Duct Airflow Calculator
Duct Calculation Info
Air Flow (CMH) = Cross Section (㎡) × Velocity (m/s) × 3600
Complete Guide to Duct Airflow Calculations
Relationship Between Air Velocity and Flow Rate
In duct systems, air velocity and flow rate are the most fundamental yet critical design parameters. Understanding their relationship is key to efficient ventilation system design.
Understanding the Basic Formula
Flow rate (Q) is calculated using the formula "Q = V × A", where Q is flow rate (㎥/h or CMH), V is air velocity (m/s), and A is duct cross-sectional area (㎡). This formula represents the volume of air passing through the duct per unit time. To convert to CMH, multiply ㎥/s by 3,600.
Practical Calculation Example
Consider a circular duct with a diameter of 200mm and air velocity of 5m/s. First, calculate the cross-sectional area: A = π × (0.1)² = 0.0314㎡. Then calculate the flow rate: Q = 0.0314 × 5 × 3,600 = 565 CMH. If the diameter increases to 300mm, the cross-sectional area becomes 0.0707㎡, resulting in a flow rate of 1,271 CMH at the same velocity.
Real-World Application
When designing an office ventilation system with 20 occupants requiring 30 CMH per person, the total required flow rate is 600 CMH. Using a 250mm diameter duct (cross-sectional area 0.049㎡), the required velocity would be 600 ÷ 0.049 ÷ 3,600 = 3.4m/s. This falls within the recommended velocity range (2~8m/s), making it an appropriate design. Duct fabrication costs vary by diameter and length, with typical galvanized 250mm duct costing ₩15,000~₩25,000 per meter.
CMH (Cubic Meter per Hour) Unit
CMH stands for Cubic Meter per Hour and is the most widely used airflow unit in the ventilation industry, representing cubic meters per hour.
Definition and Importance of CMH
CMH indicates the volume of air moving through ducts or vents in one hour. This unit is essential for evaluating and designing building ventilation. For example, a 100㎡ office with 2.5m ceiling height has an indoor air volume of 250㎥. If the goal is one air change per hour, the required flow rate is 250 CMH.
Conversion with CFM
North America primarily uses CFM (Cubic Feet per Minute). The conversion formula is "1 CMH = 0.588 CFM" or "1 CFM = 1.699 CMH". For instance, 1,000 CMH equals approximately 588 CFM. This conversion is necessary when reviewing specifications of imported equipment.
Ventilation Rate Standards
Korean building facility standards specify required ventilation rates by usage. Residential facilities require 0.5~1.0 air changes per hour, offices need 1.5~2.5 changes per hour, and conference rooms require 4~6 changes per hour. The required ventilation per person is typically 20~30 CMH, with conference rooms or lecture halls needing 30~50 CMH. Underground parking requires 6~10 CMH per square meter of floor area. Installing ventilation systems meeting these standards costs ₩50,000~₩150,000 per square meter depending on building scale.
Ventilation System Design Standards
Effective ventilation system design balances indoor air quality assurance with energy efficiency.
Indoor Air Quality Standards
According to the Indoor Air Quality Control Act for multi-use facilities, carbon dioxide concentration must be maintained below 1,000ppm. Particulate matter (PM10) should be below 100㎍/㎥, and fine particulate matter (PM2.5) below 50㎍/㎥. Adequate ventilation is essential to meet these standards. If indoor CO2 levels rise 600ppm above outdoor levels, ventilation is considered insufficient.
Required Ventilation Calculation Methods
Several methods exist for calculating ventilation requirements. First, the occupancy-based method uses "Q = N × q" (N: number of people, q: required rate per person). A lecture hall with 50 people requiring 40 CMH each needs 2,000 CMH total. Second, the air change method uses "Q = V × n" (V: indoor volume, n: air changes per hour). A 500㎥ office targeting 2 air changes per hour needs 1,000 CMH.
Design Considerations
Outdoor conditions, indoor pollutant sources, and usage patterns must all be considered. Spaces with cooking facilities require 10~20 times higher ventilation than typical offices. Seasonal temperature differences are also important; in summer, heat recovery ventilation should be considered for cooling load management. While heat recovery ventilators cost 2~3 times more initially than standard fans (₩80,000~₩200,000 per ㎡), they can reduce annual energy costs by 30~50%.
Duct Size Selection Methods
Proper duct sizing directly impacts system efficiency, noise levels, and installation costs, making it a critical design phase.
Diameter and Cross-Section Calculation
The cross-sectional area of circular ducts is calculated using "A = π × (D/2)²" (D: diameter). Once required flow rate and target velocity are determined, the necessary cross-sectional area can be calculated inversely: "A = Q ÷ (V × 3,600)". For example, to deliver 1,500 CMH at 6m/s velocity requires A = 1,500 ÷ (6 × 3,600) = 0.069㎡, which converts to a circular duct diameter of approximately 297mm. In practice, the standard size of 300mm would be selected.
Pressure Loss Considerations
Pressure loss through ducts consists of friction loss and local loss. Friction loss is proportional to duct length and to the square of velocity. Typically, 0.1~0.2 mmAq pressure loss per meter in straight ducts is appropriate. Local losses from elbows, branches, and dampers must also be considered, with each 90-degree elbow causing pressure loss equivalent to 3~5m of straight duct.
Economic Analysis
Increasing duct diameter raises material and space costs but reduces pressure loss, lowering operating costs. Choosing 300mm instead of 250mm diameter increases duct costs by ₩5,000~₩8,000 per meter, but reduces pressure loss by approximately 40%, decreasing fan power. Calculating 10-year electricity savings shows sufficient return on initial investment. Rectangular ducts offer better space utilization but have 20~30% higher pressure loss than circular ducts.
Noise Issues from Air Velocity
Noise generated in duct systems directly affects occupant comfort, with air velocity being a major contributing factor.
Recommended Velocity Ranges
Recommended velocities by application are determined by noise standards. Main ducts typically use 6~10m/s, branch ducts 4~8m/s, and areas near outlets 2~5m/s. Quiet spaces like libraries or recording studios limit even main ducts to 4~6m/s. When velocity exceeds 8m/s, noise increases sharply, and above 10m/s, duct vibration and air noise reach severe levels.
Noise Generation Mechanisms
Duct noise is classified into three types. First, turbulent flow noise from airflow increases proportional to the sixth power of velocity. Doubling velocity increases noise by approximately 18dB. Second, structural noise from duct wall vibration is particularly severe in thin sheet metal ducts. Third, mechanical noise from fans, dampers, and other mechanical components. Noise significantly increases during sharp direction changes at elbows or branches.
Noise Reduction Measures
Reducing velocity is most effective. Increasing duct diameter by 20% during design reduces velocity by about 30% and noise by 5~8dB. Installing attenuators can reduce broadband noise by 20~30dB, costing ₩150,000~₩300,000 per meter. Lining duct interiors with sound-absorbing material or wrapping exteriors with soundproofing is also effective. Installing 1~2m of flexible duct at fan outlets blocks vibration transmission. Selecting diffuser-type outlets minimizes turbulence.
HVAC System Efficiency Optimization
Optimizing HVAC (Heating, Ventilation, Air Conditioning) system efficiency is key to reducing energy costs while maintaining comfortable indoor environments.
Energy Saving Strategies
Fan power is proportional to the cube of flow rate, so reducing flow by 20% saves approximately 49% energy. Implementing Variable Air Volume (VAV) systems adjusts flow based on load, saving 30~50% annual energy. VAV system construction costs an additional ₩20,000~₩40,000 per ㎡ compared to constant volume, but large buildings can recover investment within 2~3 years. Using inverter-controlled fans enables efficient operation through speed adjustment, with 20~40% power savings versus standard motors.
Maintenance Planning
Regular maintenance is essential for maintaining system efficiency. Filters should be replaced every 3~6 months; when clogging exceeds 30%, fan load increases 15~25%. Duct interior cleaning every 2~3 years prevents flow reduction from dust accumulation. Fan belts and bearings require annual inspection; improper belt tension reduces efficiency by 10~15%. Regular maintenance contracts cost ₩1,000,000~₩3,000,000 annually for small to medium buildings.
Performance Improvement Methods
Various methods exist for improving existing systems. Replacing dampers with automatic control enables precise temperature control in each zone. Adding heat recovery ventilators recovers exhaust heat, reducing heating/cooling loads by 30~40%. Sealing duct joint leaks recovers 10~20% flow loss, with sealing costs of ₩5,000~₩15,000 per ㎡. Implementing IoT-based smart control systems enables real-time monitoring and optimal operation, recovering initial investment within 5 years through energy savings.