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January 3, 2005
Air Sampling - Early Detection by VESDA
By Vision Systems
Mission-critical refers to the operations that are critical to an organization's ability to carry out its mission. In other words, mission-critical operations are those operations that are essential to an organization's ability to perform its intended function. A mission-critical facility is one that guarantees it will continue to operate, regardless of external conditions.
A critical banking facility is an example of such a facility that must maintain operation 24 hours a day 7 days a week. In fact, a minor interruption in service, or loss of data could seriously impact the operational continuity resulting in economic loss especially during high transaction periods.
72% of mission-critical applications experience nine hours of downtime per year1. 90% of businesses go bankrupt within two years of a significant failure.
The average cost per hour of downtime for a financial brokerage house is estimated at US$6.5 M3. Of the companies that experience a disaster but have no tested business recovery plans in place, only one in ten are still in business two years later.
The biggest risk to continuous operation within a computer room after a fire is the smoke damage to electrical equipment, not the flames. This paper discusses smoke detection systems and their role in prevention of fire and smoke contamination within a mission-critical facility.
The fire risk within today's Data Center
Today's computing technology is becoming smaller and therefore requires less space, but the heat being dissipated by the digital hardware is also increasing. The result is that the heat density on the chip and in the cabinet is growing at an unprecedented rate. By illustration:
The average Intel 486 CPU consumes about
10 W, the latest Pentium 4 consumes 100 W.
With the processing density and power consumption of blade servers it is not uncommon for standard 47 U cabinets to consume in excess of 21 KW….that's a lot of heat!!
This high heat load requires significant cooling via the computer room air conditioning (CRAC) system to remove the heat generated within equipment cabinets. Failure to cool this equipment will result in equipment over-heating and provide the potential for a fire.
Mechanical cooling and airflow movement is an essential parameter within the fire detection design.
The Detection Strategy
Within a data center the type of smoke generated and the dynamics of the airflow create a challenge for the fire engineer to design an effective fire detection system. It is the detection of smoke that is the most critical part of the fire protection system. Detection systems serve the basic function of alerting occupants within the building of a fire and are used to activate other systems such as mechanical exhaust and fire suppression systems.
The traditional smoke detectors known as Early Warning Smoke Detectors (EWSD) or conventional spot type detectors are of ionization or photoelectric type. Ionization type detectors were designed to detect very small particles such as the type produced by flammable liquids. Photoelectric detectors detect larger particles such as those produced by materials like plastics. Given this fact photoelectric detectors are more suitable to detect the fire type we expect within a computer facility, however there are other factors contributing to photoelectric detector's deficiency within these environments.
Within the fire industry detectors are categorized as Early Warning Smoke Detection (EWSD) and Very Early Warning Smoke Detection (VEWSD). In fact some people use these terms very loosely and do not differentiate the two correctly. An EWSD system provides detection of a fire condition prior to the time that it becomes threatening to the occupants of a building. Generally this is the time that smoke is visible. Let's use the example of a paper basket fire within a standard office. Seconds after the paper has ignited, smoke will generate and rise to the ceiling.
This visible and hot smoke will eventually enter the smoke detection chamber and trigger the alarm to alert the occupants that a fire has commenced. In contrast, if a computer terminal within the same room had a fault within the electronics resulting in a thermal event; it may smolder for hours before a flame ignites. We refer to the smoldering stage as the incipient stage to a fire. During this incipient stage the human eye will not see the particles but the human nose may smell them. EWSD are not sensitive enough to detect smoke at the incipient stage of an electrical-type fire. Only a VEWSD will detect an incipient fire and thus the term “VERY EARLY WARNING”.
This stage of a fire could last for hours or even days.
Spot type smoke detectors are ‘passive' detectors in that they wait for smoke and rely on the airflow to transport the smoke to the detector. Therefore their performance is affected by high airflow. Since the rate of smoke generation in a smoldering fire is relatively small, and the airflow velocity in the room is quite high, the movement of smoke is dominated by the airflow of the mechanical systems. Furthermore the smoke generated during the incipient stage is not hot therefore there is very little thermal lift. This often prevents smoke from moving directly to the ceiling, where spot type detectors are located, causing the smoke to dissipate more widely. An Air Sampling smoke detection system is ‘active', constantly sampling the air from multiple points throughout the environment. It is not totally dependant on thermal energy to transport the smoke to the detector.
The effects of smoke contamination
So why is the detection of smoke at the earliest possible stage important? Because the biggest risk to the continuous operation within a computer room facility is the smoke damage to electrical equipment, not the fire. In fact according to the USA Federal Commission of Communications, 95% of all damage within these facilities is non-thermal.
The by-products of smoke from PVC and digital circuit boards are gases such as HCL and these gases will cause corrosion of IT equipment. Graph 1 depicts the increased risk of failure possible with an increase of particulates in a computer room. Even at 16 micrograms per square centimeter there is moderate corrosion with long-term effects on electronics, at 30 micrograms/cm2 the corrosion is active and the effects are short term. Above this the damage to equipment is detrimental to ongoing performance.
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Graph 1: Contamination and failure probability |
Air Sampling Smoke Detection – how it works
Air Sampling smoke detection systems are quite different from conventional spot type smoke detectors. Air Sampling systems typically comprise a number of small-bore pipes laid out above or below a ceiling in parallel runs, some meters apart. Small holes, also some feet apart, are drilled into each pipe to form a matrix of holes (sampling points), providing an even distribution across the ceiling. Air or smoke is drawn into the pipework through the holes and onward to a very sensitive smoke detector mounted nearby, using the negative pressure of an aspirator (air pump).
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Illustration 1: Air Sampled through a capillary and sample point |
The VESDA aspirating smoke detector is a form of air pollution monitor. It has sensitivity some hundreds of times higher than conventional smoke detectors, yet its false alarm rate is exceptionally low (according to independent surveys). This reliability comes from its high immunity to the major sources of false alarms–dust, draughts and electrical interference. Accordingly, the entire zone is monitored for the early symptoms of overheating materials, possibly hours before a fire develops. This generally allows plenty of time for human intervention, or automatic intervention by the operation of an electric circuit breaker for example (which removes the source of heat - the electric current). The primary role of Air Sampling smoke detection is, therefore, fire prevention.
Vision Systems
700 Longwater Drive
Norwell , Massachusetts 02061
www.vesda.com
www.visionusa.com/vesda
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