When calculating and installing ventilation, much attention is paid to the amount of fresh air supplied through these channels. For calculations, standard formulas are used, which well reflect the relationship between the dimensions of the exhaust devices, the speed of movement and the air flow rate. Some norms are spelled out in SNiPs, but most of them are of a recommendatory nature.
General principles of calculation
Air ducts can be made of different materials (plastic, metal) and have different shapes (round, rectangular). SNiP regulates only the dimensions of the exhaust devices, but does not standardize the amount of supplied air, since its consumption, depending on the type and purpose of the room, can vary greatly. This parameter is calculated using special formulas that are selected separately. The norms are established only for social facilities: hospitals, schools, preschool institutions. They are spelled out in SNiPs for such buildings. At the same time, there are no clear rules for the speed of air movement in the duct. There are only recommended values and norms for forced and natural ventilation, depending on its type and purpose, they can be viewed in the corresponding SNiPs. This is reflected in the table below. Air speed is measured in m / s.
The data in the table can be supplemented as follows: with natural ventilation, the air speed cannot exceed 2 m / s, regardless of its purpose, the minimum allowable is 0.2 m / s. Otherwise, the renewal of the gas mixture in the room will be insufficient. With forced exhaust, the maximum allowable value is considered to be 8 -11 m / s for main air ducts. You should not exceed these standards, since this will create too much pressure and resistance in the system.
Calculation formulas
To carry out all the necessary calculations, you need to have some data. To calculate the air speed, you need the following formula:
ϑ = L / 3600 * Fwhere
ϑ - air flow velocity in the pipeline of the ventilation device, measured in m / s;
L - the consumption of air masses (this value is measured in m3/ h) in the section of the exhaust shaft for which the calculation is performed;
F - cross-sectional area of the pipeline, measured in m2.
This formula is used to calculate the air speed in the duct, and its actual value.
All other missing data can be derived from the same formula. For example, to calculate the air flow, the formula must be transformed as follows:
L = 3600 x F x ϑ.
In some cases, such calculations are difficult or time consuming. In this case, you can use a special calculator. There are many similar programs on the Internet. For engineering bureaus, it is better to install special calculators that have greater accuracy (subtract the thickness of the pipe wall when calculating its cross-sectional area, put more digits in pi, calculate a more accurate air flow, etc.).
Knowing the speed of air movement is necessary in order to calculate not only the volume of the gas mixture supplied, but also to determine the dynamic pressure on the channel walls, friction and resistance losses, etc.
Some helpful tips and notes
- there will be no losses or the need to lay an additional ventilation pipeline to ensure the required air flow, if the dimensions of the room do not allow for large ducts;
- smaller pipelines can be laid, which in most cases is easier and more convenient;
- the smaller the channel diameter, the cheaper its cost, the price of additional elements (dampers, valves) will also decrease;
- the smaller size of the pipes expands the possibilities of installation, they can be positioned as needed, practically without adjusting to external constraining factors.
However, when laying air ducts of a smaller diameter, it must be remembered that with an increase in air speed, the dynamic pressure on the pipe walls increases, the resistance of the system also increases, and accordingly a more powerful fan and additional costs will be required. Therefore, before installation, it is necessary to carefully carry out all the calculations so that the savings do not turn into high costs or even losses, because a building that does not comply with SNiP standards may not be allowed to operate.
