Shedding Light on the Power of Beam Smoke Detection
The technical evolution of life-safety tools offers the promise of making our world a better-protected place. A prime example: traditional heat-sensing smoke detectors and the emergence of the projected smoke beam.
The simple sophistication of the beam smoke detector has greatly enhanced fire protection and made installing fire safety solutions a breeze in settings that once proved tedious.
But like any tool, the projected beam smoke detector will fail badly if not properly applied.
To spotlight the critical aspects of implementation, the following report discusses mechanics behind projected smoke beams, advancements in design, code compliance and installation methods.
Environmental Factors Dictate Which Type of Beam to Deploy
There are two types of smoke beams in use today: through-beam and retro-reflective.
Adverse environments can prompt the use of these devices, such as where spot-type smoke detectors could be damaged and malfunction. There is the potential of conventional detectors to false alarm from airborne dust and other pollutants. Aesthetics can also justify the use of smoke beam devices.
Through-beam sensors are mounted one in front of the other and prove to be excellent for harsh environments, says Ryan Carr, an engineering technology major at South Plains College in Levelland, Texas.
“This type of sensor is used to detect objects at long ranges,” he says. “The disadvantage is the fact that you have to wire both the receiver and emitter.”
The retro-reflective sensor utilizes a reflector to reflect the light beam back to the sensor. “When [something obstructs] the light beam, the receiver changes the state of the output contacts,” Carr says.
Thus, the primary advantage of the retro-reflective device is a relatively quick installation since wiring is required at only one end of the beam.
No matter which technology is deployed, the obscuration of the infrared (IR) light beam is integral to smoke detection.
Co-authors Charles Aulner and Bryan McLane explain in “Fire Alarm Systems Design & Installation,” published by the National Training Center in Las Vegas: “Light is projected from the transmitter to the receiver, and as long as the receiver can see the transmitter, the receiver is in a non-alarm condition. If the receiver loses sight of the transmitter for a short period of time, it goes into alarm.”
In the past, it was accepted that through-beam systems provided superior linear coverage. That didn’t preclude the use of its retro-reflective cousin, but it did limit the number of applications deployed by fire technicians. That’s changed. As explained in the following section, retro-reflective smoke beams are now on par with through-beam systems in this regard.
Through-beam detectors still have their advantages, though, according to Bob Selepa, an electrical engineer with System Sensor in St. Charles, Ill.
“You can do some things with a dual-ended beam detector that you cannot do with a retro-reflective model,” he says. “For example, you can shoot through a narrow pathway. With a retro-reflective unit, you have to be concerned about reflection, so you need a straight shot with a good bit of clearance on each side of the beam.”
Improved Beam Smoke Devices Use LED’s for Easy Alignment
The most recent developments in beam smoke technology offer improved alignment techniques and the emergence of retro-reflective detection ranges comparable to through-beam systems. Beam alignment with vintage through-beam systems involved the use of mirrors and a meter.
“With a traditional, older transmitter and receiver you have to go to the receiver and look in a set of mirrors to aim it at the transmitter,” says Jim Mottorn, project manager for fire products with Bosch Security in Fairport, N.Y. “Then you have to go to the transmitter and look into another set of mirrors to aim it at the receiver. You may have to do this several times until the signal peeks.”
Alignment can be tedious and time-consuming using this method, especially if the device is placed high up in a hard-to-reach ceiling. Through the years, however, manufacturers have worked to expedite the process. Most newer makes and models use a set of LEDs that light up when the signal is satisfactory, says Mottorn. In some cases a readout indicates signal strength.
Selepa explains that System Sensor units can be aligned by either a course adjustment using the visual method or an LED display. The LED display provides the installer with a two-digit readout that indicates the strength of the returning light beam, providing a more reliable alignment than a set of mirrors with a meter or even a series of LED point indicators, Selepa says.
Another technology advance is the adoption of the retro-reflective technique for use in projected beam smoke detectors. In burglar alarm-type photoelectric beams, the retro-reflective method usually means a shorter detection path than the through-beam system. This was meant to assure stability in the presence of condensation, fog, snow, rain and other environmental contaminants.
Today’s retro-reflective beam smoke detectors, however, can achieve a longer detection range because of an improved reflector assembly. When used in long-range applications, a larger assembly is employed to assure more signal return, thus they are more forgiving when the transmitter/receiver unit is slightly misaligned with the reflector.
“You can be off as much as 10° and you can still make it work,” says Selepa. “You can pretty much site it with your eye and hit it most of the time.”
Beams Offer Versatility for Stair Towers and Smooth Ceilings
Smoke beam detectors are commonly installed so that the IR beam is parallel to the ceiling surface. However, they can be installed vertically and at other angles, as a situation demands. A good example of this is an exceptionally high stair tower where the best approach for smoke detection might be to shoot a vertical beam top to bottom.
When in close proximity to people, such as a stair tower, there is always a chance that the IR beam could be blocked. NFPA 72, Section A.5.7.3.4.8, 2002 Edition, suggests that smoke beam detectors not produce an alarm condition should this occur: “Where the light path of a projected beam-type detector is abruptly interrupted or obscured, the unit should not initiate an alarm. It should give a trouble signal after verification of blockage.”
Most, if not all, smoke beam makers comply with this suggested tactic.
Most of the time beam devices are placed on ceilings. Here an advantage is realized because a single smoke beam can be used instead of spot-type smoke detectors.
According to Section 5.7.3.4.5: “A projected beam-type smoke detector shall be considered equivalent to a row of spot-type smoke detectors for level and sloping ceiling applications.”
In large room applications, placement can be a bit complicated. According to NFPA 72, Section A.5.7.3.4, 2002 Edition, it’s suggested that on smooth ceilings beam units can be installed parallel to one another with no more than 60 feet in between. The starting distance from each sidewall, according to the same code section, should be no more than half of this distance, or 30 feet (see drawing on page 74).
Wall mount applications are probably most common, assuming a distance of 12-18 inches from beam to ceiling. But beam smoke units can be mounted on the ceiling as well.
According to Section A.5.7.3.4, NFPA 72, 2002 Edition, “In some cases, the light beam projector is mounted on one end wall, with the light beam receiver mounted on the opposite wall. However, it is also permitted to suspend the projector and receiver from the ceiling at a distance from the end walls not exceeding one-quarter the selec
ted spacing.”
This and all other mounting parameters should always be performed according to the manufacturer’s installation instructions.
Consult Code for Placement and Coverage on Sloped, Shed Ceilings
As covered in the last section, the most common mounting method employed by fire technicians is placing them on high, smooth ceilings. The parameters set forth by NFPA 72 also include peaked and sloped surfaces.
For peaked ceilings, the first beam smoke detector should be positioned within 3 feet of the peak (Section 5.7.3.5, NFPA 72, 2002 Edition). Additional detectors should be positioned according to the common, smooth ceiling detection distance, which is usually 60 feet from the first. This type of installation is common in cathedral ceilings in churches.
NFPA 72 also covers the issue of placement on shed-type ceiling surfaces. Here the first beam smoke detector should be placed within 3 feet of the highest point of the surface. When additional units are necessary, code says to follow the same horizontal spacing rule previously covered, which is usually 60 feet between beam devices.
Whether installed on the ceiling or wall, fire technicians must also consider the distance between the IR beam and any stationary or temporary wall partitions within the protected area. This is because of how smoke commonly stratifies at ceiling level.
According to NFPA 72, Section 5.7.3.8, 2002 Edition: when a wall partition exists below the 18-inch mark relative to the ceiling, no adjustment in smoke beam placement is necessary. However, when a wall partition exists closer than 18 inches, the effects of smoke travel must be looked at as the installer seeks to reduce the spacing of detectors.
A reduction in detector spacing is necessary because partitions can affect the ceiling jet as it flows across the surface of the ceiling. For further instruction on space reduction, consult Section 5.7.3.8 of the National Fire Alarm Code Handbook, 2002 Edition, published by NFPA.
Beam Smoke Detectors Are Less of a Hassle Than Spot-type
Smoke beam detectors can be used effectively above suspended ceilings to cover vast expanses; a big advantage over conventional spot-type detectors. Fire technicians also commonly install them under raised floors, such as computer rooms. According to Section 5.7.3.7, NFPA 72, 2002 Edition, protected areas such as these are to be considered as separate rooms. In other words, you cannot install smoke beams under a raised computer floor and not protect the area within the computer room itself.
In this case, spot-type smoke detectors would be installed within the computer room, while beam units could be used above the suspended ceiling and below the raised flooring. Utilizing smoke beams in both of these instances makes for easy installation and servicing.
Where spot-type smoke detectors are employed under a raised floor, considerable time can be spent looking for an analog-type smoke detector that has gone into alarm. (In addressable systems, assuming the as-built drawings are detailed enough and readily available, finding a lone detector shouldn’t be too difficult.)
Ready and immediate identification is rarely an issue with smoke beam detectors because in many applications, a remote test station is used. In most instances, the installer will place this device in an easy-to find location.
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