Experimental Study on the Leakage Characteristics of Filter Cassettes
Journal
Aerosol and Air Quality Research
Journal Volume
22
Journal Issue
8
Date Issued
2022
Author(s)
Abstract
Hazardous aerosol particles harmful to human health can be found in various workplaces. Filter cassettes are often used for air sampling to measure dust concentration in the air. However, filter cassettes are prone to leakage if not correctly assembled, potentially causing exposure data to be underestimated. This study aims to examine all potential leak routes, mechanisms, and contributions to the leakage to find solutions to minimize the leaks. The membrane filter and the support pad were held using 37-mm transparent styrene two-piece filter cassettes of two brands (A and B). They were assembled with a homemade manual presser, which can apply a controlled force onto the cassette from 40 to 350 N. To measure the direct force required to press on the top piece and tightly clamp both the filter and the backup pad, the top part of a typical cassette A was filed to prevent contact and friction with the bottom piece. The interface of the top and bottom pieces of the cassette was sealed from the outside with a sealing shrink band to prevent external leakage. Ambient aerosols were used as challenge agents. The aerosol number concentrations upstream and downstream of the filter cassette were measured using a condensation particle counter. The sampling time was set at one minute. The pressure drop across the filter cassette was measured using a pressure transducer calibrated using an inclined manometer. A sampling flow of 0.3 L min–1 in the condensation particle counter (CPC) was chosen for low filter penetration. The results showed that the filed cassette A suffered from external leakage, regardless of whether it held the filter and/or the backup pad. For the normal cassette A, leakage was a function of the assembly press force. When the assembly force increased from 150 to 210 N, total aerosol penetration (through filer, pad, and other leaks) decreased sharply from 2.1% to 0.004%. Sealing with a shrink band could further reduce aerosol penetration to 0.001%, while the pressure drop rose to 17 mmH2 O. The re-assembling experiments showed that the service life decreased with increasing press force applied. For cassette A, the service life was 14 times when re-assembled with 210 N, while the service life dropped to 6 times when the force was 300 N. In conclusion, sufficient press force is needed for cassette assembly to prevent both internal and external leakages. Despite the only subtle difference in cassette dimensions, the minimum assembly forces required for the two cassette brands differ, 210 N for brand A and 330 N for brand B. Use of a sealing shrink band is strongly recommended because it helps reduce external leakage, provides space for labeling, and prevents sample contamination during filter retrieval.
Subjects
Filter cassette; Sealing band; Press force; Cassette assembly; Leakage; BYPASS LEAKAGE; QUANTIFICATION; EFFICIENCY
SDGs
Publisher
Taiwan Association for Aerosol Research
Type
journal article