Single-Pass vs. Recirculating Airflow Design in Cleanrooms

What Is an AC Cleanroom? (From an Engineer’s Perspective)

Imagine standing in a semiconductor facility in Shenzhen during July. Outside, it is a suffocating 35°C with 90% humidity. Inside the cleanroom, the sensitive lithography equipment demands a flat 22°C and exactly 45% relative humidity. If that humidity drifts by 5%, the photoresist chemicals fail. An AC cleanroom system is the machine that bridges that massive gap. It forces air through HEPA filters 30 to 60 times an hour (Air Changes Per Hour, or ACH). The question is: once that expensive, treated air leaves the room, do you throw it away (exhaust it) or clean it and use it again (recirculate)?

Core Objectives of Cleanroom HVAC Design

Every kilowatt your chiller consumes must be justified. In cleanroom HVAC design, we balance four pillars. If you prioritize “Safety” above all else (e.g., in a BSL-3 lab), efficiency drops. If you prioritize “Energy” (e.g., in generic packaging), you introduce contamination risks that would be unacceptable for sterile injectables. The single pass vs recirculating cleanroom debate is essentially a debate about where you sit on this spectrum.

What Is a Single-Pass Cleanroom HVAC System?

In our industry, we call this “Once-Through” air. A single-pass cleanroom system is an energy hog, but sometimes it’s the only safe option. The system pulls in 100% fresh outside air, cools/heats/filters it, pushes it into the room, and then sucks 100% of it out through the exhaust. Think of it like cooling your house with the windows open and the AC on max. You are constantly trying to condition new, hot, humid air.

Why do we do it? Safety. If you are manufacturing a cytotoxic cancer drug, or handling volatile solvents, you cannot risk taking air from that room and pumping it back into a supply duct that feeds other rooms. The risk of cross-contamination or explosion is too high. Single-pass guarantees that whatever happens in the room, leaves the building.

What Is a Recirculating Cleanroom HVAC System?

This is the workhorse of modern manufacturing. In a recirculating cleanroom, we treat the air as a precious asset. Once we have spent money cooling and filtering the air to 22°C and ISO 7 standards, we want to keep it. We capture about 80-90% of the air leaving the room and loop it back to the AHU. We mix in just enough fresh outside air (Make-Up Air) to keep the room pressurized and provide oxygen for the operators. The AHU only has to “top up” the cooling, rather than starting from scratch with hot outside air.

The Benefit: Massive OPEX reduction. In a humid climate like Southern China, a recirculating AC cleanroom can cut energy bills by 50-60% compared to single-pass.

Project Spotlight: “Project Sunshine” – Pharmaceutical Solid Dose Plant

Location: Jiangsu Province, China
Facility Size: 2,500 m² (ISO 7 & ISO 8 Areas)
Challenge: The client produces oral solid dosage (OSD) tablets. The granulation process generates heavy dust, and the coating process uses volatile organic solvents. The client originally requested a full single pass cleanroom system for safety, but the energy modeling showed operational costs would exceed their budget by 40%.

Deiiang’s Solution: Zoning & Hybrid Airflow

We didn’t force a binary choice between single pass vs recirculating cleanroom. Instead, we dissected the facility layout:

1. Zone A (Corridors, Packaging, Blending): These areas have low risk. We installed a highly efficient recirculating AC cleanroom system (85% return air) to handle the bulk of the facility’s heat load.
2. Zone B (Granulation & Coating): These are high-risk zones. We designed isolated, single-pass air handlers specifically for these rooms. To recover energy, we installed “run-around coil” heat recovery systems, capturing thermal energy from the exhaust air to pre-treat the incoming fresh air without any risk of cross-contamination.

The Result: The hybrid cleanroom HVAC design met all GMP validation requirements for cross-contamination control while reducing the projected annual energy consumption by 35% compared to the original single-pass proposal.

1. Safety vs. OPEX

This is the classic trade-off. Single-pass is the ultimate safety net. If a filter leaks in a single-pass system, the dirty air goes outside. If a filter leaks in a recirculating system, that dirty air might end up in the product in the next room. However, safety costs money. In cleanroom HVAC design, moving from recirculation to single-pass typically triples the thermal load on the chiller.

2. Energy Consumption Comparison

Let’s run the numbers for a standard 1,000 m² facility in Southern China (high humidity). The difference in cooling load is stark.

• Single-Pass: Treating 100% hot/humid outside air. Approx. Load: 1,200 kW.
• Recirculating (20% OA): Treating only 20% outside air; the rest is cool return air. Approx. Load: 400 kW.

The ROI on a recirculating AC cleanroom is often less than 12 months simply due to electricity savings.

3. Regulatory Compliance (Local & International)

Deiiang navigates regulations globally. In China, environmental bureaus are cracking down on VOC emissions. A single pass cleanroom exhausting solvent-laden air will now require expensive RTO (Regenerative Thermal Oxidizer) treatment equipment. Recirculating air with carbon filtration can sometimes avoid this capital cost. In Europe, energy directives (EPBD) practically mandate heat recovery on any large air handling system. We tailor the cleanroom HVAC design to the local code, ensuring you pass inspection the first time.