Pressure dependent vs pressure independent airflow control

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.
• Non- linear flow devices
• Low flexibility. Future adjustment is limited
• No inherent monitoring or alarming
• Often not possible to measure flow rates accurately during test and balance

Pressure independent

Applied using a VAV box or venturi valve

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 pressurised spaces e.g., labs, hospitals, and save mechanical and building capital cost. The alternative is individual exhaust assemblies.

Disadvantages

• Extra capital cost for the contractor compared to a simple blade damper
• Maintenance of airflow sensors (inspection, cleaning and calibration)

Related

“Properly designed feedback control provides safe, efficient, and consistent systems.”

John Penny, QCxP