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. 28 ECOLIBRIUM • MARCH 2018 FEATURE 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 Residual Oil Construction Distilate Oil Marine Aviation (Aircraft and Combustion Dust Combustion Vessels airport ground equipment) Other Commercial Natural Gas/LPG Wood On Road Road Non-Road Electric PM2.5 Cooking Combustion Combustion Vehicles Dust Equipment Generation 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Figure 1: PM2.5 Emissions Sources in New York. MARCH 2018 • ECOLIBRIUM 29 FEATURE 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.