This checklist is based on the following assumptions:
AS1668.2 prescribes a minimum exhaust airflow rate through kitchen hoods depending on the size of the hood, the type of hood and the appliances under the hood (process cooking type). Refer to AS1668.2: 3.5 for details.
Alternatively for a proprietary kitchen hood (commonly a low flow hood with jet capture technology) more detailed calculations are available – often based off the German guideline VDI 2052 and the German standard DIN 18869. Refer to AS1668.2: 3.6 for details.
It is important to use the kitchen hood exhaust airflow rate specified by AS1668.2 as a minimum. This will ensure adequate removal of contaminants from the kitchen and help reduce the exhaust air temperature to enable effective treatment.
The first and often the most under rated stage of a successful kitchen exhaust filtration system is effective particle load reduction at the hood. Removing a significant load of particles at the hood, particularly the larger ones, will significantly reduce the maintenance costs on equipment downstream.
UV-C lamps are common in the hood and reduce grease and odour through a mechanism known as photolysis. They also generate ozone which will oxidise some of the contaminants. UV-C lamps should be seen as a load reduction mechanism only as the power of the photolysis and oxidation alone is not 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.
A high efficiency hood filter is crucial to significantly reducing particle load. Ensure that you ask the manufacturer for the efficiency as a function of particle size 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 can be made between capture efficiency and energy efficiency.
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”. 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 filter 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.
Due to the clause being located in AS/NZS1668.1 instead of AS1668.2 it is often overlooked.
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.
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 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.
Single pass kitchen exhaust systems are typically designed to operate at 1.8m/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.
System maintenance frequency will be affected by particulate load. Generally, higher particulate loads will require more frequent system maintenance than lighter particulate loads. Different particulate filtration technologies also have varied maintenance requirements.
Consider using technologies with a lower maintenance cost when the particulate load is expected to be high to reduce ongoing costs – this includes an ESP with a PLC automatic wash system.
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.
For HVAC filters, higher temperatures can lead to deforming of the filter and therefore bypass of particulates.
As a blanket rule, when exhaust air temperatures are over 55°C, particulate filtration systems are likely to fail.
If temperature is expected to be over 55°C, consider putting a wet scrubber in the hood or duct prior to the filtration system.
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 granulated media with an air velocity of 1.8m/s.
Ozone should be used with care, as it is harmful to human health. To meet AS 1668.2 requirements that no residual ozone remains in the final exhaust air, one must provide control systems that detect and alter ozone generation as the amount required varies with cooking load. Alternatively, activated carbon can be placed downstream to adsorb residual ozone.
Ozone is a viable solution for odour control if one of these control mechanisms is in place and there is at least 2 – 5 seconds of residence time in the duct work before exhaustion/carbon filtration to allow sufficient oxidation to occur.
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 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.
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 will be spent trying to break down these larger particles instead of the small odour molecules.
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 be below 55°C.
A constant flow controller on the fan will ensure that the ventilation system for the kitchen exhaust will maintain a commissioned air flow rate when the static pressure losses in the system increase. Static pressure losses will increase when filtration systems in both the hood and the duct become dirty.
A constant flow controller works by measuring the pressure drop over the fan. By using this information along with the fan curve which is mapped in the controller, It is able to ramp the fan speed up or down depending on what is required to achieve a pre-determined flow rate.
Treatment of kitchen exhaust with an effective system and a UL1046 rated hood filter will lead to a system with a large pressure drop. This can often result in a loud fan. As the exhaust is being ventilated horizontally, it is likely in a sensitive area.
Therefore the amount of noise generated by the fan at the exhaust louvre needs to be considered. This is by either using a silencer, or particular type of treatment technology with a lower pressure drop.
Ducts must be cleaned by regulations (AS1851:2012), so in the building phase it is essential that duct cleaning ports are inserted in compliance with design standards. It is also important that they be practically accessed, which is often a tricky thing to achieve.
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 by regulations (AS1851:2012). It is also a requirement of AS1668.2 to maintain the performance of a kitchen exhaust treatment system.
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; 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.
For medium to heavy particulate loads, the maintenance requirements for particulate filtration technologies may be:
For medium to heavy particulate loads, the maintenance requirements for odour control technologies may be:
Written by Jonathan Bunge (M.ENG Chemical) and Shannon Roger (B.Ed) for Airepure Australia 2017. Published in Hotel Engineer Volume 22, Number 2, July 2017.