09 November 2018

Commercial Kitchen Exhaust Filtration Systems

Design Checklist for Commercial Kitchen Exhaust Filtration Systems

This checklist is based on the following assumptions:

 Assumptions

Kitchen exhaust discharge treatment is required as a Performance Solution.

This because the Deemed-to-Satisfy Solution, which requires kitchen exhaust to be discharged vertically at 5m/s and not within 6m of a sensitive receptor, is not achievable.

Without a Performance Solution for kitchen exhaust discharge, low level or horizontal discharge may result in odour and particulate contamination impact on adjacent occupants or properties.

HOOD FILTRATION

How much air should be extracted through the hood?

AS1668.2-2012 Amendment 2 identifies kitchen exhaust hoods as a form of “Local Exhaust”. The Standard provides designers with a choice of two methods for determining kitchen exhaust hood airflows.

The simplified prescriptive procedure prescribes a minimum exhaust airflow rate through kitchen hoods depending on the size of the hood, the type of hood and the process cooking type (Refer to AS1668.2: 3.5 for details).

Alternatively, a more involved procedure requiring comprehensive testing by a proprietary hood manufacturer is possible. Once the performance of these engineered hoods is proven, the designer may adopt the manufacturers required airflows. For this process, the hood designers often refer to the ASHRAE handbook HVAC Applications, the German guideline VDI 2052 or the German standard DIN 18869 (Refer to AS1668.2: 3.6 for details).

It is important to provide an adequate kitchen exhaust hood airflow rate calculated from either AS1668.2 or one of the recommended alternative guidelines for adequate removal of contaminants from the kitchen.

What is the efficiency rating of my hood filtration system?

The first and often the most under rated stage of a successful kitchen exhaust filtration system is effective particle load reduction at the hood itself. Removing a significant load of particles at the hood, particularly the larger ones, can significantly reduce the maintenance costs on equipment downstream.

UV-C Lamps

UV-C lamps are provided in the hood plenum to reduce grease and odour through a mechanism known as photolysis. They also generate ozone which will oxidise some of the contaminants. UV-C lamps are a load reduction mechanism only, as the power of the photolysis and oxidation alone may not be enough to reduce the exhaust to a compliant level.

Ensure that you ask the manufacturer for the efficiency as a function of particle size to enable simple comparisons with other technologies. Also note that the lamps must be cleaned regularly to maintain the effectiveness of this technology.

Hood Filters

A high efficiency hood filter can assist with significantly reducing particle load. Ensure that you ask the manufacturer for the efficiency as a function of particle size for the grease filter cassettes to enable simple comparisons between products.

A higher pressure drop normally correlates to a higher efficiency as the air is more obstructed. A trade off may have to be made between capture efficiency and energy efficiency.

What is an UL1046 rated hood filter, and why would I require one?

Unlike AS1530.1 which simply tests the material of the filter for combustibility, the UL1046 test determines the abilities of hood filters to “Limit the projection of flames downstream when subjected to flames on the upstream face, after having been loaded with grease in a manner representative of cooking that produces a grease-rich exhaust”[1]. This is a critical component of a fire safe kitchen when flames (gas or solid fuel) are present in the cooking process.

According to AS/NZS1668.1:2015 clause 6.2.9 if you have kitchen exhaust ductwork that is longer than 10 metres and you have a flame under the hood, either from solid fuel or gas, you must have a UL1046 rated device in your system. As none of the particulate filters downstream are likely to be UL rated, this clause is most commonly achieved with an UL1046 rated hood filter cartridge.

It is important to consider both AS/NZS1668.1 and AS1668.2 in kitchen exhaust design. AS1668.1 is part of the NCC (2016), and fire related insurance claims may become void if this clause is not complied with. The typical honeycomb style filter may not comply with UL1046. Please check with your filter manufacturer.

 PARTICULATE COLLECTION

What is the system efficiency rating at 0.3μm, and why is 0.3μm important?

Typically to achieve a sufficient amount of particulate removal, a minimum efficiency of 95% at 0.3μm is desired. The reason 0.3μm is chosen as a measurement point is the fact that it is typically the hardest particle size to catch, and every other particle size will have a greater efficiency for the same system.

The wise buyer and specifier will always judge system performance with ratings at 0.3μm (not ratings at 0.01μm). In addition to this, smoke is of particle size 0.3μm-1μm so it is important to have a high efficiency at this particle size.

What is the recommend system velocity to ensure adequate performance?

Single pass kitchen exhaust air filtration systems are typically designed to operate at <1.85m/s (650L/s per 600 x 600mm area) to allow enough residence time for the technologies to effectively remove the smoke, grease, particulates and odours. When increasing the velocity through a system to reduce its size, the efficiency and performance of the system will be directly affected – and this may result in a non-compliant system. It will also increase the energy required for the fan.

For electrostatic precipitation filtration (ESP) the recommendations are:

  • Single pass ESP maximum velocity of 1.85m/s (higher velocity likely results in efficiency below 95% at 0.3 micron)
  • Double pass ESP maximum velocity of 2.5m/s (higher velocity likely results in efficiency below 95% at 0.3 micron)
  • Combined ESP + HVAC filters can achieve 95% at 0.3 micron at 2.5m/s +, however calculations for each situation are required.

Multi-stage particulate filter media systems should be selected with a maximum velocity of 1.85m/s for optimal balance of performance and pressure loss (higher velocity results in excessive pressure drop and associated fan energy use).

What is the effect of various particulate loads on my system?

Particulate load generally does not negatively affect the filtration performance of the particle filtration systems (except hood mounted UV). However, system maintenance frequency will be affected. Generally, higher particulate (contaminant) loads will require more frequent system maintenance than lighter loads.

Different particulate filtration technologies also have varied maintenance requirements resulting from the loading capacity of the filter module selected. It can be worthwhile to compare the replacement, energy and module cost to arrive at the optimal product and lifecycle.

It is important to consider using technologies with a lower maintenance cost when the particulate load is expected to be high to reduce overall ongoing costs – this includes the use of an ESP with a PLC automatic wash in place system.

How does temperature affect particulate filtration?

If the air temperature is too hot, what would be a particulate at ambient temperatures may pass through the filtration system as a vapour, before condensing back to a particulate when exhausted to the ambient air outside.

For filtration by an electrostatic precipitator, the temperature of the air has a large impact on the resistivity of the air. In this case a higher temperature means a higher resistance. If the air/particle has more resistance, the charge efficiency decreases and therefore particle migration velocity and collection efficiency decreases.[2]

For media type filters, higher temperatures can lead to deforming of the filter and therefore bypass of particulates.

As a general rule, when exhaust air temperatures are over 55°C, the performance of particulate filtration systems may drop below levels required.

If temperature is expected to be over 55°C, consider including a wet scrubber in the hood or duct prior to the filtration system to evaporatively cool the air.

 ODOUR CONTROL

 What is the efficiency (residence time) of activated carbon?

The efficiency of activated carbon is highly dependent on residence time. The higher the mass of carbon in the filter and/or the slower the air passes through the filter, the greater the residence time. Assuming there is no bypass, a residence time of 0.08 seconds results in an initial contact efficiency on odours of 99%+.

This is normally achieved with 450mm deep modules with four 25mm thick V-banks per 610x610mm filter filled with 35kg of granulated media and a duct velocity of 1.8m/s.

Are safety control systems in place for ozone?

If being used to control odours, ozone should be used with care as it is harmful to human health at levels consistently above 50ppb.[3],[4]  Safe Work Australia enforces a 100ppb peak limit for human exposure[5], and AS1668.2 re-enforces this.

To ensure that no residual ozone remains in the final exhaust air discharge to atmosphere, either control systems that detect and alter ozone generation with cooking load or activated carbon placed downstream to adsorb residual ozone are required. Provision of activated carbon would generally be required to assure ozone removal from the discharge airflow and in the event of failure of the sensor or control technology.

Ozone is a viable solution for odour control if these control mechanisms are in place and there is at least 2 – 5 seconds of residence time in the duct work before exhaust and the fail-safe activated carbon bed.

 Why do odour control systems require a high level of protection from particulates?

Ideally, a kitchen exhaust treatment system should be designed to remove particulates (such as grease, oil and smoke) to a high level before removing the odour. Effective odour removal technologies will provide greater performance and endurance when protected from grease, oil and smoke particulates. These contaminations will overload, impede and reduce the efficiency of odour control systems, rendering their odour removal properties as ineffective.

A minimum particulate removal efficiency of 95% at 0.3𝜇𝑚 is desired to ensure adequate protection

Activated carbon

Activated carbon works by adsorbing odour molecules onto active sites throughout the porous carbon. When the outside of the granular activated carbon becomes coated in particulates (grease and smoke), access to the active sites inside the porous carbon are blocked and no longer accessible by the odours. This severely reduces the effective life of the carbon and the efficiency, resulting in the requirement for frequent replacement of the carbon.

Carbon can preferentially absorb water over certain odours and so with humid kitchen exhaust air flows the odour removal performance may be reduced. The use of catalytic type medias such as potassium permanganate impregnated alumina that react with and convert contaminants may be required.

Ozone / UV

Ozone works by oxidising odour molecules. Grease and smoke particles are considerably larger than odour molecules and require more “oxidising power” to break down. If grease or smoke is still present in the exhaust air, the ozone may be trying to break down these larger particles instead of the small odour molecules.

How does temperature affect odour filtration?

If the exhaust air temperature is above 55°C there is an increased likelihood that the particulate filtration system will fail. This leads to particulate bypass which will affect the performance of the odour control system.

Adsorption of odours onto activated carbon is also reduced at higher temperatures, when the temperature is high enough it will even cause desorption. To ensure adsorption occurs, the temperature of the exhaust air must generally be below 55°C.

FAN

 Does my fan have a constant flow controller?

A constant flow controller on the fan will allow the ventilation system to maintain the commissioned air flow rate as the static pressure losses in the filtration system increase. Static pressure losses increase when filtration systems in both the hood and the duct become dirty.

A constant flow controller works by measuring the airflow rate of the fan using various methods. It is then able to control the fan speed up or down using the variable speed drive depending to maintain a pre-determined air flow rate.

Alternatively if specialised demand control ventilation kitchen hoods are being used, the fan control can be integrated directly with these products.

Is fan noise a factor?

Treatment of kitchen exhaust with an effective system and a UL1046 rated hood filter will lead to a system with a relatively high pressure drop. This can often result in excess fan noise if the exhaust is being ventilated horizontally, this likely could be in a noise sensitive area.

Therefore, the amount of noise generated by the fan at the exhaust louvre needs to be managed by either using a silencer, or other noise attenuation treatment technology.

 ONGOING MAINTENANCE

 Are duct cleaning ports readily accessible?

 Various regulations require kitchen exhaust ducts to be cleaned, so in the building phase it is essential that duct cleaning ports are inserted in compliance with AS1668.1. It is also important that they be practically accessible.

Are compliant service programs in place for exhaust filtration systems, ducts and fans?

Every effort to design a successful commercial kitchen exhaust can be defeated by an inadequate service program. The various filter sections, ducts and fans will become coated by contamination over time. If these are not serviced, the chance of fire and other issues is amplified considerably.

Ducts must be cleaned as per various regulations across Australia. Commentary in AS1668.2 reinforces this and the continued performance of kitchen exhaust treatment systems through maintenance for a Performance Solution.

In reality there is significant cost involved in the maintenance of any kitchen exhaust system, and the users, facility managers and owners must be made aware of these costs and resist the tendency to “short-change” budgets in this area – as responsibilities to; fire safety, public health and council compliance are important. Similarly, regular maintenance of fans, electrical systems and the hood filters is essential to have the system operational at the intended flow rates over time.

How much will my system cost to maintain?

This is a difficult question to answer as it is dependent on cuisine, cooking frequency, quality of hood filters and even the temperature that the food is cooked at. The below frequencies are rough values from industry experience and due to the high level of variable factors, they cannot be relied on with certainty.

For medium to high particulate loads, the maintenance requirements for particulate filtration technologies may be:

  • Multi-stage filter pack system (low capital / medium maintenance cost)
    Pre-filter change every 1-2 months
    Bag filter change every 2-4 months
    Final filter change every 4-6 months
  • Manual clean ESP (medium capital / high maintenance cost)
    Manual clean of cells every 5-14 days
  • Automatic wash ESP (high capital / low maintenance cost)
    Automatic wash on PLC to clean cells once every 3-7 days
    Manual clean once per 12 months

For medium to high particulate loads, the maintenance requirements for odour control technologies may be:

  • Carbon (low/medium capital cost)
    Replace every 12 months
  • Ozone (medium/high capital cost – ozone control system required)
    Parts (varies)
    Calibration of ozone sensor regularly (public health risk) or replacement/ maintenance of the activated carbon bed

Note that for light cooking loads the maintenance can roughly be halved and for very high cooking loads, the maintenance is at least doubled.

Written by Jonathan Bunge (M.ENG Chemical) and Shannon Roger (B.Ed) for Airepure Australia 2018.  Published in Ecolibrium Volume 17.10, November 2018, p30-34.

Technical review by:
• Peter Mathieson, F.AIRAH,technical director, Aurecon Australia.
• Brett Fairweather, M.AIRAH, mechanical engineering consultant, It’s Engineered.

Jonathan Bunge, Affil.AIRAH, is a graduate of Melbourne University boasting a Master’s in Chemical Engineering, and has gained engineering experience within petrochemical facilities, the coal industry and the gaseous filtration industry. His role with Airepure Australia enables him to specialise in the control of gaseous emissions, hazardous and pungent odours, laboratory fume exhaust solutions as well as commercial cooking and industrial process exhaust solutions.

Shannon Roger has gained over 15 years’ experience writing content for the Australian HVAC industry. In recent years, she has written several technical articles with Airepure colleagues that have been published in various engineering industry publications; including healthcare, facility management and HVAC.

Peter Mathieson, F.AIRAH,is a mechanical engineer with over 30 years of mechanical services design experience. Over that period he has designed many kitchen exhaust installations, many with discharge filtration. Peter says his design of kitchen ventilation has improved with experience and is with reference to the current ASHRAE research.

Brett Fairweather, M.AIRAH, is a mechanical engineering consultant with 20 years of mechanical services experience in the design and delivery of projects. He is committed to training and support of other practitioners in the industry and volunteers his time to many committees responsible for the ongoing development of standards, codes and regulations relating to building services.

[1] UL 1046: Standard for Grease Filters for Exhaust Ducts, Edition 4, 2010
[2] Neundorfer. ESP Design Parameters and Their Effects on Collection Efficiency 1998.
[3] Lippmann M. Health effects of ozone a critical review. Japca. 1989;39(5):672-95.
[4] Stedman, J. et al, Emergency hospital admissions for respiratory disorders attributable to summer time ozone episodes in Great Britain., Thorax, vol. 11, no. 52, pp. 958-63, 1997.
[5] Safe Work Australia, Workplace Exposure Standards for Airborne Contaminants, Safe Work Australia, 2013.

Contact

Airepure Australia
1300 886 353
info@airepure.com.au

About the Author

Jonathan Bunge-Airepure Australia

Jonathan Bunge
(M.Eng Chemical)
Product Specialist
Airepure Australia

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