CA1050294A - Deep fat fryer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A food cooker is illustrated in the form of a deep-fat fryer, in which the fat is isothermally heated to an operating temperature below a temperature range in which an unacceptably high rate of undesirable decom-position products formation occurs in the cooling liquid. The heat transfer process used is vaporization and condensation of a confined liquid essentially isothermally and substantially at the working temperature. Condensation of the heat transfer vapor takes place at an outer surface of the container for the fat. Heat is applied to the heat transfer liquid remote from the con-tainer, which having no heating element or elements intruding into its in-terior is easy to clean. Heat is transferred to the cooking fat by conden-sation of the heat transfer vapor essentially only as it is used by the load being cooked. Hot spots are minimized, so that the formation of undesirable decomposition products of the cooking fat is retarded. The volume of cooking fat required to cook a given load can be reduced, and its useful life extended.

Description

lOSO;~34 This invention relates to deep fat fryers.
Food preparation apparatus wherein heat is transferred from a heat source to a food preparation surface by a vaporized working fluid is dis-closed and claimed in United States Patent No. 3,948,244, issued April 6, 1976 to L. J. Lazaridis, E. F. Searight, and P. K. Shefsiek, which is assigned to the assignee of the invention disclosed and claimed herein. As is explained in that earlier patent, a heat transfer liquid is confined in a sealed enclosure which extends from the heat source to the food preparation surface. The heat transfer liquid is vaporized at a surface adjacent the heat source and fills the sealed enclosure. It then condenses on a portion of the sealed enclosure adjacent the food preparation surface and releases heat energy. The apparatus used in this form of heat transfer is sometimes called a "heat pipe". Advantages of this mode of food preparation over prior modes involving use of heat sources applied directly ~o cooking surfaces or to oodstufs are stated in the earlier patent.
Deep fat frying of foodstuffs is done essentially by immersing the foodstuffs in a vat of heated oil. Deep fat frying equipment that is presently available involves the application of a heat source directly to one side of a partition the opposite side of which is in direct contact with the frying oil. For example, many gas-fired fryers have burner tubes running through the oil fat. These tubes ordinarily have a burner at one end and combustion products travel from the burner through the tube so that heat is transferred from the tube to the oil in the vat. The tubes are understand-ably hotter at the burner end than at the exhaust end. Although this may result in an average temperature which satisfies the requriements for heat transfer from the tube to the oil, it unavoidably createsJ at the hot end, a temperature sufficiently high to produce a substantially larger amount of oil breakdown than that which would be associated with a temperature equiva-lent to the average temperature of the burner tubes. Further, a vat fitted 3~ ~i.th electric heating coils or elements will heat oil contained in it only -1- ~;
. ~

in the vincinity close to the heating elements. Such electric elements tend to overheat the cooking oil in this close vicinity. Most efficient use of elec:tric elements occurs when they are immersed in the cooking oil vat where they require significant space and create objectionable cleaning problems.
In the absencc of either immersed gas tubes or immersion electric heaters, the usual procedure is just to expose a pot or vat to direct flame.
There must be balance, or "trade-off" of two conflicting para-meters, one against the other. On the one hand, it is desirable to cook the food at as high a temperature as possible in order to avoid excessive absorp-tion of the oil by the food being cooked; e.g.: in order to have a relative-ly dry and crisp donut, instead of a soggy, oily or greasy donut. On the other hand, it is advantageous to maintain the cooking oil at as low a tem-perature as possible in order to minimize the formation of undesirable re-action prodtlcts in the oil.
Desp fat frying equipment that is currently available does not achieve desired temperature uniformity and control. The formation of unde-sirable reaction products of frying oil is a problem which exists with cur-rently available apparatus. This problem is exacerbated by the occurence of hot spotS, or relatively hot zones, which may be from any one of several sources, not limited to the inherent properties of existing heating methods.
One theory concerning the deleterious effects of hot spots in deep fat fryers suggests that particles of food materials and the like may adhere to the heat transfer surface inside the vat, and act there as a thermal insulator so that heat is not transferred from the wall to the oil. In addition, oil held by these particles cannot move away from the "insulated" spots, becoming hotter and causing this oil and oil in contact with it to decompose. In the best of prior situations, the mechanical process of such heating is based on localiz-ed over-heating of cooking oil which then heats the contents by convection.
Deterioration of cooking oil is temperature dependent. It occurs continuously as a function of temperature, usually an exponential function, lO~OZ94 the rate at which undesirable reaction products are produced increasing ex-ponentially in relation to temperature rise.
Generally, chemical breakdown of thermally unstable liquid oils is exponential with increasing temperature, and it is predictable that oxidation or other decomposition at localized hot spots will far exceed the decomposi-tion at normal cooking temperatures. If, for example, localized hot spots ex-ceed the average surface temperature by 25F or more, they will contribute a disproportionally high amount of thermal decomposition.
Replacement of oil that has formed an excess of undesirable reac-tion products is both time consuming and costly. It necessitates, among other things, that the vat be cleaned, and this task is not easy when the vat includes burner tubes or immersion heaters. It is apparent that these tubes make it difficult to clean the vat around and under them.
Apparatus is herein disclosed for substantially isothormal heating of a vat containing cooking liquid, such as deep fat frying oil. The appara~us uses a mechanism operating on what is sometimes known as the heat pipe principle to provide the desired temperature uniformity by employing one or more surfaces of the vat itself as the heat pipe condenser. The oil is thereby heated by an essentially isothermal surface, and the formation of hot spots, or relatively hot zones is minimized, and may for all practical pur-poses be considered to be substantially eliminated. Particles of food mater-ials and the like that adhere to the vat walls will not result in oil break-down because, as a covered spot on a wall becomes insulating, it accepts less and less vapor condensation from the heat pipe, and it accepts, therefore, less and less heat energy. The temperature cannot rise above the condensation temperature of the isothermal vapor. The fact that heat is not being trans-ferred away from the spot is no longer important because heat transfer to the spot is reduced. Normal cooking oil such as, for example, cottonseed oil, other vegetable oils and animal fats can be used for long periods of time without developing a rancid odor or taste which will occur in conventional 105()'~
direct immersion heating equipment.
~ ith the risks of hot spots out of the way, it is now possible to transfer heat at maximum heat flux from the entire heat transfer surface namely the walls of the vat, to the cooking oil. This facilitates quick recovery.
The amount of oil which is absorbed by the food in deep fat frying appears to be related inversely to the oil temperature. When prior deep fat frying apparatus is used and cold food is placed in the oil, the temperature of the oil drops and a relatively large amount of oil absorption occurs be-fore cooking temperature is recovered. On the other hand, in the apparatus of the present invention, which employs essentially isothermal transfer of heat to the oil at a desired working temperature uniformly over the entire heating surface, rapid heat transfer to the oil is enhanced so that the tem-perature of the oil does not drop as far as in prior apparatus and recovers more quietly. The result is a reduction in the amount of oil absorbed by the ood. Another result is faster cooking of the food. This results not only in better tasting food (e.g.: a relatively dry and crisp donut), but also in reduced oil consumption, since less oil is removed from the vat with the food, and replacement of the oil is required less frequently.
The invention accordingly, envisions a deep fat fryer comprising:
a relatively deep cooking vat adapted to contain and to heat for immersion cooking an edible, thermally decomposable cooking oil which is rapidly de-composed at temperatures substantially above normal cooking temperatures, the internal portions o~ said vat being free from obstructions and open at its top; a jacket forming a sealed chamber at least partially surrounding said vat and sealing therein a heat transfer liquid volatile at normal cook-ing temperature, the interior of said chamber being essentially devoid of substances which are not condensible at operating conditions; heating means;
means defining a heating zone for the heat transfer liquid, said means in-cluding a first heat transfer wall and a second heat transfer wall, each of 1050;~S~4 said heat transfer walls forming a portion of said chamber, said first heat transfer wall being adjacent said heating means to conduct heat from said heating means to said heat transfer liquid to vaporize said liquid, said second heat transfer wall being a wall of said cooking vat adapted to be in direct liquid-to-wall contact with said cooking oil to transfer heat rapidly from said wall to said cooking oil; said chamber defining a free path between said first heat transfer wall and said second heat transfer wall to permit vaporiz-ed heat transfer liquid to pass from said first heat transfer wall to said second heat transfer wall, and to permit condensed heat transfer liquid to re-turn to said first heat transfer wall. Preferably, the cooking temperature ofa cooking oil when used in the vat is maintained in the range between approx-imately 350 F and 400P.
The "heat pipe" principle employed in this invention uses a sealed space filled essentially entirely with liquid and gas phases of a vaporizable fluid, to the exclusion of non-condensing gases such as air. At working temperatures there is always a significant vapor pressure, and if liquid is vaporized at one location in the sealed space, it condenses at any cooler location with releases of the energy equivalent of the heat of vaporization.
Consequently essentially isothermal conditions are favored and heat supplied to the liquid phase is immediately available at any cool surface. The system used in this invention is intended to operate at a relatively low pressure, usually but not necessarily, less than one atmosphere.
Embodiments of the inv~ntion will now be described, by way of example, with reference to the accompaning drawings, in which:-Figure 1 is a general isometric view of a food cooker according tothe invention;
Figure 2 is a top plan view, partly broken away, of Figure l;
Figure 3 is a side view, partly broken away, of Figure l;
Figure 4 is a top view of Figure 1, showing the formation of the bottom wall of the cooker;

~050;~94 Figure 5 is a fragmentary enlarged cross-section of a heat-transfer component of Figure l;
Pigure 5A is a fragmentary enlarged cross section of a heat transfer component according to another embodiment of the invention; and Figure 5B is a fragmentary enlarged cross section of a heat transfer component according to a further embodiment of the invention.
In Figure l, is illustrated the food cooker generally designated lO according to one embodiment of the invention. The cabinet of the food cooker has side walls ll, a back wall 12 and a front panel generally designa-ted 13. A bottom segment 14 of the front wall is removablel permitting access to the internal mechanism. An upper segment 15 of the front panel 13 is positioned to contain control mechanisms such as for example, a temperature control knob 16 associated with thermostatic control 8 and temperature sensor 9. A chimney or exhaust port 17 in the back wall 12 permits exit of burned com-bustion gases.
The cooking chamber or vat, generally designated 20, is defined by the two side walls ll, a rear panel 21 and a front wall 18 ~see Figure 3) terminating in an upper shelf 22. The cooking vat is adapted and intended to contain a cooking oil or other cooking ~quid up to a level generally approx-Zo imately equal to the shelf 22 and a corresponding upper shelf 24 on back panel 21. Within the cooking vat there are no obstructions or working parts.
The cooking vat, accordingly, is essentially a container of desired size and shape for a cooking oil adapted to receive, as is conventional in the art, a food holding wire basket ~not shown) or other mechanism for insertion or immersion in cooking oil contained within the vat.
In Figure 2 and Figure 3 are shown the deep fat fryer generally designated 10, as in Figure 1, and illustrating the cooking vat 20, including side walls ll, back wall 12, front wall 18 and front shelf 22 and rear panel 24. Within the cabinet in ~ront of the cooking vat 20 is a motor 25 operating a blower fan 26 adapted to receive combustible gas from a gas pipe 27 feeding ~050294 through a main shut off valve 28. Suitable gas fuel and air pipes and chambers include an air mix control 19 for mixing gas and air conventionally. A
pressure regulator 28a operates to provide desired gas pressure to a *hermo-statically-operated gas valve 28b, which may be operated from a safety cir-cuit~ and after it, just prior to the air mix control 19, is a zero governor 28c providing an output pressure that is equal to ambient, to maintain the air-gas ratio at the burner at a desired essentially constant value. The gas-air mixture from the blower 26 is passed into a fan shaped conduit for distribu-tion across the area of the vat of the food cooker and to a burner 30 mounted at the entrance of conduit 29 to a heating chamber at the bottom of the vat.
Also shown is the chimney 17, front panel 15 and tempera~ure control knob 16.
This temperature control knob operates a thermostat to open or close valve 28b in response to the temperature of cooking oil in the vat. The heating chamber as hereinafter described is positioned underneath most of the area of the bottom of vat 20. The heating wall 32 may occupy less than all the bottom of the vat, leaving a well 40 at one portion. This provides for collecting in well 40 solid particles which collect in the oil. A drain 43 at the bottom of well 40 is adapted to be opened by valve 42 to empty used cooking oil for cleaning.
In Figure 4, which is a top view of the food cooker illustrated in Figures 1, 2 and 3 are shown the side walls 111 back wall 12, chimney 17, shelf 22, front panel 15 and control knob 16 as previously described. In the top view, as shown in Figure 4, can be seen bottom panel 32 of the cooking vat. Attention is directed to the corrugations in the surface configuration of this bottom panel, as illustrated in larger size in Figure 5. There shown in Figure 5, is a doublè walled bottom panel or heat transfer chamber generally designated 33 to accomplish isothermal heat transfer. The upper surface of this heat transfer chamber 33 is a corrugated panel 32 forming the bottom of the cooking vat. Braces 39 separate and support panels 32 and 34. The lower wall or bottom of the heat transfer chamber 33 is again a corrugated metal ~OS0'~94 sheet. Contained within the heat transfer chamber 33 is a quantity of a liqu.id 35 which may as desired be sufficient to completely fill the corrugated or roughened contour of sheet panel 34. The edges and corners of the heat transfer chamber 33 are thoroughly and completely sealed and the open space within the heat transfer chamber is substantially completely free of non-condensable gaseous material.
In Figures 5A and 5B are showTI other embodiments of the invention.
In Figures 5A and 5B heat transfer chamber 33 comprises a corrugated bottom wall or sheet panel 34 holding liquid 35. Braces 39 support and separate upper panels; in Figure 5A, upper panel 32a is smooth and flat, and in Figure 5B, upper panel 32b is corrugated. In each of Figure 5, Figure 5A and Figure 5B bottom panel 34 is corrugated to provide extra heat transfer surface to accomplish extremely rapid heat transfer from the heat source to liquid 35.
Referring again to Figure 3, the bottom 32 of the cooking vat or the heat transfer chamber 33 is positioned directly over burner 30. A heat chamber including vertical baffles 37 and upper baffles 36 extends substan-tially completely under the bottom of the vat to provide a relatively broad hot air space to permit heating of the heat transfer chamber, the whole being mounted on supports 41. At the rear of this space, an exhaust 38 is position-ed to convey burned gas into chimney 17 and thus out of the food cooking de-vice. Support members 39 within the heat transfer chamber serve to maintain the desired sturdiness and separation between the upper wall 32 of the heat transfer chamber, which wall forms the bottom of the cooking vat, and the lower wall 34 of the heat transfer chamber. Being positioned between the hot air chamber and the vat itself, wall 34 is the "evaporator" or heat input surface of the heat transfer chamber and the upper wall 32 of the heat trans-fer chamber is the "condenser" or heat output wall of the heat transfer chamber.
In use and operation, the gas pipe 27 is connected to a source of combustible gas and motor 25 is connected to an electric outlet. The cooking vat 20 is filled with a cooking oil (not shown) to a level slightly below lOSO;~94 shelves 22 and 24. The blower is turned on, the burner lighted and the liquid 35 in heat transfer chamber 33 virtually immediately is strongly heated.
The ~heat transfer chamber is substantially free from non-condensable material, and accordingly, there is a uniform vapor pressure of liquid 35 in the vapor space. Whenever the condenser 32 of the heat transfer chamber is colder than the evaporator 34 of the heat transfer chamber, this vapor in the vapor space continuously condenses on the condenser, thus releasing its energy or heat and consequently, heating the wall and the cooking oil within the cooking vat.
The entire condenser 32 is constantly heated by an extremely rapid flow of heat to and through the wall 32. If for some reason, absorption of heat into the oil is blocked as, for example, by food which drops to the bottom of the vat or which is caused to adhere to the bottom of the vat, the result is not localized overheating of wall 32, but instead, isothermal transfer of heat.
If the heat is not absorbed at any specific location, the wall inherently remains at the same temperature as the other portions of the wall. ~leat ceases to be transferred through the wall at such a localization of heat transfer and instead, the heat is isothermally applied to the other areas of the wall 32. ronsequently, heating energy is imparted to the cooking oil at all locations where circulation is free to permit this heat to be spread throughout the volume of the cooking oil. Burning of the food or overheating of the oil cannot occur at any location except if the entire volume of the oil is burned or overheated.

_9._

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A deep fat fryer comprising: a relatively deep cooking vat adapted to contain and to heat for immersion cooking an edible, thermally decompos-able cooking oil which is rapidly decomposed at temperatures substantially above normal cooking temperatures, the internal portions of said vat being free from obstructions and open at its top; a jacket forming a sealed chamber at least partially surrounding said vat and sealing therein a heat transfer liquid volatile at normal cooking temperature, the interior of said chamber being essentially devoid of substances which are not condensible at operating conditions; heating means; means defining a heating zone for the heat trans-fer liquid, said means including a first heat transfer wall and a second heat transfer wall, each of said heat transfer walls forming a portion of said chamber, said first heat transfer wall being adjacent said heating means to conduct heat from said heating means to said heat transfer liquid to vaporize said liquid, said second heat transfer wall being a wall of said cooking vat adapted to be in direct liquid-to-wall contact with said cooking oil to transfer heat rapidly from said wall to said cooking oil; said chamber defining a free path between said first heat transfer wall and said second heat transfer wall to permit vaporized heat transfer liquid to pass from said first heat transfer wall to said second heat transfer wall, and to permit condensed heat transfer liquid to return to said first heat trans-fer wall.

2. A deep fat fryer according to Claim 1 wherein said second heat transfer wall extends along the bottom of said vat across less than all of said bottom, further comprising a recess at the bottom of said vat adjacent said second heat transfer wall, whereby heating said oil causes convection currents for carrying contaminants in said oil to said recess.

3. A deep fat fryer according to Claim 2 wherein said heating means comprises a gas burner and means to direct hot combustion gases to said first heat transfer wall.