Introduction
The air exchange rate (LWR), also known as air changes per hour, ACH for short, is one of the central parameters when designing a clean room or a clean process zone. It indicates how often the entire air volume of a room is replaced by filtered air per hour. If you plan too few air changes, you risk particle accumulation and quality problems. Anyone who plans too much pays unnecessarily high operating costs over the long term.
This article shows which guidelines apply to which ISO class, why the air velocity at ISO 5 is more important than the ACH number, and how you can actually calculate the required air flow rate for your system.
What is the air exchange rate — and how do you calculate it?
The air exchange rate describes the ratio between the filtered air volume supplied every hour and the volume of the room:
ACH = Filtered air volume per hour (m³/h) ÷ room volume (m³)
Example: A room with a volume of 56 m³, to which 2,520 m³ of filtered air per hour is supplied, has an air exchange rate of 45 ACH. This corresponds to typical guidelines for ISO 7.
The challenge lies in finding the right target size: What ACH number is actually required for your ISO class — and how do you achieve it with the right hardware? 【1】【3】
Air exchange rate according to ISO class: overview of indicative values
The following table summarizes the practice-relevant ACH guidelines in accordance with DIN EN ISO 14644-1. The exact values are not rigid limits of the standard, but design guidelines that must be adjusted by the specialist planner depending on the process, staffing and room geometry.【1】【4】
Note: The table values are planning guidelines. Certification is based on proof of the actual particle concentration in accordance with DIN EN ISO 14644-1:2015, not the ACH number alone. 【1】
The additional VDI Guideline 2083 Part 1 in force in Germany adopts the ISO classification and supplements it with aspects such as energy efficiency and industry-specific information for microelectronics and photonics.【5】
Laminar flow: When air velocity is more important than ACH
At ISO 5 and below, the ACH number is largely irrelevant as a planning variable — the flow rate is decisive.
Unidirectional flow (laminar flow) works on a different principle than turbulent mixed ventilation: The filtered air flows downwards from the HEPA filter surface in a uniform, parallel flow and removes particles directly from the working area instead of just diluting them.
The target value: According to DIN EN ISO 14644-3, the recommended air velocity for unidirectional flow is 0.45 m/s ± 20% — measured 150 to 300 mm below the filter outlet surface. This corresponds to a range of 0.36 to 0.54 m/s. The EU GMP Guide Annex 1 (2022) confirms this guideline for open cleanroom applications.【2】【6】
Why this value? If the speed is too low, particles can rise or stagnate in the outflow. If the speed is too high, turbulences occur, which draw particles from the edge zone into the working area. 0.45 m/s is the compromise determined experimentally.
Consequence for interpretation: Who a Laminar Flow Box or a cleanroom cabinsconfiguration for ISO 5 plans, specifies the filter coverage area and FFU performance based on the target speed — not on the basis of an ACH number. The high air exchange rates (240-480 ACH) are then calculated as a result of laminar flow, not vice versa.【2】【3】
Factors influencing the air exchange rate
The indicative values in the table are starting points, not absolute values. Three factors determine whether the theoretical ACH number is sufficient in practice:
Staffing and processes
Humans are the biggest source of particles in cleanrooms: through skin flakes, protective clothing fibers and movement. A single person who behaves calmly emits around 100,000 particles ≥ 0.3 µm per minute. During physical activity, this value is ten times higher.【6】
In practice, this means: For a single workstation in an ISO 7 zone, 30-40 ACH is often sufficient. If the same zone is running with five employees and component handling, the guidelines must be adjusted upwards.
Differential pressure
Clean rooms are operated at a slight excess pressure compared to the environment (typically at least 10 Pa compared to the adjacent area).【3】This excess pressure prevents uncontrolled air from entering through joints or openings. A Nordair Systems cleanroom cabin is designed as an overpressure system: The filter fan units permanently pump more filtered air into the cabin than can escape through leaks.
The pressure cascade (cleanest zone with highest pressure, adjacent lock with medium pressure, environment with ambient pressure) is a decisive planning feature that is directly related to the ACH design.【3】【5】
GMP requirements (a side issue for most Nordair customers)
Anyone who produces drugs or medical devices is subject to the EU GMP guidelines with classes A-D. These classes correlate with ISO classes, but also define microbiological limits and have more detailed monitoring and qualification requirements.【6】At this point, it should be noted that Nordair Systems does not yet cover the GMP requirements.
For Nordair Systems customers in photonics, precision engineering or metal processing, GMP requirements are generally not relevant. ISO 14644 and VDI 2083 are the relevant framework.
Practical example: Calculate air exchange rate and translate it to hardware
Let's say you're planning a clean island for component handling and packaging after the cleaning plant — target class ISO 7:
Step 1: Determine room volume
- Area: 5 m × 4 m = 20 m²
- Room height: 2.8 m
- Volume: 20 m² × 2.8 m = 56 m³
Step 2: Set target ACH
- ISO 7 guideline: 30-60 ACH
- For a workplace with 2 people and moderate activity: planning value 45 OH
Step 3: Calculate required airflow
- Air flow = 56 m³ × 45 ACH = 2,520 m³/h
Step 4: Derive hardware
- A Nordair systems cleanroom cabin With suitably designed filter fan units, this throughput provides the required filter coverage for ISO 7.
- The exact FFU configuration depends on room geometry, filter coverage and return air flow — this is determined in the design consultation.
The key point: The calculation is not complex. The errors usually arise not with the formula, but with the assumption of the boundary conditions - incorrect staffing, missing differential pressure buffer, underestimated process particles.【1】【4】
Figure: One cleanroom cabin by Nordair Systems
What air exchange rate does a lock or changing room need?
Locks and changing rooms are not clean rooms, but critical transition zones. Your task is to minimize the transfer of particles from the environment into the clean room.
As a guideline, locks and changing rooms are typically 10—20 AH (ISO 8 level). This corresponds to the lowest end of the cleanroom classification, but a multiple of the ACH rate of normal office or production ventilation.【1】【3】
The pressure hierarchy is decisive here: The clean room has the highest overpressure, the changing room has a medium overpressure compared to the environment. Air always flows from clean to less clean. Never the other way around. This applies regardless of whether the door to the lock has just been opened or not.
Conclusion: Air exchange rate is a design parameter — not a certification value
The common confusion: Many believe that reaching a specific ACH number means meeting an ISO class. That is not true. The ISO class is proven by particle counting — not by measuring the air exchange rate.【1】
The ACH number is the tool for reliably reaching and maintaining the particle class in practice. Anyone who knows the guidelines and realistically assesses the influencing factors (staffing, process particles, differential pressure) plans a system that works in operation instead of one that has to be retrofitted during the first real production run.
Unsure what air exchange rate your process requires? We help with the design — specifically, without overengineering.
Source citations
- DIN EN ISO 14644-1:2015 — Clean rooms and associated clean room areas. Part 1: Classification of air purity based on particulate concentration. Beuth Verlag, Berlin.
- DIN EN ISO 14644-3:2019 — Clean rooms and associated clean room areas. Part 3: Test methods. Beuth Verlag, Berlin. (Includes 0.45 m/s ± 20% guideline for unidirectional flow.)
- DIN EN ISO 14644-4:2022 — Clean rooms and associated clean room areas. Part 4: Planning, construction and initial operation. Beuth Verlag, Berlin.
- Whyte, W. (2011). Cleanroom Technology: Fundamentals of Design, Testing and Operation (2nd ed.). Wiley & Sons (Standard reference for design calculations, including staffing and particulate emissions.)
- VDI 2083 Part 1 — Clean room technology: Air particle purity classes. Association of German Engineers, Düsseldorf. (National supplement to ISO 14644-1 with industry-specific notes for microelectronics and photonics.)
- European Commission, EU GMP Guide Annex 1 (2022): Manufacturing of sterile medicinal products. European Medicines Agency, Amsterdam. (Guideline 0.36—0.54 m/s for laminar flow applications in open GMP A/B areas.)
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