Polymer heat exchanger design for condensing boiler applications

Condensing boilers achieve very high efficiency levels by recovering both sensible heat and water vapor latent heat from the flue gas. Research since the 1980's has focused on corrosion in such condensing heat exchangers related to the acidic condensate and material selection. Polymers in condensing heat exchangers have been considered to avoid the cost and corrosion concerns of metallic designs. Past efforts have shown that polymers offer the advantage of corrosion resistance and cost, however, lower thermal conductivity limited their application. More recent developments have introduced thermally conductive polymers which now offer promising conductivity values. This project focused on the evaluation of a thermally conductive polymer heat exchanger for this application. Computational fluid dynamic results indicated thermal conductivity values of stainless steel, a typical heat exchanger material, do not need to be achieved for similar heat transfer performance. An increase in thermal conductivity from about 10 times that of the base polymer can achieve an overall heat exchanger effectiveness similar to that achieved with stainless steel. A polymer compositethermal conductivity of approximately 2.5 W/m.K would be adequate. Thermally conductive polymer materials are now commercially available which offer values up to 20 W/m.K. In this work, one Nylon-12 and one thermally conductive polymer composite heat exchanger prototypes were constructed for a condensing boiler application. Tests demonstrated that good overall heat transfer performance was achieved. The lower thermal conductivity of the polymer heat exchanger will lead to higher surface temperatures and lower water condensation rates. (C) 2016 Elsevier Ltd. All rights reserved.