Fouling of polymeric hollow fiber heat exchangers by air dust

Abstract

Currently, liquid-to-gas heat exchangers in buildings, domestic appliances and the automotive industry are mainly made of copper and aluminum. Using plastic instead of metal can be very beneficial from an economic and environmental point of view. However, it is required that a successful plastic design meets all the requirements of metal heat exchangers. The polymeric hollow fiber heat exchanger studied in this work is completive to common metal finned heat exchangers. Due to its unique design (the use of thousands of thin-walled microtubes connected in parallel), it achieves a high level of compactness and thermal performance, low pressure drops and high operation pressure. This paper focuses on an important aspect of heat exchanger operation— its fouling in conditions relevant to building and domestic application. In heating, ventilation and air conditioning (HVAC) and automotive and domestic appliances, outdoor and domestic dust are the main source of fouling. In this study, a heat exchanger made of polymeric hollow fibers was tested in conditions typical for indoor HVAC equipment, namely with the 20 °C room air flowing through the hot water coil (water inlet 50 °C) with air velocity of 1.5 m/s. ASHRAE test dust was used as a foulant to model domestic dust. A polymeric heat exchanger with fibers with an outer diameter of 0.6 mm (1960 fibers arranged into 14 layers in total) and a heat transfer area of 0.89 m2 was tested. It was proven that the smooth polypropylene surface of hollow fibers has a favorable antifouling characteristic. Fouling evolution on the metallic heat transfer surfaces of a similar surface density was about twice as quick as on the plastic one. The experimental results on the plastic heat exchanger showed a 38% decrease in the heat transfer rate and a 91% increase in pressure drops after eighteen days of the experiment when a total of 4000 g/m2 of the test dust had been injected into the air duct. The decrease in the heat transfer rate of the heat exchanger was influenced mainly by clogging in the frontal area because the first layers were fouled significantly more than the deeper layers.