Air sampling smoke detector system send the air it draws from protected areas to highly sensitive detector with the help of a pipe network to analyse and gives a warning signal. These systems are advantageous in terms of performance, installation cost and regular maintenance. It is preferred in situations where conventional systems are not sufficient; where there are entrance-maintenance problems, in areas requiring high protection (atrium, high structures, structure openings, in complex roof structures, in dusty areas, in cold warehouses, in warm areas, in places where the installation is to be kept invisible...), in extreme environmental conditions (like dust, dirt). 2 types of systems are used. In the primary air sampling systems, it is appropriate to used one detector per 1500 cubic metre volume. In some extreme conditions such as clean room etc., this volume can be reduced for each detector. If secondary sampling system is used, the amount of volume per detector used by normal detectors can be used. (For high sensitivity in normal conditions, holes can be drilled in the pipe with 3-5 metre intervals).
Primary sampling system: It operates with the assumption that any smoke particle in the protected area will pass through the ventilation system. It is used in situations when air speed is more than 1m/h. When operated with air conditioning systems, it shows optimum performance. (Performance limited to air) Cold smoke detection occurring in the first instances of a fire not reaching up to the ceiling and caused by a small sized problem is an advantage. If the detector is to be assembled out of the protected room, it would be more suitable to turn the outlet of detector back to the protected room. In this way, sample air - or maybe smoke- will be prevented to pollute other areas and pressure distribution between detector placements in the protected room will be allowed to be balanced. External pollution entering from the ventilation may cause false alarms. With the help of a reference detector observing this, redundant operation of the alarm is prevented.
Secondary sampling system: Analysis is realised with the sampling holes of the pipe network. It is used in situations where there is slow air movement. It consists of sampling holes positioned in the same position and uses the same design criteria with the normal detectors. Hole diameters are determined with hydraulic calculation program. In rooms with too high ceiling (high warehouses, atrium …), usage of multilevel sampling holes is preferred.
Design criteria for many LPCB approved detector systems are as following:
- Max. total pipe length 200m
- Max. single pipe length 100m (the time from sample entrance till detector should be limited to 120sn)
- Generally red ABS is preferred as pipe material; however pipes of other origins enabling 20-22 mm nominal diameter and laminar flow can also be used.
- Pipe lengths should be jointed without the existence of T connections.
High sensitivity enables early detection of potential fire situations which may cause loss of life and property and enables to avoid any possible damages. Smoke detector sensitivity is defined with obscuration per metre (obs% per meter) unit. Namely, the obscuration degree of a light beam for a given percentage in one metre is determined. There are three defined sensitivity degrees:
Normal: Sensitivity of normal point smoke detector (5%obs per meter)
Increased: Better than 2%
High: Better than 0,8%
Sensitivity that can be achieved by using many point smoke detectors is possible to be achieved with a single high sensitivity air sampling detector. Apart numerically less usage of air sampling detectors when compared to point detectors, its advantage is to be able to make arrangements in places where point detectors cannot be used because of ambient restrictions. Also including pipe system, its installation and maintenance is cheaper and easier than point detectors. Determination of detector sensitivity is closely related to the number of sample holes in the pipe network. If a detector is set to alert for 0,05% smoke intensity, average system sensitivity will be 1% in a 20 hole system (0,05x20). For instance, When smoke reaches 3 holes, the sensitivity will reach to 0,33% for 1/3. Namely, sensitivity will show an increase directly proportional to the number of holes the smoke reaches.
In air sampling detectors, preferred detectors are the light sampler ones. They operate free of problems against any pollution and has a long operating life. In very dirty and dusty areas, there are not any detectors showing better performance than light sampler detectors.
Light sampler detectors: Air sample entering the detection zone will be sent to high energy light source. If there is smoke particle in the air, it will cause the sampling of the light. Sampled light amount will be proportional to the smoke amount and it will be analysed by solid state light receiver. In order to be protected from dusts, mixed filter or dust obstructive should be used.
Artificial intelligence system used in air sampling system re-sets the sensitivity by taking the smoke amount in the ambient into account and can maintain this sensitivity independently from ambient conditions.
In this sense, it quickly adjusts to the changing ambient conditions. These products are produced in a way to require low maintenance frequency and interfaces are provided for the user to see the error when there is a problem.
Installation cost and installation ease are big advantages of air sampling systems. Moreover in rooms where the system is requested to be invisible, this problem can be solved with coupling covers. Point detectors normally require test and maintenance every six months. In places where there are many point detectors or in rooms where entrance is restricted or blocked, this is not any practical. By placing out of the protected room, air sampling systems can easily be subject to maintenance.