Tuesday, 05 November 2024
In Part 10 of the series on air filtration, Dr Iyad Al-Attar points out the fact that each filter has several efficiencies depending on the particle size distribution, thus challenging the basic assumption applied to filters and the flow rate used in testing. He also envisages a future where the definition of filtration efficiency is more nuanced and accurate.
THE DEFINING MOMENT
In the late 1700s, the Scottish inventor and mechanical engineer, James Watt (1736-1819) developed the concept of horsepower to rate his steam engine’s capabilities[1]. He calculated horsepower to be 33,000 lbs ft/minute, based on deducing the horse’s speed while performing an exhausting activity and estimating the horse’s pulling force. The unit was globally implemented to measure the output of engines, turbines, electric motors and other machinery. The definition of the unit varied between geographical regions. The term “horsepower” has become a standard measurement of engine capacity, and the SI unit of power, the Watt, was named after him[2, 3].
In simple terms, “horsepower” is a measure of work done over time. Today, the term “horsepower” is commonly heard and used especially when we plan to purchase a car. The term may loosely mean anything the salesman chooses to define it as. Therefore, caution must be exercised in using the term. The mechanical horsepower is equivalent to 745.7 watts, while the metric horsepower of 75 kgf-m per second is approximately equivalent to 735.499 watts. The boiler horsepower is used for rating steam boilers, and is equivalent to 34.5 pounds of water evaporated per hour at 212F, or 9,809.5 watts. One horsepower for rating electric motors is equal to 746 watts[4].
A similar analogy can be made while considering the efficiency of an air filter. Akin to the term “horsepower”, “efficiency”, too, can be interpreted differently in the field of air filtration. A given filter could have different readings for arrestance, atmospheric dust-spot, particle size and fractional efficiencies. In fact, all these efficiencies, whether they are based on weight or particle size concept, carry the % symbol.
MINUTE DETAILS OF PARTICLES
Let’s start by considering the difference between the weight concept of the arrestance test and particle size. Let’s assume, for instance, that we have 1,001 particles challenging a hypothetical filter. The particle distribution consists of 10 particles of (10 µm) and 1,000 particles of (1 µm), as shown in Figure 1. If the filter captures only 10 micron particles, it would have 91% arrestance by weight. On the other hand, it would be one per cent efficient by particle size, assuming that all the 1 µm particles will pass through the filter.
While arrestance tests the ability of a filter to capture the largest atmospheric dust particles, it does not provide a representative performance indication in capturing finer particles. Clearly, there is no consideration of particle size in arrestance test. Therefore, it should only be considered when mass of dust in the air is the primary concern. For relatively finer particles (respirable-size range), the arrestance test fails to distinguish between filters.
“Dust Spot Efficiency”, on the other hand, assesses the filter’s ability to capture particles by virtue of the light transmission through previously evaluated target paper[5]. Another means of evaluating efficiency is “Fractional Efficiency” or penetration, where uniform-sized particles are fed into the air filter to determine the particles removed (percentage) by the air filter, using a particle counter. A sample of the fractional efficiency output is shown in Table 1 for short range of particle size and for five different flow rates.
Taking into account the implications of Table 1, the issue gains significance when giving due consideration to the FAQs: What is the efficiency of “X” or “Y” filter? What filter efficiency is appropriate for an office or home? At this point, we ought to pause to ask: Which “efficiency” do we actually refer to? Which definition of efficiency are we going to choose to represent a given filter?
It is evident from Table 1 that each filter has several efficiencies, depending on the particle size distribution challenging the filter and the flow rate used in testing. Just between a narrow particle size range of 0.065 to 0.2 µm for a given flow rate, the filter has 18 different efficiencies. The filter is rated at the Most Penetrating Particle Size (MPPS) where the filter scores its lowest efficiency (maximum penetration). Therefore, it is imperative that we operate this filter at the rated flow rate. When using the filter at higher flow rate than the rated one, the efficiency drops, MPPS shifts and the filter performance deviates from the test report accompanying a newly purchased filter.
NANOFIBRES IN AIR FILTRATION
Recently, the emphasis has shifted towards developing different ways to enhance filtration performance of fibrous filters. Evidently, filtration designers and manufacturers envisage ways to improve the efficiency of air filters without having to increase the pressure drop. Their dream is to reach an ideal situation of lowering pressure drop and improving filtration efficiency simultaneously. The introduction of nanofibre has given some hope to such dreams. The use of nanofibre has, in fact, proved invaluable for many industries, such as aviation (in aircraft cabins), healthcare (in hospitals) and in Space programmes (in spacecraft).
Nanoweb is a layer of very fine polymeric fibres with nominal diameter of 0.2 µm. It is important to note that nanofibre is a loosely defined term, with some definitions labeling it as a fibre with diametre less than 0.3µm, while other definitions pegging it to less than 0.1 µm diametre.[6] Nanoweb has a significantly tighter distribution of fibre diametres than for glass fibres or meltblown fibres.
An important method of enhancing the performance of the filter media is coating the surface of lower efficiency substrate media with a nanofibre layer (Figure 2). Usually, the substrate filter media have good mechanical strength and relatively modest filtration efficiencies. The newly created composite media provides good separation capabilities and good efficiency. The efficiency of a nanofibre web is achieved through purely mechanical filtration mechanisms. It does not degrade under varying ambient conditions, as charged meltblowns are expected to do.[7]
An important advantage of the nanofibre web is its extreme layer thinness, compared to a charged meltblown layer, as shown in Figure 3. This is certainly an advantage in many industrial applications. The submircon fibre diametre range provides a very high surface area, which in turn, grants an enhancement in the filtration efficiency in interception and inertial regimes at relatively negligible decrease in permeability.[8]
Although there are many advantages of using nanofibres, there are some challenges involved that would make it unsuitable as standalone filtration media. The weak mechanical strength and high density of the nanoweb makes using it as a standalone material in wide filtration applications questionable. Also, the high packing density of the nanofibre material introduces a huge permeability challenge. The increase in packing density enhances the efficiency at the expense of increasing the pressure due to permeability reduction. But the question remains: Would the rate of the increase in efficiency be faster or slower than the increase of the pressure drop, given the relationship between permeability and pressure drop of the filter? This relationship, obviously, is of critical importance. According to Darcy’s Law, pressure drop is inversely proportional to the filter’s permeability and directly proportional to flow rate, as shown in Figures 4 and 5 respectively.
THE ETERNAL QUEST
Every time I address a different aspect of air filtration, my conviction about the quest for further development in the field is re-emphasised. Saving the horsepower of the industrial application requires utilising aerodynamic and efficient filtration technologies. Living in an era of green buildings and renewable energy, where saving every single iota of energy matters down the street and around the globe, it could be possible that, one day, a concept of filtration horsepower is developed to describe the filter with the highest efficiency and the lowest pressure drop.
The writer is Regional Director, Middle East, and International Consultant, EMW Filtertechnik, Germany. He can be contacted at iyad.al-attar@emw.de
The writer is Regional Director, Middle East, and International Consultant, EMW Filtertechnik, Germany. He can be contacted at iyad.al-attar@emw.de
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