How to properly distribute fire alarms.
How to properly distribute fire alarms
It is especially difficult to solve the problem of proper placement of fire alarms in cases of non-standard layout, room size, and specific operation of the alarms themselves.
Arrangement of fire alarms
The operation of heat and smoke detectors depends on convection, which carries hot gas and smoke from the fire source to the detector. Fire detector placementwith sensors of this type should be based on the need to limit the time spent on this movement and on the condition of a sufficient concentration of combustion products at the location of the sensor. Hot gas and smoke will generally concentrate in the highest parts of the room, so this is where heat and smoke sensors should be located. Since smoke and hot gases from the fire rise upward, they are diluted by clean and cold air that enters the convective stream. Consequently, with an increase in the height of the room, the size of the fire sufficient to activate the heat or smoke sensors quickly increases. To some extent, this effect can be compensated for by using more sensitive sensors. Linear smoke sensors with an optical beam are less sensitive to the effect of a high ceiling than point-type sensors, since with an increase in the smoke-filled space, the length of the beam affected by the smoke increases proportionally.In addition, when the convection current captures ambient air, the gases cool. If the ceiling is high enough and the ambient temperature at the top of the room is high, the temperature of the gas-smoke mixture can drop to the ambient temperature at a level below the ceiling. This is possible if the air temperature in the room increases with height, for example, as a result of heating by the sun, the air at higher levels can be higher than the temperature of the smoke. Then a layer of smoke will form at this level before it reaches the ceiling, as if the room had an «invisible ceiling» at a certain height. If installation of fire alarmsis carried out at the expected stratification level, and stratification does not occur, or occurs at a higher level, detection can be dangerously late, since the relatively narrow convection jet can «bypass» the detectors. Eventually, as the fire grows and more heat is released, the convection jet will overcome the thermal barrier and the ceiling-mounted detectors will be operational, albeit at a later stage of the fire than if no stratification had occurred. Fire Detector Placement, heat and smoke detectors, should be carried out taking into account the structure of the expected air flows in the room. Air conditioning and ventilation systems with a high level of air exchange can adversely affect the capabilities of the sensors, creating an influx of fresh air to them, and an outflow of heated air, smoke and gases from combustion, or diluting smoke and hot gases from the hearth. Smoke detectormay be installed to monitor smoke in ventilation ducts. Basically, such sensors should help prevent the spread of smoke through the ventilation system, any recirculation should be stopped in the event of a fire. Smoke detector can be connected to a fire alarm system, but if smoke detectors have normal sensitivity, they cannot be a satisfactory means of detecting a fire in the area from which the air comes, since the smoke is diluted by the extracted clean air.
Layout of fire detectors for different types of ceilings
If it is not possible to install linear smoke detectors under the ceiling, they can be placed at a level lower than 600 mm from the ceiling. However, with such placement of detectors, the protected area is significantly reduced and is up to 12.5% of the installation height in each direction from the optical axis (Fig. 6. With increasing height, smoke disperses over a larger area, therefore, it is more economical to install linear optical detectors at the highest possible height. For example, when installed at a height of 4 m, for reliable detection of the source, the distance between the optical axes should be no more than 1 m, when installed at a height of 20 m, respectively, no more than 5 m. The layout of fire detectors in large industrial premises is characterized by the fact that there are usually beams of considerable height on the ceiling. Arrangement of detectorsin this case it must be carried out in accordance with clause 12.20. NPB 88-2001*: «point smoke and heat fire detectors must be installed in each ceiling compartment with a width of 0.75 m or more, limited by building structures (beams, purlins, slab edges, etc.) protruding from the ceiling by more than 0.4 m. If building structures protrude from the ceiling by more than 0.4 m, and the compartments formed by them are less than 0.75 m in width, the area monitored by fire detectors, specified in Tables 5, 8, is reduced by 40%. If there are protruding parts from 0.08 to 0.4 m on the ceiling, the area monitored by fire detectors, specified in Tables 5, 8, is reduced by 25%. For a honeycomb ceiling, installation of fire alarms on a beam is allowed with a relatively small cell width, less than four times the beam height, or on a ceiling with a cell width greater than four times the beam height. Here, the beam height limit of 600 mm appears (unlike our 400 mm), but the relative beam height is also taken into account — an additional limit of 10% of the room height.