The 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). Determine the direction of the aerodynamic calculation - from the most distant and loaded area to the fan. When in doubt when determining a direction, consider all possible options.

The calculation begins with a remote area: determine the diameter D (m) of the round or the area F (m 2) cross section rectangular duct:

Table. Required hourly consumption fresh air, m 3 /h (cfm)

According to Appendix H, the nearest standard values ​​are taken: D st or (a x b) st (m).

Actual speed (m/s): or
Hydraulic radius rectangular air ducts(m):

Reynolds criterion: Re = 64100 x D st x U fact (for rectangular ducts D st = D L).

Hydraulic friction coefficient: λ = 0.3164 x Re - 0.25 at Re ≤ 60000, λ = 0.1266 x Re - 0.167 at Re Pressure loss in the design area (Pa): where is the sum of the local resistance coefficients in the air duct section.

Local resistances at the border of two sections (tees, crosses) are assigned to the section with lower flow rate. Local resistance coefficients are given in the appendices.

Diagram of the supply ventilation system serving a 3-story administrative building.

Table 1. Aerodynamic calculation

No. of plots	flow L, m 3 / h	length L, m	U re k, m/s	section a x b, m	U f, m/s	D l , m	Re	λ	Kmc	losses on the site?р, pa
PP grate at outlet				0.2 x 0.4	3,1	-	-	-	1,8	10,4
1	720	4,2	4	0.2 x 0.25	4,0	0,222	56900	0,0205	0,48	8,4
2	1030	3,0	5	0.25 x 0.25	4,6	0,25	73700	0,0195	0,4	8,1
3	2130	2,7	6	0.4 x 0.25	5,92	0,308	116900	0,0180	0,48	13,4
4	3480	14,8	7	0.4 x 0.4	6,04	0,40	154900	0,0172	1,44	45,5
5	6830	1,2	8	0.5 x 0.5	7,6	0,50	234000	0,0159	0,2	8,3
6	10420	6,4	10	0.6 x 0.5	9,65	0,545	337000	0,0151	0,64	45,7
6a	10420	0,8	Yu.	ø 0.64	8,99	0,64	369000	0,0149	0	0,9
7	10420	3,2	5	0.53 x 1.06	5,15	0,707	234000	0.0312 x n	2,5	44,2
Total losses: 185 Note. For brick channels with an absolute roughness of 4 mm and U f = 6.15 m/s, correction factor n = 1.94 (Table 22.12.).

The air ducts are made of galvanized sheet steel, the thickness and size of which correspond to approx. N from . The material of the air intake shaft is brick. Adjustable grilles of the PP type with possible sections: 100 x 200 are used as air distributors; 200 x 200; 400 x 200 and 600 x 200 mm, shading coefficient 0.8 and maximum air outlet speed up to 3 m/s.

The resistance of the insulated intake valve with fully open blades is 10 Pa. The hydraulic resistance of the heating unit is 100 Pa (according to a separate calculation). Filter resistance G-4 250 Pa. Hydraulic resistance of the muffler 36 Pa (according to acoustic calculation). Based on architectural requirements, rectangular air ducts are designed.
The cross-sections of brick channels are taken according to table. 22.7.

Local resistance coefficients.

Section 1. PP grid at the outlet with a cross section of 200 x 400 mm (calculated separately):
Dynamic pressure:

Lattice KMC (Appendix 25.1) = 1.8.
Pressure drop in the grid: Δр - рД x KMC = 5.8 x 1.8 = 10.4 Pa.
Design fan pressure p: Δр vent = 1.1 (Δр air + Δр valve + Δр filter + Δр cal + Δр muffler) = 1.1 (185 + 10 + 250 + 100 + 36) = 639 Pa.
Fan flow: L fan = 1.1 x Lsyst = 1.1 x 10420 = 11460 m 3 /h.

Selected radial fan VTs4-75 No. 6.3, version 1: L = 11500 m 3 /h; Δр ven = 640 Pa (fan unit E6.3.090 - 2a), rotor diameter 0.9 x D pom, rotation speed 1435 min-1, electric motor 4A10054; N = 3 kW installed on the same axis as the fan. Unit weight 176 kg.
Checking fan motor power (kW):
According to the aerodynamic characteristics of the fan, n fan = 0.75.

Table 2. Determination of local resistances

No. of plots	Type of local resistance	Sketch	Angle α, deg.	Attitude			Rationale	KMS
				F 0 /F 1	L 0 /L st	f pass /f stv
1	Diffuser		20	0,62	-	-	Table 25.1	0,09
	Retraction		90	-	-	-	Table 25.11	0,19
	Tee-pass		-	-	0,3	0,8	Adj. 25.8	0,2
							Σ	0,48
2	Tee-pass		-	-	0,48	0,63	Adj. 25.8	0,4
3	Branch tee		-	0,63	0,61	-	Adj. 25.9	0,48
4	2 bends	250 x 400	90	-	-	-	Adj. 25.11
	Retraction	400 x 250	90	-	-	-	Adj. 25.11	0,22
	Tee-pass		-	-	0,49	0,64	Table 25.8	0,4
							Σ	1,44
5	Tee-pass		-	-	0,34	0,83	Adj. 25.8	0,2
6	Diffuser after fan		h=0.6	1,53	-	-	Adj. 25.13	0,14
	Retraction	600 x 500	90	-	-	-	Adj. 25.11	0,5
							Σ	0,64
6a	Confusion in front of the fan			D g =0.42 m			Table 25.12	0
7	Knee		90	-	-	-	Table 25.1	1,2
	Louvre grille						Table 25.1	1,3
							Σ	1,44

Krasnov Y.S., "Ventilation and air conditioning systems. Design recommendations for industrial and public buildings", Chapter 15. "Thermocool"

With this material, the editors of the magazine “Climate World” continue the publication of chapters from the book “Ventilation and air conditioning systems. Design guidelines for production
water and public buildings.” Author Krasnov Yu.S.

The 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 distant and loaded area to the fan. When in doubt when determining a direction, consider all possible options.

The calculation begins with a remote section: determine the diameter D (m) of the round or the area F (m 2) of the cross section of the rectangular air duct:

The speed increases as you approach the fan.

According to Appendix H, the nearest standard values ​​are taken: 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 air duct section.

Local resistances at the border of two sections (tees, crosses) are assigned to the section with lower flow rate.

Local resistance coefficients are given in the appendices.

Diagram of the supply ventilation system serving a 3-story administrative building

Calculation example

Initial data:

No. of plots	flow 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 area Δр, pa
PP grid at the outlet				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 size of which correspond to approx. N from. The material of the air intake shaft is brick. Adjustable grilles of the PP type with possible sections: 100 x 200 are used as air distributors; 200 x 200; 400 x 200 and 600 x 200 mm, shading coefficient 0.8 and maximum air outlet speed up to 3 m/s.

The resistance of the insulated intake valve with fully open blades is 10 Pa. The hydraulic resistance of the heating unit is 100 Pa (according to a separate calculation). Filter resistance G-4 250 Pa. The hydraulic resistance of the muffler is 36 Pa (according to acoustic calculations). Based on architectural requirements, rectangular air ducts are designed.

The cross-sections of brick channels are taken according to table. 22.7.

Local resistance coefficients

Section 1. PP grid at the outlet with a cross section of 200×400 mm (calculated separately):

No. of plots	Type of local resistance	Sketch	Angle α, deg.	Attitude			Rationale	KMS
				F 0 /F 1	L 0 /L st	f pass /f stv
1	Diffuser		20	0,62	-	-	Table 25.1	0,09
	Retraction		90	-	-	-	Table 25.11	0,19
	Tee-pass		-	-	0,3	0,8	Adj. 25.8	0,2
							∑ =	0,48
2	Tee-pass		-	-	0,48	0,63	Adj. 25.8	0,4
3	Branch tee		-	0,63	0,61	-	Adj. 25.9	0,48
4	2 bends	250×400	90	-	-	-	Adj. 25.11
	Retraction	400×250	90	-	-	-	Adj. 25.11	0,22
	Tee-pass		-	-	0,49	0,64	Table 25.8	0,4
							∑ =	1,44
5	Tee-pass		-	-	0,34	0,83	Adj. 25.8	0,2
6	Diffuser after fan		h=0.6	1,53	-	-	Adj. 25.13	0,14
	Retraction	600×500	90	-	-	-	Adj. 25.11	0,5
							∑=	0,64
6a	Confusion in front of the fan			D g =0.42 m			Table 25.12	0
7	Knee		90	-	-	-	Table 25.1	1,2
	Louvre grille						Table 25.1	1,3
							∑ =	1,44
Table 2. Determination of local resistances

Krasnov Yu.S.,

1. Friction losses:

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

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

Permissible speed method

Note: speed air flow in the table it 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, their equivalent diameters must be found using the table below.

Notes:

• If there is not enough space (for example, during reconstruction), rectangular air ducts are chosen. 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, coefficient of air friction on the surface), it is possible to calculate the pressure loss in the system at the designed air flow.

Total pressure loss (in kg/sq.m.) is calculated using the formula:

where R is the pressure loss due to friction per 1 linear meter of the air duct, l is the length of the air duct in meters, z is the pressure loss due to local resistance (with a variable cross-section).

1. Friction losses:

In a round air duct, pressure loss due to friction P tr is calculated as follows:

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

where x is the friction resistance coefficient, l is the length of the air duct in meters, d is the diameter of the air duct in meters, v is the air flow speed in m/s, y is the air density in kg/cub.m., g is the acceleration of free fall (9 .8 m/s2).

Note: If the duct has a rectangular rather than a round cross-section, the equivalent diameter must be substituted into the formula, which for an air duct with sides A and B is equal to: deq = 2AB/(A + B)

2. Losses due to local resistance:

Pressure losses due to local resistance are calculated using the formula:

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

where Q is the sum of the local resistance coefficients in the section of the air duct for which the calculation is being made, v is the air flow speed in m/s, y is the air density in kg/cub.m., g is the acceleration of gravity (9.8 m/s2 ). Q values ​​are presented in tabular form.

Permissible speed method

When calculating the air duct network using the permissible speed method, the optimal air speed is taken as the initial data (see table). Then the required cross-section of the air duct and the pressure loss in it are calculated.

Procedure for aerodynamic calculation of air ducts using the permissible speed method:

Draw a diagram of the air distribution system. For each section of the air duct, indicate the length and amount of air passing in 1 hour.

We start the calculation from the areas farthest from the fan and the most loaded.

Knowing the optimal air speed for a given room and the volume of air passing through the air duct in 1 hour, we will determine the appropriate diameter (or cross-section) of the air duct.

We calculate the pressure loss due to friction P tr.

Using 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 following branches of the air distribution network is determined as the sum of pressure losses in the areas located before this branch.

During the calculation process, it is necessary to sequentially link all branches of the network, equating the resistance of each branch to the resistance of the most loaded branch. This is done using diaphragms. They are installed on lightly loaded areas of air ducts, increasing resistance.

Table maximum speed air depending on the requirements for the air duct

Constant head loss method

This method assumes a constant loss of pressure per 1 linear meter of air duct. Based on this, the dimensions of the air duct network are determined. The method of constant pressure loss is quite simple and is used at the stage of feasibility study of ventilation systems:

Depending on the purpose of the room, according to the table of permissible air speeds, select the speed on the main section of the air duct.

Based on the speed determined in paragraph 1 and based on the design air flow, the initial pressure loss is found (per 1 m of duct length). The diagram below does this.

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 length of the system by the pressure loss from step 2. The pressure loss at the diffusers is added to the resulting value.

Now, using the diagram below, determine the diameter of the initial air 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 pressure loss and diameter of air ducts

The pressure loss diagram shows the diameters of round ducts. If rectangular ducts are used instead, their equivalent diameters must be found using the table below.

Notes:

If space allows, it is better to choose round or square air ducts;

If there is not enough space (for example, during reconstruction), rectangular air ducts are chosen. As a rule, the width of the duct is 2 times the height).

In the table, the horizontal height of the air duct is indicated in mm, the vertical width is indicated, and the table cells contain the equivalent diameters of the air ducts in mm.

The programs can be useful to designers, managers, and engineers. Basically, to use the programs it is enough Microsoft Excel. Many program authors are unknown. I would like to acknowledge the work of these people who, using 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 cannot absolutely trust the program; check its data.

Sincerely, site administration

It is especially useful for engineers and designers in the field of designing engineering structures and sanitary systems. Developer Vlad Volkov

An updated calculator was sent by user ok, for which Ventportal thanks him!

A program for calculating the thermodynamic parameters of moist air or a mixture of two streams. Convenient and intuitive interface; the program does not require installation.

The program converts values ​​from one measurement scale to another. The "Transformer" knows the most commonly used, less common and outdated measures. In total, the program database contains information about 800 measures, many of which have brief information. There are possibilities to search the database, sort and filter records.

The Vent-Calc program was created for the calculation and design of ventilation systems. The program is based on the methodology hydraulic calculation air ducts according to the Altschul formulas given in

A program for converting various units of measurement. Program language - Russian/English.

The program algorithm is based on the use of an approximate analytical method for calculating changes in air condition. The calculation error is no more than 3%

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 m2. In addition, for rectangular ducts, the equivalent diameter, mm, is calculated. Squares have 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 pressure loss due to friction 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 made of other materials are used, it is necessary to introduce a correction for roughness β w. It depends on the absolute equivalent roughness of the air duct material K e and the value 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 plastic ducts β w = 1. More detailed values ​​of β w can be found in table 22.12. Taking into account this amendment, the updated friction pressure loss Rlβ w, Pa, is obtained by multiplying Rl by the value β w.

Then the dynamic pressure in the area, Pa, is determined. Here ρ in is the density of transported air, kg/m3. Usually they take ρ in = 1.2 kg/m 3.

The names of resistances (bend, tee, cross, elbow, grille, lampshade, umbrella, etc.) available in this area are written in the “local resistance” column. In addition, their quantity and characteristics are noted, by which the CMR values ​​for these elements are determined. For example, for a round outlet this is the angle of rotation and the ratio of the radius of rotation to the diameter of the air duct r/d, for a rectangular outlet - the angle of rotation and the dimensions of the sides of the air duct a and b. For side openings in an air duct or channel (for example, at the location where the air intake grille is installed) - the ratio of the opening area to the cross-section of the air duct f hole /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 relate to the one of these sections with less air flow L. The difference between tees and crosses on a pass 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 depicted by thin arrows. In addition, it is signed where exactly in each option the trunk, passage and branch of the tee are located for the right choice relations 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 areas to which the tees in question belong are indicated with check marks. 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 aerodynamically linking the secondary sections (see below). In this case, the same tee in the main direction can be taken into account as a tee for passage, and in the secondary direction - as a branch with a different coefficient.

Approximate ξ values ​​for commonly encountered 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 amount as for the corresponding tees.

Values ​​of ξ of some local resistances.

Name of resistance	KMS (ξ)	Name of resistance	KMS (ξ)
Round bend 90 o, r/d = 1	0.21	Fixed grille RS-G (exhaust or air intake)	2.9
Rectangular bend 90 o	0.3 … 0.6
Tee on passage (discharge)	0.25 … 0.4	Sudden expansion
Tee on branch (pressure)	0.65 … 1.9	Sudden contraction	0.5
Tee on passage (suction)	0.5 … 1	First side opening (entrance to the air intake shaft)	2.5 … 4.5
Tee on branch (suction)	–0.5 * … 0.25
Ceiling light (anemostat) ST-KR,ST-KV	5.6	Rectangular elbow 90 o	1.2
Adjustable grille RS-VG (supply)	3.8	Umbrella over the exhaust shaft	1.3

*) negative CMR can occur at low L o /L s due to the ejection (suction) of air from the branch by the main flow.

More detailed data for KMS are shown in tables 22.16 - 22.43. After determining the value of Σξ, the pressure loss at local resistances, Pa, and the total pressure loss in 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 determined total resistance ventilation network ΔР network = Σ(Rlβ w + Z). The ΔР value of the network serves as one of the initial data for selecting a fan. After selecting a fan in supply system an acoustic calculation of the ventilation network is made (see Chapter 12) and, if necessary, a muffler is selected.

The calculation results are entered into a 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 line that are not included in the main direction are linked (see example in paragraph 2.3). The calculation of the linked sections is carried out in the same sequence as for the main direction, and is recorded in the table in the same form. The linkage is considered completed if the sum of pressure losses Σ(Rlβ w + Z) along the tied sections deviates from the sum Σ(Rlβ w + Z) along the parallel connected sections of the main direction by no more than ±10%. Parallel connected sections are considered to be sections along the main and linked directions from the point of their branching to the end air distributors. If the diagram looks as shown in the following figure (the main direction is highlighted with a thick line), then linking direction 2 requires that the value of Rlβ w + Z for section 2 be equal to Rlβ w + Z for section 1, obtained from the calculation of the main direction, with an accuracy ±10%.

Purpose	Basic Requirement
	Silence	Min. head loss
	Main channels	Main channels		Branches
		Inflow	Hood	Inflow	Hood
Residential premises	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
Stores	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 location 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 is the loss of air pressure in the design section, R is the loss in linear meter section, L – total length of air ducts on the section, Z – losses in additional fittings of the ventilation system.

To calculate pressure loss in a round duct, the formula Ptr is used. = (L/d×X) × (Y×V)/2g. X is the tabulated coefficient of air friction, depends on the material of the air duct, L is the length of the design section, d is the diameter of the air duct, V is the required air flow speed, 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.

Table No. 2. Equivalent diameters of round air ducts for square ones

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

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

Table No. 3. Pressure loss at bends

To determine pressure losses in diffusers, data from table No. 4 is used.

Table No. 4. Pressure loss in diffusers

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

Table No. 5. Diagram of air pressure loss in straight air ducts

All individual losses in a given section of the air duct are summarized and adjusted with table No. 6. Table. No. 6. Calculation of flow pressure reduction in ventilation systems

During design and calculations, existing regulations recommend that the difference in pressure loss between individual sections should not exceed 10%. The fan must be installed in the area of ​​the ventilation system with the highest resistance; the most distant air ducts must have minimal 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.