July 1-6, 2007
Microstructure devices are well known for their excellent performance with regard to heat and mass transfer. Microstructured heat exchangers show significant advantages in comparison with conventional heat exchangers. The unique properties of a microreaction system show high overall heat transfer coefficients for example. Small characteristic dimensions are in the order of a few hundred μm (Schubert et al., 1998, 2001; Worz et al, 1998). Due to the small dimensions, an increased pressure drop is combined with excellent heat transfer properties. But the small channels are prone to fouling processes. The accumulation of crystalline deposits is a severe problem. An additional wall layer causes a decrease of the overall heat transfer coefficient. Therefore the attention has been directed to the reduction of possible fouling processes within the channels.
The fouling can be subdivided in two parts: First into the induction period and second into the so-called fouling period itself (Lund et al., 1981; Forster et al., 1999a, 1999b, and 2000).
For the investigations a special electrically heated micro heat exchanger with changeable foils of different surface materials (stainless steel, FEP, DLC) was developed. The foil temperature is electronically controlled to a constant level of 100°C.
A solution of calciumnitrate/sodiumhydrogencarbonate is pumped under laminar flow conditions through the channels of the microstructured part. The high temperature causes the precipitation of solid calciumcarbonate on the surface.
The results for all materials, the uncoated stainless steel and the DLC and FEP coated heat transfer surfaces, show typical fouling behaviour with an induction period, followed by an asymptotic built-up of the deposited calcium carbonate. The fouling plot of DLC and FEP coated heat transfer surfaces contradict the hypothesis that the use of such materials leads to much longer induction periods. There is no influence of the surface material found on the induction period and the gradient of the fouling period in the laminar flow regime.
W. Benzinger, J.J. Brandner, U. Schygulla, and K. Schubert, "INFLUENCE OF DIFFERENT SURFACE MATERIALS ON THE FOULING PROCESS IN A MICROSTRUCTURED HEAT EXCHANGER UNDER LAMINAR REGIME" in "Heat Exchanger Fouling and Cleaning VII", Hans Müller-Steinhagen, Institute of Technical Thermodynamics, German Aerospace Centre (DLR) and Institute for Thermodynamics and Thermal Engineering, University of Stuttgart, Germany; M. Reza Malayeri, University of Stuttgart, Germany; A. Paul Watkinson, The University of British Columbia, Canada Eds, ECI Symposium Series, (2007). http://dc.engconfintl.org/heatexchanger2007/50