(A) Background:
Before global cleanroom classifications and standards were adopted by the International Standards Organization (ISO), the U.S. General Service Administration’s standards (known as FS209E) were applied virtually worldwide. However, as the need for international standards grew, the ISO established a technical committee and several working groups to delineate its own set of standards.
Difference between FS209E AND ISO 14644-1

(B) Airborne Particulate Cleanliness Class Comparison:

 
(C) Airborne Particulate Cleanliness Classes (by cubic meter):

Important Points:

1. In cleanrooms, particulate concentration changes over time — from the construction and installation of equipment to its operational status.
2. ISO delineates three cleanroom classification standards:

• as- built
• at- rest and
• operational

 
(D) ISO 14644-2: It describes the type and frequency of testing required to conform to certain standards.
The below mentioned tables indicate mandatory and optional tests:

1. Required Testing (ISO 14644-2)

2. Optional Testing (ISO 14644-2)

3. In addition to ISO 14644-1 and ISO 14644-2, eight other cleanroom standards documents exist, as well as three specific to bio-contamination applications.

 (E) ISO and Federal Air Change Rates for Cleanrooms:
What is Air Change Rate (ACR)?
Ans: A critical factor in cleanroom design is controlling air-change per hour (ACH). This refers to the number of times each hour the filtered outside air replaces the existing volume in a building or chamber. A lower ACR often resulted in cleaner air.
Example: – In a normal home, an air-conditioner changes room air 0.5 to 2 times per hour. In a cleanroom, depending on classification and usage, air change occurs anywhere from 10 to more than 600 times an hour.
Use of Air Change Rate (ACR)?
 Ans: ACR is a prime variable in determining ISO and Federal cleanliness standards. To meet optimal standards, ACR must be painstakingly measured and controlled.
Name of the factors due to ACR varies from One Clean- Room to Other?
Ans:  The responsible factors are given below:

• Internal equipment,
• Staffing and operational purpose.
• Everything depends on the level of outside contaminants trying to enter the facility versus the level of contaminants being generated on the inside.

Note: Low-end figures within each contamination class generally indicate air velocity and air change requirements for an as-built or at-rest facility—where no people are present and no contaminating processes under way. When there are people and processes producing contaminants, more air changes are required to maintain optimal cleanliness standards. For instance, some manufacturers insist on as many as 720 air changes per hour to meet Class 10 standards.
Name of the factors required for determining the number of Air Changes for particular application?
Ans: The various factors are mentioned below:

• The number of personnel,
• Effectiveness of garbing protocol, frequency of access, and
• Cleanliness of process equipment.

                       Recommended following ranges based on FS209E classifications

Describe three abiding principles of ACR?
Ans: The three abiding principles of ACR are:

• Lower air change rates result in smaller fans, which reduce both initial investment and construction cost.
• Fan power is proportional to the cube of air change rates or airflow. A 30-percent reduction in air change rate results in a power reduction of approximately 66 percent.
• By minimizing turbulence, lower airflow may improve cleanliness.

(F) Federal and ISO Ceiling Fan Coverage Specifications:

• Achieving the optimal air change rate requires proper ceiling fan coverage.
• The cleanest modular cleanroom incorporates filter/fan units (FFUs) in every 2’ x 4’ (610 mm x 1219 mm) ceiling bay.
• This near-100% coverage provides a laminar flow of filtered air to quickly remove contaminants from the room, thus meeting FS209E standards for Class 10 and ISO Class 1 standards.
• Such coverage, especially in a large cleanroom, can lead to higher energy consumption, thus increasing costs for both initial construction and ongoing operation.
• In most cases, a smaller percentage of ceiling coverage produces adequate cleanliness.

Before deciding on the appropriate velocity and air changes for your application, it is recommended to evaluate some factors carefully and name of factors are:

• Number of personnel,
• Effectiveness of garbing protocol,
• Access frequency and
• Cleanliness of process equipment.

Important Point: Once the required air change figure is established; the number of required FFUs can be determined using this formula:
No. of FFUs = (Air Changes/Hour ÷60) x (Cubic ft. in room÷ 650*)
*CFM output of a loaded FFU
Example: Meeting Class 100 standards using the low-end air change recommendation (240/hour) inside a 12’ x 12’ x 7’ (3302 mm x 3302 mm x 2134 mm) cleanroom, with 1008 cu. ft. of volume, requires 6 FFUs. To meet the same standard using the high-end air change recommendation (480/hour) requires 12 FFUs.
(G) Positive Pressure: Cleanrooms are designed to maintain positive pressure, preventing “unclean” (contaminated) air from flowing inside and less-clean air from flowing into clean areas. The idea is to ensure that filtered air always flows from cleanest to less-clean spaces.
Range of differential air- pressure between spaces: 0.03 to 0.05 inches water gauge is recommended.
Note: In order to minimize disruptions to these cascading pressures when doors are opened, air locks are often specified between rooms of differing ISO cleanliness levels. Automated fan controls simplify pressure balancing by allowing fan speed adjustments at a centralized console panel.                          
(H) Laminar and Turbulent Air Flow:

 
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