Stoves and furnaces – Liquid heater – Open-top vessel that may include lid
Reissue Patent
1994-12-13
2001-01-09
Lo, Weilun (Department: 3748)
Stoves and furnaces
Liquid heater
Open-top vessel that may include lid
C126S11000R, C126S392100, C126S09900D, C237S053000, C165S153000
Reissue Patent
active
RE037009
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to heat exchangers for furnaces. More specifically, the field of the invention is that of heat exchanger tubes which provide passageways for heated flue gases within furnace heat exchangers.
2. Prior Art
Tubular conduits are used in heat exchangers to provide an interior conduit for flue gases and exterior heat transfer surface for circulating air. The interior conduits may be formed from metallic clam-shell plates wherein two clam-shell plate surfaces are connected together to form the conduits, or the tubular conduits may comprise metal tubes. Within furnaces, such tubular conduits provide a passageway for flue gases, the heated products of combustion, which flow through the heat exchanger. The flue gas flow transfers heat to the material defining the passageway which then transfers the heat to air circulating over and around the heat exchanger.
The heat exchanger contains the flue gas flow. An inducer fan draws the combustion gases from a gas burner through the passageway to an exhaust system. The inducer fan insures that the heated flue gases are constantly flowing through the heat exchanger during the operation of the furnace, providing sufficient air for combustion. Also, a circulator fan is disposed adjacent the heat exchanger to drive a flow of circulation air over and around the tubular conduit and into the interior of the building being heated.
Typically, a furnace's heat exchanger tubular conduit contains one or more elongate portions which are disposed perpendicularly to the flow of circulation air. This arrangement allows the circulation air flow to impact on the exterior surfaces of the heat exchanger conduits to promote heat exchange. Also, the conduits generally include two or more elongated sections connected by bend sections so that the interior flow of heated flue gas is disrupted and impacts interior surfaces of the tubes to promote further heat exchange. However, a problem with prior art heat exchanger conduits involves the inefficiency in the amount of heat transferred from the heated flue gases to the circulating air.
Various structures exist which increase heat transfer efficiency. For example, one known configuration includes a plurality of indentations within the wall of the heat exchanger for disrupting the flow over the indentations. A problem with this configuration is that although disruption is caused within the flow along the inner surfaces of the wall, the flow in the center may only be minimally effected. Another known configuration includes tubes which have curved or polygonal walls varying in cross-sectional shape over the length of the tube. A problem with this configuration is the expense involved in manufacturing tubes which vary in cross-sectional shape over their length.
Inlets and outlets of the heat exchanger conduits are attached to a heat exchanger panel so that the burners, inducer and circulator fans, and the exhaust system can be conveniently attached to the heat exchanger. The heat exchanger conduits are disposed within the heat exchanger and arranged so that the circulator fan drives air over the conduits. For the clam-shell configuration, the plates are disposed generally perpendicularly to the direction of circulation air flow. The problem with the clam-shell configuration is that the flow produced by the circulator fan is only minimally disrupted in the spaces between the plates. For the tubular configuration, generally cylindrical elongated portions of the tubes may be disposed so that a direct line of sight is blocked along the direction of circulation flow. A problem with the tubular configuration is that the first row of cylindrical elongated portions causes a high pressure drop in the circulation flow resulting in the circulation flow only minimally wrapping around the other row or rows of elongated portions so that hot spots develop on the downstream elongated portions. Also, a relatively large circulation fan must be used to provide a sufficiently strong flow of circulation air following the high pressure drop.
What is needed is a heat exchanger element which more efficiently transfers heat from the heated flue gases to the circulation air.
Another need is for a heat exchanger element in which the flow in the center of the conduit is more effectively disrupted.
A further need is for such a heat exchanger element which is less expensive to manufacture.
A still further need is for a heat exchanger conduit for a furnace which promotes circulation air flow around the exterior of the conduit and minimizes the occurrence of hot spots.
Also needed is a heat exchanger conduit for a furnace which reduce the pressure drop of the circulation air across the heat exchanger conduits.
SUMMARY OF THE INVENTION
The present invention is a heat exchanger tube which includes an enhanced portion which is narrowed to have a smaller cross-sectional area than the cylindrical flue portion of the tube. The enhanced portion promotes heat transfer by accelerating and disrupting the flow of the flue gases. Also, the heat transfer properties of the present invention are improved by increasing the amount of internal heat transfer surface in comparison to total volume in the enhanced portion.
To further improve the efficiency of heat transfer, the enhanced portion includes turbulators for disrupting and radially mixing the heated flue gases which flow within the tube. The turbulators may take the form of indentations formed on the sides of the tube, or as an insert shaped and positioned in the tube to effect most of the flow.
A bend portion of the tube joins the flue and enhanced portions, and decreases in cross-sectional area from the flue portion to the enhanced portion. This gradual narrowing of the bend portion accelerates the heated flue gas flow so that it strikes the turbulators at a greater velocity.
For improving the flow characteristics of the circulating air around the exterior of the tube, the enhanced portion has a relatively thin width. This narrow profile allows circulation air to pass around the enhanced portion with a relatively small pressure drop which provides a more complete heat transfer at the outer surfaces of the flue portion.
The enhanced portion has a generally elliptical shape and has a major axis disposed at a slight angle relative to the plane defined by the central axes of the flue and enhanced portions. Within the casing of the heat exchanger, the tubes are positioned side by side and angled slightly from the vertical plane. Disposed in this manner, the major axes and therefore the exterior surfaces of the enhanced portions are generally parallel to the flow direction of the circulating air and the enhanced portions do not block direct flow to the exterior of the generally cylindrical flue portions. This arrangement decreases the pressure drop in the circulating air as it passes over the enhanced portion. The resulting flow over the flue portion transfers more of the exterior of the flue portion and thereby lessens the chance of developing hot spots.
The present invention provides improved heat transfer characteristics by shaping the tubular heat exchanger to increase heat transfer internally and externally. Internally, the heated flue gases are accelerated by the narrowing of the enhanced portion, and the enhanced portion has a greater ratio of surface area to internal volume which increases heat transfer efficiency. Externally, the circulation air is provided a flow path which decreases the initial pressure drop after passing over the enhanced portion and increases the scraping of the flue portion so that the circulation air absorbs more heat and hot spots do not develop on the heat exchanger.
The present invention is, in one form, a heat exchanger element in a furnace including a burner, an exhaust system, and a heat exchanger defining an internal air circulation area. The heat exchanger element is in the form of an elongated tube and comprises an inlet, an outlet, a flue portion, and an enhanced portion. The inlet is operably
Baker & Daniels
International Comfort Products Corporation (USA)
Lo Weilun
LandOfFree
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