‌Fan Filter Unit (FFU): Applications, Classification, and Structure

‌1. Introduction & Basic Concept

FFU (Fan Filter Unit), is a modular unit (or can be used individually) designed to provide clean air. Widely deployed in cleanrooms, clean workbenches, clean production lines, modular cleanrooms, and local Class 100 environments.

FFU contains both primary and high-efficiency filters. A fan draws air from the top through these filters. The filtered clean air is then discharged uniformly across the outlet face at a velocity of ‌0.45 m/s ±20%‌. This makes it suitable for achieving high-grade cleanliness in various environments, supplying quality clean air to spaces of different sizes and cleanliness levels. In new construction or renovation of cleanrooms/workshops, FFUs can upgrade cleanliness, reduce noise and vibration, lower overall costs significantly, and are convenient for installation and maintenance—making them an ideal component for contamination-controlled environments.

An installed FFU looks like this:

‌2. FFU Applications, Classifications, and Structure

‌2.1 The Cleanroom Concept

Example: A Class F5 cleanroom employs an MAU + FFU + DC system: a Make-up Air Unit + Fan Filter Unit + Dry Coil system. Layout is as illustrated below:

‌2.2 Applications and Advantages

Typical cleanroom systems include ducted systems, FFU systems, and axial fan systems.
Advantages of FFU systems compared to ducted systems:

⑴ ‌Flexibility‌
⑵ ‌Reusability‌
⑶ ‌Negative Pressure Ventilation‌
⑷ ‌Shortened Construction Cycles‌
⑸ ‌Reduced Operating Costs‌
⑹ ‌Space Savings

Cleanrooms typically rated at Class 1000 (FS209E standard) or ISO 6 and above often utilize FFU layouts. Localized purification environments like clean cabinets or clean benches are also commonly implemented using FFUs.

‌Key advantages promoting FFU adoption:

1. ‌Easy Replacement, Installation, and Relocation:‌ As self-powered, modular units with readily replaceable filters, FFUs are not restricted by location. Within a cleanroom, they allow for zonal control and can be easily replaced or moved as needed.
2. ‌Negative Pressure Ventilation:‌ A distinctive feature of FFUs. Their ability to generate static pressure ensures the cleanroom maintains positive pressure relative to the outside environment. This simplicity and security in containment design is due to external particles being unable to infiltrate.
3. ‌Shortened Construction Periods:‌ FFU installation eliminates the need for ductwork fabrication and installation, speeding up the project timeline.
4. ‌Lower Operating Costs:‌ Although initial investment for an FFU system may be higher than a ducted system, it demonstrates significant advantages in long-term operation: energy efficiency and minimal maintenance.
5. ‌Space Efficiency:‌ Compared to other systems, FFU systems occupy less vertical space within the supply air plenum and essentially no floor space inside the cleanroom itself.

‌2.3 Classification of FFUs

FFUs are classified based on several criteria:

（1）By Overall Cabinet Dimensions (mm):‌ Based on center-to-center distance of ceiling grid T-bar.

• 1200 x 600 (Code: 42)
• 1200 x 900 (Code: 43)
• 1200 x 1200 (Code: 44)
• 600 x 600 (Code: 22)
• 750 x 1500 (Code: 25)
• Custom Sizes (Code: CS)

（2）By Cabinet Material:

• Standard Coated Steel Plate (Galvanized, Galvalume, Powder-Coated) (Code: G)
• Stainless Steel Plate (Code: S)
• Aluminum Plate (Code: A)
• Others (Code: O)

（3）By Motor Type:

• AC Motor (Single-phase AC: Code A1; Three-phase AC: Code A3)
• EC (Electronically Commutated / Brushless DC Motor) (Code: EC)

 （4）By Control Method (for AC units):

• Single-mode (Constant Speed) (Code: S)
• Multi-step Speed Control (Code: M)
• Continuous Speed Regulation (e.g., Voltage or Frequency Control) (Code may be omitted)

‌（5）By External Static Pressure Capability:

• Standard Static Pressure Type (Code: S; may be omitted)
• High Static Pressure Type (Code: H)

（6）By Filter Efficiency:

• HEPA Filter (Code: H)
• ULPA Filter (Code: U)
• If equipped with a pre-filter at the inlet: add Code P. Omitted if no pre-filter.

‌2.4 FFU Structure

(1) General Appearance:‌ Types include ‌Split Design‌ (easier filter replacement, reduces installation labor) and ‌Integral/Monolithic Design‌ (improves sealing to prevent leakage, aids in noise and vibration reduction).

(2)Basic Structure of FFU Enclosure

       Basic Components:‌ An FFU primarily consists of four parts:

1. Cabinet/Enclosure:‌ Typically made of coated steel, aluminum alloy, or stainless steel. Functions as structural support for the fan/impeller assembly, guide ring, and airflow straightening device.
2. Guide Vanes / Airflow Straightener:‌ An internal device surrounding the fan exit to ensure uniform airflow distribution.
3. Fan Unit:‌ Includes fan and motor. Motor types as classified above (Single-phase AC, Three-phase AC, or DC/EC).
4. Control Components:‌ For AC FFUs: multi-step speed controllers or variable speed controllers. For EC/DC systems: control logic is integrated into the motor, often managed via dedicated software, computers, control gateways, and network lines for remote control and monitoring.

2.5 FFU Key Internal Components:

(‌1) Common materials for enclosure ：Galvalume, 430 stainless steel, aluminum alloy‌

(2) Fan & Motor Assembly：

• External rotor rear drive type （Recommended use） : Compact structure, high efficiency, long service life, high static pressure, and low noise.
• traditional internal rotor forward tilt type : Complex structure, low efficiency, short lifespan, low static pressure, and high noise.

(3) Motor (EC/Brushless Type Explanation)

• AC motors: Simple in structure, easy to manufacture, reliable in operation, inexpensive, and long-lasting; available in single-phase and three-phase versions, they generate magnetic force through power commutation.
• DC motors: Complex in structure, expensive, and prone to failure; they generate magnetic force through the directional rotation of the armature, commonly known as brushed motors.

#Motor (EC/Brushless Type Explanation):‌ Structurally, brushless (EC) motors share similarities with brushed motors, having a rotor and stator, but in a reversed configuration. In a brushed motor, the rotor is the wound coil connected to the output shaft, while the stator contains permanent magnets. In a brushless motor, the rotor holds the permanent magnets (attached to the housing and output shaft), and the stator houses the winding coils. The “brushes” and “commutator” used for changing the electromagnetic field in brushed motors are eliminated, hence the name “brushless” or EC motor. These are commonly used in applications demanding precise and high-speed control, such as model aircraft and precision instruments.

         Common Motor Faults:

• a. Three-phase power loss (Single phasing): Possibly due to grid supply issue.
• b. Bearing failure: Mechanical fault.
• c. Overload: e.g., Fan wheel obstructed.
• d. Phase imbalance: Unbalanced loads across three phases.

(4) Filters:

• Pre-filter:‌ Removes larger particulates. Often non-woven fabric, primary filter mesh with frame.
• ‌High-Efficiency Filter:‌ HEPA or ULPA filter. Example: H14, efficiency 99.999% @ 0.3 µm.
• Chemical Filter:‌ Removes ammonia, boron, organic gases, etc. Usually installed at the air intake, similar to pre-filter installation.

3. FFU Specifications, Parameters, and Selection Guidelines

General specifications considered during design/manufacture:

• Dimensions:‌ Must match ceiling grid/module.
• ‌Material:‌ Depends on environmental requirements and cost considerations.
• ‌Face Velocity:‌ Typically 0.35~0.45 m/s. Significant impact on energy consumption.
• ‌External Static Pressure:‌ Must overcome system pressure drop (ductwork, return air path resistance).
• ‌Filter:‌ Selected based on required cleanliness class.
• Motor:‌ Based on power supply characteristics, required power, bearing life.
• Noise:‌ Must comply with cleanroom noise level requirements.

3.1 ‌FFu Key Technical Parameters:

1. Face Velocity:‌ Generally ranges 0-0.6 m/s. For 5-step speed control, approximate velocities per step: 0.3-0.35-0.4-0.45-0.5 m/s. For variable speed: 0-0.6 m/s.
2. Power Consumption:‌ AC systems: ~100-300W; DC/EC systems: ~50-220W. DC systems typically consume 30-50% less power than comparable AC units.
3. Velocity Uniformity:‌ Refers to the consistency of air velocity across the FFU face. Critical in high-grade cleanrooms to prevent turbulence. Determined by superior fan, filter, and diffuser design/manufacturing. Testing involves measuring velocity at 6-12 evenly distributed points; the max/min deviation from average should not exceed ‌±20%‌.
4. External Static Pressure (ESP):‌ Also called Available Static Pressure. Impacts FFU lifespan. Related to fan capability. Typically required to be ≥90 Pa at 0.45 m/s face velocity.

5. ‌Total Static Pressure (TSP):‌ Also Total Pressure. The maximum static pressure the FFU can generate at zero airflow (max power). AC units: ~300 Pa; DC/EC units: 500-800 Pa. At a given velocity: ‌TSP = ESP + Filter Pressure Drop‌ (resistance of the filter at that velocity).

6. Noise Level:‌ Generally between 42-56 dBA. Often specified at 0.45 m/s face velocity and 100 Pa ESP. DC units are typically 1-2 dBA quieter than equivalent AC units.
7. Vibration Velocity:‌ Typically <1.0 mm/s.
8. Standard Dimensions (Basic Module – Centerline of ceiling grid):

   Grid Module (W x L mm)	Approx. Imperial (ft)	FFU Cabinet Size (W x L mm)	Filter Size (W x L mm)
   1200 x 1200	4′ x 4′	1175 x 1175	1170 x 1170
   900 x 1200	3′ x 4′	875 x 1175	870 x 1170
   600 x 1200	2′ x 4′	575 x 1175	570 x 1170
   600 x 900	2′ x 3′	575 x 875	570 x 870
   600 x 600	2′ x 2′	575 x 575	570 x 570

   Note 1: Width/Length dimensions are generally standardized; thickness may vary by manufacturer.

   Note 2: Custom sizes are possible but may affect lead time and cost.

3.2 ‌Selection of HEPA Filter Efficiency

Note 1: Cleanroom classification depends on both filter efficiency and air change rate (supply airflow). High-efficiency filters with insufficient airflow cannot achieve the target class.
Note 2: EN1822 is the current prevalent European standard.
Note 3: CRAA is a draft industry standard by the China Refrigeration and Air-Conditioning Industry Association.
Note 4: FS209E is a historical US Federal Standard, now obsolete but still referenced in some regions.

3.3 ‌Motor Selection (AC vs. EC)

(1) AC Motor Selection:‌ AC-FFUs, often controlled by manual speed controllers, have a lower initial cost. Commonly used in cleanrooms with fewer than ~200 FFUs.

(2) EC (DC Brushless) Motor Selection:‌ EC-FFUs are suitable for large-scale cleanroom facilities with hundreds or thousands of units. They enable intelligent control of each FFU’s operation and fault status via computer software, reducing maintenance costs. A single software instance can control multiple master gateways, each managing up to 7935 FFUs.

• EC-FFUs are ‌>30% more energy efficient‌ than AC-FFUs, leading to substantial annual energy savings in large systems.
• EC-FFUs also operate at ‌lower noise levels‌.

‌3.4 Energy Consumption Comparison (Example for 1175x1175mm FFU)

Comparison of four configurations (H14 filter, 90 Pa ESP):

Key Takeaways:

1. EC systems are more energy-efficient than AC systems.
2. For the same motor type, systems with ‌PTFE filters‌ are more efficient than those with ‌glass fiber filters‌. At 0.45 m/s, the EC system with glass fiber consumes ~46W (30.8%) more power than with PTFE.

4. FFU Installation Guide

‌4.1 Prerequisites, Environment & Attire

1. The cleanroom must be ‌sealed‌, with the Make-up Air Unit (MAU) supplying clean air for approximately ‌3 days‌ prior. The interior must be clean, dust-free, and all ceiling grid must be installed and leveled.
2. Installers ‌must wear cleanroom garments and gloves‌ to prevent contamination of FFU cabinets and filters.
3. Recommended crew size: ‌6 persons per team‌.

‌4.2 Assembling FFU Cabinet with HEPA Filter

HEPA filters use ultra-fine glass fiber media, which is fragile and prone to damage/leakage. Handle with extreme care during transport, unpacking, and installation. Avoid dropping, impacts, and never touch the filter media with fingers or objects.

Steps for Cabinet-Filter Assembly:

1. Lay a plastic sheet on a flat, spacious area. Place the FFU cabinet vertically on it (side without CLIPs facing down). Loosen the screws on the 6 CLIPs (three on each side).
2. Position the HEPA filter flush against the FFU cabinet. ‌Handle the filter ONLY by its frame.‌ Ensure correct orientation: the ‌airflow arrow‌ on the filter frame must point ‌OUTWARD‌ (direction of filtered air discharge).
3. Hook each CLIP onto the filter’s groove and tighten sequentially. After installation, check that the joint between cabinet and filter is ‌even and tight‌.

‌Steps for Mounting Assembled Unit to Ceiling Grid:

1. Remove the “Butterfly Clips” from the ceiling grid’s T-bar.
2. Place the assembled FFU on a lifting platform (scissor lift). Position the lift so the unit is centered under the grid opening. Slowly raise the unit.
3. Re-install the Butterfly Clips onto the T-bar.
4. Slowly lower the FFU assembly onto the Butterfly Clips.
5. Remove the lifting platform. The FFU installation is complete.
6. Verify the assembly sits ‌evenly and snugly‌ on the grid. Proceed with electrical connection.

Common Questions Brief Analysis

1.Noise levels often exceed designers’ predictions:

Adjust airflow during testing; use sound-absorbing materials; analyze areas of unusual noise variation from the windmill.

2.Installation difficulties:

a. The FFU size does not match the T-Bar size;

b. The hook-shaped fasteners cannot be fixed in 100% coverage area;

3.Motor selection:

a. Three-phase AC motors are prone to phase loss and burnout;

b. Single-phase AC motors are bulky and prone to load imbalance;

c. DC motors are expensive to manufacture and maintain;

d. If motor efficiency is the primary consideration, a three-phase AC motor should be chosen;

e. If ease of maintenance is a priority, a single-phase AC motor should be chosen;

f. If cleanliness requirements are a priority, a DC motor should be chosen;

g. In group control, single-phase motors only allow for individual three-speed adjustments, while three-phase motors allow for group-wide frequency conversion speed control;

Note: With the improvement and application of modern frequency conversion technology and phase loss protection technology, three-phase motors will be more widely used in AC fans.

4.Harmonics:

Cause: Primarily due to the DC motor rectifier generating voltage at a non-fundamental frequency;
Harm: It can cause an increase in system current; it can cause control system malfunctions; it can increase the temperature of electrical components, reducing their lifespan;

5.Power connection method:

1. Terminal block: Time-consuming connection, generally a standard accessory, commonly used in small cleanrooms;

2. Plug: Simple and quick connection, generally an optional accessory, commonly used in large cleanrooms;

6.Filter:

Whether there are requirements for low or no boron, PTFE type filters are boron-free.

7.Certification:

Whether CE/UL/FM certification is required; motors and filters can be certified.