Consulting-Specifying Engineer - November 2008 - (Page 30)

Ceiling exhaust inlet Ceiling exhaust inlet Smoke layer Figure 2 This shows the flow pattern near a ceiling exhaust inlet with no smoke layer. Source: John Klote equivalent diameter concept. This research is not applicable to slot exhausts, however, and the methods and associated equations in this article are not recommended for inlets that are more than five times longer than they are wide. Research is needed regarding plugholing and slot inlets. One suggestion for systems with slot exhausts is to use scale modeling or CFD modeling. 40,000 ft Figure 3 This shows the flow pattern near ceiling exhaust inlets with a smoke layer. Source: John Klote NFPA 92B requires that the ratio of the depth of smoke layer below the exhaust inlet to the equivalent diameter be greater than two. In general, exhausts located in ceilings tend to be more effective than exhausts located in walls, and a location factor is used to account for this. A location factor of 1.0 is used for the exhaust inlets centered more than twice the inlet diameter from the nearest wall. A location factor of 0.5 is used for exhaust inlets centered less than twice the inlet diameter from the nearest wall. Figure 4 shows the critical flow rate for a location factor of 1.0. The critical flow rate for a location factor of 0.5 is half of that shown in Figure 4. As expected, Figure 4 shows that the critical exhaust is larger for greater smoke layer depths and for hotter smoke layers. In this figure, the temperature rise of the smoke layer is the smoke layer temperature minus the ambient temperature. Separation between inlets If two exhaust inlets are located near each other, the flow around them can be much like the flow around a single inlet. Inlets must have sufficient separation to prevent such adverse interaction. The minimum separation distance depends on the flow rate at the inlet as shown in Figure 5. Because plugholing can cause occupants to be exposed to smoke, it must be prevented. When atrium smoke control analysis is done by high-quality scale model The equivalent diameter is: Di = 2ab a+b Where: Di = equivalent diameter a = length of the inlet b = width of the inlet 9. 5 ay er be lo w in 35,000 le t= 9 ft 8.5 ft Critical exhaust (cfm) t 8f 30,000 De pth of sm ok el f 7.5 7 ft t 25,000 20,000 6.5 f t 15,000 6 ft 5.5 ft 10,000 5 ft 4.5 ft 4 ft 5,000 0 20 40 60 80 100 Temperature rise of smoke layer (F) Figure 4 Shown is the critical exhaust for a ceiling-mounted inlet located away from walls (location factor of 1.0). Source: John Klote 30 Consulting-Specifying Engineer • NOVEMBER 2008

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2008

Consulting-Specifying Engineer - November 2008 Contents Viewpoint News M/E Roundtable Comissioning Lighting Controls Prevent Plugholing: Smoke Control Done Right Commercial Geothermal Heating and Cooling System Design Codes &�Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - November 2008

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