Duct Detectors

[Charles Jones]

If duct detectors are rated to operate in units from 300-4000cfm, what do you do for a unit that is 15,000cfm? A detector is required, but the cfm exceeds the rated velocity for the detector.

 

[gadfly56]

If duct detectors are rated to operate in units from 300-4000cfm, what do you do for a unit that is 15,000cfm? A detector is required, but the cfm exceeds the rated velocity for the detector.

You are confusing velocity (ft/min) with volumetric flow rate (ft³/min). The detector is listed for 300-4,000 ft/min. That 4,000 ft/min is equivalent to 45 miles/hr. If the supply duct coming off that unit were 20" x 40", the velocity in the duct would be 2.700 ft/min. Design guides (McGill, for one) suggest that a 15,000 cfm supply should aim for about 1,500 ft/min as a design. They don't show any units approaching 4,000 ft/min until the unit capacity is 100,000 cfm.