Fixed Temperature Heat Detectors Engineering Essay

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This is the most common type of heat detector. Fixed temperature detectors operate when the heat sensitive element reaches a set operating temperature. Thermal lag delays the accumulation of heat at the sensitive element so that a fixed-temperature device will reach its operating temperature sometime after the surrounding air temperature exceeds that temperature. The most common fixed temperature point for electrically connected heat detectors is 58°C. Recent technological developments have enabled the perfection of detectors that activate at a temperature of 47°C, increasing the available reaction time and margin of safety.

Rate-of-rise heat detectors

Rate-of-Rise (ROR) heat detectors operate on a rapid rise in element temperature of 12° to 15°F (6.7° to 8.3°C) increase per minute, irrespective of the starting temperature. This type of heat detector can operate at a lower temperature fire condition than would be possible if the threshold were fixed. Rate of rise detectors may not respond to low energy release rates of slowly developing fires. To detect slowly developing fires combination detectors add a fixed temperature element that will ultimately respond when the fixed temperature element reaches the design threshold.

Smoke detector

A smoke detector is a device that detects smoke typically as an indicator of fire. Commercial, industrial, and mass residential devices issue a signal to a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible and/or visual alarm from the detector itself.

Smoke detectors are typically housed in a disk-shaped plastic enclosure about 150 millimetres (6 in) in diameter and 25 millimetres thick, but the shape can vary by manufacturer or product line. Most smoke detectors work either by optical detection (photoelectric) or by physical process (ionization), while others use both detection methods to increase sensitivity to smoke. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. However, in many single family detached and smaller multiple family housings, a smoke alarm is often powered only by a single disposable battery.

Optical Smoke Detector with the cover removed.

Optical

An optical detector is a light sensor. When used as a smoke detector, it includes a light source (incandescent bulb or infrared LED), a lens to collimate the light into a beam, and a or photodiode photoelectric sensor at an angle to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber across the path of the light beam, some light is scattered by the smoke particles, directing it at the sensor and thus triggering the alarm.

Also seen in large rooms, such as a gymnasium or an auditorium, are devices to detect a projected beam. A unit on the wall sends out a beam, which is either received by a receiver or reflected back via a mirror. When the beam is less visible to the "eye" of the sensor, it sends an alarm signal to the fire alrm control panel.

Optical smoke detectors are quick in detecting particulate (smoke) generated by smoldering (cool, smoky) fires. Many independent tests indicate that optical smoke detectors typically detect particulates (smoke) from hot, flaming fires approximately 30 seconds later than ionization smoke alarms.

They are less sensitive to false alarms from steam or cooking fumes generated in kitchen or steam from the bathroom than are ionization smoke alarms. For the aforementioned reason, they are often referred to as 'toast proof' smoke alarms.

Ionization

This type of detector is cheaper than the optical detector; however, it is sometimes rejected because it is more prone to false (nuisance) alarms than photoelectric smoke detectors . It can detect particles of smoke that are too small to be visible. It includes about 37kBg of radioactive americium-241 (241Am), corresponding to about 0.3µg of the isotope.The radiation passes through an , an air-filled ionization chamber between two electrods, and permits a small, constant current between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this current, setting off the alarm.

An alpha emitter, has a half-life of 432 years. This means that it does not have to be replaced during the useful life of the detector, and also makes it safe for people at home, since it is only slightly radioactive. Alpha radiation, as opposed to beta and gamma, is used for two additional reasons: Alpha particles have high ionization, so sufficient air particles will be ionized for the current to exist, and they have low penetrative power, meaning they will be stopped by the plastic of the smoke detector and/or the air. About one percent of the emitted radioactive energy of 241Am is gamma radiation.

Air-sampling

An air-sampling smoke detector is capable of detecting microscopic particles of smoke. Most air-sampling detectors are aspiration smoke detectors, which work by actively drawing air through a network of small-bore pipes laid out above or below a ceiling in parallel runs covering a protected area. Small holes drilled into each pipe form a matrix of holes (sampling points), providing an even distribution across the pipe network. Air samples are drawn past a sensitive optical device, often a solid-state laser, tuned to detect the extremely small particles of combustion. Air-sampling detectors may be used to trigger an automatic fire response, such as a gaseous fire suppression system, in high-value or mission-critical areas, such as archives or computer server rooms.

Most air-sampling smoke detection systems are capable of a higher sensitivity than spot type smoke detectors and provide multiple levels of alarm threshold, such as Alert, Action, Fire 1 and Fire 2. Thresholds may be set at levels across a wide range of smoke levels. This provides earlier notification of a developing fire than spot type smoke detection, allowing manual intervention or activation of automatic suppression systems before a fire has developed beyond the smoldering stage, thereby increasing the time available for evacuation and minimizing fire damage.

Carbon monoxide and carbon dioxide detection

A carbon monoxide detector or CO detector is a device that detects the presence of the carbon monoxide (CO) gas in order to prevent carbon monoxide poisoning . CO is a colorless and odorless compound produced by incomplete combustion. It is often referred to as the "silent killer" because it is virtually undetectable without using detection technology. Elevated levels of CO can be dangerous to humans depending on the amount present and length of exposure. Smaller concentrations can be harmful over longer periods of time while increasing concentrations require diminishing exposure times to be harmful.

CO detectors are designed to measure CO levels over time and sound an alarm before dangerous levels of CO accumulate in an environment, giving people adequate warning to safely ventilate the area or evacuate. Some system-connected detectors also alert a monitoring service that can dispatch emergency services if necessary.

While CO detectors do not serve as smoke detectors dual smoke/CO detectors are also sold. Smoke detectors detect the smoke generated by flaming or smoldering fires, whereas CO detectors go into alarm and warn people about dangerous CO buildup caused, for example, by a malfunctioning fuel-burning device. In the home, some common sources of CO include open flames, space heaters, water heaters, blocked chimneys or running a car inside a garage.

Aspirating smoke detector

An aspirating smoke detector (ASD), consists of a central detection unit which draws air through a network of pipes to detect smoke. The sampling chamber is based on a nephelometer that is capable of detecting the presence of smoke particles suspended in air by detecting the light scattered by them in the chamber.

In most cases aspirating smoke detectors require a fan unit to draw in a representative sample of air from the protected area through its network of pipes, such as is the case for Wagner and Xtralis ASD systems.

Aspirating smoke detectors are highly sensitive, and can detect smoke before it is even visible to the human eye. They are not recommended for use in unstable environments due to the wide range of particle sizes that are detected. This does not mean they can't be used in dusty/dirty environments, as long as the levels of aerosol remain stable.

Commercial smoke detectors

Commercial smoke detectors are either conventional or analog addressable, and are wired up to security monitoring systems or fire alarm control panel (FACP). These are the most common type of detector, and usually cost a lot more than a household smoke alarms. They exist in most commercial and industrial facilities, such as high rises, ships and trains. These detectors don't need to have built in alarms, as alarm systems can be controlled by the connected FACP, which will set off relevant alarms, and can also implement complex functions such as a staged evacuation.

Conventional

The word Conventional is slang used in to distinguish the method used to communicate with the control unit from that used by addressable detectors whose methods were unconventional at the time of their introduction. So called "Conventional Detectors" cannot be individually identified by the control unit and resemble an electrical switch in their information capacity. These detectors are connected in parallel to the signaling path . so that the current flow is monitored to indicate a closure of the circuit path by any connected detector when smoke or other similar environmental stimulus sufficiently influences any detector. The resulting increase in current flow is interpreted and processed by the control unit as a confirmation of the presence of smoke and a fire alarm signal is generated.

Addressable

This type of installation gives each detector on a system an individual number, or address. Thus, addressable detectors allow an FACP, and therefore fire fighters, to know the exact location of an alarm where the address is indicated on a diagram.

Analog addressable detectors provide information about the amount of smoke in their detection area, so that the FACP can decide itself, if there is an alarm condition in that area (possibly considering day/night time and the readings of surrounding areas). These are usually more expensive than autonomous deciding detectors.

Flame ionization detector

A flame ionization detector (FID) is a type of gas detector used in gas chromatography. The first flame ionization detector was developed in 1957by scientists working for the in CSIRO.

The detection of organic compounds done with flame ionization. Biochemical compounds such as proteins, nucleotides, and pharmaceuticals can be studied with flame ionization as well as other detectors, like thermal conductivity,thermionic, or electrolytic conductivity due to the presence of nitrogen, phosphorus, or sulfur atoms or because of the universality of the thermal conductivity detector. However, typically the biochemical compounds have a greater amount of carbon present than other elements. This means that a particular compound may be more easily detected using flame ionization over the other methods because of higher carbon concentration and also flame ionization's sensitivity.

Reffrences

^ "HYPERLINK "http://www.wpi.edu/offices/president/recipients.html"Recipient ProfilesHYPERLINK "http://www.wpi.edu/offices/president/recipients.html"". Worcester Polytechnic Institute. http://www.wpi.edu/offices/president/recipients.html. Retrieved 2010-04-20. 

^ "HYPERLINK "http://www.springerlink.com/content/h1427k29l5417444/"Journal: Fire Technology, Volume 20 #1HYPERLINK "http://www.springerlink.com/content/h1427k29l5417444/"". Springer. 1984-02. http://www.springerlink.com/content/h1427k29l5417444/. Retrieved 2010-04-20. 

^ "HYPERLINK "http://www.sti.nasa.gov/tto/spinfaq.htm"Scientific and technical information FAQHYPERLINK "http://www.sti.nasa.gov/tto/spinfaq.htm"". NASA Center for AeroSpace Information (CASI). http://www.sti.nasa.gov/tto/spinfaq.htm. Retrieved 2008-01-20. 

^ Residential Smoke Alarm Performance, Thomas Cleary, Building and Fire Research Laboratory, National Institute of Standards and Technology, UL Smoke and Fire Dynamics Seminar. November, 2007.

^ a b Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings, http://www.fire.nist.gov/bfrlpubs/fire07/art063.html, Bukowski, Cleary et al

^ "HYPERLINK "http://www.cns-snc.ca/ecc/smoke_am241.pdf"Smoke detectors and americium-241 fact sheetHYPERLINK "http://www.cns-snc.ca/ecc/smoke_am241.pdf"". Canadian Nuclear Society. http://www.cns-snc.ca/ecc/smoke_am241.pdf. Retrieved 2009-08-31. 

^ Julie Louise Gerberding (2004-04). "HYPERLINK "http://www.atsdr.cdc.gov/toxprofiles/tp156.pdf"Toxicological Profile For AmericiumHYPERLINK "http://www.atsdr.cdc.gov/toxprofiles/tp156.pdf"" (PDF; 2.1MiB). United States Department of Health and Human Services/Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp156.pdf. Retrieved 2009-08-29. 

^ New York City Fire Department. "HYPERLINK "http://www.nyc.gov/html/fdny/html/safety/firesafety_carbon_monoxide.shtml"Carbon monoxide poisoningHYPERLINK "http://www.nyc.gov/html/fdny/html/safety/firesafety_carbon_monoxide.shtml"". http://www.nyc.gov/html/fdny/html/safety/firesafety_carbon_monoxide.shtml. Retrieved 2008-10-20.

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