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Purification of air conditioning ventilation system: Classification of air ducts
2026/10/20
Understanding the classification of ducts is the basis for designing and constructing the ventilation system. Generally, ducts can be classified according to four dimensions: material, cross-sectional shape, working pressure and functional use.
1. Classification by material
Material determines the strength, corrosion resistance, temperature resistance and cost of the duct. It is the most core classification method.
1.1 Metal ducts
Metal ducts have high strength, good air tightness and strong pressure resistance, and are the most commonly used form in air conditioning systems.
Galvanized steel ducts: The most commonly used, suitable for general air supply and exhaust systems in air conditioning, with galvanized surface for rust prevention.
Stainless steel ducts: Corrosion-resistant, high hygiene level, often used in kitchen exhaust, food and medicine, etc., in clean or corrosive environments.
Aluminum ducts: Lightweight, corrosion-resistant, often used in conveying special media or for systems with weight sensitivity.
Color-coated steel (color steel) ducts: Combines strength and decoration, often used for exposed ducts.
1.2 Non-metallic ducts
Non-metallic ducts are lightweight and corrosion-resistant, widely used for corrosive gas transportation or concealed projects with limited space.
Glass fiber reinforced plastic ducts: Corrosion-resistant, light weight, but relatively weak in rigidity.
PVC/plastic ducts: Acid and alkali resistant, often used in chemical ventilation.
Inorganic glass fiber reinforced plastic, glass fiber board, glass fiber composite ducts: Often used in corrosive or need for insulation and integration occasions.
1.3 Flexible ducts
Flexible ducts can be bent and are convenient for installation, often used as connection pieces between equipment and ducts or end branch pipes.
Canvas hoses (soft joints): Used for connection between fans, equipment and ducts, serving as vibration isolation and error compensation.
Metal hoses: High temperature resistance, good strength.
Flexible insulation hoses: Self-insulating layer, often used for fan coil units, end air outlet branch pipes.
2. Classification by cross-sectional shape
Cross-sectional shape affects the hydraulic characteristics, occupied space and processing technology of the duct.
Rectangular ducts (square): Convenient for layout close to the floor or wall, saving space, and are the most commonly used form in air conditioning systems.
Circular ducts: Good hydraulic characteristics, low pressure loss along the way, often used for main ducts of air supply and exhaust and systems with sensitivity to pressure loss.
Elliptical / flat circular ducts: Balance the low resistance of circular ducts and the low net height requirement of flat ducts, often used in areas with limited ceiling height in suspended ceilings.
3. Classification by working pressure
According to standards such as "Ventilation and Air Conditioning Engineering Construction Quality Acceptance Specification" (GB 50243), ducts are classified by working pressure into three levels. The pressure level directly determines the thickness of the sheet, the spacing of the flange and the requirements for tightness (air leakage rate).
Low-pressure ducts: Working pressure P ≤ 500 Pa, used in general air supply and exhaust systems in air conditioning.
Medium-pressure ducts: 500 Pa < P ≤ 1500 Pa, used in systems with higher pressure requirements.
High-pressure ducts: P > 1500 Pa, often used in high-speed air supply, smoke exhaust and other systems with high requirements for tightness.
The higher the pressure level, the higher the requirements for sheet thickness, connection strength and air leakage rate control.
4. Classification by functional use
According to the air flow function it undertakes in the ventilation system, ducts can be classified as:
Supply ducts: Supply treated air or fresh air to the room.
Return ducts: Draw indoor air back to the air conditioning unit for circulation processing.
Exhaust ducts / Smoke exhaust ducts: Excluding indoor polluted air or smoke generated by fire, smoke exhaust ducts usually require high temperature resistance and high tightness.
Fresh air ducts: Introducing fresh outdoor air to meet the requirements of fresh air volume.
Key points for practical application
Based on the above classification, in actual engineering, several principles should be followed:
The main air ducts of air conditioners are mostly made of galvanized steel rectangular ducts. The main supply and exhaust air ducts can adopt circular ducts to reduce resistance.
Flexible soft joints should be installed at the connection points between the ducts and the fans, the equipment, etc., to reduce vibration and compensate for installation errors.
Smoke exhaust ducts are required to be resistant to high temperatures and have high tightness. They are usually made according to the standards of high-pressure ducts.
Non-metallic ducts are suitable for corrosive environments. Flexible insulation hoses are mostly used in the branch ducts of terminal air outlets.
