MIRROR: G K BHAT
Each device got its own limitation. It does not mean that we limit to the limitation of the devices. According to our requirements we should redesign the equipment and expand our limitation to fit the necessity. Like big ware houses, aircraft hangers etc. are big halls and to cover such places we need wider angle beam detectors. In our previous topic we discussed the spot beam detector and it limitation. To overcome the limitation we can go for a beam detector which can cover wider area.
limitation of the projected beam smoke detectors is that they are a line-of-sight devices is and are therefore subject to interference from any object or person which might enter the beam path. This makes their use impractical in most occupied areas with normal ceiling heights. However, many facilities have areas where beam smoke detectors are not only acceptable, but are the detector of choice. High ceiling areas such as atriums, lobbies, gymnasiums, sports arenas, museums, church sanctuaries, as well as factories and warehouses might be candidates for beam smoke detectors. Many of these applications present special problems for the installation of spot-type detectors, and even greater problems for their proper maintenance. The use of beam smoke detectors in many of these areas may reduce these problems since fewer devices may be required, and the devices can be mounted on walls, which are more accessible than ceilings. Application for areas like these are explained in the NFPA 92B, Guide for Smoke Management in Malls, Atria and Large Areas.
High air movement areas present a special problem for detecting smoke for both spot-type and beam smoke detectors because the propagation of smoke developing under normal conditions may not occur. High air velocity may blow smoke out of the sensing chamber of a spot-type detector. Careful consideration should be given to the spot-type detector’s performance where air velocities exceed 300 feet per minute (fpm) or when air changes in the protected area exceed 7.5 changes per hour. Beam smoke detectors are not tested for listing purposes for stability in high air flow because high air movement does not have as great an effect on a beam smoke detector. A beam smoke detector’s sensing range can be as long as a football field (maximum beam range is typically 330’) not the one or two inch dimension of a spot-type sensing chamber. It is therefore less likely that smoke will be blown out of the beam smoke detector’s sensing range. Although reduced spacing is not required in high air flow areas, attention should be given to the anticipated behaviour of smoke in these applications.
Stratification occurs when smoke is heated by smouldering or burning materials and becomes less dense than surrounding cooler air. The smoke rises until there is no longer a difference in temperature between the smoke and the surrounding air. See NFPA 72-2002, A.188.8.131.52. Therefore, stratification may occur in areas where air temperature may be elevated at the ceiling level, but especially where there is a lack of ventilation. On smooth ceilings, beam smoke detectors should generally be mounted a minimum of 12 to 18 inches from the ceiling. In many cases, however, the location and sensitivity of the detectors shall be the result of an engineering evaluation that includes the following: structural features, size and shape of the room and bays, occupancy and uses of the area, ceiling height, ceiling shape, surface and obstructions, ventilation, ambient environment, burning characteristics of the combustible materials present, and the configuration of the contents in the area to be protected. The results of an engineering evaluation may require an installation farther from the ceiling to defeat the effects of stratification or other obstructions. NFPA 72, 2002 Section A.184.108.40.206 – “In high ceiling areas, projected beam-type detectors at different levels also should be considered.” (See also NFPA 92B 2000, section 3.4)
Due to the inherent capabilities of projected type beam detectors they are often installed in locations where spottype detection is impractical. Projected type beam smoke detectors are ideally suited for environmental conditions that might include high ceilings, dusty and dirty environments, or environments that experience temperature extremes. Often these conditions present special problems for the installation of spot-type detectors and even greater problems for their proper maintenance. Due to the inherent flexibility of mounting locations and large coverage area of projected type beam detectors often the conditions above can be addressed or minimized. Some examples of applications for beam detectors might include freezers, aircraft hangars, cold storage warehouses, shipping warehouses, enclosed parking facilities, sporting arenas and stadiums, concert halls, barns, or stables. Some of these environments might be considered too hostile for spot-type smoke detectors. If the environment is considered to be hostile, more insensitive alarm threshold settings should be used.
Note: During our discussion of the topic we will be coming across with the words like NFPA and UL standards. These are some standards accepted globally. We will discuss the same in our future topics separately
Will be continued..
G K Bhat