Indirect gas-fired condensing furnace

Abstract

An indirect gas-fired condensing furnace assembly and method includes a primary heat exchanger, a secondary heat exchanger, and a tertiary heat exchanger. The secondary heat exchanger assembly may be an intermediate single-pass tubular heat exchange section made from a corrosion-resistant material. The tertiary heat exchanger assembly may be a single-pass tubular heat exchanger section with a corrosion resistant material. The tertiary heat exchanger assembly may include a plurality of fins. The primary heat exchanger assembly may include a plurality of aligned tubes wherein each tube includes a first straight portion, an intermediate portion, and a second straight portion such that the primary heat exchanger tubes surround the secondary and the tertiary heat exchanger assemblies such that the airflow is configured to traverse the primary heat exchanger assembly, the secondary heat exchanger assembly, and the tertiary heat exchanger assembly in various directions such that a risk of condensation freezing within the secondary and tertiary heat exchange assemblies is reduced.

Having thus described the invention, I claim: 
     
         1 . A condensing furnace assembly comprising:
 a primary heat exchanger assembly;   a secondary heat exchanger assembly; and   a tertiary heat exchanger assembly.   
     
     
         2 . The condensing furnace according to  claim 1 , further comprising a burner assembly, a manifold assembly, and a combustion air device. 
     
     
         3 . The condensing furnace according to  claim 1 , wherein the secondary heat exchanger assembly is an intermediate single-pass tubular heat exchange section made from a corrosion-resistant material. 
     
     
         4 . The condensing furnace according to  claim 1 , wherein the tertiary heat exchanger assembly is a single-pass tubular heat exchanger section with a corrosion resistant material. 
     
     
         5 . The condensing furnace according to  claim 4 , wherein the tertiary heat exchanger assembly includes a tube having a plurality of fins. 
     
     
         6 . The condensing furnace according to  claim 1 , wherein the primary heat exchanger assembly further comprises a plurality of aligned tubes wherein each tube includes a first straight portion, an intermediate portion, and a second straight portion such that the primary heat exchanger tubes surround the secondary and the tertiary heat exchanger assemblies. 
     
     
         7 . The condensing furnace according to  claim 6 , wherein the primary heat exchanger assembly, the secondary heat exchanger assembly, and the tertiary heat exchanger assembly are oriented such that airflow can traverse over the first straight portion of the primary heat exchanger tubes to the secondary and tertiary heat exchanger assemblies to the second straight portion of the primary heat exchanger tubes. 
     
     
         8 . The condensing furnace according to  claim 6 , wherein the primary heat exchanger assembly, the secondary heat exchanger assembly, and the tertiary heat exchanger assembly are oriented such that airflow can traverse over the second straight portion of the primary heat exchanger tubes to the secondary and tertiary heat exchanger assemblies to the first straight portion of the primary heat exchanger tubes. 
     
     
         9 . The condensing furnace according to  claim 1 , wherein the turndown ratio of the condensing furnace is about 5 to 1. 
     
     
         10 . A condensing furnace assembly comprising:
 a primary heat exchanger assembly including a plurality of aligned tubes wherein each tube includes a first straight portion, an intermediate portion, and a second straight portion;   a secondary heat exchanger assembly including a plurality of aligned tubes; and   a tertiary heat exchanger assembly including a plurality of aligned tubes wherein the primary heat exchanger tubes surround the secondary and the tertiary heat exchanger assemblies;   wherein an airflow is be selectively bi-directional to traverse the primary, secondary and tertiary heat exchanger assemblies in a first direction and a second direction opposite the first direction such that the risk of condensate freezing is reduced.   
     
     
         11 . The condensing furnace of  claim 10  wherein the primary, secondary and tertiary heat exchanger assemblies are oriented to allow an airflow in the first direction to traverse over the first straight portion of the primary heat exchanger tubes to the secondary and tertiary heat exchanger assemblies to the second straight portion of the primary heat exchanger tubes and to allow the airflow in the second direction to traverse over the second straight portion of the primary heat exchanger tubes to the secondary and tertiary heat exchanger assemblies to the first straight portion of the primary heat exchanger tubes. 
     
     
         12 . The condensing furnace of  claim 10  wherein the plurality of tubes of the tertiary heating assembly comprise a plurality of fins. 
     
     
         13 . The condensing furnace according to  claim 10 , wherein the turndown ratio of the condensing furnace is about 5 to 1. 
     
     
         14 . A method of operating a condensing furnace assembly configured to reduce the risk of condensate freezing, the steps comprising:
 providing a condensing furnace assembly comprising:
 a primary heat exchanger assembly including a plurality of aligned tubes wherein each tube includes a first straight portion, an intermediate portion, and a second straight portion; 
 a secondary heat exchanger assembly including a plurality of aligned tubes; and 
 a tertiary heat exchanger assembly including a plurality of aligned tubes wherein the primary heat exchanger tubes surround the secondary and the tertiary heat exchanger assemblies; and 
 a burner assembly, 
   introducing combustion gas into and through the primary heat exchanger assembly, the secondary heat exchanger assembly, and the tertiary heat exchanger assembly such that at least a portion of the combustion gas is converted into a condensate liquid; and   traversing an airflow in a first direction over the heat exchanger assemblies relative to the condensing furnace.   
     
     
         15 . The method of operating a furnace assembly of  claim 14  further comprises modulating the burner assembly to combust gas above a dew point temperature of exhaust gases of the combustion gases wherein the combustion gases are introduced into and through the primary heat exchanger assembly above the dew point temperature while the airflow traverses over the primary heat exchanger assembly. 
     
     
         16 . The method of operating a furnace assembly according to  claim 15  further comprises traversing an airflow over the heat exchanger assemblies in a second direction, opposite the first direction, relative to the condensing furnace. 
     
     
         17 . The method of operating a furnace assembly according to  claim 14 , wherein the airflow traversing over the heat exchanger assemblies has a temperature that is less than 0° F. 
     
     
         18 . The method of operating a furnace assembly according to  claim 14  further comprises modulating a combustion air device to draw the combustion gases through the primary heat exchanger assembly, secondary heat exchanger assembly and tertiary heat exchanger assembly such that the combustion gases are exhausted from the furnace assembly. 
     
     
         19 . The method of operating a furnace assembly according to  claim 14  wherein a portion of the combustion gas is converted into the condensate liquid as the combustion gas in within the secondary heat exchanger or tertiary heat exchanger. 
     
     
         20 . The method of operating a furnace assembly according to  claim 14  further comprises maintaining operation of the condensing furnace at a turndown ratio of about 5:1.