Smoke extraction in industrial kitchens - 2021

How to calculate the air flow to extract and how to install the hoods

Smoke extraction in industrial kitchens

It is obvious that in all kitchen, or place where a cooking or smoke generation is carried out, these must be extracted to the outside so that they do not accumulate making it impossible for the Cooks, or what would be worse, that they also spread to the dining room.

1. Cácalculation of an extractionón

The air flow to extract from a kitchen It will be determined by many factors, but here we will focus only on the basics, which on the other hand will guarantee us total success.

  1. We will consider that for one extractor hood normal (non-inductive), fumes cannot spread to other areas, a certain air current must be established towards the hood throughout its free perimeter. This means that the hood will draw air from any part of its perimeter where there is no wall.
  2. The suction power of a hood must always be weighted by these factors that we now develop and determine, by this we mean how negative it is that less than necessary is sucked in as more than necessary, generating energy consumption and air currents. unnecessary. This comment is essential to introduce this second point that despite its importance, usually goes unnoticed and that is the height of the bell.

smoke extraction in industrial kitchens

The hood (we will consider a hypothetical simile) behaves like a vacuum cleaner that we all use in our homes to suck dust from carpets and rugs. Surely we have realized that the closer we bring it to the ground the more it sucks, but that when separating it a few centimeters it does not suck anything.

Thus we can understand to what extent this height is decisive in the capture of the hood. Now, empirically, we can establish that the correct height from the hood to the cooking zone is 1 meter.

smoke extraction in industrial kitchens

If for any reason the hood is mounted higher or lower, the corresponding correction factor will be applied.

  1. As we mentioned in the first point, we establish a certain approach air current to the hood. This air current (or its speed) defines the coefficient "K", which is obtained experimentally and although as a general rule it has a fixed value, depending on the type of cooking it may vary.

The coefficient "K" for wall hoods will be 0,30-and for central hoods it will be 0,25. When the extraction is from a grill (a lot of smoke production) K = 0,35. Extreme cases such as wood-fired chicken rotisseries K = 0,40.

  • Formula

Applying the previously defined parameters We can develop the following formula. Flow to extract = (free perimeter in meters) x (height from hood to cooking zone in meters) (coefficient "K") x 3.600 (multiply by 3.600 because the result will be in m3 / h. (1 hour has 3.600 seconds ).

We note that the result of this formula could vary in some cases, in which for some reason a more personalized study had to be carried out.

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Calculation Example 1

4.000 × 900 wall hood in which one side rests on the wall.

Formula: perimeter x height x 0,3 x 3.600 = flow 4,9 x 1 x 0,3 x 3.600 = 5.292 m3 / h.

Calculation Example 2

Suppose that the same hood above (Example 1) is installed at a height higher than the normal 1,2 meters.

4,9 x 1,2 x 0,3 x 3.600 = 6.350 m3 / h.

As we can see, the height is a determining factor in the correct operation of the hood, since with only a height difference of 0,2 m (20 cm), we are forced to extract 1.058 m3 / h. more (20%).

Calculation Example 3

This time we will apply the formula on a central bell that was 4 meters long by 2,1 deep.

Free perimeter (all) = 12,2 m., For which we have:

Flow = 12,2 x 0,25 x 3.600 = 10.980 m3 / h.


This section is not intended to give lessons to assemblers and professionals in general, but simply to dictate a series of basic rules that in some cases particularly affect our products.

To get a better understanding, we will divide this part into five points:

  1. Measurements: as previously mentioned in the "Calculation of an extraction" section, the height at which a hood is mounted is essential for its correct operation, for which we remind you that it will be 1 meter between the front of the hood. and the cooking surface, which is the same, 1,90 meters from the ground. To clarify these concepts, look at Figures 5, 6, 7 and 8 in which these ideas are graphically reflected applied to the different types of hood.
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smoke extraction in industrial kitchens

  1. Fixation: due to its different characteristics, we will proceed to the study independently of the three basic types of hood:
      • Wall hoods: they have work built-in fixation, based on double-stud plates, which fit into the fittings provided on the hood.

Thanks to the particular shape of these plates, the hood fits perfectly against the wall. As a safety measure, it is recommended to fix with two other plugs in the lower part of the same using the holes provided for this purpose. Figures 9 and 10 show in detail the shape and measurements to which these plates should be attached.

smoke extraction in industrial kitchens

  • Central bells: in the design of these bells, special attention was paid to the support structure, achieving a work innovative plenum-frame in the shape of a “V”, which, integrated within the hood itself, acts as a resistant frame while providing us with an ideal anchoring area.

3. Basic connections: we include this topic, due to the amount of confusion we find ourselves with, probably caused by the different ways in which each manufacturer usually delivers their products.

There are two types of duct coupling for hoods, depending on the model: For hoods with a “Monoblock” built-in fan, a coupling hopper will be needed to transform the rectangular outlet, natural in centrifugal fans, to the circular duct normally used.

It is not correct to directly couple the duct to the mouth of the fan as it would lose performance and make noise. In the rest of the models with a remote fan, the issue is significantly simplified since it can be exited directly with a circular duct.

smoke extraction in industrial kitchens

The part used for this purpose is called the crown sleeve and it will be used whenever you want to attach a circular duct to any flat surface. From what has been said above, we will understand that the crown sleeve will also serve to enter the ventilation box.

4. Legislation: To summarize the legislation that affects smoke extraction, we would need a complete book, and when it was finished the legislation would have already changed, so we will simply limit ourselves to citing some important nuances that we will divide into four points to facilitate understanding:

  • Filters: The law is very strict in this regard, forcing any public premises that have kitchen, to have a work extraction with metal filters (class M0), with an inclination equal to or greater than 45 °. Interestingly, nothing says their type, shape or effectiveness, or if they should be clean.
  • Bell: It will be built in class M0 material (non-porous), and located more than 50 cm from any unprotected combustible material. Of course, gas pipes cannot pass through the interior of the hood.

On the other hand, according to the low voltage electrotechnical regulations and considering the hood as an element of danger, it will be necessary for any electrical element located inside it to be highly secure or explosion-proof.

It will be mandatory to equip the hood with work of fire extinguishing in all kitchens whose dimensions are greater than 50 m2, or 25 m2 in the case of hospitals (be careful with open kitchens). It is strictly forbidden to install fire dampers.

  • Ducts: The system will be independent of any extraction or ventilation and exclusive for each local kitchen. They will be made of M0 class material and will have inspection logs in each horizontal section of a maximum of 3 m.

They will not have fire dampers inside. When they pass through facades, their distance to any area of ​​the same that is not at least PF 30 will be 1,5 m., At least.

The final exit of the duct or chimney will be located at a minimum distance from any neighboring point of greater height, of 10 m., In case the cooking is with gas, and 20 m, in the case that it is with charcoal or wood.

  • Ventilation units: they will be capable of operating at 400 ° C for at least 90 minutes and their connection to the pipes will be watertight.

I am a dreamer and in my dreams I believe that a better world is possible, that no one knows more than anyone, we all learn from everyone. I love gastronomy, numbers, teaching and sharing all the little I know, because by sharing I also learn. "Let's all go together from foundation to success"
smoke extraction in industrial kitchens