What is a Friction Loss in HVAC?
Friction loss in HVAC (Heating, Ventilation, and Air Conditioning) refers to the pressure drop due to air resistance as it moves through ductwork, fittings, and components. It is a critical factor in designing efficient HVAC systems.
Read More about: Friction Loss in an HVAC System.
How to Calculate Friction Loss in HVAC Systems?
Friction loss in HVAC ductwork is calculated using formulas, charts, or specialized software. The most common method is using the Darcy-Weisbach equation or duct friction loss charts based on airflow and duct properties.
Friction Loss can be calculated by using:
- Using the Darcy-Weisbach Equation
- Using the HVAC Friction Loss Chart
- Using the Equal Friction Method (Simplified Approach)
- Online Calculators & Software
1. Using the Darcy-Weisbach Equation
Where:
- hf = Friction loss (feet or meters of air)
- f = Friction factor (depends on duct material and roughness)
- L = Length of duct (feet or meters)
- V= Velocity of air (feet per second or meters per second)
- D = Duct diameter (feet or meters)
- g= Acceleration due to gravity (32.2 ft/s² or 9.81 m/s²)
This equation is more precise but requires knowledge of the friction factor, which can be found in ductwork charts.
2. Using the HVAC Friction Loss Chart
A friction loss chart (like an ASHRAE duct calculator) provides values based on:
- Duct diameter
- Airflow rate (CFM – cubic feet per minute).
- Air velocity.
Steps to Use the Chart:
- Find the airflow (CFM) of your system.
- Locate the duct size (diameter or rectangular dimensions).
- Find the corresponding friction loss per 100 feet of duct.
- Adjust for actual duct length by using:
Where:
- L = total duct length in feet
- Friction loss is in inches of water gauge (in. w.g.)
3. Using the Equal Friction Method (Simplified Approach)
Many HVAC designers use the equal friction method, which assumes a constant friction loss per 100 feet of duct (typically 0.1 in. w.g. per 100 ft). This simplifies duct sizing but may not be as precise.
4. Online Calculators & Software
To save time, you can use HVAC friction loss calculators or duct sizing software like:
- ASHRAE Duct Calculator
- HVAC duct sizing apps
- Engineering toolbox friction loss calculator
Example Calculation Using a Chart:
- Given:
- Airflow = 1,000 CFM
- Duct size = 10-inch round
- Duct length = 50 feet
- From the chart: Friction loss = 0.08 in. w.g. per 100 ft
- Calculation:
Final Answer:
Total friction loss = 0.04 in. w.g. for 50 feet of 10-inch round duct carrying 1,000 CFM of airflow.
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Conclusion
Friction loss is an important factor in HVAC design to ensure proper airflow and efficiency. It can be calculated using equations, charts, or software, depending on the level of accuracy needed. Proper duct sizing and smooth airflow paths help reduce friction loss and improve system performance.
