High efficiency convection oven

Processes and systems for improvements on the efficiencies of convection ovens utilizing exhaust heat recuperation in the form of heat exchangers and premixed combustion systems, and a combination thereof. The heat exchangers and premixed combustion systems allow for increased efficiency of use of fuel and gaseous mixtures within convection oven cavities for improved heating capabilities and decreased efficiency losses.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates generally to ovens and, more particularly, to convection ovens having or exhibiting high operating efficiencies through exhaust heat recuperation.

Description of Related Art

The convection oven as a product and process, in the small (residential/restaurant) and large (industrial baking) context, is covered by a substantial body of known art. This known art includes gas fired convection ovens such as U.S. Pat. No. 5,460,157 which teach what is referred to as a “typical” or “conventional” convection oven in this document. U.S. Pat. No. 7,699,237 for a method for controlling exhaust flow from a cooking cavity of a baking oven concerns the use of variable speed exhaust rates to affect rates of various stages of the baking process. U.S. Pat. No. 8,146,488 is directed to a cooking oven with a premix burner for boilers and relates to the use of a premixed burner, adapted from boilers, to ovens cooking with steam generators. U.S. Pat. No. 4,337,585 is directed to heat recovery and an air preheating apparatus for textile dryer ovens and concerns high-temperature heat recovery from a large industrial oven, using “a plurality of thermal mass disks which operate as heat sponges to pick up heat from exhausts as it leaves the dryer and subsequently to put that heat into entering replacement air.”

The basic components and operation of commercial convection ovens are for the most part very similar. For example, commercial convection ovens tend to have induced-draft burners in the bottom of the oven and a fan built into the oven that pulls air from the burner/combustion area through the sides of the oven into the back and into the oven cavity before they are pushed out of the flue of the oven. Commercial convection ovens generally operate in the 40-50% cooking efficiency range. Generally, such cooking equipment include low thermal efficiencies and higher flue gas temperatures due to the higher temperature heat sink (oven cavities are 250-500° F., fryer oil is ˜350° F.) and, as a result, thermal efficiency improvements generally require heat recovery from this high temperature exhaust.

A simple illustration of this for convection ovens follows. The temperature of the oven cavity establishes an effective efficiency limitation on the process, regardless of how effectively or quickly the burner heats the oven contents. For example and as shown inFIG.1, an oven cavity at 350° F. with an expected 100° F. temperature difference from the hot flue gases to the cavity will necessarily have a thermal efficiency of no greater than 75%, which with typical cycling losses yields an operating efficiency of no more than 67%.

There is a need and a demand for convection ovens, such as for commercial applications, in which convection ovens exhibit or provide increased, desirably significantly increased, operating efficiencies.

SUMMARY OF THE INVENTION

This invention provides a new convection oven assembly or system as well as a new process of convection oven operation having or exhibiting improved or increased, and preferably desirably high, operating efficiencies. Specifically, a preferred convection oven assembly utilizes exhaust heat recuperation, transferring otherwise wasted heat from exiting flue gases to incoming combustion air to a fuel-fired combustion process. Exhaust heat recuperation is currently in use in large scale applications such as industrial furnaces, large stationary engines, process heaters, and other large combustion equipment.

As described in greater detail below, the subject invention development desirably employs and integrates exhaust heat recuperation with convection ovens to achieve operating efficiency gains that have hereto before been unattainable or unrealizable.

As used herein the term ‘thermal recuperation’ generally refers to the recycling or recovering of energy, e.g., heat, such as might normally be rejected as waste heat, back to the combustion apparatus, e.g., the convection oven.

DETAILED DESCRIPTION

The subject invention development acknowledges the operating efficiency limitations of conventional prior art commercial convection ovens such as shown inFIG.1and introduces heat recovery external to the oven cavity, such as shown inFIG.2, to increase thermal/operating efficiencies to 85%/77% respectively, a 10% increase. To achieve this, the subject invention development integrates heat recuperation with a convection oven design (integrating recovery/primary HXs, as shown inFIG.2) in novel ways for partially-premixed combustion systems, with an inducer blower downstream that “pulls” combustion gas around at least one side of the oven106and into the cavity, acting as a primary heat exchanger, and then “pushes” resulting flue gases112out of a flue110from an oven cavity100. Also, for more advanced premixed combustion systems that may have “push-only” or “push-pull” designs, the premixed system has the added benefit of better system control through input modulation, better controlling oven temperature and eliminating the large temperature swings around a set point currently experienced with traditional convection ovens. In addition to cooking efficiency, no matter whether food is being cooked or the oven is in standby at a set temperature with no cooking, there is always energy being lost. A great deal of oven energy use is energy lost while no cooking is taking place. The fan on a conventional convection oven is often running, pulling heated air out of the oven even when the burners are not running and no food is being cooked. This results in ovens commonly experiencing very high standby energy losses which account for a large percentage of the overall oven energy use. The incorporation and use of the subject heat recovery system helps to recover heat during standby oven use as well as further decreasing overall energy use of the oven.

In accordance with one aspect of the subject invention development, a novel method of internal heat recuperation is employed to increase the operating efficiency of a commercial convection oven, such as commonly used in restaurants and other food service establishments. In one embodiment, a portion of the exhaust heat, i.e., energy that is normally rejected as waste heat is recycled back to the combustion process via incoming combustion air. In one embodiment, this ‘thermal recuperation’ is physically performed by a first heat exchanger (HX)102that is physically integrated with an oven cavity100and in which an exiting flue gas112exhaust stream and incoming combustion114air communicate via heat transfer. Those skilled in the art and guided by the teachings herein provided will understand and appreciate that many HX designs may apply (counter/co-flow, finned tube/shell-and-tube, etc.). This is expected to be an inexpensive addition to conventional convection ovens, as an example one manifestation of this concept would have tubing on the top, sides and/or bottom of the oven to transfer the heat with little additional cost to the oven design and potentially utilizing existing air-moving equipment (blowers/fans) to drive the process.

Preheating incoming combustion air114by cooling exiting flue gases112to practical temperature limits is expected to boost the cooking efficiency of the oven by 3-10%.

In accordance with another aspect of the subject invention development, a premixed combustion system is used or employed as opposed to the more common induced-draft combustion systems, to provide even better temperature control along with the efficiency boost in the oven. The use of a premix combustion system226within a convection oven permits not only improved control of the cooking process, but also the ability to dramatically reduce the emissions of criteria air pollutants, particularly oxides of nitrogen (NOx) and carbon monoxide (CO). Modulation of the premixed combustion system226can reduce cycling energy losses, which are estimated as 8-10%.

In accordance with another aspect of the subject invention development, the recycling of a portion of the exhaust heat back to the combustion process via incoming combustion air, such as discussed above, is desirably combined with the use of a premixed combustion system314as opposed to the more common induced-draft combustion systems, to provide even better temperature control along with the efficiency boost in the oven.

The following three embodiments represent various combinations of an improved convection oven.

As shown inFIGS.3A and3B, an Exhaust Heat Recuperator (EHR) such as in the form of one or a series of heat exchangers102is integrated with or separate from the overall convection oven that capture otherwise wasted heat from the exiting flue gases112to the incoming combustion air114. In one embodiment of this invention, a recuperative heat exchanger102is separate from an oven cavity100or cooking cavity of the convection oven, and a combustion air manifold122conveys preheated air to the combustion cavity, as shown inFIGS.2-3B. Based on estimations and limited laboratory testing, an EHR can improve convection oven operating efficiency by up to 10%. The EHR can be comprised of simple flue-to-air heat exchangers to minimize material/manufacturing cost and added pressure drop, such as tube-in-tube, shell-and-tube, finned-tube, utilization of turbulators/inserts or other means of increased surface area, and other methods. While the oven cavity100is insulated, it will lose heat and it may be advantageous to have combustion gas104pass over an exterior of the oven cavity100to capture otherwise wasted skin losses, prior to absorbing heat from the flue gases112within the cavity itself and then the first heat exchanger102.

This concept can be readily integrated into a conventional convection oven design, which uses a blower/inducer108at an entrance118of the oven cavity100, which maintains a suitable pressurization within the oven cavity100and promotes internal circulation, shown specifically inFIG.3B, for improved cooking/temperature control. Preferably, a variable speed inducer blower108: 1) will draw combustion gas104into the oven cavity100through the at least one side106of the oven cavity100and, after combustion in a combustion cavity124, draw hot products of combustion (i.e. flue gases112) into the oven cavity100for circulation; and 2) push out the flue product gases112through another side of the oven cavity100via the flue110to an exhaust vent under slightly positive pressure. Before reaching the combustion cavity124, the combustion air114is mixed with a fuel116. The combination of the combustion air114and the fuel116allow the combustion reaction to take place in the combustion cavity124to create the resulting combustion gas104. Integration of the EHR, which necessitates a combustion air intake manifold122; means of conveying combustion gas104from the EHR to the oven cavity100; and an additional restriction within the EHR for exiting flue gases prior to exit, will likely require the use of a more powerful blower inducer as a secondary inducer120as a part of the oven cavity100.

Shifting from a partially-premixed, induced draft combustion system to a premixed combustion system226inFIGS.4A and4B, within an oven cavity200the convection oven in isolation has clear advantages. In summary, an electronically-controlled blower202“pushes” a fuel/air mixture into a high-efficiency premix burner220which provides a controlled hot stream of flue gases204to be drawn into the oven cavity200. The premixed combustion system226premixes a fuel supply214and an air supply216prior to introduction to the oven cavity200. The premixed combustion system226draws the fuel supply214and the air supply216into the oven cavity200by utilizing the blower202, the premix burner220and a fuel and air mixer210, which may exist immediately upstream or downstream (as shown) of the blower202.

The subject embodiment has a number of key and unique advantages. The premix burner220can be designed in such a manner to take advantage of the precise fuel/air mixture flow control by working in conjunction with the fuel and air mixer210so as to reduce NOx and CO emissions. The premix burner220as part of a combustion cavity218is placed relative to the oven cavity200to receive the fuel supply214and the air supply216once they have been combined via the fuel and air mixer210. The burner220may be composed of perforated, high-temperature resistant metal, sintered, woven, or knitted metal fiber mats, porous ceramic or metallic foamed solids, or other material. Through this, the convection oven using the premixed combustion system226can achieve low NOx and CO emission rates not feasible with conventional induced-draft, partially-premixed combustion systems.

The premixed combustion system226, including the electronically-controlled blower202with variable speeds, fuel and air mixer210with proportional control (pneumatic or electronic fuel metering), and the premix burner220, will be capable of modulation. This modulation, increasing or decreasing the heating rate with time, will enable the oven to more tightly maintain oven cavity200temperatures and vary the heat input to the oven with time as required by the cooking or baking process as per electronic control. Modulation, when controlled effectively, also will reduce cycling losses and increase operating efficiency. Additionally, by virtue of a compact flame structure with the premix burner220, this can better enable safer combustion and oven operation with improved ignition and flame sensing controls for the combustion cavity218.

Additionally, through improved fuel supply214and air supply216mixing control, the convection oven can operate with less excess combustion air. Typically convection ovens operate with large amounts of dilution, 10-16% stack O2on a dry basis, by virtue of the partially-premixed combustion system and to minimize CO emissions. With the premix burner220, this dilution can be dramatically reduced, which improves combustion efficiency, reduces the volume of the air supply216required and thus the blowers202/219energy to move it, and can more effectively cook a product through a better balance of heating and drying. On the latter, flue gases204with a lower excess air supply216level will have a higher concentration of water vapor (a higher dew point) and dry product at a slower rate relative to the heating rate, assuming the design flows flue gases204through the oven cavity200. Coupling the precise control of the premix combustion process with independent control of the oven cavity200circulation, permits reducing excess aeration for sake of efficiency while permitting the oven to circulate combustion gas230both within and outside the cavity200to promote effective cooking.

It is generally advisable that the oven cavity200not be excessively pressurized. This is for at least two reasons, oven doors commonly do not seal perfectly and in normal operation kitchen staff will open/close the doors while the oven is operating. A pressurized oven cavity would promote leakage from the front and, upon opening the door, push hot gases onto oven operators. As a result, it is required that care be taken with the degree of pressurization of the combustion cavity218and that the blower/inducer219downstream of the combustion cavity218will be required.

Sizing the blower/inducer219for a ‘balanced’ design, such that the combustion cavity218is slightly pressurized to overcome the flue gas204pathway to the oven cavity200(and any upstream combustion air ducting) but the blower/inducer219at or downstream of the oven cavity200ensures the oven at an acceptable level of pressurization. This can be further ensured by a differential pressure sensor206between the oven cavity200and an installation space228.

Improved sealing of a door224of the oven and an operational lock222on the door224to prevent opening until the combustion system is off or at a sufficiently low heating rate setting will also assist in maintaining a desired pressurization.

With a focused intake of the air supply216, into the combustion premix burner220, care will be required to avoid drawing in grease/dust laden air into the burner220. As a result, it may be advantageous to use inlet filtering of the air supply216and/or draw the air supply216from a specific location (rear of oven, etc.) to limit issues arising.

This additional embodiment shown inFIGS.5A and5Bis in effect combining items 1) and 2), which in addition to all the advantages described above, permit additional advantages.

The subject embodiment, as shown inFIGS.5A and5B, has potential for improving operating efficiency as the solutions are additive in nature, an exhaust heat recuperator (EHR)302increasing steady state efficiency by 3-10% while a premixed, modulating combustion system314reduces the system cycling losses, further increasing operating efficiency in an oven cavity300by up to an additional 8%. The oven cavity300is therefore a hybrid of the oven cavities in 1) and 2).

The premixed combustion system314will effectively meter combustion gases304, and combustion air318that is necessary for complete combustion, controlling the volume of air318and flue gases330flowing through the EHR302and the premixed combustion system314. To aid in this control, a bypass326may be located on the EHR302as an alternate to a flue exit328also on the EHR302. This limits the EHR302physical size and/or air-moving equipment necessary to overcome the pressure drop of the EHR302. The premix combustion system314will first mix fuel316and air318before the EHR302transports combustion gases304throughout the oven cavity300.

With the EHR302, the convection oven will necessarily collect and transport combustion gases304upstream of the oven cavity300, and with the premixed combustion system314a downstream blower/inducer308can overcome the pressure drop associated with this. The downstream blower/inducer308can pull the combustion gases304through a side306of the oven cavity300; and then subsequently push the combustions gases304out of a flue310through the oven cavity300. This means coupling the EHR302and combustion air manifold312with the premixed combustion system314will, in most cases, require a combustion blower322at or upstream of where the fuel316and air318is mixed together prior to this mixture reaching a premix burner332and then a combustion cavity324. Where air moving equipment is located, the downstream blower/inducer308will be necessary at or upstream of the oven cavity300. This arrangement can act to decouple the circulation within the oven from the movement of flue gases330, which may be advantageous for improving cooking times by stabilizing cavity temperatures and reducing the boundary layer thickness on a product320inside the oven cavity300.

In the small (residential/restaurant) and large (industrial baking) context, the subject invention desirably incorporates or utilizes: (a) premixed combustion with precise heating control; (b) exhaust heat recuperation to combustion air for partially-premixed and premixed combustion applications; and (c) the system controls required as a method of use in ways that have hereto before not been described by the prior art. More particularly, for example, 1) the subject invention development provides high-efficiency and goes beyond baking-only applications; 2) the designs and operation of the subject invention development are not applied to ovens with steam generators; and 3) the subject invention development neither uses thermal storage or other batch process for heat recuperation.