Evaluation of the exposure of humans to ultrafine, airborne particles is an important aspect of health in the workplace, especially in cases where nano-particles are present. However, portable sampling devices for efficiently collecting ultrafine particles in a worker's breathing zone are not readily available. The present study describes the design and development of a portable sampler for collecting particulates in the breathing zone, as a possible tool for this purpose. The design is based on the use of an ”Inertial Filter” to separate various-sized nano-order particles. Inertial filters consisting of SUS fiber felt (fiber diameter 5.6-13.5 μm) placed in circular nozzles (3-6 mm diameter with 4.5 mm length) were used. To achieve the smallest dp50 under the allowable pressure drop of a portable pump, the influence of fiber loading on separation performance and pressure drop were investigated. The influence of particle loading was also examined in relation to pressure drop and separation performance. The smallest dp50 under the allowable pressure drop (5.7 kPa at 6 L/min) for the battery pump employed was ~140 and 200 nm respectively for SUS fibers of 5.6 and 9.8 μm diameter (particle volume fraction ~0.013). The change in separation performance due to particle loading was confirmed to be acceptable for use under the present conditions. Under these conditions, a sufficient amount of particles can be collected for chemical analyses, e.g., particle-bound PAHs after 6-8 hours of sampling. Hence, the developed sampler has the potential for use in evaluating exposure to ultrafine particles in the breathing zone in the workplace.

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Development of a Personal Sampler for Evaluating Exposure to Ultrafine Particles

We investigated submicron-sized aerosol filtration and the pressure drop of an electret filter called a “Flimmer filter.” The fibers of the filter are aligned parallel to the direction of the airflow, unlike conventional fibrous filters or conventional electret filters. Lab-scale tests were performed first in a laboratory duct system for submicron particle removal efficiency and pressure drop of the filter. Then, Field tests were conducted in an apartment home using two portable aerosol spectrometers and with a Flimmer filter installed at the terminal of a duct within a mechanical ventilation system. The removal efficiencies at the face velocity of 1.0 m/s for 0.4 μm and 0.6 μm were 52% and 65%, respectively. The removal efficiency for PM1.0 was about 51%. Through an adapted mass balance model, indoor particle concentrations both in number and mass were predicted. The predicted results for the temporal variations of 0.4 and 0.6 μm sized particle, and PM1.0 correlated well with the results obtained from the field tests. When the face velocity was 1.0 m/s, which is the nominal operating condition of the test filter, the pressure drop was 11.5 Pa, which is relatively lower than the pressure drops of other conventional fibrous filters or conventional electret filters having the same filtration efficiency.

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Characteristics of submicron-sized aerosol filtration and pressure drop of an electret filter installed in an air diffuser in a residential apartment unit

As a tool to evaluate the characteristics of aerosol nano-particles, a high-volume air sampler for the collection of nano-particles was developed based on the inertial filter technology. Instead of the webbed fiber geometry of the existing inertial filter, wire mesh screens alternately layered using spacing sheets with circular holes aligned to provide multi-circular nozzles were newly devised and the separation performance of the filter was investigated experimentally. The separation performance was evaluated for a single-nozzle inertial filter at different filtration velocities. A webbed stainless steel fiber mat attached on the inlet surface of the developed inertial filter was discussed as a pre-separator suppressing the bouncing of particles on meshes. The separation performance of a triple-nozzle inertial filter was also discussed to investigate the influence of scale-up on the separation performance of a multi-nozzle inertial filter. The influence of particle loading on the pressure drop and separation performance was discussed. A supplemental inlet for the nano-particle collection applied to an existing portable high-volume air sampler was devised and the consistency with other types of existing samplers was discussed based on the sampling of ambient particles. The layered-mesh inertial filter with a webbed stainless steel fiber mat as a pre-separator showed good performance in the separation of particles with a dp50 ranging from 150 to 190 nm keeping the influence of loaded particles small. The developed layered-mesh inertial filter was successfully applied to the collection of particles at a dp50 ∼ 190 nm that was consistent with the results from existing samplers.

Development of a high-volume air sampler for nanoparticles.

4 – Selected Operations Involving Transport of a Single Fluid Phase through a Porous Medium

5 – Selected Operations Involving Transport of a Single Fluid Phase through a Porous Medium

By using beds of glass fiber with adiameter of 4-270μ as shown in Table 1, we investigated the various pressure drop characteristics and obtained the following results.(1) Pressure drops of the fiber mats decrease with the reduction of the area ratio Ai/A0, i.c. area ratio of masked face area to initial face area of the fiber mat. And the pressure drop Δpf changes with filter thickness L or fiber diameter Df (Figs. 5, 6, 7 & 8). (2) From the definition of a drag coefficient of a fiber mat, the experimental drag coefficient CD, were obtained, and Correlated with Reynolds number NRe. (Fig. 9) The results are represented by the empirical Eq. (5) or (6).In case 10-3 NRe 1.5×1010-3CDe=0.6+4.7/√NRe+11NRe Eq. (5)(3) In Fig. 9, the curved line shows the vaLues calculated from Eq. (5), which are in agreement with the experimental data. Comparison was made between these values and those obtained from other investigators' equations, and the Iberalls' equation was found to be in agreement with ours in case NRe 1. When NRe 1, however, all the values obtained from other investigators' equations deviated from the experimental data.(4) The pressure drops of the fiber mats were proportional to (1-e)m in our experiments (Eq. 7). When this relation is introduced in to Eq. (4), in the place of (1-e), we may call it a modifid drag coefficient CDm. The correlations of CDm vs. NRe can be given by Fig. 12 in the same way as by Fig. 9, and the experimental data fall exactly on the straight line when NRe 1(CDm∝NRe-1), and they show tendencies similar to frictional factors of acircular pipe for the turbulent region when NRe 1 (Eq. 8).

N. Kimura

Papers

Experimental Studies on the Pressure Drop Characteristics of Fiber Mats