What Is a Stacked Pass Box?

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1. Definition — What Is a Stacked Pass Box?

A stacked pass box — also referred to as a double-deck pass box or vertical pass-through chamber — is a cleanroom material transfer device that integrates two independently sealed pass-box chambers into a single vertical assembly, one mounted above the other, sharing a common structural frame. Each chamber functions as a controlled-transfer interface between two adjacent rooms of differing or equal cleanliness classifications, with interlocked doors on both the “dirty” and “clean” sides preventing simultaneous opening and thereby maintaining the pressure cascade between zones. The stacked configuration multiplies transfer capacity within the same floor footprint — delivering two independent transfer points in the wall area that a single-chamber pass box would occupy — making it the rational specification for facilities where material-flow volume exceeds the throughput of a single chamber but where wall space for additional side-by-side pass boxes is unavailable.

SCT manufactures stacked pass boxes in both static and dynamic configurations. A static stacked pass box relies on mechanical or electromagnetic door interlocking and UV-C germicidal irradiation (254 nm, typically 2 × 8 W lamps per chamber) for contamination control — suitable for transfers between rooms of equal ISO classification where active air filtration is not required. A dynamic stacked pass box integrates a HEPA H14 recirculation system within each chamber, delivering ISO 5 (Class 100 / GMP Grade A) air cleanliness at ≥0.45 m/s uniform downflow velocity, for transfers from uncontrolled or lower-classification zones into ISO 7 or ISO 5 cleanrooms. Both configurations are fabricated from AISI 304 stainless steel (316L optional) with coved-corner internal geometry, flush-mounted doors with tempered-glass viewing panels, and a microprocessor-based control system managing interlock logic, UV cycle timing, and — on dynamic models — fan-speed and filter-pressure-drop monitoring.

 

Quick Facts — SCT Stacked Pass Box: Key Specifications

 

2. The Engineering Rationale — Why Stack Two Pass Boxes Vertically?

The stacked pass box addresses a constraint that cleanroom facility planners encounter repeatedly: wall length is finite, but material-transfer throughput requirements grow with production volume. In a pharmaceutical facility, raw ingredients enter the Grade C preparation zone through pass boxes from the uncontrolled warehouse corridor. In-process samples move from Grade C to Grade B through a second set of pass boxes. Finished products exit to packaging through a third set. As the number of transfer points multiplies, the cumulative wall length consumed by single-chamber pass boxes becomes a binding constraint on cleanroom layout — particularly in retrofits of existing buildings where structural columns, existing doorways, and equipment clearances restrict available wall segments.

The stacked configuration solves this by utilizing vertical space — the dimension that is most abundant in a cleanroom with a standard 2.4–3.0 m ceiling height — rather than horizontal wall length. Two independent pass-box chambers, each with its own interlocked doors, control system, and (on dynamic models) independent HEPA recirculation loop, are mounted in a common stainless steel frame occupying the same wall penetration and floor footprint as a single-chamber unit of equivalent plan-area dimensions. The upper chamber typically serves lighter, smaller-format transfers (sterile components, sampling containers, documentation), while the lower chamber — positioned at a more ergonomic height — handles heavier loads (bulk raw-material containers, equipment change parts, waste-out transfer). This assignment is not dictated by the hardware; both chambers are engineered to the same cleanliness and structural specification. It is an operational choice enabled by the dual-chamber architecture.

A secondary benefit — often underappreciated during the design phase but immediately valued during operation — is transfer segregation. With two independent chambers, material inflow and outflow can be physically separated: the upper chamber dedicated to incoming sterile materials, the lower chamber to outgoing waste or samples. This eliminates the operational conflict where a single-chamber pass box is occupied for a waste-out transfer while incoming sterile materials queue in the uncontrolled corridor — a bottleneck that directly impacts production throughput in single-chamber installations. The stacked configuration provides N+1 transfer redundancy within a single wall penetration: if one chamber is undergoing a UV cycle or is temporarily out of service for filter replacement, the other chamber remains operational.

 

3. Static vs. Dynamic Stacked Pass Box — Selection Criteria by Application

The selection between a static and dynamic stacked pass box is determined by the cleanliness gradient between the two rooms being connected. The table below summarizes the decision logic:

Note: The cost multiplier of 1.8–2.5× for the dynamic variant reflects the addition of the HEPA filter, centrifugal fan, perforated diffuser plate, DOP/PAO test ports, and fan-speed control electronics per chamber. For a two-chamber stacked unit, both chambers are typically specified to the same type (both static or both dynamic), though mixed configurations — upper chamber dynamic, lower chamber static — are available on custom order for applications with distinct transfer requirements per chamber.

 

4. Installation, Operation & Maintenance — Engineering Considerations

Wall penetration and sealing. The stacked pass box is installed through a wall penetration cut to the external dimensions of the common frame, with a continuous perimeter seal — typically silicone or EPDM gasket, compatible with the facility's cleaning and sterilization chemistry — between the pass-box flange and the cleanroom wall panel. The sealing must maintain the pressure differential between the two adjacent rooms (typically 10–15 Pa for a one-class gradient) without leakage. SCT provides a factory-welded perimeter mounting flange with pre-drilled bolt holes on the common frame to facilitate airtight installation. For facilities subject to VPHP or ClO₂ bio-decontamination, the perimeter seal material must be specified for chemical compatibility — standard silicone gaskets degrade under repeated VPHP exposure and should be upgraded to EPDM or fluorosilicone for these applications.

Operational sequence (dynamic variant, Grade C → Grade B transfer). (1) Operator on the Grade C side initiates the transfer cycle via the touchscreen controller; the Grade C-side door interlock releases. (2) The operator opens the Grade C-side door, places the material inside the chamber, and closes the door. Both doors are now locked. (3) The HEPA recirculation fan ramps to rated speed; uniform downflow at 0.45 m/s ±20% is established across the chamber cross-section. The particle concentration within the chamber decays to ISO 5 within ≤60 seconds (validated during commissioning per ISO 14644-3). Simultaneously, the UV-C lamps activate for the programmed cycle duration (typically 10–15 minutes for surface decontamination). (4) On cycle completion, the Grade B-side door interlock releases. An audible/visual indicator signals to the Grade B-side operator that the transfer is ready. (5) The Grade B-side operator opens the door, retrieves the material, and closes the door. The system returns to standby with both doors locked. At no point during the sequence can both doors be open simultaneously — this is enforced by the hardware interlock logic, independent of operator compliance.

Routine maintenance. (a) Monthly: clean chamber interior surfaces with 70% IPA or facility-approved disinfectant; inspect door gaskets for cuts, compression set, or chemical degradation; verify UV lamp operation via controller self-diagnostic. (b) Quarterly: clean or replace G4 pre-filter (dynamic variant); verify door-interlock function — attempt to open both doors simultaneously and confirm hardware lockout; inspect and tighten frame mounting bolts. (c) 6-monthly: HEPA filter integrity test via DOP/PAO aerosol photometer scan per ISO 14644-3 Annex A.6 — leak criterion ≤0.01% of upstream challenge concentration at any scan point. (d) Annual: replace UV lamps (regardless of apparent function — UV-C output degrades with age even if the lamp visibly illuminates); verify fan-speed and airflow uniformity (dynamic variant); re-certify chamber cleanliness classification per ISO 14644-1.

5. Frequently Asked Questions — Stacked Pass Box Selection & Operation

Q1: When should I specify a stacked pass box instead of two separate single-chamber pass boxes?

A stacked pass box is the correct specification when: (a) available wall length is constrained — the facility has insufficient uninterrupted wall length to accommodate two single-chamber units side by side (a single-chamber pass box with 600 × 600 mm internal dimensions requires approximately 1,000–1,200 mm of wall length including frame flanges and door-swing clearance on both sides; two side-by-side units require roughly 2,200–2,500 mm). A stacked unit delivers two chambers within approximately 1,200 mm of wall length. (b) Transfer segregation is operationally desirable — inflow and outflow can be assigned to separate chambers, eliminating the single-chamber queuing bottleneck. (c) Vertical workflow is advantageous — lighter items at upper-chamber height, heavier items at ergonomic lower-chamber height. Two separate single-chamber units remain the correct specification when: the two transfer points must be physically separated at different locations on the cleanroom perimeter (e.g., raw-materials entry on the north wall, waste exit on the south wall, per unidirectional workflow design), or when future expansion flexibility requires the ability to relocate or repurpose individual pass boxes independently.

Q2: Can the two chambers in a stacked pass box have different specifications — e.g., upper chamber dynamic, lower chamber static?

Yes. SCT manufactures mixed-configuration stacked pass boxes on custom order. A common mixed-configuration application is a pharmaceutical cleanroom where the upper chamber serves as a dynamic pass box for the transfer of sterile components from Grade C into Grade B (requiring HEPA-filtered active airflow to protect the higher-classification zone), while the lower chamber serves as a static pass box for the transfer of packaged finished product from Grade B out to Grade C (no active filtration required because the transfer is from cleaner to less-clean, and the product is sealed in its final packaging). The two chambers share a common structural frame and control power supply but have independent door-interlock logic, independent UV timers, and — on the dynamic chamber only — an independent HEPA recirculation fan and filter. Mixed configurations are priced on a project-specific basis; the cost is approximately the average of the static and dynamic single-chamber prices plus a shared-frame integration charge.

Q3: What size stacked pass box do I need? How are internal chamber dimensions determined?

Internal chamber dimensions are determined by the largest single item that must pass through the transfer point, plus clearance for gloved-hand manipulation. For pharmaceutical applications transferring 5–20 L carboys or sterile bags, a 600 × 600 × 600 mm internal chamber (W×D×H) per deck is typical. For electronics manufacturing transferring wafer cassettes or component trays, 500 × 500 × 500 mm is common. For bulk-container transfer in food or industrial cleanrooms, 800 × 800 × 800 mm or larger is specified. SCT manufactures to custom dimensions per the client's User Requirement Specification (URS); the standard six-model LCTW series covers internal dimensions from 400 × 400 × 400 mm to 1,000 × 1,000 × 1,000 mm per chamber. The key dimensional constraint for stacked configurations is the total external height — the external height of a two-chamber stacked unit is approximately 2× the single-chamber external height plus the shared-frame depth (typically 50–100 mm). For a 600 × 600 × 600 mm internal chamber with 820 × 660 × 1,150 mm external per chamber, the stacked unit total height is approximately 2,400–2,500 mm — compatible with a standard 2.6–3.0 m cleanroom ceiling. Custom reduced-height chambers are available for facilities with lower ceiling heights; contact SCT with your available wall height and required internal dimensions for a feasibility assessment.

Q4: How is the UV sterilization cycle validated, and what documentation does SCT provide for GMP-regulated facilities?

SCT provides the following documentation package for each stacked pass box delivered to GMP-regulated facilities: (a) Factory Acceptance Test (FAT) report — door interlock function (simultaneous-open attempt × 10 cycles, zero failures), UV lamp irradiance measurement (μW/cm² at chamber center, per lamp manufacturer's photometric data), fan airflow and velocity uniformity (dynamic variant only, 6-point grid measurement), HEPA filter integrity (factory DOP/PAO scan). (b) Material certificates — EN 10204 3.1 for all stainless steel components (304 or 316L), UV lamp specification sheet, HEPA filter test certificate per EN 1822-1, gasket/seal material conformity statement. (c) IQ/OQ protocol templates — Installation Qualification (serial-number verification, dimensional verification against approved drawing, utility connection verification) and Operational Qualification (door-interlock challenge test, UV cycle-timer accuracy, fan-speed/airflow verification, HEPA integrity field test, chamber cleanliness classification at rest). UV sterilization cycle efficacy is validated by the end-user's microbiology laboratory using biological indicators (typically Bacillus atrophaeus spores, 10⁶ population, ≥3-log reduction per cycle) placed at multiple locations within the chamber — including corners and the area directly beneath the HEPA diffuser plate — to map the UV-C dose distribution. SCT's IQ/OQ documentation provides the test templates and acceptance criteria; the end-user executes the PQ (Performance Qualification) under their own validation protocol. This division of validation responsibility aligns with ISPE Baseline Guide Volume 5 (Commissioning and Qualification, 2nd Edition) and is standard practice for GMP cleanroom equipment procurement.

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

Standard-configuration stacked pass boxes — static or dynamic, AISI 304 SS, standard chamber dimensions (500–800 mm internal), electromagnetic interlock, touchscreen controller — ship within 30–40 calendar days from order confirmation (FOB Shanghai). Custom configurations — 316L stainless steel, non-standard internal dimensions, mixed static/dynamic chambers, or integration with facility BMS via Modbus RTU — add 10–15 working days to the manufacturing cycle. Ocean freight transit times: 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). Each unit is shipped fully assembled and factory-tested — no on-site chamber assembly is required beyond frame mounting, perimeter sealing, and electrical connection. SCT provides complete export documentation including commercial invoice, packing list, bill of lading, FAT report, material certificates, and CE Declaration of Conformity. For current lead times and a project-specific quotation, contact SCT at admin@sctcleanroom.com or via WhatsApp at +86 15306200553.

 

The stacked pass box is not the most technically complex device in the cleanroom equipment catalog — that distinction belongs to the intelligent FFU with its Modbus-integrated per-unit airflow control, or to the air shower with its Siemens PLC-governed high-velocity nozzle array. But in terms of operational impact per unit of wall area consumed, it is among the most value-dense specifications a cleanroom planner can make. Two independent transfer chambers in one wall penetration. Static or dynamic, 304 or 316L, standard or custom dimensions. N+1 transfer redundancy without consuming additional floor space. SCT's stacked pass box platform, manufactured in Suzhou under ISO 9001 quality management with CE certification, delivers this capability in a factory-tested, fully assembled unit — ready for wall-mount installation, electrical connection, perimeter sealing, and immediate integration into the facility's contamination-control sequence. For a project-specific technical consultation, chamber-dimension verification against your material-transfer URS, or a current FOB quotation, contact the SCT engineering team at admin@sctcleanroom.com or via WhatsApp at +86 15306200553.

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