Activated Carbon Development for a Powered Air Purifying Respirator
Discover how a respirator manufacturer partnered with Haycarb for expert recommendations and tailored carbon samples for OV/AG filters in a Powered Air Purifying Respirator (PAPR) system, aiming for NIOSH certification. Haycarb provided optimized carbon samples, tested against various gases to ensure effective filtration and compliance in supporting reliable respiratory protection.
Background
The design of gas and vapour filters for Powered Air Purifying Devices (PAPRs) is always a complex process, with several iterative steps with attendant testing and evaluations to be followed. Haycarb has long experience in this field, supplying and supporting leading PPE companies with high quality coconut shell carbon for decades. Our team of Chemists and Engineers is well rehearsed in the processes of prototype carbon production, based on the requirements and test feedback from customers.
Applications
In general, Powered Air Purifying respirators are widely used when respiratory protection is required for long periods of the working day. They have the distinct advantage of reducing the burden of wearing respiratory protection for workers, by minimising the work of breathing, and providing a cooling air flow. In some cases, by the use of hood or helmet headtops, they remove the need for a tight face-seal. They can also aid the integration of other protective modes such as eye, face, skin and head protection, through the availability of multi-mode devices, certified to multiple applicable standards.
The use of PAPRs, sometimes called powered respirators, has expanded greatly over the years, with many different applications in industries such as metals (smelting, refining, welding etc), pharmaceutical production, chemical processing, painting and finishing, asbestos and mould abatement, agriculture, and many others. In most countries, if they are to be used in the workplace, they require certification to recognised national or international standards. The best known of these are the US NIOSH (National Institute for Occupational Safety and Health), promulgated via 42CFR84, and the European standards EN12941 and EN12942.
Types of Gas/Vapour Filters
In common with negative pressure respirator standards, there are requirements for the actual filter elements, which are generally replaceable spares. Firstly, they may be defined as either particle filters, or gas/vapour filters, or combination filters, which offer both filtration modes. The usual media choice for gas and vapour filters is activated carbon. Beyond this, gas and vapour filters are then defined by type, according to the type of gas they are intended to remove. Examples of types of filters include OV (Organic Vapour), and AG (Acid Gas), in 42CFR84, or the well-known A, B, E and K types in European standards. OV or A type filters will require a high surface area base carbon for the filters, usually coconut based, whereas the other types will require impregnation to allow chemisorption processes to take place within the filter bed during use.
Airflow Considerations
The filter development is a critical element in any powered respirator design. It is one of the main components of the system, together with the blower, battery, headtop, and any hose or valve systems. The performance of a system relies on delivery of the correct airflow into the headtop at all times during the use period, and manufacturers must ensure that their system delivers this airflow right through the design duration of the system. Some systems are designed to be reactive, delivering air on demand, following the wearer’s breathing pattern. In all cases, the filter is a resistive element in the system, and the pressure drop therefore needs to be very carefully matched to ensure the blower is capable of delivering the correct flow. The higher the resistance of the filter, the higher the power requirement for the blower, which then leads to additional weight, especially of the battery.
Filter Capacity & Considerations
The capacity of the filter to remove harmful gases and vapours has also to be tested under the standards. There is a defined minimum filter life against the representative gas test chemicals. Typically, the capacity of the filter needs to be tested at the system flowrate, which needs to be determined in advance.
Once the system flowrate is known, the test flowrate for the filters can be determined (with flow divided between the number of filters in the system, typically 1-3). The type classification is determined by the designer, who decides which they want their filter to meet, depending on the target market.
The final decision is the class, which is a measure of the filter capacity. In the European standard there are 3 capacity classes (1,2, and 3, in increasing order of gas capacity), and it must be borne in mind that these are defined differently from the classes in the negative pressure standards (such as EN14387). The higher the capacity class of the filter, the longer the filter will last in a given concentration of contaminant, although the filters and the system then get heavier.
Cooperative Development
A filter designer, representing a respirator manufacturer, contacted us to request recommendations and samples of carbon for OV/AG filters for a Powered Respirator Systems to be certified under the NIOSH system. We were able to help them through this process by following certain key steps;
Initial Volume Calculation
A rough calculation of the volume of the required filter bed was initially made, taking account of the required flowrates and filter types. This is based on in-house data and experience.
Prototype Development
The filter designer would then normally make rapid prototype filter bodies which can be filled with carbon, although a certain amount of checking can also be done by using refillable carbon vessels in our own laboratory.
Carbon Sizing and Pressure Drop Considerations
The next stage is to establish the potential carbon sizing, noting that the smaller the carbon mesh size, the more efficient will be the contaminant adsorption, whilst the pressure drop would also be higher. Achieving the correct balance here is especially critical in powered respirator systems. This work can be done more cheaply through the supply of 2-3 different size ranges of base carbon, which the manufacturer can then use to check pressure drop and system flowrate. The results from the trials can then be extrapolated to get the ideal carbon mesh size range. Since the impregnation will not significantly affect the pressure-drop, there is no need to produce such at this stage.
Prototype Impregnations and Gas Life Testing
Once the mesh size has been initially selected, prototype impregnations can be done. It is then possible to check the approximate expected gas life in the laboratory by using refillable containers and testing at a calculated flowrate to replicate the dwell time expected in the final filter. Alternatively, if prototype filter bodies are available, they can be used to check actual filter performance, provided that there is confidence the filters are correctly sealed at all joints and the attachment point.
In this case, the filling method is very important, to ensure that the fill of carbon is completely level, and there are no voids or channels. Alternative methods include forms of vibration filling, or “snowstorming”. In addition, a measure of compression is required to ensure the carbon bed is properly immobilised in the filter. The benefit of using prototype filter bodies is that the air flow pattern through the filter will accurately represent the final design, taking into account any blind spots, or inherent flow restrictions. The design of the plenum between the filter bed and the attachment point can make a significant difference here.
Performance Testing and Optimization
Once prototype filter bodies are available, performance against the various gases can be checked. In this case, an OV/AG classification was requested, so the filters had to be tested against Cyclohexane, as well as representative acid gases such as chlorine and sulphur dioxide. Since the requirement was for NIOSH certification, humidified conditioning of the filters is required prior to some of the tests, and this is always the worst-case scenario for OV, in particular.
Initial feedback suggested that the OV performance would fall significantly short of the requirement, however it was also found that there was excess capacity against chlorine and sulphur dioxide. There can be a trade-off in this case, and we were able to quickly produce new prototype carbon samples in our laboratory. The amount of impregnant was reduced, this having the effect of increasing OV performance whilst reducing the acid gas capacity. Two rounds of such testing were required to reach the final ideal carbon for the application.
In other scenarios of this type, it may be necessary to adjust the activity level of the carbon, to make changes to its surface chemistry, or to alter relative levels of the different impregnants used in multigas type filters. In all such cases our ability to quickly make samples at laboratory scale will assist the designer.
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