Translation of air ducts from linear meters into square meters. How to calculate the cross-section and diameter of the air duct

When the duct parameters (their length, cross-section, friction coefficient of air against the surface) are known, it is possible to calculate pressure losses in the system at the designed air flow.

P = R * l + z,

where R  - loss of frictional pressure per 1 running meter of duct, l z  - loss of pressure on local resistance (with variable cross-section).

1. Loss of friction:

In a circular duct, the frictional pressure loss Ptr  are:

Ptr = (x * l / d) * (v * v * y) / 2g,

where x  - coefficient of friction resistance, l  - the length of the duct in meters, d  - diameter of the duct in meters, v y g  - acceleration of gravity (9.8 m / s2).

Comment:  If the duct has not a round but a rectangular section, the equivalent diameter must be substituted in the formula, which for the duct with sides A and B is: dEq = 2AB / (A + B)

2. Losses on local resistance:

Loss of pressure on local resistance is calculated by the formula:

z = Q * (v * v * y) / 2g,

where Q  - the sum of the coefficients of local resistance in the duct section for which the calculation is made, v  - speed of air flow in m / s, y  - density of air in kg / m3., g  - acceleration of gravity (9.8 m / s2). Values Q  are contained in a table form.

Method of permissible speeds

When calculating the network of air ducts, the optimal air speed is taken as the initial data by the permissible speed method (see table). Then, the desired section of the duct and the pressure loss in it are considered.

Procedure for aerodynamic calculation of air ducts by the method of permissible speeds:

1. Draw a diagram of the air distribution system. For each section of the duct specify the length and amount of air passing in 1 hour.
2. Calculation begins with the farthest from the fan and the most loaded areas.
3. Knowing the optimal air speed for a given room and the volume of air passing through the air duct in 1 hour, determine the appropriate diameter (or cross-section) of the duct.
4. Calculate the pressure loss for friction Ptr.
5. According to tabular data, we determine the sum of the local resistances Q and calculate the pressure loss for local resistances z.
6. The available pressure for the following branches of the air distribution network is defined as the sum of the pressure losses in the sections located before this branch.

In the process of calculation, it is necessary to consistently link all the branches of the network, equating the resistance of each branch to the resistance of the most loaded branch. This is done with the help of diaphragms. They are installed on lightly loaded sections of ducts, increasing resistance.

Table of the maximum air speed depending on the duct requirements

Method of constant head loss

Diagram of the determination of head loss and diameter of ducts

Use of rectangular ducts

The diameter of the circular ducts is indicated in the pressure loss diagram. If instead of them air ducts are used rectangular section, it is necessary to find their equivalent diameters using the table below.

Remarks:

1. If space allows, it is better to choose round or square air ducts.
2. If space is not enough (for example, during reconstruction), choose rectangular ducts. Typically, the width of the duct is 2 times the height). In the table, the duct height in mm is indicated in horizontal, the width in the vertical direction, and in the cells of the table, there are equivalent duct diameters in mm.

Table of equivalent duct diameters

Comments:

• Factors affecting the size of air ducts
• Calculation of airway dimensions
• Selection of dimensions for real conditions

To transfer the supply air or exhaust air  Air ducts of various configurations, shapes and sizes are used from ventilation installations in civil or industrial buildings. Often they have to be laid on existing premises in the most unexpected and cluttered places. For such cases, the correct cross-section of the duct and its diameter play a crucial role.

Factors affecting the size of air ducts

It is not a big problem to successfully lay ventilation systems on newly designed or newly constructed facilities - it is enough to reconcile the location of systems with respect to workplaces, equipment and other engineering networks. In existing industrial buildings, this is much more difficult to do because of the limited space.

This and several other factors affect the calculation of the diameter of the duct:

1. One of the main factors is the flow of supply or exhaust air per unit time (m 3 / h), which must pass this channel.
2. The throughput also depends on the air speed (m / s). It can not be too small, then by calculation the size of the air duct will be very large, which is economically impractical. Too high speed can cause vibrations, increased noise and power ventilation system. For different parts of the supply system, it is recommended to take a different speed, its value lies in the range from 1.5 to 8 m / s.
3. The material of the duct is important. Usually it is galvanized steel, but other materials are also used: various types of plastics, stainless steel or black steel. The latter has the highest surface roughness, the resistance to flow will be higher, and the channel size will have to be taken more. The diameter value should be selected according to the regulatory documentation.

Table 1 shows the normal size of ducts and the thickness of the metal for their manufacture.

Table 1

Note: Table 1 reflects the normal not completely, but only the most common channel sizes.

Air ducts produce not only round, but also rectangular and oval shape. Their dimensions are taken through the equivalent diameter value. Also, new methods of making channels allow the use of a metal of smaller thickness, while increasing the speed in them without the risk of causing vibrations and noise. This applies to spiral-wound air ducts, they have high density and stiffness.

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Calculation of airway dimensions

First you need to determine the amount of supply or exhaust air, which must be delivered through the channel into the room. When this value is known, the cross-sectional area (m 2) is calculated by the formula:

In this formula:

• θ - air velocity in the channel, m / s;
• L - air consumption, m 3 / h;
• S is the cross-sectional area of ​​the channel, m 2;

In order to associate the units of time (seconds and hours), the number 3600 is present in the calculation.

The diameter of the circular duct in meters can be calculated from the area of ​​its cross-section by the formula:

S = π D 2/4, D 2 = 4S / π, where D is the diameter of the channel, m.

The procedure for calculating the size of the air duct is as follows:

1. Knowing the air flow in this area, determine the speed of its movement, depending on the purpose of the channel. As an example, we can take L = 10 000 m 3 / h and a speed of 8 m / s, since the branch line is a main line.
2. Calculate the cross-sectional area: 10 000/3600 x 8 = 0.347 m 2, the diameter will be 0.665 m.
3. On the normal take the nearest of two sizes, usually take the one that is larger. Next to 665 mm there are diameters of 630 mm and 710 mm, it should take 710 mm.
4. In the reverse order, the actual velocity of the air mixture in the air duct is calculated to further determine the fan output. In this case, the cross section will be: (3.14 x 0.71 2/4) = 0.4 m 2, and the real speed is 10 000/3600 x 0.4 = 6.95 m / s.
5. In the event that it is necessary to lay a channel of rectangular shape, its dimensions are selected according to the calculated cross-sectional area equivalent to the round one. That is, calculate the width and height of the pipeline so that the area is 0.347 m 2 in this case. It can be an option of 700 mm x 500 mm or 650 mm x 550 mm. Such air ducts are installed in cramped conditions, when the space for laying is limited by technological equipment or other engineering networks.

The parameters of microclimate indicators are determined by the provisions of GOST 12.1.2.1002-00, 30494-96, SanPin 2.2.4.548, 2.1.2.1002-00. Based on the existing government regulations, the Code of Practice SP 60.13330.2012 was developed. The speed of air must ensure that the existing norms are met.

What is taken into account in determining the air velocity

For correct execution of calculations, designers must fulfill several regulated conditions, each of them is of equally important importance. Which parameters depend on the speed of the air flow?

Noise level in the room

Depending on the specific use of the premises, sanitary standards set the following maximum sound pressure levels.

Table 1. Maximum values ​​of noise level.

Exceeding parameters is allowed only in the short-time mode during start / stop ventilation system  or additional equipment.
The level of vibration in the roomDuring operation of the fans, vibration is produced. Vibration indicators depend on the material of the manufacture of air ducts, the methods and quality of vibration damping gaskets and the speed of air flow through the air ducts. General vibration indicators can not exceed the limits set by state organizations.

Table 2. Maximum values ​​of permissible vibration.

In the calculations, the optimal air speed is selected, which does not enhance the vibration processes and associated sound oscillations. The ventilation system must maintain a certain microclimate in the premises.

The values ​​for the flow velocity, humidity and temperature are given in the table.

Table 3. Microclimate parameters.

Another indicator taken into account during the calculation of the flow velocity is the frequency of air exchange in ventilation systems. In view of their use, sanitary standards establish the following requirements for air exchange.

Table 4. Multiplicity of air exchange in various rooms.

Algorithm of calculationsThe air velocity in the air duct is determined taking into account all the above conditions, technical data are specified by the customer in the design and installation of ventilation systems. The main criterion for calculating the flow velocity is the multiplicity of the exchange. All further approvals are made by changing the shape and cross-section of the air ducts. The flow rate can be taken from the table depending on the speed and diameter of the duct.

Table 5. Air consumption, depending on the flow velocity and the diameter of the duct.

Self-calculation

For example, in a room with a volume of 20 m 3 according to the requirements health standards  For effective ventilation it is necessary to provide a three-time change of air. This means that at least one hour through the duct must pass at least L = 20 m 3 × 3 = 60 m 3. The formula for calculating the flow velocity is V = L / 3600 × S, where:

V - velocity of air flow in m / s;

L - air flow in m 3 / h;

S is the cross-sectional area of ​​ducts in m 2.

Take a circular air duct Ø 400 mm, the cross-sectional area is:

In our example, S = (3.14 × 0.4 2 m) / 4 = 0.1256 m 2. Accordingly, to ensure the necessary multiplicity of air exchange (60 m 3 / h) in a circular air duct Ø 400 mm (S = 0.1256 m 3), the airflow velocity is: V = 60 / (3600 × 0.1256) ≈ 0.13 m / s.

With the help of the same formula, at a predetermined speed, it is possible to calculate the volume of air moving along the ducts per unit time.

L = 3600 × S (m 3) × V (m / s). The volume (consumption) is obtained in square meters.

As already described before, the noise levels of ventilation systems depend on air speed. To minimize the negative impact of this phenomenon, engineers made calculations of the maximum permissible air velocities for different rooms.

The same algorithm determines the air velocity in the duct when calculating the heat supply, sets the tolerances to minimize losses for building maintenance during the winter period, and selects the fans in terms of power. Air flow data are also required to reduce pressure loss, and this allows to increase the efficiency of ventilation systems and reduces the consumption of electrical energy.

The calculation is carried out for each individual section, taking into account the obtained data, the parameters of the main lines for diameter and geometry are selected. They must be able to pass the evacuated air from all the individual rooms. The diameter of the air ducts is chosen in such a way as to minimize noise and resistance losses. For calculating the kinematic scheme, all three parameters of the ventilation system are important: the maximum volume of pumped / discharged air, the speed of movement air masses  and the diameter of the air ducts. Work on the calculation of ventilation systems are classified as difficult from the engineering point of view, they can be performed only by professional specialists with special education.

To ensure constant values ​​of air velocity in channels with different cross-sections, the following formulas are used:

After calculation for final data, the nearest values ​​of standard pipelines are taken. Due to this, the time for mounting the equipment is reduced and the process of its periodic maintenance and repair is simplified. Another plus is a reduction in the estimated cost of the ventilation system.

For air heating of residential and production facilities  speed are adjusted taking into account the temperature of the coolant at the inlet and outlet, to uniformly dissipate the flow warm air  The scheme of installation and the sizes of ventilating lattices is thought over. Modern systems  air heating automatic adjustment  speed and direction of flows. The air temperature can not exceed + 50 ° C at the outlet, the distance to the workplace is not less than 1.5 m. The speed of air masses is regulated by the current state standards and industry acts.

During the calculations, at the request of customers, the possibility of installing additional branches can be taken into account, for this purpose, a stock of equipment productivity and channel capacity is provided. Flow rates are calculated in such a way that, after increasing the capacity of the ventilation systems, they do not create additional sound load on the people present in the room.

The choice of diameters is made from the minimum acceptable, the smaller the dimensions - the universal ventilation system, the cheaper it is to manufacture and install it. Local suction systems are calculated separately, they can work both in stand-alone mode and can be connected to existing ventilation systems.

State regulatory documents set the recommended speed of movement, depending on the location and destination of air ducts. When calculating, you must adhere to these parameters.

Inside the premises, air can not move at a speed of more than 0.3 m / s, a short-term exceeding of the parameter is not more than 30%. If there are two systems in the room, the air speed in each of them must provide at least 50% of the calculated volume of supply or removal of air.

Firefighters put forward their demands on the speed of movement of air masses in air ducts, depending on the category of premises and features technological process. The standards are aimed at reducing the speed of smoke or fire spreading through the air ducts. If necessary, valves and cut-offs must be installed on the ventilation systems. The devices are triggered after the sensor signal or manually by the responsible person. In a single ventilation system, only certain groups of rooms can be connected.

In a cold period of time in heated buildings, the air temperature as a result of the operation of the ventilation system can not be lower than the normalized. Normalized temperature is provided before the start of the work shift. In a warm period these requirements are not relevant. The movement of air masses should not impair the standards stipulated by SanPin 2.1.2.2645. To achieve the desired results, during the design of systems, the diameter of the ducts, the power and the number of fans and the flow rate change.

Accepted design data on the parameters of movement in ducts should provide:

1. Performance of microclimate parameters in premises, support of air quality within the regulated limits. At the same time, measures are taken to reduce unproductive heat losses. Data is taken from both existing normative documents, and from the technical task of customers.
2. The speed of movement of air masses in working areas should not cause drafts, provide an acceptable comfort of stay in the room. Mechanical ventilation is provided only in cases where it is impossible to achieve the desired results at the expense of the natural. In addition, mechanical ventilation must be installed in workshops with harmful working conditions.

During the calculation of the air flow in systems with natural ventilation  the average annual value of the difference between the density of the internal and external air is taken. The minimum actual performance data should provide acceptable standard values ​​for the air exchange rate.