Effect of Automatic Sprinkler Protection on Smoke Control Systems

ecause each data point represents an early stage in thefires, such that the preceding histories <strong>of</strong> the crib fires weremore comparable than in the tests in the one-story room.Again, the dashed diagonal line linking the unsprinklered tothe sprinklered-shielded points shows a strong relationshipbetween the spray density and the HRR <strong>of</strong>a fire,From the dashed line in Figure 3, it can be conservativelyestimated that the HRR <strong>of</strong> the fires would be reducedto less than SO% <strong>of</strong> the nonsprinklered rate at an approximatedensity <strong>of</strong> 0.11 gprn!ft2 (4.5 Llmin·m 2 ). That HRR(for the 1,200-lb crib) was about 97S Btu/s (1,02S kW).For ihe 700-lb (318-kg) wood cribs in the Tower tests(Figure 4), the same SO% reduction in HRR (to S70 Btu/s[600 kW]) occurred at a density <strong>of</strong> 0.13 gprn!ft 2 (S.3Llmin·m 2 ). It is noteworthy that the reduced, sustained heatrelease rates at comparable densities were in proportion tothe mass <strong>of</strong> fuel within the shielded area.Two tests, one in the one-story room and the other inthe Tower, were conducted with the shielding removed sothat the sprinkler spray could extinguish the fires. In bothcases, the fires were extinguished in less than 12 minutes.The HRR values shown in Figures 3 and 4 for the unshieldedfires were peak values, measured just beforesprinklers activated, which then reduced to zero as the firewas extinguished.This leads to the question <strong>of</strong> what level <strong>of</strong> fire controlis expected from a typical sprinkler system designed to meetNFPA Standard 13 (NFPA 1991). That standard distinguishesbetween "fire control" and "fire suppression" andrecognizes that the sprinkler system is deemed to haveachieved .fire control if it limits the size <strong>of</strong> the fire, decreasesthe HRR, and controls ceiling temperatures. The limitson the size <strong>of</strong> the fire (area) are not stated but clearly willbe some fraction <strong>of</strong> the "design area," which is the areaprotected by the maximum number <strong>of</strong> sprinklers permitted(by the NFPA Standard 13 design criteria) to open. Thecontrol over ceiling temperatures also relates to the designarea: combustion gases must be cooled within its boundaryso as not to activate sprinklers outside the design area. Theintended degree <strong>of</strong> control over the heat release rate canthen be inferred: the HRR must be small enough that gastemperatures higher than the sprinkler temperature rating donot spread outside the design area.These experiments demonstrated that sprinklers substantiallyreduced the heat release rates <strong>of</strong> the wood crib fires,even though they were operating at lower spray densitiesthan would be normal for such fuel loads. They alsodemonstrated that the magnitude <strong>of</strong> the heat release ratedepends on the amount <strong>of</strong> fuel present within the shieldedarea and that the fuel within that area will continue burninguntil it is mostly consumed. Knowledge <strong>of</strong> the maximumarea <strong>of</strong> shielding possible in a sprinklered building (excludinga gross error in the system design) and the weight andtype <strong>of</strong> fuel likely to be held within the shielded area couldbe used to estimate the maximum beat release in a sprinklered,shielded fire.It was mentioned earlier that the statistical review done500by Morgan and Hansell (198S) concluded that fires in asprinklered <strong>of</strong>fice building were likely to be between 28Sand 2,8SO Btu/s (300 and 3,000 kW), with a higher probability<strong>of</strong> occurrence <strong>of</strong> fires in the low end <strong>of</strong> the range.The results <strong>of</strong> this series <strong>of</strong> full-scale tests indicate thatshielded fires with heat release rates as high as 97S Btu/s(1,02S kW) are possible, depending on the amount <strong>of</strong> fueland the sprinkler density. The wood cribs in these tests,however, particularly in the one-story room, represented asubstantial fuel mass arranged in an ideal burning formation.Yet, only two sprinklers, operating at minimumdensities appropriate for their listing, kept the HRR below9SO Btu/s (1,000 kW). These measurements suggest thatshielded fires in sprinklered <strong>of</strong>fice buildings are likely tohave heat release rates below 9SO Btu/s (1,000 kW). Theresults also imply that for an occupancy in which shieldedfires are a possibility, design densities higher than theminimum recommended values for unshielded conditionswould be beneficial.<strong>Effect</strong>s <strong>of</strong> <strong>Sprinkler</strong>s on Heat Fluxto SurroundingsFigure S shows the intensity <strong>of</strong> radiant heat energy inthe one-story test room, approximately 6.5 ft (2 m) fromthe wood crib, at mid-height <strong>of</strong> the room, measured atdifferent spray densities. In addition to removing convectiveheat energy from the hot gases given <strong>of</strong>f by the fire,sprinklers reduce fire severity by absorbing radiant beatenergy in the fire compartment. In an unsprinklered fire,heat radiates from the visible flames and from the hot gaslayer that develops in the upper portion <strong>of</strong> the room. Thisradiant heat reflects from surrounding surfaces back to the'¢:~1.41.21.0 -l! o.s -"' ,;=li: 0.6'a!u:I:0.40.20...,.,0.0 '0.00Figure 5<strong>Sprinkler</strong> spray density Lpm/ml2 3 4 s 6 7 8' ' ' ' '1614- 12.,~..,.1 Me10~8 ,;=li:6 'a!:!I ...y·' ' '4y ' y,Yy' ' '2''• 0o.os 0.10 O.lS 0.20<strong>Sprinkler</strong> spray density USgpmlft'Heat flux at mid-height on the south wall <strong>of</strong>the one-story test room versus sprinkler spraydensity.ASH RAE Transactions: Research... ~