1. 呼氣閥動態洩漏測試系統的研發 2. 呼吸防護具新式高保護係數呼氣閥之研發

Abstract

Part I Respirators are commonly used to protect workers from inhaling air contaminants, especially when engineering and work practice controls fail to reduce workers’ exposure to acceptable levels. In general, there are three potential routes of leakage into respirators: filter penetration, facial seal leak and exhalation valve leak. Normally, exhalation valve leaks less compared to the other two leak routes. However, when a higher protection (more filter material and better respirator fit) is needed, the valve leakage becomes more important because of the leakage re-distribution. Current static certification test employed by USA and Australia requires leakage into new exhalation valves should not exceed 30 mL/min at a constant suction head of 25 mmH2O. This static test may not able to reflect the overall leak characteristics of exhalation valve when used under practical cyclic flow mode. Therefore, a dynamic leakage test system needs to be developed. A dynamic leakage test apparatus developed in this work consisted of an aerosol generation system, a breathing machine, and a cylinder simulating respirator cavity. The cylinder cavity was connected to aerosol chamber with an exhalation valve as the interface. With the breathing simulator functioning, the air resistance caused by the filter medium can be created by controlling the clean air flow supplied to the cylinder cavity. A condensation particle counter was used to monitor the aerosol concentration inside the cylinder cavity. The results showed the static leakage correlated well with the dynamic test data. However, the dynamic method is apparently more versatile and should not be substituted or replaced by the static test method. For example, static method cannot be used to study the effect of aerosol loading on the valve membrane and/or valve seat, which is likely to occur under cyclic flow. Also, the modification of the valve cover showed significant decrease in valve leakage, but the static test method was insensitive to the benefit. Part II Respirator is the last resort to protect workers against air contaminants. However, the accumulated heat and humidity often make wearers feel uncomfortable, especially for workers wearing respirators for a long period of time. Respirators equipped with an exhalation valve reduce the level of discomfort. In general, there are three routes of leakage on respirators, namely filter penetration, facial seal leak and exhalation valve leak. Usually, the exhalation valve leakage is the least among all potential leak pathways. Nevertheless, when a higher protection is needed, the exhalation valve leakage will become more important due to the increase of air resistance resulted from the thicker filter medium and better fit of the respirator to the wearer’s face. The static leakage test method currently employed by USA and Australia cannot reflect the leak characteristics when the respirators are used under practical cyclic flow mode. In order to characterize the leak behavior of exhalation valve under cyclic flow and design new exhalation valve, a dynamic leakage test system was built. By adding a piece of foam filter, honeycomb, or simply a tube, on top of the valve membrane, the new device was found to reduce the mass change between the clean exhaled breath and the contaminated ambient air. That means lower exhalation valve leakage, and therefore higher protection. To make the respirators user friendly, i.e., handy, portable, miniaturized, the use of highly porous materials, such as foam filters (100 ppi and above) is recommended. The used of porous materials or strengthening devices not only reduce the mass exchange but also help filter the contaminants if they are aerosol particles. The addition of this foam certainly increased the air resistance during exhalation, but the benefit is tremendous decrease in exhalation valve leakage. For example, under breathing flow of 10.5 L/min (tidal volume of 0.7 L, and breathing frequency of 15 breath/min), the maximum exhalation resistance increase from 4.8 to 6 mm H2O, a 25% increase, but the exhalation valve leakage decreased from 0.45 to 0.0005%, a 99.99% decreases by the use of the foam disc with 80 ppi and 24 mm thickness.