THE OFFICIAL JOURNAL OF AIRAH MARCH 2018 · VOLUME 17.2 RRP $14.95 PRINT POST APPROVAL Ecolibrium NUMBER PP352532/00001
Common good A Games facility built for all. FEATURE
Kitchen exhaust filtration design Part 1
In the first half of a two-part series on kitchen exhaust aesthetic or comfort issue. But how do we approach kitchen exhaust filtration? design, we discuss kitchen exhaust filtration, and Australian standards compared to benchmark practices within the US. WHY FILTER Using first-principle analysis, critical design parameters are KITCHEN EXHAUST? provided for kitchen exhaust filtration design in Australia. Within Australia, there are standards that govern kitchen ventilation. By Jonathan Bunge, Affil.AIRAH. These include council and state regulations, local, state and federal Primarily due to health concerns shown that in the past emissions fire codes and most significantly and odour complaints, the requirement from barbeque grills in New York AS/NZS 1668.1[2] and AS 1668.2[3] to effectively filter kitchen exhaust have been the sole attributor to 400 – all of which are referred to by the discharge to a non-objectionable effluent deaths annually[1]. Filtering kitchen NCC (National Construction Code) is increasing. Research out of the US has exhaust could no longer be solely an for deemed-to-satisfy solutions.
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that emissions from char broilers (grills) in New York contributed Are we just to more than 12.5 per cent of PM2.5 concerned about attributable deaths annually in the period 2005–2007. This equates to odour and the visual 400 deaths per year[1]. If all char broilers eyesore of smoke, had effective kitchen exhaust filtration installed and maintained, it is estimated or the health nearly 350 of these deaths could have been prevented[1].
of the general Another study completed at a similar population? time in New York suggested that 20 per cent of all PM2.5 particles in the air are from commercial cooking; this is more than the amount attributed Failure to satisfy all of these requirements to on-road vehicles at 17 per cent[4]. will result in a system that does not comply with AS 1668.2:2012. Separately, researchers at the University of California estimated that “the average These requirements aren’t always diesel-engine truck on the road today practical or cost-effective though, would have to drive for 10 miles (16km) particularly when working with existing on the freeway to put out the same buildings. For these situations the NCC mass of particles as a single charbroiled makes a performance solution pathway (grilled) hamburger patty.[5]” available to us, with a verification process prescribed for soundly engineered This in turn led to an amendment to designs. AS 1668.2 offers some non- New York law, effective as of May 6, 2016, mandatory commentary to guide that prohibits the operation of any new designers towards potential solutions. commercial char broiler (grill) and any existing chain-driven commercial The NCC and AS 1668.2:2012 also char broiler (grill) to cook more than require designers to consider situations 875 pounds (400kg) of meat per week where exhaust may pose a danger or unless it has an emissions-control device nuisance. Exhaust filtration systems that meets the requirements established can help manage these risks. by the Commissioner of the Department AS 1668.2:2012 identifies all kitchen One of Australia’s motives for kitchen of Environmental Protection (DEP)[1]. exhaust over 1,000L/s as “Type B exhaust filtration is to avoid nuisance Objectionable Effluent”. In addition complaints and provide odour-free to the exhaust discharge requirements air discharge. Yet it is interesting WHAT IS SUITABLE for other exhausts, requires that this that in the US (where kitchen exhaust FILTRATION FOR AN is then: regulation and technology is typically ENGINEERED SOLUTION? • Discharged vertically more advanced), certain places enforce Under AS 1668.2 the only deemed- the filtration of kitchen exhaust • Discharged at no less than 5m/s to-satisfy solution is to provide an to improve ambient air quality and exhaust that complies with a type-B • Discharged at least 6m away therefore the health of the population. from boundaries (including street objectionable effluent as described boundaries), outdoor air intakes A study from the Department of Health above, in addition to all the requirements and natural ventilation openings. and Mental Hygiene in New York estimated for non-objectionable discharges.
Buildings Traffic Non-Road Emissions Sources
R O C D O M A A C D C V O
ommer ial Natural as ood n Road Road Non Road le tri M oo in ombustion ombustion ehi les ust uipment eneration
Figure 1: PM2.5 Emissions Sources in New York.
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However, due to increasing urban that odours are removed to a reasonable In a simplistic way, rather than acting as density, and available space for level, a significant level of particle a sieve, the HEPA relies on the probability mechanical services within buildings, filtration also needs to be in place. that as a particle passes through the exhausting vertically from the roof fibres, they will either impact onto a fibre It turns out that the level of particle more than 6m away from a boundary (A) or come within 1 particle diameter filtration required to produce cost- is often not possible. of a fibre and be drawn onto a fibre effective odour filtration is also through interception(B). To overcome this, an engineered the level required to eliminate smoke solution might include a filtration system as a visual pollutant, and significantly Naturally as the particle size decreases, to clean the air to a “required” level. reduce PM2.5 emissions. the probability of a particle passing The definition of this “required” level through the filter without impaction The empirically successful number is unclear – while some recommendations or interception increases. However, as used in the US and Asia is a minimum are present as commentary after clause the particle size decreases below 1μm, particle filtration efficiency of 95 per cent 3.10.3 in AS 1668.2, this still doesn’t the particle undergoes diffusion (C), at 0.3μm (micron). give us a clear deemed-to-satisfy otherwise known as Brownian motion[6]. solution. It also doesn’t necessarily give Brownian motion is where the particle us a concrete, repeatable engineered WHY 0.3μm? is becoming closer in size and mass to solution that will always be acceptable The rating of the performance of filters gaseous molecules in the air. This causes to neighbours, councils and building for kitchen exhaust filtration at the the effect of collisions and interactions certifiers, especially given the variability particle size of 0.3μm is for two reasons: of the particle with gas molecules to in contaminant level of different • 0.3μm is known as the most increase, causing the particle to move commercial kitchen exhausts. penetrating particle size (MPPS) radially to the direction of bulk airflow. As an industry, we need to define • 0.3μm is a common particle size This results in a significant increase a clearer picture of what the requirements for smoke. in residence time and randomness for kitchen exhaust filtration are in of path for the particle though the filter; Australia. Are we just concerned about Most Penetrating and therefore, far greater opportunity odour and the visual eyesore of smoke, Particle Size (MPPS) for the particle to be captured in the or the health of the general population The particle size of 0.3μm (MPPS) filtration process. (and PM2.5-related deaths)? is typically selected as the test Figure 3 shows the overall minima point for rating filtration efficiency. The most cost-effective method can in fractional efficiency, which is This is because particles above and actually address all three concerns. between 0.1–0.4μm. This is where below this size (0.3μm) are generally the improvements in filtration due easier to capture[6]. THE SCIENCE BEHIND to diffusion are yet to offset the reduction The science behind the capture in interception and impaction due KITCHEN EXHAUST of particles on a traditional HVAC to smaller particle sizes. filter illustrates why 0.3μm is the most FILTRATION Similarly, for an electrostatic precipitator penetrating particle size. There are three Kitchen exhaust filtration can be broken (ESP), as the particle size decreases, main filtration mechanisms: impaction, up into two clear aspects: particulate the charge held decreases by a greater interception and diffusion. filtration, which in this case includes amount. This results in a lower filtration grease, smoke and free moisture; and The filtration mechanisms are efficiency. However, when the particle gaseous filtration, which includes odours. best explained using a HEPA filter as size decreases past 1μm, this reduction an example. Despite having a filtration in charge held is offset by the increased The first rule for kitchen exhaust efficiency of 99.97 per cent+ at 0.3μm, residence time in the ESP from the filtration is that effective and sustained HEPA filters don’t operate as a sieve effects of diffusion and the minimum gas-phase/odour filtration cannot be where only particles smaller than at 0.3μm remains. achieved without an adequate level the smallest gap will pass through. of prior effective particulate filtration. We therefore cannot assume that an Instead they comprise a mesh efficiency specified at 0.01μm indicates of fibres, with gaps between fibres Additionally, particles (such as smoke) a higher efficiency for all particle sizes are also odorous. Therefore, to ensure over 1μm not uncommon[7] [8]. Impaction Interception Diffusion
Figure 2: Filtration mechanisms.
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or EN779 (2012)[11] test products over the range of 0.3–10μm, so extracting the performance of the product at 0.3μm from these tests is also reasonable. The ASHRAE 52.1 (1992)[12] standard is obsolete. ASHRAE has since released TOTAL various versions of ASHRAE 52.2 to supersede 52.1, which reflects the growing understanding of filter testing methodology. ASHRAE 52.1 Diffusion Interception (1992) dust spot efficiency tested the Fractional efficiencyFractional filter’s performance at approximately 0.7–0.8μm[13]. It has been shown mathematically that a filter with an ASHRAE 52.1 dust spot efficiency of 90–95 per cent would test 65 per cent Particle size, μm efficient at 0.3μm[13], a critical difference given the importance of removing particles Figure 3: The impact of diffusion, interception and impaction on overall filtration efficiency. of this size for kitchen exhaust filtration. It is also important to check that the larger than 0.01μm. We want to ensure compare system efficiency at a particle velocity and/or airflow in the filtration size of 0.3μm. we are testing products at their worst system designed is the same as the test performing point – 0.3μm – and this The EN1822:5 (2009)[9] is widely recognised report, because most filtration products principle may be applied to many types as the best standard for testing a product – especially ESPs – drop significantly of particle filtration technologies. If you at its MPPS, this is normally close to in filtration performance as the velocity want to compare apples to apples, always 0.3μm. The ASHRAE 52.2 (2017)[10] of air increases.
Equilibrium HP Be Impressed 210X150mm 3mmBleed v1.indd 1 19/10/2015 12:17 pm FEATURE
Gaseous filtration and particles Below is a brief description of two M common methods of gaseous filtration M used to control odour, and how poor prior particle filtration will render them ineffective. Part 2 in the series on kitchen exhaust filtration design will go into
further detail on these technologies. Adsorbent/chemisorbent media Adsorbent media, commonly activated carbon, consists of micropores within larger granules. Small odorous gas molecules find their way inside these pores where various forces adsorb the
gas molecules to the media, therefore removing them from the airstream. If the media granule is subject to particle contamination, the outer surface of granule will become coated, creating a seal. This results in a poor performance due to the unused pores being inaccessible and going to waste. Ozone Ozone works by breaking down Figure 4: Particle mass distributions during the grilling of 50g of bacon on a gas stove[14]. molecules into smaller and smaller molecules with oxidation until eventually simple, common odourless gases are Smoke particles • Common odour control/gaseous left such as CO2, N2, H2O and O2, etc. filtration mechanisms don’t perform Smoke particles are normally 0.3-1μm in When relatively large particles such [6],[14] effectively or economically when size , and this is another significant as grease and smoke are present in reason why 0.3μm is chosen to compare there are too many particles present in the air. the air, which are 100 to 1,000,000+ system efficiency within kitchen exhaust times larger[15] than odorous gas filtration systems. A system that delivers • 99.97 per cent at 0.3μm while better molecules, the ability of the ozone to poor efficiency at 0.3μm will lead to poor for performance, poses economic break down the small odour molecules filtration of smoke, a common visual and footprint issues. 95 per cent is can be reduced. This is because the and odorous pollutant present in kitchen regarded as the sweet spot for a high oxidative power of ozone will be absorbed exhaust. Smoke can also be a significant level of filtration before extensive by large particles which may not be fully health risk when present in ambient air. design changes are required. broken down to a simple odourless gas, Figure 4 shows the mass of particles generated when cooking bacon on a gas stove. There is a significant mass of particles, mostly smoke, generated at 0.3μm, which is particularly noticeable AIRFLOW when the food is cooked at a high heat. The second peak of particles at 7–10μm is likely comprised of fine grease aerosols[15].
WHY 95 PER CENT? The rating of the performance of filters for kitchen exhaust filtration of 95 per cent at the particle size of 0.3μm Gas Molecules is done for three reasons: • 95 per cent filtration of 0.3μm particles ensures the air is mostly clear of smoke and almost entirely clear of particles of a smaller and larger size as Activated carbon discussed previously with impaction, interception and diffusion. Figure 5: The adsorption of gas molecules within the pores of activated carbon.
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particularly given the short residence [5] L. Bose, “Controlling the [16] N. Mehio, D. Sheng and J. De-en, time in kitchen exhaust filtration Emissions of Charbroiled Burgers,” “Quantum mechanical basis for kinetic and available concentration of ozone. 18 September 2012. [Online]. diameters of small gaseous molecules,” The tiny odorous molecules are therefore Available: https://ucrtoday.ucr. The Journal of Physical Chemistry, effectively “hidden” and can remain edu/8896. [Accessed 11 May 2017]. vol. 118, no. 6, pp. 1150-1154, 2014. in the airstream along with the particles [6] M. K. Owen and D. S. Densor, [17] W. H. Organization, “Ambient which are not fully broken down. “Airborne particle sizes and sources air pollution: A global assessment of found in indoor air.,” Atmospheric exposure and burden of disease.,” 2016. CONCLUSION Environment., vol. Part A., no. General Topics, 26(12), Nuisance odours are not the only factor pp. 2149–2162., 1992. About the Authors when considering options to improve [7] T. F. Scientific, “Fibremetric the ambient air quality from discharged Written by: – SEM image of hepa filter,” cooking fumes. The health of the general • Jonathan Bunge, Affil.AIRAH, national Thermo Fisher Scientific, [Online]. population is now also being taken into product manager, Airepure Australia. Available: https://www.phenom- consideration. By understanding the Technical review by: first principles of particle and gaseous world.com/software/fibermetric. [Accessed 29 January 2018]. • Brett Fairweather, M.AIRAH, filtration, we can make better decisions mechanical engineering consultant, regarding kitchen exhaust filtration [8] A. Mason, S. Wylie, It’s Engineered. applications to form an economic A. Thomas, H. Keele, A. Shaw • Peter Mathieson, F.AIRAH, and effective solution. and A. Al-Shamma’a, “HEPA technical director, Aurecon Australia. Filter Material Load Detection Jonathan Bunge, Affil.AIRAH, The World Health Organisation Using a Microwave Cavity Sensor,” estimates that approximately 3 million is a graduate of Melbourne University International Journal on Smart boasting a Master’s in Chemical deaths are attributable solely to ambient Sensing and Intelligent Systems, vol. 3, Engineering, and has gained engineering [16] (outdoor) air pollution each year . no. 3, pp. 322-337, 2010. experience within petrochemical facilities, Kitchen exhaust has been identified in the coal industry and the gaseous filtration poor ambient air quality. Increasing [9] BSI, BS EN 1822-5:2009 High industry. His role with Airepure Australia awareness of the effect of poor ambient efficiency air filters (EPA, HEPA enables him to specialise in the control air quality on human health could see and ULPA). Determining the efficiency of gaseous emissions, hazardous and filtration of kitchen exhaust discharges of filter elements, BSI, 2009. pungent odours, laboratory fume exhaust becoming increasingly important [10] ASHRAE, ASHRAE 52.2:2017 solutions as well as commercial cooking and industrial process exhaust solutions. in the future. Method Of Testing General Ventilation Air-Cleaning Devices For Removal Brett Fairweather, M.AIRAH, Part 2 in next month’s Ecolibrium Efficiency By Particle Size, ASHRAE, is a mechanical engineering consultant uses the first principles of filtration 2017. with 20 years of mechanical services reviewed here to analyse some common experience in the design and delivery kitchen exhaust filtration technologies [11] BSI, BS EN 779:2012 Particulate of projects. He is committed to training including hood filters, ultra-violet (UV), air filters for general ventilation. and support of other practitioners in the multi-stage filter packs, electrostatic Determination of the filtration industry, and volunteers his time to many committees responsible for the ongoing precipitators (ESPs), activated carbon performance, BSI, 2012. development of standards, codes and and ozone. The focus will be placed on [12] ASHRAE, Standard 52.1-1992 – regulations relating to building services. how we can economically and effectively Gravimetric and Dust-Spot Procedures achieve a particulate filtration efficiency Peter Mathieson, F.AIRAH, for Testing Air-Cleaning Devices Used is a mechanical engineer with over 30 years of 95 per cent at 0.3μm with sufficient in General Ventilation for Removing of mechanical services design experience. downstream odour control. ❚ Particulate Matter, ASHRAE, 1992. Over that period he has designed many [13] Camfil Farr, ASHRAE Testing kitchen exhaust installations, many with discharge filtration. Peter says his design for HVAC Air Filtration – A Review REFERENCES of kitchen ventilation has improved with of Standards 52.1-1992 & 52.2-1999, [1] Department of Enviromental experience, and is with reference to the Camfill Farr, 2002. Protection, Section 1403 (c) of current ASHRAE research. the New York Charter and Sections [14] G. Buonanno, L. Morawska 24-105 and 24-149.4 of the New York and L. Stabile, “Particle emission City Administrative Code., New factors during cooking activities,” York: Department of Enviromental Atmospheric Environment 43, Would you like to know more? Protection, 2016. vol. 43, p. 3235–3242, 2009. Part 2 reviews the likely performance [2] Standards Australia, [15] The Australian Institue of AS/NZS 1668.1, SAI Global, 2015. of common filtration methods for kitchen Refrigeration, Air Conditioning exhaust, and aims to uncover economic [3] Standards Australia, and Heating, Fire safety – Kitchen and effective choices for kitchen exhaust AS 1668.2, SAI Global, 2012. hood exhaust systems Understanding applications that will achieve the [4] N. Y. Health, “New York City and addressing the special fire risks design performance parameters Community Air Survey,” inherent in commercial kitchen established in part 1. New York, 2015. ventilation systems, AIRAH, 2016.
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