Commercial kitchen exhaust systems require specialist equipment and design. What are the key issues relating to the application of these systems to buildings?
An exhaust hood in compliance with the Australian standards must be provided where the cooking appliances in a room, have a total maximum power input exceeding eight kilowatts for electrical, or a total gas input of 29 megajoules per hour for a gas appliance. There may also be other cases where complying hoods are required, including where the total electrical power or gas input exceeds certain limits based on the room floor area, where a grease interceptor is deemed to be required by the municipal council or water authority, or where the building permit requires the provision of this type of appliance.
Kitchen exhaust systems are required to comply with Australian Standard AS/NZS1668.2 The use of ventilation and air conditioning in buildings - Ventilation design for indoor air contaminant control, and also AS/NZS1668.1 The use of ventilation and air conditioning in buildings - Fire and smoke control in buildings.
In addition to this, municipal councils, retail precinct managers or building owners, may require additional measures with regards to design, access, safety systems, frequency of maintenance and reporting requirements.
Types of systems
Commercial type kitchen exhaust hoods come in many different configurations, including to suit the kitchen layout and cooking equipment provisions. The majority of hoods are mounted in a location where the cooking appliances are positioned in a location with one, two or three open sides. Some hoods are designed to sit above appliances with no side walls and this is classified as an island type configuration.
The smaller the number of open sides to the hood, the less amount of air is required for the hood as the closed sides act to restrict spillage of cooking vapours at the edges of the hood.
Some hoods not only provide exhaust but include make up air supply, which is normally used to limit spillage of cooking vapours at the edges of the hood and thereby allow reduced exhaust rates. The make-up air also reduces the quantity of conditioned air lost from the space due to the exhaust. This can provide significant reduction in room air conditioning/air tempering energy costs.
There are also specialised vented ceilings which are engineered to provide efficient capture of cooking vapours and provide flexibility in kitchen equipment layout. These are particularly suited to large commercial kitchens which include significant number of cooking appliances.
All complying kitchen exhaust hoods include high efficiency grease removal filters (except for hoods capturing clean heated air or steam without grease content). Some hoods also include UV treatment, ozone injection, water scrubbers or other means to reduce any grease by-pass of the filters in to the exhaust ductwork system. This reduces the frequency of duct cleaning required and reduces the fire risk as a result of grease build up in ductwork.
Design and installation
Selection of the most appropriate kitchen exhaust hood should be left with specialist engineering or kitchen consultants to ensure they are sized correctly, are suitable for the type of cooking to be performed, have a reduced energy cost, and reduce the potential deposit of grease within the exhaust ductwork system.
The exhaust ductwork, exhaust fan and associated components should be designed and selected by a specialist designer.
Fire suppression systems can be provided to kitchen exhaust hoods to prevent potential fire at the cooking appliance or hood from spreading in to the ductwork system.
It is critical that kitchen exhaust systems are designed to provide ready access for regular cleaning. This includes access panels in ductwork, safe access to all cleaning points in the ductwork, and minimisation of bends and horizontal run of ductwork.
There are very many examples of kitchen exhaust systems which have limited access for cleaning and poor design, including long horizontal sections, bends, dampers, silencers and other surfaces which tend to collect grease particles as they are transported along the ductwork.
Fire risk and safety measures
Kitchen exhaust systems present a significant fire risk to buildings. Build-up of grease, high temperatures from cooking activities and potential for ignition sources to be sucked through the hood can lead to a fire within the duct which will travel through the building via the duct and potentially ignite materials and surfaces in proximity to the ductwork system.
There are many documented cases of significant fires in buildings which were initiated via poorly maintained kitchen exhaust hoods and ductwork. Responsibility for maintenance of the kitchen exhaust system by commercial food concessions operating within a leased space in a building is sometimes a neglected element. In some cases, maintenance and cleaning cannot be adequately performed due to poor system design and safe access provision.
Responsibility and liability
Responsibility for the fire and life safety aspects of a building remain with the building owner. This includes kitchen exhaust systems which are located within the building. It is therefore very important for building owners to understand what systems are installed in their building, who is maintaining the system, what the system configuration is, whether it can be accessed and cleaned adequately, and what records are being provided to verify that regular cleaning is being undertaken.
Regular inspection of the system via an independent professional contractor is recommended to ensure building owners are ensuring fire risk is being monitored and minimised.
Dos and don’ts
DO check whether you actually require a commercial grade exhaust system in compliance with As1668.2. The difference in cost between a domestic type non complying kitchen exhaust and a complying exhaust system can be significant. Clients should be aware of the cost, spatial and maintenance access requirements which are required for commercial kitchen installations, as well as the need to ensure on-going maintenance for the life of the system. Assessments should be undertaken by a registered building surveyor or engineer.
DO have the system selected and designed by professional contractors and consultants who have experience with the specific requirements of these installations and can provide design compliance sign off for the system.
DO provide adequate space and pathways through the building to limit the length of ductwork, reduce number of changes in direction and reduce extent on horizontal run of ductwork.
DO ensure exhaust discharge points do not cause nuisance to occupied areas surrounding the area and are located well away from air intake sources, including openable windows and doors.
DO include fire suppression systems where possible to reduce the risk of a fire entering the ductwork system and igniting grease deposited within the duct.
DO check that maintenance of the kitchen exhaust system is being undertaken in an adequate manner by the kitchen operator of maintenance contractor. Poor maintenance will significantly increase risk of fire in the system, thereby increasing risk of fire to the building elements. Building owners and managers should establish guidelines for kitchen exhaust system design and maintenance in their buildings and should seek and maintain records of system cleaning by the maintenance contractor.
DON’T skimp on the quality of the hood. Higher quality engineered exhaust hoods can save significant costs over the life of the system due to reduction in energy lost due to reduced air extraction rate, and improved fume capture and reduced grease bypass, and can significantly reduce maintenance costs.
DON’T locate kitchens in areas where there is reduced space for locating kitchen exhaust ductwork and which require long and convoluted pathways for the kitchen exhaust.
Further information can be obtained from the AS1668.2 standard and from suppliers of commercial kitchen hoods.
AIRAH (Australian Institute of Refrigeration, Air Conditioning and Heating) have produced a technical bulletin which includes contributions from a range of industry professionals. You can download a free copy via their website.