1. What is an Air Velocity and Flow Calculator?

Definition: This calculator estimates air velocity (feet per minute, FPM) and volumetric flow (cubic feet per minute, CFM) in a duct based on pressure measurements and duct dimensions.

Purpose: It helps HVAC professionals and engineers determine airflow characteristics for system design, maintenance, and optimization, ensuring efficient ventilation and air quality.

2. How Does the Calculator Work?

The calculator uses the following method, based on Pitot tube measurements:

Formulas: \[ P_v = P_m - P_s \] \[ V = 4005 \sqrt{P_v} \] \[ A_{\text{rectangular}} = \text{width} \times \text{height} \] \[ A_{\text{circular}} = \pi \times \left(\frac{\text{diameter}}{2}\right)^2 \] \[ Q = V \times A \] Where:

• \( P_v \): Velocity pressure (in H₂O)
• \( P_m \): Measured pressure (in H₂O)
• \( P_s \): Static pressure (in H₂O)
• \( V \): Air velocity (FPM)
• \( \rho \): Air density (0.075 lb/ft³ at 70°F, 29.92 in Hg)
• \( A \): Cross-sectional area (ft²)
• \( Q \): Volumetric flow (CFM)

Unit Conversions:

• Pressure:
• 1 in Hg = 13.595 in H₂O
• 1 Pa = 0.0040146 in H₂O
• 1 psi = 27.68 in H₂O
• Dimensions:
• 1 cm = 0.0328084 ft
• 1 m = 3.28084 ft
• 1 in = 0.0833333 ft

Steps:

• Enter measured and static pressures and select their units (in H₂O, in Hg, Pa, or psi).
• Select duct shape (rectangular or circular).
• Enter duct dimensions (width and height for rectangular, diameter for circular) and select their units (cm, m, in, ft).
• Convert pressures to in H₂O and dimensions to ft.
• Calculate velocity pressure (\( P_v = P_m - P_s \)).
• Compute air velocity using the velocity formula.
• Calculate duct area based on shape.
• Compute volumetric flow (\( Q = V \times A \)).
• Display results, rounded to 2 decimal places.

3. Importance of Air Velocity and Flow Calculations

Accurate airflow measurements are crucial for:

• Efficiency: Optimizing HVAC system performance and energy use.
• Air Quality: Ensuring adequate ventilation for occupant health.
• System Health: Preventing issues like filter clogging or duct blockages.

4. Using the Calculator

Examples:

• Example 1 (Rectangular Duct): Measured pressure = 2.5 in H₂O, static pressure = 2.0 in H₂O, duct width = 60 cm (1.9685 ft), height = 45 cm (1.4764 ft):
  • Velocity pressure: \( 2.5 - 2.0 = 0.5 \, \text{in H₂O} \)
  • Velocity: \( 4005 \sqrt{\frac{0.5}{0.075}} \approx 4005 \sqrt{6.667} \approx 10338.77 \, \text{FPM} \)
  • Area: \( 1.9685 \times 1.4764 \approx 2.9056 \, \text{ft}^2 \)
  • Flow: \( 10338.77 \times 2.9056 \approx 30045.91 \, \text{CFM} \)
• Example 2 (Circular Duct): Measured pressure = 0.1324 in Hg (1.8 in H₂O), static pressure = 0.1103 in Hg (1.5 in H₂O), diameter = 12 in (1 ft):
  • Convert pressures: \( 0.1324 \times 13.595 \approx 1.8 \, \text{in H₂O}, 0.1103 \times 13.595 \approx 1.5 \, \text{in H₂O} \)
  • Velocity pressure: \( 1.8 - 1.5 = 0.3 \, \text{in H₂O} \)
  • Velocity: \( 4005 \sqrt{\frac{0.3}{0.075}} \approx 4005 \sqrt{4} \approx 8010 \, \text{FPM} \)
  • Area: \( \pi \times (1/2)^2 \approx 3.1416 \times 0.25 \approx 0.7854 \, \text{ft}^2 \)
  • Flow: \( 8010 \times 0.7854 \approx 6289.05 \, \text{CFM} \)

5. Frequently Asked Questions (FAQ)

Q: Why measure both measured and static pressure?
A: Measured pressure includes both static and velocity components. Subtracting static pressure gives velocity pressure, which is used to calculate air velocity.

Q: Why is an averaging Pitot tube recommended?
A: Air velocity varies across a duct due to friction at the walls. An averaging Pitot tube with multiple sensing points provides a more accurate average velocity.

Q: Can this calculator replace professional equipment?
A: No, this is an estimate. For precise measurements, use calibrated sensors like Setra’s SRIMV and consult HVAC professionals for system design.

Reference

https://www.setra.com/blog/how-to-measure-velocity-and-flow

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