Ductwork engineering design and data references

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This post was last edited by hyqboom on 2017-10-30 at 16:19. Basic knowledge of duct design: When air used for renting out in small offices flows through ducts, if there are bends, tees, or changes in the cross-section of the ducts, this causes changes in the airflow pattern, resulting in impedance. What are the methods for duct design? There are three methods for duct design: a. Velocity method ； b. Equal friction method c. Static pressure recovery method a. Velocity method. When designing air ducts using this method, one first selects the initial wind speed; then, by referring to tables based on the required air volume, the dimensions of the circular ducts and the friction losses are determined. Therefore, the designer must determine the wind speed in each section of the duct on their own in order to consult tables and find the dimensions and friction losses of circular ducts. Therefore, this method requires extensive design experience and knowledge, and it is generally rarely used. b. Equal friction method: In this approach, it is assumed that each point along the entire duct system has the same friction loss per unit length, and the size of the ducts is calculated based on this assumption. This method is simple to apply, but it results in poor airflow balance; therefore, airflow regulators need to be installed in each branch duct to adjust the airflow as needed. c. Static pressure recovery: This method takes advantage of the fact that as the wind speed in each branch duct decreases, the static pressure increases; this increase in static pressure precisely compensates for the friction losses incurred in the subsequent ducts. Therefore, each branch duct has the same impedance, allowing the system air volume to balance the supply air volume. 1. Basic aspects of duct design: ① Determining the shape of the ducts and selecting their dimensions. ②Calculate the pressure loss of the air duct. By calculating the frictional pressure loss and local pressure loss along the duct, the dimensions of the duct are finally determined and the fan is selected. The pressure loss ΔP (Pa) of the air ducts is given by ΔP = ΔPm + ΔPj, where ΔPm represents the frictional pressure loss along the duct (Pa), and ΔPj represents the local pressure losses in the duct (Pa). Note: The wind speed should be appropriate, the outlets should be evenly distributed, there should be no short circuits between supply and return outlets, and the wind speed at the return outlets must not exceed 1.5 m/s.

2. Standard specifications for air ducts:
- Circular air ducts: Outer diameter D (mm)
Basic series: 100, 80, 60; Auxiliary series: 500, 480, 420, 360, 320, 280, 240, 220, 210, 190, 170, 150, 130, 120, 110, 100
- Rectangular air ducts: Outer dimensions (mm) (length × width)
120 × 120, 160 × 120, 160 × 160, 200 × 120, 200 × 160, 200 × 200,
250 × 120, 250 × 160, 250 × 200, 250 × 250,
320 × 160, 320 × 200, 320 × 250,
630 × 500, 630 × 630, 800 × 320, 800 × 400, 800 × 500, 800 × 630, 800 × 800,
1000 × 320, 1000 × 400, 1000 × 500, 1000 × 630, 1000 × 800, 1000 × 1000,
320 × 320, 400 × 200, 400 × 250, 400 × 320, 400 × 400,
500 × 200, 500 × 250, 500 × 320, 500 × 400, 500 × 500,
630 × 250, 630 × 320, 630 × 400,
1250 × 400, 1250 × 500, 1250 × 630, 1250 × 800, 1250 × 1000,
1600 × 500, 1600 × 630, 1600 × 800, 1600 × 1250,
2000 × 800, 2000 × 1000, 2000 × 1250

Conversion table from circular air ducts to rectangular air ducts:
5, 10, 15, 20, 25, 30, 35, 40, 45, 50
5 ~ 130: 22.2, 22.5, 22.8, 23.1; 33.9, 34.4, 34.9, 35.3;
43.3, 43.9, 44.5, 45.2; 51.4, 52.2, 52.9, 53.6;
58.6, 59.5, 60.4, 61.2; 65.2, 66.2, 67.2, 68.1;
71.3, 72.4, 73.5, 74.5; 76.9, 78.1, 79.3, 80.5;
82.2, 83.5, 84.8, 86.1; 87.2, 88.6, 90.0, 91.3
155, 160, 165, 170, 175: 23.4, 23.7, 23.9, 24.2, 24.5;
35.8, 36.2, 36.7, 37.1, 37.5;
45.7, 46.3, 46.9, 47.5, 48.0;
54.4, 55.1, 55.7, 56.4, 57.1;
62.1, 62.9, 63.7, 64.4, 65.2;
69.1, 70.0, 70.9, 71.8, 72.6;
75.6, 76.6, 77.6, 78.5, 79.5;
81.6, 82.7, 83.8, 84.9, 85.9;
87.3, 88.5, 89.7, 90.8, 91.9;
92.6, 93.9, 95.2, 96.4, 97.6

180, 185, 190, 195, 200, 210 ~ 300: 24.7, 25.0, 25.3, 25.5, 25.8;
37.9, 38.3, 38.7, 39.1, 39.5;
48.5, 49.1, 49.6, 50.1, 50.6;
57.7, 58.4, 59.0, 59.6, 60.2;
66.0, 66.7, 67.4, 68.1, 68.8;
73.5, 74.3, 75.1, 75.9, 76.7;
80.4, 81.4, 82.2, 83.1, 84.0;
86.9, 87.9, 88.9, 89.9, 90.8;
93.0, 94.1, 95.2, 96.3, 97.3;
98.8, 100.0, 101.2, 102.3, 103.4

3. Considerations for air duct design:
① The materials used for air ducts are usually composite ducts or galvanized steel sheets (with paint applied). In cases where building space or utility trenches are available, reinforced concrete or brick ducts can also be used (this is common in ventilation systems for textile industries) ； Its surface should be sanded smooth, and painting is required for higher standards. The floor ducts need to be waterproofed. Ductwork in rooms with corrosive gases can be made of plastic or fiberglass. ②The shape of air ducts is generally circular or rectangular. Circular ducts have high strength and require less steel (with the exception of circular flexible ducts, which have higher resistance), but they take up more space; their elbows and tees need to be placed at greater distances from each other. Rectangular air ducts are widely used in air conditioning systems due to their advantages such as occupying less space, being easy to install, and presenting a neat appearance when installed openly. The aspect ratio of rectangular air ducts should be below 2.5. (High-speed air ducts should preferably use circular spiral ducts.) Elliptical spiral ducts combine the advantages of circular and rectangular ducts, making them an ideal type of duct; however, they are less commonly used at present due to the need for specialized equipment for their fabrication. ) ③ The wall thickness of air ducts can be selected according to the table below: Wall thickness of steel plate air ducts (mm), Diameter of circular air ducts or length of the longer side of rectangular air ducts (mm): ≤200, 220–500, 560–1120, 1250–2000. For low-speed air ducts, the thickness is 0.5 mm; for high-speed air ducts, it is 0.8 mm, 0.75 mm, 1.0 mm, or 1.2 mm respectively. Here’s some information on the distinction between high-speed and low-speed air ducts: These are differentiated based on wind speed – air ducts with a wind speed of 15 m/s or less are considered low-speed air ducts, while those with a wind speed above 15 m/s are classified as high-speed air ducts. When classified by pressure, low-pressure air ducts have a pressure of 3 inWG or less, medium-pressure air ducts have a pressure of 3–6 inWG, and high-pressure air ducts have a pressure of 6–12 inWG. What is the width-to-height ratio of a duct? The width-to-height ratio of a duct is the ratio of its longer side to its shorter side ； The aspect ratio is a very important factor to consider when designing air ducts; an excessively large aspect ratio increases the thickness of the metal sheet and the project costs, so it is necessary to control the aspect ratio during design. ④When the length of the long side of each rectangular duct is greater than 1 m and the duct is relatively long, reinforcement measures such as those shown in the figure below or other similar forms should be adopted. ⑤Duct flanges can be used as shown in the table below. Steel plate duct flanges: For rectangular ducts, the length of the longer side (in mm) is ≤630, 800–1250, 1600–2000. The material specifications for the flanges are as follows: L25×3, L30×4, L40×4. For circular ducts, the diameter (in mm) is ≤140, 150–280, 300–500, 530–1250, 1320–2000; corresponding flange specifications are L20×4 and L25×4. ⑥ Wind speed inside the ducts: The recommended wind speeds are shown in the table below: Wind speed in low-speed ducts (m/s); allowable noise level indoors in dB(A); wind speed in main ducts; wind speed in branch ducts; wind speed at the fresh air inlet: 25–35, 35–50, 50–65, 65–85; respectively, 3–4, 4–7, 6–9, 8–12. Noise levels: ≤2, 2–3, 2–5, 5–8. For high-speed ducts, the maximum wind speed; air volume range (m3/h); maximum wind speed (m/h): 1700–5000 with a wind speed of 12.5 m/h; 5000–10000 with a wind speed of 10000–17000 m/h; 17000–25500 with a wind speed of 17.5 m/h; 25500–42500 with a wind speed of 22.5 m/h; 42500–68000 with a wind speed of 22.5 m/h; 68000–100000 with a wind speed of 22.5 m/h. The wind speed in the fresh air inlets of high-speed ducts, as well as in the branch ducts after the air handling equipment and silencing boxes, is the same as that in low-speed ducts. ⑦Requirements for the layout and fabrication of air ducts (for details, please refer to the relevant course materials from Nan She). 1. Air ducts should be arranged neatly and aesthetically, and in a way that facilitates maintenance and testing. It should be considered in conjunction with other pipelines to prevent adverse effects between hot and cold pipes. During design, consideration should be given to the ease of installing and removing various pipes ； ⒉When arranging air ducts, local resistance should be minimized as much as possible. The central radius of curvature of the elbow should be no less than the diameter or side length of the duct; generally, 1.25 times the diameter or side length can be used. For large-diameter air ducts, guide vanes can be used to reduce resistance; streamlined shapes are preferred for these vanes, and they can also be designed as single pieces for convenience. Its dimensions can be made as shown in the figure below. Its local drag coefficient is ζ=0.1 for streamlined blades and ζ=0.35 for single-blade types. For circular ducts, as well as T- or Y-shaped connections, the angle between the branches should not exceed 45° ； For square ducts, the elbows at T-joints or cross-joints should have the same radius of curvature as the elbows themselves. Behind the elbows and tees, it is advisable to connect branch pipes with straight pipes that have 4–5 equivalent diameters. If this is not possible, the elbows and tees should be designed with guide vanes to prevent excessive resistance and airflow deviation. The correct installation of tees in rectangular ducts is shown in the example diagram below: The diameter change in ducts should be achieved through gradually expanding or contracting sections; the expansion angle on each side of an expanding section should not exceed 15°, while the contraction angle on each side of a contracting section should not exceed 30°. The fan outlet should be connected to the elbow in the direction of the fan blades; when it is necessary to connect in the opposite direction to the impeller rotation or to use a bypass elbow, guide vanes should be installed inside the elbow. 3. Elastic gaskets such as rubber, sponge rubber, or oil-impregnated cardboard should be placed between the duct flanges to prevent air leakage. Asbestos rope should not be used as a gasket. Ducts and duct connections should have no visible holes. ⑧Duct painting (for informational purposes only): 1. Apply rust-proof paint (either red lead rust-proof paint, iron oxide rust-proof paint, or aluminum powder-iron oxide phenolic rust-proof paint) in two coats to the inner surface of thin steel ducts and to the outer surface of ducts that require insulation. The uninsulated outer surface of the thin steel sheet air duct should be coated with one layer of rust-inhibiting primer and two layers of oil-based topcoat ； The color of the paint should match the painting and decoration of the room. 2. Galvanized thin steel sheets can be used without painting. However, the damaged bite marks need to be painted; painting is also required when rust is detected during construction. 3. Remove rust and oil before painting. What are the design considerations regarding duct air velocity? Duct air velocity affects duct size, frictional impedance, and noise vibration. When the wind speed increases, the diameter of the air ducts becomes smaller, but both their frictional resistance and noise levels increase. Therefore, the appropriate wind speeds for different locations can be referred to in the table below: Table of maximum wind speeds for low-speed air ducts. Application areas: Control factor – noise; Main air ducts (FPM); Control factor – duct friction coefficient. Main air ducts, branch air ducts, supply air (FPM), return air (FPM). Residential buildings, apartments, hotel bedrooms, hospital wards: 1300, 1200. Offices, reception rooms, libraries, theaters, large auditoriums: 1100. Ordinary offices, fine dining restaurants, high-end department stores, banks, regular department stores, self-service restaurants. Factories: 2500, 3000, 2200. Air duct design procedure using the equal-friction method: The air duct design procedure using the equal-friction method is as follows: a. Determine the air volume and wind speed of the main air ducts. b. Using the air volume and wind speed values of the main duct, refer to the friction loss table for circular ducts to determine the diameter of the main duct’s circular section and its friction loss value. Circular duct friction loss table: c. By referring to the air volume of each branch duct and the friction loss values, the circular duct friction loss table is used to determine the diameter of the circular ducts for each branch duct. d. Use the circular-to-square duct conversion table to determine the length and width of the square duct based on the diameter of the circular duct. Example: There is a duct system in which the air volume of the main duct is 30,000 CMH and the wind speed in the main duct is 10 m/s. The air volume of the branch ducts is 10,000 CMH each. Determine the dimensions of each duct. a. The air volume of the main duct is 30,000 CMH, with an air velocity of 10 m/s. According to the friction loss table for circular ducts, the friction loss is 0.085 MMAQ/m; the diameter of the circular duct is 103 cm. b. Based on an air volume of 10,000 CMH for the branch duct and a friction loss of 0.085 mmag/m, the circular diameter of the branch duct is 68 cm as determined from the friction loss table for circular ducts. C. Convert the diameter of the circular duct to the dimensions of a rectangular duct by referring to the conversion table for circular-to-rectangular ducts (see the table above). Assuming the height of the main duct is 50 cm and its width is 200 cm, the dimensions of the rectangular duct will be 200×50 cm. Assuming the height of the branch duct is 40 cm and its width is 140 cm, the dimensions of the rectangular duct are 140×40 cm. Selection of fresh air and arrangement of inlets: 1. Selection of fresh air volume: A. Choose based on a minimum fresh air volume of 30 m3/h per person. B. Choose based on the number of air changes per room. For ordinary areas, an air exchange rate of 3 to 5 times per hour is sufficient. 2. Location of fresh air inlet: ⑴ It should be placed in a relatively clean outdoor area, and the concentration of harmful substances in the outdoor air at the inlet should not exceed 30% of the maximum allowable concentration in the indoor working area. ⑵When arranging, ensure that the exhaust and intake vents are kept as far apart as possible. The air intake should be below the exhaust outlet for harmful substances. ⑶To avoid inhaling dust from the outdoor ground, the bottom of the air inlet should not be lower than 2m above the outdoor ground level ； When installed in green areas, it should also not be lower than 1m. ⑷To keep the temperature of the outdoor air drawn in during summer lower, the air inlet should be located in the shaded part of the building, preferably on the north wall, and should be avoided on the roof and west wall. 3. Other requirements for new air inlets: (1) The air inlet should be equipped with louvers to prevent rainwater from entering; fixed louvers should be used, and waterproof louvers should be employed in rainy areas. ⑵To prevent birds from entering, a metal mesh should be installed inside the louvers. ⑶For centralized systems that use a large amount of fresh air during the transition season, it is advisable to install two fresh air inlets; one of them should be a minimum-sized inlet, with its area determined based on the minimum required amount of fresh air ； The other is a new air inlet with variable air volume; its area is calculated by subtracting the minimum air volume from the system’s maximum air volume (allowing for a higher wind speed). Duct design example: For instance, an air handling unit (manufacturer’s model, omitted); the air volume is 2000 m3/h. What should be the specifications of the main duct? Taking the wind speed as 5.5 m/s, the cross-sectional area of the air duct is: 2000 m3/h ÷ 3600 ÷ 5.5 m/s = 0.1010101 m2 = 101010 mm2 ； Duct size: 500mm*200mm ； The wind speed is calculated based on the duct dimensions: 2000 m3/h ÷ 3600 ÷ 0.5 m ÷ 0.2 m = 5.56 m/s, which is within the specified range; therefore, a duct of 500 mm * 200 mm is selected.