Comfort and Airflow) Airflow Measurements and Instrumentation

 

1. Basic Tools and Principles for Measuring Airflow

• Efficiency Ratings: Ensure any airflow instrumentation meets OEM (Original Equipment Manufacturer) efficiency ratings.
• Selection of Tools: Choose tools that best meet specific needs; they may perform various tasks and require proper training.
• Calibration:
  • Analog Anemometers: Can be field calibrated (e.g., adjusting the screw on a differential pressure gauge).
  • Electronic Equipment: Often self-calibrates at startup but may require periodic recalibration by the manufacturer.

2. Measuring Velocity and Airspeed

• Importance of Velocity: Determines airflow volume, measured in feet per minute (fpm).
• Instruments Used:
  • Anemometers/Velometers: Measure instantaneous velocity. Transversing the duct helps obtain an average velocity.
  • Types of Anemometers: Analog, digital, thermal, and rotating vane.
  • Measuring CFM (Cubic Feet per Minute):
    • Take multiple readings for accuracy.
    • Use fpm readings and the free area factor of the register or grill to calculate CFM.
    • Free area factor usually ranges from 92% to 98% of the actual area.

3. Types of Anemometers

• Digital Anemometer: Can average readings over time.
• Thermal (Hot Wire) Anemometer: Has a probe placed in the airstream to measure velocity.
• Rotating Vane Anemometer: Measures air velocity with rotating vanes.

4. Understanding Pressure in Ducts

• Static Pressure: Resistance to airflow, exerted outward in the duct.
• Velocity Pressure: Moving pressure in the direction of airflow.
• Total Pressure: Sum of static and velocity pressures.
• High Static Pressure: Indicates low airflow; the duct cannot move the air introduced by the blower.
• Calculating Air Volume: Use the formula CFM = Area (sq. ft) * Velocity (fpm).

5. Tools for Measuring Pressure

• Pitot Tubes and Manometers: Measure pressure in inches of water column in larger ducts.
• Inclined Manometer: High precision, used for very small pressure measurements.
• U-Tube Manometer: Measures both positive and negative suction pressures, often needing calibration.
• Diaphragm Type Differential Pressure Gauge: Measures the difference between two pressures.
• Absolute Pressure (psia) vs. Gauge Pressure (psig): Absolute pressure uses absolute zero; gauge pressure uses atmospheric pressure as zero.

6. Airflow Hoods

• Function: Provide the most accurate measurement of CFM and fpm.
• Usage: Covers the entire opening of the register or grill.

7. Calculating CFM Without Airflow Hood

• Formula: CFM = (Heat Output in BTUs/hr) / Temperature Rise (TD) * 1.08
• Application: More common in electric heat furnaces.

8. Impact of Humidity on Airflow

• High Relative Humidity: Makes air heavier, which may affect airflow performance.
• Airflow Adjustment: Necessary in environments of extreme humidity or dryness to maintain performance.