Clean room air exchange volume, parameters and calculation
Why can't there be a unified standard for clean room air exchange?
There are many parameters obtained from experiments and standards for clean room air exchange (ACR). Many of those documents date back more than 20 years. Commonly referenced standards are ISO 146144-4 for hourly air exchanges and wind speed, and Federal standard 209E for ceiling coverage ratio of air filters.
Many charts found on the internet reflect obscure interpretations of industry standards, or sometimes data found from outdated standards. Many of them are combinations of results and graphs from the following documents:
- IEST-RP-CC012
- ISO 14644-4: Design, Construction, & Start Up
- Fed. Standard 209E (Expiry)
- ASHRAE
What makes calculating cleanroom air exchange so troublesome?
- Outdated, incomplete data, missing or mixed content
- Incorrect clean room calculation method
- Incorrect input of input parameters, such as ceiling height and wind direction blow.
- Refer to standards that are outdated or have no scientific basis.
Why is the amount of gas exchange important?
The amount of air exchange is an important component in determining the design and standards for an HVAC clean room. Total air flow, direction, and air exchange efficiency have far-reaching impacts on cleanroom performance and costs. The efficiency and cost of a cleanroom will ultimately determine the level of profitability for investing in a cleanroom.
Ventilation technique is the main method of controlling contamination for particulate dust as well as bacteria. However, the requirement from the operator for safety and comfort during use is also a requirement that a top-rated clean room supplier must achieve.
ISO definition of gas exchange volume
Defined in ISO 146144-4: the amount of air exchange is expressed by the number of air exchanges per unit of time and is calculated as the quotient between the volume of air supplied in a unit of time divided by the volume of the clean room or clean zone.
How to choose the right amount of air exchange for a Clean Room
First, the designer must establish the necessary cleanliness level of the room based on ISO standards for particulate dust control.
- Number of air exchanges (ACH)
- Average wind velocity
- Coverage ratio of air filters
“The truth is that there is no simple way to relate a room's cleanliness class to a specific cleanroom air velocity or air exchange rate because of the complex factors that must be considered during design and operation. However, the use of cleanroom air velocity and/or air exchange volume to calculate the total required air flow rate has become common in practice and in industry publications. ASHRAE and IEST.”
Lawrence Berkeley National Laboratory
The clean room airflow direction of HVAC design will determine how to calculate the appropriate amount of air exchange. Turbulence, turbulence, production equipment and pressure differentials all affect the amount of air exchange because the air discharged and returned to the room will interact with each other throughout the cleanroom.
The ISO standard outlines a single calculation method for cleanrooms with unidirectional airflow and cleanrooms with anisotropic airflow.
- In clean rooms with anisotropic and mixed wind, use the method of calculating the number of air exchanges (ACH)
Calculate the Air Exchange Rate (ACR) in a clean room with multi-directional wind (ISO 6 – ISO 8)
The table below is quoted from ISO 14644-4:2004 Design, Construction, & Start Up.
The specifications below are based on a ceiling height of 3m.
Ceiling height 3m ACH low level (m3/m2 x h) ACH high level (m3/m2 x h) ISO 6 70 160 ISO 7 30 70 ISO 8 10 20 Formula to calculate the number of air exchanges (ACH)?
Calculate ACR with ACH when the clean room has anisotropic or mixed wind.
ACH calculation formula for clean rooms:
Number of air exchange times (times/hour) = air flow (m3/hour) / Room volume (m3)
Below, we will present the fundamentals of air exchange quantities, but also provide a more logical outline of standard air exchange parameter tables for specific cleanrooms. Most importantly, we will help you understand calculating the appropriate amount of air exchange based on the industry standards of your facility's cleanrooms.
- Most clean rooms with anisotropic or mixed air are ISO 5 - ISO 8 clean rooms.
- When calculating the number of air exchange times (ACH), the ceiling height calculation must be adjusted.
- Application: Pharmaceutical, medical health, biological sciences, packaging, isolation, treatment...
Do not use wind speed to calculate air exchange in clean rooms with anisotropic wind?
If the air supply to the cleanroom is anisotropic, the wind velocity parameter will not be able to provide an accurate sample of the inferred data. Turbulence, turbulence, production equipment, and pressure differentials all affect velocity data because the air discharged and returned to the room will interact with each other throughout the cleanroom. The data read will not be representative of actual room conditions. Measured anisotropic air flow coupled with mean velocity parameters often produce confusing, misleading or irrelevant data.
Calculation of Air Exchange Volume for ISO 1-5 Cleanrooms
In a clean room with unidirectional wind, the amount of gas exchange is calculated by sampling the wind speed.
It is recommended that in clean rooms with isotropic air flow (ISO Classes 1 – 5), the amount of air exchange is calculated by the average velocity of the supply air flow. In some cases, ISO 6 cleanrooms may also require unidirectional airflow.
What is isotropic wind?
Isotropic wind moves parallel horizontally or vertically throughout a space, usually at a speed of 60 - 90 ft/min (180 - 270 m/min). Airflow maintains an angle of no more than 18 degrees in space moving parallel at a high enough speed to sweep away dust particles before they stick to surfaces.
Isotropic wind parameter table (Average wind speed)
Velocity Threshold ISO 5 ISO 4 ISO 3 ISO 2 Low speed (m/s) 0.2 0.3 0.3 0.3 Voice high speed (m/s) 0.5 0.5 0.5 0.5 Clean room application for Average Wind Velocity Principle
- Standard-compliant facilities
- Clean room with unidirectional wind
- Wind speed independent of ceiling height
- Used for manufacturing electronic components and microbiology
- Uniform wind velocity throughout the entire space
- Measuring wind blowing across the entire cross-section of the clean zone
- Stable wind velocity from 60 - 90 ft/min and almost in parallel direction
- Directional airflow can be blown both vertically and horizontally.
Air filter coverage density of unidirectional airflow clean room
In most cases, a clean room with unidirectional airflow requires an air quality of ISO 5 or higher and has an air filter ceiling coverage density close to 100%; Air filter coverage density is not a very specific metric. Vent coverage density is often referenced in IEST, FED, ASHRAE standards, but is not found in the latest ISO design documents.
As mentioned, the number of air exchanges per hour of a clean room with isotropic wind is calculated by the wind speed parameter. This is covered in the IES RP CC 002-86 standard “Laminar Flow Clean Air Devices”, which calculates the appropriate wind speed of a clean room with isotropic air to be 90 ft/min.
20% of all clean room measurements with unidirectional wind often take the standard wind speed of 90 ft/min. An air filter with fan can achieve 90 ft/min allowing designers to simply proceed with installing air filters with fans until the required level is achieved. The number of air filters required may vary depending on the facility type. A facility can have up to 100 zoned areas, with different systems for each zone.
Air filter coverage density for clean rooms with anisotropic air flow
Air filter coverage density is a clean room standard mentioned in Fed standard 209E (now expired) and other documents.
The most commonly asked question in clean room construction is: "How much will this cost?". Multiplying the basic cost of an air filter by the number of filters per unit volume simplifies cleanroom cost calculations. One can consider calculating air filter coverage density as a reference tool in the early design stage.
If you do not know the number of air filters needed for a clean room, calculating the initial cost for a clean room is challenging. Air filter efficiency and cost vary widely, so estimating air filter coverage is useful for cleanroom professionals after other criteria have been established.
Air filter coverage chart simplifies the work between designers and users (buyers) and makes quotation more transparent. The final effect is still formed from many different factors.
In short: Calculation of air filter coverage density is not mentioned in ISO clean room design standards. This is a versatile tool for professionals to quickly estimate construction costs. Filter coverage provides a simple way to calculate cleanroom construction costs during the preliminary design process, but is considered secondary to cleanroom efficiency and performance.
Formula to calculate air filter coverage density
Number of FFU = (Number of air exchanges / minute) x (Room volume (m3) ÷ FFU velocity (m/min))
Clean room gas exchange quantity and parameter table of United States Pharmacopoeia (USP) standards
USP 797 and USP 800 cleanrooms for sterile and hazardous drug manufacturing processes also have unique gas exchange requirements. In fact, a range hood in the main control area affects the overall air exchange of the whole room. Sterile and toxic substances have ventilation requirements that are much different than those in microelectronics manufacturing cleanrooms.
Factors in clean room gas exchange calculation
These models are the most popular because they are easy to look up at the three-head design stage. The final calculation requires more careful tools to achieve ISO cleanroom recovery levels, to ensure that airborne particulate contamination levels rarely exceed standards. For example, a decay equation measures the grain between the first and second test sampling. This helps designers understand dust particle levels after doors or walkways open or close. Other factors, such as dust dispersion from people and machinery, additional air cleaning devices, surface adhesion, and air handling procedures also influence dust particle formation.
Why Does the Air Exchange Rate Change Despite ISO Cleanroom Standards?
The requirement for the number of air exchanges of a clean room is largely determined by the degree of contamination. A clean room built based on low-level gas exchange standards can still be guaranteed if in the same production process, that clean room has fewer people working inside.
Facilities also contribute to solving the particulate problem by increasing the amount of air exchange, or sometimes with built-in air flow changers. Some cleanroom facilities choose demand-controlled filtration systems to optimize air circulation based on real-time particle counts. Likewise, adjustable air filters can also pre-set parameters in case the cleanroom is left vacant for long periods of time. Because cleanrooms are often powered 24/7, reducing fan load can help reduce overall energy costs.
Why is the gas exchange of a Clean Room a series of parameters?
In nearly all examples, the amount of exchange as displayed by a range of parameters ranges from low to high for a simple reason... it is not an exact science.
The high level air exchange (48 ACH) of an ISO 8 clean room is nearly 9 times higher than the low level (5 ACH) according to ISO 146144-4 standard. This does not provide clarity for contractors or project leaders where budgets, product safety or operating costs are paramount.
The air exchange quantity tables provide a range of parameters because they do not take into account the complexity of achieving the final cleanliness level of a particular cleanroom. Furthermore, some facilities may over-standardize their air handling systems to make temperature and humidity control more convenient. However, in most cases, facilities tend to be built to lower standards to optimize gas efficiency. However, the air exchange parameter table allows for an overall estimate of the size and number of dust filters, which is an important factor in the overall cost per unit area.
A reliable test result requires a reliable test method.
It is important to know that every calculation method is suitable for every facility. Depending on the design of the clean room and HVAC equipment, calculation methods require clarity and rationality.
Standards establish principles and best practices, however, cleanroom gas exchange rates vary more than one might think. Project leaders and contractors often lack data to compare internal changes. Factors include the use of an air handler instead of a modular air filter system, filter and motor efficiency, variation in air exchange per zone, and total pressure loss.
“There is no consensus on a recommended amount of air exchange. Most specifications suggest a range of parameters, which are often very wide and designers, who need to choose an appropriate amount of air exchange to size the equipment, are not provided with clear guidance clear.”
US Energy Research AdministrationWhy do facilities avoid a common standard parameter table for Gas Exchange Quantity?
A strictly designed air exchange rate will require a more powerful HVAC system, greater air filter sub-package density, and increase system operating costs. Sometimes a manufacturing process is rapidly transitioning toward making smaller, more sensitive components, or a facility is expecting increased output and product sensitivity in the near future. An oversized air handling system can easily accommodate air demands while scaling production that requires a higher level of cleanliness or includes an increase in the number of clean rooms.
Gas exchange volume 2.0
A cleanroom design that combines analysis and consideration of all factors between manufacturers, architects, engineers, HVAC specialists and system operators. Gas exchange data sheets often provide arguments and considerations but do not include a traceable technical basis for pollution control. The initial calculation of air exchange must reflect final conditions and adjust for less obvious parameters such as compensation for heating, air leakage, and recovery rate.
The air exchange parameter table is an important tool during the initial discussion, however, efficiency and cleanliness are parameters that are much more difficult to grasp than room area, average wind speed, average air flow, number of air exchanges. Experienced cleanroom designers understand that other factors such as choosing the right fan size, coverage density, and room design will have poor results if not considered early. A cleanroom requires many initial assessments and decisions that depend on each other to have a solid foundation.
Conclusion
- In clean rooms with anisotropic or mixed wind, use the method of calculating the number of air exchanges (ACH)
The basic function of each cleanroom is unique to its manufacturing process, and so is the science that goes into designing them. For most facilities, a dialogue with an environmental management expert is the fastest way to go from a clean room idea to a clean room project.
The information here will help you do two things: clarify and explain the gas exchange calculation method, and understand what is most applicable to clean rooms. However, it is not a requirement for your cleanroom.
Abridged translation: Khanh An
