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πŸ’¨ Duct Airflow Calculator

Air Flow (CMH)
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Cross Section β€”

Duct Calculation Info

Air Flow (CMH) = Cross Section (㎑) Γ— Velocity (m/s) Γ— 3600.

GUIDE

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01

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 Γ— A, where Q is flow rate (γŽ₯/h or CMH), V is air velocity (m/s), and A is duct cross-sectional area (㎑). To convert to CMH, multiply γŽ₯/s by 3,600. A 200mm duct at 5m/s gives A = Ο€ Γ— (0.1)Β² = 0.0314㎑ and Q = 0.0314 Γ— 5 Γ— 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.

02

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 250γŽ₯; 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.

03

Ventilation System Design Standards

Effective ventilation design balances indoor air quality with energy efficiency. Carbon dioxide should stay below 1,000ppm, PM10 below 100㎍/γŽ₯, and PM2.5 below 50㎍/γŽ₯. Ventilation can be sized by occupancy (Q = N Γ— q) or air changes (Q = V Γ— n). A 50-person hall needing 40 CMH each requires 2,000 CMH; a 500γŽ₯ 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.

04

Duct Size Selection Methods

Proper duct sizing directly impacts efficiency, noise, and installation cost. Circular duct area is A = Ο€ Γ— (D/2)Β², and required area is A = Q Γ· (V Γ— 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.

05

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.

06

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.