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BACKGROUND OF THE INVENTION
Food preparation apparatus wherein heat is transferred from a heat source
to a food preparation surface by a vaporized working fluid is disclosed
and claimed in an earlier application for Letters Patent Ser. No. 165,569
of Lazaridis, Searight and Shefsiek, filed July 23, 1971, now abandoned,
which is assigned to the same assignee as the present application. As is
explained in that earlier application, 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 to cooking surfaces or to foodstuffs are stated in the earlier
application.
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 understandably hotter at the burner end than at the exhaust end.
Although this may result in an average temperature which satisfies the
requirements for heat transfer from the tube to the oil, it unavoidably
creates, 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 equivalent to the average temperature of the
burner tubes. Further, a vat fitted with electric heating coils or
elements will heat oil contained in it only in the vicinity close to the
heating elements. Such electric elements tend to overheat the cooking oil
in this close vicinity. Most efficient use of electric elements occurs
when they are immersed in the cooking oil vat where they require
significant space and create objectionable cleaning problems. In the
absence 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 parameters, 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 absorption of
the oil by the food being cooked; e.g.: in order to have a relatively 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
temperature as possible in order to minimize the formation of undesirable
reaction products in the oil.
Deep fat frying equipment that is currently available does not achieve
desired temperature uniformity and control. The formation of undesirable
reaction products of frying oil is a problem which exists with currently
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 localized 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, the rate at which undesirable reaction products are produced
increasing exponentially 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 decomposition at normal cooking temperatures. If, for example,
localized hot spots exceed the average surface temperature by 25.degree. F
or more, they will contribute a disproportionally high amount of thermal
decomposition.
Replacement of oil that has formed an excess of undesirable reaction
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.
BRIEF SUMMARY OF THE INVENTION
Methods and apparatus are herein disclosed for substantially isothermal
heating of a vat containing cooking liquid, such as deep fat frying oil.
The apparatus uses a mechanism 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
purposes be considered to be substantially eliminated. Particles of food
materials and the like that adhere to the vat walls will not result in oil
breakdown 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 transferred 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 direct immersion heating equipment.
With 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
before cooking temperature is recovered. On the other hand, in the methods
and 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 temperature of the oil does not drop as far as in
prior apparatus and recovers more quickly. The result is a reduction in
the amount of oil absorbed by the food. 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 method of heating, with an
isothermal surface, a thermally decomposable liquid such as oil for deep
fat frying of foodstuffs, comprising the steps of providing a vat for the
cooking liquid which vat has a bottom and side walls, placing the cooking
liquid in the vat, locating a vaporizable liquid in the absence of vapors
which remain in gas phase at the operating temperature in the vicinity of
the exterior of the vat, confining the vaporizable liquid so that its
vapors will come in contact with said exterior, heating the vaporizable
liquid sufficiently to cause vaporization and condensation thereof, such
condensation occuring at the exterior of the vat at a temperature in a
range sufficiently high to heat the cooking liquid in the vat to an
operating temperature but below a temperature range in which an
unacceptably high rate of undesirable reaction products formation occurs
in the cooking liquid. Preferably, the cooking temperature of a cooking
oil when used in the vat is maintained in the range between, approximately
350.degree. F and 400.degree. F.
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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a general isometric view of a food cooker according to the
invention;
FIG. 2 is a top plan view, partly broken away, of FIG. 1;
FIG. 3 is a side view, partly broken away, of FIG. 1;
FIG. 4 is a top view of FIG. 1, showing the formation of the bottom wall of
the cooker;
FIG. 5 is a fragmentary enlarged cross-section of a heat-transfer component
of FIG. 1;
FIG. 5A is a fragmentary enlarged cross section of a heat transfer
component according to another embodiment of the invention; and
FIG. 5B is a fragmentary enlarged cross section of a heat transfer
component according to a further embodiment of the invention.
DESCRIPTION OF THE DRAWINGS
In FIG. 1, is illustrated the food cooker generally designated 10 according
to one embodiment of the invention. The cabinet of the food cooker has
side walls 11, a back wall 12 and a front panel generally designated 13. A
bottom segment 14 of the front wall is removable, 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 combustion gases.
The cooking chamber or vat, generally designated 20, is defined by the two
side walls 11, a rear panel 21 and a front wall 18 (see FIG. 3)
terminating in an upper shelf 22. The cooking vat is adapted and intended
to contain a cooking oil or other cooking liquid up to a level generally
approximately 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 FIG. 2 and FIG. 3 are shown the deep fat fryer generally designated 10,
as in FIG. 1, and illustrating the cooking vat 20, including side walls
11, back wall 12, front wall 18 and front shelf 22 and rear panel 24.
Within the cabinet in front 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 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 thermostatically-operated gas valve 28b, which may be
operated from a safety circuit, 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 distribution 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 temperature control knob 16. This
temperature control knob operates a thermostat to open or close a 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 FIG. 4, which is a top view of the food cooker illustrated in FIGS. 1, 2
and 3 are shown the side walls 11, back wall 12, chimney 17, shelf 22,
front panel 15 and control knob 16 as previously described. In the top
view, as shown in FIG. 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 FIG. 5. There shown in
FIG. 5, is a double 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 sheet. Contained within the heat transfer chamber 33 is a quantity
of a liquid 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 FIGS. 5A and 5B are shown other embodiments of the invention. In FIGS.
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 FIG. 5A, upper panel 32a is smooth and flat, and in FIG. 5B,
upper panel 32b is corrugated. In each of FIG. 5, FIG. 5A and FIG. 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 FIG. 3, the bottom 32 of the cooking vat or the heat
transfer chamber 33 is positioned directly over burner 30. A heat chamber
including verticle baffles 37 and upper baffles 36 extends substantially
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
positioned to convey burned gas into chimney 17 and thus out of the food
cooking device. 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 transfer 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 electrical outlet. The
cooking vat 20 is filled with a cooking oil (not shown) to a level
slightly below 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. Heat 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. Consequently, 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.
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