Two-stage heat exchanger

A furnace comprising an exterior housing having air distribution means for discharging air heated by said furnace;

• a primary heat exchanger comprising a combustion chamber having a top and a bottom and at least one sidewall for burning a fluid fuel yielding hot gaseous products of combustion positioned within said housing;
• a secondary heat exchanger in communication with said combustion chamber and adapted to receive air in turbulent flow thereabout, for receiving hot gaseous products of combustion from the combustion chamber;
• a tertiary heat exchanger in communication with said secondary heat exchanger for receiving hot gaseous products of combustion from the secondary heat exchanger at a temperature above the dewpoint of any condensible gases, said tertiary heat exchanger comprising a pair of spaced apart opposed manifolds, a plurality of parallel, equispaced finned tubes interconnecting said manifolds, and drainage means for draining condensate from said tertiary heat exchanger, said tertiary heat exchanger arranged in the exterior housing whereby air to be heated passes said tertiary heat exchanger before contact with the secondary heat exchanger whereby said gaseous products of combustion are cooled in said tertiary heat exchanger to about 120.degree. F. for condensation of at least a portion of condensible gases for substantial recovery of latent heat of condensation of said condensible products of combustion and transfer of said heat to the discharging air.

A furnace including primary, secondary and tertiary heat exchangers comprising a pair of spaced apart opposed manifolds, a plurality of parallel, equispaced finned tubes interconnecting said manifolds, and drainage means for draining condensate from said tertiary heat exchanger, said tertiary heat exchanger arranged in the exterior housing whereby air to be heated passes said tertiary heat exchanger before contact with the secondary heat exchanger whereby said gaseous products of combustion are cooled in said tertiary heat exchanger to about 120.degree. F. for condensation of at least a portion of condensible gases for substantial recovery of latent heat of condensation of said condensible products of combustion and transfer of said heat to the discharging air, a manifold, an inlet conduit for communicating said manifold to the secondary heat exchanger, an exhaust conduit for discharge of combustion products from said manifold, drainage means for discharge of condensate from said manifold, and a plurality of parallel, equispaced, upstanding thermosiphons, each having a lower evaporator section and an upper condenser section and a low-boiling liquid therein mounted in said manifold whereby the lower evaporator sections of the thermosiphons are seated in the manifold for exposure of hot combustion gases thereto for vaporization of the liquid in the thermosiphons and the upper condenser sections of the thermosiphons are exposed to air to be heated for transfer of heat to said air by condensation of vaporized liquid in the thermosiphons.