Prevent plugholing: Smoke control done right
Many engineers do not know that plugholing can result in occupants being exposed to smoke, but there is a method to prevent this. Plugholing is the pulling of “fresh” air into a smoke exhaust, which can happen when the smoke exhaust flow rate is relatively high, as shown in Figure 1. When fresh air is pulled into the smoke exhaust, the amount of smoke exhausted is reduced, the smoke...
View the full story , including all images and figures, in our monthly digital edition .
Many engineers do not know that plugholing can result in occupants being exposed to smoke, but there is a method to prevent this. Plugholing is the pulling of “fresh” air into a smoke exhaust, which can happen when the smoke exhaust flow rate is relatively high, as shown in Figure 1. When fresh air is pulled into the smoke exhaust, the amount of smoke exhausted is reduced, the smoke layer descends more than intended, and occupants can be exposed to smoke. NFPA 92B has equations to help engineers prevent such failures, and the 2006 International Building Code
In this article, the term “atrium” is used in a generic sense to mean any large volume space including enclosed shopping malls, arenas, exhibition halls, and airplane hangars. The idea of atrium smoke control is that smoke rises from a fire, forms a layer of hot smoke under the ceiling, and is exhausted from this smoke layer to provide tenable conditions below the smoke layer for occupant evacuation.
The number and location of smoke exhaust inlets are the keys to preventing plugholing. When engineers use too few exhaust inlets and locate them poorly, plugholing can result in system failure. This article presents a quick graphic method to determine whether plugholing is a potential problem with your smoke control design. This graphic method is based on the equations listed in Table 1.
The graphic method and the underlying equations are intended for use with the algebraic equation method of atrium smoke control 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.
In 1974, Spratt and Heselden 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.
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 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.
The equivalent diameter is:
Di = equivalent diameter
a = length of the inlet
b = width of the inlet
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 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.
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 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.
|V max||maximum volumetric flow rate without plugholing at T s (cfm)|
|T s||absolute temperature of the smoke layer (°R)|
|T o||absolute ambient temperature (°R)|
|d||depth of smoke layer below the lowest point of the exhaust inlet (ft)|
|γ||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)|
|D i||diameter of the inlet (ft)|
|a||length of the inlet (ft)|
|b||width of the inlet (ft)|
|S min||minimum edge-to-edge separation between inlets (ft)|
|Ve||volumetric flow rate of one exhaust inlet (cfm)|
|K||convective fraction (2 for plugholing applications)|
|Q c||convective portion of heat release (Btu/s)|
|C p||specific heat of plume gases (0.24 Btu/lb-°R)|
|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.|