Evaluation of a submicron particulate monitor

A Sample Extraction / Dilution System (SEDS) was built at The University of Tennessee Space Institute (UTSI) to measure particle size distributions and mass loadings in a high number density environment The SEDS extracts a sample of particle laden gas from a high particle concentration environment and dilutes it while preventing acid and water condensation. After dilution, a commercially available particle sizing instrument can be used for number density and mass loading measurements. The major advantage of the SEDS over currently available techniques for measuring high loading environments is its on-line, near real-time response to particle number density fluctuations. The performance limitations and uncertainties of the SEDS is evaluated, both theoretically and experimentally. The theoretical evaluation is done with supporting experiments by considering each contributing element to the measurement individually. By propagating the errors through a Taylor series expansion, the largest contributing factor to the overall uncertainty can be identified. From this investigation, the largest error source was the unsteady sample flow of the commercial particle sizing instruments. The uncertainty limit is estimated to be ±32%. When the error due to the varying flow rates is removed, the uncertainty limit reduces to ±20%. Experiments were carried out at the U. S. Department of Energy Coal Fired Flow Facility (CTFF) adjacent to UTSI to evaluate directly the SEDS performance. The purpose of these tests was to compare measurements made with the SEDS to those made with an accepted standard particle sampling method (5 stage cyclone). However, during the tests, the sample orifice and the tubing within the SEDS plugged gradually. This shut off the flow of particles. When a larger orifice was installed to prevent plugging, the particle counters became saturated. The SEDS also had difficulty in drawing a representative sample from the flue duct. The pressure inside the flue at the CFFF stays at about 8% below atmospheric pressure. The pumps on the SEDS could not adequately sample the gas from this low pressure region. Because of these difficulties, no direct comparison could be made between the SEDS and the 5 stage cyclone. To evaluate fully the SEDS, the plugging needs to be eliminated by redesigning the extraction system. This should promote better sampling from the duct. Also, by modifying the individual components of the SEDS and the particle sizing instruments, the accuracy of the system will be improved.