The fire is affected by factors such as the type of combustible substances, the distribution of combustible substances, the nature of the fire, the fire load, the ambient temperature, and the supply of fresh air. Generally speaking, the occurrence and development of fire can be divided into four stages:
1.Initial period: The fire hasn’t formed yet, only a certain amount of smoke has appeared, and no property damage has been caused.
2.Early period: The fire begins to form, the smoke increases, the indoor temperature rises, and flames begin to appear, causing a little property damage
3.Middle period: The fire has formed, the indoor temperature is high, and the burning begins to accelerate, causing great property damage.
4.Late period: The fire is spreading.
In the fire alarm control system, choosing suitable smoke detectors, heat detectors and flame detectors is very important. The detectors should be chosen according to the developing features of the fires that may occur in the detection area, the nature and quantity of combustibles, the height of the building, the environmental features of the place, the airflow conditions, and possible interference sources causing a false alarm.
The following are three ways to choose the right detectors:
1.Choose According to The Forming and Development Features of Fire
Choosing according to the forming and development features of fire is the core. It usually follows the following principles:
(1)There is a smoldering stage in the early period of fire (such as cotton and linen fabric fire and wood fire), which will generate a large amount of smoke and a small amount of heat. In this case, people will usually choose smoke detectors to detect fire. The specific smoke detection type and sensitivity level of the smoke detector should be determined according to the specific place.
(2)When the fire develops rapidly, and there is a large amount of flame radiation and a small amount of smoke heat, you should choose the flame detector. In this case, people will usually use point-type UV flame detectors or point-type ultraviolet and infrared flame detectors. Their effectiveness depends on the optical sensitivity, detection angle, response time and installation location. Take explosion-proof point-type UV flame detector as an example. Its sensitivity level is grade I, its detection angle is 120°, and when facing a 0.3m×0.3m n-heptane flame, its response time is less than or equal to 10 seconds.
Ultraviolet and Infrared Flame Detector UV Flame Detector
(3)When the fire develops fast, it will generate a large amount of heat and smoke and flame radiation. At this point, you should use the combination of heat detector, smoke detector ,flame detector and beam detector.
heat detector beam detector
(4)When the detectors are connected with the fire extinguishing equipment, the linkage control signal must be sent after the double alarm signal is confirmed or even after the delay alarm is confirmed.
The places that must use the double alarm signals are usually important and dangerous. The double alarm signals are given by the same type of combination or different type of combination of smoke detectors, heat detectors and flame detectors. The same type of combination means the same type of detectors having two outputs with different sensitivities. The different type of combination means using the combined detectors and non-combined detectors.
(5)In places that have flammable gas or flammable liquid or vapor, you should use flammable gas detectors to detect early fire.
(6)In places where the features of fire forming are unpredictable, you can do a simulation test, and choose the detectors according to the test results.
2.Choose According to The Building’s Height
Limiting the use height of the detectors is to make sure their sensitivity and effectiveness can be fully brought into play. Normally, the installation height of the smoke detector is ≤ 12m, and the heat detector is ≤ 8m. The installation height of the flame detector is determined by its optical sensitivity range. The sensibility of the detectors should increase as the building height increases. It is worth a mention that the shape of the ceiling of the building (spire, vault, etc.) and the uneven ceiling of a large space have an impact on determining the building’s height. People should choose detectors according to their coverage and protecting radius.
3.Choose Under Various Environmental Conditions
When the air humidity is less than 95%, it generally does not affect the detector. But when there is atomized smoke or condensation, it will affect the sensitivity of smoke detectors and flame detectors. When there is smoke or ash, it will directly affect the use of smoke detectors. For heat detectors and flame detectors, they should avoid wet dust. The light interference doesn’t have much effect on smoke detectors and heat detectors, but it will affect the flame detectors.
