Consulting-Specifying Engineer - November 2008 - (Page 29)

trol analysis. Alternate methods like computational fluid dynamics (CFD) and scale modeling can simulate plugholing when it occurs, making a separate plugholing analysis unnecessary. To be capable of simulating plugholing and other fire-related processes, these alternate methods must be high-quality analyses that depend on skill and modeling technique. Historical background In 1974, Spratt and Heselden3 conducted a series of scale model experiments of plugholing in the United Kingdom. This project resulted in a relationship for the maximum volumetric flow at a smoke exhaust inlet that can happen without plugholing. In this article, this maximum volumetric flow is called the critical exhaust. ASHRAE sponsored research about plugholing at the National Research Council of Canada4,5,6. This Canadian research consisted of full-scale experiments and computer simulations. The 2005 version of NFPA 92B has conservative plugholing equations; earlier versions had less conservative plugholing equations. Here, we’ll discuss the 2005 version of NFPA 92B. The plugholing equations are also in a book by John Klote and Douglas Evans published by the International Code Council7. Figure 2 shows the flow pattern where there is no smoke layer and a uniform temperature under the ceiling. In this figure, air flows equally toward the exhaust in all directions. This is similar to what is called sink flow in fluid dynamics. The flow pattern that occurs when a smoke exhaust is working properly is very different, as shown in Figure 3. The buoyancy of the smoke layer works to force the smoke into the exhaust and prevent air from below the smoke layer from being exhausted. The buoyancy increases as the smoke layer temperature and depth increase. Based on this, it seems that the critical exhaust also would increase with increasing smoke layer temperature and smoke layer depth. In fact, the research projects mentioned above have verified this. Plugholing of Air Into Smoke Exhaust Figure 1 Plugholing can result in exposing occupants to smoke. Source: John Klote Number of exhaust inlets To prevent plugholing, the idea is to choose enough smoke inlets so that the smoke exhaust at each smoke inlet is low enough. In this article, the maximum volumetric flow rate without plugholing is called the critical exhaust. The critical exhaust depends on three factors: the temperature rise of the smoke layer above ambient, the depth of smoke layer below the exhaust inlet, and the exhaust location. The plugholing research was done for round exhaust inlets, and it can be extended to other inlets to some extent by the Table 1 Plugholing equations VMAX = 452 d5/2 Ts To To Smin = 0.065Ve Variables: Vmax Ts To d 1/2 () 1/2 Di = 2ab a+b KQc mCp Ts = To + maximum volumetric flow rate without plugholing at Ts (cfm) absolute temperature of the smoke layer (°R) absolute ambient temperature (°R) depth of smoke layer below the lowest point of the exhaust inlet (ft) location factor (1.0 for inlets centered more than 2 times Di from the nearest wall, 2 for exhaust inlets centered closer than 2 times Di from the nearest wall) Di a b Smin Ve K Qc Cp m diameter of the inlet (ft) length of the inlet (ft) width of the inlet (ft) minimum edge-to-edge separation between inlets (ft) volumetric flow rate of one exhaust inlet (cfm) convective fraction (2 for plugholing applications) convective portion of heat release (Btu/s) specific heat of plume gases (0.24 Btu/lb-°R) mass flow rate of smoke plume (lb/s) Consulting-Specifying Engineer • NOVEMBER 2008 29

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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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