Patent Application
20130213378
The present disclosure pertains to burners and particularly to burners for heat exchangers. More particularly, the disclosure pertains to burner manifolds for the heat exchangers.
The disclosure reveals a burner system for a central furnace. The system may have a wedged or other shaped burner box designed to reduce acoustic resonance. An air-fuel mixer or mixing component, such as a venturi, may be attached to a smaller end of the burner box at about a right angle relative to a direction of a gas and air mixture leaving the larger end of the box. The mixer may provide the gas and air mixture to the box. A burner head may be attached to the larger end of the box. The burner head may be sufficient for virtually all heater sections of a heat exchanger. A spacer and an orifice shield or plate may be situated between the burner head and the heat exchanger. A draft fan may pull the gas and air mixture from the mixer and through the box and the burner head. The mixture may be ignited into a flame which is pulled into the sections of the tube or clamshell based heat exchanger. The fan may be situated at the exhaust of the heat exchanger. Some of the exhaust from a flue of the exchanger may be re-circulated by being mixed with air to the mixer.
The present apparatus may solve a problem of reducing NOx emissions on a central furnace to less than 14 nanograms per Joule (ng/J) of useful heat. The apparatus may use a premix burner with a 1:1 gas air valve and flue gas recirculation to achieve sub 14 nanograms per Joule (ng/J) of useful heat of NOx emissions. The apparatus may also use a detected partition panel temperature to insure that the system is functioning properly.
The present apparatus may relate to technology disclosed in U.S. Pat. No. 6,923,643, issued Aug. 2, 2005, and entitled “Premix Burner for Warm Air Furnace”, and in U.S. Pat. No. 6,880,548, issued Apr. 19, 2005, and entitled “Warm Air Furnace with Premix Burner”. U.S. Pat. No. 6,923,643, issued Aug. 2, 2005, is hereby incorporated by reference. U.S. Pat. No. 6,880,548, issued Apr. 19, 2005, is hereby incorporated by reference.
The apparatus may have a premix burner structure constructed with 45 degree angle convolutions. The convolutions may be used to increase surface area resulting power inputs similar to inshot burners. The air and fuel may be supplied using a 1:1 gas valve and the mixer. The air/fuel mixture (i.e., premix) may be introduced into a box/manifold to which the burner head is assembled. Flue gases may be recirculated by running a pipe from the flue to the inlet of the mixer. The recirculation may be controlled by an orifice which is sized to provide the correct amount of flue products to achieve the desired emissions. Partition panel temperature may be monitored to insure proper combustion. A high partition panel temperature may indicate high burner CO2 or low flue gas recirculation.
The present approach may incorporate a burner solution designed to bolt onto existing warm air furnace heat exchangers with no modifications to the “hot” side of the furnace. Gas (e.g., natural, LP, butane, or the like) may enter the gas valve. The gas valve may regulate the gas pressure. The mixer may mix gas and air. A gas orifice and an air orifice contained in the mixer may be sized to obtain combustion CO2 ranging from 5 to 7 percent for low NOx emissions and up to 9 percent for increased combustion efficiency.
The gas/air mixture may be admitted into the burner box at an angle of 90 degrees or other angle relative to the burner head to enhance mixing of the gas and air and to reduce the length of the assembly. The burner box may be wedge shaped. The depth and width (aspect ratio) of the burner box may be designed to reduce acoustic resonance of the premix burner. The box does not necessarily have internal features to shape or distribute the gas/air mixture. Large input furnace models may include a baffle inside the burner box to aid in distribution of the gas and air.
The burner head may be a FeCrAl alloy fiber layer, such as a mat, weave, or knit of fibers, strands, wires, or the like. The layer does not necessarily have features to shape or distribute the flame and requires no supporting substrate. The fibers, strands, or wire-like materials may have about a 0.009 inch diameter, but may have other diameters. Other shapes of the layer material may be used. Other materials may incorporate Kanthal™, Fecralloy™, and the like. Even non-metal fibers or wires may be used. The material of fibers, strands, wires and the like should be able to withstand temperatures greater than 1800 degrees F.
Burner design may consist of one burner head for all of the heat exchanger sections as opposed individual burners within or for each heat exchanger section. There may instead be a burner header for each sub-group of sections.
A FeCrAl alloy fiber layer, as an example, may create a very small pressure drop of in the range of 0.2-0.5″ WC. Nominal thickness of the layer may range between 0.01 and 0.10. An example thickness may be 0.035″. A flame may be shaped by a negative pressure created by an induced draft blower drawing the flame and combustion products through the orifice shield and heat exchanger. The burner head may be spaced away from the heat exchanger by a burner front spacer which can also contain the igniter, flame sensor and viewport. The igniter may be a hot surface or direct spark. The direct spark version may use a single rod for ignition and flame sensing. A temperature sensor may be used to detect unsafe or abnormal operating conditions of the burner.
An orifice shield may be in front of the heat exchanger. The orifice shield may prevent overheating the partition panel with flame impingement or radiant energy from the burner. The orifice shield may also help shape the flame.
The primary heat exchanger may be a tube or clamshell construction with multiple parallel paths and with or without a secondary tube and fin heat exchanger. Combustion products may flow inside the heat exchanger, and circulating air may flow over the outside of the heat exchanger. Circulating blower outlet may be turned 180 degrees from a current configuration to direct circulating air flow to the front end of the heat exchanger. The design may or may not necessarily include baffling within the heat exchanger to direct air flow across specified sections of the tube or clamshell.
A summary of additional information may incorporate: 1) Premix burner lighting at approx 50 percent of full rate; 2) Design and application may include control of the inducer fan speed; 3) Burner design may or may not include a fixed or variable firing rate control; 4) Use of an electronic or mechanical choke of the mixer to control the gas/air mixture; 5) Use of a pressure switch to time the point at which gas flows for during the ignition sequence; 6) Solution may or may not utilize a single, two-stage, or modulating atmospheric gas valve or a 1:1 premix gas/air control; 7) Application may or may not include a flue sensing device to determine CO2, burner temperature, or flue temperature to tune the gas/air mixture; 8) Use of a mass flow sensor, for example, Helga trim (i.e., a Honeywell™ electronic gas/air control mass flow sensor) to monitor emissions; 9) Use of a gas valve (e.g., a Honeywell PX42 pneumatic 1:1) in combination with a stepper motor control throttle within the mixer to control gas/air mixture; and 10) Use of an adjustable choke controlling the combustion air of an atmospheric valve application.
The system may also have an addition of flue gas recirculation through a fixed orifice. The orifice may be sized for 5 to 10 percent flue gas recirculation.
An orifice shield 33 may be positioned at the front of spacer 27 and at a heat exchanger 34. The flame may be drawn into a multiple tube or clamshell structure of the exchanger. The flame may be drawn in and through the heat exchanger 34 by an induced draft blower 35. Blower 35 may pull out exhaust or flue gas 36 into a flue 37. A circulating blower 38 may pull in return air 39 and move the air through heat exchanger 34. From heat exchanger 34 may be heated air.
A gas and air mixture may be drawn from the mixer 24 into and through a wedged-shaped box manifold 25. The mixture may be drawn through a burner head 26, which may be a layer such as a mesh, fiber mat, or woven or knit fibers, after which the mixture can be ignited into a flame. The flame may be drawn through a front burner spacer 27 and an orifice shield 33. The flame may be further drawn in as separate flames 46 through tubes 45 of heat exchanger 34. A circulating blower may pull in return air 39 and push the air by hot tubes 45 to result in heated air 41 which exits the exchange port out of a port 47 to various vents or the like for heating a space or spaces. Flames 46 in tubes 45 may result in burnt gases 36 which are drawn out through flue 37 by fan 35. Fan 35 may be an induced draft blower. Fan 35 may be modulated or varied in speed. Fan 35 may force much flue gas 36 out of the system via flue 37 to the outside. Some of flue gas 36 may be re-circulated with air 23, as noted herein.
To recap, a furnace burner system may incorporate an air-fuel mixer, a burner box coupled to the mixer, a burner head coupled to a first open end of the burner box, a spacer coupled to an output side of the burner head, an orifice shield coupled to an output side of the spacer and an input side of a heat exchanger, and an igniter situated between the burner head and the orifice shield. The heat exchanger may have a tube or clamshell structure.
The burner box may be funnel-shaped and have a wider portion in a direction toward the burner head and a narrower portion in a direction toward the mixer. A direction of a gas and air mixture input to the burner box may be different from a direction of the gas and air mixture out from the burner box. The mixer may have an input to receive a portion of exhaust gas from the heat exchanger for recirculation.
The mixer may incorporate a gas orifice and an air orifice. The gas and air orifices may be sized to minimize combustion CO2 for decreasing NOx emissions. The burner head incorporate a FeCrAl alloy fiber mat.
The furnace burner system may further incorporate a blower to provide a below atmospheric pressure in a plurality of sections of the tube or clamshell structure of the heat exchanger to draw the gas and air mixture into the burner box and pull a flame at the burner head through the orifice shield into the plurality of sections.
An approach for achieving a low-emissions furnace, may incorporate drawing an air and gas mixture into a manifold, drawing the air and gas mixture from the manifold through a burner head and a spacer, igniting the air and gas mixture in the spacer with an igniter into a flame, and drawing the flame from the spacer through a plurality of sections of a heat exchanger and drawing exhaust gases out from the heat exchanger. The drawing of the air and gas mixture, the flame, and exhaust gases may be performed with an air mover. A portion of the exhaust gases may be re-circulated into the air and gas mixture.
An air-fuel mixer may be coupled to the manifold. The air and gas mixture may be drawn into the manifold from the mixer.
The flame may be kept on a side of the burner head towards the heat exchanger when being drawn from the spacer into the plurality of sections. A section may be a tube. A temperature sensor may be situated in the spacer. A temperature indication from the temperature sensor may provide a condition of combustion of the mixture and/or a condition of the air and gas mixture.
The manifold may incorporate an enclosure wall from the mixer to the burner head. The manifold may also incorporate an input at the mixer and an output at the burner head. An area of an opening of the output may be greater than an area of an opening of the input. A cross-section area virtually perpendicular to a line between the intake and the output may increase proportionally relative to a distance from the intake area towards the output of the manifold.
A direction of the air and gas mixture coming in through the input of the manifold may be at an angle between 60 and 120 degrees relative to a direction of the air and gas mixture going through the output of the manifold.
A burner assembly may incorporate a manifold box having an input port and output port, an air-fuel mixer coupled to the input port, a burner head coupled to the output port, a spacer coupled to the burner head, and a one-to-multiple flame conformer coupled to the spacer. The one-to-multiple flame conformer may incorporate a plate having a plurality of openings, coupled to the spacer. Each opening of the plurality of openings may be aligned with and coupled to a first end of a section of a plurality of sections of a heat exchanger.
The burner assembly may further incorporate an air mover having an input connected to second ends of the plurality of sections. An air tube may be coupled to an intake of the mixer and to an air supply. An output tube may be coupled to the intake of the mixer and an output of the air mover. The output tube may have a flow limiting orifice situated in series with the output tube, and the intake of the mixer may be coupled to a fuel valve and fuel supply port.
A direction of flow at the input port of the burner assembly may be different than a direction of flow at the output port. The manifold box of the burner assembly may have two or more slanted sides opening further away relative to one another, from the input end towards the output end in a form of a funnel.
In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense.
Although the present system and/or approach has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the related art to include all such variations and modifications.
