If you can’t stand the heat, the ventilation isn’t performing

In a commercial kitchen, adequate ventilation is essential in order to control health, safety and hygiene risks. Admittedly it’s a challenge to properly ventilate this environment as it requires designers and specifiers to extract and replace large volumes of air through ductwork and place equipment in very restricted spaces, however, there are minimum standards that we must adhere to.

The Health and Safety Executive (HSE) highlights a number of objectives that commercial ventilation systems should achieve. They must: dilute and remove cooking odours and other by-products of combustion; provide sufficient air for safe combustion at gas burning appliances; introduce cool clean air and remove hot air to ensure a comfortable working environment; be able to be kept clean from fat residues to avoid efficiency loss and fire risks, and only feature canopies made from non-combustible materials.

When specifying a commercial kitchen it is the job of a ventilation engineer to consider not only kitchen usage, the building limitations and equipment capacity, but also factors such as extraction, make up air, ventilated ceilings, odour control and fire suppression.

Endorsed standard
To inform this task, building on the core HSE guidance, is the industry specification DW172 Standard for Kitchen Ventilation Systems, which is not only endorsed by the HVCA, but also recognised by CIBSE, HSE, BSRIA, the Association of British Insurers and the Loss Prevention Council. The successor to the original DW171 guidance, the new specification includes legislative changes, new standards and all the requirements for design, installation and maintenance of kitchen ventilation systems.

When it comes to extracting air, it’s not just a case of looking for the simplest way possible. For instance, scientists have learnt that the degradation of certain organic materials generate carcinogenic fumes. Therefore it’s important to design a system that avoids extracting air through the breathing zones of staff operating the cooking equipment.

Calculating extract flow rates should be done using the Thermal Convection Method, which acknowledges each piece of equipment, its surface area and fuel type. Coupled with the type and style of canopy used, this method gives an accurate and consistent extract flow rate. It also generates the supply air flow rate, which should be a maximum of 85 per cent of the extract rate.

Make up air, to replace the contaminated extracted air, should be mechanically introduced, filtered and tempered to assure a minimum entry temperature of 10°C when delivered via a canopy, and 16°C when delivered via ceiling grilles. A fan powered system provides positive control and, therefore, is the preferred method to use.
The most common way to introduce make up air via a canopy is through a perforated front panel, allowing cool air to tumble onto those operating the cooking equipment.

There is a common design that introduces air inboard of the canopy, this method has various names including ‘induction’, ‘entrainment’ and ‘capture air stream’. These methods can improve the capture and containment of a canopy, but any inboard air shall be added to the calculated extract flow rate.

Gas supply
Both the British Standard BS6173:2001 and the HSE require that commercial ventilation systems are interlocked with the gas supply to ensure safe operation. This means the ventilation system has to be proven to both extract and replace air before the gas supply can be activated. This interlocking system also cuts off the gas should the extract or supply air system fail during operation. The kitchen ventilation contractor is specifically responsible for providing the air proving mechanism.

Regarding the dimensions of extraction equipment, there should be a minimum 300mm to the front, side and, where appropriate, back of any canopy. The front dimension should be extended to 600mm for combination ovens and certain bakery ovens. This ensures that the large quantity of steam emitted when opening oven doors of this type is contained within the canopy. The underside of the canopy should be between 2,000mm and 2,100mm above the finished floor level.

Cleaning filters
For the grease filter, the minimum distance between the lowest edge of the filter and the top of a cooking surface should be 450mm. This avoids the risk of excessive temperatures or fire in the filter which could cause the extracted grease to vaporise and pass through to the ductwork. Only stainless steel is suitable for the canopy fabrication as it is non-combustible, and mesh filters can only be used as a secondary method of grease extraction as they offer no fire barrier.

Ventilated ceilings are becoming a popular alternative to canopies and have gained favour with catering colleges, training kitchens and some government properties. There are two types of ventilated ceiling available – modular cassette and plenum. They can vary between 2,200mm and 3,500mm in height, but generally 2500mm is optimum. The new specification states that open plenums should not be used as part of an extract system where grease laden air is being extracted. P

Material used to manufacture a ventilated ceiling should also be stainless steel. Some manufacturers use aluminium in the supporting frame, this form of construction should meet with the approval of the client or specifier.

Assessing risk
Operators of commercial kitchens have a duty under health and safety law to assess fire risk in the ventilation system, particularly those created by excessive build up of cooking oil deposits. If suitable precautions are not undertaken, and in the event of personal injury or death resulting from a fire associated with a poorly maintained system, charges of corporate liability or manslaughter may be brought against the kitchen operator.

The considerable amount of flammable grease and related particles within extract systems which, combined with the chance of fire ignition caused by cooking equipment, creates a hazard level above that normally encountered in other ventilation systems. It is imperative that kitchen ventilation provides adequate protection to ensure the safety of occupants and fire fighting personnel, and limits property damage.

There is increasing demand for odour control systems within commercial kitchen ventilation. Each site needs to be considered on its own merits, as one solution is not a cure-all. Various filtration systems are available, including three stage filters, electrostatic precipitators, activated carbon filters and UV-C light.

Kitchen ventilation systems represent one of the largest uses of energy in a commercial food service facility. With this in mind, adhering to an approved standard for designing and specifying these systems is paramount if we are to maximise efficiency, comfort and safety.  

For more information:
Web: www.hvca.org.uk