The removal of fumes and steam from cooking and industrial processes should be done as near to the source as possible. Warm fumes and steam rise quickly and spread over a comparatively large area of the kitchen and must be 'picked up' and removed quickly.

To deal with this sort of local problem by increasing the general ventilation rate of the room is not always economical or convenient due to the large volume of air extraction necessary to reduce the spread of the fumes. In such cases a canopy, or hood, would be fitted directly above the equipment and overlapping it by up to 300 mm all round to collect the fumes. The canopies and fans should be of sufficiently large capacity to 'hold' and carry away the fumes without undue spillage from the mouth of the canopy.

To achieve this, the velocity of the air through the open area between the canopy and the equipment must be sufficiently high to draw in fumes near the edge of the equipment against the eddying effects of local draughts which could be caused by the movement of people around the equipment.

Air Volume

Where the items of cooking equipment to be placed under a canopy are known, the total of the volumes of air required for each piece of equipment will constitute the extract volume to be provided by the canopy extract fan/s. (See Table 1 for volume of air required for cooking equipment). Where the equipment is not known, the formula shown below can be used. This formula uses the base area of the canopy, rather than the open perimeter area used in earlier formulae, and more closely matches the volume of hot air rising from the cooking equipment. The volumes obtained by this formula should be regarded as minimums and no harm will result if they are increased by 50%.

Vol. m³/s = L.metres × W.metres × K, where K =

0.25 for domestic
0.30 for Light Commercial
0.40 for Commercial and Light Industrial
0.50 for Heavy Commercial and Industrial (Welding, etc.)
(The factor K represents the face velocity (m/s) of the airflow at the canopy)

Plastic flexible ducting should not be used to extract from kitchen canopies, as it is very difficult to clear and would constitute a fire hazard. Steel ductwork should be used, with adequate access panels for cleaning. In special cases, flexible metal ducting could be used, but only where it is short enough to be easily dismantled for cleaning or replacement.

Example

A canteen kitchen (equivalent to a light commercial kitchen) is to have a canopy 3 m x 1.25 m covering cooking equipment not yet specified.

L = 3m
W = 1.25m
K = 0.3

Air req. m³/s = 3 × 1.25 × 0.3 = 1.125 m³/s (4050m³/h)

Other points to consider:

a)	Minimum height from floor to underside of canopy 2m
b)	Air replacement based on 75-85% of extracted air
c)	Temperature of replacement air must not be below 10oC when coming into contact with cooked food
d)	Maximum duct velocity 6m/sec

Commercial Kitchens

Efficient ventilation is an important factor in kitchen design. It must effectively remove cooking fumes and odours and the products of combustion given off by gas cooking apparatus, and ensure an adequate supply of comparatively fresh replacement air without creating uncomfortable draughts.

The main points when preparing a scheme are:

1)	Give an adequate air flow. Use a minimum ventilation rate of 25 ACH for commercial kitchens, increasing these figures as necessary to deal with higher than average loading and cooking equipment. When calculating the amount of air necessary to give the selected ACH it is usual to base the volume of the kitchen on a height of 3m. This will automatically compensate for different ceiling or roof heights by increasing the ventilation rate for a low ceiling, and reducing it for a high ceiling.
2)	Specific Volumes for Cooking Equipment. Current practice for commercial kitchen ventilation extends the guidelines for sizing ventilation schemes. Whilst retaining the minimum of 20-30 ACH, specific quantities of air to be provided for each piece of cooking apparatus are now available. Therefore, when the details of the equipment are known, a more accurate assessment of the air volume required can be made. These requirements can result in substantially higher rates of extraction than the minimum rates, and will take much of the uncertainty out of deciding by how much the minimum must be exceeded. The volumes can be used for determining both general extraction and canopy extraction requirements. Table 1:  Kitchen Cooking Equipment Volume Requirements Apparatus m³/h m³/s l/s ft³/min Cookers per m² 1080 0.3 300 640 Pastry ovens 1080 0.3 300 640 Fish fryers 1620 0.45 450 950 Grills 900-1080 0.25-0.3 250-300 530-640 Steak grills 1620 0.45 450 950 Salamanders 1620 0.45 450 950 Boiling pans (140-180 litre) 1080 0.3 300 640 Steamers 1080 0.3 300 640 Sink (sterilising) 900 0.25 250 530 Bain Marie 720 0.2 200 420 Tea sets 540-900 0.15-0.25 150-250 320-530 Alternatively, calculations can be based on the number of meals prepared per hour, multiplied by 10-15, to give an extract volume in litres per second. This method highlights the different requirements between, for example, an expensive restaurant with one sitting per table per evening, to steak bars with around 3-4 sittings/table/evening, to pizza restaurants (semi fast food) with 5-6 and a burger bar. The amount of cooking, hence air movement required, increases the faster the food.
3)	Locate extract units as high as possible and as near the source of the fumes as convenient. Hot moist fumes from cooking operations rise fast to ceiling level, and unless they are removed quickly from that level they will spread over ceiling, walls and windows depositing the moisture content and grease as it condenses on the cooler surfaces. Roof lights and lantern lights are sometimes an ideal location for extract units in a commercial kitchen as they are usually over some cooking equipment at or near the centre of the kitchen, and it is a simple matter to fit roof fans in the glazing. If due to some obstruction it is not possible to site the unit at high level directly above the cooker, then keep it at high level and move it a foot or two to one side. This is better than putting the unit immediately above the cooker but only half way up the wall, as the velocity of the steam and fumes would carry them past the unit to ceiling level where they would spread horizontally and hang about for some time before cooling sufficiently to drop to the level of the extract point. This is a common fault in domestic kitchens, the low siting of the unit sometimes allowing cooking fumes to float through the top of a doorway before they can sink low enough to be extracted by the fan.
4)	Use canopies over 'heavy' cooking equipment, particularly in commercial kitchens, to collect and 'hold' the fumes at source. Estimate the total volume of air required for the kitchen, subtract the volume required for the canopy, then allow units over the wash-up and food preparation sinks to make up the difference. Canopy grease filters are necessary to remove the bulk of the oil and fat droplets from the air before it passes along ducting and through extract fans.
5)	Ensure ample air replacement openings, well distributed to eliminate local draughts and to spread the supply of fresh air. Some air replacement from adjoining rooms is not a disadvantage as the flow of air through the doorways will reduce the possibility of fumes from the kitchen passing through to these adjoining rooms. Extract units should be switched on as soon as any cooking apparatus is in use to prevent a build-up of hot fumes, and should be left running for 20 to 30 minutes after cooking is finished to clear away any residual fumes and hot air convected from the cooker surfaces.

Check that sufficient free area for air replacement has been provided for fuel burning equipment, including gas cookers.

CIBSE Guide B2 states that:

A supply of fresh air is required for:
  a) Human respiration
  b) Removal of pollutants (odours, cigarette smoke, moisture)
  c) Combustion appliances
  d) Thermal comfort
  e) Smoke clearance (fire)

In a commercial kitchen, the principal objective is to enable the occupants to 'pursue their working activities in comfort'. "If you cannot stand the heat, get out of the kitchen", has no foundation. It is a legal requirement to provide correct working conditions for staff. It is also a legal requirement to provide healthy conditions for patrons.

The Heating and Ventilating Contractors Association has recently published DW/171 Standard for Kitchen Ventilation Systems (ISBN 0-903783-29-0). It is recommended that this publication is referred to for all aspects of Commercial Kitchen ventilation, canopies, filtration and ducting.