utilized not merely to maintain the temperature of hot foods as described ` above, but also to reconstitute cold food to the desired serv-ing temperature. A cold dinner plate for example at a temper-ature of 40F.placed on heater 104 may be heated to the 140F. -150F. range in less than one hour. Such a system thus will enable institutions to buy frozen meals from independent food preparing firms and eliminate its own cooking facilities. Essen-tially, the establishment utilizing the system would set up the tray with cold food and after a prescribed period the reconstitu-ted food would be delivered to the patient. It is within the scope of this invention that the cart could during the initial heating period be connected to line voltage through a trans-former and then during the delivery period be switched to itsown power pack.
~ While in the preferred form of this invention PTC pellet ;- heaters are utilized as the heat source, it should be appreciated that other heaters may be used such as resistance heaters. Some of these are so inexpensive that they may be considered dispos-~ able, and if the tray is made from inexpensive materials, such ;~ as corrugated or low cost plastic the entire tray could be dis-`-: posable. Printed circuit heaters as well as foil heaters of the '" ' type shown in United States Patent No. 3,397,301 fall within the class of resistance heaters contemplated. These heaters get hot when electric current is passed through them. These heaters pro-vide a fixed amount of heat, assuming that the voltage source is ~ixed. To control the heat output of resistance heaters a thermo-stat may be used to turn the heaters on and off at selected low and high temperature levels. Thus a resistance heater and thermo-stat combination may be used in place of the PTC pellets of the ; preferred form of this invention.
It is contemplated within the scope of this invention that each patient may himself have a special tray support which has an electrical terminal adapted to connect with the contact rivets on the tray to further energize the heaters 104 and 106 to continue to maintain the food at the desired temperature when at the patient's station. In this connection, it will be noted that in the preferred form, both the bowl B and the plate P are provided with covers so as to better retain the heat generated through the introduction of heat from the spot heaters to the food.
It will be appreciated from the foregoing description that because of the localized nature of the heaters, the heat generated by them will not elevate the temperature of other foods on the tray either in separate dishes or on the tray surface 120.
For example, a cup containing a cold beverage will not be warmed by the heaters, nor will the butter on the bread dish BD melt because of the generation of heat at the heaters 104 and 106.
Further re, the dessert dish DD which may contain Jello (Trade Mark) or some other cold food will not be subjected to heat from :. ~or)s~-j4 cr~ f /y heaters 104 and 106 and Gcnso~uently the dessert will also remain at the desired temperature.
Because the heat is transmitted by conduction from the heaters to the foods in the dishes or bowls on them, little or no loss of efficiency will occur in the heating process even with the introduction of a fan or other air circulator which would prevent a build up of heat, food odors and condensation in the compart-ments of the cart. This circulation will assist in maintaining the t~ ' lower temperatures o the other foods by preventing an increase in the ambient temperature.
From the foregoing description it will be appreciated that the advantages sought by the present invention are achieved.
Of particular importance is the fact that the food may be served to the patient in conventional dishes and bowls so that the pa-tient need not be made to feel "different". The trays themselves although internally markedly different from trays heretofore used do not have a "foreign" or strange appearance. And of primary importance is the fact that the food is served to the patient at the desired temperature. And while in the foregoing descrip-tion the use of plastic dishes and plates is suggested it should be appreciated that regular chinaware or glassware may be used with equal facility, albeit the added expense of washing is in-troduced.

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It should also be appreciated that the tray may be dec-~`~ orated by coloring, texturing or printing so as to make them most attractive. The aluminum discs can be porcelainized or enameled ....
so as to match or contrast with the main tray body 108. And the - 20 heaters in no way interfere with any other functional feature ; that may be advantageously incorporated into the tray.

While the cart is described as having a control panel 46 ; with button actuated switches for the heaters, it will be appre-~ ciated that the heaters could be made to operate automatically .. :~
~ without the usual manually operated buttons. For example, a .:~
, thermostatic element could be positioned at the aluminum disc and sense the temperature of a cold or hot dish placed on the - disc, and open or close the heater circuits as desired.
Because numerous modificatic~s may be made of this in-vention without departing from its spirlt, we do not intend to , limit the scope of this invention to the embodiments illustrated and described. Rather, it is intended that the scope of this in-vention be determined by the appended claims and their equivalents.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A food serving tray comprising a horizontal support comprising a thermally and electrically insulating material and an upturned edge portion, an opening in the support generally corresponding in size to the bottom surface of a food container, a circuit assembly base secured to the bottom of the support and aligned with the opening, a disc on the tray made of good heat conductive material and generally conforming in shape to the opening and aligned with the opening, said disc and base forming a closed cavity in the tray with the disc forming a portion of a top surface of said tray, an electric heating element heat trans-fer device secured to the tray and in contact with the bottom of the disc and with its bottom spaced from the base and lying in the cavity, and an electrical contact in the tray and electric-ally connected to the heat transfer device by conductors in the cavity for carrying energy to the device to energize it.

2. A tray as defined in claim 1, further characterized by said device being a positive temperature coefficient heater.

3. A tray as described in claim 2 further characterized by a rib in the upper surface of the tray adjacent the edge of the opening for defining the periphery of the heated area of the tray and for retaining a food container on the support in contact with the disc.

4. A tray as described in claim 3, further characterized by said disc being made of aluminum.

5. A tray as described in claim 2,further characterized by said base and disc sealing the heater in the tray so that it may be immersed in water for washing.

6. A method of serving food at diverse locations from a central location comprising the steps of:
filling individual serving dishes with the food to be served, providing individual trays each for a full service of dishes for one person and arranging the dishes on the trays, at least one of said trays comprising a horizontal support com-prising a thermally and electrically insulating material, an opening in the support generally corresponding in size to the bottom surface of a food container, a circuit assembly base secured to the bottom of the support and aligned with the opening, a disc on the tray made of good heat conductive material and generally conforming in shape to the opening and aligned with the opening, said disc and base forming a closed cavity in the tray with the disc forming a porting of a top surface of said tray, an electric heating element heat transfer device secured to the tray and in contact with the bottom of the disc and lying in the cavity, and an electrical contact in the tray and electrically connected to the heat transfer device by conductors in the cavity for carrying energy to the device to energize it, and selectively heating only portions of the trays that carry the dishes containing the food to be served at an elevated temperature to the exclusion of the other portions of the trays by placing said trays on a cart having a self contained energy source to direct energy to the trays for the selective heating of only portions thereof and transporting said cart from the central location to the diverse locations while maintaining said heating.

7. A method of serving tood at diverse locations from a central location comprising the steps of:
providing a vehicle having an independent energy source, mounting on the vehicle at the central location a plurality of trays, at least one of said trays comprising a horizontal support comprising a thermally and electrically insulating material, an opening in the support generally corres-ponding in size to the bottom surface of a food container, a circuit assembly base secured to the bottom of the support and aligned with the opening, a disc on the tray made of good heat conductive material and generally conforming in shape to the opening and aligned with the opening, said disc and base form-ing a closed cavity in the tray with the disc forming a porting of a top surface of said tray, an electric heating element heat transfer device secured to the tray and in contact with the bottom of the disc and lying in the cavity, and an electrical contact in the tray and electrically connected to the heat transfer device by conductors in the cavity for carrying energy to the device to energize it, the heat transfer device of the trays being operatively connected to the energy source when the trays are on the vehicle, placing food on the trays with selected foods in heat exchange relationship with the devices, and moving the vehicle from the central location to the diverse locations while maintaining said heat exchange relationship to heat the food and removing the trays from the vehicle for serving to individuals at the diverse locations.

8. A method in accordance with the method of claim 6, wherein the positions of said trays that are heated are heated by means of a positive temperature coefficient heater underlying each selected portion of the tray and sealed in the trays, said food being heated to a temperature in the range of from 130°F to 170°F.

9. A method in accordance with the method of claim 8, wherein said temperature is in the range of from 140°F to 160°F
and maintained at that temperature by said self-contained energy source.

10. A method of serving food at diverse locations from a central location comprising the steps of:
filling individual serving dishes with the food to be served, providing individual trays each for a full service of dishes for one person and arranging a plurality of said dishes on each tray, at least one o* said trays comprising a horizontal support comprising a thermally and electrically insulating material, an opening in the support generally corresponding in size to the bottom surface of a food container, a circuit assembly base secured to the bottom of the support and aligned with the opening, a disc on the tray made of good heat conductive material and generally conforming in shape to the opening and aligned with the opening, said disc and base forming a closed cavity in the tray with the disc forming a porting of a top surface of said tray, an electric heating element heat transfer device secured to the tray and in contact with the bottom of the disc and lying in the cavity, and an electrical contact in the tray and electrically connected to the heat transfer device by conductors in the cavity for carrying energy to the device to energize it, and selectively heating, only portions of each tray that carry dishes containing food to be served at an elevated temperature, to the exclusion of other portions of the trays by placing said trays on a cart having a self-contained energy source to provide heating and to direct energy to the trays for the selective heating of only portions thereof up to the time the food is served and transporting said cart from said central location to said diverse locations while maintaining said heating,

11. An immersible serving and food temperature controlling tray structure comprising: a tray body of thermally and electri-cally insulating material having a top layer for receiving food containers and a bottom layer, at least one heat exchange sur-face portion of thermally conductive material integrally set and sealed in said top layer, peripheral bead means surrounding said heat exchange surface portion for conformally receiving and indexing a food container over said heat exchange surface por-tion and substantially preventing lateral movement of said container on said top layer, a sealed cavity defined in said tray between said top and bottom layers thereof, an electric heating element secured to the tray in said cavity juxtaposed with the underside of said heat exchange surface portion and spaced from said bottom layer to minimize the transfer of heat through said bottom layer, exposed contact means sealed in said tray for connection to a source of electric power, and connector means extending from said contact means through said tray between said top and bottom layers and into said sealed cavity to said electric heating element.

12. A tray as defined in claim 11, wherein said tray body is comprised of plastic material; and said heat exchange surface portion comprises a metallic sheet.

13. A tray as defined in claim 11, wherein said peripheral bead means comprises an upstanding continuous ridge of thermal insulating material integral with said top layer and defining a receiving socket for a conformally shaped food container.

14. A tray as defined in claim 11, wherein said electric heating element comprises heater means for maintaining a pre-determined temperature in a food container placed on said heat exchange surface.

15. A tray as defined in claim 14, wherein said heating element is a positive temperature coefficient electric heating element.