Smoke detector

[sfxf]

Per "1997 Standard Mechanical Code", recirculating air system with a fan capacity great than 2000 cfm shall automatically shut down by means of an approved smoke detector placed in the return air stream. Could anyone explain to me why only return air stream, not discharge air stream also? How to detect the smoke in the discharge side?
How to detect the spark of the motor in a blow-through air-handling unit?

Thanks in advance!

 

[IRstuff]

I would think that the objective is to stop air circulation when a fire starts in the structure, not in the equipment.

If there are filters in the system, the discharge stream may not have sufficient particulate matter to trigger the smoke detector.

TTFN

 

[lilliput1]

Fire & smoke generation will most probably occur in the room. The smoke detector is placed on the return side so it will see room air with smoke, not diluted with outdoor air, if it were at the supply side. If the AHU is > 15,000 CFM and serves multiple floors, it is required to provide smoke detectors at each floor before the duct connects to the return air riser. If the AHU is > 15,000 CFM smoke dampers are required at the AHU return and supply ductwork.

 

[ChasBean1]

The older version of the BOCA Mechanical Code (1993) had the smokes in supply air >2,000 cfm and in return air ducts for systems >15,000 cfm. The newer version of the BOCA Code, The International Mechanical Code (1996) specifies return detectors in systems >2,000 cfm, as you indicated.

However, NFPA-90A (2002), if followed in your area, still requires that that a duct smoke be placed "downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)."

I would do both to be safe. The supply smoke protects you if the supply motor, belt, or bearing smokes itself...

 

[DouginMB]

Be careful spec'ing smoke detectors in the supply duct for air handlers with electric strip heaters. During the summer months dust will accumulate on the heaters. The first time the heaters are energized in the fall or winter, the dust will burn off and cause a nuisance trip of the smoke detector. This isn't so bad if its a stand-alone smoke detector that simply shuts down the air handler, but if it's addressable (tied into the central alarm system), it can be a real hassle.

 

[ChasBean1]

Likewise caution downstream of steam humidifiers...

 

[sfxf]

Thank you all for the responses.

But what is the best way to install the smoke detector in the discharge side of the fan so it can hold 50" static pressure?

 

[ChasBean1]

50" is high. I don't know what the pressure limits are for duct-mounted detectors. Usually the detector is mounted on the outside of the duct in a chamber that uses velocity pressure to drive flow across the detector. Normal air velocity range is 300-4,000 feet per minute. The first page of this link has a pretty good sketch:

http://www.hochiki.com/literature/installation/ha-06-073.pdf

 

[sfxf]

Thanks!