Preventing cleanroom contamination from fire
BY MARVIN SPEHAR
One of the most important aspects of operating a cleanroom environment is the effort taken to maintain its integrity. Facilities develop rigid standards and protocols at great financial expense to help prevent compromising their cleanroom. In the event of smoke, the expense of decontamination is costly in materials, downtime and loss of production.
Decontamination is one of the many challenges of operating a cleanroom. Studies indicate that the average cost of decontamination is in excess of $4 million for each event, with the potential for an extended period of downtime exceeding several weeks.
These catastrophic and costly events can be minimized with planning and by using the proper fire detection methods. One of the least understood factors in the compromising of a cleanroom is the effect of contaminated particulate matter on equipment and product resulting from a small fire event. Even a small, contained fire has the ability to cause an extensive amount of contamination. This event will most likely halt production and require unwanted expenditures for decontamination.
The detection of any fire threat is only as good as the available technology. Over the past few years, many facility operators have installed new laser-based early warning air-sampling smoke detection systems. Many of these operators believe their systems are state-of-the-art; and they are—to a degree. In the last decade, dramatic leaps have been made in technology, bringing earlier warning fire detection into the cleanroom marketplace. Throughout the years, fire detection has evolved through many types of detection, from heat to smoke and then laser detection.
This newer laser technology has allowed detection devices to alarm much earlier than the previous methods, and detect smoke that is virtually invisible to the human eye. But there is one major inherent flaw in these new smoke detection systems, and ironically enough, it is the smoke. The same smoke required to activate a laser detection device is what's contaminating the protected cleanroom.
Many companies that have accepted laser detection believe the earliest possible warning is being provided. Laser detectors can detect smoke particles as small as .2 to .3 µm in high enough concentration, which is just below the threshold of the size particle seized by HEPA filters. Unfortunately, this is smoke after a fire event has started—the same smoke that will contaminate a cleanroom.
The laser detector's operating principle is a beam of light that determines particulate matter in the air flowing through the detector. As particles pass through the laser beam, the light is refracted and then reflected to a photo receiver that indicates a physical presence in the sampled air. This presence may be smoke, dirt, dust, gases or any foreign object—all which are interpreted as smoke by the detector.
Even with the air sample being filtered by the detector, there is potential for false or nuisance alarms due to the operating principle of its technology. In Europe, some of these same laser detectors are being called environment monitors, rather than smoke detectors, due to their inability to distinguish the difference among smoke, dust and pollutants.
Smoke, flame and heat are the typically recognized stages of a fire. But there is one precursor to these three stages—the incipient or overheating stage, which is seldom discussed and commonly misunderstood. It's the stage where combustion begins, at the very onset of any potential threat, before smoke appears.
The fire's incipient stage begins at a material's Thermal Particulate Point (TPP) in air. The TPP is the specific temperature at which materials begin to chemically break down via thermal degradation and begin to emit invisible particles of combustion, comprised mostly of carbon molecules. This stage can propagate for hours, even days, before any smoke appears.
Particles released at the TPP of any material can be as small as .0025 µm, which is 1/100th the size of the average smoke particle. For instance, PVC insulation has a TPP of 290 degrees, at which point it begins to thermally degrade and emit combustion particles. These invisible particles have a zero percent obscuration per foot (obs/ft) density, which is the decrease in light transmission due to the opaque nature of smoke, at a distance of one foot. And that's undetectable by laser technologies.
Lasers systems can detect to .0015 percent obs/ft, which undoubtedly is very sensitive, taking into consideration that spot detectors require 1.7 percent obs/ft or higher. As a material continues to overheat to the point of a potential fire threat, it releases these invisible incipient particles at a rate of 109/sec/cm3. For example, one microgram of material can produce 1014 particles in a matter of seconds. Detecting these particles can give you true early warning fire detection before there are any signs of smoke, flame or heat, and before a cleanroom becomes contaminated.
Cloud chamber technology, however, is able to detect these invisible, incipient particles of combustion. The cloud chamber detector is based on the theory of airborne condensation nuclei and dates back to 1841 with the work of James Espy.
In the late 1890s, physicist C.T.R. Wilson invented the "Wilson Cloud Chamber" in an attempt to track cosmic particles emitted from space and alpha particles emitted from the radioisotope Radium. Although he was successful, Wilson did not realize the importance of his discovery in the detection of incipient particles of combustion.
The principle behind the "Wilson Cloud Chamber" is to humidify a sample of air and expose it to a rapid expansion. A barometric phenomenon then occurs where the particulate matter in the sample act as seeds to form microscopic water droplets around each carbon molecule. This process amplifies the size of the particulate from something that was below the wavelength of light (invisible) to a size far above the wavelength of light, making it easily to detect and quantify.
One byproduct of this type of physics is that false alarms are virtually impossible. By controlling the amount of pressure in the sample, the cloud chamber controls the size of the condensate, which is done at a level far below the size of dust, dirt and other contaminants.
In the last 20 years, microprocessors and advanced circuitry have made cloud chamber technology a reliable and cost-effective early warning, fire detector. The detector draws air samples, through tubing or pipe from remote locations, back to the detector for analysis.
The operating principle behind cloud chamber detection makes it the absolute earliest detection possible. Since cloud chamber technology is not affected by dirt, dust, humidity, or temperature it is virtually free of false and nuisance alarm yet offers unparalleled detection sensitivity.
In comparing technologies (see Figure 1), laser detection is an effective early warning smoke detector for environments where smoke can be tolerated and where the occasional false alarm from dust or dirt is acceptable. Cloud chamber detection, on the other hand, is used where smoke is not acceptable, environmental issues are a concern, and the absolute earliest warning possible of an impending fire threat is paramount.
Life safety issues, preventing loss of data, decreasing production downtime, preventing contamination and stopping business interruption are the most important factors to consider when determining which technology you should choose to safeguard your facility.
Taking into consideration the methods of early warning fire detection available on the market today, laser detection will not spare your cleanroom of contamination. It will, however, warn of a fire threat that has reached the smoky, smoldering stage prior to ignition, minimizing damage or containing it to one area. Detecting a fire threat before contamination, however, is not only crucial in today's facilities, but also fundamental in cost containment and preventing a catastrophic event.
A negative to any early warning air-sampling fire detector is locating the source of the smoke or incipient particles after a fire has begun. Air sampling in large open areas will not identify the exact location of the threat, even though it will isolate it to a particular zone or area. To overcome the obstacle of locating a potential fire threat, a cloud chamber detector manufacturer has developed a hand-held unit utilizing cloud chamber technology. With a higher concentration of invisible, incipient combustion particles near the threat itself, firet can be easily detected using a portable cloud chamber detector.
When comparing technologies, a cloud chamber detector will notify personnel of a fire threat hours before it actually occurs. Laser-based fire detection, on the other hand, will only notify personnel that the integrity of the cleanroom has already been compromised.
MARVIN A. SPEHAR is director of business development of SAFE Fire Detection, Inc. (Charlotte, N.C.)