Patent Application
20060169271
1. Field of the Invention
This invention relates to a cooking oven for cooking of food products using a synthetic thermal fluid.
2. Description of Prior Art
Large scale, batch cooking ovens for cooking food, such as linear cooking ovens, typically include a conveyor or conveyor belt for conveying food to be cooked from an inlet to an outlet and through a cooking chamber. A heat source, air mover and heat exchanger are typically provided within the cooking chamber for cooking the food provided on the conveyor.
Existing cooking systems, including linear ovens, fryers and heated platen cooking systems, typically use Paratherm NF; a mineral oil-based thermal fluid for heat exchange within the cooking chamber, grill and/or fryer. This thermal fluid limits cooking temperatures to below 600 degrees F. because the Paratherm NF oil will break down if used at temperatures continuously above 600 degrees F. with an oil-air interface.
It is therefore an object of this invention to provide a cooking oven that provides an elevated temperature cooking range using a synthetic oil thermal fluid.
It is another object of this invention to provide a cooking device that is heated by thermal fluid at temperatures between approximately 600 and 750 degrees F., and more preferably between approximately 600 and 650 degrees F.
A cooking device according to one preferred embodiment of this invention includes a housing having an inlet and an outlet, said housing enclosing a chamber. The chamber may comprise a cooking chamber, a deep fat fryer reservoir or a grill enclosure. A heat exchanger is preferably positioned in fluid communication with the chamber and includes a circulating flow of synthetic thermal fluid. The synthetic thermal fluid, or heating oil, preferably circulates within the heat exchanger at a temperature between approximately 600 and 750 degrees F.
The invention preferably additionally includes a system for protecting the synthetic thermal fluid from degradation including a sealed inert gas atmosphere, such as nitrogen; a bypass filter to remove thermal fluid sludge; and a periodically electronic initiated purge of volatile decomposition products. A separate local loop for fryers to provide a lower temperature food safe heating may also be necessary. Such a separate local loop may be used in a thermal fluid system that also heats ovens that demand higher temperatures.
The above-mentioned and other features and objects of this invention will be better understood from the following detailed description taken in conjunction with the drawing wherein:
According to a preferred embodiment of this invention, conveyor 40 operates continuously whenever cooking oven 10 is in operation. Thus, cooking oven 10 may be referred to as a continuous oven. Uncooked food is loaded onto conveyor 40 continuously at inlet 17 and transported through chamber 30 and emerges cooked, at the outlet end of the oven.
Alternatively, or as described in more detail below, in addition, a cooking system according to this invention may further comprise fryer 20, such as a deep fat fryer or a grill arrangement such as a heated platen thermal heated cooking system. In such embodiments, chamber 30 may comprise a fryer reservoir or a grill enclosure. Any such cooking system that relies upon a traditional mineral oil such as Paratherm NF oil for heat transfer would benefit from the subject invention.
According to one preferred embodiment of this invention shown in
Heat exchanger 60 is preferably positioned in fluid communication with chamber 30, as shown in
Heat exchanger 60 may be a flat plate heat exchanger, fin and tube heat exchanger or similar heat exchanger that preferably includes an internally circulating flow of thermal fluid. Flat plates may include a hollow portion or channel for containing and transporting a thermal fluid, specifically a synthetic heating oil. The synthetic heating oil preferably passes through channels or tubes within heat exchanger 60 and serves to elevate the temperature of the heat exchanger plates or fins to a desired temperature or temperature range. A preferred synthetic heating oil may comprise SLF1 HTF 32 from Synthetic Laboratories & Filters, or similar such synthetic heating oil that permits operating temperatures up to approximately 650 degrees F. Silicon based thermal fluid oils such as Syltherm 800 can operate to 750 degrees F. Thermal fluid heating permits a higher humidity oven atmosphere than conventional gas-fired heating systems because of the absence of moisture diluting entering combustion air.
Depending upon the flow pattern of air within cooking oven 10, heat exchanger 60 may be positioned in various parts of cooking oven for optimum performance. As a result of the described configuration of cooking oven 10, heat exchanger 60 may direct air flow from an upper portion of chamber 30 to a lower portion of chamber 30 and across food products and/or from one end of chamber 30 to the opposite end and across food products.
Chamber 30 generally includes a platform in line with one or more air manifolds 70, an air mover positioned over the platform and, preferably at an end of heat exchanger 60 positioned adjacent to the platform wherein air from the air mover is induced and/or forced through heat exchanger 60 and through air manifold 70 across the platform. According to such an embodiment, the platform may comprise conveyor 40, cooking surface, or any other surface requiring redirected heat from an air mover and/or heat source to another location within chamber 30.
More specifically, according to one preferred embodiment of this invention, heat exchanger 60 is positioned adjacent conveyor 40 and induces air through air manifold 70 and across food items on conveyor 40. In addition, the air may travel through a damper system and/or a plurality of manifolds, whereupon such air is impinged on the food items and the cooled return air is then returned through the air manifold.
According to one preferred embodiment of this invention, thermal fluid heater 80 is connected within heat exchanger 60. Thermal fluid heater 80 preferably operates at temperatures exceeding approximately 600 degrees F. and thereby maintains a suitable temperature of the synthetic thermal fluid to between approximately 600 degrees F. and 750 degrees F.
The upper limit of most practical cooking devices is between approximately 500 degrees F. and 550 degrees F. Also, a 100 degree F. minimum temperature differential between thermal fluid and chamber temperature is normally required to achieve desired heating capacity. Thus, a preferred thermal fluid temperature according to a preferred embodiment of this invention is in the range of 600 to 650 degrees F.
In fryers 20 or in combination cooking oven 10/fryer 20 systems, thermal fluid temperatures are increased to increase oven efficiency, the heat flux in the fryer heat exchanger inlets will become large. This causes the frying oil next to the heat exchanger to approach the same temperature as the thermal fluid. At this condition, premature frying oil damage may occur.
The use of thermal fluid heated fryers in food processing plants has lengthened the safe life of frying oil. The thermal fluid heated fryers eliminated the hot spots on the old gas fryer tubes and thus avoided forming carcinogenic decomposition chemicals in the frying oil.
According to a preferred embodiment of this invention, such as shown schematically in
Accordingly,
According to a preferred embodiment of this invention, zone control valves of both ovens 10, 10′ and fryers 20,20′ supply a variable amount of the heated thermal fluid to ovens 10, 10′ and fryers 20, 20′ to maintain constant frying oil or constant oven atmosphere temperature. As discussed above, traditionally, the thermal fluid temperature was maintained within a range of approximately 500 degrees F. to approximately 530 degrees F. for fryers. The development of thermal fluid heated ovens required and permitted a higher thermal fluid temperature than traditional fryers. Such thermal fluid heated ovens have been rated as high for use with thermal fluid as hot as 572 degrees F. Such thermal fluid heated ovens typically require a very hot fluid because the fryers are heating frying oil while ovens are heating air.
Prior to the invention described herein, prior art thermal fluid heaters have been limited to approximately 572 degrees F. output temperature. As a result, thermal fluid heated ovens may struggle to produce adequate heat. The present invention enables thermal fluid heaters to obtain temperatures of approximately 590 degrees F. up to approximately 620 degrees F. or higher for supplying thermal fluid at such temperatures to both ovens 10 and fryers 20. Such temperatures, without additional controls, may result in damaging frying oil temperatures in the boundary layer of the thermal fluid heat exchanger.
Put another way, as thermal fluid temperatures are increased to increase oven efficiency, the heat flux in the heat exchanger inlets of fryer 20 will become large. This causes the temperatures of the frying oil next to the heat exchanger to approach the temperatures of the thermal fluid. At this condition, premature frying oil damage may be likely.
According to one preferred embodiment of this invention shown schematically in
A preferred embodiment of this invention having the fryer local loop preferably uses a separate fryer local loop to provide a lower, food safe temperature heat source to fryers 20, 20′ while providing efficient high temperature fluid to ovens 10, 10′ from a single thermal fluid heater 80. The system according to this embodiment further provides more accurate, stable fryer temperature control when used with PID control loops due to lower fryer supply temperatures.
Benefits of the subject invention and the resulting higher operating temperatures include a reduction in heat exchanger size and thermal piping size and cost while maintaining a cooking system having the same heat capacity and operating temperatures in the 500 to 550 degrees F. range. Food products, such as sausage, are cooked with beneficial appearance and taste as a result of elevated cooking temperatures. Capacity and throughput of food product through the cooking system is improved as a result of larger thermal capacity of heat exchanger 60 and synthetic thermal fluid.
Operation at very high temperatures often breaks down the thermal fluid molecules to lighter, volatile fractions that are preferably allowed to accumulate in expansion tank 110 connected with respect to the cooking device. Expansion tank 110 is preferably covered with a Nitrogen blanket to eliminate oxidation. Periodic flushing of expansion tank 110, such as once per month, is recommended to prevent a buildup of volatiles. Prompting for this flushing may be included in control software. An example of such control software is LINK™ software supplied by FMCTI. In addition, pressure regulator 115 may be connected within the system to maintain desired operating pressures within the system.
At high temperatures, some thermal fluid molecules polymerize to form sludge. The high temperature system according to this invention preferably includes bypass filter 120 and strainer 125 to permit sludge removal.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that this invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.
