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PART 7: Calculating Airflow, Pressure, and Selecting VAV Boxes

In HVAC system designs using VAV Boxes (Variable Air Volume Terminal Units), accurate calculation of airflow, pressure, and selecting the appropriate VAV type is essential for achieving operational efficiency, energy savings, and desired indoor air quality.

📊 Airflow Design: The Foundation for Sizing VAV Units

• Maximum airflow (Qmax) Determined based on the cooling (or heating) load of each zone: 

          Cooling Load

Qmax = ---------------------------

           1.2 x (Tsupply - Tzone)

• Where:

  • $T_{supply}\approx {13}^{\circ }CT\_\{supply\}\; \backslash approx\; 13^\backslash circ\; C$$T_{supply}\approx {13}^{\circ }CT\_\{supply\}\; \backslash approx\; 13^\backslash circ\; C$ Tsupply ~13°C (standard cooling system)
  • 1.2 is the air-specific heat conversion factor

   

• Minimum airflow (Qmin) Ensures fresh air intake (according to ASHRAE 62.1 / QCVN regulations):
  • Offices: 0.6–0.8 L/s·m²
  • Per person: ~10 L/s·person

💨 Pressure Design: A Balance Between Engineering and Practicality

• Ideal operating pressure difference (ΔP) for VAV: 50–250 Pa
• Must be sufficient for accurate VAV flow measurement and within expected system pressure losses

📦 VAV Selection Criteria

• Types of VAV:
  • Single Duct: standard cooling zones
  • Fan-Powered: zones requiring pressurization or supplementary heating
  • VAV with Heater: heating-required zones

• Product Certifications: Priority given to AHRI 880 or equivalent for accuracy, noise control, and air tightness

• Noise Levels: Compliant with ASHRAE 130 and AHRI 885 Target noise criteria: NC25–30 at Qmax

📚 Reference Standards & Resources

• AHRI 880 – Performance Rating of VAV Terminals
• ASHRAE 62.1 / 90.1 / Applications Handbook
• SMACNA HVAC Duct Design Manual
• AMCA 300 / ISO 3741 – Sound Standards

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