How to Perform Air Balance for HVAC Systems?

In biopharmaceutical projects, air balance is often a confusing task. Most people underestimate the steps, duration, and potential issues involved in this process, which is by no means a simple undertaking. Before delving into specific details, let us first ask: What is the ultimate goal of "air balance" in biopharmaceutical facilities? On the surface, the objective seems straightforward: to adjust and balance the flow rates of fresh air, supply air, return air, and exhaust air in accordance with the design specifications of all Air Handling Unit (AHU) systems across the facility. In reality, however, this is merely a means to an end. Our ultimate goal is to meet GMP requirements: cleanroom cleanliness and pressure cascade.

I. Concept of Air Balance

Air balance is divided into two phases: static balance and dynamic balance.

II. Static Balance

Static balance is the first activity conducted after system startup and AHU testing. The purpose of this initial phase of balancing is to adjust the flow rates of fresh air, supply air, return air, and exhaust air to match the design values. This is achieved by regulating manual dampers, Constant Air Volume (CAV) boxes, and Variable Air Volume (VAV) boxes within the ductwork. In addition, it involves adjusting the air flow at each supply air outlet and return/exhaust air inlet on the room side.

It is important to note that room pressure does not need to be a concern during the static balance phase. If there are VAV boxes in the return/exhaust air system, they are fixed/locked at the design flow rate. In other words, the system operates in a static mode and does not respond to any changes in room pressure. Static balance is performed on a system-by-system basis.

A successful static balance for a system requires a balance report that includes at least the following items:

• Air Changes per Hour (ACH)
• Fresh air flow rate of the AHU
• Supply air flow rate (main and branch ducts)
• Return air flow rate (main and sub-branch ducts)
• Exhaust air flow rate (main and sub-branch ducts)
• Air flow at each room outlet and inlet
• HEPA filter face velocity

Once all systems have achieved static balance (i.e., all the above items meet the acceptance criteria), the process proceeds to the next phase: dynamic balance.

III. Dynamic Balance

Before initiating dynamic balance, all AHU systems must have completed static balance. This is a critical prerequisite for entering this phase.

What takes place during dynamic balance? You simply unlock the VAV boxes (if applicable) to operate under room pressure conditions and begin monitoring room pressure. As we all know, pressure cascade (a minimum differential pressure of 10–15 Pa between rooms of different cleanliness grades) is a critical requirement for biopharmaceutical facilities.

If VAV boxes are not installed, adjustments must be made to CAV boxes or manual dampers on the return/exhaust air side to achieve the design pressure. During dynamic balance, the core objective is to achieve the correct room pressure, which ideally should be accomplished by adjusting the return/exhaust air flow rate within the room. Occasionally, it may be necessary to adjust the supply air flow rate set during static balance, but this should be considered a last resort.

Once the required pressure is achieved and stabilized, the process is nearly complete. A dynamic balance report should include the following elements:

• Room pressure (minimum 24-hour trend data)
• Air Changes per Hour (ACH)
• Fresh air flow rate of the AHU
• Supply air flow rate (main and branch ducts)
• Return air flow rate (main ducts)
• Exhaust air flow rate (main ducts)
• Air flow at each room outlet and inlet
• HEPA filter face velocity

If any values fail to meet the acceptance criteria, further adjustments are required. Dynamic balance demands great patience from the team, as it involves finding the optimal balance between fresh air supply, recirculated air volume, differential pressure, air flow rate, and operational costs.

Final Remarks

The cleanroom environment of the pharmaceutical industry is vital to product quality. Both air volume and air changes per hour are mandatory testing items specified in GMP regulations. Air balance is directly related to the purification system's ability to consistently and stably maintain the required production environment and operate normally.

Precise air balance also serves as an energy-saving measure. If the supply air volume far exceeds the actual demand, or if excessive positive pressure leads to significant leakage of clean air, substantial energy waste will occur. Through meticulous adjustments to enable the system to "supply air on demand", it is possible to significantly reduce operational costs while ensuring compliance with environmental standards.

Suzhou Pharma Machinery Co.,Ltd.

2026/01/05

Gino