Learn how to accurately calculate the supply airflow (CFM) of an Air Handling Unit (AHU) using differential pressure and K-Factor. Includes real-life example, formula, and field applications.
How to Calculate Supply CFM of an AHU Using Differential Pressure and K-Factor
In HVAC design, installation, and commissioning, accurate airflow measurement is essential especially when dealing with large Air Handling Units (AHUs). One widely used method is calculating Supply CFM using Differential Pressure (ΔP) and a device-specific K-Factor. This method is practical, accurate, and often used in Testing, Adjusting, and Balancing (TAB) procedures.
This guide walks you through the step-by-step process, along with a real-life example to make the concept clear and field-applicable.
What Is a K-Factor in AHU Airflow Measurement?
The K-Factor is a constant provided by the manufacturer of an airflow measuring device such as:
- Flow rings
- Pitot tube arrays
- Averaging flow grids
- Venturi tubes
This constant correlates the square root of differential pressure to the actual air volume flow rate.
CFM Calculation Formula Using K-Factor
CFM=K×√(ΔP)
Where:
- CFM = Airflow in Cubic Feet per Minute
- K = K-Factor (device-specific constant)
- ΔP = Differential Pressure (in inches of water column or "in. w.c.")
Tools Needed
- A differential pressure manometer or digital pressure sensor
- Access to the airflow measuring station (inside the AHU or supply duct)
- Manufacturer-provided K-Factor for the installed device
Real-Life Example: AHU in a Hospital Operating Theater
Let's say you're on-site at a hospital during HVAC commissioning. You're checking if the AHU is delivering the correct airflow to maintain positive pressure in the operating room.
Example Data:
- Device installed: Averaging Flow Grid
- K-Factor provided: 1250
- Measured Differential Pressure (ΔP): 0.64 in. w.c.
Step-by-Step Calculation:
Result: The AHU is supplying 1000 CFM, which you can now compare with design specifications.
Step-by-Step Procedure for Field Use
1.Verify Airflow Measuring Device
Ensure the AHU has a properly installed device designed for differential pressure-based airflow measurement.
2. Obtain the K-Factor
Find the K-Factor from the product manual, manufacturer's data sheet, or sticker on the device.
3. Measure Differential Pressure (ΔP)
Use a calibrated manometer or digital pressure sensor. Measure pressure between high-pressure and low-pressure taps on the airflow station.
4. Apply the Formula
Insert values into the equation:
CFM=K×√(ΔP)
5. Compare with Design Flow
Use the calculated CFM to verify compliance with design airflow, make balancing decisions, or adjust damper settings.
Field Considerations for Accurate Results
Why Use the K-Factor Method?
Benefits:
1. Non-intrusive measurement
2. Ideal for large airflow systems
3. Faster than traditional methods
4. Minimal disturbance to system operation
5. Good for AHUs, RTUs, VAV systems, etc.
Where Is This Method Commonly Used?
1. Commercial HVAC commissioning
2. Hospital and cleanroom systems
3. Industrial process ventilation
4. Variable air volume (VAV) system verification
5. Energy audits and retro-commissioning
Final Thoughts
Using Differential Pressure and K-Factor to calculate AHU supply CFM is a reliable and efficient technique widely adopted in the HVAC industry. It saves time, reduces manual errors, and supports precise system balancing—especially when traditional velocity tools or flow hoods are impractical.
Whether you're an HVAC design engineer, TAB technician, or facility manager, mastering this method enhances your ability to ensure airflow accuracy, system performance, and indoor comfort.
){kind=link}
0 Comments
Please avoid posting spam links in the comment section.