Calculation of air duct network resistance online calculator. Determination of dynamic pressure in the air duct. Behavior of the environment inside the air duct

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

Recommended air speed in air ducts:

Determination of air speed in air ducts:

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

Where L- air flow, m 3 / h;
F- channel cross-sectional area, m2.

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

Recommendation 2.
In systems with long air ducts and big amount 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, air ducts of a smaller cross-section can be used.

Example of calculation of a ventilation system:
The calculation must begin by drawing up a sketch of the system indicating the locations of air ducts, ventilation grilles, fans, as well as the lengths of the air duct sections between the tees, then determine the air flow in each section of the network.

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

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

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

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

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

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

Section 6: the air flow through this section will be 1570 m 3 /h. We take the diameter of the air duct to be 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 loss in the network elements: in the CP 315/900 silencer (16 Pa) and in check valve KOM 315 (22 Pa). We will also determine the pressure loss in the taps to the grilles (the total resistance of 4 taps will be 8 Pa).

Determination of pressure loss at bends of air ducts

The graph allows you to determine the pressure loss in the outlet based on the bend 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 of 500 m3/h. To do this, we find the intersection of the vertical line corresponding to our air flow with the inclined line characterizing the diameter of 250 mm, and on the vertical line on the left for a 90° outlet we find the value of pressure loss, which is 2 Pa.

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

Now let’s sum up all the pressure loss values ​​for straight sections of air ducts, network elements, bends and grilles. The desired value is 186.3 Pa.

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

Determination of pressure losses in air ducts.

Determination of pressure loss in a check valve.

Selection of the required fan.

Determination of pressure loss in silencers.

Determination of pressure losses at bends of air ducts.

Determination of pressure loss in diffusers.

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:

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

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

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.

This method assumes a constant pressure loss of 1 linear meter 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:

The head loss diagram shows the diameters of round ducts. If ducts are used instead rectangular section, then you need to find their equivalent diameters 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).

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
agricultural 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:

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; 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 200×400 mm (calculated separately):

No. of plots	View 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.,

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 coefficient of friction resistance, 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 acceleration 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 of this premises 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

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

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 air 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

Using rectangular 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).

The table shows the height of the air duct in mm along the horizontal line, its width in the vertical line, and the cells of the table contain the equivalent diameters of the air ducts in mm.

Table of equivalent duct diameters

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 fluid during calculations. And this is completely acceptable, because too much pressure does not form in the air ducts. In fact, pressure is formed as a result of air friction against the walls of the channels, as well as when resistance of a local nature appears (these include pressure surges at places where direction changes, when connecting/disconnecting air flows, in areas where control devices or same where the diameter of the ventilation duct changes).

Note! The concept of aerodynamic calculation includes determining the cross-section of each section of the ventilation network that ensures the movement of air flows. Moreover, the pressure generated as a result of 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 at the time of calculation. Below are situations that are often encountered in such cases.

1. The cross-sectional area of ​​the cross-channels in the ventilation system is already known; it is necessary to determine the pressure that may be required in order to required quantity gas moved. This often happens in those air conditioning lines where the cross-sectional dimensions were based on technical or architectural characteristics.
2. We already know the pressure, but we need to determine the cross-section of the network to provide the ventilated room with the required volume of oxygen. This situation is inherent in networks natural ventilation, in which the existing pressure cannot be changed.
3. We do not know about any of the indicators, therefore, we need to determine both the pressure in the main and the cross-section. This situation occurs in most cases in the construction of houses.

Features of aerodynamic calculations

Let's get acquainted with general methodology carrying out this kind of calculations provided that both the cross section and the pressure are unknown to us. Let’s immediately make a reservation 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, which will contain a list of all network elements, as well as their exact dimensions. In accordance with the ventilation system plan, the total length of the air ducts is calculated. After this, the entire system should be divided into segments with homogeneous characteristics, according to which (only separately!) the air flow will be determined. What is typical is that 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 movement of air flows, as well as a permanent flow rate. All the obtained indicators must be entered into the axonometric diagram already mentioned above, and then, as you probably already guessed, you need to select the main highway.

How to determine the speed in ventilation ducts?

As can be judged from everything said above, as the main highway it is necessary to choose the chain of successive sections 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 section (and these include air flow, section length, its serial number, etc.), they should also be entered into the calculation table. Then, when the application is completed, the shape of the cross-section is selected and its cross-sections and 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 rate in the selected area;
• VT is the speed at which air masses move through this area (measured in meters per second);
• FP is the cross-sectional area of ​​the channel we need.

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

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

LP/ FF = VФ.

Having received the required speed indicator, 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 section, they should be calculated separately and then summed up into a common indicator. Then, by summing up the local resistance and losses due to friction, the total losses in the air conditioning system can be obtained. In the future, this value will be used to calculate the required quantity gas masses in ventilation ducts.

Air heating unit

Previously, 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 read this information

How to calculate the pressure in the ventilation network

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

Н x g (РН – РВ) = DPE.

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

• N in in this case indicates the difference in the elevations of the mine mouth and the intake grid;
• RV and RN are an indicator of gas density, both outside and inside the ventilation network, respectively (measured in kilograms per cubic meter);
• Finally, DPE is an indicator of what the natural available pressure should be.

We continue to analyze 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 outside temperature is taken as plus 5 degrees, regardless of the specific region of the country in which construction works. And if the temperature outside is lower, then as a result the injection into the ventilation system will increase, which, in turn, will cause the volumes of incoming air masses to be exceeded. And if the outside temperature, on the contrary, is higher, then the pressure in the line will decrease because of this, although this trouble, by the way, can be compensated for by opening the vents/windows.

As for main task 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, as an option, at least equal to it. For greater clarity, we present the point 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 air speed due to local resistance;
• ? – this is the value, more precisely, the coefficient of the roughness of the walls in the pipeline;
• l is another simple value that indicates the length of the selected section (measured in meters);
• Finally, R is the friction loss index (measured in pascals per meter).

Well, we’ve sorted that 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 the 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 indicator will be as follows:

• for plaster using reinforcing mesh – 1.48;
• for slag gypsum - about 1.08;
• for ordinary brick - 1.25;
• and for slag concrete, respectively, 1.11.

Speed ​​– 0.8 meters per second

Here the described indicators will look like this:

• for plaster using 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 using reinforcing mesh – 1.84;
• for slag gypsum – 1.18;
• for ordinary brick - 1.50;
• and, therefore, for slag concrete it is about 1.31.

And the last indicator of speed.

Speed ​​– 1.60 meters per second

Here the situation will look like this:

• for plaster using 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've sorted out the roughness, but it's worth noting one more thing important point: in this case, it is advisable to take into account a small margin, ranging from ten to fifteen percent.

Understanding the general ventilation calculations

When performing an aerodynamic calculation of air ducts, you must take into account all the characteristics of the ventilation shaft (these characteristics are given below in the form of a list).

1. Dynamic pressure (to determine it, the formula is used - DPE?/2 = P).
2. Air mass flow (it is designated by the letter L and measured in cubic meters per hour).
3. Pressure loss due to air friction against the internal walls (denoted by the letter R, measured in pascals per meter).
4. Diameter of air ducts (to calculate this indicator, the following formula is used: 2*a*b/(a+b); in this formula, the values ​​a, b are the cross-sectional dimensions of the ducts and are measured in millimeters).
5. Finally, speed is V, measured in meters per second, which we mentioned earlier.

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

Step one. First, you should determine the required channel area, for which the formula below is used:

I/(3600xVpek) = F.

Let's understand the values:

• 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 assumed, for mines - about 1.5 meters).

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

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

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

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

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

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 calculations it is also necessary to take into account the indicator of vacuum or pressure, which must be added to the overall loss indicator.

Video - How to make calculations using the VIX-STUDIO program

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 rooms are ventilated for a particular channel cross-section. And the effective functioning of ventilation, in turn, ensures maximum comfort of your stay in the house.

Example of calculations. The conditions in this case are as follows: the building is of an administrative nature, has three floors.

The pressure distribution in the ventilation system must be known when setting up and regulating the system, when determining flow rates in individual sections of the system, and when solving many other ventilation problems.

Pressure distribution in ventilation systems with mechanical stimulation of air movement. Consider an air duct with a fan (Fig. XI.3). In section 1-/ the static pressure is zero (i.e. equal to the air pressure at the level of the air duct). The total pressure in this section is equal to the dynamic pressure рді, determined by formula (XI.1). In section II-II, the static pressure рстіі>0 (numerically equal to the pressure loss due to friction between sections II-II and I-/). With a constant cross-section of the air duct, the static pressure line is straight. The total pressure line is also straight,

Parallel to the rst line. The vertical distance between these lines determines the dynamic pressure pDi.

In the diffuser located between sections II-II and III-III, a change in flow rate occurs. The dynamic pressure decreases as the air flows. In this regard, the static pressure changes and may even increase, as shown in the figure (рстіі>рстііі).

The total pressure in section III-III, created by the fan, is lost through friction Drtr and in local resistances (diffuser Lrdif, at the exit of Arnykh). The total pressure loss on the discharge side is:

The static pressure outside the duct on the suction side is zero. In the immediate vicinity of the opening within the suction plume, the air flow already has kinetic energy. The vacuum within the suction plume is insignificant.

At the entrance to the air duct, the flow speed increases, and therefore increases kinetic energy flow. Therefore, according to the law of conservation of energy, the potential energy of the flow must decrease. Taking into account pressure losses L/?POt in any section on the suction side

Per = 0 - rd - Drpot - (XI. 24)

In the suction duct, as well as on the discharge side, the total pressure is equal to the difference in pressure at the beginning of the duct and the pressure loss up to the section in question:

Rp = 0-DrpOt. (XI.25)

From formulas (XI.24) and (XI.25) it follows that in each section of the air duct on the suction side the values ​​p0t and pp are less than zero. By absolute value static pressure is greater than total pressure, however, formula (XI.2) is also valid for this case.

The static pressure line runs below the total pressure line. The sharp decrease in the static pressure line after section VI-VI is explained by the narrowing of the flow at the entrance to the air duct due to the formation of a vortex zone. Between cross sections V-V and IV-IV the diagram shows a confuser with a rotation. The decrease in the static pressure line between these sections occurs due to an increase in both the flow velocity in the confuser and the pressure loss. Diagrams of static pressure in Fig. XI.3 are shaded.

At point B, the lowest total pressure value in the air duct system is observed. Numerically, it is equal to the pressure loss on the suction side:

A - full and static in the discharge air duct; b - the same, in the suction air duct; c - dynamic in the discharge air duct; g - dynamic in the suction air duct

The fan creates a pressure drop equal to the difference between the maximum and minimum value total pressure (rll - Rpb)> increasing the energy of 1 m3 of air passing through it by the amount

The pressure created by the fan is spent on overcoming the resistance to air movement through the air ducts:

Rveit = DRvs + Drnagn. (XI. 27)

Professor P.N. Kamenev proposed constructing pressure diagrams on the suction air duct from absolute zero pressure (absolute vacuum). In this case, the construction of the lines rst. abs and rp. abs fully corresponds to the case of injection.

Pressures in air ducts are measured with a micromanometer. To measure static pressure, the hose from the micromanometer is connected to a fitting attached to the wall of the air duct, and to measure total pressure - to a pitot pneumometric tube, the hole of which is directed towards the flow (Fig. XI.4, a, b).

The difference between the total and static pressure is equal to the value of the dynamic pressure. This difference can be measured directly with a micromanometer, as shown in Fig. XI.4, c, d. The speed, m/s, is determined from the rd value:

V = V2prfp, (XI. 28)

By which the air flow in the duct is calculated, m3/h:

L = ZbООу/. (XI. 29)

Pressure distribution in ventilation systems with natural air movement. The features of such systems are vertical arrangement their channels in the building, low values ​​of available pressures and, consequently, low velocities. The operation of systems with natural air movement depends on the design features of the system and building, the difference in the density of external and internal air, wind speed and direction. However, when choosing design dimensions individual elements ventilation systems (sections of channels and shafts, areas of louvered grilles) it is enough to carry out calculations for the case when the building does not affect the work.

A - diagrams of absolute aerostatic pressures in the channel, closed with plugs 1 - inside the channel; 2 - outside the channel; b - diagram excess pressure in the same channel; c - diagrams of excess pressure during air movement through the channel; d - diagrams of excess pressure in the shaft and in the “wide channel” connected to it; d-diagrams of excess pressure in the channel and shaft in the presence of a branch; e - diagrams of excess pressures with natural impulse of air movement in the ventilation system multi-story building; g - diagrams of excess pressure under mechanical stimulation of air movement; (рst> Рп~ line, respectively, of static and total pressure inside the channel and shaft; Рн - line of static pressure outside the channel and shaft)

Let us consider the simplest case, when a vertical channel of height Yak, filled warm air with temperature tB, closed at the top and bottom with plugs. The channel is surrounded by outside air with temperature ta.

Let us assume that the pressure inside and outside the channel at the level of its top is equal to pa (to ensure this condition, it is enough to leave a small hole in the top plug). Then, in accordance with Pascal's law, the absolute pressure at any level (at a distance h from the top of the channel) is equal to: outside pst n=pa4-^rn£, and inside pstk=pa4--hpBg. The distribution of absolute pressures inside the channel (line 1) and outside it (line 2) is shown in Fig. XI.5, a.

In the “channel - ambient air” system, you can use conditional values ​​of excess pressure, i.e., conditionally take the aerostatic pressure inside the channel at any level as zero. The diagram of these pressures outside the channel has the shape of a triangle (Fig. XI.5,6J. The base of the triangle

Drk = Nk Drg

Is the available pressure, Pa, which determines the movement of air through the channel.

When air moves through the channel (Fig. XI.5, c), pressure losses are the sum of losses at the inlet, friction and outlet. In Fig. XI.5, c shows the distribution of total and static pressures (in excess pressures relative to the conditional zero). Dynamic pressure pd is equal to the difference between pp and pst. The static pressure (its diagram is shaded in the figure) along the entire length of the channel is less than the excess aerostatic pressure outside the channel pH. In some cases, ZONES WITH Рst >рн may be observed in the channel. For example, in the channel before the narrowing (Fig. XI.5, d), under certain conditions, the static pressure can exceed the pH pressure. Polluted air will leak through leaks in this channel area.

If vertical ventilation duct combines two (Fig. XI, 5, (3) or more (Fig. XI.5, e) branches, it is recommended to connect them not at the level of the air inlet into the branch, but slightly higher (one, two floors or more). This recommendation is given taking into account the accumulated operating experience. When connecting a branch at the level of point A instead of the level of point B, the available pressure Drotv increases (see Fig. XI.5, e); therefore, the channel resistance and the stability of the system also increase.

In Fig. XI.5, e, f static pressure diagrams are shaded. The total pressure decreases in height to the value of the losses at the outlet, and the dynamic pressure at a constant cross-section of the channel increases in height, since after connecting the branch, the flow rate in the channel increases.

Recently, ventilation systems with vertical channels and mechanical stimulation of air movement have been introduced. In these systems, air moves under the influence of a fan and gravitational forces. The construction of pressure distribution in such systems is similar to that discussed above. The peculiarity is that the static pressure in front of the fan is determined by the vacuum created by the fan (see diagram in Fig. XI.5,g). In this case, the available pressure for air movement in the system is

The basis for designing any utility networks is the calculation. In order to correctly design a network of supply or exhaust air ducts, you need to know the air flow parameters. In particular, it is required to calculate the flow rate and pressure loss in the channel for correct selection fan power.

In this calculation, an important role is played by such a parameter as the dynamic pressure on the walls of the air duct.

Behavior of the environment inside the air duct

A fan that creates an air flow in a supply or exhaust air duct communicates this flow potential energy. During movement in the limited space of the pipe, the potential energy of the air partially transforms into kinetic energy. This process occurs as a result of the influence of flow on the channel walls and is called dynamic pressure.

In addition to it, there is also static pressure, this is the effect of air molecules on each other in a flow, it reflects its potential energy. The kinetic energy of the flow is reflected by the dynamic impact indicator, which is why this parameter is included in the calculations.

At constant flow air, the sum of these two parameters is constant and is called full pressure. It can be expressed in absolute and relative units. The reference point for absolute pressure is complete vacuum, while relative pressure is considered starting from atmospheric pressure, that is, the difference between them is 1 Atm. As a rule, when calculating all pipelines, the value of the relative (excess) impact is used.

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Physical meaning of the parameter

If we consider straight sections of air ducts, the cross-sections of which decrease at a constant air flow, then an increase in flow speed will be observed. In this case, the dynamic pressure in the air ducts will increase, and the static pressure will decrease, the magnitude of the total impact will remain unchanged. Accordingly, for a flow to pass through such a narrowing (confuser), it should initially be informed required amount energy, otherwise consumption may decrease, which is unacceptable. By calculating the magnitude of the dynamic impact, you can find out the amount of losses in this confuser and correctly select the power of the ventilation unit.

The reverse process will occur if the channel cross-section is increased at a constant flow rate (diffuser). The speed and dynamic impact will begin to decrease, the kinetic energy of the flow will turn into potential. If the pressure developed by the fan is too high, the flow rate in the area and throughout the system may increase.

Depending on the complexity of the circuit, ventilation systems have many turns, tees, narrowings, valves and other elements called local resistances. The dynamic impact in these elements increases depending on the angle of attack of the flow on inner wall pipes. Some system components cause a significant increase in this parameter, for example fire dampers, in which one or more dampers are installed in the flow path. This creates increased flow resistance in the area, which must be taken into account in the calculation. Therefore, in all of the above cases, you need to know the magnitude of the dynamic pressure in the channel.

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Parameter calculations using formulas

In a straight section, the speed of air movement in the air duct is constant, and the magnitude of the dynamic impact remains constant. The latter is calculated by the formula:

Рд = v2γ / 2g

In this formula:

• Рд — dynamic pressure in kgf/m2;
• V—air velocity in m/s;
• γ—specific air mass in this area, kg/m3;
• g is the acceleration due to gravity, equal to 9.81 m/s2.

You can also get the value of dynamic pressure in other units, in Pascals. There is another variation of this formula for this:

Рд = ρ(v2 / 2)

Here ρ is the air density, kg/m3. Since there are no conditions for compression in ventilation systems air environment to such an extent that its density changes, it is assumed to be constant - 1.2 kg/m3.

Next, we should consider how the magnitude of the dynamic impact is involved in the calculation of channels. The point of this calculation is to determine losses in the entire supply system or exhaust ventilation to select the fan pressure, its design and engine power. The calculation of losses occurs in two stages: first, the losses due to friction against the channel walls are determined, then the drop in air flow power in local resistances is calculated. The dynamic pressure parameter is involved in the calculation at both stages.

The frictional resistance per 1 m of a circular channel is calculated by the formula:

R = (λ / d) Рд, where:

• Рд — dynamic pressure in kgf/m2 or Pa;
• λ—friction resistance coefficient;
• d is the diameter of the duct in meters.

Friction losses are determined separately for each section with different diameters and expenses. The resulting R value is multiplied by the total length of the channels of the calculated diameter, the losses due to local resistances are added and the total value for the entire system is obtained:

HB = ∑(Rl + Z)

Here are the parameters:

1. HB (kgf/m2) - total losses in the ventilation system.
2. R is friction loss per 1 m of a circular channel.
3. l (m) - length of the section.
4. Z (kgf/m2) - losses in local resistances (bends, crosses, valves, etc.).

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Determination of local resistance parameters of the ventilation system

The magnitude of the dynamic impact also takes part in determining the Z parameter. The difference with the straight section is that in different elements of the system the flow changes its direction, branches, and converges. In this case, the medium interacts with the inner walls of the channel not tangentially, but under different angles. To take this into account, in calculation formula you can enter trigonometric function, but there are a lot of difficulties here. For example, when passing a simple 90⁰ bend, the air turns and presses against the inner wall at at least three different angles (depending on the design of the bend). There are a lot of more complex elements in the air duct system, how to calculate the losses in them? There is a formula for this:

1. Z = ∑ξ Рд.

In order to simplify the calculation process, a dimensionless local resistance coefficient is introduced into the formula. It is different for each element of the ventilation system and is a reference value. The coefficient values ​​were obtained by calculations either empirically. Many manufacturing plants producing ventilation equipment, conduct their own aerodynamic research and product calculations. Their results, including the coefficient of local resistance of the element (for example, fire damper), are included in the product passport or placed in technical documentation on your website.

To simplify the process of calculating ventilation duct losses, all dynamic impact values ​​for different speeds are also calculated and tabulated, from which they can simply be selected and inserted into formulas. Table 1 shows some values ​​for the most commonly used air velocities in air ducts.