Fritz Ebert

In situ optical particle counter with improved coincidence error correction for number concentrations up to 107 particles cm−3

Abstract An optical particle counter of the type first described by Umhauer (1983) was modified for the purpose of in situ measurements at high concentrations to achieve very low coincidence errors. Therefore, the optically defined measuring volume was reduced to 36 × 52 × 35 μm 3 and additionally a digital signal processing (DSP) as described by Sachweh (1991) was applied to measure the fraction of coincident signals for different aerosols (quartz, Monospheres and glycerin) and number concentrations up to 2 × 10 6 particles cm −3 . Thus, errors from coincidence events in particle size distribution and number concentration could be corrected successfully. The fraction of coincident signals could directly be measured by the DSP. The results were found to be in good agreement with the theoretical model derived by Raasch and Umhauer (1984). From these experiments the upper concentration limit could be assessed to be approximately 10 7 particles cm −3 . A lower 50% size detection limit of 0.2 μm was also determined for PSL spheres in comparison with a condensation particle counter. Performance comparisons are also reported for a commercial HC15 counter (Polytec) based on the same operating principle but “normal” sensing volume of 110 × 220 × 220 μm 3 .

Aerosol Measurement in Low‐Pressure Systems with Standard Scanning Mobility Particle Sizers

The scanning mobility particle sizer (SMPS) is one of the best known instruments for measuring particle size distributions in the submicron range. The SMPS consists of two parts: an electrostatic aerosol classifier (differential mobility particle analyser, DMA), followed by a counting device, in general a condensation particle counter (CPC). Unfortunately, commercial measurement devices such as the TSI DMA Model 3071 and the TSI CPC Model 3022 (TSI Inc., St. Paul, MN, USA), can be used only at nearly atmospheric pressure in the sampling line or in slight overpressure mode, but not in low-pressure systems. A modification in the sampling line is shown which enhances the operating range of a standard SMPS system to low pressure. Samples taken under standard and low-pressure conditions show good agreement in the measured particle size distributions and concentration. The behaviour observed in experimental studies agrees well with theoretical predictions.

On Flow Fields in Tubular Heat Exchangers – Numerical Calculations and Flow Visualization

Tubular heat exchangers belong to heat equipment, which is frequently used in industry. During their design it is usually assumed that nearly the same velocity prevails in all pipes of a bundle. Because of the abrupt changes of cross section in the inlet chamber the fluid cannot follow the chamber contour and flow separation occurs. Vortices are formed in the inlet chamber, which lead to different flow rates into the tube bundle. A method for the hydrodynamic optimization of tubular heat exchangers is represented within the framework of this work, which consists of measurements on a real tubular heat exchanger, visualization of the flow field in a water channel and numerical calculations with CFD programs. It will be shown what velocity profile appears in a bundle of pipes, and which parameters have an influence on it. Using the results obtained on the distribution of the flow medium, it is possible to estimate the effect of the nonhomogeneity on heat transfer, dissipation and fouling.

EXPERIMENTAL INVESTIGATION OF A WET DUST SCRUBBER: DUST COLLECTION BASED ON TURBULENT DIFFUSION

ABSTRACT The “nozzle scrubber” is a wet scrubber in which the scrubbing water is dispersed in the dust laden gas stream by means of one or more pneumatic nozzles. This scrubber is distinguished by an excellent collection efficiency for submicron dust at an unusually low energy and water consumption. No well-defined theory exists for this process. The collection efficiency in the “nozzle scrubber” depends primarily on turbulent diffusion respectively on the interaction of particles and droplets induced by turbulence, and not on inertial separation as in the case of the venturi scrubber. A light scattering device was used to measure the particle distributions. The experimental set-up was built up in a technical scale. The influence of operation parameters, especially water consumption, residence time, and pressurized air, on the grade efficiency has been demonstrated by their systematic variation. The contribution of turbulent diffusion to the collection efficiency has been confirmed.