Air velocity in the duct: flow rates in ventilation

Microclimate, ventilation systems in residential or industrial premises, affects the well-being and performance of people. To create comfortable living conditions are developed rules governing the composition of the air. Agree, regular ventilation is vital.

We'll show you what should be the speed of the air in the duct. We advise you what to do, that the air flow has always been fresh and meets hygiene standards. Here you will find a detailed description of calculation methods and listing rules of selection of optimal air.

Proposed to educate information is based on a normative reference data. For the practical development of calculation methods are examples. Text material is supplemented by illustrations and videos to facilitate the perception of a difficult topic.

The content of the article:

  • The importance of air for man
  • The rules for determining the air velocity
    • №1 - sanitary norms noise
    • №2 - vibration
    • №3 - ventilation rate
  • An algorithm for calculating the air speed
  • The recommended standards of air exchange rate
  • Subtleties duct selection
  • Conclusions and useful videos on the topic

The importance of air for man

For construction and sanitary norms, every residential or production facility is necessary to ensure ventilation system.

Its main purpose - the preservation of the balance of the air, creating a favorable working and relaxing climate. This means that in an atmosphere where people breathe, shall be observed excess heat, moisture, pollution of various kinds.

Violations in the organization of ventilation systems lead to the development of infectious diseases and diseases of the respiratory system, reduce immunity to the premature spoilage of food.

The overly moist and warm environment of rapidly evolving pathogens, on walls, ceilings, and even furniture there are pockets of mold and mildew.

ventilation system

The scheme of ventilation in a two-story private house. The ventilation system is equipped with energy-saving air handling unit with heat recuperator which allows reuse of the heat output from the outside of the building

One of the conditions for maintaining a healthy balance of the air is the correct design of the ventilation system. Each part of the air exchange network should be selected based on the volume of air spaces and characteristics therein.

Suppose, in a small apartment is well-established supply and exhaust ventilation, while in the manufacturing plants required the installation of equipment for the forced air.

During the construction of houses, public institutions, shops companies are guided by the following principles:

  • each room must be provided with a ventilation system;
  • hygienic air conditions must be observed;
  • enterprises should install the device, enhancing and regulating the air speed; in dwellings - conditioners or fans provided insufficient ventilation;
  • in rooms for different purposes (for example, in the wards for patients and operating room or in the office and in the smoking room) must be equipped with different systems.

To ventilation to meet these conditions, you need to make calculations and pick up the equipment - air devices and ducts.

also, when ventilation device you need to choose the right air intake, to prevent the admission of polluted streams back into the room.

Designated discharge and intake

In the process of ventilation project for a detached house, apartment building or industrial premises calculate the volume of air and the scheduled place of ventilation equipment installation: voduhoobmennyh units, air conditioners, ducts

The size of the ducts (including brownies mines) depends on the effectiveness of ventilation. Let us see what are the rules of the air flow speed in the ventilation specified in the health documentation.

image gallery

Photo of

The ventilation system in the attic

The speed of movement of the air ducts is calculated for channel systems in which air is supplied or removed for the construction of pipe paths

Equipment for ventilation

At the air velocity has a greater influence capacity of the equipment used in the construction of a mechanical ventilation system

Plastic rectangular ducts

The calculations required to take into account the material from which the ducts, the shape and size of their cross section

Local resistance ducts

The calculation must take into account local resistance, including Elbows, crosses, built-in filters channel tees and air diffusers

The ventilation system in the attic

The ventilation system in the attic

Equipment for ventilation

Equipment for ventilation

Plastic rectangular ducts

Plastic rectangular ducts

Local resistance ducts

Local resistance ducts

The rules for determining the air velocity

Air speed is closely related with terms such as noise and vibration level in the ventilation system. The air passing through the channels creates a certain noise and pressure which increases with the number of turns and bends.

The greater the resistance in the tubes, the lower air speed and the higher is the fan efficiency. Consider the rules associated factors.

№1 - sanitary norms noise

Standards specified in SNIP relate to residential buildings (private houses and apartment buildings), public and industrial type.

In the table below, you can compare the rates for rooms of various types, as well as areas adjacent to buildings.

Allowable noise standards

Part of the table №1 Snip-2-77 of the section "Protection against noise". The maximum permissible limits related to the night-time, below the daily values, and norms for the adjacent areas is higher than for premises

One reason for the increase in accepted norms just might be incorrectly designed duct system.

Sound pressure levels are represented in another table:

pressure level Standards

When commissioning the ventilation or other equipment associated with the provision of friendly, healthy indoor climate, only allowed a brief rise marked noise parameter

№2 - vibration

Power fan operation is directly related to levels of vibration.

The maximum vibration threshold depends on several factors:

  • duct size;
  • quality gaskets providing reduced vibration level;
  • Pipe manufacturing material;
  • air flow rate passing through the channels.

Standards which should be followed when selecting the ventilation devices and the calculations relating to air ducts are provided in the following table:

Maximum allowable local vibration values

Maximum allowable local vibration value. When you check the actual rates are higher standards, then the duct system is designed with the technical deficiencies that must be corrected, or the fan power is too great

Air velocity in mines and channels should not affect the increase in vibration performance, as in the related parameters of sound vibrations.

№3 - ventilation rate

Air purification occurs by a process of air exchange, which is divided into natural or forced.

In the first case it is carried out when opening the doors, transoms, panes, windows (known as aeration) or simply by infiltration through cracks at the junctions of walls, doors and windows, in the second - using air conditioners and ventilation equipment.

change of air in the room, utility room or workshop several times an hour, to the degree of contamination of air masses must take place was acceptable. Number of shifts - a multiplicity value, as needed to determine air speed in the air ducts.

Multiplicity is calculated by the following formula:

N = V / W,


  • N - ventilation rate, time of 1 hour;
  • V - the amount of clean air, the filling room for 1 h, m³ / h;
  • W - the volume of the room in m³.

Not to perform additional calculations, the average multiplicity of indicators collected in the table.

For example, for premises suitable ventilation rate in the following table:

Number of air

Judging from the table, the frequent change of air in the room is required if it is characterized by high humidity or temperature - e.g., in the kitchen or bathroom. Accordingly, when insufficient natural ventilation in the premises set data devices forced circulation

What happens if the ventilation rate standards are not adhered to, or will be, but not sufficiently?

There will be one of two things:

  • Multiplicity below normal. Fresh air stops to replace polluted, whereby the room is increased concentration of harmful substances, bacteria, pathogens, hazardous gases. The amount of oxygen is important for the human respiratory system is reduced, and carbon dioxide, in contrast, increases. Humidity rises to a maximum, which is fraught with the appearance of mold.
  • Multiplicity above normal. It occurs if the air speed of moving in the channels exceeds the norm. This adversely affects the temperature regime: the room just did not have time to heat up. Overly dry air stimulates the skin and respiratory system.

To air changes comply with sanitary norms, install, remove or adjust the ventilation equipment, and if necessary, replace the air ducts.

An algorithm for calculating the air speed

Taking into account the above conditions and specifications are specifically taken the room, you can determine the characteristics ventilation systemAnd to calculate the velocity of the air in the pipes.

Should be based on the number of air, which for these calculations is the determining value.

handy table to refine the flow parameters:

Airflow duct in the parameters

The table shows the dimensions of air ducts with rectangular cross-section, i.e. their length and specified width. For example, using channels 200 mm x 200 mm at a speed of 5 m / s air flow of 720 m³ / h

To make your own calculation, you need to know the amount of space and the rate of air exchange rate for the room or the room of a given type.

For example, the parameters must be found for the studios with kitchenette totaling 20 m³. Take the minimum value of the multiplicity for the kitchen - 6. It turns out that for 1 chasa air channels must move around L = 20 m³ * 6 = 120 m³.

Also, you need to know Square ductsInstalled in the ventilation system. It is calculated by the following formula:

S = πr2 = Π / 4 * D2,


  • S - cross-sectional area of ​​the duct;
  • π - the number of "pi", a mathematical constant equal to 3.14;
  • r - radius of the duct;
  • D - the diameter of the cross section of the duct.

Assume that the diameter of the circular shape of the duct is 400 mm, it is substituted into the formula and we obtain:

S = (3,14 * 0,4²) / 4 = 0.1256 m²

Knowing the cross-sectional area and the flow rate, we can calculate the speed. The formula for calculating the air flow rate:

V = L / 3600 * S,


  • V - air flow rate (m / s);
  • L - Air flow (m³ / h);
  • S - sectional area of ​​the air ducts (air ducts) (m²).

Substituting known values, we get: V = 120 / (3600 * 0.1256) = 0.265 m / s

Therefore, to ensure the required ventilation rate (120 m3/ H) using a circular duct with a diameter of 400 mm, is required to install equipment to increase air flow rate to 0.265 m / s.

It should be remembered that the factors described above - the parameters of vibration and noise - are directly dependent on the speed of air movement.

If the noise exceeds the norm indicators have lower speed, thus increasing the cross section of the duct. In some cases it is sufficient to install a pipe of different material or fragment replaced curved channel on the line.

The recommended standards of air exchange rate

During the drafting of the building to perform the calculation of each individual site. In the production of this plant, in residential buildings - apartments, in a private house - floor blocks or separate rooms.

Before installing the ventilation system is known, what routes and the size of the main thoroughfares of what geometry is needed ventsWhat size pipe is optimal.

Air ducts of circular cross-section

Do not be surprised overall dimensions of the duct in catering establishments or other institutions - they are designed to output a large number of used air

Calculations related to the movement of air currents in residential and industrial buildings, belong to the category of the most difficult, so they are obliged to engage experienced and qualified professionals.

The recommended air velocity in the duct is indicated in SNIP - the state regulatory documentation and the design and delivery of objects is oriented on it.

Air velocity by portions

The table shows the parameters that should be followed when constructing ventilation system. Numbers indicated speed of the air masses to the mounting location of channels and gratings in conventional units - m / s

It is believed that the indoor air velocity should not exceed the value of 0.3 m / s.

Exceptions are temporary technical circumstances (e.g., repair, installation and other construction equipment.), During which the parameters can exceed the limits up to 30%.

In large volume area (garage, manufacturing plants, warehouses, sheds) often instead of a single ventilation system there are two.

The load is halved, therefore, and the air velocity are selected so as to provide 50% of the calculated displacement volume of air (or removing contaminated feed grade).

In the event of force majeure is necessary sharp change in air speed or complete suspension of the ventilation system.

For example, fire safety requirements air flow rate is reduced to a minimum in order to prevent the spread of fire to adjacent rooms and smoke during a fire.

To this end, the duct and transition sections are mounted and the valves shut off.

Subtleties duct selection

Knowing the results of aerodynamic calculations, it is possible to choose the right air duct parameters - or rather, the diameter of circular and rectangular cross sections dimensions. In parallel, you can select the device forced air (Fan) and determine the pressure loss in the air movement through the channel.

Knowing the value of air flow rate and the speed of its movement, it is possible to determine what section ducts required.

For this formula is taken inverse formula to calculate the air flow rate:

S = L / 3600 * V.

Using the result, it is possible to calculate the diameter:

D = 1000 * √ (4 * S / π),


  • D - the diameter of the cross section of the duct;
  • S - sectional area of ​​the air ducts (air ducts) (m²);
  • π - the number of "pi", a mathematical constant equal to 3.14 ;.

The resulting number is compared with factory standards, admitted according to GOST, and selecting the closest diameter of the product.

If you want to choose a rectangular rather than round ducts, it is necessary to determine the length / width of the body instead of the diameter.

When choosing guided by the approximate cross-section, using the principle of a * b ≈ S and the sizes of the table provided by the manufacturer. We remind you that according to the norms ratio of width (b) and length (a) should not exceed 1 to 3.

Air ducts with rectangular section

Ducts with rectangular or square cross-section has an ergonomic shape that allows you to set them up against the wall. This is used, furnished pets and drawing the tube over masking structures or ceiling mounted above the kitchen cabinets (mezzanine)

Conventional rectangular channels Standards: minimum size - 100 mm x 150 mm maximum - 2000 mm x 2000 mm. Round ducts are good because they possess less resistance, respectively, have the minimum noise level indicators.

In recent years, especially for applications inside apartment produce convenient, safe and Lightweight plastic boxes.

Conclusions and useful videos on the topic

Helpful videos teach you to work with physical quantities and help to better understand how the ventilation system works.

Video # 1. Calculation of natural ventilation parameters using a computer program:

Video # 2. Useful information about the ventilation device in a newly built private house:

Article information may be used for informational purposes and in order to understand better the work of the ventilation system.

For a more accurate calculation of air velocity at the home communications design recommend appeal to engineers who know the nuances of the ventilation device and help you choose the right size ducts.

Wishes to share a personal experience gained with the device ducts, interesting facts and specific information, please write in the comments box located below. Ask questions on the controversial moments. We or visitors with pleasure pouchastvuem in the discussion.

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