INTEGRAL HEATING EQUIPMENT DEVELOPMENT

 

JOHN M. MAHLUM

 

3M Company

[177] The 3M Co. is pleased to have been invited to say a few words in explanation of integral heating. A number of speakers have referred to our development as a new concept for cooking food and they mentioned our laboratory work as well as our activities with the commercial airline industry

Frankly, it is much too early in the development of this concept to articulate specifically on costs, performance under varied conditions, etc., and, therefore, we are not yet at the point where we are actively selling this method in any specific form to the public. We do have great confidence in the potential of this technology. We do have behind us a substantial amount of laboratory work and, as reported by others, a successful evaluation by American Airlines of this system. Tests have been run for 7 weeks under normal operating conditions where our system has been used to reconstitute typical airline frozen meals with fine results in terms of quality of food and performance of the equipment.

Integral heating is accomplished from a resistive coating applied to a surface area. The coating can be applied in a variety of manners, its composition is of a variety of materials, and it is applied to become an integral portion of the surface areas from which it is intended to deliver heat directly to food. It uses the principle of low watt density, is unrestricted in terms of the surface area required, can accomplish contours of any kind, and basically has the capability for "putting the heat where you want it. " It is not restricted to one composition of materials and, of course, the choice of materials and the manner of processing same is proprietary to 3M.

Integral heating provides

(1) More efficient heat transfer. The low mass and large surface area is supplemented by choice of low-thermal-capacity materials which give up their heat quickly to their surroundings, i. e., food.

 

(2) Quick response. The surface heats extremely fast and also cools extremely fast when energy inputs are reduced. It is therefore controllable and responsive to critical demands.

 

(3) Minimal residual heat problems. Conventional flame or high-watt-density heating elements, such as a calrod type, are nonexistent. Heat developed on the cooking surface is removed by the food, with the "heating surface" temperature climbing the temperature scale parallel to the food being heated and rarely exceeding a surface temperature of more than 10 percent that of the food.

 

[178] (4) Easy programming. Control of the input energy with electric-electronic components permits a variety of performance levels. This, in turn, makes it possible to match the requirement of the food with simple control adjustments giving the integrally heated 3M system the maximum possible versatility.

In lay terms we may say that, in the average home, two-thirds of the energy of all conventional cooking devices is wasted. Integrally heated surfaces operate at better than 90-percent efficiencies. It can also be said that integral heating operates with the fast response of gas, with the convenience of electricity, and without the danger of either.

As a means of explaining the system further, we will use the 3M in-flight food service system as a reference. The hardware for this system has been developed and used in flight tests and can be said to be generally commercially acceptable. Such a system can operate in commercial aircraft and military aircraft; aboard ships and submarines; in land-based facilities such as hospitals, colleges, and restaurants; and in almost any facility where for some reason there is an advantage to preparing the food, chilling or freezing it, and bringing it back to edible temperature at a later date or at a remote location from that where it was prepared.

The design of the system includes the following equipment:

(1) An outer oven shell. This unit is thermally insulated and can be installed permanently or temporarily into the area where you wish to do your cooking. The control units are appended to this shell and this becomes your operational center. Let me emphasize that this is not a classic oven - the walls do not get hot and there is no induced air circulation.

 

(2) An interior oven shell or rack. This rack contains electrodes which carry the electric energy to the dish; it has a series of trays which slide in and out and nest the dish. It also serves as the means by which the dish and food is moved to and from the kitchens.

 

(3) A casserole dish. This is the item which is integrally heated. It transforms the electric energy to heat and delivers the heat to the food. A combination of these three pieces of equipment in a variety of styles makes up the system. Each of the three components is necessary for the system to operate properly.

I would like to comment now on general performance requirements in developing any new food service system. There is always an interchange phase of going from the old method to the new, We found it essential that this system of ours be versatile. We therefore have programmed it to handle all three types of food preparation - frozen, chilled, and previously cooked and held as warm. Performance in general had to be fast and we have established a parameter of reconstitution of 10 to 12 oz of frozen food mixes (i. e., meat, vegetable, and starch) in 15 min or less, and we also built into the system the capacity of holding the desired temperature for a substantial period of time. Also the unit is capable of being utilized as either an oven or refrigerator, or a combination of these two.

The reliability of the system and its parts is of course a primary concern. It is necessary to have interlocking components, that is, a free interchange of parts to the system from one unit to the other. All parts have been designed to withstand physical demands and operational demands of both refrigeration and heating plus those of the more mundane facts of life such as commercial [179] dishwashing facilities. It is interesting to note that in this system we are limiting the potential liability or failure of the oven to function down to the lowest possible minimum. That is, each individual service of a meal is where the energy transfer is made, and therefore it is most likely that our liability is related to that one unit rather than to the total system.

There are many advantages that can be built into a new system and we have attempted to appraise these. Of course, the concern for storage and hence the need for stackable units is a consideration in any high volume operation. We have selected stainless-steel welded construction for its obvious characteristics of strength, cleanliness, and general acceptability when exposed to food environments.

The casserole was designed to withstand tremendous physical abuse and choice of a china or glass porcelain innerface was made because of its heatproof, stainproof, scratchproof, odorproof, rustproof, fadeproof, and ageproof properties. Not the least of its properties is its low bacteria retention level, which is superior to that of almost any other material. The casserole was designed to withstand cryogenic temperatures as low as -350° F and will operate at surface temperatures of 600° F, which is the generally accepted maximum cooking temperature. We have, in fact, eliminated the intense heat source or high-watt density factor found in most ovens.

Our total performance is accomplished with a lower power consumption. We use a given amount of power to get the job done and this is a prerequisite. However, our efficiency results in less power waste; therefore, the power consumption per unit time is substantially less than that Or conventional ovens.

Since there is no concern for airflow within the oven shell, the oven itself can be of almost any size and shape. Styling is also a factor and the system permits choice of size, shape, and color and generally is unrestricted. Use of modern updated electric-electronic materials permits versatility in controls and performance of the system.

I would like to make a few statements on the general premise from which we justify our performance claims. Table I is a study of the heat balance required to process a typical 10-oz food mix from a 0° F storage temperature to a 180° F "piping hot" condition. Note that less than 5 percent of the total BTU requirements are needed to raise the food temperature from 0° to 32° F. Approximately 45 percent of the energy is required to accomplish the heat of fusion or melting and an additional 45 percent is required to accomplish the sensible heat from 32° to 180° F. Approximately 8 percent of the total energy demand is used in bringing the casserole dish to proper temperature, and, thus, it can be said that in this example approximately 92 percent of the energy drawn is put to a worthy cause, that of heating the food. Using materials of low thermal capacity is working in favor of the system. I might point out that in a frozen storage condition the food will also stay cold longer inasmuch as the casserole dish does not deliver heat to the food and thus war, it up. One might compare the casserole dish to a thermos bottle. It works ideally whether the end product is to be hot or cold.

Table II indicates that we can transfer the power or watts required for a given meal to a heating-time relationship and we say arbitrarily that if you deliver 240 W for 15 min you will accomplish the delivery of BTU's consistent with our previous heat balance study.

In any and all heating systems it is necessary to transfer energy from a source to that which you want to receive it. Figure 1 is an enlargement of a casserole or heating surface bottom; the circles indicate the four points of temperature level. We use a typical convection oven as an illustration. At the top we will assume that we have a 1200° to 1800° F calrod unit. We depend on air to bring this heat to the proximity of our container carrying the food. In passing through this air space, it eventually encounters a stationary air film. As indicated, this is a thin film of air surrounding the surface of any material. There is, of course, a temperature drop at the point [181] where it hits this air film. At this point a severe loss of efficiency, or a large [delta] T, is realized.

This large temperature drop is inherent in all air-conducting systems and it is minimized somewhat by high-velocity air movement. At the point where the energy or heat reaches the casserole or dish bottom, it passes through the casserole bottom at some predictable rate and emerges to contact the food. The net heat or BTU absorption of the food is a small portion of that generated at the power source. The 3M Co. is successfully introducing the source temperature at the casserole bottom, completely bypassing air films, and therefore realizing a very low [delta] T drop from its delivering point until it reaches the food. In addition to that, as we have mentioned before, the thermal capacity of the dish is restricted. This principle guides our thinking in developing integrally heated devices. We look, forward to any comments you might have regarding possible applications of this principle.

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