Sampling, isokinetic aerosol

Sampling. Four aerosol samples were collected isokinetically on July 26, 1979, at ports on the inlet duct of the ESP at Plant A two were taken with 62-mm fluoropore filters and two were taken with the University of Washington MKV Cascade impactor (1 ). At plant D, samples were collected over a 6-day period at ports both in the outlet duct of the ESP (i.e., upstream of the FGD system) and at the 91-m level of the stack. Eleven fluoropore filter, 1 MKV, and 4 MKIII impactor samples at each location, giving a total of 22 filter, and 8 MKIII samples. A single MKV sample was also collected in-stack at reduced pressure at plant D during the 6-day period. Polycarbonate material coated with apiezon L vacuum grease and 62 or 47-mm-diam, l-pm pore Fluoropore filters were used as back-up filters in the MKV impactor. 

Figure 1. Representation of an aircraft aerosol sampling inlet showing flow streamlines for subisokinetic, superisokinetic, and isokinetic sampling. Under subisokinetic conditions large particles will be oversampled under superisokinetic conditions small particles will be oversampled and under isokinetic conditions, the particle size distribution will be preserved.

In order to test the validity of the VELUT model it was used to generate predicted isokinetic values of VE differential sampling efficiency for values of MMAD and GSD corresponding to experimental values reported in the literature. A comparison of the experimental data obtained by Carson and Lynch (10) using mono-disperse aerosols of dioctylphthalate, with the predictions of VELUT for the same conditions is given in Table I. This limited data is in excellent agreement with the predictions of VELUT. 

Figure 2. Diagram of a typical sharp-edged probe for isokinetic sampling of aerosol particles from an aircraft. The probe is sized to provide 500 L/m at a sampling velocity of 50 mis. The 7° expansion of the probe behind the probe tip is to slow the air velocity. The interior of the probe is polished to a mirror...

A particle sampling and measuring system was set up to characterize the suspended particles in the biomass combustion Fig. ). The system was capable of on-line measuring of the concentration number and particle size distribution in the micron and submicron range. In addition, mass concentration, particle morphology and chemical composition were obtained. The aerosol is first extracted from the duct and then led to the sampling station through an isokinetic probe equipped with an automatic control unit. 

Self-Adapting Capability. The range of aerosol concentrations that can be handled in a fully automated unit can be expanded markedly if sampling times and conditions can be adjusted in accord with the actual aerosol content of the atmosphere or on other measured parameters. With such a feature, automatic isokinetic adjustments become possible. Such adjustments allow a marked increase in the quantitative accuracy of the calculation of actual aerosol concentrations in the air from the measured aerosol amounts. 

Figure 2. The effects of sa7npling under isokinetic (A), low-flow (B), and high-flow (C) conditions. The cross-hatched regions represent the sampling orifice large and small symbols represent large and small aerosol particles, respectively solid lines are air streamlines ar dashed lines are heavy particle trajectories.

Isokinetic flow can be achieved if the sampling system monitors the ambient wind velocity and adjusts the pumping speed as appropriate. Unfortunately, this system exacerbates the dynamic-range problem because the system samples faster in high winds, when the aerosol concentration tends to be higher. 

Aerosol monitor(s), automatic concentration range problems, 64 concepts, 63 data storage, 62 filter-paper tape, 66-67 flow conditions, varying, 65 isokinetic flow, 64 sample manipulation, 66 sampling time, 65-66... 

Work with slurries requires that the slurries are first nebulized and behave just as solutions with respect to the sample introduction into the aerosol. From electron probe micrographs of aerosol particles sampled on Nuclepore filters under isokinetic conditions, it was found that at nebulizer gas flows of 3 L/min, being typical of plasma spectrometry but far below those for flame atomic absorption, particles with a diameter of up to 15 pm can be found in the aerosol (Fig. 44) [117]. This would imply that powders with a grain size of up to about 15 pm could still be nebulized as could a solution. This, however, is not true as the mass distribution in the case of powders may be quite different in the slurry and in the aerosol, as shown for the case of SiC (Fig. 45) [118]. The nebulization limitations for the case of slurry nebulization thus must be investigated from case to case and leads to certain types of restrictions. 

In the applications of gas-solid flows, measurements of particle mass fluxes, particle concentrations, gas and particle velocities, and particle aerodynamic size distributions are of utmost interest. The local particle mass flux is typically determined using the isokinetic sampling method as the first principle. With the particle velocity determined, the isokinetic sampling can also be used to directly measure the concentrations of airborne particles. For flows with extremely tiny particles such as aerosols, the particle velocity can be approximated as the same as the flow velocity. Otherwise, the particle velocity needs to be measured independently due to the slip effect between phases. In most applications of gas-solid flows, particles are polydispersed. Determination of particle size distribution hence becomes important. One typical instrument for the measurement of particle aerodynamic size distribution of particles is cascade impactor or cascade sampler. In this chapter, basic principles, applications, design and operation considerations of isokinetic sampling and cascade impaction are introduced. 

The applications of isokinetic sampling cover but are not limited to the sampling of aerosols such as flu gas in chimney, soots (unbumed carbons) from diesel engine exhaust, dusts suspended in the atmosphere, and fumes from various sprayers measurements of particulate mass fluxes in pneumatic transport pipelines and other particulate pipe flows solid fuel (also some liquid fuels) distributions in furnaces, engines, and other types of combustors and calibrations of instruments for the measurements of particle mass concentrations. Isokinetic sampling can also be applied to flows with liquid droplets. In this case, the droplet sample is usually collected by an immiscible liquid (Koo et al., 1992 Zhang and Ishii, 1995). 

Isokinetic sampling is for the direct measurements of particulate mass fluxes or concentrations of aerosols (assuming no slip between aerosols and the gas flow) in gas-solid suspension flows. This is accomplished by inserting a thin-wall tube into the particulate suspension flow to draw samples at the isokinetic condition and by passing the collected particles into a sampling train. Typical... 

There are two kinds of solids suction isokinetic and nonisokinetic. In isokinetic sampling, the gas suction velocity is adjusted to equal the local gas velocity. This method was mainly applied to aerosol or other... 

---