Considering the vast number of hotels and commercial kitchens in Australia, one would expect that exhaust treatment solutions for these applications would be routine. However, whilst the aims of good kitchen design and the effective treatment and exhaust of cooking fumes, are easy to express – the diverse range of sites and environments, create a remarkably complex situation.
Different cuisine types produce varying amounts of moisture, grease, smoke and odour and the resulting cooking fumes comprise a combination of solid particles, liquid droplets and vapour / gaseous phase contaminants. Various kitchen exhaust treatment technologies are available to choose from – each with pros and cons affecting the cost versus performance scale.
This article seeks to set out the broad aims and parameters of commercial kitchen exhaust design within hotels to ensure exhausted cooking fume matter complies with relevant standards to reduce fire risk and will appease sensitive receptors to the exhaust odours.
Within Australia, there are standards that govern kitchen ventilation including council and state regulations, local, state and federal fire codes and most significantly AS/NZS 1668.1[i] and AS 1668.2[ii] – all of which are referred to by the BCA (Building Code of Australia).
Kitchen exhaust hoods have three (3) major functions; the first and most obvious one is to ensure all of the cooking fumes generated by cooking processes are captured by the hood. The second and third is to ensure enough dilution air is captured by the hood to reduce the temperature of the exhaust air and the contaminant concentration of the exhaust air.
Effective treatment of kitchen exhaust requires air temperature in in the duct to be under 50°C and at low contaminant concentrations; which are both functions of adequate dilution air from the hood.
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).
Excerpt of AS1668.2: 22.214.171.124 Hood Type Nomenclature ii
Note the hood type 7 (proprietary hoods) are calculated using alternate proven and tested standards. They will generally have a flow rate of 30-40% less than AS1668.2 suggests for a regular hood, as vendors maintain that the jet flow technology enables the capture of contaminants with a lower amount of air. However the reduction of dilution air which will affect temperature and contaminate concentration which may decrease the efficiency of downstream treatment systems.
Whilst it is recommended to use the kitchen hood exhaust airflow rate specified by AS 1668.2 as a minimum, designing for a higher airflow rate than required will provide your system more chance of success and decrease the possibility of such issues arising in the future.
The Australian Standards 1668.2 has clear guidelines to determine if treatment is needed, and the extent of treatment (if required). Some key elements are:
Table 1 – Minimum Separation Distances from Discharges to Intakes, Boundary or Natural Ventilation Device[iii]
Additionally, in any treatment system, an odour control stage relies on particles being removed – before the odour control section. It is difficult to quantify odour removal, but the starting point is always to remove particles and measure the success of particle removal at the 0.3 micron level, as particles of size 0.3 microns are the hardest to capture.
It should be remembered that even when the standards have been followed, additional treatment may be enforced by local councils if odour complaints are received. This applies to both horizontal and vertical exhaust systems, and can become very costly for the system owner to rectify.
Whilst the end goal for any kitchen exhaust system is to ensure that no smoke, grease or odour is exhausted into sensitive locations, the optimal type of treatment system selected is dependent on the contamination level and the airflow rate.
A combination of kitchen exhaust treatment technologies is often employed to achieve the most cost effective, high performing results. These technologies target particulates (P) and/or odour (O) and include hood filters (P), ultra violet light (UV-C) (PO), ozone (O), electrostatic precipitators (ESP) (P), multi-staged filter packs (P), activated carbon (O), wet scrubbers (PO) and dilution / dispersion (O). Wet scrubbers are tailored for solid fuel applications – this is not common in hotels and therefore will not be covered in this article.
Diagram of recommended kitchen exhaust treatment systems by flow rate and contaminant level
Kitchens operating within hotels will have similar flow rates and contaminant levels to most commercial kitchens, making the multi-staged filter pack or an electrostatic precipitator a viable and cost effective selection for particulate filtration. This system can easily be paired up with other technologies such as a high efficiency hood filters and UV to increase effectiveness and reduce maintenance costs.
Kitchens operating with low particulate contaminations levels should consider using a multistage filter pack, which are effective at treating smaller volumes of low to moderately contaminated air with a relatively low capital cost. The filter replacement and energy consumption costs when combined with the capital cost provide a cost effective solution for low to moderate kitchens.
Kitchens operating with high particulate contaminations levels should consider using ESP’s, which are effective at treating large volumes of highly contaminated air with relatively low operating costs. This is typically due to a maintenance regime consisting of cleaning instead of replacement.
Technologies such as high efficiency hood filters and UV-C may be effective in reducing particulate loads on other treatment systems but are not seen as a complete solution and come with some risks that must be controlled. Many UV systems introduce Ozone into the system, which has been found to be injurious to health at levels consistently above 50 ppb.[iv]
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 such as activated carbon will provide greater performance and endurance when protected from grease, oil and smoke particulates. These contaminations will impede and reduce the efficiency of activated carbon, rendering its odour removal properties as ineffective.
To ensure effective odour removal and to save on unnecessary replacement costs, a treatment system that incorporates adequate protection of activated carbon filters or media from grease, oil and smoke particulates is recommended. A recommended protection requirement is a system prior to the carbon which removes minimum 95% of particles at 0.3 microns, this is usually achieved with a well-designed ESP or a HEPA filter.
Whilst activated carbon is the preferred method of odour abatement, this is often paired with UV-C lamps and ozone to reduce load. UV-C lamps reduce grease and odour through a mechanism known as photolysis as well as generating ozone along with other ozonolysis methods such as corona discharge. Ozone should be used with care, as it is harmful to human health and Safe Work Australia TWA exposure standard limits are 0.1 ppm (0.2 mg/m3[v]).
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.
UV-C and by extension ozone should only be seen as a viable solution for odour control if one of these control mechanism are 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.
Excerpt of AS1668.2: 126.96.36.199 Cooking Process Type Nomenclature
Multi-stage filter pack systems are ideal for kitchens with a low to moderate contaminant loading, namely cooking process types 1, 2, 3 and 7 according to AS1668.2. These systems typically offer a low capital cost solution with higher operating costs from the static and filter change outs. Change out cycles can be reduced by pairing this system with an efficient hood filter or appropriately designed UV-C lamps prior to the filter pack.
Typical Multi-Stage Filter Pack Configuration
These systems are typically designed with four (4) stages of filtration consisting of three (3) sequential stages of particulate filtration and one (1) final stage of odour control.
It is possible for these systems to handle kitchen exhaust with a higher contaminant loading, however this will increase the change out frequency required.
ESP’s are ideal for kitchens with a higher contaminant loading, namely cooking process types 4, 5 and 6 according to AS 1668.2.
ESP’s use electrostatic charges to ionize particles initially before collection on plates of the opposition charge. If the ESP is doing its job – there will be extensive contamination trapped on the plates of the ESP. These plates must be cleaned / regenerated periodically to remove grease and smoke particles. An ESP is able to clean air of all contaminant levels, however the required frequency of cell cleaning directly relates to the contaminant load; a higher contaminant load means more frequent washing of the cells to maintain performance of the system.
Manual washes of ESP cells may need to take place anywhere from daily to monthly depending on load; and a short wash cycle will add significant maintenance costs, particularly if the treatment system is placed in a hard to access location.
More capital intensive ESP systems will have programmed wash systems to ensure the system is automatically maintained for optimum performance, this ensures lower operation costs over time. An automatic wash system for the ESP can be programmed to run daily, weekly, fortnightly etc. This can extend the frequency required for a manual clean from 1 – 5 years depending on the contaminant load.
Typical Large Scale Self-Washing ESP System Configuration
Large scale, self-washing ESP systems may include the following options:
Kitchen exhaust systems are typically designed to operate around 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. Do not be tempted to raise the air velocity above 1.8m/s to reduce the size of the treatment system, as this will reduce the efficiency of the system and directly increase energy and maintenance costs. For example; a multi-staged filter pack system* running at 1.8m/s will clean the exhaust air more effectively and cost up to $1K less in energy costs annually compared to a system running at 2.5m/s. (*2,500L/s system, hotel hours of operation).
The particle size of 0.3 micron is typically selected as the test point for rating filtration efficiency because particles above and below this size are generally easier to capture – and these are the most “elusive”. This principle applies to all technologies (filters, ESPs, UV, ozone), so if you want to compare apples to apples, always compare system efficiency at a particle size of 0.3 microns.
Most Penetrating Particle Size – Fractional Efficiency by Particle Size
The particle size of smoke (an easily visualised pollutant) ranges between 0.3 to 1.0 microns. To ensure an adequate amount of smoke particles are captured, you would require high efficiency filtration at this particle size – 95% efficiency at 0.3 microns.
It seems surprising, but a system which is rated as 95% efficient at 0.01 microns is actually inferior to a system that is rated as 95% efficient at 0.3 microns. This is due Brownian motion (diffusion) which describes the motion of extremely small particles and how it differs from bulk flow. It demonstrates why smaller 0.01 micron particles are easier to trap than the 0.3 micron particles. In fact, particles of 0.01 microns are as easy to catch as particles of 10 microns. The wise buyer and specifier will always judge system performance with ratings at 0.3 micron – the most challenging particles to capture.
Additionally, air velocity directly impacts system efficiency. For example; an ESP operating at 3.5m/s air velocity would only be 40% efficient at 0.3 microns – even though it rates at 95% efficiency at 0.01 microns. This same ESP operating at a lower velocity – 1.8m/s would rate at 95% efficiency at 0.3 microns – thus be comparatively effective. Therefore, look for ratings at the hardest particle size (0.3 micron) and at a sensible air flow velocity (around 1.8m/sec).
Despite operating at the desired velocity, the efficiency of a system can be severely impacted by its location. If the treatment system is placed directly after or before duct bends with short transitions; the flow of air will not be evenly distributed through the treatment system, rendering a portion of the system useless. It is recommended to use industry standard transition sizes and allow 2 – 4 duct diameters of straight run either side of the treatment system.
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 (AS/NZS 1668.1:2015), 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.
It is 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.
In the design phase – attention should be given to some of the potential cost savings that can be achieved for “long-term-operation” with larger capital investments at the building stage, e.g. auto-washing filter systems vs manually changed filter systems.
Whilst there are many factors affecting the design, implementation and maintenance of a successful kitchen exhaust treatment system; there are guidelines and sound recommendations available to assist with your compliance to relevant standards and local council regulations, as well as your specific objectives for performance and cost.
Key considerations include:
Written by Jonathan Bunge (M.ENG Chemical), Shannon Roger (B.Ed) and Dr Allan Heckenberg (PhD) for Airepure Australia 2016, and published in The Hotel Engineer, Vol 21 No 2, July 2016.
[i] Australia S. The use of ventilation and air conditioning in buildings. Part 1: Fire and smoke control in buildings: SAI Global 2015
[ii] Australia S. The use of ventilation and air conditioning in buildings. Part 2: Mechanical ventilation in buildings: SAI Global Limited 2012
[iii] Yeung L, To W. Size distributions of the aerosols emitted from commercial cooking processes – indoor and built environment. 2008;17(3):220-9.
[iv] Stedman JR, Anderson HR, Atkinson RW, Maynard RL. Emergency hospital admissions for respiratory disorders attributable to summer time ozone episodes in Great Britain. .Thorax. 1997 Nov;52(11):958-63.
[v] Based on TWA (time weighted average) over 8 hour day, 5 days per week.