💨 Duct Airflow Calculator

Air Flow (CMH)

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Duct Calculation Info

Air Flow (CMH) = Cross Section (㎡) × Velocity (m/s) × 3600.

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Relationship Between Air Velocity and Flow Rate

In duct systems, air velocity and flow rate are the most fundamental yet critical design parameters. Flow rate (Q) is calculated using Q = V × A, where Q is flow rate (㎥/h or CMH), V is air velocity (m/s), and A is duct cross-sectional area (㎡). To convert to CMH, multiply ㎥/s by 3,600. A 200mm duct at 5m/s gives A = π × (0.1)² = 0.0314㎡ and Q = 0.0314 × 5 × 3,600 = 565 CMH. At 300mm, area becomes 0.0707㎡ for 1,271 CMH at the same velocity. For an office of 20 occupants needing 30 CMH each, total is 600 CMH; with a 250mm duct (0.049㎡) the required velocity is 3.4m/s, within the recommended 2~8m/s range.

CMH (Cubic Meter per Hour) Unit

CMH stands for Cubic Meter per Hour, the most widely used airflow unit in ventilation. A 100㎡ office with 2.5m ceiling holds 250㎥; targeting one air change per hour requires 250 CMH. North America uses CFM, where 1 CMH = 0.588 CFM and 1 CFM = 1.699 CMH. Residential facilities require 0.5~1.0 air changes per hour, offices 1.5~2.5, and conference rooms 4~6. Per-person ventilation is typically 20~30 CMH, conference rooms 30~50 CMH, and underground parking 6~10 CMH per square meter of floor area.

Ventilation System Design Standards

Effective ventilation design balances indoor air quality with energy efficiency. Carbon dioxide should stay below 1,000ppm, PM10 below 100㎍/㎥, and PM2.5 below 50㎍/㎥. Ventilation can be sized by occupancy (Q = N × q) or air changes (Q = V × n). A 50-person hall needing 40 CMH each requires 2,000 CMH; a 500㎥ office at 2 changes per hour needs 1,000 CMH. Spaces with cooking facilities need 10~20 times higher ventilation, and heat recovery ventilation should be considered in summer.

Duct Size Selection Methods

Proper duct sizing directly impacts efficiency, noise, and installation cost. Circular duct area is A = π × (D/2)², and required area is A = Q ÷ (V × 3,600). Delivering 1,500 CMH at 6m/s requires A = 0.069㎡, about 297mm diameter, so the standard 300mm is selected. Friction loss is proportional to length and to the square of velocity, with 0.1~0.2 mmAq per meter appropriate for straight ducts. Each 90-degree elbow causes pressure loss equivalent to 3~5m of straight duct. Larger diameters reduce pressure loss and operating cost.

Noise Issues from Air Velocity

Duct noise directly affects occupant comfort, with velocity a major factor. Main ducts typically use 6~10m/s, branch ducts 4~8m/s, and outlet areas 2~5m/s; libraries and studios limit even main ducts to 4~6m/s. Above 8m/s noise rises sharply. Turbulent noise scales with the sixth power of velocity, so doubling velocity adds about 18dB. Increasing diameter by 20% reduces velocity by about 30% and noise by 5~8dB. Attenuators can cut broadband noise by 20~30dB.

HVAC System Efficiency Optimization

Optimizing HVAC efficiency reduces energy costs while keeping comfort. Fan power scales with the cube of flow, so reducing flow by 20% saves about 49% energy. VAV systems adjust flow to load for 30~50% annual savings, and inverter-controlled fans save 20~40% power. Filters should be replaced every 3~6 months; clogging over 30% raises fan load 15~25%. Clean ducts every 2~3 years, and sealing joint leaks recovers 10~20% of lost flow.