Furnace heat exchanger coupling

A coupling to connect a primary heat exchanger to a condensing heat exchanger of a furnace includes a coupling box extending between a primary heat exchanger and a condensing heat exchanger. The coupling box defines a flow path for flue gas between a primary heat exchanger outlet and a condensing heat exchanger inlet. A tube sheet is located at a distance from the tube sheet in the coupling box. The liner and the tube sheet define an insulating liner space therebetween reducing condensation on the liner.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to furnaces. More specifically, the present disclosure relates to a coupling between heat exchanger components of a multi-position gas furnace.

In a typical furnace, a fuel, for example, natural gas, is combusted in a burner. The byproduct of combustion, called flue gas, is routed through one or more heat exchangers which extract the heat therefrom. In a condensing gas furnace there are two types of heat exchangers: a primary heat exchanger (PHX) and a secondary or condensing heat exchanger (CHX). The PHX accounts for most of the efficiency of the furnace by reducing the heat of the flue gas from the flame temperature to a temperature well above the dew point temperature of the water in the flue gas. The flue gas heats the surface of the PHX and air is blown across the exterior of the PHX thus removing heat from the PHX by convection. Efficiency is measured by the amount of heat energy that is transferred out of the flue gas compared to the amount of heat energy that remains in the flue gas as it leaves the heat exchanger. It can be determined roughly by knowing how much air and gas enters and is burned in the PHX, and the temperature of the gas leaving the PHX. The CHX makes up the remainder of the furnace efficiency by reducing the flue gas temperature below the dew point of the flue gas and thus taking advantage of the latent heat from the water byproduct of combustion.

The PHX and CHX are connected via a coupling box, with flow from the PHX proceeding through the coupling box and into the CHX. Without adequate distribution of the flue gas through the coupling box, localized spots on parts of the coupling box may fall below the dew point temperature of the flue gas causing there to be localized condensation prior to the flue gas entering the CHX tubes. Baffles and other means to distribute the flue gas to all parts of the coupling box will increase internal pressure drop, which requires a larger combustion blower. The moisture deposits outside of the CHX reduce the effectiveness of the CHX and also can cause corrosion of components of the coupling box and/or PHX. Further, there is a desire to reduce the size of the furnace, if possible, to reduce cost and household space occupied by the furnace. If a furnace requires a larger combustion blower, the size of the blower will limit how much the overall size of the furnace can be reduced.

The furnace being a multiposition appliance must perform its intended function in the 4 major positions of upflow, laid horizontally on its left side, laid horizontally on its right side, and upside down, or downflow. If orientation was limited to a single position the CHX could be designed to provide a natural slope that would allow condensate formed in the tubes to drain by gravity in the direction of the slope. However, in order to accommodate multiple positions (that is more than 2 positions) the CHX must be designed to be level, with no intended slope within the appliance. Generally as the furnace operates and the combustion blower is operating to pull the gas through the CHX, it will provide enough flow velocity to assist with water drainage to the intended front of the furnace. When the furnace shuts off, there is no air flow through the CHX, and water remaining in the tubes will migrate to either the inlet or the outlet. If the furnace is installed in a residence in a way that the CHX tubes slope gradually backwards in the unit, gravity will tend to lead the remaining water to the back of the furnace where it can pool and cause a leak, or corrosion on parts that are not intended to handle condensate.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a coupling to connect a primary heat exchanger to a condensing heat exchanger of a furnace includes a coupling box extending between a primary heat exchanger and a condensing heat exchanger. The coupling box defines a flow path for flue gas between a primary heat exchanger outlet and a condensing heat exchanger inlet. A tube sheet is located at the condensing heat exchanger inlet and a liner is located at a distance from the tube sheet in the coupling box. The liner and the tube sheet define an insulating liner space therebetween reducing condensation on the liner.

According to another aspect of the invention, a heat exchanger assembly for a furnace includes a primary heat exchanger and a condensing heat exchanger in flow communication with the primary heat exchanger. A coupling connects the primary heat exchanger to the condensing heat exchanger and includes a coupling box extending between the primary heat exchanger and the condensing heat exchanger and defining a flow path for flue gas between a primary heat exchanger outlet and a condensing heat exchanger inlet. A tube sheet is located at the condensing heat exchanger inlet and a liner is located at a distance from the tube sheet in the coupling box. The liner and the tube sheet define an insulating liner space therebetween reducing condensation on the liner.

DETAILED DESCRIPTION OF THE INVENTION

Shown inFIG. 1is an embodiment of a furnace10. The furnace10includes a burner12for combusting a fuel such as natural gas. Flue gas14exits the burner12and flows through a primary heat exchanger (PHX)16. The PHX16is a gas-to-gas heat exchanger in which the flue gas14flowing through the PHX16transfers thermal energy to the surface of the PHX16. The thermal energy is then dissipated from the surface of the PHX16into a flow of air18urged across the exterior of the PHX16by, for example, a blower20. At an exit of the PHX16, a temperature of the flue gas14is still at a level that exceeds a dew point temperature at which moisture will condense out of the flue gas14. The flue gas14then flows into a condensing heat exchanger (CHX)22in which the temperature of the flue gas14is lowered and walls of the CHX22are below the dew point causing condensation of the flue gas14and further removing thermal energy from the flue gas14.

Referring toFIGS. 2 and 6, the PHX16and CHX22are connected via a coupling box24. The coupling box24is defined on one side by a PHX outlet26and a CHX inlet28, and on another side by a coupling pan30. The coupling pan30is connected at a first end32to the PHX16and a second end34to the CHX22. The coupling pan30is formed with multiple depths to accommodate a protrusion of the PHX outlet26and preserve a consistent coupling box depth36. A transition38is located between the PHX16and CHX22at a location to reduce cold spots in the coupling box24. Further, the second end34is sloped, and pan edges62are tapered, shown best inFIG. 6, to improve manufacturability of the coupling pan30and to improve thermal energy distribution in the coupling box24.

Referring toFIG. 3, a containment plate40is located in the coupling box24at the CHX inlet28. The containment plate40has one or more plate openings42therethrough to allow the flow of flue gas14through the coupling box24and into the CHX22. The containment plate40includes a lip44extending from the plate opening42to direct the flue gas through the plate opening42. As shown inFIG. 4, the lip44extends toward the coupling pan30and defines a lip gap46between the lip44and the coupling pan30. The lip gap46is sized to provide a small resistance to the flow and thereby force the flue gas14to spread out from side to side in the coupling box24to provide thermal heating of the coupling pan30and thereby reduce cold spots. The lip gap46is also sized for balance so as not to increase resistance to flow of the flue gas14by too great amount and thereby maintain a lower pressure drop across the PHX16and CHX22. Maintaining a lower pressure drop helps to reduce the size of the combustion blower20needed for the furnace10and as a result increases effectiveness of the PHX16and CHX22.

Referring toFIGS. 5a-5d, the CHX22comprises a plurality of CHX tubes48which extend through a tube sheet50at a tube inlet. To accommodate the CHX tubes48passing through the tube sheet50, the tube sheet50includes a plurality of tube sheet openings54, each having a tube sheet collar56which extends outwardly toward the coupling box24. The tube sheet50is secured to the CHX tubes48at the tube sheet collar56. A liner58is located over the tube sheet50defining a liner space60between the liner58and the tube sheet50. Similar to the tube sheet50, the liner58includes a plurality of liner openings62though which the CHX tubes48pass. Each liner opening62includes a liner collar64which extends toward, and in some embodiments, abuts the tube sheet collar56. The liner58is secured to the CHX tubes48at the liner collar64. The liner collar64extending toward the tube sheet collar56ensures that the liner space60will be a desired size, while preventing damage to the liner58when assembling the liner58to the CHX22and the tube sheet50. The liner space60serves to insulate the liner58from the CHX22, thus preventing condensation on the liner58.

In some embodiments, the liner58and the containment plate40are formed from a corrosion-resistant material such as 29-4C steel or the like to define a corrosion-resistant containment space66for collection of condensate in the event of, for example, back flow from the CHX tubes48. The plate opening42is sized and positioned to prevent water that collects in a containment area, defined by the liner58and the containment plate40, from flowing over the plate opening42in the case that the CHX tubes48are not level because the furnace10is installed at an angle such that the front of the furnace is about ½-inch higher than the back of the furnace, or about 1° in some embodiments, in any position which the furnace10is installed. Thus the furnace10may be located and out of level in any position while preventing corrosion in the heat exchanger portion of the furnace10. It is to be appreciated that it may be desired to utilize angles other than 1 to achieve a desired out of level position of the furnace10.