Consulting-Specifying Engineer - November 2008 - (Page 32)

12 10 8 6 4 2 located in the ceiling of the atrium? The bottom of the inlet is 9 ft above the bottom of the smoke layer. The temperature difference is still 30 F. From Figure 4 the critical exhaust is 26,000 cfm for an inlet located away from a wall. The total exhaust divided by the exhaust per inlet is 108,000/26,000 = 4.2. So 5 inlets are used. For 5 inlets, the exhaust flow rate per inlet is 21,600 cfm, and the minimum separation distance from Figure 5 is 9.6 ft. An exhaust inlet 4 ft long by 4 ft wide is selected. The equivalent diameter of this inlet is Di = 2ab/(a + b) = 2(4)(4)/(4 + 4) = 4 ft. The ratio d/Di = 9/4 = 2.25. Because this ratio is greater than 2, it is acceptable. Klote recently retired from his own consulting engineering firm, John H. Klote Inc. He has 19 years of experience with research in smoke control and other areas of fire protection engineering with the National Institute of Standards and Technology. He also developed and teaches a series of smoke control seminars for the Society of Fire Protection Engineers. References 1. NFPA 92B, Standard for Smoke Management Systems in Malls, Atria, and Large Spaces, NFPA, Quincy, Mass., 2005. 2. International Building Code. International Code Council, Country Club Hills, Ill., 2006. 3. Sprat, D. and Heselden, A.J.M. Effective Extraction of Smoke from a Thin Layer Under a Ceiling. Fire Research Note 1001, February 1974. 4. Lougheed, G.D. and Hadjisophocleous, G.V. Investigation of Atrium Smoke Exhaust Effectiveness. ASHRAE Transactions, Volume 103, 1997, pp. 519-533. 5. Hadjisophocleous, G.V., Lougheed, G.D. and Cao, S. Numerical Study of the Effectiveness of Atrium Smoke Exhaust Systems. ASHRAE Transactions, Volume 105, 1999, pp. 699-715. 6. Lougheed, G.D., Hadjisophocleous, G.V., McCartney, C. and Taber, B.C. Large-scale Physical Model Studies for an Atrium Smoke Exhaust System. ASHRAE Transactions, Volume 105, 1999, pp. 676-698. 7. Klote, J. H. and Douglas H. Evans. A Guide to Smoke Control in the 2006 IBC. International Code Council, Country Club Hills, Ill., 2007. Minimum separation between inlets (feet) 0 0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 Smoke exhaust at an inlet (cfm) Figure 5 This figure shows the minimum separation distance between exhaust inlets. Source: John Klote tests or high-quality CFD, separate analysis of plugholing is not needed. The graphic method and the underlying equations are not meant for slot exhausts, and analysis by scale modeling or CFD is suggested. Plugholing can be avoided by: • Providing a sufficient number of exhaust inlets so that the exhaust at each inlet does not exceed the critical exhaust • Keeping the exhaust inlets away from each other by at least the minimum separation distance • Making sure that the ratio of the depth of the smoke layer below the exhaust inlet to the equivalent diameter is greater than two. Example For a particular design the ambient temperature is 74 F and the smoke layer temperature is 104 F. The exhaust inlets are located in the side walls of the atrium, and the bottom of the inlet is 7 ft above the bottom of the smoke layer. The total smoke exhaust is 108,000 cfm. Part 1. Determine how many exhaust inlets are needed, calculate the minimum separation between the inlets, and check the inlet size. The temperature difference is 104 – 74 = 30 F. The smoke layer depth below the inlet is 7 ft. From Figure 4 the critical exhaust would be 14,000 cfm for an inlet located away from a wall. Because this inlet is located in the wall, the critical velocity is half that from Figure 4, which is 7,000 cfm. The total exhaust divided by the exhaust per inlet is 108,000/7,000 = 15.4. So 16 inlets are used. For 16 inlets, the exhaust flow rate per inlet is 6,750 cfm, and the minimum separation distance from Figure 5 is 5.3 ft. An exhaust inlet 4 ft long by 1 ft wide is selected. The equivalent diameter of this inlet is Di = 2ab/(a + b) = 2(4)(1)/(4 + 1) = 1.6 ft. The ratio d/Di = 7/1.6 = 4.4. Because this ratio is greater than 2, it is acceptable. Part 2. For the atrium above, how would the calculations change if the inlets were 32 Consulting-Specifying Engineer • NOVEMBER 2008

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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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