AMCA International inmotion Magazine - Fall 2011 - (Page 15)

applicable to atria with a very large volume above the highest occupied ﬂoor. Natural smoke venting consists of opening a number of large vents in an atrium roof, and smoke the smoke ﬂows naturally out the vents. When conditions permit, natural venting is used in Europe, the United Kingdom and Australia. However, natural venting is not common in the United States. Plume Dynamics Smoke rises above a ﬁre in a plume, and a smoke layer forms under the ceiling of the atrium. As the smoke rises, it pulls air from the surroundings into the plume. It follows that the mass ﬂow of the plume increases with height, and the temperature of the plume decreases with height. The plume temperature and ﬂow rate depend on the clear height and the heat release rate (HRR) of the ﬁre. The clear height is the height from the bottom of a ﬁre to the bottom of the smoke layer, as shown in Figure 1. Plume dynamics have been the subject of many fullscale ﬁre experiments, and a number of equations have been developed to describe how plumes work. Method of Analysis The two most commonly used methods of analysis for atrium smoke control are the algebraic equation method and computational ﬂuid dynamics (CFD). The algebraic equation method consists of a set of equations that include the plume equations mentioned above. This method is in NFPA 92B,1 and it is explained in more detail by Klote and Evans2 and Klote and Milke.3 These equations can be programmed into a spread sheet program like Excel so that calculations can be done quickly. The large amounts of exhaust ﬂow for an atrium are shown in Figure 2. The ﬂows in this ﬁgure were calculated by the algebraic equation method for an axisymmetric plume and an ambient temperature of 70°F. An axisymmetric plume is one that forms inside the large atrium space away from walls, and other kinds of plumes can be analyzed with the algebraic equation method. In Figure 2, it can be seen that an atrium with a clear height of 40 ft and a design ﬁre of 5,000 Btu/s, a smoke exhaust of about 160,000 cfm would be needed. Figure 3 shows that this exhaust would be at about 155°F. The temperatures of Figure 3 were calculated for the same conditions as Figure 2. The amount of exhaust ﬂow calculated by the algebraic equation method can be different from that calculated by CFD modeling by as much as 15%. Often a CFD analysis results in lower smoke exhaust because CFD is more realistic and is not based on conservative assumptions. The idea of CFD modeling is to divide a space into a large number of small cells, and use a computer to solve the governing equations for the ﬂows, pressures and temperatures in that space. The governing equations are a system of non-linear, partial differential equations that cannot be programmed into a spread sheet. W W W. A M C A . O R G Fire dynamic simulator (FDS) is a CFD model that was developed at the National Institute for Standards and Technology (NIST) speciﬁcally for ﬁre applications.4 FDS is available from the NIST at no cost. Because FDS is extensively used around the world for ﬁre applications, it has become the de facto standard CFD model for atrium smoke control. The disadvantage with CFD modeling is that a single simulation takes many hours of computer time, and a CFD analysis of an atrium smoke control system requires a number of simulations. Makeup Air Makeup air can be supplied by a fan system, or it can be through open vents to the outside. Makeup air needs to be well below the smoke layer. It is recommended that makeup air for fan powered smoke exhaust systems be designed at 85% to 95% of the exhaust. The idea is that the remaining air (5% to Figure 2: Atrium exhaust fan ﬂow rate. Figure 3: Atrium exhaust fan temperature. A M C A I N T E R N AT I O N A L inmotion FA L L 2 0 1 1 15 http://WWW.AMCA.ORG

Table of Contents for the Digital Edition of AMCA International inmotion Magazine - Fall 2011

AMCA International inmotion Magazine - Fall 2011
President’s Message
News
Code Watch
Fan Efficiency Grades: Answers to Frequently Asked Questions
Fans and Atrium Smoke Control
Fire Dampers and Smoke Dampers: The Difference is Important
Fan and Damper Guide Specs Update
Calendar

AMCA International inmotion Magazine - Fall 2011