Imagine air moving through a cleanroom as a supply chain. The air-handling unit pulls in outside air and conditions it. Ductwork distributes it through the building. Coarse filters catch the bulk of the particles. But there is a final step — a piece of equipment that delivers HEPA-filtered air directly into the critical zone — that is often the least-discussed and most-misunderstood part of the system.
That final step is the HEPA box, also called a filter box or supply-air inlet. It is the terminal device in a cleanroom air-distribution system: a self-contained unit that houses the HEPA filter, an air-volume damper, and a diffuser — all delivering clean, unidirectional air into the room at a controlled velocity and volume.
SCT's HEPA box is designed as a terminal supply-air solution for modular cleanroom systems, suitable for pharmaceutical, laboratory, electronics, hospital, food-processing, and medical-device applications. This guide explains how it works, where it sits in the cleanroom hierarchy, and how to specify the right model for your environment.
1. Terminal Filtration: Where the HEPA Box Fits in the Air System
To understand a HEPA box, it helps to understand where it sits relative to the rest of the air-handling architecture. A cleanroom air system typically follows a staged filtration hierarchy:
• Primary filtration: Outside air intake with pre-filter (G4) to remove large particles. Coarse protection for the downstream equipment.
• Secondary filtration: Medium-efficiency filters (F7/F8) in the air-handling unit. Removes the majority of fine particulate before the air reaches the final stage.
• Terminal filtration: The HEPA box or FFU at the room ceiling. The last line of defence — delivering HEPA-filtered air directly into the critical zone.
The HEPA box is a ceiling-mounted terminal device that combines three functions in one compact housing: the HEPA filter itself, a volume-control damper for air-balancing, and a diffuser to spread the clean air evenly across the room. By contrast, a fan filter unit (FFU) adds a motorised fan to the same housing — it generates its own airflow rather than relying on ducted supply pressure.
Both approaches achieve HEPA-grade terminal filtration, but they suit different system architectures. A ducted HEPA box is the natural choice when you want to balance air volume centrally through the HVAC, and when the air-handling unit already provides sufficient differential pressure. An FFU is preferred when independent, decentralised airflow control is needed — common in modular cleanrooms where the ceiling grid doubles as both supply and filtration.
2. Anatomy of a HEPA Box: What Is Inside
A HEPA box is deceptively simple from the outside — a rectangular housing mounted at the ceiling — but its internal components each serve a distinct and necessary function:
• HEPA Filter — H13/H14/U15/U16. The core element. H13 (≥ 99.99% at 0.3 μm) and H14 (≥ 99.995%) are the standard grades for most cleanroom applications. U15 and U16 offer higher efficiency for ISO 3–5 environments. SCT's HEPA box supports all four grades, with filter efficiency ranging from 99.95% to 99.99995%.
• Volume-Control Damper. Located at the air-intake collar, the damper adjusts the volume of air entering the box, allowing fine-tuning of room air-balance without redesigning the ductwork. Top or side inlet positions are both available.
• Diffuser Plate. Installed at the air-exit face, the diffuser converts the high-velocity airstream from the HEPA filter into a uniform, low-turbulence unidirectional supply pattern. Without the diffuser, the clean air exits as a concentrated jet — creating draughts and uneven coverage.
• Housing (SS304 or powder-coated). The structural frame must be rigid enough to hold the filter seal under continuous airflow and cleanable enough for GMP environments. SS304 is standard; SS316 is available for VHP or chemically aggressive suites.
3. HEPA Box Specifications: SCT Standard Range
The table below summarises the standard SCT HEPA box range. Custom dimensions and configurations are available for project-specific requirements.
| Model | Dimensions (W × D × H, mm) | Air Volume (m³/h) | HEPA Grade |
| SCT-HB01 | 625 × 625 × 300 | 500–800 | H13 / H14 / U15 / U16 |
| SCT-HB02 | 625 × 625 × 400 | 800–1,200 | H13 / H14 / U15 / U16 |
| SCT-HB03 | 900 × 600 × 400 | 1,000–1,500 | H13 / H14 / U15 / U16 |
| SCT-HB04 | 1,200 × 600 × 400 | 1,500–2,000 | H13 / H14 / U15 / U16 |
| SCT-HB05 | 1,200 × 900 × 400 | 2,000–2,500 | H13 / H14 / U15 / U16 |
| SCT-HB06 | 1,200 × 1,200 × 400 | 2,500–3,000 | H13 / H14 / U15 / U16 |
| Custom | Project-specific | Up to 5,000+ | Any grade per ISO 29463 |
Selecting the right model comes down to three inputs: the required air volume per unit (derived from the room's air-change rate and ceiling coverage plan), the ceiling grid dimension, and the cleanliness class — which determines the HEPA filter grade. SCT engineers can run the air-balance calculation and propose the correct model based on your room layout.
4. HEPA Box vs FFU: Choosing the Right Terminal Device
The two most common terminal supply-air devices for modular cleanrooms are the HEPA box and the fan filter unit (FFU). Choosing between them is one of the most common specification questions we receive — so here is a practical decision framework.
| Criteria | HEPA Box | Fan Filter Unit (FFU) |
| Air-source | Ducted supply from AHU | Self-contained motorised fan |
| Best for | Centralised HVAC systems; retrofits | Modular cleanrooms; independent zoning |
| Air-volume control | Damper at box + central HVAC | Variable-speed (EC motor) at each FFU |
| Energy profile | Lower total energy (no fan per unit) | Higher energy (fan in every ceiling tile) |
| Maintenance | Filter replacement only; no fan service | Fan motor + filter replacement |
A practical rule: if your cleanroom runs off a central air-handling unit that already provides sufficient differential pressure, a ducted HEPA box is the simpler, lower-energy solution. If your project uses a modular ceiling grid where each tile needs its own airflow source — particularly for ISO 5–8 cleanrooms with independent room pressurisation requirements — an FFU ceiling is typically the better fit.
5. Applications by Industry
SCT's HEPA box serves a wide range of cleanroom environments. Here is how the selection criteria shift across the main application sectors:
• Pharmaceutical (ISO 14644 ISO Class 5–8 / GMP Grade A–D). Terminal filtration at the ceiling is the primary supply mechanism. HEPA H13/H14 is standard; U15/U16 for Grade A/ISO 5 zones. The damper must allow precise air-volume setting to maintain the required air-change rate and positive pressurisation.
• Laboratory (research & testing). Variable occupancy and equipment heat loads make damper-adjustable supply a practical advantage. HEPA box provides stable, tunable airflow without redesigning the ductwork when the room configuration changes.
• Electronics & Semiconductor (ISO 3–7). High air-change rates and ultra-clean supply are essential. U15/U16 HEPA grades are common at the upper end. The diffuser plate's uniform airflow distribution prevents dead zones that could allow particles to settle on wafers or PCB assemblies.
• Hospital & Healthcare (negative-pressure isolation, OR ventilation). HEPA box supply supports ISO 5/Grade A zones within operating theatres and isolation rooms. Side-inlet configurations allow flexible routing in tight service plenum spaces.
• Food Processing & Medical Devices.Moderate cleanliness classes (ISO 7–8) with a need for cost-effective, reliable terminal filtration. A ducted H13 HEPA box delivers the required performance at a lower total cost than a full FFU ceiling.
6. What Happens After Installation: Air-Balance and Integrity Testing
A HEPA box that is installed but not tested is an unknown quantity. After installation, two verification steps are non-negotiable for a compliant cleanroom:
• Air-volume verification. Using a balometer or pitot-tube traverse at the diffuser face, confirm that each HEPA box is delivering its design air volume. Deviation of more than ±10% from the specification should trigger damper re-adjustment. Incorrect airflow means the room's air-change rate and pressurisation are wrong.
• HEPA filter integrity test (DOP/PAO scan). With the diffuser plate removed or a probe inlet fitted, an aerosol challenge test is performed upstream of the filter. A handheld particle counter scans the downstream face for any leakage. A passing result confirms that the filter media, the seal, and the housing are all intact. This test is required by ISO 14644-3 and EU GMP Annex 1.
Together, these two tests confirm that the terminal supply-air system is delivering the right quantity of air, at HEPA quality, without bypass leakage. Without them, the cleanroom's performance claim rests on the datasheet alone — not on verified reality.
7. Frequently Asked Questions About HEPA Boxes
Q: What is a HEPA box and how does it work?
A: A HEPA box is a ceiling-mounted terminal filtration device that combines a HEPA filter, volume-control damper, and diffuser in one compact housing. Filtered air from the central HVAC ductwork passes through the HEPA filter inside the box, then exits through the diffuser as uniform, low-turbulence supply air into the cleanroom. It is the final filtration stage in a staged air-handling system.
Q: What is the difference between a HEPA box and an FFU?
A: A HEPA box relies on ducted supply pressure from the central AHU; it has no fan of its own. An FFU contains an integrated motorised fan that generates airflow independently, making it suitable for modular cleanrooms where decentralised airflow control is needed. HEPA boxes generally consume less total energy; FFUs offer more independent zoning.
Q: What HEPA filter grade does a cleanroom need?
A: ISO 8 / GMP Grade D environments typically use H13 (99.99% @ 0.3 μm). ISO 7 / Grade C commonly uses H13 or H14 (99.995%). ISO 5 / Grade A/B require H14 minimum, often U15/U16 for the most critical zones. The required grade is driven by the cleanroom classification and the air-change rate, not by the HEPA box itself.
Q: Can a HEPA box be used in a VHP environment?
A: Standard HEPA boxes with powder-coated housings are not suitable for VHP decontamination suites. For VHP environments, specify a stainless-steel (SS304 or SS316) housing with a tempered-glass or VHP-compatible diffuser plate. SCT offers VHP-resistant configurations for pharmaceutical Grade A/B applications.
Q: How do I select the right HEPA box model?
A: The selection is driven by three inputs: the required air volume per unit (derived from the room's air-change rate and coverage plan), the ceiling grid dimension, and the cleanliness class. SCT engineers can run the air-balance calculation and recommend the correct model from the SCT-HB01 to HB06 range or a custom configuration.
8. Why SCT Cleanroom for HEPA Boxes?
• Complete grade range — H13 through U16.SCT HEPA boxes support H13, H14, U15, and U16 filter grades as standard, covering ISO Class 3 through ISO Class 8 applications with a single product family.
• Modular system compatibility.Designed for seamless integration with cleanroom ceiling systems — both new builds and retrofits. Standard footprints match common 600 × 600 mm and 1,200 × 600 mm ceiling grids, with custom dimensions available for non-standard layouts.
• CE-certified with full documentation.Every SCT HEPA box ships with CE Declaration of Conformity, HEPA filter integrity test certificates, material certificates, and dimensional drawings — the documentation package required for GMP and ISO 14644 qualification files.
Specify the Terminal Device That Matches Your System
The HEPA box is the least visible part of a cleanroom air system and often the last thing specified — but it is the device that actually delivers clean air into the critical zone. Choosing the right model, filter grade, and configuration upfront avoids air-balance problems, compliance gaps, and re-work during commissioning.
SCT provides free air-balance calculations and model selection based on your room layout and cleanliness class. Whether you are specifying a new modular cleanroom or upgrading terminal filtration in an existing facility, our engineering team can recommend the correct HEPA box configuration and supply the complete documentation package for your qualification file.
Contact SCT Cleanroom Today
Website: www.sctcleanroom.com
Email: admin@sctcleanroom.com
WhatsApp: +86 15306200553
Post time: Sep-07-2026