Limits and Use of Polymeric Hollow Fibers as Material for Heat Transfer Surfaces

Usage of plastic tubes as heat transfer surface is not a completely new issue. In 2001 a nylon car radiator was tested. Similar heat exchangers using polymeric tubes with wall thickness about 1 mm are used now in chemical industry. Low overall heat transfer coefficient of the heat exchangers with plastic tubes is cause dominantly by thick heat transfer wall and low thermal conductivity. Polymeric hollow fiber heat transfer surfaces were presented for the first time in 2004 in shell and tube heat exchanger. The paper describe progress in study of these heat transfer surfaces since that time. Typical outer diameter of fibers is in the range from 0.6 to 1.3 mm and the thickness of the wall is about 10 % of the outer diameter. Reduction of the wall thickness to about 0.1 mm reduce significantly thermal resistance. Another advantage of hollow fibers is physical fact that convective heat transfer coefficient grows with diminishing the heat transfer surfaces. Hollow polymeric fibers are made by extrusion, and production technology limits the choice of suitable polymers. Nevertheless, there are number of materials that can be extruded in small dimensions with operation temperature from 80°C to 230°C. Even if the fibers have wall thickness 0.08 mm to 0.15 mm it can withstand internal pressure of 5 to 20 MPa at room temperatures and some materials pressure of 10 MPa at temperature 120°C. Three materials of heat transfer surfaces are discussed in the paper. Polypropylene can be used for long term load by temperatures to 80°C and internal pressure of 1 MPa. This material is suitable for variety of purposes including food and chemical industry. Higher temperatures are demanded for car industry where heat exchangers should work up to 130 °C. Typical materials for medium temperature range is polyamide. Thermoplastic polymer polyether ether ketone (PEEK) can be used for extrusion of hollow fibers used for application where temperatures up to 230°C are demanded. These heat exchangers were used for flue gas condensation and for heat exchangers steam / liquid.