A Novel Furnace Design Utilizing a Low Temperature Plastic Condensing Heat Exchanger

Abstract

The initial phase of a research and development program for the Consumers' Gas Co. and the Federal Department of Energy, Mines and Resources to design a condensing heat exchanger/gas fired residential air furnace has been com­pleted. Progress to date has resulted in a novel design utilizing a relatively low temperature plastic material for the last stage heat exchanger. To utilize this low temperature plastic, a method of reducing the temperature of the flue gas entering the final heat exchanger was devised using a unique flue gas recirculation process.

Heat transfer calculations and pressure drop prediction methods have indicated that the design is sound and can easily be accommodated in a residential furnace with only moderate increase in cost and space requirements. The existing design is also well suited to incorporation as a retrofit package and this is also being pursued.

Based on the calculated performance, a condensing heat exchanger was sized, fabricated and installed on a con­ventional 80,000 BTU/hr input gas fired residential fur­nace. The initial experimental tests have given very en­couraging results. Based on a final flue gas exit temp­erature of 85F with an excess air condition of 25%, these initial tests yielded a furnace efficiency of approximate­ly 97%. Although combustion air preheat has not been em­ployed in these initial tests, this feature is planned as part of the prototype design.