Rangehoods are a staple feature in all residential projects, but their importance is magnified in professional kitchens. Unlike stovetops, ovens and other appliances, which can be troubleshot in isolation, the exhaust hood is only one component of a commercial kitchen ventilation (CKV) system.
This ventilation system is, in turn, part of a larger chain where the kitchen hood is ultimately linked to a building’s overall HVAC strategy.
“Commercial kitchens, as opposed to residential kitchens, can be considered a large processing plant,” Managing Director of Air & Odour Management (AOM), Sven Bolomey, explains. “During cooking processes, they generate large volumes of air contaminants and heat. It is essential in a HVAC strategy of a restaurant that these two elements are dealt with accordingly.”
Without an exhaust system as a ‘first step’ to regulate and remove the hot air and airborne contaminants produced by cooking – studies have found that cooking releases some of the same pollutants found in smog – a commercial kitchen becomes unworkable. Not only does an ineffective hood contribute to a less hygienic kitchen environment and health issues, its failure could affect a venue’s customers and even neighbouring tenancies or apartments.
In this kitchen ventilation starter pack, we explore the different factors designers should consider when selecting the right hood for their next project.
STANDARD VS PROPRIETARY HOODS
According to Bolomey, there are two main hood types on the market that specifiers can choose from. The first are standard hoods, which are not typically built by specialised kitchen exhaust equipment suppliers but must still meet Australian Standards AS 1668.1:2015. These standard hoods – sometimes labelled stainless steel boxes – are often supplied to smaller restaurants that do not require extensive or large exhaust systems.
Then there are proprietary hoods, also known as performance or low velocity hoods produced by specialised manufacturers. These hoods boast to be more efficient by including features specifically designed to optimise performance. For example, AOM’s range of high efficiency proprietary hoods feature a certified honeycomb filter that has been tested to stop flames from penetrating into the hood and exhaust duct system. Proprietary hoods are also capable of incorporating other innovations like automation, touch screen commands and control panels.
One of AOM’s innovations is a high efficiency kitchen exhaust hood that includes the best possible filtration equipment (electrostatic precipitators) and odour mitigation methods (ozone injection) within the kitchen exhaust canopy so that the filtration is done as close as possible to the source of the contamination. This hood has been tested by the University of Sydney and is used in cases of difficult discharge points. Pictured is Burger Project in Sydney. Photography by Kimberly Low
Companies like Stoddart also manufacture exhaust hoods to specific requirements, such as the Halton ventilated ceiling solution. Designed for installation over cooking equipment like char grills, induction cooking and teppanyaki plates, this ventilated ceiling system extracts cooking pollutants while creating an air curtain to reduce the effect of cross breezes in an open kitchen.
The Halton ventilated ceiling solution was installed at The Ternary Restaurant in Darling Harbour, Sydney. Its jet air system creates the right ‘air’ balance between the kitchen environment and the outside area, while aesthetically blending into the flowing design of the restaurant’s refurbished area.
Separately, hoods may also be categorised according to their use purpose – whether they are to be installed over cooking equipment that produce heat and grease-laden effluent, or over equipment that produce just heat and condensation. Oven and condensate hoods, which are traditionally exhaust-only canopy hoods, are examples of the latter.
DETERMINING EXHAUST RATE
Perhaps the most fundamental step to specifying the right commercial kitchen hood is calculating a kitchen’s exhaust needs.
Take the example of a steakhouse and a sushi bar. The former’s kitchen is expected to generate stronger thermal plumes and grease than the latter. Similarly, a wok and gas char-broiler will naturally have higher exhaust requirements compared to an electric and gas steamer. The types and number of equipment and cooking that will take place in the kitchen necessarily informs what exhaust hood is finally specified.
The calculation of exhaust airflow rate is detailed in Australian Standard AS 1668.2-2012, which sets out the “requirements for mechanical air-handling systems that ventilate buildings and car parks, and for ventilation based on the need to control odours, particulates and specific gases”. Section 3 focuses on exhaust systems, including kitchen exhaust hoods and airflow.
Exhaust rates are affected by hood style and geometry. A wall mounted canopy hood, for instance, is expected to require less exhaust than island hoods. Interior angles close to, or at the capture edge of a hood, is also typically understood to improve performance.
According to a 2003 whitepaper by Energy Design Resources, “capture and containment performance may be enhanced with active low-flow, high-velocity air jets along the perimeter of the hood”. In most cases, side or end panels further permit a reduced exhaust rate, as all of the replacement air is drawn across the front of the equipment, improving containment of the effluent plume generated by the hot equipment.
REPLACEMENT AIR DISTRIBUTION POINTS
Extracting hot air and contaminants is just one half of a good CKV system. The other half is all about how makeup air (MUA) will be introduced into the kitchen to strike the right ‘air balance’.
“The idea that by not installing a dedicated makeup air supply, the operator is going to save money (in both first cost and operating cost) is short sighted,” a California Energy Commission design guide notes.
“If replacement air doesn’t come in, that means it doesn’t go out the exhaust hood and problems begin. Not only will the building pressure become too ‘negative’, the hood may not capture and contain cooking effluents due to reduced exhaust flow.
“We have all experienced the ‘can’t-open-the-door’ syndrome because the exhaust fan is sucking too hard on the inside of the restaurant.”
Replacement air may be introduced from a variety of distribution points, from service doors and drive through windows, to the rangehood itself. MUA can be introduced through a hood with an integrated supply plenum or external supply plenum, with multiple studies and guides recommending designers choose the latter when working with full-service and institutional kitchens with larger exhaust requirements.
Using a hood with an external supply plenum has advantages when used over equipment that produces a lot of heat and smoke, as its greater hood volume (represented by the shaded area) can hold more smoke and heat until they are exhausted.
However, there are challenges associated with introducing an independent or external MUA supply. If located too close to rangehoods, makeup air distribution points could interfere with the hood’s ability to capture and contain. For example, a fan in the kitchen pointing at the stove actually reduces hood performance and makes capture of plumes difficult.
“The primary recommendation for minimizing the impact that locally supplied MUA will have on hood performance is to minimize the velocity (fpm) of the makeup air as it is introduced near the hood,” a Food Service Technology Center (FSTC) guide prepared by Architectural Energy Corporation and Fisher-Nickel recommends.
“This can be accomplished by minimizing the volume (cfm) of makeup air through any one pathway, by maximizing the area of the grilles or diffusers through which the MUA is supplied, or by using a combination of pathways.”
Locally supplied makeup air with higher temperatures may also affect hood performance, since the buoyancy of hot air impacts the dynamics of air movement around the hood. In more temperate climates such as Australian summers, evaporative cooling may be an effective method of maintaining MUA temperatures within a range comfortable enough for kitchen staff, and which does not hamper hood performance.
Also important is considering where the hot air and contaminants are going to be discharged to while still meeting the requirements of codes and standards. – a task that’s becoming more difficult in metropolitan areas
“With more and more restaurants using more ‘extreme’ cooking styles – solid fuel including large fire pits inside restaurants, large chargrills, use of sauces and spices directly on open flame kitchen equipment et cetera – these high contaminated exhaust cannot be discharged directly into the local environment,” Bolomey says.
“We have been involved in many projects where local residents have complained about restaurants. In city centres, discharge points often need to be engineered so as to not become a nuisance.”
This implies filtration and odour control equipment must be installed in the exhaust system. This could lead to further design challenges, such as in restaurants located within heritage buildings.
As with all, if not most, products in the AEC industry, proper cleaning and servicing (O&M) of the specified rangehood is vital to ensuring it works as required in the long haul. Maintenance prevents issues such as the build-up of grease in ducts, the discharge of contaminated air into the local environment, failure of the exhaust system, and other health and safety risks including the risk of fire. Ensure that your supplier will be involved even after the commissioning and installation of the specified kitchen rangehood.
Finally, go where the experience is. While most specifiers may work on a handful of kitchen projects a year, specialised suppliers often handle over a hundred cases and will have vast experience in the different types of projects and equipment requirements.
Depending on facility layouts, a CKV system may account for up to 50 percent of a building’s HVAC load. Some specification tips to maximise efficiency include:
- Choosing a hood with interior angles that are close to or at the capture edge of the hood
- Ensuring the hood has variable speed fans and idle conditions to minimise operating costs
- Specifying different ventilation rates for hoods or hood sections over different equipment
- Placing heavier duty appliances in the centre of the hood instead of at its edge