What Is a HEPA Box?

1. Definition — What Is a HEPA Box (Terminal HEPA Air Supply Unit)?

A HEPA box — also referred to as a HEPA filter box, terminal HEPA housing, HEPA air supply inlet, or static-pressure plenum air supply unit — is a self-contained, ceiling-mounted or duct-end terminal device that integrates four functional components into a single factory-fabricated assembly: (1) an air inlet connection (top or side configuration, square or round flange) that receives conditioned supply air from the main AHU duct network; (2) a static-pressure plenum chamber — a steel box that equalizes incoming duct velocity pressure into uniform static pressure across the entire upstream face of the filter, eliminating velocity hot-spots that would cause premature localized filter loading; (3) a HEPA (H13 or H14) or ULPA (U15) filter element — factory-installed and gasket-sealed against the plenum's discharge flange; and (4) a diffuser plate or air-outlet panel — perforated, louvered, or directional — that shapes the discharge airflow pattern for the intended cleanroom application.

Unlike a Fan Filter Unit (FFU), which carries its own motor-driven fan and generates its own static pressure, a HEPA box is a passive terminal device — it relies entirely on an upstream air handling unit to deliver air at sufficient duct static pressure to overcome the combined resistance of the duct network, the plenum box, the HEPA media, and the diffuser plate. This makes the HEPA box the rational specification for cleanrooms served by a central AHU with adequate fan external static pressure (ESP) — typically ≥300 Pa available at the most remote terminal — where the capital cost and maintenance burden of individually powered FFUs are not justified by the application. For cleanrooms up to approximately ISO 7 (Class 10,000 / GMP Grade C), HEPA-box-based air distribution remains the predominant global design specification, representing an estimated 60–70% of terminal air supply installations in pharmaceutical, electronics, and laboratory cleanrooms worldwide.

 

Quick Facts — SCT HEPA Box: Key Specifications & Model Range

 

SCT-HB Series — Standard Model Range & Performance Data

Note: Non-standard dimensions, custom airflow ratings, stainless steel construction, and insulation-layer variants are available on project-specific order. All models support H13 or H14 filter grade, top or side air inlet, and multiple diffuser types. Contact SCT for custom-dimension quotation.

 

2. How a HEPA Box Works — Airflow Path, Static Pressure Recovery & Filtration Mechanics

The HEPA box performs three sequential aerodynamic functions that, together, transform the high-velocity, unevenly distributed air arriving from the supply duct into the clean, uniformly distributed air required at the cleanroom ceiling plane: velocity-to-pressure conversion, particulate filtration, and discharge-pattern shaping.

Velocity-to-pressure conversion (plenum function). Supply air enters the HEPA box through the inlet flange — typically at a duct velocity of 4–8 m/s, with a non-uniform velocity profile across the duct cross-section (higher velocity at the center, lower near the walls). The air discharges into the plenum chamber — a significantly larger cross-sectional area than the inlet duct — where the sudden expansion causes the velocity to drop by a factor of 5–10×. Per Bernoulli's principle, this velocity reduction is accompanied by a static pressure recovery that distributes the available pressure uniformly across the entire upstream face of the HEPA filter. Without this plenum, the filter would experience a non-uniform velocity profile — high in the center, low at the edges — causing the central region of the media to load with particulate at 2–3× the rate of the perimeter, shortening the filter's service life and creating a non-uniform discharge velocity into the cleanroom. The plenum depth on SCT-HB models (380–450 mm) is dimensioned to achieve ≥90% velocity uniformity (standard deviation/mean ≤0.15) across the filter face at rated airflow, verified by computational fluid dynamics (CFD) simulation during product development.

Particulate filtration (HEPA function). The now-uniform airflow passes through the HEPA filter media — a pleated, hydrophobic glass-fiber mat with a large extended surface area (typically 15–25 m² of media per m² of filter face area, depending on pleat depth and spacing). The media captures particles via three mechanisms: inertial impaction (dominant for particles ≥1 μm), interception (dominant for 0.3–1 μm), and Brownian diffusion (dominant for particles ≤0.1 μm). The Most Penetrating Particle Size (MPPS) — typically 0.1–0.3 μm — is the particle diameter at which the combined capture efficiency of all three mechanisms is at its minimum. An H14-rated filter achieves ≥99.995% efficiency at the MPPS, meaning ≤5 particles out of every 100,000 entering the media pass through. Critically, this efficiency applies only to air that passes through the media — hence the importance of the continuous gasket or gel seal between the filter frame and the plenum discharge flange, which is factory-assembled and leak-tested before the unit leaves SCT's manufacturing facility.

Discharge-pattern shaping (diffuser function). The filtered air exits through the diffuser plate, which serves two purposes: it protects the downstream face of the HEPA media from accidental contact or damage, and it shapes the discharge airflow pattern for the intended cleanroom application. A perforated-panel diffuser produces a uniform, low-velocity downward discharge suitable for general ISO 7–8 cleanroom ventilation. A 4-directional (4D) louvered diffuser directs airflow laterally in four quadrants, suitable for perimeter-zone air distribution in rooms with ceiling return grilles at the walls. A 1-directional (1D) diffuser produces a unidirectional air curtain suitable for pass-through openings or zone-boundary separation. The diffuser type is specified per room on the mechanical air distribution schedule, not uniformly across the project — different rooms in the same cleanroom facility may require different diffuser configurations depending on their equipment layout, heat-load distribution, and airflow pattern requirements.

 

3. How to Specify a HEPA Box — Five Engineering Selection Parameters

Selecting the correct HEPA box model for a cleanroom project requires matching five interdependent engineering parameters to the mechanical design schedule and the application's regulatory requirements:

a) Rated Air Volume vs. Room Air Change Rate. The HEPA box's rated air volume (m³/h at specified initial pressure drop) must match the room's design supply airflow, which is calculated as: Room Volume (m³) × Design Air Change Rate (ACH). For an ISO 7 pharmaceutical preparation room of 100 m³ at 40 ACH, the total supply airflow is 4,000 m³/h. Using SCT-HB04 units rated at 1,500 m³/h each, three units (4,500 m³/h total) with balancing dampers provide the required airflow with approximately 12% margin for filter loading and future re-balancing. Oversizing — specifying a unit rated significantly above the design airflow — should be avoided because operating a HEPA box at <50% of its rated flow can result in insufficient plenum pressurization, poor velocity uniformity across the filter face, and a discharge velocity too low to overcome the room's thermal buoyancy currents. The correct specification is the smallest model whose rated airflow equals or modestly exceeds (≤25% margin) the room's design airflow per terminal.

b) Air Inlet Configuration — Top vs. Side Connection. Top-inlet (PDC-type) HEPA boxes receive supply air from the ceiling plenum or from ductwork routed above the cleanroom ceiling, with the inlet flange on the top face of the plenum box. This is the most common configuration for cleanrooms with an accessible ceiling plenum and ductwork in the interstitial space above. Side-inlet (PLC-type) HEPA boxes receive supply air from a duct connected to the side face of the plenum box. This configuration is specified when the supply ductwork is routed in the wall or when ceiling-plenum access is restricted (e.g., in a facility with a structural slab directly above the cleanroom ceiling with insufficient plenum height). The side-inlet variant is available with an integral butterfly-type volume control damper — galvanized or stainless steel — that allows individual terminal airflow balancing from within the cleanroom without accessing the ceiling plenum.

c) Filter Grade Selection — H13 vs. H14 vs. ULPA U15. H13 (≥99.99%) is technically sufficient for ISO 7–8 cleanrooms per ISO 14644-4 guidance, and it offers approximately 20–25% lower initial pressure drop (≤120 Pa vs. ≤220 Pa for H14 at rated flow), which translates to lower AHU fan energy consumption over the filter's service life. H14 (≥99.995%) is the default specification for ISO 5–6 cleanrooms and GMP Grade A/B pharmaceutical environments, where the margin between achieved and permitted particle concentration is narrower. U15 ULPA (≥99.9995% at 0.12 μm) is specified for semiconductor photolithography below 28 nm node, advanced optical manufacturing, and ISO 3–4 cleanrooms. For most pharmaceutical, medical device, and electronics assembly applications, H14 is the conservative default — the energy penalty of the higher pressure drop is justified by the reduced contamination risk and the simplified filter-inventory management of using a single filter grade across the facility.

d) Diffuser Type vs. Room Airflow Pattern. The diffuser selection directly affects the cleanroom's airflow pattern at the working plane. A perforated-panel (TP) diffuser produces a uniform, low-velocity downward discharge — ideal for general ISO 7 cleanroom ventilation where turbulent dilution is the dominant cleaning mechanism. A 4-directional (4D) diffuser directs airflow toward the room perimeter, suitable for cleanrooms with wall-mounted return grilles where the airflow must sweep the perimeter before returning. A 1-directional (1D) diffuser is used for air-curtain applications at pass-through openings or zone boundaries. The diffuser selection should be specified room-by-room on the mechanical schedule, not applied uniformly, because airflow pattern requirements vary with room geometry, equipment heat load, and occupancy.

e) Body Material — Powder-Coated Steel vs. Stainless Steel. Cold-rolled steel with electrostatic powder coating (white/RAL 9010, anti-corrosion, acid-resistant) is the standard specification for electronics, general laboratory, and ISO 7–8 pharmaceutical cleanrooms — it provides adequate corrosion resistance for environments cleaned with 70% IPA and quaternary ammonium compounds. AISI 304 stainless steel (TIG-welded, polished finish) is specified for ISO 5 / GMP Grade A/B pharmaceutical cleanrooms subject to routine VPHP bio-decontamination, and for food-processing cleanrooms with wash-down cleaning protocols. AISI 316L stainless steel is specified for facilities using chlorine dioxide (ClO₂) gas sterilization or peracetic acid fogging — the molybdenum content provides the additional pitting-corrosion resistance required for repeated exposure to oxidizing sterilants. The material cost increment from powder-coated steel to 304 SS is approximately 25–40% per unit; the increment from 304 to 316L is approximately 15–20%. SCT manufactures all three grades and provides EN 10204 3.1 material certificates with each shipment.

 

4. Industry Applications — Where HEPA Boxes Deliver Cleanroom Air Supply

Pharmaceutical GMP cleanrooms (Grade A–D): HEPA boxes serve as the terminal air supply devices in non-unidirectional (turbulent dilution) cleanrooms — Grades B, C, and D — where ceiling coverage is typically 15–40% and the cleaning mechanism is dilution ventilation rather than piston flow. For Grade A (ISO 5) unidirectional zones, FFU ceiling arrays are the correct specification; HEPA boxes are used in the surrounding Grade B background environment that supports the Grade A zone. The room-side-replaceable filter design, integrated DOP/PAO test ports, and availability of AISI 304/316L construction make SCT-HB models suitable for GMP-regulated facilities requiring documented filter integrity testing and material traceability.

Electronics & semiconductor manufacturing: HEPA boxes in ISO 6–8 electronics cleanrooms provide terminal filtration for assembly bays, test stations, and packaging areas where the particulate threat is primarily inert dust and fiber contamination rather than viable microorganisms. The electrostatic powder-coated steel body (± conductive coating available on request) prevents galvanic corrosion in environments with ESD-sensitive components.

Hospital operating rooms & healthcare facilities: HEPA boxes in operating-room ceilings deliver H14-filtered supply air directly over the surgical field, forming a sterile air column that displaces airborne microorganisms away from the open surgical site. The 4-directional diffuser configuration is commonly specified for OR applications, directing filtered air to cover the full sterile field with the surgical team positioned within the clean-air zone.

Biotechnology & research laboratories: HEPA boxes in BSL-2 and BSL-3 laboratories provide terminally filtered supply air to containment zones where exhaust air is separately HEPA-filtered before atmospheric discharge. The airtight welded plenum construction and continuous gasket seal prevent contaminated room air from bypassing the filter and entering the supply ductwork during fan-off or maintenance conditions.

Food processing & sterile packaging: AISI 304 stainless steel HEPA boxes with wash-down-compatible diffuser panels are specified for food-processing cleanrooms where the ceiling-level equipment is subject to periodic wet-cleaning with detergent solutions and low-pressure water spray. The stainless steel construction and fully sealed diffuser-to-plenum interface prevent moisture ingress into the plenum chamber and filter media.

 

5. Frequently Asked Questions — HEPA Box Selection, Installation & Maintenance

Q1: What is the difference between a HEPA box and a Fan Filter Unit (FFU)? Which should I specify?

A HEPA box is a passive terminal device — it contains a HEPA filter, plenum chamber, and diffuser, but has no internal fan. It relies entirely on the central AHU to deliver air at sufficient duct static pressure (typically ≥300 Pa ESP at the most remote terminal). An FFU contains its own motor-driven fan and generates its own static pressure, operating independently of the AHU's fan capacity. Specify HEPA boxes when: (a) the facility has a central AHU with adequate available ESP, (b) the cleanroom classification is ISO 6–8 (turbulent dilution ventilation), (c) capital cost per terminal is a primary constraint (HEPA boxes are 50–70% less expensive per unit than equivalent-sized FFUs), and (d) individual terminal airflow modulation is not required — all terminals on the same AHU zone operate at the same duct pressure. Specify FFUs when: (a) ceiling coverage ≥60% is required (ISO 5 unidirectional flow), (b) per-zone or per-terminal airflow modulation is needed for pressure-cascade control, or (c) the facility does not have a central AHU with the available ESP to drive a passive terminal array. HEPA boxes and FFUs can coexist in the same facility — HEPA boxes in ISO 7–8 background zones, FFUs in ISO 5 critical zones — sharing a common make-up air AHU and BMS platform.

Q2: Can SCT HEPA boxes be supplied with ULPA (U15) filters for semiconductor applications?

Yes. The SCT-HB model range supports filter upgrades from the standard H13 or H14 to U15 ULPA (≥99.9995% at 0.12 μm MPPS, per EN 1822-1). The plenum chamber, gasket sealing surface, and clamping mechanism are identical across filter grades — the ULPA upgrade is a filter-element substitution that does not require a different housing model. The practical considerations for ULPA specification are: (a) initial pressure drop increases by approximately 30–50% compared to an H14 filter of the same face dimensions (typically 280–350 Pa vs. ≤220 Pa at rated flow) — the central AHU must have sufficient available ESP to accommodate this additional resistance at every terminal on the zone; (b) ULPA media is more sensitive to handling damage and requires careful unpacking and installation procedures; (c) the factory lead time for ULPA-filter-equipped HEPA boxes is typically 7–10 working days longer than for standard H14 units, reflecting the ULPA media procurement and certification cycle. Contact SCT for a ULPA-option quotation with your project's airflow schedule and duct-static-pressure budget.

Q3: How is filter replacement performed on an SCT HEPA box — do I need ceiling-plenum access?

SCT HEPA boxes are designed for room-side filter replacement — the filter element is removed and replaced from within the cleanroom, without requiring access to the ceiling plenum above. The procedure: (a) isolate the AHU supply to the zone or close the integral damper (if equipped); (b) remove the diffuser plate — typically secured by quarter-turn fasteners, captive screws, or hinge-latch mechanisms that do not require tools and do not generate particulate (no drilling, no screw-thread debris); (c) release the filter clamping mechanism — cam-lock levers or spring-loaded toggle clamps — and lower the spent filter from the plenum face; (d) clean the gasket sealing surface with 70% IPA wipe and inspect for damage or corrosion; (e) install the replacement filter, ensuring the gasket is correctly seated and the clamping mechanism is uniformly engaged (tighten bolts in a star pattern to the specified torque if a bolted-clamp design); (f) re-attach the diffuser plate; (g) perform a DOP/PAO aerosol integrity test on the newly installed filter per ISO 14644-3 Annex A.6 before returning the zone to service. The room-side-replaceable design eliminates the need to shut down adjacent cleanroom zones for ceiling-plenum access and reduces filter-change downtime from 2–4 hours (ceiling-access method) to approximately 30–45 minutes per unit.

Q4: Can the SCT HEPA box be used in a ceiling with limited plenum height?

Yes. The SCT-HB03 and SCT-HB05 models feature a reduced-height plenum (380 mm) specifically designed for installations where the interstitial space between the cleanroom ceiling and the structural slab above is constrained. For installations with extremely limited plenum height (<400 mm), the side-inlet (PLC-type) configuration routes the supply duct connection to the side face of the plenum rather than the top face, eliminating the vertical clearance required for a top-entry duct elbow. For custom applications where the standard 380 mm plenum height still exceeds available clearance, SCT can fabricate a custom-dimension plenum with a reduced height — typically down to 300 mm minimum — with a correspondingly larger plan-area footprint to maintain adequate plenum volume for static-pressure recovery and velocity uniformity. Custom plenum dimensions add approximately 7–12 working days to the manufacturing cycle and are priced on a project-specific basis. Provide the available plenum height, target airflow per terminal, and inlet-configuration preference when requesting a custom-dimension quotation.

Q5: What is the typical lead time and export logistics for SCT HEPA box orders?

Standard-configuration HEPA boxes — SCT-HB series, cold-rolled steel with powder coating, H14 HEPA filter, perforated-panel diffuser, top-inlet configuration — ship within 20–30 calendar days from order confirmation (FOB Shanghai). Ocean freight transit times are destination-dependent: 18–22 days to major European ports (Rotterdam, Hamburg), 25–30 days to US West Coast (Los Angeles/Long Beach), and 30–35 days to Middle East ports (Jebel Ali, Dammam). Custom configurations — stainless steel body, H13 or U15 filter grade, side-inlet with integral damper, custom plenum dimensions, or non-standard diffuser type — add 7–15 working days to the manufacturing cycle depending on the specific combination of options. All HEPA boxes are individually factory-tested for filter integrity before packing and are shipped with the HEPA filter pre-installed and gasket-sealed — ready for duct connection, diffuser attachment, and in-situ integrity verification on site. SCT provides export documentation including commercial invoice, packing list, bill of lading, filter test certificates (per EN 1822-1), material certificates (EN 10204 3.1 for SS variants), and CE Declaration of Conformity where applicable. For current lead times and a project-specific quotation, contact SCT at admin@sctcleanroom.com or via WhatsApp at +86 15306200553.

 

The HEPA box — the passive terminal air supply device that converts duct velocity into uniform filtered downflow — remains the workhorse of cleanroom air distribution worldwide. It is not as technologically visible as the intelligent FFU with its Modbus interface, nor as physically imposing as the air shower with its interlocked stainless steel doors. But in the majority of ISO 6–8 cleanrooms, it is the HEPA box that delivers the filtered air — silently, reliably, and with no moving parts to maintain — for the 3–5 years between scheduled filter replacements. SCT's HB series, manufactured in Suzhou with CE-certified quality management, provides six standard models covering 500–3,000 m³/h per terminal in powder-coated steel or stainless steel, with H13/H14/U15 filter grades, top or side inlet, and multiple diffuser configurations — a specification breadth that allows the mechanical engineer to select the correct terminal for each room on the air distribution schedule without compromise. For project-specific technical consultation, custom-dimension quotation, or filter-replacement program planning, contact the SCT engineering team at admin@sctcleanroom.com or via WhatsApp at +86 15306200553.

 www.sctcleanroom.com  |  ✉️ admin@sctcleanroom.com  |   WhatsApp: +86 15306200553

#HepaBox  #HepaAirBox  #SupplyAirInlet  #HvacHepaBox  #CleanroomEquipment  #HEPAFilter  #CleanroomCeiling  #TerminalHEPA  #CleanroomHVAC