ISO 5 Cleanroom Airflow Design: Why Unidirectional (Laminar) Flow is Mandatory

iso 5 sets a range for airborne particle concentration classes, but does not prescribe a specific airflow velocity or pattern. Nevertheless, in many cases, and possibly in strict accordance with the relevant guidelines, airflow that is unidirectional is utilized to move airborne contaminants away from the safety zones of critical operations. Air velocity should be established based on process needs, airflow qualifications, and risk assessment, rather than considering 0.45 m/s as a blanket solution across projects.

What is the intention of ISO 5?

ISO 5 is a particle concentration classification.

ISO 14644-1 describes the upper limits of airborne particle concentration. It is not prescriptive in terms of a specific fan, a particular filter brand, or a certain airflow. It is concerned with the definitions of cleanliness and the determination of particles by their numbers at given sizes.

ISO 5 is often informally called Class 100. However, they arise from separate standards and cannot be treated as interchangeable. ISO 5 is based on measurements in the asbuilt, at rest or operational states. Pharmaceutical projects also need microbiological supervision in addition to particle counting.

SEO Question: Are Class 100 and ISO 5 the same standard? How many particles of size 0.5 μm are allowed in ISO 5? Is the Standard ISO 5/ Class 5 for Laminar Airflow?

What Standard ISO 5 does not define by itself

Standard ISO 14644-1 does not define:

• 0.45 m/s airflow velocity.
• Vertical, unidirectional airflow in each and every room.
• Certain percentage of ffu coverage.
• Certain airflow exchange rates.
• Specific Types of HEPA or ULPA filters.

These aspects must be determined based on process risk, personnel actions, the arrangement of equipment, the status of the room, applicable regulations, the time necessary for the room to achieve the desired environment, and operational demands. Unidirectional airflow is a response to these factors and is not arbitrary.

ISO 5, EU gmp grade a, and application-specific requirements

Never mix different standards. ISO 5 pertains to non-viable particle cleanliness, while EU GMP Grade A concerns unidirectional airflow protection in high-risk zones of aseptic processing. iso 14644-4 deals with design, construction, and the start-up of controlled environments. Custom requirements may also include FDA, PIC/S, user requirement specifications (URS), or local building ordinances. ISO 5 explains the risks and how clean air needs to protect the process.

Industry focus: semiconductor and biopharma and precision optics have different requirements for filters and different critical particle sizes.

What is unidirectional airflow? Is it laminar flow?

Unidirectional airflow in engineering

Airflow enters from a large, filtered supply surface, and during its journey in parallel streamlines in the critical zone, it carries the contaminants toward an exhaust or return opening. The objective of directional airflow of an enclosure is consistent contaminant removal. Local disturbances are caused by equipment, personnel, lighting, and process tools.

How does unidirectional airflow achieve the goal of contamination removal?

The system has four distinct mechanisms at work: HEPA/ULPA filtration, uniform supply of air, transport of contamination away from the critical zone, and return flow at low levels or from the opposite wall.

There are three principal mechanisms at work:

• Displacement: Replaces contaminated air with clean air.
• Sweeping: Reduces the residence time of particles in proximity to the product.

Proposed image: Unidirectional airflow removes particles from an iso 5 cleanroom.

Why does turbulent or mixed airflow fail in critical ISO 5 zones?

Mixed airflow typically results in particle recirculation, wakes behind equipment, dead zones beneath workbenches, and the re-entrainment of particles from ceiling diffusers. As an example, airflow patterns may even cause some local reading to be “in control”, while the reading in those same locations may be “out of control”. The table below describes some behaviors typical of each of the flow regimes.

Why Unidirectional Airflow Is Often Essential for ISO 5

The Infinite Challenge

The challenge presented with ISO 5 is not one of simply cleaning a room, but one of maintaining the level of contamination control while personnel and equipment are allowed to move freely within the cleanroom, thus generating particles. To that end, a unidirectional airflow design will ensure a short, predictable exit path for contaminates. This design serves to continuously protect the critical zone.

Protection of the Critical Work Zone

iso 5 classifications can be achieved anywhere in the clean room, but can still be inadequate to protect the products at the point of exposure. One must conduct a risk assessment on exposed product surfaces, filling needles, wafers, optical components, assembly stations, and clean transfer zones. The laminar flow of ISO 5 is adopted in cleanrooms by directing the clean air stream right over the critical task being performed.

Rapid Recovery Time (100:1 Cleanup Ratio)

Unidirectional flow has rapid recovery capability after contamination as one of its major operational benefits. For a well designed vertical unidirectional system, the 100:1 recovery time (time taken to bring down the concentration of contaminants to one percent of the level of the initial spike) is about 15 to 20 minutes. On the other hand, cleanrooms that employ mixed flow or turbulence usually take 45 minutes or longer to achieve the same recovery ratio.

The rapid recovery capability of unidirectional flow is important for compliance to the EU GMP Annex 1, which brings in the requirement for dynamic recovery for Grade A zones. The rapid sweeping capability of unidirectional flow ensures that the temporary disturbances (opening a door, transferring materials, or adjusting the position of personnel) do not pose a risk to the safety of the product for prolonged periods. Deiiang™ field data have indicated recovery times of less than 16 minutes for systems with well designed return ducts and stable velocity profiles, thereby meeting the stringent requirements of the regulators.

Faster recovery after contamination events

Directional flow is capable of quickly restoring clean conditions after personnel movement, door openings, and maintenance activities. Though recovery times must be assessed individually in a project specific manner, unidirectional flow design allows a faster recovery time of clean conditions.

More Consistent Qualification

Testing and qualification of stable airflow patterns is simplified with controlled air flows. air velocity tests, airflow visualization, HEPA integrity tests, particle counting, and recovery tests are all made easier when the direction of airflow is known, controlled, and remains stable. A cleanroom is not made successful simply because air flows through a HEPA filter. A cleanroom is successful because the filtered, controlled clean air is directed to the critical zone.

Is 0.45 m/s Airflow in an ISO 5 Cleanroom Mandatory?

The Long Answer

No, 0.45 m/s does not have to be the value for all ISO 5 projects. It is a design value and a value typically used in practice. The determination of air velocity for cleanrooms is dependent on the risk of a given standard and where the measurements would be taken, filter loading, and on-site airflow studies. The value for cleanroom air velocity for an ISO 5 Cleanroom should be substantiated, not assumed.

Interpreting “0.45 m/s ±20%”

According to Deiiang™ FFU technical data, the nominal face velocity is 0.45 m/s ±20%, which means approximately 0.36 to 0.54 m/s. The ffu unit draws air from the upper section of the unit, passes it through a HEPA filter and discharges it through the unit’s face. These units are used in the semiconductor, optoelectronics, precision instruments, biopharmaceutical, hospital, and food processing industries. Note that this is a design reference and not a design per ISO 14644-1.

Factors that change the optimum velocity

The optimum velocity for a system needs to account for:

• Risk of contamination and the sensitivity of the product
• Distance from filter face to working surfaces
• Blockages from equipment and wake zones
• Thermal plumes from humans and equipment
• Density of personnel and their activities
• Location of return air openings
• Sensitivity of the system to turbulence

Optimum velocity is determined by many factors and is not a simple problem of the faster the better. A higher velocity means more disruptive flows, more vortices and air turbulence, and higher filter pressure drops. A higher velocity also increases the noise and energy consumption of the system.

Air Velocity and air changes Per Hour

When dealing with unidirectional flow systems, using air changes per hour (ACH) alone can be very misleading. Use the following relationship:

Even with an ACH, a poor return design can create stagnant areas. Class 100 flow pattern air changes need to be verified by measurements of flow velocity and smoke tests.

Vertical vs. Horizontal Unidirectional Air Flow

Vertical Unidirectional Air Flow

In this system, air moves downward from ceiling filters, passes through the work area, and exits from low wall or floor returns. This system tends to be the best option for open spaces with multiple personnel and equipment, in semiconductor and electronics manufacturing, and in spaces with changing process layouts. The removal of particulates is aided by gravity, coverage is simple with modular FFU ceilings, and protection of large critical zones is more feasible. The system also has its limits. These include the depth of the ceiling plenum, limited zones of protection for tall equipment, and a complex design for floor returns.

Horizontal Unidirectional Air Flow

This system is based on horizontal laminar flow workstations that are a part of the Deiiang™ product line. In this system, air flows horizontally from a rear HEPA filter face to the front. This is ideal for workbenches, small clean air zones, and clean air zones for specific process steps. The system is a fully integrated unit which has a low-noise centrifugal fan, plenum, and a high efficiency filter. Potential risks that can occur with the system include personnel being positioned upstream of a product, a blockage to air flow, and a loss of velocity uniformity over longer distances.

Selection Comparison

Designing the ISO 5 Air Flow Path

Step 1: Define the Critical Zone

This is the area (often in a cleanroom) where personnel will be working the closest to the product. Drawings should include critical zones, personnel, and airflow direction.

Step 2—Locate clean air supply above or upstream

When it comes to FFUs or HEPA supply outlets, we have to show greater importance to the critical zone over the ceiling grid aesthetics. Deiiang™ HEPA supply outlets are offered in both top-entry and side-entry plenum models, and their HEPA filters can be removed from below in the cleanroom. The components of these supply units are a housing frame, insulation, a baffle, a sealing strip, a pressure bar, a filter, and a diffuser plate. The decorative ceiling is mounted on independent suspension rods, thus the rods do not transfer load to the ceiling.

Step 3—Position returns downstream

To avoid short-circuiting, blockage from equipment, or locating within the product and clean supply air, optimize placement of low-wall returns, floor returns, and opposite-wall returns. Don't depend on a single corner return for large spaces. Control return velocity to avoid local entrainment.

Step 4—Control obstructions and wake zones

Personnel, equipment, open doors, and material bins can all be disruptive to clean air flow. Analyses of upstream wake zones, top wake zones, and downstream wake zones, can help determine the need for additional FFU coverage or flow straightening.

Step 5—Account for heat loads

Thermal plumes created by equipment and personnel can diminish downward flow. Sensible heat, surface temperature, personnel, localized heating sources, and the ability of air to maintain direction should all be considered. In particular, for complicated projects CFD is beneficial; however, it cannot ｒｅｐｌａｃｅ on-site tests for smoke and particulate matter.

Step 6—Maintain pressure and airtightness

According to the provided construction references, the connections of clean room duct and component constructions should be air tight and clean. Elastic, airtight, and non-particle emitting gaskets should be used, and foam plastic, cardboard, and asbestos rope should be avoided. Clean air valves should be cleaned and sealed before being installed. Unidirectional airflow design relies upon pressure differentials, and small leaks at walls, doors, windows, filter frames, and ceiling joints can greatly affect performance.

FFU, HEPA, and ULPA Selection for iso class 5 cleanrooms

Understanding Deiiang FFUs

A Deiiang™ FFU incorporates a fan located above the unit, which pulls air through a HEPA or ULPA filter and then releases the filtered air across the entire outlet face. Used in conjunction, several FFUs create a continuous clean air supply zone within a modular cleanroom ceiling. Controllers enable the adjustment of FFUs’ speed at an individual or group level. (Refer to Optimizing Flow for Better Performance.)

Deiiang FFU Control Options

Validation strategies allow for independent adjustment of different zones, re-balancing of filters as pressure drop increases, and night or non-production modes. Group controls keep track of operating states. Actual project model and protocol determine the final functionality.

H13, H14, or ULPA?

From Deiiang™ product data:

• H13: 99.97%–99.99% at 0.3 µm
• H14: 99.995%–99.999% at 0.3 µm
• Rated airflow accommodates 550–2,500 m³/h, depending on size
• Initial pressure drop is typically ≤200–220 Pa
• Final resistance is typically 400–600 Pa
• Media consists of ultra-fine glass fiber paper and PU seal

ULPA Options (U15, U16, U17) are tested at 0.12 µm and are appropriate for cleanrooms and ultra-clean labs. However, ULPA is not automatically required for ISO 5. Filter grade, quantity, face velocity, and leak integrity determine filter performance. Always refer to EN 1822 MPPS classification independently of 0.3 µm efficiency.

Filter pressure drop and FFU fan reserve

FFUs must achieve the target airflow even as filters become loaded with particles. Consider initial and final filter resistances, static pressure reserve from the fan, filter change triggers, and cleanroom requirements. Velocity of the air in cleanrooms to achieve ISO 5 must be sustainable through the service life of the filters.

Ceiling Coverage and Structural Coordination

Does ISO 5 require 100% FFU coverage?

Not exactly. Coverage is determined by the extent of the critical zone, airflow modeling, process risk, and on-site assessment. It is not determined by a fixed percentage of the ISO class. There are three common types of approaches that include full ceiling coverage, partial coverage of critical zones, and localized clean zones or clean rooms and booths.

Deiiang ceiling grid data

Deiiang™ T-grid products include 1,200 x 600 mm and 1,200 x 1,200 mm modules. Models MCS-55A, MCS-60, and MCS-70A are available in varying heights. The blank panel central load test value is 4,000 N, with deflection measures of approximately 0.84 to 0.97 mm, with residual deformation of about 0.05 to 0.08 mm. The figures are for reference only, as actual structural design must consider suspension points, spans, maintenance load, weight of the FFUs, seismic factors, and local regulations.

Installation details that preserve airflow performance

FFUs and HEPA supply outlets require independent suspension. Sealing is required for the ceiling joints and filter frame. Joints and frames can be sealed with plenum sealing. Top entry and side entry plenums can be used. Bottom access plenums allow filters to be changed from the clean room. Cutouts of ceilings and frames must match the plenum with sealing to allow a bypass of the filter media.

Recommended illustration: FFU/T-grid layout, HEPA outlet suspension, filter replacement flow diagram.

Common ISO 5 Airflow Design Failures

How to Validate an ISO 5 Unidirectional Airflow System

Pre-commissioning checks to be undertaken:

• FFU installation orientation.
• Filter type and quantity.
• Gasket continuity.
• Dampers.
• Return air openings which should be unobstructed.
• Ceiling joints and sealing.
• Sensor and control wiring.
• Construction debris and cleanliness of site.

Testing air velocity and flow rate

Document the measurement plane, gauging system, grid, data collection, instrument calibration status, room state, and air flow measurement for iso 14644-3.

Multi-point measurements of air flow rate should be taken at working height at the filter face.

Measurements should include data collection/sampling of average, min, max, flow rate deviation and measurement comparison against control setpoints.

Record the measurement plane, grid, instrument calibrations, state room occupancy, air flow measurement control status, and the state of the construction.

Post clean room construction site surveys for air flow and construction site debris collection

Use a systematic approach for air flow studies, establish the norm for a clean for a planned state, then analyze the stagnation flow and reverse flow conditions, and then re-test. Abnormal operating conditions should include the placement and movement of personnel, opening/closing of doors, and placement and movement of equipment.

HEPA filter integrity testing

Integrity testing confirms filter integrity and integrity of the gasket and frame. Integrity testing of Deiiang™ liquid-seal HEPA outlets includes PAO test ports. It does not guarantee compliance. Integrity testing is performed to confirm tightness of the frame and media.

Particle concentration testing

Integrity testing should be conducted in accordance with the validation plan developed for the project. Integrating detailed procedures should not be done if the procedures will be obsolete.

Operational Monitoring

Ongoing checks: differential pressure between rooms, FFU operational status, velocity of critical zone, filter pressure drop, trends in particles, performance of alarm systems, deviations, and maintenance re-validations.

Cleanroom Operational Monitoring Issue

Step 1: Actor Description

Name: Lin
Title: Facility and quality manager
Industry: Precision optical manufacturing

Step 2: Background

Lin’s company is currently setting up a new manufacturing line for precision lens coating and inspection. This line will require a new ISO 5 critical work zone with an extremely clean environment. Product surfaces can be damaged by 0.3 µm and 0.5 µm particles. Lin has limited ceiling space to work with. There will be tall coating equipment and inspection instruments that will generate heat. Management will have to take a hands-on approach to control energy and maintenance costs.

1st Question: What does providing ISO 5 mean with regard to setting the velocity of the cleanroom air to 0.45 m/s?

What Lin Saw On Site

Clear vertical shadows of cleanroom air flow were observed. Inspectors were seen blocking the upstream flow of air to the product. Return air grilles were directly blocked by material storage racks. H14 filters may cause a zonal imbalance through pressure drop.

Lin’s Approach

1. Mark areas of product exposure and locations of personnel
2. Analyze the wakes caused by tall equipment
3. Prioritize the placement of FFUs above the critical zone
4. Adjust the low walls of the returns
5. Balance the zonal air flows using group control
6. Validate the operations of personnel using smoke flow studies

7. Use particle monitoring results only (and not the single velocity number) to determine compliance.

Deiiang Project Case Study - ISO 5 Precision Manufacturing Cleanroom

Project Overview

• Industry / Location: Precision Optoelectronic Coating Lines, Singapore
• Clean Area: 280 m²
• Ceiling Height: 3.2 m
• Target Class: ISO 5 (operational)
• Primary Process: Coating and Inspection of Optical Lenses
• Project Type: Retrofit (upgrade of turbulent to vertical unidirectional)
• Year of Delivery: 2025

Four Required Project Images

• Image 1: Site of construction showing the old layout of equipment and turbulent diffusers. Alternative: Site of ISO 5 cleanroom awaiting Deiiang Airflow upgrade.
• Image 2: Installation of Deiiang™ FFU and T-grid. Alternative: Installation of Deiiang FFU ceiling grid for ISO 5 cleanroom.
• Image 3: Testing of airflow and smoke demonstration in progress. Alternative: Testing of unidirectional airflow and smoke in ISO 5.
• Image 4: Cleanroom showing new FFU ceiling and low wall returns. Alternative: Deiiang ISO 5 cleanroom completed with vertical unidirectional airflow.

Project Challenges

• Turbulent system fell short of ISO 5 during the movement of the operators.
• Coating machines of considerable height created significant wake zones.
• Ceiling plenum height of 600 mm posed a limitation to ducted solutions.
• Production could only be halted for a maximum of 48 hrs during retrofit.
• Existing low wall returns were positioned too high (350 mm above floor).

Deiiang’s Solution

• Installed 84 (1200 mm x 1200 mm) FFUs with H14 filters and DC EC motors.
• FFU placement performed with higher density using CFD optimization over critical coating stations.
• Return grills were lowered to 120 mm from the finished floor and balancing dampers were added.
• RS485 group control was utilized for precise control, adjusting the velocity of air within the zones to within 0.42 and 0.48 m/s.
• Installation and validation were completed within a 46-hour timeframe to reduce downtime to a minimum.

Verified results show the following:

“The Deiiang solution gave us the cleanroom we needed without interrupting our production schedule. The 14-minute recovery time is a game-changer for our coating yield.” — Plant Manager, Singapore

ISO 5 Airflow Design Checklist

Design phase

Confirm applicable standard and room state

Define the critical zone

Map personnel and material paths

Document equipment dimensions and heat loads

Select vertical or horizontal unidirectional flow

Calculate effective supply area and airflow volume

Plan downstream return placement

Check filter pressure drop and fan reserve

Plan maintenance access and filter change paths

Complete CFD if required

Installation phase

Verify FFU orientation

Inspect ceiling and filter seals

Confirm independent suspension

Clean ducts and dampers

Check returns are clear

Validate controls and communications

Record filter serial numbers and locations

Qualification phase

HEPA integrity testing

Multi-point velocity testing

Airflow visualization

Particle concentration testing

Differential pressure testing

Recovery testing

Noise and temperature/humidity checks

Close deviations with corrective actions

Frequently Asked Questions

Conclusion—Design for the Critical Zone, Not Just the Classification Label

ISO 5 is not a simple formula of "0.45 m/s + HEPA filter." Reliable design integrates the critical zone, contamination sources, supply coverage, return path, filter performance, equipment heat loads, and dynamic operations. The value of unidirectional airflow design is not creating perfectly undisturbed air; it is keeping contaminants moving in a predictable direction away from the product.