Smoke detectors sense the presence of smoke particles. In order for a smoke detector to sense these particles, smoke must travel from the point of origin to the detector. When evaluating a particular building or location for detector layout, likely fire locations should first be determined, and paths of smoke travel from each of these fire locations should be determined. Wherever practical, actual field tests should be conducted. The most desired location for smoke detectors would be the common points of intersection of smoke travel from fire locations throughout the building. Ceiling height, construction, and ventilation play significant roles in smoke detector performance.
Smoke detectors re-spond to the solid and liquid aerosols produced by a fire. Each type responds differently to different types of smoke.
Smoke Detectors applications
Smoke detectors have a wide range of fire sensing applications and are best suited to detect slow, smoldering fires such as wood pyrolysis and smoldering cotton.
Smoke Detectors types
Ionization Smoke Detector
Ionization smoke detectors use an ionization chamber and a source of ionizing radiation to detect smoke. This type of smoke detector is common because it is inexpensive and better at detecting the smaller amounts of smoke produced by flaming fires.
The principle of using a small amount of radioactive material to ionize the air between two differentially charged electrodes to sense the presence of smoke particles. Smoke Particles entering the ionization volume decrease the conductance of the air by reducing ion mobility. The reduced conductance signal is processed and used to convey an alarm condition when it meets preset criteria.
Inside the ionization detector is a small amount (perhaps 1/5000th of a gram) of Americium-241. The radioactive element americium has a half-life of 432 years, and is a good source of alpha particles.
An ionization chamber is very simple. It consists of two plates with a voltage across them, along with a radioactive source of ionizing radiation.
Photoelectric Light-Scattering Smoke Detector:
In a photoelectric light-scattering smoke detector, a light source and a photosensitive sensor are arranged so that the rays from the light source do not normally fall on the photosensitive sensor. When smoke particles enter the light path, some of the light is scattered by reflection and refraction onto the sensor, causing the detector to initiate a fire alarm signal.
Photoelectric Linear Projected Beam Smoke Detector
In a photoelectric linear projected beam smoke detector, a light source and a photo-sensitive sensor are arranged across a protected space so that the rays from the light source normally fall on the photosensitive
sensor. When the smoke particles enter the light path, the intensity of the light is reduced, causing the detector to initiate a fire alarm.
Air-sampling Smoke Detector:
In an air-sampling smoke detector, a system of tubing and sampling ports draws a sample of air from a protected space into a detection unit. When smoke particles in the air sample enter a detection chamber, the presence of
the particles causes the detector to initiate a fire alarm signal.
Air Duct-type Smoke Detector
Detects smoke for the purpose of controlling the propagation of smoke through the heating, ventilation and air conditioning system (HVAC). This helps prevent possible panic and damage from
distribution of smoke and gaseous products. These detectors only detect smoke when smoke is circulation in the duct.
Spacing of smoke detectors
The spot type smoke detector spacing recommendation of 30 ft. (9.1 m) is based upon the detector installation on a smooth ceiling that is 10 ft. (3 m) high.
smoke detector coverage is a circle whose radius is 0.7 times the listed spacing. Since all of the area within the detector’s circle of coverage is suitable for detecting smoke from fire.
Unlike heat detectors, smoke detectors are not given a listed spacing. It is recommended that smoke detectors be installed on S = 30 ft. (9.1 m) centers, on smooth ceilings.
Exposed solid joists
Exposed solid ceiling joists may impede the flow of smoke to detectors. When spacing spot type smoke detectors, a joist is defined as any solid member extending 8 in. (20 cm) or more down from the ceiling and spaced less than 3 ft.
(1 m) apart.
Exposed beams
Beams are defined as any members extending 8 in. (20 cm) or more down from the ceiling and spaced more than 3 ft. (1 m) apart . The spacing of smoke detectors must be reduced in the direction perpendicular to the beam. Detectors may be mounted on the bottom of the beams that are less than 12 in. (30 cm). If the beams are greater than 18 in. (46 cm) deep, each bay created by the beams must have at least one detector mounted on the ceiling. Smoke detectors should be mounted on the ceiling within each bay if the ratio of beam depth (D) to ceiling height (H), D/H, is greater than 0.1, and the ratio of beam spacing (W) to ceiling height (H), W/H, is greater than 0.4. Smoke detectors should be mounted on the bottom of each beam if either the ratio of beam depth (D) to ceiling height (H), D/H, is less than 0.1, or the ratio of beam spacing (W) to ceiling height (H), W/H, is less than 0.4.
Sloped ceilings
Rooms with peaked ceilings must have the first row of detectors placed within 3 ft. (1 m) (measured horizontally) of the ceiling peak. Additional detectors, if required, must be spaced based upon the horizontal projection of the ceiling and ceiling construction. This modification of spacing for smoke detectors on sloped ceilings is identical to that used for spot type heat detectors.
Rooms with shed ceilings having a slope greater than 1 ft. in 8 ft. (1 m in 8 m) must have the first row of detectors within 3 ft. (0.9 m) of the high end of the ceiling. Additional detectors, if required, must be spaced based upon the horizontal projection of the ceiling and ceiling construction. For roofs having a slope less than 30 degrees, horizontal spacing must be adjusted according to the height of the peak. For roofs having a slope greater than 30 degrees, horizontal spacing must be adjusted according to the average sloped ceiling height. These modifications of spacing for smoke detectors on shed ceilings is identical to that used for spot type heat detectors.
High air movement
The use of spot type smoke detectors in areas of high air movement (greater than 300 ft./min. [1.5 m/sec.]) requires a suitable reduction in detector spacing to maintain detector performance.
smoke detectors installation
references
- smoke detectors manufacturers
- A Practical Guide to Fire Alarm Systems Provided as a service to the insurance – Third Edition by Central Station Alarm Association