Differential pressure control is one of the most critical elements of any purification air conditioning system. Effective laboratory pressure difference control ensures a reasonable airflow organization within the purification zone, which is the only way to reliably meet cleanroom purification and process requirements. Today, BOKA looks at laboratory pressure difference control methods, comparing the two main strategies used in cleanroom and lab HVAC design: passive control and active control.
What Is Differential Pressure Control in Cleanroom Systems?
A differential pressure control system manages the infiltration or exfiltration of air between adjacent spaces. By maintaining the correct pressure relationship between rooms, facilities prevent unwanted particles or contaminants from migrating across zones. Strategies generally fall into two categories: passive control (Constant Air Volume) and active control (Variable Air Volume).
Constant Air Volume (CAV): A Passive Pressure Control Strategy
CAV is a passive method that uses a manual air volume control valve to balance air supply and exhaust. The desired pressure difference is created by supplying slightly less (or more) air than is exhausted - this "residual air" produces the target differential. CAV is simple and cost-effective to install, but it comes with notable limitations for laboratory pressure difference control:
Fixed rates - supply and exhaust volumes must stay constant at all times, limiting flexibility.
Limited scalability - exhaust equipment (e.g., biosafety cabinets) can't be added or removed without rebalancing, constraining future expansion.
High energy costs - systems must be sized for full load with a large margin for filter degradation, so they run continuously at high capacity and cost.
Frequent maintenance - as fans, filters, and dampers degrade, the system often needs rebalancing.
Higher noise - continuous full-volume operation raises noise levels.
If these limitations aren't acceptable, CAV may not be the right strategy. Many facilities now improve CAV reliability with a pressure-independent constant air volume device, such as a Venturi valve, on supply and exhaust ducts. This lets airflow adjust dynamically within a set range, largely eliminating the effect of static pressure fluctuations and delivering more stable, constant flow. For buyers evaluating CAV equipment, Venturi valves are worth prioritizing when long-term stability and lower maintenance are priorities.
Variable Air Volume (VAV): An Active Pressure Control Strategy
VAV is an active strategy that continuously adjusts supply or exhaust airflow through an electric air volume control valve to maintain the desired pressure in real time. Because VAV responds dynamically, it typically offers better energy efficiency, lower noise, and more flexibility for future changes than CAV. Active VAV methods fall into two types:
- Pure differential pressure control (DP): directly monitors and controls the pressure differential between spaces.
- Residual air volume / flow tracking control (AV): tracks supply and exhaust airflow rates to calculate and maintain the target residual pressure.
For labs with frequently changing exhaust loads - multiple biosafety cabinets or fume hoods, for example - VAV generally offers a more resilient, future-proof approach, though at a higher upfront cost than CAV.
Choosing the Right Strategy
Both CAV and VAV can effectively control air infiltration and seepage to keep laboratory pressure differentials in spec. Choose CAV with pressure-independent valves for smaller, stable facilities where budget is the main constraint. Choose VAV when you need dynamic response, lower long-term energy costs, or room to expand exhaust equipment over time.