Painful conflicts and problems in HVAC design often arise due to misunderstanding of the design requirements.

When designing HVAC systems for pharmaceutical cleanrooms, one parameter that often creates confusion is Air Changes Per Hour (ACPH). ACPH significantly affects duct sizing, filter selection, air handling units (AHUs), space planning, energy consumption—and ultimately, cost.

The higher the design ACPH, the larger the air distribution system required, leading to greater energy use and installation costs. Conversely, lower ACPH means smaller systems and reduced energy and capital costs.

Large pharmaceutical companies often follow internal global engineering guidelines that specify minimum air changes for various cleanroom grades. For instance, a Grade C cleanroom might require a minimum of 25 or 30 ACPH. Some guidelines mention acceptable tolerances; others don’t. And that’s where confusion begins for HVAC designers.

Suppose you’re tasked with designing the HVAC system for a suite of Grade C cleanrooms. The guideline states: “Minimum 30 ACPH for Grade C.”

What should you design for?

Many designers assume the obvious answer is 30 ACPH. Seems obvious, right?

The problem arises when “minimum” is mistaken for “design.” There is a key difference between the minimum acceptable and what you actually design for.

If you design for exactly 30 ACPH, while 30 is also the minimum requirement, here’s what can happen:

1. All equipment—filters, grilles, ductwork, VAVs/CAVs—will be selected for exactly 30 ACPH. This can be a constraint for future upward adjustments.
2. During commissioning, the air balancing contractor will target 30 ACPH, but most contractors allow a tolerance of ±10%. If the final result is 29.5 ACH, the balancer may consider it acceptable. However, you’ve now dropped below the minimum requirement—a failure.
3. The qualification team will reject any room with <30 ACPH, even if particle counts, temperature, and humidity are all within limits. For them, “minimum” is not negotiable.
4. If you try to ramp up ACPH post-design to above the minimum, you may hit capacity limits in your air distribution system and/or Air Handling Unit (AHU). Even if you succeed, expect 20% higher pressure drops and 30% more energy consumption than originally estimated.

Some may argue that typical design margins can absorb such increases. But design margins are intended for unforeseen issues, not predictable and preventable design gaps.

To avoid unnecessary confusion, here is one proven way to go about it:

1. Review the client’s guidelines carefully. Do they mention:
   • Design ACPH (single number)?
   • Design ACPH (a range)?
   • Minimum ACPH with tolerance?
   • Minimum ACPH without tolerance?
2. If it’s the last item in the above list, you have two options:
   • Design with a margin (e.g., 10% higher, meaning 33 ACH for a 30 ACPH minimum), or
   • Clarify the expectations upfront with the client’s global engineering team.

It may seem like a small issue, but multiply this across 30–40 cleanrooms, and it can result in major confusion during commissioning—and potentially, failures during qualification.

Good design starts by clarifying what’s actually required.

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The design ACPH depends not just on guidelines, but also on:

• Room classification
• Room cooling load
• Direct exhaust from the room
• Rate of particle generation inside the room
• Room volume and layout

Also, worth noting: ACPH itself does not directly determine room cleanliness. Cleanliness depends on:

• Total supply airflow volume
• Ventilation effectiveness
• Particle generation rate

However, ACPH does impact the room recovery time after a contamination event.

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