10 Tips for designing HVAC ventilation systems-Part One

1. Basic content of air duct design

① Determine the shape of the air duct and select the size of the air duct.

② Calculate the pressure loss of the air duct. Through the calculation of the pressure loss along the way (friction) and local pressure loss of the air duct, the size of the air duct is finally determined and the fan is selected.

Air duct pressure loss ΔP (Pa)

The pressure loss of the air duct: ΔP=ΔPm+ΔPj

In the formula: ΔPm-pressure loss along the air duct (Pa);

ΔPj- the local pressure loss of the duct (Pa).

2. Uniform specifications for air ducts

3. Air duct design considerations:

① Air duct materials are generally painted thin steel plates or galvanized thin steel plates. Reinforced concrete or brick air ducts can also be used for building space or trenches; the surface should be smoothed, and paint is required for high requirements. The trench air duct shall be waterproofed. Air ducts placed in rooms with corrosive gases can be made of plastic or fiberglass.

② The shape of the air duct is generally round and rectangular. The circular air duct has high strength and low steel consumption, but takes up a large effective space, and the elbow and tee need a long distance. Rectangular air ducts have the characteristics of small effective space, easy layout, and more beautiful surface installation, so air-conditioning air ducts mostly use rectangular air ducts. The aspect ratio of the rectangular air duct should be below 2.5. The high-speed air duct should adopt circular spiral air duct. The elliptical spiral duct has the advantages of circular and rectangular ducts, and is an ideal duct. However, due to the need for special equipment for processing, it is rarely used at present.

③The wall thickness of the air duct can be selected according to the following table:

The wind speed in the fresh air inlet pipe of the high-speed air duct, the air handling equipment and the rear branch air duct of the silencing static plenum is the same as that of the low-speed air supply.

4. Arrangement and production requirements of air ducts:

1. The air duct should be neatly arranged, beautiful and convenient for maintenance and testing. It should be considered uniformly with other pipelines, and adverse effects between hot and cold pipelines should be prevented. When designing, the convenience of assembly and disassembly of various pipelines should be considered;

2. When the air duct is arranged, local resistance should be reduced as much as possible. The radius of curvature of the center of the elbow should not be smaller than the diameter or side length of the air duct, generally 1.25 times the diameter or side length. Large-section air ducts can be used as guide vanes in order to reduce resistance. The guide vanes are preferably streamlined, and can also be made into a single piece for convenience. Its dimensions can be produced as shown below.

Its local drag coefficient is ζ=0.1 for streamlined blades and ζ=0.35 for single-piece blades.

For round ducts, the included angle of the tee or cross should not be greater than 45°; for square ducts, the elbow of the tee or cross should have the same radius of curvature as the elbow. Behind the elbow and the tee, it is better to have a straight pipe with 4 to 5 equivalent diameters and then connect the branch pipe. If it is impossible, make the elbow and the tee with guide vanes to prevent excessive resistance and airflow deflection .

A pre-insulated duct making machine is an important tool to improve your duct making efficiency. There are many pre-insulated duct making machines in the market. Please make sure that you chooes the right pre insulated duct making machine for your ductworks. If you use a pre insulated duct making machine that is not user-friendly, you may end up spending more time for your pr-insulated duct making process.

Is the tee installation of the rectangular duct correct?

The variable diameter of the air duct should be made into expander and reducer. The expansion angle of each side of the expander should not be greater than 15°, and the contraction angle of each side of the reducer should not be greater than 30°.

The outlet of the fan should be connected to the elbow along the direction of the fan blades. When it is necessary to reverse the direction of the impeller or connect to the elbow, there should be guide vanes in the elbow.

5. Spare parts should put in place.

Elastic gaskets, such as rubber, sponge rubber, oil-soaked cardboard, etc., should be placed between the flanges of the air duct to prevent air leakage. Asbestos rope should not be used as gasket. Air ducts and duct connections should have no visible holes.

Please use a CNC duct fabrication machine for duct fabrication. Duct fabricaiton machine is a CNC machine used to cut pre-insulated ductworks. If you use a duct fabrication machine for your pre-insulated ductwork, your duct making efficiency will be improved greatly.

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