Air Duct

Engineering Tip #101: Pressure Dependent Vs Pressure Independent

Pressure Dependent

Applied using blade dampers

Advantages

  • Straightforward, simple design
  • Low cost of controls
  • Minimum requirement

Disadvantages

  • Stability is compromised because the system is pressure dependent. Changes and fluctuations create improper flow rates and pressurization problems. Rebalancing is required at every damper location when changes are made or fan system performance degrades.
  • The system must be rebalanced frequently
  • Low flexibility. Future adjustment is limited.
  • No inherent monitoring or alarming.
  • Often not possible to measure flow rates accurately during commissioning

Pressure Independent

Applied using VAV boxes or Venturi Valves

Advantages

  • Accurate and stable flow control (within ranges)
  • Eliminates the need for rebalancing – the system is pressure independent and maintains steady flow through system changes, filter loading and HVAC system degradation.
  • Opportunity to manifold exhausts in laboratories and save mechanical capital cost. The alternative can be costly individual exhaust assemblies.

Disadvantages

  • Extra capital cost for the contractor compared to a simple blade damper
  • Maintenance of airflow sensors on VAV boxes (inspection, cleaning and calibration)
Pressure Independent Air Control Valve – Phoenix Controls Accel II

Our goal is to provide you the information to make up your own mind.