In Situ Determination of Fractional Efficiency in General Ventilation Filters
The purpose of the EN 4/10 research paper was what many of us in the industry of air filtration have looked for as a method of validating the performance of one filter system to another in real time. Fractional or particle size efficiency of filters in use in the end users application is difficult to predict with laboratory style testing where the conditions and parameters are controlled. At the same time interpreting the results of testing performed in the actual environment can be very different than what we expected from the filter.
First lets review what the EN 4/10 guidelines are:
- Particle counter with sampling probes upstream and downstream of the filter
- Preset distances from the filter in test
- Valves to switch from up/downstream
- Computer – to track multiple counts
- Diluter – a device to reduce the concentration down to the particle counter limits
- Pump – may be necessary to achieve isokinetic flow to the sampler
- Determine whether ambient aerosol is significantly different from the calibration aerosol (procedure to make ‘lab’ like corrections)
- Moisture check to make sure within the instrument capabilities
EN 4/10 is specifically designed to target the particle sizes from 0.2 – 1.0 microns. Although there is a provision for the ‘greater than one micron size’, this size category is regarded as having unreliable data reporting. Sampling lines should be of a material that will not allow the particulates to adhere to the inner wall surfaces of the hose. The sample probe itself should have a sharp edge at air entry and be a minimum of 8mm (appx 3/8 inch) in diameter. The valves should be of the same diameter as the tubing to reduce the possibility of creating a pressure blockage or particle retention point.
In Situ sampling requires the ability to install a probe upstream and downstream of the target filter. This requires installation without leakage from the insertion of the test lines. The filter system should be sealed to prevent leakage of the framing members and/or frames to filters.
Efficiency determination is made as the result of a minimum of twelve counts, of a minimum of 20 seconds each, conducted successively upstream and downstream of the filter. There are some provisions for purge included for the lines between samples. The reported efficiency is essentially a computed average of the upstream vs downstream concentrations. Although the procedure outlined allows for the averaging of two of the upstream measurements to one downstream measurement and not conversely.
Potential For Errors
- Humidity may have an influence on the instruments if operating beyond their limits. It can also coerce variations in the efficiency and particle behavior.
- Temperature could have problems with instruments in cold conditions.
- Aerosol Composition the refractive index, density, and shape of the particles can cause variations from test aerosol calibration.
- Uneven Aerosol Distribution will occur and will show variations in time.
- Turbulent Airflow has the potential to distort the results with non-directional airflow patterns.
- Coincidence errors are the limitations of the particle counter itself.
What we find from the outline of Eurovent 4/10 – 1996 is an effort to take the laboratory out into the field and determine fractional efficiency. Many references are made in correlation to “in the lab conditions”. This is not surprising as lab folks look for repeatability in any thing they do as a methodology. What is difficult to accept from their work is the complete disregard to the particulate larger than one-micron. This is a large part of our market segment and to disregard the values found, In Situ appears as a choice, to ignore the potential for unloading or releasing of particulates from the media. Previously published papers have documented for example the release of fiber from unsupported air-laid microglass media. The phenomenon of agglomeration of smaller particles can lead to the inability of a filter to retain the larger created particle due to the media velocities or turbulence within the system.
Additionally in most scientific studies when taking numerous measurements those that are statistically out of proportion from the rest are discarded while the samples used for calculation in the EN4/10 are not. This leads to some calculations that are not really descriptive of the actual performance. It is important that each number or set of values makes sense with the sampling techniques and that human errors of statistical significance are eliminated.
It is not possible to get good representative values when inserting a hose into the side of a duct without sealing the penetration, as there will be turbulence caused by the air entering the duct through the remaining orifice space. Additionally the size of the plenum area upstream and downstream must be the same otherwise there will be different values in the face velocity, which will alter the particle capture ability.
While there are definitive issues with the degree of parameters set in this document it remains the genesis of creating a teachable method of measure for those without practical experience using a laser particle counter in the field. As we look at the potential for this kind of field measuring of performance verification and recognize that this was done in 1996 the future will be brighter with the emergence of particle counters that are more user friendly and easier to transport with the likes of the ARTI and Lighthouse versions.
Each of these devices allow lightweight multiple channel-sizing particle counts with very economical purchase prices. One of the features that make them valuable is that they can be set up to data-log and make several counts in the same area without physical attention.
There is still no shortcut or substitute for experience in diagnosis of what the data means once collected. This is profound when in the Eurovent 4/10 examples where, at the 3.00 to 4.50 micron size, the filter in test is negatively efficient while calculated at 83% at 0.75 to1.00 micron. This is not the possibility of unreliable information if it is true through all 12 counts! (page 22)This phenomenon has been witnessed in several lower efficiency filters and reported as unloading or sloughing, should it be any different in high efficiency filters, or In Situ testing?