Aspects of fouling in case of heat exchangers with polluted gas

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.This paper presents and discusses various aspects of fouling in case of heavily polluted flue gas (and/or off-gas) coming especially from incinerators/waste-to-energy systems. A long-term experience and know-how from this area as well as continuing research and development brings new insights into manufacturing of equipment and their operating. Polluted off-gas causes high propensity to fouling and necessity of consequent cleaning. In some cases, it is not possible to utilize a conventional approach. Introductory part of the paper provides a description of various types of units for thermal processing of wastes (including sludge and contaminated biomass). Fouling is classified according to various cases of industrial applications connected with easy/difficult cleaning. It is shown how fouling can influence geometry of heat exchangers and their selection. In the following part, several industrial cases are shown taking into account aspects like: • solid particles (ash and flying ash) in the gas, and adapting design according to their concentration; • species contained in flue gas which can chemically react and create fouling deposits on heat transfer surfaces; • potential corrosion between the fouling layer and heat transfer surface caused by local temperature decrease. The above aspects are clearly illustrated through industrial applications as follows: • heat recovery system of unit for the thermal treatment of sludge coming from pulp and paper production; • boiler systems in incineration plants and chemical industry plants. There are various methods to reduce fouling. We have utilized very efficient approach combining intuitive design and sophisticated tools based on CFD (Computational Fluid Dynamics). However, fouling cannot be eliminated completely therefore various efficient methods (mostly tailor-made ones) are utilized (like common mechanical cleaning, air guns, controlled local explosion) for cleaning of surfaces. There is also an effort to develop a mathematical model for fouling prediction, and selection of the most economically acceptable systems connected with current research and development in the field. However, it is difficult to validate the models. Fouling in the field described in the paper is in fact a “never ending story”.pm201

Circadian rhythms in Mexican blind cavefish Astyanax mexicanus in the lab and in the field

Biological clocks have evolved as an adaptation to life on a rhythmic planet, synchronising physiological processes to the environmental light–dark cycle. Here we examine circadian clock function in Mexican blind cavefish Astyanax mexicanus and its surface counterpart. In the lab, adult surface fish show robust circadian rhythms in per1, which are retained in cave populations, but with substantial alterations. These changes may be due to increased levels of light-inducible genes in cavefish, including clock repressor per2. From a molecular standpoint, cavefish appear as if they experience ‘constant light’ rather than perpetual darkness. Micos River samples show similar per1 oscillations to those in the lab. However, data from Chica Cave shows complete repression of clock function, while expression of several light-responsive genes is raised, including DNA repair genes. We propose that altered expression of light-inducible genes provides a selective advantage to cavefish at the expense of a damped circadian oscillator