REFERENCE MATERIAL

Cleanroom

A room, enclosure, and/or other special environment where contaminants are controlled within specific limits. Ultraclean, particle-free air is forced into the cleanroom under positive pressure to control the airborne particulate levels and in some cases temperature, humidity and other environmental variables may also be controlled.

Cleanroom Classification

 A cleanroom class is defined by the number of (typically) one half micron size particles found in a cubic foot or cubic meter of air. For example, a “US Customary” Class 100 (ISO 5) cleanroom would need to create and maintain an environment in which there are no more than 100 one half (0.5) micron and larger size particles per cubic foot of air sample. In most cases, Class 100 areas are considered to be sterile, as well. A Class 10,000 cleanroom is an order of magnitude cleaner than a Class 100,000 cleanroom, and so on. Airflow, by Cleanroom Class is specified in ISO Standard 14644-4.

Class 100,000 (ISO 8 / M 6.5) – Requires 10 - 20 air changes per hour*

Class 10,000 (ISO 7 / M 5.5) – Requires 30 - 70 air changes per hour*

Class 1,000 (ISO 6 / M 4.5) – Requires 70 - 160 air changes per hour*

Class 100 (ISO 5 / M 3.5) – Requires 40 -100 feet per min.** (400 to 480 air changes per hour***)

Class 10 (ISO 4 / M 2.5) - Requires 60 - 100 feet per min.** (540 to 600 air changes per hour***)

Class 1 (ISO 3 / M 1.5) - Requires 60 - 100 feet per min.** (540 to 600 + air changes per hour***)

*= Nonunidirectional dilution airflow for 9’ high Class 100,000 to Class 1,000 cleanrooms

**= Unidirectional displacement airflow for Class 100 to Class 1 cleanrooms

***= Estimated Rule of Thumb for 9’ high cleanrooms

Containment Booth

A room or enclosure with negative pressure to prevent powder or vapor from escaping to the surrounding area.  A containment booth may also provide filtered air to control contaminants and protect operators.  Air can be recirculated or 100% exhausted.

Down Flow Containment Booth

A personnel safety containment booth that provides Class 100 unidirectional down flow air and negative pressure to the outside.  An OEL specification is provided.

OEL – Occupational Exposure Limit

An Occupational Exposure Limit is an upper limit on the acceptable concentration of a hazardous a substance in workplace air for a particular material or class of materials.  It is typically set by competent national authorities and enforced by legislation to protect occupational safety and health.  It can be a tool in risk assessment and in the management of activities involving handling of dangerous substances.  There are many dangerous substances for which there are no formal occupational exposure limits.

TLV – Threshold Limit Value

The Threshold Limit Value of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse health effects.  TLV is a reserved term of the American Conference of Governmental Industrial (ACGIH). 

The TLV is an estimate based on the known toxicity in humans or animals of a given chemical substance and the reliability & accuracy of the latest sampling and analytical methods.  It is not a static definition since new research can often modify the risk assessment of substances and new laboratory or instrumental analysis methods can improve analytical detection methods.  The TLV is a recommendation by ACGIH, with only a guideline status.  As such, it should not be confused with exposure limits having a regulatory status, like those published and enforced by the Occupational Safety and Health Administration (OSHA). 

Definition – The TLV for chemical substances is defined as a concentration in air, typically for inhalation or skin exposure.  Its units are in parts per million of air (ppm) and in milligrams per cubic meter (mg/m³) for particulates.

Three types of TLVs for chemical substances are defined:

1. Threshold Limit Value – Time Weighted Average (TLV-TWA): average exposure on the basis of an 8h/day, 40h/week work schedule.
2. Threshold Limit Value – short-term exposure limit (TLV-STEL): spot exposure for a duration that cannot be repeated more than 4 times per day.
3. Threshold Limit Value – Ceiling (TLV-C) absolute exposure limit that should not be exceeded at any time. 

HEPA Filters

High Efficiency Particulate Air filters are extended life, pleated media cartridges in rigid frames having minimum particle arrestance efficiencies exceeding 99.97% down to 0.3 micron. The recommended air flow rate is typically 90 - 100 feet per minute, plus or minus 20%.

Unidirectional (“Laminar”) Air Flow

A continuous stream of particulate-free air which is forced under constant velocity and positive pressure in a single direction through the entire cross section of a clean space in parallel streamlines to avoid turbulence and eddy currents and eliminate contaminants by displacement (that is by pushing particles and vapors out with a cascade of ultraclean air). The first Class 100 clean devices were laminar flow benches and, although designs have changed somewhat over time, physics and the operational basics have not. Unidirectional air flow and the displacement principle are critical elements of every Class 100 – Class 1 clean space and laminar flow device. Maintaining these high cleanliness levels, and especially to avoid turbulence and eddy currents, requires nearly 100%, or equivalent, HEPA filter coverage and air flowing at a typical average velocity of 90 feet per minute, plus or minus 20%.

Nonunidirectional (“Turbulent”) Air Flow

Cleanrooms use turbulent airflow systems to clean particulate air and maintain a clean environment. It occurs when there is an irregularity such as a disruption in the surface across which the fluid is flowing, which alters the direction of movement.

Microns

Very small particulate sizes, expressed in micrometers (millionths of a meter). A micron is about 39 millionths of an inch (.000039”).

Hazardous Locations

Hazardous locations are operating environments in which explosive or ignitable vapors or dust is present or is likely to become present.

CLASS II LOCATIONS

Class II locations are those that are hazardous because of the presence of combustible dust.

Class II, Division 1 locations are those where combustible dust may be in suspension in the air under normal conditions in sufficient quantities to produce explosive or ignitable mixtures.

Class II, Division 2 locations are those where combustible dust will not normally be in suspension nor will normal operation put dust in suspension, but where accumulation of dust may interfere with heat dissipation from electrical equipment.