Ph.D. S. B. Gorunovich, PTO engineer, Ust-Ilimskaya CHPP, branch of OAO Irkutskenergo, Ust-Ilimsk, Irkutsk region.
Statement of a question
It is known that many enterprises that in the recent past had reserves of heat and electrical energy, insufficient attention was paid to its losses during transportation. For example, various pumps were included in the project, as a rule, with a large power reserve, pressure losses in pipelines were compensated by an increase in supply. The main steam pipelines were designed with jumpers and long lines, allowing, if necessary, to transport excess steam to neighboring turbine units. During the reconstruction and repair of transmission networks, preference was given to the versatility of schemes, which led to additional tie-ins (fittings) and jumpers, the installation of additional tees and, as a result, to additional local losses. full pressure. At the same time, it is known that in long pipelines at significant medium velocities, local losses of total pressure (local resistances) can lead to significant losses in costs for consumers.
At present, the requirements of efficiency, energy saving, total optimization of production make us take a fresh look at many issues and aspects of the design, reconstruction and operation of pipelines and steam pipelines, therefore, taking into account local resistances in tees, forks and fittings in hydraulic calculations pipelines becomes an urgent task.
The purpose of this work is to describe the most commonly used tees and fittings at power engineering enterprises, exchange experience in the field of ways to reduce local resistance coefficients, methods comparative evaluation the effectiveness of such activities.
To assess local resistance in modern hydraulic calculations, they operate with a dimensionless coefficient of hydraulic resistance, which is very convenient because in dynamically similar flows, in which the geometric similarity of sections and the equality of Reynolds numbers are observed, it has the same value, regardless of the type of liquid (gas) , as well as on the flow velocity and transverse dimensions of the calculated sections.
The hydraulic resistance coefficient is the ratio of the total energy (power) lost in a given section to the kinetic energy (power) in the accepted section or the ratio of the total pressure lost in the same section to the dynamic pressure in the accepted section:
where p total - lost (in this area) total pressure; p is the density of the liquid (gas); w, - speed in the i-th section.
The value of the drag coefficient depends on which design speed and, therefore, to which section it is reduced.
Exhaust and supply tees
It is known that a significant part of local losses in branched pipelines are local resistances in tees. As an object that is local resistance, the tee is characterized by the branch angle a and the ratio of the cross-sectional areas of the branches (lateral and straight) F b / F q , Fh / Fq and F B / Fn. In the tee, the flow rates Q b /Q q , Q n /Q c and, accordingly, the speed ratios w B /w Q , w n /w Q can change. The tees can be installed both in the suction sections (exhaust tee) and in the discharge sections (supply tees) in case of flow separation (Fig. 1).
The resistance coefficients of exhaust tees depend on the parameters listed above, and the inlet tees of the usual form - practically only on the branch angle and the ratio of velocities w n /w Q and w n /w Q, respectively.
The drag coefficients of conventionally shaped exhaust tees (without rounding and no flare or contraction of side branch or straight run) can be calculated using the following formulas.
Resistance in the side branch (in section B):
where Q B \u003d F B w B, Q q \u003d F q w q - volumetric flow rates in section B and C, respectively.
For type F n =F c tees and for all a, the values of A are given in Table. one.
When the ratio Q b /Q q changes from 0 to 1, the drag coefficient varies from -0.9 to 1.1 (F q =F b , a=90 O). Negative values are explained by suction action in the line at small Q B .
It follows from the structure of formula (1) that the drag coefficient will rapidly increase with a decrease in the cross-sectional area of the nozzle (with an increase in F c /F b). For example, when Q b /Q c =1, F q/F b =2, a=90 O, the coefficient is 2.75.
It is obvious that a decrease in resistance can be achieved by reducing the angle of the side branch (choke). For example, when F c =F b , α=45 O, when the ratio Q b /Q c changes from 0 to 1, the coefficient changes in the range from -0.9 to 0.322, i.e. its positive values decrease by almost 3 times.
The resistance in the forward passage should be determined by the formula:
For Fn=F c type tees, K P values are given in Table. 2.
It is easy to verify that the range of change in the drag coefficient in the forward pass
de when changing the ratio of Q b /Q c from 0 to 1 is in the range from 0 to 0.6 (F c =F b , α=90 O).
Reducing the angle of the side branch (choke) also leads to a significant reduction in resistance. For example, when F c =F b , α =45 O, when the ratio Q b /Q c changes from 0 to 1, the coefficient changes in the range from 0 to -0.414, i.e. with an increase in Q B, a "suction" appears in the direct passage, further reducing the resistance. It should be noted that dependence (2) has a pronounced maximum, i.e. maximum value the drag coefficient falls on the value of Q b /Q c =0.41 and equals 0.244 (at F c =F b , α =45 O).
The resistance coefficients of supply tees of normal shape in turbulent flow can be calculated using the formulas .
Side branch resistance:
where K B - flow compression ratio.
For type Fn=F c tees, the values of A 1 are given in Table. 3, K B =0.
If we take F c \u003d F b , a \u003d 90 O, then when the ratio Q b /Q c changes from 0 to 1, we obtain coefficient values in the range from 1 to 1.2.
It should be noted that the source provides other data for the coefficient A 1 . According to the data, A 1 =1 should be taken at w B /w c<0,8 и А 1 =0,9 при w B /w c >0.8. If we use the data from , then when the ratio Q B /Q C changes from 0 to 1, we obtain coefficient values in the range from 1 to 1.8 (F c =F b). In general, we will get slightly higher values for the drag coefficients in all ranges.
The decisive influence on the growth of the drag coefficient, as in formula (1), is exerted by the cross-sectional area B (fitting) - with increasing F g /F b, the drag coefficient increases rapidly.
Resistance in the straight passage for supply tees of the type Fn=Fc within
The values of t P are indicated in Table. four.
When the ratio Q B /Qc(3) changes from 0 to 1 (Fc=F B, α=90 O), we obtain coefficient values in the range from 0 to 0.3.
The resistance of conventionally shaped tees can also be markedly reduced by rounding off the junction of the side branch with the prefabricated hose. In this case, for exhaust tees, the angle of rotation of the flow should be rounded (R 1 in Fig. 16). For inlet tees, the rounding should also be done on the separating edge (R 2 in Fig. 16); it makes the flow more stable and reduces the possibility of it breaking away from that edge.
In practice, the rounding of the edges of the conjugation of the generatrix of the side branch and the main pipeline is sufficient when R / D (3 = 0.2-0.3.
The above formulas for calculating the resistance coefficients of tees and the corresponding tabular data refer to carefully manufactured (turned) tees. Manufacturing defects in tees made during their manufacture (“failures” of a side branch and “overlapping” of its section by an incorrect wall cut in a straight section - the main pipeline) become a source of sharp increase hydraulic resistance. In practice, this happens with a poor-quality tie-in into the main pipeline of the fitting, which occurs quite often, because. "factory" tees are relatively expensive.
The gradual expansion (diffuser) of the side branch effectively reduces the resistance of both exhaust and supply tees. The combination of rounding, bevelling and expanding the side branch further reduces the resistance of the tee. The resistance coefficients of improved tees can be determined from the formulas and diagrams given in the source. Tees with side branches in the form of smooth bends also have the least resistance, and where practicable, tees with small branch angles (up to 60 °) should be used.
In turbulent flow (Re>4.10 3) the drag coefficients of the tees depend little on the Reynolds numbers. During the transition from turbulent to laminar, there is an abrupt increase in the drag coefficient of the side branch both in the exhaust and supply tees (by about 2-3 times).
In calculations, it is important to take into account in which section it is reduced to the average speed. There is a link in the source about this before each formula. The sources give a general formula, which indicates the rate of reduction with the corresponding index.
Symmetrical tee when merging and splitting
The resistance coefficient of each branch of a symmetrical tee at the confluence (Fig. 2a), can be calculated by the formula:
When the ratio Q b /Q c changes from 0 to 0.5, the coefficient changes from 2 to 1.25, and then with an increase in Q b / Q c from 0.5 to 1, the coefficient acquires values from 1.25 to 2 (for the case F c =F b). Obviously, dependence (5) has the form of an inverted parabola with a minimum at the point Q b /Q c =0.5.
The resistance coefficient of a symmetrical tee (Fig. 2a) located in the injection (separation) section can also be calculated using the formula:
where K 1 \u003d 0.3 - for welded tees.
When the ratio w B /w c changes from 0 to 1, the coefficient changes in the range from 1 to 1.3 (F c =F b).
Analyzing the structure of formulas (5, 6) (as well as (1) and (3)), it can be seen that a decrease in the cross section (diameter) of the side branches (sections B) adversely affects the resistance of the tee.
The flow resistance can be reduced by a factor of 2-3 when using tees-forks (Fig. 26, 2c).
The drag coefficient of a tee-fork during flow separation (Fig. 2b) can be calculated by the formulas:
When the ratio Q 2 /Q 1 changes from 0 to 1, the coefficient changes in the range from 0.32 to 0.6.
The resistance coefficient of the tee-fork at the merger (Fig. 2b) can be calculated by the formulas:
When the ratio Q 2 /Q 1 changes from 0 to 1, the coefficient changes in the range from 0.33 to -0.4.
A symmetrical tee can be made with smooth bends (Fig. 2c), then its resistance can be further reduced.
Manufacturing. Standards
Industry energy standards prescribe for pipelines of low-pressure thermal power plants (at operating pressure P slave.<22 кгс/см 2 и температуре среды t<425 О С) использовать тройники сварные по ОСТ34-42-762
OST34-42-765-85. For higher environmental parameters (P work b.<40 кгс/см 2) изготавливают тройники из углеродистых и кремнемарганцовистых сталей: штампованные по ОСТ108.720.01, ОСТ108.720.02-82; сварные по ОСТ108.104.01 - ОСТ108.104.03-82; с обжатием (с вытянутой горловиной) по ОСТ108.104.04, ОСТ108.104.05-82. Из хромомолибденованадиевых сталей изготавливают тройники: штампованные по ОСТ108.720.05, ОСТ108.720.06-82; сварные по ОСТ108.104.10 - ОСТ108.104.12-82; с обжатием (с вытянутой горловиной) по ОСТ108.104.13 - ОСТ108.104.15-82 для паропроводов высокого давления (с параметрами Р раб. до 255 кгс/см 2 и температурой t до 560 О С). Существуют соответствующие нормативы и для штуцеров.
The design of tees manufactured according to the existing (above) standards is far from always optimal in terms of hydraulic losses. Only the shape of stamped tees with an elongated neck contributes to a decrease in the coefficient of local resistance, where a rounding radius is provided in the side branch according to the type shown in Fig. 1b and fig. 3c, as well as with end compression, when the diameter of the main pipeline is slightly smaller than the diameter of the tee (as shown in Fig. 3b). Forked tees are apparently made to order according to "factory" standards. In RD 10-249-98 there is a paragraph devoted to the calculation of the strength of tees-forks and fittings.
When designing and reconstructing networks, it is important to take into account the direction of movement of the media and the possible ranges of flow rates in tees. If the direction of the transported medium is clearly defined, it is advisable to use inclined fittings (side branches) and forked tees. However, there remains the problem of significant hydraulic losses in the case of a universal tee, which combines the properties of supply and exhaust, in which both merging and separation of the flow are possible in operating modes associated with a significant change in flow rates. The above qualities are typical, for example, for switching nodes of feed water pipelines or main steam pipelines at TPPs with "jumpers".
It should be borne in mind that for steam and hot water pipelines, the design and geometric dimensions of welded pipe tees, as well as fittings (pipes, branch pipes) welded on straight sections of pipelines, must meet the requirements of industry standards, norms and specifications. In other words, for critical pipelines, it is necessary to order tees made in accordance with the specifications from certified manufacturers. In practice, in view of the relative high cost of "factory" tees, tie-in fittings are often performed by local contractors using industry or factory standards.
In general, the final decision on the tie-in method should be taken after a comparative feasibility study. If a decision is made to carry out the tie-in “on their own”, the engineering and technical personnel need to prepare a choke template, calculate the strength (if necessary), control the quality of the tie-in (avoid “failures” of the choke and “overlap” its section with an incorrect wall cut in a straight section) . It is advisable to make the internal joint between the metal of the fitting and the main pipeline with a rounding (Fig. 3c).
There are a number of design solutions to reduce hydraulic resistance in standard tees and line switching assemblies. One of the simplest is to increase the size of the tees themselves to reduce the relative velocities of the medium in them (Fig. 3a, 3b). At the same time, tees must be completed with transitions, the expansion (constriction) angles of which are also advisable to choose from a number of hydraulically optimal ones. As a universal tee with reduced hydraulic losses, you can also use a forked tee with a jumper (Fig. 3d). The use of tees-forks for switching nodes of highways will also slightly complicate the design of the node, but will have a positive effect on hydraulic losses (Fig. 3e, 3f).
It is important to note that with a relatively close location of local (L=(10-20)d) resistances of various types, the phenomenon of interference of local resistances takes place. According to some researchers, with the maximum convergence of local resistances, it is possible to achieve a decrease in their sum, while at a certain distance (L = (5-7) d), the total resistance has a maximum (3-7% higher than the simple sum) . The reduction effect could be of interest to large manufacturers ready to manufacture and supply switching units with reduced local resistances, but applied laboratory research is required to achieve a good result.
Feasibility study
When making a constructive decision, it is important to pay attention to the economic side of the problem. As mentioned above, "factory" tees of a conventional design, and even more so made to order (hydraulically optimal), will cost significantly more than a tie-in fitting. At the same time, it is important to roughly evaluate the benefits in case of reducing hydraulic losses in a new tee and its payback period.
It is known that pressure losses in station pipelines with normal media flow rates (for Re>2.10 5) can be estimated by the following formula:
where p - pressure loss, kgf / cm 2; w is the speed of the medium, m/s; L - deployed length of the pipeline, m; g - free fall acceleration, m/s 2 ; d - design diameter of the pipeline, m; k - coefficient of friction resistance; ∑ἐ m is the sum of local resistance coefficients; v - specific volume of the medium, m 3 / kg
Dependence (7) is usually called the hydraulic characteristic of the pipeline.
If we take into account the dependence: w=10Gv/9nd 2 , where G is the consumption, t/h.
Then (7) can be represented as:
If it is possible to reduce the local resistance (tee, fitting, switching unit), then, obviously, formula (9) can be represented as:
Here ∑ἐ m is the difference between the local resistance coefficients of the old and new nodes.
Let us assume that the hydraulic system "pump - pipeline" operates in nominal mode (or in a mode close to nominal). Then:
where P n - nominal pressure (according to the flow characteristic of the pump / boiler), kgf / cm 2; G h - nominal flow rate (according to the flow characteristic of the pump / boiler), t / h.
If we assume that after replacing the old resistances, the “pump-pipeline” system will remain operational (ЫРn), then from (10), using (12), we can determine the new flow rate (after reducing the resistance):
The operation of the "pump-pipeline" system, the change in its characteristics can be visualized in Fig. four.
Obviously, G 1 >G M . If we are talking about the main steam pipeline that transports steam from the boiler to the turbine, then by the difference in flow rates ЛG=G 1 -G n it is possible to determine the gain in the amount of heat (from the turbine extraction) and / or in the amount of generated electric energy according to the operating characteristics of this turbine.
Comparing the cost of a new node and the amount of heat (electricity), you can roughly estimate the profitability of its installation.
Calculation example
For example, it is necessary to evaluate the cost-effectiveness of replacing an equal tee of the main steam pipeline at the confluence of flows (Fig. 2a) with a forked tee with a jumper of the type indicated in fig. 3y. Steam consumer - heating turbine PO TMZ type T-100/120-130. Steam enters through one line of the steam pipeline (through a tee, sections B, C).
We have the following initial data:
■ design diameter of the steam pipeline d=0.287 m;
■ nominal steam flow rate G h =Q(3=Q^420 t/h;
■ nominal pressure of the boiler Р н =140 kgf/cm 2 ;
■ specific volume of steam (at P ra b=140 kgf/cm 2 , t=560 o C) n=0.026 m 3 /kg.
We calculate the resistance coefficient of a standard tee at the confluence of flows (Fig. 2a) using the formula (5) - ^ SB1 = 2.
To calculate the resistance coefficient of a tee-fork with a jumper, assume:
■ division of flows in the branches occurs in the proportion Q b /Q c «0.5;
■ the total resistance coefficient is equal to the sum of the resistances of the inlet tee (with a 45 O branch, see Fig. 1a) and the branch tee at the confluence (Fig. 2b), i.e. interference is neglected.
We use formulas (11, 13) and get the expected increase in consumption by G=G 1 -G n = 0.789 t/h.
According to the regime diagram of the T-100/120-130 turbine, a flow rate of 420 t/h can correspond to an electrical load of 100 MW and a thermal load of 400 GJ/h. The relationship between flow and electrical load is close to directly proportional.
The gain in electrical load can be: P e \u003d 100AG / Q n \u003d 0.188 MW.
The heat load gain can be: T e \u003d 400AG / 4.19Q n \u003d 0.179 Gcal / h.
Prices for products made of chromium-molybdenum-vanadium steels (for tees-fork 377x50) can vary widely from 200 to 600 thousand rubles, therefore, the payback period can only be judged after a thorough market research at the time of the decision.
1. This article describes various types of tees and fittings, gives a brief description of the tees used in pipelines of power plants. Formulas for determining the coefficients of hydraulic resistance are given, ways and means of their reduction are shown.
2. Prospective designs of tees-forks, a switching unit for main pipelines with reduced coefficients of local resistance are proposed.
3. Formulas are given, an example and the expediency of a technical and economic analysis is shown when choosing or replacing tees, when reconstructing switching units.
Literature
1. Idelchik I.E. Handbook of hydraulic resistance. M.: Mashinostroenie, 1992.
2. Nikitina I.K. Handbook of pipelines of thermal power plants. Moscow: Energoatomizdat, 1983.
3. Handbook of calculations of hydraulic and ventilation systems / Ed. A.S. Yuriev. S.-Pb.: ANO NPO "World and Family", 2001.
4. Rabinovich E.Z. Hydraulics. Moscow: Nedra, 1978.
5. Benenson E.I., Ioffe L.S. Cogeneration Steam Turbines / Ed. D.P. Elder. M: Energoizdat, 1986.
The calculation of supply and exhaust air duct systems is reduced to determining the dimensions of the cross-section of the channels, their resistance to air movement and linking the pressure in parallel connections. The calculation of pressure losses should be carried out using the method of specific friction pressure losses.
Calculation method:
An axonometric diagram of the ventilation system is built, the system is divided into sections, on which the length and flow rate are plotted. The design scheme is shown in Figure 1.
The main (main) direction is selected, which is the longest chain of successively located sections.
3. Sections of the highway are numbered, starting from the section with the lowest flow.
4. The dimensions of the cross section of the air ducts on the calculated sections of the main are determined. We determine the cross-sectional area, m 2:
F p \u003d L p / 3600V p ,
where L p is the estimated air flow in the area, m 3 / h;
According to the found values F p ] the dimensions of the air ducts are taken, i.e. is F f.
5. The actual speed V f, m/s is determined:
V f = L p / F f,
where L p is the estimated air flow in the area, m 3 / h;
F f - the actual cross-sectional area of the duct, m 2.
We determine the equivalent diameter by the formula:
d equiv = 2 α b/(α+b) ,
where α and b are the transverse dimensions of the duct, m.
6. The values of d eq and V f are used to determine the values of specific friction pressure losses R.
The pressure loss due to friction in the calculated section will be
P t \u003d R l β w,
where R is the specific friction pressure loss, Pa/m;
l is the length of the duct section, m;
β w is the roughness coefficient.
7. The coefficients of local resistances are determined and the pressure losses in local resistances in the section are calculated:
z = ∑ζ P d,
where P d - dynamic pressure:
Pd \u003d ρV f 2 / 2,
where ρ is the air density, kg/m3;
V f - the actual air speed in the area, m / s;
∑ζ - the sum of the CMR on the site,
8. Total losses are calculated by sections:
ΔР = R l β w + z,
l is the length of the section, m;
z - pressure loss in local resistances in the section, Pa.
9. Pressure losses in the system are determined:
ΔР p = ∑(R l β w + z),
where R is the specific friction pressure loss, Pa/m;
l is the length of the section, m;
β w is the roughness coefficient;
z - pressure loss in local resistances in the area, Pa.
10. Branches are being linked. Linkage is made, starting with the longest branches. It is similar to the calculation of the main direction. The resistances in all parallel sections must be equal: the discrepancy is not more than 10%:
where Δр 1 and Δр 2 are losses in branches with higher and lower pressure losses, Pa. If the discrepancy exceeds the specified value, then a throttle valve is installed.
Figure 1 - Calculation scheme of the supply system P1.
The sequence of calculation of the supply system P1
Plot 1-2, 12-13, 14-15,2-2',3-3',4-4',5-5',6-6',13-13',15-15',16- 16':
Plot 2 -3, 7-13, 15-16:
Plot 3-4, 8-16:
Plot 4-5:
Plot 5-6:
Plot 6-7:
Plot 7-8:
Plot 8-9:
local resistance
Plot 1-2:
a) at the exit: ξ = 1.4
b) bend 90°: ξ = 0.17
c) tee for straight passage:
Plot 2-2’:
a) branch tee
Plot 2-3:
a) bend 90°: ξ = 0.17
b) tee for straight passage:
ξ = 0,25
Plot 3-3':
a) branch tee
Plot 3-4:
a) bend 90°: ξ = 0.17
b) tee for straight passage:
Plot 4-4’:
a) branch tee
Plot 4-5:
a) tee for straight passage:
Plot 5-5’:
a) branch tee
Plot 5-6:
a) bend 90°: ξ = 0.17
b) tee for straight passage:
Plot 6-6’:
a) branch tee
Plot 6-7:
a) tee for straight passage:
ξ = 0,15
Plot 7-8:
a) tee for straight passage:
ξ = 0,25
Plot 8-9:
a) 2 bends 90°: ξ = 0.17
b) tee for straight passage:
Plot 10-11:
a) bend 90°: ξ = 0.17
b) at the exit: ξ = 1.4
Plot 12-13:
a) at the exit: ξ = 1.4
b) bend 90°: ξ = 0.17
c) tee for straight passage:
Plot 13-13’
a) branch tee
Plot 7-13:
a) bend 90°: ξ = 0.17
b) tee for straight passage:
ξ = 0,25
c) branch tee:
ξ = 0,8
Plot 14-15:
a) at the exit: ξ = 1.4
b) bend 90°: ξ = 0.17
c) tee for straight passage:
Plot 15-15’:
a) branch tee
Plot 15-16:
a) 2 bends 90°: ξ = 0.17
b) tee for straight passage:
ξ = 0,25
Plot 16-16’:
a) branch tee
Plot 8-16:
a) tee for straight passage:
ξ = 0,25
b) branch tee:
Aerodynamic calculation of the supply system P1
|
Consumption, L, m³/h |
Length, l, m |
Duct dimensions |
Air velocity V, m/s |
Losses per 1 m length R, Pa |
Coeff. roughness m |
Friction loss Rlm, Pa |
CMR sum, Σξ |
Dynamic pressure Rd, Pa |
Local resistance losses, Z |
Pressure loss in the section, ΔР, Pa |
||||||||||||
|
Sectional area F, m² |
Equivalent Diameter |
|||||||||||||||||||||
Let us perform the discrepancy of the supply system P1, which should be no more than 10%.
Since the discrepancy exceeds the allowable 10%, it is necessary to install a diaphragm.
I install the diaphragm in the area 7-13, V = 8.1 m / s, P C = 20.58 Pa
Therefore, for an air duct with a diameter of 450, I install a diaphragm with a diameter of 309.
|
Purpose |
Basic requirement | ||||
| Noiselessness | Min. head loss | ||||
| Main channels | main channels | Branches | |||
| tributary | Hood | tributary | Hood | ||
| Living spaces | 3 | 5 | 4 | 3 | 3 |
| Hotels | 5 | 7.5 | 6.5 | 6 | 5 |
| Institutions | 6 | 8 | 6.5 | 6 | 5 |
| Restaurants | 7 | 9 | 7 | 7 | 6 |
| The shops | 8 | 9 | 7 | 7 | 6 |
Based on these values, the linear parameters of the air ducts should be calculated.
Algorithm for calculating air pressure losses
The calculation must begin with drawing up a diagram of the ventilation system with the obligatory indication of the spatial 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, the losses are determined using the formula P = L × R + Z, where P is the air pressure loss in the calculated section, R is the loss per linear meter of the section, L is the total length of the air ducts in the section, Z is the loss in the additional fittings of the system ventilation.
To calculate the pressure loss in a circular duct, the formula Ptr is used. = (L/d×X) × (Y×V)/2g. X is the tabular coefficient of air friction, depends on the material of manufacture of the duct, L is the length of the calculated section, d is the diameter of the duct, V is the required air flow rate, Y is the air density, taking into account temperature, g is the acceleration of fall (free). If the ventilation system has square air ducts, then table No. 2 should be used to convert round values to square ones.
Tab. No. 2. Equivalent diameters of round ducts for square
| 150 | 200 | 250 | 300 | 350 | 400 | 450 | 500 | |
| 250 | 210 | 245 | 275 | |||||
| 300 | 230 | 265 | 300 | 330 | ||||
| 350 | 245 | 285 | 325 | 355 | 380 | |||
| 400 | 260 | 305 | 345 | 370 | 410 | 440 | ||
| 450 | 275 | 320 | 365 | 400 | 435 | 465 | 490 | |
| 500 | 290 | 340 | 380 | 425 | 455 | 490 | 520 | 545 |
| 550 | 300 | 350 | 400 | 440 | 475 | 515 | 545 | 575 |
| 600 | 310 | 365 | 415 | 460 | 495 | 535 | 565 | 600 |
| 650 | 320 | 380 | 430 | 475 | 515 | 555 | 590 | 625 |
| 700 | 390 | 445 | 490 | 535 | 575 | 610 | 645 | |
| 750 | 400 | 455 | 505 | 550 | 590 | 630 | 665 | |
| 800 | 415 | 470 | 520 | 565 | 610 | 650 | 685 | |
| 850 | 480 | 535 | 580 | 625 | 670 | 710 | ||
| 900 | 495 | 550 | 600 | 645 | 685 | 725 | ||
| 950 | 505 | 560 | 615 | 660 | 705 | 745 | ||
| 1000 | 520 | 575 | 625 | 675 | 720 | 760 | ||
| 1200 | 620 | 680 | 730 | 780 | 830 | |||
| 1400 | 725 | 780 | 835 | 880 | ||||
| 1600 | 830 | 885 | 940 | |||||
| 1800 | 870 | 935 | 990 |
The horizontal is the height of the square duct, and the vertical is the width. The equivalent value of the circular section is at the intersection of the lines.
Air pressure losses in bends are taken from table No. 3.
Tab. No. 3. Loss of pressure on bends
To determine the pressure loss in the diffusers, the data from Table No. 4 are used.
Tab. No. 4. Pressure loss in diffusers
Table No. 5 gives a general diagram of losses in a straight section.
Tab. No. 5. Diagram of air pressure losses in straight air ducts
All individual losses in a given section of the duct are summarized and corrected with Table No. 6. Tab. No. 6. Calculation of the flow pressure drop in ventilation systems
During design and calculations, existing regulations recommend that the difference in pressure loss between individual sections should not exceed 10%. The fan should be installed in the section of the ventilation system with the highest resistance, the most distant air ducts should have the minimum resistance. If these conditions are not met, then it is necessary to change the layout of air ducts and additional equipment, taking into account the requirements of the regulations.
Programs can be useful to designers, managers, engineers. Basically, Microsoft Excel is enough to use the programs. Many authors of programs are not known. I would like to note the work of these people who, based on Excel, were able to prepare such useful calculation programs. Calculation programs for ventilation and air conditioning are free to download. But don't forget! You can not absolutely trust the program, check its data.
Sincerely, site administration
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Especially useful for engineers and designers in the design of engineering structures and sanitary systems. Developer Vlad Volkov |
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An updated calculator was sent by the user ok, for which Ventportal thanks him! |
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A program for calculating the thermodynamic parameters of humid air or a mixture of two streams. Convenient and intuitive interface, the program does not require installation. |
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The program converts values from one scale to another. The "transformer" knows the most commonly used, less common and obsolete measures. In total, the program database contains information about 800 measures, many of them have a brief reference. There are possibilities of searching in the database, sorting and filtering records. |
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The Vent-Calc program was created for the calculation and design of ventilation systems. The program is based on the method of hydraulic calculation of air ducts according to the Altshul formulas given in |
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A program for converting various units of measurement. program language - Russian/English. |
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The algorithm of the program is based on the use of an approximate analytical method for calculating the change in the state of the air. The calculation error is no more than 3% |
With this material, the editors of the journal “Climate World” continue to publish chapters from the book “Ventilation and air conditioning systems. Design recommendations for
water and public buildings”. Author Krasnov Yu.S.
Aerodynamic calculation of air ducts begins with drawing an axonometric diagram (M 1: 100), putting down the numbers of sections, their loads L (m 3 / h) and lengths I (m). The direction of the aerodynamic calculation is determined - from the most remote and loaded section to the fan. When in doubt, when determining the direction, all possible options are calculated.
The calculation starts from a remote section: the diameter D (m) of a round or the area F (m 2) of the cross section of a rectangular duct is determined:
The speed increases as you get closer to the fan.
According to Appendix H, the nearest standard values are taken from: D CT or (a x b) st (m).
Hydraulic radius of rectangular ducts (m):
 |
where - the sum of the local resistance coefficients in the duct section.
Local resistances at the border of two sections (tees, crosses) are attributed to the section with a lower flow rate.
Local resistance coefficients are given in the appendices.
Scheme of the supply ventilation system serving the 3-storey administrative building
Calculation example
Initial data:
| No. of plots | supply L, m 3 / h | length L, m | υ rivers, m/s | section a × b, m |
υ f, m/s | D l ,m | Re | λ | kmc | losses in the section Δр, pa |
| outlet grating pp | 0.2 × 0.4 | 3,1 | — | — | — | 1,8 | 10,4 | |||
| 1 | 720 | 4,2 | 4 | 0.2 × 0.25 | 4,0 | 0,222 | 56900 | 0,0205 | 0,48 | 8,4 |
| 2 | 1030 | 3,0 | 5 | 0.25×0.25 | 4,6 | 0,25 | 73700 | 0,0195 | 0,4 | 8,1 |
| 3 | 2130 | 2,7 | 6 | 0.4×0.25 | 5,92 | 0,308 | 116900 | 0,0180 | 0,48 | 13,4 |
| 4 | 3480 | 14,8 | 7 | 0.4×0.4 | 6,04 | 0,40 | 154900 | 0,0172 | 1,44 | 45,5 |
| 5 | 6830 | 1,2 | 8 | 0.5×0.5 | 7,6 | 0,50 | 234000 | 0,0159 | 0,2 | 8,3 |
| 6 | 10420 | 6,4 | 10 | 0.6×0.5 | 9,65 | 0,545 | 337000 | 0,0151 | 0,64 | 45,7 |
| 6a | 10420 | 0,8 | Yu. | Ø0.64 | 8,99 | 0,64 | 369000 | 0,0149 | 0 | 0,9 |
| 7 | 10420 | 3,2 | 5 | 0.53×1.06 | 5,15 | 0,707 | 234000 | 0.0312×n | 2,5 | 44,2 |
| Total losses: 185 | ||||||||||
| Table 1. Aerodynamic calculation | ||||||||||
The air ducts are made of galvanized sheet steel, the thickness and dimensions of which correspond to app. N from . The material of the air intake shaft is brick. Adjustable gratings of the PP type with possible sections are used as air distributors: 100 x 200; 200 x 200; 400 x 200 and 600 x 200 mm, shade factor 0.8 and maximum outlet air velocity up to 3 m/s.
The resistance of the insulated intake valve with fully open blades is 10 Pa. The hydraulic resistance of the air heater installation is 100 Pa (according to a separate calculation). Filter resistance G-4 250 Pa. The hydraulic resistance of the muffler is 36 Pa (according to acoustic calculation). Based on architectural requirements, rectangular ducts are designed.
Cross-sections of brick channels are taken according to Table. 22.7.
Local resistance coefficients
Section 1. RR grating at the exit with a section of 200 × 400 mm (calculated separately):
| No. of plots | Type of local resistance | Sketch | Angle α, deg. | Attitude | Rationale | KMS | ||
| F0/F1 | L 0 /L st | f pass / f st | ||||||
| 1 | Diffuser |
 |
20 | 0,62 | — | — | Tab. 25.1 | 0,09 |
| Withdrawal |
 |
90 | — | — | — | Tab. 25.11 | 0,19 | |
| Tee-pass |
 |
— | — | 0,3 | 0,8 | App. 25.8 | 0,2 | |
| ∑ = | 0,48 | |||||||
| 2 | Tee-pass |
 |
— | — | 0,48 | 0,63 | App. 25.8 | 0,4 |
| 3 | branch tee |
 |
— | 0,63 | 0,61 | — | App. 25.9 | 0,48 |
| 4 | 2 outlets | 250×400 | 90 | — | — | — | App. 25.11 | |
| Withdrawal | 400×250 | 90 | — | — | — | App. 25.11 | 0,22 | |
| Tee-pass |
 |
— | — | 0,49 | 0,64 | Tab. 25.8 | 0,4 | |
| ∑ = | 1,44 | |||||||
| 5 | Tee-pass |
 |
— | — | 0,34 | 0,83 | App. 25.8 | 0,2 |
| 6 | Diffuser after fan |
 |
h=0.6 | 1,53 | — | — | App. 25.13 | 0,14 |
| Withdrawal | 600×500 | 90 | — | — | — | App. 25.11 | 0,5 | |
| ∑= | 0,64 | |||||||
| 6a | Confuser in front of the fan |
 |
D g \u003d 0.42 m | Tab. 25.12 | 0 | |||
| 7 | Knee | 90 | — | — | — | Tab. 25.1 | 1,2 | |
| Louvre grille | Tab. 25.1 | 1,3 | ||||||
| ∑ = | 1,44 | |||||||
| Table 2. Determination of local resistances | ||||||||
Krasnov Yu.S.,
„Ventilation and air conditioning systems. Design recommendations for industrial and public buildings”, chapter 15. “Thermocool”
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