Creating comfortable conditions for staying indoors is impossible without aerodynamic calculation of air ducts. Based on the data obtained, the diameter of the pipe section, the power of the fans, the number and characteristics of the branches are determined. Additionally, the power of the heaters, the parameters of the inlet and outlet openings can be calculated. Depending on the specific purpose of the rooms, the maximum allowable noise level, the frequency of air exchange, the direction and speed of flows in the room are taken into account.
Modern requirements for are prescribed in the Code of Rules SP 60.13330.2012. The normalized parameters of microclimate indicators in rooms for various purposes are given in GOST 30494, SanPiN 2.1.3.2630, SanPiN 2.4.1.1249 and SanPiN 2.1.2.2645. During the calculation of indicators ventilation systems all provisions must be taken into account without fail.
Aerodynamic calculation of air ducts - algorithm of actions
The work includes several successive stages, each of which solves local problems. The data obtained is formatted in the form of tables, on the basis of which schematic diagrams and graphs are drawn up. The work is divided into the following stages:
- Development of an axonometric diagram of air distribution throughout the system. On the basis of the scheme, a specific calculation method is determined, taking into account the features and tasks of the ventilation system.
- An aerodynamic calculation of air ducts is performed both along the main lines and along all branches.
- Based on the data obtained, a geometric shape and cross-sectional area of air ducts are determined technical specifications fans and heaters. Additionally, the possibility of installing fire extinguishing sensors, preventing the spread of smoke, the possibility of automatically adjusting the ventilation power, taking into account the program compiled by users, is taken into account.
Development of a ventilation system scheme
Depending on the linear parameters of the scheme, the scale is selected, the spatial position of the air ducts, the points of attachment of additional technical devices, existing branches, places of supply and air intake.
The diagram indicates the main highway, its location and parameters, connection points and specifications branches. Features of the location of the air ducts take into account the architectural characteristics of the premises and the building as a whole. While compiling supply scheme the calculation procedure starts from the point farthest from the fan or from the room for which it is required to provide the maximum air exchange rate. While compiling exhaust ventilation the main criterion is the maximum values for the air flow rate. The common line during calculations is divided into separate sections, while each section must have the same cross-sections of air ducts, stable air consumption, the same materials of manufacture and pipe geometry.
The sections are numbered in sequence from the section with the lowest flow rate and ascending to the highest. Next, the actual length of each individual section is determined, the individual sections are summed up and the total length of the ventilation system is determined.
When planning the ventilation scheme, they can be taken as common for such premises:
- residential or public in any combination;
- industrial, if they belong to group A or B according to the fire category and are located on no more than three floors;
- one of the categories industrial buildings categories B1 - B4;
- categories of industrial buildings B1 and B2 can be connected to one ventilation system in any combination.
If the ventilation systems completely lack the possibility of natural ventilation, then the scheme should provide for the mandatory connection of emergency equipment. The power and installation location of additional fans are calculated according to general rules. For premises with openings that are constantly open or open if necessary, the scheme can be drawn up without the possibility of a backup emergency connection.
Systems for exhausting polluted air directly from technological or working areas must have one backup fan; the device can be put into operation automatically or manually. The requirements apply to working areas of the 1st and 2nd hazard classes. It is allowed not to provide a backup fan on the installation diagram only in the following cases:
- Synchronous stop harmful production processes in case of malfunction of the ventilation system.
- IN industrial premises separate emergency ventilation with its own air ducts. The parameters of such ventilation must remove at least 10% of the volume of air provided by stationary systems.
The ventilation scheme should provide for a separate possibility of showering on workplace with high levels of air pollution. All sections and connection points are indicated on the diagram and are included in the general calculation algorithm.
It is forbidden to place air intake devices closer than eight meters horizontally from garbage dumps, car parking places, roads with heavy traffic, exhaust pipes and chimneys. Reception air devices subject to protection special devices from the windward side. Resistance values of protective devices are taken into account during aerodynamic calculations common system ventilation.
Air flow pressure loss calculation Aerodynamic calculation of air ducts for air losses is done in order to right choice sections to ensure the technical requirements of the system and the choice of fan power. Losses are determined by the formula:
R yd - the value of specific pressure losses in all sections of the duct;
P gr – gravitational air pressure in vertical channels;
Σ l - the sum of the individual sections of the ventilation system.
The pressure loss is given in Pa, the length of the sections is determined in meters. If the movement of air flows in ventilation systems occurs due to the natural pressure difference, then the calculated pressure drop Σ = (Rln + Z) for each individual section. To calculate the gravitational pressure, you need to use the formula:
P gr – gravitational pressure, Pa;
h is the height of the air column, m;
ρ n - air density outside the room, kg / m 3;
ρ in - air density inside the room, kg / m 3.
Further calculations for systems natural ventilation are performed according to the formulas:
Definition cross section air ducts
Determination of the speed of movement of air masses in gas ducts
Calculation for losses due to local resistances of the ventilation system
Determination of loss to overcome friction
Determination of the air flow velocity in the channels
The calculation begins with the most extended and remote section of the ventilation system. As a result of aerodynamic calculations of air ducts, the required mode of ventilation in the room should be provided.
The cross-sectional area is determined by the formula:
F P = L P / V T .
F P - cross-sectional area of the air channel;
L P is the actual air flow in the calculated section of the ventilation system;
V T - the speed of movement of air flows to ensure the required frequency of air exchange in the required volume.
Taking into account the results obtained, the pressure loss is determined during the forced movement of air masses through the air ducts.
Correction coefficients are applied for each material for the manufacture of air ducts, depending on the indicators of surface roughness and the speed of movement of air flows. Tables can be used to facilitate aerodynamic calculations of air ducts.
Tab. No. 1. Calculation metal air ducts round profile.
Table number 2. Values of correction factors taking into account the material of manufacture of air ducts and the speed of the air flow.
The roughness coefficients used for calculations for each material depend not only on its physical characteristics, but also on the speed of air flow. The faster the air moves, the more resistance it experiences. This feature must be taken into account during the selection of a specific coefficient.
Aerodynamic calculation for air flow in square and round ducts shows various indicators speed of movement of the flow at the same cross-sectional area of the conditional passage. This is explained by differences in the nature of vortices, their significance and ability to resist movement.
The main condition for the calculations is that the air velocity constantly increases as the area approaches the fan. With this in mind, requirements are imposed on the diameters of the channels. In this case, the parameters of air exchange in the premises must be taken into account. The locations of the inflow and outlet of the flows are selected in such a way that people staying in the room do not feel drafts. If a direct section fails to achieve a regulated result, then diaphragms are inserted into the air ducts with through holes. By changing the diameter of the holes, an optimal adjustment of the air flows is achieved. Diaphragm resistance is calculated by the formula:
The overall calculation of ventilation systems should take into account:
- Dynamic pressure of the air flow during movement. The data are consistent with the terms of reference and serve as the main criterion during the selection of a particular fan, its location and principle of operation. If it is impossible to provide the planned modes of operation of the ventilation system with one unit, several units are installed. The specific location of their installation depends on the features circuit diagram air ducts and allowable parameters.
- The volume (flow rate) of air masses moved in the context of each branch and room per unit of time. Initial data - the requirements of the sanitary authorities for the cleanliness of the premises and features technological process industrial enterprises.
- Inevitable pressure losses resulting from vortex phenomena during the movement of air streams at different speeds. In addition to this parameter, the actual cross section of the duct and its geometric shape are taken into account.
- Optimum speed of air movement in the main channel and separately for each branch. The indicator affects the choice of fan power and their installation locations.
To facilitate the production of calculations, it is allowed to use a simplified scheme; it is used for all rooms with non-critical requirements. To guarantee desired parameters selection of fans by power and quantity is done with a margin of up to 15%. A simplified aerodynamic calculation of ventilation systems is performed according to the following algorithm:
- Determination of the cross-sectional area of the channel depending on the optimal speed of the air flow.
- Selection of a standard channel section close to the calculated one. Specific indicators should always be selected upwards. Air ducts may have increased technical indicators, it is prohibited to reduce their capabilities. If it is impossible to select standard channels in specifications their production according to individual sketches is envisaged.
- Checking the indicators of air movement speed, taking into account the actual values of the nominal section of the main channel and all branches.
The task of aerodynamic calculation of air ducts is to provide the planned indicators of ventilation of premises with minimal loss of financial resources. At the same time, it is necessary to achieve a reduction in the labor intensity and metal consumption of construction and installation works, ensuring the reliability of operation installed equipment in various modes.
Special equipment must be mounted in accessible places, it must be freely accessible for routine technical inspections and other work to maintain the system in working condition.
According to the provisions of GOST R EN 13779-2007 for calculating the ventilation efficiency ε v you need to apply the formula:
with EHA- indicators of the concentration of harmful compounds and suspended solids in the exhaust air;
With IDA- the concentration of harmful chemical compounds and suspended solids in the room or working area;
c sup- indicators of pollution coming from the supply air.
The efficiency of ventilation systems depends not only on the power of the connected exhaust or blowing devices, but also on the location of air pollution sources. During the aerodynamic calculation, minimum performance indicators for the system should be taken into account.
Specific power (P Sfp > W∙s / m 3) of fans is calculated by the formula:
de R - power electric motor installed on the fan, W;
q v - air flow rate supplied by fans during optimal operation, m 3 / s;
∆ p is an indicator of the pressure drop at the inlet and outlet of air from the fan;
η tot is the overall efficiency for the electric motor, air fan and air ducts.
During calculations, the following types of air flows are taken into account according to the numbering on the diagram:
Scheme 1. Types of air flows in the ventilation system.
- External, enters the air conditioning system from the external environment.
- Supply. Air flows into the duct system after pre-training(heating or cleaning).
- The air in the room.
- flowing air currents. Air moving from one room to another.
- Exhaust. Air vented from a room to the outside or into a system.
- Recirculation. Part of the flow returned to the system to maintain the internal temperature at setpoints.
- Removable. Air that is expelled from the premises irrevocably.
- secondary air. Returns back to the room after cleaning, heating, cooling, etc.
- Air loss. Possible leaks due to leaky air duct connections.
- Infiltration. The process of entering the air into the premises in a natural way.
- Exfiltration. Natural air leakage from the room.
- Air mixture. Simultaneous suppression of several streams.
Each type of air has its own state standards. All calculations of ventilation systems must take them into account.
After choosing the diameter or cross-sectional dimensions, the air speed is specified: , m/s, where f f is the actual cross-sectional area, m 2 . For round ducts 
, for square 
, for rectangular m 2 . In addition, for rectangular ducts, the equivalent diameter, mm, is calculated. Square has an equivalent diameter equal to side square.
You can also use the approximate formula 
. Its error does not exceed 3–5%, which is sufficient for engineering calculations. The total friction pressure loss for the entire section Rl, Pa, is obtained by multiplying the specific losses R by the length of the section l. If air ducts or channels from other materials are used, it is necessary to introduce a correction for roughness βsh. It depends on the absolute equivalent roughness of the duct material K e and the value of v f.
Absolute equivalent roughness of air duct material:
Correction values β w:
| V f, m/s | β w at values of K e, mm | |||
| 1.5 | ||||
| 1.32 | 1.43 | 1.77 | 2.2 | |
| 1.37 | 1.49 | 1.86 | 2.32 | |
| 1.41 | 1.54 | 1.93 | 2.41 | |
| 1.44 | 1.58 | 1.98 | 2.48 | |
| 1.47 | 1.61 | 2.03 | 2.54 |
For steel and vinyl ducts, βsh = 1. More detailed values of βsh can be found in table 22.12. With this correction taken into account, the adjusted friction pressure losses Rlβ sh, Pa, are obtained by multiplying Rl by the value β sh.
Then the dynamic pressure in the section is determined, Pa. Here ρ in is the density of the transported air, kg / m 3. Usually take ρ in \u003d 1.2 kg / m 3.
The column “local resistances” contains the names of resistances (elbow, tee, cross, elbow, grate, ceiling, umbrella, etc.) available in this area. In addition, their number and characteristics are noted, according to which the CMR values are determined for these elements. For example, for a round bend it is the angle of rotation and the ratio of the radius of rotation to the diameter of the duct r/d, for a rectangular bend it is the angle of rotation and the dimensions of the sides of the duct a and b. For side openings in an air duct or duct (for example, at the installation site of an air intake grille) - the ratio of the opening area to the cross section of the air duct f resp / f o. For tees and crosses on the passage, the ratio of the cross-sectional area of the passage and the trunk f p / f s and the flow rate in the branch and in the trunk L o / L s is taken into account, for tees and crosses on the branch - the ratio of the cross-sectional area of the branch and the trunk f p / f s and again, the value of L o /L s. It should be borne in mind that each tee or cross connects two adjacent sections, but they refer to one of these sections, in which the air flow L is less. The difference between tees and crosses on a run and on a branch has to do with how the design direction runs. This is shown in the following figure.
Here, the calculated direction is depicted by a thick line, and the directions of air flows are shown by thin arrows. In addition, it is signed exactly where in each option the trunk, passage and branch of the tee are located for the correct choice of the ratios f p / f s, f o / f s and L o / L s. Note that in supply systems, the calculation is usually carried out against the movement of air, and in exhaust systems, along this movement. The sections to which the considered tees belong are indicated by checkmarks. The same applies to crosses. As a rule, although not always, tees and crosses on the passage appear when calculating the main direction, and on the branch they appear when aerodynamic linking of secondary sections (see below). In this case, the same tee in the main direction can be considered as a tee per passage, and in the secondary direction - as a branch with a different coefficient.
Approximate values of ξ for common resistances are given below. Grilles and shades are taken into account only at the end sections. The coefficients for crosses are taken in the same size as for the corresponding tees.
Values ξ of some local resistances.
| Name of resistance | KMS (ξ) | Name of resistance | KMS (ξ) |
| Elbow round 90 o, r/d = 1 | 0.21 | Grille unregulated RS-G (exhaust or air intake) | 2.9 |
| Rectangular elbow 90 o | 0.3 … 0.6 | ||
| Tee in the passage (injection) | 0.25 … 0.4 | sudden expansion | |
| Branch tee (pressure) | 0.65 … 1.9 | sudden constriction | 0.5 |
| Tee in the passage (suction) | 0.5 … 1 | First side opening (inlet to the air intake shaft) | 2.5 … 4.5 |
| Branch tee (suction) | –0.5 * … 0.25 | ||
| Plafond (anemostat) ST-KR,ST-KV | 5.6 | Rectangular elbow 90 o | 1.2 |
| Adjustable grate RS-VG (supply) | 3.8 | Umbrella over the exhaust shaft | 1.3 |
*) negative CMR can occur at small L o /L s due to the ejection (suction) of air from the branch by the main flow.
More detailed data for the CCM are indicated in tables 22.16 - 22.43. After determining the value of Σξ, the pressure losses at local resistances , Pa, and the total pressure losses at the section Rlβ w + Z, Pa are calculated. When the calculation of all sections of the main direction is completed, the values of Rlβ w + Z for them are summed up and the total resistance of the ventilation network ΔР of the network = Σ(Rlβ w + Z) is determined. The ΔР value of the network serves as one of the initial data for fan selection. After selecting a fan in the supply system, an acoustic calculation of the ventilation network is made (see chapter 12) and, if necessary, a silencer is selected.
The calculation results are entered in the table in the following form.
After calculating the main direction, one or two branches are linked. If the system serves several floors, you can select floor branches on intermediate floors for linking. If the system serves one floor, branches from the main that are not included in the main direction are linked (see the example in clause 2.3). The calculation of linked sections is carried out in the same sequence as for the main direction, and is recorded in the table in the same form. Linking is considered completed if the sum of pressure losses Σ(Rlβ w + Z) along the linked sections deviates from the sum Σ(Rlβ w + Z) along the parallel connected sections of the main direction by no more than ±10%. Sections along the main and linked directions from the point of their branching to the end air distributors are considered to be connected in parallel. If the circuit looks like the one shown in the following figure (the main direction is highlighted with a thick line), then direction 2 alignment requires that the value of Rlβ w + Z for section 2 is equal to Rlβ w + Z for section 1, obtained from the calculation of the main direction, with an accuracy ±10%.
|
Purpose |
Basic requirement | ||||
| Noiselessness | Min. head loss | ||||
| Main channels | main channels | Branches | |||
| tributary | Hood | tributary | Hood | ||
| Living spaces | 3 | 5 | 4 | 3 | 3 |
| Hotels | 5 | 7.5 | 6.5 | 6 | 5 |
| Institutions | 6 | 8 | 6.5 | 6 | 5 |
| Restaurants | 7 | 9 | 7 | 7 | 6 |
| The shops | 8 | 9 | 7 | 7 | 6 |
Based on these values, the linear parameters of the air ducts should be calculated.
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, losses are determined using the formula P = L × R + Z, where P are losses air pressure in the design section, R - losses per linear meter of the section, L - 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 ducts, then to translate round values in squares, use table No. 2.
Tab. No. 2. Equivalent diameters of round ducts for square
| 150 | 200 | 250 | 300 | 350 | 400 | 450 | 500 | |
| 250 | 210 | 245 | 275 | |||||
| 300 | 230 | 265 | 300 | 330 | ||||
| 350 | 245 | 285 | 325 | 355 | 380 | |||
| 400 | 260 | 305 | 345 | 370 | 410 | 440 | ||
| 450 | 275 | 320 | 365 | 400 | 435 | 465 | 490 | |
| 500 | 290 | 340 | 380 | 425 | 455 | 490 | 520 | 545 |
| 550 | 300 | 350 | 400 | 440 | 475 | 515 | 545 | 575 |
| 600 | 310 | 365 | 415 | 460 | 495 | 535 | 565 | 600 |
| 650 | 320 | 380 | 430 | 475 | 515 | 555 | 590 | 625 |
| 700 | 390 | 445 | 490 | 535 | 575 | 610 | 645 | |
| 750 | 400 | 455 | 505 | 550 | 590 | 630 | 665 | |
| 800 | 415 | 470 | 520 | 565 | 610 | 650 | 685 | |
| 850 | 480 | 535 | 580 | 625 | 670 | 710 | ||
| 900 | 495 | 550 | 600 | 645 | 685 | 725 | ||
| 950 | 505 | 560 | 615 | 660 | 705 | 745 | ||
| 1000 | 520 | 575 | 625 | 675 | 720 | 760 | ||
| 1200 | 620 | 680 | 730 | 780 | 830 | |||
| 1400 | 725 | 780 | 835 | 880 | ||||
| 1600 | 830 | 885 | 940 | |||||
| 1800 | 870 | 935 | 990 |
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 general diagram loss in a straight line.
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 It is recommended that the difference in pressure loss between individual sections should 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.
With this material, the editors of the journal “Climate World” continue to publish chapters from the book “Ventilation and air conditioning systems. Design recommendations for
water and public buildings.” Author Krasnov Yu.S.
Aerodynamic calculation of air ducts begins with drawing an axonometric diagram (M 1: 100), putting down the numbers of sections, their loads L (m 3 / h) and lengths I (m). The direction of the aerodynamic calculation is determined - from the most remote and loaded section to the fan. When in doubt, when determining the direction, all possible options are calculated.
The calculation starts from a remote section: the diameter D (m) of a round or the area F (m 2) of the cross section of a rectangular duct is determined:
The speed increases as you get closer to the fan.
According to Appendix H, the nearest standard values are taken from: D CT or (a x b) st (m).
Hydraulic radius of rectangular ducts (m):
where is the sum of the local resistance coefficients in the duct section.
Local resistances at the border of two sections (tees, crosses) are attributed to the section with a lower flow rate.
Local resistance coefficients are given in the appendices.
Scheme of the supply ventilation system serving the 3-storey administrative building
Calculation example
Initial data:
| No. of plots | supply L, m 3 / h | length L, m | υ rivers, m/s | section a × b, m |
υ f, m/s | D l ,m | Re | λ | kmc | losses in the section Δр, pa |
| outlet grating pp | 0.2 × 0.4 | 3,1 | - | - | - | 1,8 | 10,4 | |||
| 1 | 720 | 4,2 | 4 | 0.2 × 0.25 | 4,0 | 0,222 | 56900 | 0,0205 | 0,48 | 8,4 |
| 2 | 1030 | 3,0 | 5 | 0.25×0.25 | 4,6 | 0,25 | 73700 | 0,0195 | 0,4 | 8,1 |
| 3 | 2130 | 2,7 | 6 | 0.4×0.25 | 5,92 | 0,308 | 116900 | 0,0180 | 0,48 | 13,4 |
| 4 | 3480 | 14,8 | 7 | 0.4×0.4 | 6,04 | 0,40 | 154900 | 0,0172 | 1,44 | 45,5 |
| 5 | 6830 | 1,2 | 8 | 0.5×0.5 | 7,6 | 0,50 | 234000 | 0,0159 | 0,2 | 8,3 |
| 6 | 10420 | 6,4 | 10 | 0.6×0.5 | 9,65 | 0,545 | 337000 | 0,0151 | 0,64 | 45,7 |
| 6a | 10420 | 0,8 | Yu. | Ø0.64 | 8,99 | 0,64 | 369000 | 0,0149 | 0 | 0,9 |
| 7 | 10420 | 3,2 | 5 | 0.53×1.06 | 5,15 | 0,707 | 234000 | 0.0312×n | 2,5 | 44,2 |
| Total losses: 185 | ||||||||||
| Table 1. Aerodynamic calculation | ||||||||||
The air ducts are made of galvanized sheet steel, the thickness and dimensions of which correspond to app. N out. The material of the air intake shaft is brick. Adjustable gratings of the PP type with possible sections are used as air distributors: 100 x 200; 200 x 200; 400 x 200 and 600 x 200 mm, shade factor 0.8 and maximum outlet air velocity up to 3 m/s.
The resistance of the insulated intake valve with fully open blades is 10 Pa. The hydraulic resistance of the air heater installation is 100 Pa (according to a separate calculation). Filter resistance G-4 250 Pa. Silencer hydraulic resistance 36 Pa (according to acoustic calculation). Based on architectural requirements, rectangular ducts are designed.
Cross-sections of brick channels are taken according to Table. 22.7.
Local resistance coefficients
Section 1. RR grating at the exit with a section of 200 × 400 mm (calculated separately):
| No. of plots | Type of local resistance | Sketch | Angle α, deg. | Attitude | Rationale | KMS | ||
| F0/F1 | L 0 /L st | f pass / f st | ||||||
| 1 | Diffuser |
 |
20 | 0,62 | - | - | Tab. 25.1 | 0,09 |
| Withdrawal |
 |
90 | - | - | - | Tab. 25.11 | 0,19 | |
| Tee-pass |
 |
- | - | 0,3 | 0,8 | App. 25.8 | 0,2 | |
| ∑ = | 0,48 | |||||||
| 2 | Tee-pass |
 |
- | - | 0,48 | 0,63 | App. 25.8 | 0,4 |
| 3 | branch tee |
 |
- | 0,63 | 0,61 | - | App. 25.9 | 0,48 |
| 4 | 2 outlets | 250×400 | 90 | - | - | - | App. 25.11 | |
| Withdrawal | 400×250 | 90 | - | - | - | App. 25.11 | 0,22 | |
| Tee-pass |
 |
- | - | 0,49 | 0,64 | Tab. 25.8 | 0,4 | |
| ∑ = | 1,44 | |||||||
| 5 | Tee-pass |
 |
- | - | 0,34 | 0,83 | App. 25.8 | 0,2 |
| 6 | Diffuser after fan |
 |
h=0.6 | 1,53 | - | - | App. 25.13 | 0,14 |
| Withdrawal | 600×500 | 90 | - | - | - | App. 25.11 | 0,5 | |
| ∑= | 0,64 | |||||||
| 6a | Confuser in front of the fan |
 |
D g \u003d 0.42 m | Tab. 25.12 | 0 | |||
| 7 | Knee | 90 | - | - | - | Tab. 25.1 | 1,2 | |
| Louvre grille | Tab. 25.1 | 1,3 | ||||||
| ∑ = | 1,44 | |||||||
| Table 2. Determination of local resistances | ||||||||
Krasnov Yu.S.,
1. Friction loss:
Ptr \u003d (x * l / d) * (v * v * y) / 2g,
z = Q* (v*v*y)/2g,
Permissible speed method
Note: the air flow rate in the table is given in meters per second
Using Rectangular Ducts
The head loss diagram shows the diameters of round ducts. If rectangular ducts are used instead, find their equivalent diameters using the table below.
Notes:
- If there is not enough space (for example, during reconstruction), choose rectangular ducts. As a rule, the width of the duct is 2 times the height).
Table of equivalent duct diameters
When the parameters of the air ducts are known (their length, cross section, air friction coefficient on the surface), it is possible to calculate the pressure loss in the system at the projected air flow.
The total pressure loss (in kg/sq.m.) is calculated using the formula:
where R is the friction pressure loss per 1 running meter air duct, l - air duct length in meters, z - pressure loss due to local resistances (with a variable section).
1. Friction loss:
In a round duct, friction pressure losses P tr are calculated as follows:
Ptr \u003d (x * l / d) * (v * v * y) / 2g,
where x is the coefficient of friction resistance, l is the length of the duct in meters, d is the diameter of the duct in meters, v is the air flow velocity in m/s, y is the air density in kg/m3, g is the acceleration free fall(9.8 m/s2).
Note: If the air duct has not a round, but a rectangular cross section, the equivalent diameter must be substituted into the formula, which for an air duct with sides A and B is equal to: dequiv = 2AB/(A + B)
2. Losses due to local resistance:
Pressure losses due to local resistances are calculated according to the formula:
z = Q* (v*v*y)/2g,
where Q is the sum of the coefficients of local resistances in the section of the duct for which the calculation is made, v is the air flow velocity in m/s, y is the air density in kg/m3, g is the free fall acceleration (9.8 m/s2 ). The Q values are contained in tabular form.
Permissible speed method
When calculating the duct network according to the method of permissible speeds, the initial data are optimum speed air (see table). Then, the required cross-section of the duct and the pressure loss in it are considered.
The procedure for the aerodynamic calculation of air ducts according to the method of permissible speeds:
Draw a diagram of the air distribution system. For each section of the duct, indicate the length and amount of air passing in 1 hour.
We start the calculation from the most distant from the fan and the most loaded sections.
Knowing the optimal air velocity for a given room and the volume of air passing through the duct in 1 hour, we determine the appropriate diameter (or cross section) of the duct.
We calculate the pressure loss due to friction P tr.
According to the tabular data, we determine the sum of local resistances Q and calculate the pressure loss due to local resistances z.
The available pressure for the next branches of the air distribution network is determined as the sum of the pressure losses in the sections located before this branch.
In the process of calculation, it is necessary to sequentially 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 diaphragms. They are installed on lightly loaded sections of air ducts, increasing resistance.
Table top speed air depending on the requirements for the duct
Constant Head Loss Method
This method assumes a constant pressure loss per 1 linear meter of the duct. Based on this, the dimensions of the duct network are determined. The method of constant head loss is quite simple and is used at the stage of the feasibility study of ventilation systems:
Depending on the purpose of the room, according to the table of permissible air velocities, the speed on the main section of the duct is selected.
Based on the speed determined in paragraph 1 and on the basis of the design air flow, the initial pressure loss is found (per 1 m of the duct length). This is the diagram below.
The most loaded branch is determined, and its length is taken as the equivalent length of the air distribution system. Most often this is the distance to the farthest diffuser.
Multiply the equivalent system length by the head loss from step 2. The head loss at the diffusers is added to the value obtained.
Now, according to the diagram below, determine the diameter of the initial duct coming from the fan, and then the diameters of the remaining sections of the network according to the corresponding air flow rates. In this case, the initial pressure loss is assumed to be constant.
Diagram for determining head loss and duct diameter 
The head loss diagram shows the diameters of round ducts. If rectangular ducts are used instead, find their equivalent diameters using the table below.
Notes:
If space permits, it is better to choose round or square ducts;
If there is not enough space (for example, during reconstruction), select rectangular ducts. As a rule, the width of the duct is 2 times the height).
The table shows the height of the duct in mm horizontally, the vertical width, and the cells of the table contain equivalent duct diameters in mm.
Programs can be useful to designers, managers, engineers. Basically, to use the programs it is enough Microsoft Excel. Many authors of programs are not known. I would like to note the work of these people who, based on Excel, were able to prepare such useful calculation programs. Calculation programs for ventilation and air conditioning are free to download. But don't forget! You can not absolutely trust the program, check its data.
Sincerely, site administration
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