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History And Properties Of Fire Alarms Engineering Essay

Fire alarms and how they work have always fascinated me. I believe them to be one of the most useful, necessary and life saving inventions. Statistics show that the presence of the right fire alarm in households can reduce fire related deaths significantly. Fewer people die in home fires when a smoke alarm/detector is present and activates.

- Smoke alarm present and activated: 12 deaths/1000 home fires

- No devise / alarm did not activate: 17 deaths/1000 home fires

The smoke detector and the heat detector are the 2 types used to notify people of a fire. They work hand in hand to precaution us when a fire breaks out. The use of both is necessary. For example; Smoke detectors cannot be installed in kitchens or bathrooms as they would be going off all of the time. And that is one of the cases when having a heat detector becomes useful.

Fire alarms use both audible and visual alarms to let us know when the fire alarm system has been activated. Some are horns, some have flashing strobe lights on them, and some are speakers that will provide you with recorded instructions.

All fire alarm systems are designed to be audible in all rooms, even when the doors are closed.

This report includes a search on the different types of fire alarms, while we have chosen to present the heat detector in our experiment further on.

History:

Francis Robbins Upton invented the first automatic electric fire alarm in 1890. However, home smoke detectors were not introduced until the 1960s, when they revolutionized home safety.

Duane D. Pearsall and Stanley Bennett Peterson are credited with inventing the first truly affordable home smoke detector in 1965. Their detector featured individual battery powered units that could be easily installed and replaced. These first units were made from strong fire resistant steel and shaped much like a bee's hive.

In 1975, Stanley B. Peterson; working at Duane Pearsall’s company (called the “Statitrol Corporation”) came up with the idea of mass production.

Today they are installed in 93% of US homes and 85% of UK homes. However, it is estimated that any given time over 30% of these alarms don’t work, as users remove batteries, or forget to replace them.

Smoke detectors

A smoke detector or smoke alarm is a device that detects smoke and issues an alarm to alert nearby people that there is a potential fire. Because smoke rises, most detectors are mounted on the ceiling or on a wall near the ceiling. To avoid the nuisance of false alarms, most smoke detectors are mounted away from kitchens .Smoke detectors are usually powered by one or more batteries but some can be connected directly to household wiring. 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.

Photoelectric Detectors

There are many different types of photoelectric detectors.

One is a light sensor. When used as a smoke detector it includes a light source, a lens (to collimate the light into a beam) and a photoelectric sensor at right-angles 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 into the path of the light beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased input of light into the sensor sets off the alarm.

Another type works by using a straight line infra-red beam from the sender to the receiver. When smoke enters the beam, some light is scattered which results in less light detected by the receiver. A decreased input of light into the receiver sets off the alarm.

Another type is the projected beam detector. 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 alarm control panel.

Optical smoke detectors are quick in detecting slow burning, smoky fires. They are less sensitive to false alarming from cooking or steam from the bathroom steam than are ionization smoke alarms.

Ionization detector

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

Their differences:

Both ionization and photoelectric detectors are effective smoke sensors. Ionization detectors respond more quickly to flaming fires with smaller combustion particle while the photoelectric detectors respond more quickly to fire with little smoke and no flame.

Ionization detectors are less expensive than photoelectric detectors, but some users purposely disable them because they are more likely to sound an alarm from normal cooking due to their sensitivity to minute smoke particles. However, ionization detectors have a degree of built-in security which photoelectric detectors do not. When the battery starts to fail in an ionization detector, the ion current falls and the alarm sounds, warning that it is time to change the battery before the detector becomes ineffective. Back-up batteries may be used for photoelectric detectors.

In either type of detector, steam or high humidity can lead to condensation on the circuit board and sensor, causing the alarm to sound.

HEAT DETECTORS:

Heat detectors respond to changes in temperature. If the surrounding temperature rises above a certain predetermined point, an alarm signal goes off. They are intended to especially supplement smoke detectors to want of fire and are not meant to replace smoke detectors. The mechanical heat detectors do not use electricity or batteries for its working. Automatic or wireless heat detectors detect the growing fire by using electronic sensors to detect the smoke, heat, or flames from a fire and providing an early warning. the “rate- of-rise” and “fixed” are the two main classifications of heat detectors.

Rate-of-rise heat detectors

Also known as ROR, these heat detectors react to sudden changes or rise in the surrounding temperature. Any sudden rise in temperature in the predetermined range will set off the alarm. A typical alarm may be set off when the rate of temperature rise exceeds 6.7° to 8.3°C per minute.

Fixed temperature heat detectors

Fixed temperature heat detectors detect abnormally high temperatures and are set off when their surrounding temperature reaches a fixed point. The most commonly fixed temperature point is 58°C. But recently, due to technological developments, the detectors react at a temperature of 47°C. This is very useful as it provides us with a little bit more time to escape a fire.

Their differences:

Each type has its own features, styles and advantages and one can not say that one type of heat detector must always be used instead of the other. for example, A fixed threshold detector could be used above a large, closed oven, where a ROR detector would be unable to function properly. As a rate-of-rise heat detector would be set off, due to the sudden heat transient, every time the oven door is opened. And similarly, a ROR heat detector may be preferred in a room filled with highly combustible materials. Because in case of a fire, the flames could exceed the alarm threshold in a fixed temperature heat detector due to thermal lag.

Advantages of Heat Detectors

No necessity to change batteries

No annoying beep when the battery is low

Inherently secure and reliable

Resistive against contagion like UV and IR rays

Affordable compared to other detectors

Best loop capability

Immunity towards electromagnetic interference

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