pressure in the ventilation system. Duct resistance calculation calculator. Calculation of pressure in air ducts. The sequence of calculation of the supply system P1

Lecture 2. Pressure loss in ducts

Lecture plan. Mass and volumetric air flows. Bernoulli's law. Pressure losses in horizontal and vertical air ducts: coefficient of hydraulic resistance, dynamic coefficient, Reynolds number. Loss of pressure in the outlets, local resistances, for the acceleration of the dust-air mixture. Loss of pressure in a high-pressure network. The power of the pneumatic conveying system.

2. Pneumatic parameters of air flow

2.1. Air flow parameters

Under the action of the fan, an air flow is created in the pipeline. Important parameters air flow are its speed, pressure, density, mass and volume flow of air. Air volume volumetric Q, m 3 /s, and mass M, kg/s, are interconnected as follows:

;
, (3)

where F- square cross section pipes, m 2;

v– air flow velocity in a given section, m/s;

ρ - air density, kg / m 3.

The pressure in the air flow is divided into static, dynamic and total.

static pressure R st It is customary to call the pressure of particles of moving air on each other and on the walls of the pipeline. Static pressure reflects the potential energy of the air flow in the section of the pipe in which it is measured.

dynamic pressure air flow R din, Pa, characterizes its kinetic energy in the pipe section where it is measured:

Full pressure the air flow determines all its energy and is equal to the sum of static and dynamic pressures measured in the same pipe section, Pa:

R = R st + R d .

Pressures can be measured either from absolute vacuum or relative to atmospheric pressure. If the pressure is measured from zero (absolute vacuum), then it is called absolute R. If pressure is measured relative to atmospheric pressure, then it will be relative pressure H.

H = H st + R d .

Atmospheric pressure is equal to the difference between the total pressures of absolute and relative

R atm = R – H.

Air pressure is measured by Pa (N / m 2), mm of water column or mm of mercury:

1 mm w.c. Art. = 9.81 Pa; 1 mmHg Art. = 133.322 Pa. The normal state of atmospheric air corresponds to the following conditions: pressure 101325 Pa (760 mm Hg) and temperature 273K.

Air density is the mass per unit volume of air. According to the Claiperon equation, the density of pure air at a temperature of 20ºС

kg / m 3.

where R– gas constant equal to 286.7 J/(kg  K) for air; T is the temperature on the Kelvin scale.

Bernoulli equation. By the condition of the continuity of the air flow, the air flow is constant for any section of the pipe. For sections 1, 2, and 3 (Fig. 6), this condition can be written as follows:

;

When the air pressure changes within the range of up to 5000 Pa, its density remains almost constant. Concerning

;

Q 1 \u003d Q 2 \u003d Q 3.

The change in air flow pressure along the length of the pipe obeys Bernoulli's law. For sections 1, 2, one can write

where  R 1,2 - pressure losses caused by flow resistance against the pipe walls in the section between sections 1 and 2, Pa.

With a decrease in the cross-sectional area 2 of the pipe, the air velocity in this section will increase, so that the volume flow remains unchanged. But with an increase v 2 the dynamic flow pressure will increase. In order for equality (5) to hold, the static pressure must fall exactly as much as the dynamic pressure increases.

With an increase in the cross-sectional area, the dynamic pressure in the cross section will drop, and the static pressure will increase by exactly the same amount. The total pressure in the cross section remains unchanged.

2.2. Pressure loss in a horizontal duct

Friction pressure loss dust-air flow in a direct duct, taking into account the concentration of the mixture, is determined by the Darcy-Weisbach formula, Pa

where l- length of the straight section of the pipeline, m;

 - coefficient of hydraulic resistance (friction);

R din- dynamic pressure calculated from the average air velocity and its density, Pa;

TO– complex coefficient; for roads with frequent turns TO= 1.4; for straight lines with no large quantity turns
, where d– pipeline diameter, m;

TO tm- coefficient taking into account the type of transported material, the values of which are given below:

Hydraulic resistance coefficient  in engineering calculations are determined by the formula A.D. Altshulya

, (7)

where TO uh- absolute equivalent surface roughness, K e = (0.0001 ... 0.00015) m;

d – inner diameter pipes, m;

Re is the Reynolds number.

Reynolds number for air

, (8)

where v is the average air velocity in the pipe, m/s;

d– pipe diameter, m;

 - air density, kg / m 3;

 1 – coefficient of dynamic viscosity, Ns/m 2 ;

Dynamic coefficient value viscosities for air are found by the Millikan formula, Ns/m2

 1 = 17,11845  10 -6 + 49,3443  10 -9 t, (9)

where t– air temperature, С.

At t\u003d 16 С  1 \u003d 17.11845  10 -6 + 49.3443  10 -9 16 \u003d 17.910 -6.

2.3. Pressure loss in vertical duct

Pressure loss during the movement of the air mixture in a vertical pipeline, Pa:

, (10)

where  - air density,  \u003d 1.2 kg / m 3;

g \u003d 9.81 m / s 2;

h– lifting height of the transported material, m.

When calculating aspiration systems, in which the concentration of the air mixture   0.2 kg/kg value  R under only taken into account when  h 10 m. For inclined pipeline  h = l sin, where l is the length of the inclined section, m;  - the angle of inclination of the pipeline.

2.4. Pressure loss in outlets

Depending on the orientation of the outlet (rotation of the duct at a certain angle), two types of outlets are distinguished in space: vertical and horizontal.

Vertical outlets denoted by the initial letters of words that answer questions according to the scheme: from which pipeline, where and to which pipeline the air mixture is directed. There are the following withdrawals:

- Г-ВВ - the transported material moves from the horizontal section upwards to the vertical section of the pipeline;

- G-NV - the same from the horizontal down to the vertical section;

- ВВ-Г - the same from vertical upwards to horizontal;

- VN-G - the same from vertical down to horizontal.

Horizontal outlets There are only one type G-G.

In the practice of engineering calculations, the pressure loss in the outlet of the network is found by the following formulas.

At the values of consumption concentration   0.2 kg/kg

where
- the sum of the coefficients of local resistance of branch bends (Table 3) at R/ d= 2, where R- radius of turn of the axial line of the branch; d– pipeline diameter; dynamic airflow pressure.

At values   0.2 kg/kg

where is the sum of conditional coefficients that take into account the pressure loss for turning and dispersing the material behind the bend.

Values  about conv are found by the size of the tabular  T(Table 4) taking into account the coefficient for the angle of rotation TO P

 about conv =  T TO P . (13)

Correction factors TO P take depending on the angle of rotation of the taps :


TO P

Table 3

Coefficients of local resistance of taps  O at R/ d = 2

Branch design	Rotation angle, 
Branch design
Elbows are bent, stamped, welded from 5 links and 2 cups

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:

P \u003d R * l + z,

where R is the pressure loss due to friction per 1 linear meter of the duct, l is the length of the duct in meters, z is the 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 free fall acceleration (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 air duct network using the method of permissible speeds, the optimal air speed is taken as the initial data (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 suitable diameter(or 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 of maximum air speed depending on duct requirements

Purpose	Basic requirement
	Noiselessness	Min. head loss
	Main channels	main channels		Branches
	Main channels	tributary	Hood	tributary	Hood
Living spaces
Hotels
Institutions
Restaurants
The shops

Note: the air flow rate in the table is given in meters per second

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

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 space permits, it is better to choose round or square ducts;
If there is not enough space (for example, during reconstruction), rectangular ducts are chosen. 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.

Table of equivalent duct diameters

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 by the fan. The calculation of friction losses for air in air ducts, as well as the resistance of network equipment (filter, silencer, heater, valve, etc.) can be made using the appropriate tables and diagrams specified in the catalog. The total pressure drop can be calculated by summing the resistance values of all elements of the ventilation system.

Determining the speed of air movement in the ducts:

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

where L– air consumption, m3/h; F is the cross-sectional area of the channel, m2.

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.

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.

Find out the pressure loss for sections 1-6, using the graph of pressure loss in round ducts, we determine the required diameters of the air ducts and the pressure loss in them, provided that it is necessary to ensure the permissible air velocity.

Plot 1: the air flow will be 220 m3/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 m3/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 m3/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 m3/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 m3/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 m3/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.

Determination of pressure losses on bends of air ducts.

In order for the air exchange in the house to be “correct”, an aerodynamic calculation of the air ducts is needed even at the stage of drawing up a ventilation project.

Air masses moving through the channels of the ventilation system are taken as an incompressible liquid during calculations. And this is quite acceptable, because too much pressure is not formed in the air ducts. In fact, pressure is formed as a result of air friction against the walls of the channels, and even when resistances of a local nature appear (these include it - pressure - jumps at places where the direction changes, when connecting / disconnecting air flows, in areas where control devices or where the diameter of the ventilation duct changes).

Note! The concept of aerodynamic calculation includes the determination of the cross section of each section of the ventilation network that provides the movement of air flows. Moreover, the injection resulting from these movements is also determined.

In accordance with many years of experience, we can safely say that sometimes some of these indicators are already known during the calculation. The following are situations that are often encountered in such cases.

The cross-sectional index of the cross channels in the ventilation system is already known, it is required to determine the pressure that may be required in order for the desired amount of gas to move. This often happens in those air conditioning lines where the sectional dimensions were based on characteristics of a technical or architectural nature.
We already know the pressure, but we need to determine the cross section of the network to provide the ventilated room with the required amount of oxygen. This situation is inherent in networks natural ventilation, in which the already existing pressure cannot be changed.
It is not known about any of the indicators, therefore, we need to determine both the pressure in the line and the cross section. This situation occurs in most cases in the construction of houses.

Features of aerodynamic calculations

Let us get acquainted with the general procedure for carrying out such calculations, provided that both the cross section and the pressure are unknown to us. Let's make a reservation right away that the aerodynamic calculation should be carried out only after the required volumes of air masses have been determined (they will pass through the air conditioning system) and the approximate location of each of the air ducts in the network has been designed.

And in order to carry out the calculation, it is necessary to draw an axonometric diagram, in which there will be a list of all network elements, as well as their exact dimensions. In accordance with the plan of the ventilation system, the total length of the air ducts is calculated. After that, the entire system should be divided into segments with homogeneous characteristics, according to which (only separately!) The air flow will be determined. Typically, for each of the homogeneous sections of the system, a separate aerodynamic calculation of the air ducts should be carried out, because each of them has its own speed of air flow, as well as a permanent flow rate. All the indicators obtained must be entered into the axonometric scheme already mentioned above, and then, as you probably already guessed, you need to select the main highway.

How to determine the speed in the ventilation ducts?

As can be judged from all that has been said above, it is necessary to choose as the main highway that chain of successive segments of the network that is the longest; in this case, the numbering should begin exclusively from the most remote section. As for the parameters of each of the sections (and these include air flow, the length of the section, its serial number etc.), then they should also be entered in the calculation table. Then, when the introduction is finished, the cross-sectional shape is selected and its - sections - dimensions are determined.

LP/VT=FP.

What do these abbreviations stand for? Let's try to figure it out. So in our formula:

LP is the specific air flow in the selected area;
VT is the speed at which the air masses move through this area (measured in meters per second);
FP - this is the cross-sectional area of the channel we need.

Tellingly, when determining the speed of movement, it is necessary to be guided, first of all, by considerations of economy and noise of the entire ventilation network.

Note! According to the indicator obtained in this way ( we are talking about the cross section), it is necessary to select an air duct with standard values, and its actual cross section (indicated by the abbreviation FF) should be as close as possible to the previously calculated one.

LP/ FФ = VФ.

Having received the indicator of the required speed, it is necessary to calculate how much the pressure in the system will decrease due to friction against the walls of the channels (for this, you need to use a special table). As for the local resistance for each of the sections, they should be calculated separately, and then summarized into a general indicator. Then, by summing up the local resistance and the losses due to friction, you can get the total loss in the air conditioning system. In the future, this value will be used to calculate the required amount of gas masses in the ventilation ducts.

Air heating unit

Earlier we talked about what an air-heating unit is, talked about its advantages and areas of application, in addition to this article, we advise you to familiarize yourself with this information

How to calculate the pressure in the ventilation network

In order to determine the expected pressure for each individual section, you must use the formula below:

H x g (PH - PB) \u003d DPE.

Now let's try to figure out what each of these abbreviations means. So:

H in this case denotes the difference in the marks of the mine mouth and the intake grate;
РВ and РН is an indicator of gas density, both outside and inside the ventilation network, respectively (measured in kilograms per cubic meter);
Finally, DPE is a measure of what the natural available pressure should be.

We continue to disassemble the aerodynamic calculation of air ducts. To determine the internal and external density, it is necessary to use a reference table, and the temperature indicator inside / outside must also be taken into account. As a rule, the standard temperature outside is taken as plus 5 degrees, and regardless of in which particular region of the country are planned construction works. And if the temperature outside is lower, then as a result the injection into the ventilation system will increase, due to which, in turn, the volumes of incoming air masses will be exceeded. And if the temperature outside, on the contrary, is higher, then the pressure in the line will decrease because of this, although this trouble, by the way, can be completely compensated by opening the vents / windows.

As for main task of any described calculation, then it consists in choosing such air ducts, where the losses on the segments (we are talking about the value? (R * l *? + Z)) will be lower than the current DPE indicator, or, alternatively, at least equal to it. For greater clarity, we present the moment described above in the form of a small formula:

DPE? ?(R*l*?+Z).

Now let's take a closer look at what the abbreviations used in this formula mean. Let's start from the end:

Z in this case is an indicator indicating a decrease in the speed of air movement due to local resistance;
? - this is the value, more precisely, the coefficient of what is the roughness of the walls in the line;
l is another simple value that indicates the length of the selected section (measured in meters);
finally, R is an indicator of friction losses (measured in pascals per meter).

Well, we figured it out, now let's find out a little more about the roughness index (that is?). This indicator depends only on what materials were used in the manufacture of channels. It is worth noting that the speed of air movement can also be different, so this indicator should also be taken into account.

Speed - 0.4 meters per second

In this case, the roughness index will be as follows:

for plaster with the use of reinforcing mesh - 1.48;
for slag gypsum - about 1.08;
for an ordinary brick - 1.25;
and for cinder concrete, respectively, 1.11.

Speed - 0.8 meters per second

Here, the described indicators will look like this:

for plaster with the use of reinforcing mesh - 1.69;
for slag gypsum - 1.13;
for ordinary brick - 1.40;
finally, for slag concrete - 1.19.

Let's slightly increase the speed of the air masses.

Speed - 1.20 meters per second

For this value, the roughness indicators will be as follows:

for plaster with the use of reinforcing mesh - 1.84;
for slag gypsum - 1.18;
for an ordinary brick - 1.50;
and, consequently, for slag concrete - somewhere around 1.31.

And the last indicator of speed.

Speed - 1.60 meters per second

Here the situation will look like this:

for plaster using a reinforcing mesh, the roughness will be 1.95;
for slag gypsum - 1.22;
for ordinary brick - 1.58;
and, finally, for slag concrete - 1.31.

Note! We figured out the roughness, but it is worth noting one more important point: while it is desirable to take into account a small margin, fluctuating within ten to fifteen percent.

We deal with the general ventilation calculation

When making an aerodynamic calculation of air ducts, you must take into account all the characteristics of the ventilation shaft (these characteristics are listed below).

Dynamic pressure (to determine it, the formula is used - DPE? / 2 \u003d P).
The flow of air masses (it is denoted by the letter L and is measured in cubic meters per hour).
Pressure loss due to air friction about inner walls(denoted by the letter R, measured in pascals per meter).
Air duct diameter (to calculate this indicator, the following formula is used: 2 * a * b / (a + b); in this formula, the values \u200b\u200bof a, b are the dimensions of the cross section of the channels and are measured in millimeters).
Finally, speed is V, measured in meters per second, as we mentioned earlier.

As for the actual sequence of actions in the calculation, it should look something like this.

Step one. First, the required channel area should be determined, for which the following formula is used:

I/(3600xVpek) = F.

Understanding the meanings:

F in this case is, of course, the area, which is measured in square meters;
Vpek is the desired speed of air movement, which is measured in meters per second (for channels, a speed of 0.5-1.0 meters per second is taken, for mines - about 1.5 meters).

Step three. The next step is to determine the appropriate duct diameter (indicated by the letter d).

Step four. Then the remaining indicators are determined: pressure (denoted as P), speed of movement (abbreviated as V) and, therefore, decrease (abbreviated as R). For this, it is necessary to use nomograms according to d and L, as well as the corresponding tables of coefficients.

Step Five. Using already other tables of coefficients (we are talking about indicators of local resistance), it is required to determine how much the effect of air will decrease due to local resistance Z.

Step six. At the last stage of calculations, it is necessary to determine the total losses in each individual section of the ventilation line.

Pay attention to one important point! So, if the total losses are below the already existing pressure, then such a ventilation system can be considered effective. But if the losses exceed the pressure indicator, then it may be necessary to install a special throttle diaphragm in the ventilation system. Thanks to this diaphragm, excess pressure will be extinguished.

We also note that if the ventilation system is designed to serve several rooms at once, for which the air pressure must be different, then during the calculation it is necessary to take into account the underpressure or back pressure indicator, which must be added to the total loss indicator.

Video - How to make calculations using the program "VIKS-STUDIO"

Aerodynamic calculation of air ducts is considered a mandatory procedure, an important component of planning ventilation systems. Thanks to this calculation, you can find out how efficiently the premises are ventilated with a particular section of the channels. And the effective functioning of ventilation, in turn, ensures the maximum comfort of your stay in the house.

Calculation example. The conditions in this case are as follows: an administrative building, has three floors.

The purpose of the aerodynamic calculation is to determine the pressure loss (resistance) to air movement in all elements of the ventilation system - air ducts, their fittings, grilles, diffusers, air heaters and others. Knowing the total value of these losses, you can choose a fan that can provide the required air flow. There are direct and inverse problems of aerodynamic calculation. The direct problem is solved in the design of newly created ventilation systems, which consists in determining the cross-sectional area of all sections of the system at a given flow rate through them. The inverse problem is the determination of the air flow rate for a given cross-sectional area of operated or reconstructed ventilation systems. In such cases, to achieve the required flow, it is enough to change the fan speed or replace it with a different size.

Aerodynamic calculation begins after determining the rate of air exchange in the premises and making a decision on the routing (laying scheme) of air ducts and channels. The air exchange rate is a quantitative characteristic of the ventilation system, it shows how many times during the 1st hour the volume of air in the room is completely replaced by a new one. The multiplicity depends on the characteristics of the room, its purpose and may differ several times. Before starting the aerodynamic calculation, a system diagram is created in axonometric projection and on a scale of M 1:100. The diagram highlights the main elements of the system: air ducts, their fittings, filters, silencers, valves, air heaters, fans, grilles and others. According to this scheme, the building plans of the premises determine the length of individual branches. The scheme is divided into calculated sections, which have constant flow air. The boundaries of the calculated sections are shaped elements - bends, tees and others. Determine the flow rate for each section, put it, length, section number on the diagram. Next, a trunk is selected - the longest chain of successively located sections, counting from the beginning of the system to the most remote branch. If there are several highways of the same length in the system, then the main one is chosen with big expense. The cross-sectional shape of the ducts is accepted - round, rectangular or square. The pressure loss in the sections depends on the air speed and consists of: friction losses and local resistances. The total pressure losses of the ventilation system are equal to the line losses and consist of the sum of the losses of all its calculated sections. Choose the direction of calculation - from the farthest section to the fan.

By area F determine the diameter D(for round shape) or height A and width B(for a rectangular) duct, m. The values obtained are rounded up to the nearest larger standard size, i.e. D st , A st and In st(reference value).

Recalculate the actual cross-sectional area F fact and speed v fact.

For rectangular duct determine the so-called. equivalent diameter DL = (2A st * B st ) / (Ast+Bst), m.

Determine the value of the Reynolds similarity test Re = 64100*Dst*v fact. For rectangular shape D L \u003d D st.

Friction coefficient λtr = 0.3164 ⁄ Re-0.25 at Re≤60000, λtr= 0.1266 ⁄ Re-0.167 at Re>60000.

Local resistance coefficient λm depends on their type, quantity and is selected from directories.