Aerodynamic calculation of air ducts of the supply ventilation system. Duct resistance calculation calculator. Duct pressure calculation Duct pressure drop

Ventilation calculation this is the calculation of air ducts and ventilation ducts in systems supply and exhaust ventilation . Ventilation is used to supply and remove air with temperatures up to 80°C. The calculation is made according to the method of specific pressure losses. The total pressure loss, kgf/m², in the duct network for standard air (t = 20°C and γ = 1.2 kg/m³) is determined by the formula:

p =∑(Rl+Z),

where R is the pressure loss due to friction in the calculated segment kgf / m² per 1 m; l is the length of the duct section, m; Z - pressure loss due to local resistances in the calculated segment, kgf / m².

Friction pressure loss R, kgf/m² per 1 m in round air ducts is determined by the formula R= λd v²γ2g, where λ is the coefficient of friction resistance; d is the duct diameter, m; v is the speed of air movement in the duct, m/s; γ - volumetric mass of air moving through the duct, kgf/m³; v²γ / 2g - speed (dynamic) pressure, kgf / m².

The drag coefficient is adopted according to the Altshul formula:

where Δe is the absolute equivalent surface roughness of the air duct made of sheet steel, equal to 0.1 mm; d – duct diameter, mm; Re is the Reynolds number.

For air ducts made of other materials with an absolute equivalent roughness Ke≥0.1 mm, the values ​​of R are taken with a correction factor n for friction pressure losses.

Δe value for other materials:

1. Sheet steel - 0.1mm
2. Viniplast - 0.1mm
3. Asbestos-cement pipes - 0.11mm
4. Brick - 4mm
5. Plaster on the grid - 10mm

Recommended speed of air movement in air ducts with mechanical stimulation. Industrial buildings main air ducts - up to 12 m/s, branch air ducts - 6 m/s. Public buildings main air ducts - up to 8 m/s, branch air ducts - 5 m/s.

In air ducts rectangular section the calculated value d is taken to be the equivalent diameter dev, at which the pressure loss in a round duct at the same air velocity is equal to the loss in a rectangular duct. The values ​​of equivalent diameters, m, are determined by the formula

where A and B are the dimensions of the sides of the rectangular duct. It should be borne in mind that at equal air speed rectangular duct and similar round have different air flow rates. The value of velocity (dynamic) pressure and specific friction pressure losses for round air ducts.

Pressure loss Z, kgf / m², due to local resistances is determined by the formula

Z = ∑ζ(v²γ/2g),

where ∑ζ is the sum of the coefficients of local resistances on the estimated section of the duct. If the temperature of the transported air is not equal to 20°C for pressure losses calculated by the formula p =∑(Rl+Z), it is required to enter correction factors K1 - friction, K2 - local resistance.

If discrepancies in pressure losses along the branches of the air ducts are within 10%, iris dampers should be installed.

The resistance to the passage of air in a ventilation system is mainly determined by the speed of air movement in this system. As the speed increases, so does the resistance. This phenomenon is called pressure loss. The static pressure created by the fan causes the air to move in the ventilation system, which has a certain resistance. The higher the resistance of such a system, the lower the air flow moved or. Calculation of friction losses for air in ducts, as well as resistance network equipment(filter, silencer, heater, valve, etc.) can be produced using the relevant tables and diagrams indicated in the catalog. The total pressure drop can be calculated by summing the resistance values ​​of all elements ventilation system.

Recommended air speed in ducts:

Determining the speed of air movement in the ducts:

V= L / 3600*F (m/s)

where L- air consumption, m 3 / h;
F- cross-sectional area of ​​the channel, m 2.

Recommendation 1.
Pressure loss in a duct system can be reduced by increasing the cross section of the ducts to ensure relatively uniform air velocity throughout the system. In the image we see how it is possible to achieve a relatively uniform air velocity in the duct network with minimal pressure loss.

Recommendation 2
In systems with a long duct length and a large number of ventilation grilles it is advisable to place the fan in the middle of the ventilation system. This solution has several advantages. On the one hand, pressure losses are reduced, and on the other hand, smaller ducts can be used.

An example of calculating the ventilation system:
The calculation must begin with a sketch of the system, indicating the location of the air ducts, ventilation grilles, fans, as well as the lengths of the air duct sections between the tees, then determine the air flow in each section of the network.

Let's find out the pressure loss for sections 1-6, using the graph of pressure loss in round ducts, we will determine the required diameters of the ducts and the pressure loss in them, provided that it is necessary to provide an acceptable air speed.

Plot 1: the air flow will be 220 m 3 /h. We take the diameter of the air duct equal to 200 mm, the speed is 1.95 m / s, the pressure loss will be 0.2 Pa / m x 15 m = 3 Pa (see the diagram for determining pressure losses in air ducts).

Plot 2: let's repeat the same calculations, not forgetting that the air flow through this section will already be 220 + 350 = 570 m 3 / h. We take the diameter of the duct equal to 250 mm, the speed is 3.23 m/s. The pressure loss will be 0.9 Pa / m x 20 m = 18 Pa.

Plot 3: the air flow through this section will be 1070 m 3 / h.
We take the diameter of the duct equal to 315 mm, the speed is 3.82 m/s. The pressure loss will be 1.1 Pa / m x 20 \u003d 22 Pa.

Plot 4: the air flow through this section will be 1570 m 3 /h. We take the diameter of the duct equal to 315 mm, the speed is 5.6 m/s. The pressure loss will be 2.3 Pa x 20 = 46 Pa.

Plot 5: the air flow through this section will be 1570 m 3 / h. We take the diameter of the duct equal to 315 mm, the speed is 5.6 m/s. The pressure loss will be 2.3 Pa / m x 1 \u003d 2.3 Pa.

Plot 6: the air flow through this section will be 1570 m 3 /h. We take the diameter of the duct equal to 315 mm, the speed is 5.6 m/s. The pressure loss will be 2.3 Pa x 10 = 23 Pa. The total pressure loss in the air ducts will be 114.3 Pa.

When the calculation of the last section is completed, it is necessary to determine the pressure losses in the network elements: in the silencer СР 315/900 (16 Pa) and in check valve KOM 315 (22 Pa). We also determine the pressure loss in the outlets to the grids (the resistance of the 4 outlets in total will be 8 Pa).

Determination of pressure losses at duct bends

The graph allows you to determine the pressure loss in the outlet, based on the bending angle, diameter and air flow.

Example. Let us determine the pressure loss for a 90° outlet with a diameter of 250 mm at an air flow rate of 500 m3/h. To do this, we find the intersection of the vertical line corresponding to our air flow with an oblique line characterizing a diameter of 250 mm, and on the vertical line on the left for a 90 ° outlet we find the pressure loss, which is 2Pa.

We accept for installation ceiling diffusers of the PF series, the resistance of which, according to the schedule, will be 26 Pa.

Now let's sum up all the pressure losses for straight sections of air ducts, network elements, bends and gratings. The desired value is 186.3 Pa.

We calculated the system and determined that we need a fan that removes 1570 m3 / h of air with a network resistance of 186.3 Pa. Given the characteristics required for the operation of the system, we will be satisfied with the characteristics required for the operation of the system, we will be satisfied with the fan VENTS VKMS 315.

Determination of pressure losses in air ducts.

Determination of pressure losses in the check valve.

Selection of the necessary fan.

Determination of pressure losses in silencers.

Determination of pressure losses on bends of air ducts.

Determination of pressure losses in diffusers.

It is not always possible to invite a specialist to design a system engineering networks. What to do if during the repair or construction of your facility, the calculation of ventilation ducts was required? Is it possible to make it on your own?

The calculation will make it possible to effective system, which will ensure the uninterrupted operation of units, fans and air handling units. If everything is calculated correctly, this will reduce the cost of purchasing materials and equipment, and subsequently on further maintenance of the system.

Calculation of air ducts of the ventilation system for rooms can be carried out by different methods. For example, like this:

• constant pressure loss;
• allowed speeds.

Types and types of air ducts

Before calculating networks, you need to determine what they will be made of. Now products made of steel, plastic, fabric, aluminum foil etc. Air ducts are often made of galvanized or of stainless steel, it can be organized even in a small workshop. Such products are convenient to mount and the calculation of such ventilation does not cause problems.

In addition, air ducts may differ in appearance. They can be square, rectangular and oval. Each type has its own merits.

• Rectangular allow you to make ventilation systems of small height or width, while maintaining the desired cross-sectional area.
• There is less material in round systems,
• Oval combine the pros and cons of other types.

For example, let's choose round pipes from tin. These are products that are used for ventilation of housing, office and retail space. The calculation will be carried out by one of the methods that allows you to accurately select the network of air ducts and find its characteristics.

Method for calculating air ducts by the method of constant speeds

You need to start with a floor plan.

Using all norms determine right amount air into each zone and draw a wiring diagram. It shows all gratings, diffusers, cross-section changes and taps. The calculation is made for the most remote point of the ventilation system, divided into sections limited by branches or gratings.

The calculation of the air duct for installation consists in choosing the desired section along the entire length, as well as finding the pressure loss for selecting a fan or supply unit. The initial data are the values ​​of the amount of passing air in the ventilation network. Using the scheme, we will calculate the diameter of the duct. To do this, you need a pressure loss graph.
For each type of air duct, the schedule is different. Usually, manufacturers provide such information for their products, or you can find it in reference books. Let's calculate round tin air ducts, the graph for which is shown in our figure.

Nomogram for size selection

According to the chosen method, we set the air velocity of each section. It must be within the limits for buildings and premises of the selected purpose. For the main air supply and exhaust ventilation ducts, the following values ​​are recommended:

• living quarters - 3.5–5.0 m/s;
• production - 6.0–11.0 m/s;
• offices - 3.5–6.0 m/s.

For branches:

• offices - 3.0–6.5 m/s;
• living quarters - 3.0–5.0 m/s;
• production - 4.0–9.0 m/s.

When the speed exceeds the permissible level, the noise level rises to an uncomfortable level for a person.

After determining the speed (in the example 4.0 m/s), we find the desired section of the air ducts according to the graph. There are also pressure losses per 1 m of the network, which will be needed for the calculation. The total pressure loss in Pascals is found by multiplying the specific value by the length of the section:

Manual=Man·Man.

Network elements and local resistances

Losses on network elements (lattices, diffusers, tees, turns, changes in section, etc.) are also important. For lattices and some elements, these values ​​are specified in the documentation. They can also be calculated by multiplying the coefficient of local resistance (c.m.s.) by the dynamic pressure in it:

Rm. s.=ζ Rd.

Where Rd=V2 ρ/2 (ρ is the air density).

K. m. s. determined from reference books and factory characteristics of products. We summarize all types of pressure losses for each section and for the entire network. For convenience, we will do this in a tabular way.

The sum of all pressures will be acceptable for this duct network and the branch losses must be within 10% of the total available pressure. If the difference is greater, it is necessary to mount dampers or diaphragms on the outlets. To do this, we calculate the required c.m.s. according to the formula:

ζ= 2Rizb/V2,

where Pizb is the difference between available pressure and branch losses. According to the table, select the diameter of the diaphragm.

The required diameter of the diaphragm for air ducts.

The correct calculation of ventilation ducts will allow you to choose the right fan by choosing from manufacturers according to your criteria. Using the found available pressure and the total air flow in the network, this will be easy to do.

• The performance of a system serving up to 4 rooms.
• Dimensions of air ducts and air distribution grilles.
• Air line resistance.
• Heater power and estimated electricity costs (when using an electric heater).

If you need to choose a model with humidification, cooling or recuperation, use the calculator on the Breezart website.

An example of calculating ventilation using a calculator

In this example, we will show how to calculate supply ventilation for 3 room apartment in which a family of three lives (two adults and a child). During the day, relatives sometimes come to them, so up to 5 people can stay in the living room for a long time. The ceiling height of the apartment is 2.8 meters. Room options:

We will set the consumption rates for the bedroom and the nursery in accordance with the recommendations of SNiP - 60 m³ / h per person. For the living room, we will limit ourselves to 30 m³ / h, since a large number of there are not many people in this room. According to SNiP, such air flow is acceptable for rooms with natural ventilation(you can open a window for ventilation). If we also set an air flow rate of 60 m³/h per person for the living room, then the required performance for this room would be 300 m³/h. The cost of electricity to heat this amount of air would be very high, so we made a compromise between comfort and economy. To calculate the air exchange by the multiplicity for all rooms, we will choose a comfortable double air exchange.

The main air duct will be rectangular rigid, the branches will be flexible and soundproof (this combination of duct types is not the most common, but we chose it for demonstration purposes). For extra cleaning supply air a coal-dust fine filter of the EU5 class will be installed (we will calculate the resistance of the network with dirty filters). Air velocities in ducts and allowable level we leave the noise on the gratings equal to the recommended values, which are set by default.

Let's start the calculation by drawing up a diagram of the air distribution network. This scheme will allow us to determine the length of the ducts and the number of turns that can be both in the horizontal and vertical plane (we need to count all the turns at a right angle). So our schema is:

The resistance of the air distribution network is equal to the resistance of the longest section. This section can be divided into two parts: the main duct and the longest branch. If you have two branches of approximately the same length, then you need to determine which one has more resistance. To do this, we can assume that the resistance of one turn is equal to the resistance of 2.5 meters of the duct, then the branch with the maximum value (2.5 * number of turns + duct length) will have the greatest resistance. It is necessary to select two parts from the route in order to be able to set different type ducts and different air speeds for the main section and branches.

In our system, balancing throttle valves are installed on all branches, allowing you to adjust the air flow in each room in accordance with the project. Their resistance (in the open state) has already been taken into account, since this is a standard element of the ventilation system.

The length of the main air duct (from the air intake grille to the branch to room No. 1) is 15 meters, there are 4 right-angle turns in this section. The length of the air handling unit and air filter can be ignored (their resistance will be taken into account separately), and the silencer resistance can be taken equal to the resistance of an air duct of the same length, that is, simply consider it a part of the main air duct. The longest branch is 7 meters long and has 3 right angle bends (one at the branch, one at the duct and one at the adapter). Thus, we have set all the necessary initial data and now we can proceed to the calculations (screenshot). The calculation results are summarized in tables:

Calculation of the air duct network

• For each room (subsection 1.2), the performance is calculated, the cross-section of the duct is determined, and a suitable duct of standard diameter is selected. According to the Arktos catalog, the dimensions of distribution grids with a given noise level are determined (data for the AMN, ADN, AMR, ADR series are used). You can use other gratings with the same dimensions - in this case, there may be a slight change in the noise level and network resistance. In our case, the grilles for all rooms turned out to be the same, since at a noise level of 25 dB(A) the allowable air flow through them is 180 m³/h (there are no smaller grilles in these series).
• The sum of the air flow rates for all three rooms gives us the total system performance (subsection 1.3). When using a VAV system, the system performance will be one third lower due to the separate adjustment of the air flow in each room. Next, the cross section of the main duct is calculated (in the right column - for VAV systems) and suitable rectangular air ducts are selected (usually several options are given with different aspect ratios). At the end of the section, the resistance of the air duct network is calculated, which turned out to be very large - this is due to the use of a fine filter in the ventilation system, which has a high resistance.
• We have received all the necessary data to complete the air distribution network, with the exception of the size of the main air duct between branches 1 and 3 (this parameter is not calculated in the calculator, since the network configuration is not known in advance). However, the cross-sectional area of ​​this section can be easily calculated manually: from the cross-sectional area of ​​the main duct, you need to subtract the cross-sectional area of ​​\u200b\u200bbranch No. 3. Having obtained the cross-sectional area of ​​\u200b\u200bthe duct, its size can be determined by.

Calculation of heater power and selection of air handling unit

The recommended Breezart 550 Lux model has programmable parameters (capacity and power of the heater), therefore, the performance that should be selected when setting up the remote control is indicated in brackets. It can be seen that the maximum possible power of the heater of this launcher is 11% lower than the calculated value. The lack of power will be noticeable only at outdoor temperatures below -22 ° C, and this does not happen often. In such cases, the air handling unit will automatically switch to a lower speed to maintain the set outlet temperature (Comfort function).

In the calculation results, in addition to the required performance of the ventilation system, the maximum performance of the PU at a given network resistance is indicated. If this performance turns out to be noticeably higher than the required value, you can take advantage of the possibility of programmatically limiting the maximum performance, which is available for all Breezart ventilation units. For a VAV system, the maximum performance is indicated for reference, since its performance is adjusted automatically during the operation of the system.

Calculation of the cost of operation

This section calculates the cost of electricity used to heat the air during the cold season. The costs for a VAV system depend on its configuration and mode of operation, therefore they are taken as an average value: 60% of the costs conventional system ventilation. In our case, you can save money by reducing the air consumption at night in the living room, and during the day in the bedroom.

Based on these values, one should calculate linear parameters air ducts.

Algorithm for calculating air pressure losses

The calculation must begin with drawing up a diagram of the ventilation system with the obligatory indication of the spatial arrangement of the air ducts, the length of each section, ventilation grilles, additional equipment for air purification, technical fittings and fans. Losses are determined first for each individual line, and then summed up. For a separate technological section, the losses are determined using the formula P = L × R + Z, where P is the air pressure loss in the design section, R is the loss in running meter section, L - the total length of the air ducts in the section, Z - losses in the additional fittings of the ventilation system.

To calculate the pressure loss in a circular duct, the formula Ptr is used. = (L/d×X) × (Y×V)/2g. X is the tabular coefficient of air friction, depends on the material of manufacture of the air duct, L is the length of the calculated section, d is the diameter of the air duct, V is the required air flow rate, Y is the air density, taking into account temperature, g is the acceleration of fall (free). If the ventilation system has square air ducts, then table No. 2 should be used to convert round values ​​​​to square ones.

Tab. No. 2. Equivalent diameters of round ducts for square

The horizontal is the height of the square duct, and the vertical is the width. Equivalent value round section is at the intersection of the lines.

Air pressure losses in bends are taken from table No. 3.

Tab. No. 3. Loss of pressure on bends

To determine the pressure loss in the diffusers, the data from Table No. 4 are used.

Tab. No. 4. Pressure loss in diffusers

Table No. 5 gives a general diagram of losses in a straight section.

Tab. No. 5. Diagram of air pressure losses in straight air ducts

All individual losses in a given section of the duct are summarized and corrected with Table No. 6. Tab. No. 6. Calculation of the flow pressure drop in ventilation systems

During design and calculations, existing regulations recommend that the difference in pressure loss between separate sections did not exceed 10%. The fan should be installed in the section of the ventilation system with the highest resistance, the most distant air ducts should have the minimum resistance. If these conditions are not met, then it is necessary to change the layout of air ducts and additional equipment, taking into account the requirements of the regulations.