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| Kitchen ,Canopies and Hoods ,Air Volume Calculation |
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| 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. |
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| 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. |
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| 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 = 4m
W = 1.25m
K = 0.3
Air req. m³/s = 3 × 1.25 × 0.3 = 1.5 m³/s
(5400m³/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/se |
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| 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. |
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| 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
|
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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. |
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| 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. |
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| 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. |
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